Prog. Polym. Sci. 29 (2004) 183–275
www.elsevier.com/locate/ppolysci
Hyperbranched polymers: from synthesis to applications
C. Gao*, D. Yan*
College of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, China
Received 20 August 2003; revised 20 August 2003; accepted 18 December 2003
Abstract
Over the past 15 years, hyperbranched polymers have received much attention due to their unique chemical and physical
properties as well as their potential applications in coatings, additives, drug and gene delivery, macromolecular building
blocks, nanotechnology, and supramolecular science. Hyperbranched polymers can be prepared by means of singlemonomer methodology (SMM) and double-monomer methodology (DMM). In SMM, the polymerization of an ABn or
latent ABn monomer leads to hyperbranched macromolecules. SMM consists of at least four components: (1)
polycondensation of ABn monomers; (2) self-condensing vinyl polymerization; (3) self-condensing ring-opening
polymerization; (4) proton-transfer polymerization. In DMM, direct polymerization of two suitable monomers or a
monomer pair gives rise to hyperbranched polymers. A classical example of DMM, the polymerization of A2 and Bn
ðn . 2Þ monomers, is well known. Recently, a novel DMM based on the in situ formation of ABn intermediates from
specific monomer pairs has been developed. This form of DMM is designated as ‘couple-monomer methodology’ (CMM)
to clearly represent the method of polymerization. Many commercially available chemicals can be used as the monomers in
these systems, which should extend the availability and accessibility of hyperbranched polymers with various new end
groups, architectures and properties. Because a number of comprehensive reviews have been published on SMM, research
involving DMM is emphasized here. In addition, recent developments in the modification, functionalization and application
of hyperbranched polymers are described.
q 2004 Elsevier Ltd. All rights reserved.
Keywords: Hyperbranched polymer; Dendritic polymer; Polymer brush; Modification; Functionalization; Application; Surface
Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.1. History of hyperbranched polymers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.2. Synthesis methodology. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.2.1. SMM—polycondensation of AB2 monomers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.2.2. SMM—self-condensing vinyl polymerization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.2.3. SMM—self-condensing ring-opening and proton-transfer polymerizations . . . . . . . . . . .
1.2.4. DMM—polymerization of two types of monomers . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2. ‘A2 þ B3’ methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
* Corresponding authors. Tel.: þ 86-21-54742665; fax: þ 86-21-54741297.
E-mail addresses: [email protected] (C. Gao); [email protected] (D. Yan).
0079-6700/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.progpolymsci.2003.12.002
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C. Gao, D. Yan / Prog. Polym. Sci. 29 (2004) 183–275
3. Couple-monomer methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.1. General description of CMM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2. ‘A2 þ BB20 ’ approach to hyperbranched poly(sulfone amine)s and poly(ester amine)s . . . . . . . . .
3.3. ‘A2 þ CBn’ approach to hyperbranched poly(urea urethane)s . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.4. ‘AB þ CDn/Cn’ approach to hyperbranched poly(amine ester)s and poly(amide amine)s . . . . . . .
3.4.1. ‘AB þ CDn approach’ to hyperbranched poly(amine ester)s . . . . . . . . . . . . . . . . . . . . . .
3.4.2. ‘AB þ Cn’ approach to hyperbranched poly(amide amine)s . . . . . . . . . . . . . . . . . . . . . .
3.5. ‘Ap þ CB2/Bn’ approach to hyperbranched poly(ester amide)s and polyesters . . . . . . . . . . . . . . .
3.5.1. ‘Ap þ CB2’ approach to hyperbranched poly(ester amide)s . . . . . . . . . . . . . . . . . . . . . .
3.5.2. ‘Ap þ Bn’ approach to hyperbranched polyesters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.6. ‘AAp/Ap2 þ B2’ approach to hyperbranched polyesters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.7. ‘A2 þ B2 þ BB20 ’ approach to highly branched copolymers . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4. Modification of hyperbranched polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.1. End-capping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2. Terminal grafting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.3. Surface growing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.3.1. On the surface of Au . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.3.2. On the surface of SiO2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.3.3. On the surface of Si . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.3.4. On the surface of PE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.3.5. On the other surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.4. Hypergrafting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5. Application of hyperbranched polymers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.1. Conjugated functional materials: optical, electronic and magnetic properties. . . . . . . . . . . . . . . .
5.1.1. Hyperbranched poly(phenylenevinylene) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.1.2. Hyperbranched poly(b,b-dibromo-4-ethynylstyrene) . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.1.3. Hyperbranched poly(arylene/1,1-vinylene)s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.1.4. Hyperbranched polyarylenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.1.5. Hyperbranched non-linear optical polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.1.6. Hyperbranched coumarin-containing polymers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.1.7. Hyperbranched polythiophenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.1.8. Hyperbranched polyanilines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.2. Polymer electrolytes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.3. Nanomaterials and biomaterials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.3.1. Nanomaterials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.3.2. Biomaterials. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.4. Supramolecular chemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.5. Patterning of hyperbranched polymer films . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.6. Coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.7. Modifiers and additives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.8. Other applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6. Concluding remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1. Introduction
Hyperbranched polymers are highly branched
macromolecules with three-dimensional dentritic
architecture. Due to their unique physical and chemical
properties and potential applications in various
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fields from drug-delivery to coatings, interest in
hyperbranched polymers is growing rapidly, as confirmed by theincreasing number of publications (Fig. 1).
As the fourth major class of polymer architecture,
coming after traditional types which include linear,
cross-linked and branched architectures, dendritic
C. Gao, D. Yan / Prog. Polym. Sci. 29 (2004) 183–275
Nomenclature
ACPA
AFM
ATRP
BIEM
0
4,4 -azobis(4-cyanopentanoic acid)
atomic force microscopy
atom transfer radical polymerization
2-(2-bromoisobutyryloxy)ethyl methyacrylate
bis-MPA 2,2-bis-hydroxymethyl propionic acid
BPEA 2-2-(bromopropionyloxy)ethyl acrylate
Bu4Nþ tetra-n-butylammonium
CAC
critical association concentration
CAH 2-chloroethylamine hydrochloride
CMM couple-monomer methodology
CP
chitosan powder
CPS
chloromethylstyrene
D
dendritic unit
DB
degree of branching
DBOP diphenyl(2,3-dihydro-2-thioxo-3-benzoxazolyl)phosphonate
DCC
N,N-dicylohexylcarbodiimide
DIPC diisopropylcarbodiimide
DMA dynamic mechanical analysis
DMAc N,N-dimethylacetamide
DMAP 4-(dimethylamino)pyridine
DMF N,N-dimethylformamide
DMM double-monomer methodology
DPMP diphenylmethylpotassium
DSC
differential scanning calorimetry
EG
ethylene glycol
EHBP epoxy-functionalized hyperbranched polyesters
EL
electroluminescence
EO
ethylene oxide
ESI
electrospray ionization
GTP
group transfer polymerization
L
linear unit
LALLS low-angle laser light scattering
LCST lowermost critical solution temperature
LDA
lithium diisopropylamide
MA
methyl acrylate
MAH maleic anhydride
MALDI-TOF matrix-assisted laser desorption/ionization time-of-flight
Me6-TREN tri(2-(dimethylamino)ethyl)amine
MHO 3-methyl-3-(hydroxymethyl)oxetane
MMA methyl methacrylate
Mn
number average molecular weight
185
MOI
2-methacryloyloxyethyl isocyanate
MUA mercaptoundecanoic acid
weight average molecular weight
Mw
NIR
near infrared
NMP N-methyl-2-pyrrolidone
NMR nuclear magnetic resonance
PA
polyaniline
PAA
poly(acrylic acid)
PAAM poly(allylamine)
PCL
poly(1-caprolactone)
PCS
photon correlation spectroscopy
PDI
polydispersity index ðMw =Mn Þ
PDMA poly(2-dimethylaminoethyl methacrylate)
PE
polyethylene
PEHO poly[3-ethyl-3-(hydroxymethyl)oxetane]
PEI
poly(ethyleneimine)
PEO
poly(ethylene oxide)
PG
hyperbranched polyglycerol
PL
photoluminescence
PMDETA pentamethyldiethylenetriamine
PMHO poly[3-methyl-3-(hydroxymethyl)oxetane]
PP
polypropylene
PPMA phosphorus pentoxide/methanesulfonic
acid
PPO
poly(propylene oxide)
PPV
poly( p-phenylenevinylene)
PS
polystyrene
PTBA poly(tert-butyl acrylate)
PTHF polytetrahydrofuran
PTP
proton-transfer polymerization
PVK
poly(9-vinylcarbazole)
ROMBP ring-opening multibranching polymerization
SCROP self-condensing ring-opening polymerization
SCVP self-condensing vinyl polymerization
SDS
sodium dodecyl sulfate
SEC
size exclusion chromatography
SHG
second harmonic generation
SMM single-monomer methodology
T
terminal unit
Td
decomposition temperature
TEM transmission electron microscopy
Tg
glass transition temperature
TGA
thermogravimetric analysis
THF
tetrahydrofuran
186
Tm
TPA
TPP
C. Gao, D. Yan / Prog. Polym. Sci. 29 (2004) 183–275
melting point
terephthaldehyde
triphenylphosphine
architecture consists of six subclasses: (a) dendrons and
dendrimers; (b) linear-dendritic hybrids; (c) dendrigrafts or dendronized polymers; (d) hyperbranched
polymers; (e) multi-arm star polymers; (f) hypergrafts
or hypergrafted polymers (Fig. 2) [1]. The first three
subclasses exhibit perfect structures with a degree of
branching (DB) of 1.0, while the latter three exhibit a
random branched structure. Dendrons and dendrimers
with high regularity and controlled molecular weight
are prepared step by step via convergent and divergent
approaches. A linear polymer linked with side dendrons
is called as dendronized polymer. Dendronized polymers can be obtained by direct polymerization of
dendritic macromonomers, or by attaching dendrons to
a linear polymeric core.
A number of excellent reviews have been published
on dendrimers, linear-dendritic hybrids, and dendronized polymers, covering synthesis, functionalization,
supramolecular self-assembly and applications [1 – 34].
This article reviews the synthesis, modifications, and
applications of random hyperbranched and hypergrafted polymers, focusing on the recently developed
novel synthetic strategy for hyperbranched polymers.
1.1. History of hyperbranched polymers
As shown in Table 1, showing the development
of hyperbranched polymers, highly branched
Fig. 1. Scientific publications as a function of publication year searched by SCIE (ISI web of science) with hyperbranched as the topic.
TsCl
tolylsulfonyl chloride
UV – Vis ultraviolet – visible spectroscopy
WAXS wide-angle X-ray scattering
artificial molecules have a long and complex history.
The history of hyperbranched macromolecules can be
dated to the end of 19th century, when Berzelius
reported the formation of a resin from tartaric acid
(A2B2 monomer) and glycerol (B3 monomer) [35].
Following the Watson Smith report of the reaction
between phthalic anhydride (latent A2 monomer) or
phthalic acid (A2 monomer) and glycerol (B3 monomer) in 1901 [35], Callahan, Arsem, Dawson, Howell,
and Kienle, et al. [35 –37] studied that reaction further,
obtaining results and conclusions still used today. For
example, Kienle [36] showed that the specific viscosity
of samples made from phthalic anhydride and glycerol
was low when compared to numerous specific
viscosity values given by Standinger for other
synthetic linear polymers, such as polystyrene. In
1909, Baekeland [38] introduced the first commercial
synthetic plastics, phenolic polymers, commercialized
through his Bakelite Company. The cross-linked
phenolic polymers are obtained by the polymerization
of soluble resole precursors made from formaldehyde
(latent A2 monomer) and phenol (latent B3 monomer).
Just prior to gelation, these polymers have a so-called
random hyperbranched structure.
In the 1940s, Flory et al. [39 – 43] used statistical
mechanics to calculate the molecular weight distribution of three-dimensional polymers with trifunctional and tetrafunctional branching units in the state
of gelation, and developed the ‘degree of branching’
and ‘highly branched species’ concepts. However,
both the experiments and calculations mentioned
above are based on polycondensation of bifunctional
A2 monomer with trifunctional B3 monomers, so
gelation occurs when the degree of polymerization
approaches the critical condition. In 1952, Flory [44]
developed the theory that highly branched polymers
can be synthesized without the gelation by polycondensation of a monomer containing one A
functional group and two or more B functional ones
capable of reacting with A (ABn monomer, n $ 2).
Finally, in 1982, Kricheldorf [45] obtained highly
C. Gao, D. Yan / Prog. Polym. Sci. 29 (2004) 183–275
187
Fig. 2. Schematic description of dendritic polymers.
branched polyesters by copolymerization of AB and
AB2 type monomers.
‘Hyperbranched polymer’ was first coined by Kim
and Webster [46,47] in 1988 when the authors
intentionally synthesized soluble hyperbranched polyphenylene. Since then, hyperbranched polymers have
attracted increasing attention owing to their unique
properties and greater availability as compared with
dendrimers.
The other category contains methods of the doublemonomer methodology (DMM) in which direct
polymerization of two types of monomers or a
monomer pair generates hyperbranched polymers.
Table 2 shows the synthetic methodologies and
approaches to hyperbranched polymers.
1.2. Synthesis methodology
Year
Case
Lead authors
Reference
Up to now, the synthetic techniques used to prepare
hyperbranched polymers could be divided into two
major categories. The first category contains techniques of the single-monomer methodology (SMM),
in which hyperbranched macromolecules are synthesized by polymerization of an ABn or a latent ABn
monomer. According to the reaction mechanism, the
SMM category includes at least four specific
approaches: (1) polycondensation of ABn monomers;
(2) self-condensing vinyl polymerization (SCVP); (3)
self-condensing ring-opening polymerization
(SCROP); (4) proton-transfer polymerization (PTP).
Before
1900
1901
Tartaric acid þ glycerol
Berzelius
[35]
Glycerol þ phthalic
anhydride
Phenolic þ formaldehyde
Glycerol þ phthalic
anhydride
Molecular size
distribution in theory
ABn polymerization
in theory
AB2 þ AB copolymerisation
AB2 homopolymerization
Smith
[35]
Baekeland
Kienle
[38]
[35–37]
Flory
[39–41]
Flory
[44]
Kricheldorf
Kim/Webster
[45]
[46,47]
Table 1
History of hyperbranched polymers
1909
1929–
1939
1941
1952
1982
1988–
1990
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C. Gao, D. Yan / Prog. Polym. Sci. 29 (2004) 183–275
Table 2
Synthesis strategies and approaches of hyperbranched polymers
Methodology
Approach
Lead
author
Year
Reference
SMM
Polycondensation
of ABn
SCVP
SCROP
Kim/
Webster
Fréchet
Suzuki/
Penczek
Fréchet
1988
[46,47]
1995
1992/
1999
1999
[101]
[128,131]
Jikei/
Kakimoto
1999
[159]
Emrick/
Fréchet
1999
[160]
Yan/Gao
Gao/Yan
Gao/Yan
Gao/Yan
Gao/Yan
Gao/Yan
DSM
research
2000
2000
2001
2001
2001
2001
2000
[161,162]
[163]
[164]
[164]
[164]
[164]
[165,166]
PTP
DMM
A2 þ B 3
Polycondensation
of A2 and B3
monomers
[141]
CMM
A2 þ BB20
A2 þ B2 þ BB20
A2 þ CBn
AB þ CDn
Ap þ B n
AAp þ B2
Ap þ CB2
1.2.1. SMM—polycondensation of AB2 monomers
A broad range of hyperbranched polymers, including hyperbranched polyphenylenes [46 – 49], polyethers [50 – 53], polyesters [54 – 76], polyamides
[77 – 82], polycarbonates [83], and poly(ether
ketone)s [84 – 87], are prepared via one-step polycondensation of ABn type monomers. If one group of
ABn monomers contains double or triple bonds, small
molecules may not be formed in the polymerization.
Through polyaddition of the ABn monomers, hyperbranched polyurethanes [88 – 91], polycarbosilanes
[92 – 95], polyamides [96,97], and poly(acetophenone)s [98,99] have been successfully obtained.
AB3, AB4, AB5, and even AB6 monomers are also
used to synthesize hyperbranched polymers while
controlling the branching pattern [94,100].
1.2.2. SMM—self-condensing vinyl polymerization
SCVP was invented by Fréchet and coworkers in
1995 [101]. This polymerization method is quite
versatile, as hyperbranched polymers can
be approached via polymerization of AB vinyl
monomers. In the reaction, the B groups of the AB
monomers are activated to generate the initiating Bp
sites. Bp initiates the propagation of the vinyl group A
in the monomer, forming a dimer with a vinyl group, a
growth site, and an initiating site. The dimer can
function as an AB2 monomer, and undergo further
polymerization to yield the hyperbranched polymer.
In SCVP, the activities of chain propagation of the
growth sites and the initiating sites differ, resulting in
a lower DB when compared to the DB of the
hyperbranched polymer prepared via polycondensation of AB2 monomers. The theoretical maximum DB
of SCVP is 46.5% [127]. On the other hand, SCVP
does exhibit some disadvantages. For example, side
reactions may lead to gelation, the molecular weight
distribution is usually very broad, and it is difficult to
determine DB directly via an NMR analysis. In order
to avoid crosslinking, living/controlled polymerizations such as atom transfer radical polymerization
(ATRP) and group transfer polymerization (GTP) are
combined in SCVP [121 – 126]. Monomers used in
SCVP and polymerization results such as numberaverage molecular weight ðMn Þ and polydispersity
index (PDI, Mw =Mn ) are summarized in Table 3
[101 –126].
1.2.3. SMM—self-condensing ring-opening and
proton-transfer polymerizations
Hyperbranched polyamines [128,129], polyethers
[130 – 138] and polyesters [139,140] have been
prepared through SCROP. Table 4 shows some of
the monomers employed in SCROP and the polymerization results. Finally, hyperbranched polyesters
with epoxy or hydroxyl end groups [141 – 143] and
hyperbranched polysiloxanes [144] were synthesized
through PTP. A number of reviews have been
published on hyperbranched polymers prepared by
SMM [145 –158].
1.2.4. DMM—polymerization of two types of
monomers
DMM can be divided into two main subclasses
based on the selected monomer pairs and different
reaction pathways. The classical one, the polymerization of A2 and B3 (or Bn, n . 2) monomers,
taken up in Section 2, can be called the ‘A2 þ B3’
methodology, and was first adopted intentionally
to prepare soluble hyperbranched polymers by
C. Gao, D. Yan / Prog. Polym. Sci. 29 (2004) 183–275
189
Table 3
The monomers used in SCVP and the polymerization results
Monomer
Condition
Mn (GPC)
PDI
Reference
SnCl4 Bu4NþBr2 220 to 215 8C
660,000–17,000
9.8– 2.9
[101]
ATRP
6280–1900
2.5– 1.3
[102]
Bulk polymerization at 130 8C
4280
1.4
[103]
80 8C in toluene for 40 h
4410
1.38
[104]
Photopolymerization (UV light at 20 8C)
38,900
2.03
[105]
ATRP (50 8C, conversion ¼ 87%)
27,700
2.9
[115]
ATRP (24 8C for 0.5 h in bulk)
2270
14.2
[116]
100 8C for 18 h
18,100
1.91
[117,118]
ATRP in emulsion (90 8C for 44 h)
22,100
9.5
[119]
(continued on next page)
190
C. Gao, D. Yan / Prog. Polym. Sci. 29 (2004) 183–275
Table 3 (continued)
Monomer
Condition
Mn (GPC)
PDI
Reference
ATRP (24 8C for 26 h in benzene)
1990
6.6
[116]
ATRP (24 8C for 24 h in benzene)
650
4.3
[116]
GTP at 250 8C
1630
34
[121,122]
Copolymerisation at 50 8C for 24 h
5980
1.87
[124]
Table 4
The monomers used in SCROP and the polymerization results
Monomer
Condition
Mn (GPC)
PDI
DB
Reference
Pd(0), 25 8C for 48 h in THF
1800
1.5
0.61
[128,129]
4170
1.43
0.38
[131]
BF3·OEt2, 210 to 4 8C for 48 h
1780
1.28
0.33
[136]
CH3OK, 95 8C for 12 h
6310
1.47
0.59
[137]
Sn(Oct)2, 110 8C in bulk
20,300–26,500
3.2
0.5
[139]
Sn(Oct)2, 110 8C in bulk
3000
2.8
0.5
[140]
C. Gao, D. Yan / Prog. Polym. Sci. 29 (2004) 183–275
Kakimoto [159] and Fréchet [160]. Combination of
the basic SMM synthetic principle and the multimonomer character of ‘A2 þ B3’ methodology
results in the other DMM, relevant to Section 3:
couple-monomer methodology (CMM) based on in
situ formation of ABn intermediates from specific
monomer pairs due to the non-equal reactivity of
different functional groups [161,162]. Many commercially available chemicals can be used as
reactive monomers for CMM, which should
extend the availability and accessibility of hyperbranched polymers with various new end groups,
architectures and properties, without the danger of
crosslinking [161 – 167]. One aim of this review is to
introduce in detail the double-monomer methodologies, since both ‘A2 þ B3’ and CMM have been
incompletely covered or even omitted in prior
reviews.
2. ‘A2 1 B3’ methodology
The first intentional preparation of hyperbranched
polymers via the ‘A2 þ B3’ approach was reported
by Kakimoto [159] and Fréchet [160], although the
method had been explored to prepare cross-linked
polymeric materials more than a century before
[38]. It is well known that direct polycondensation
of A2 and B3 monomers generally results in gelation
[39 – 43]. Thus, the crucial problem of this approach
is how to avoid gelation, and obtain soluble threedimensional macromolecules. Soluble hyperbranched polymers can be obtained by stopping
the polymerization through precipitation or endcapping prior to the critical point of gelation, by the
slow addition of monomer, or by using special
catalysts and condensation agents. Table 5 lists the
A2 and B3 type monomers reported in the literature
recent years.
The reaction between aromatic diamines (A2)
and aromatic tricarboxylic acids (B3) was first
investigated by Aharoni [168 –171]. However, only
insoluble networks composed of aromatic rigid
segments were obtained. Although the direct
connection of aromatic rings by amide rings led
to good heat and flame resistance, as well as
high tensile strength and modulus, the crosslinked materials possessed poor processibility.
191
Kakimoto [159] further studied the reaction, and
obtained soluble hyperbranched aromatic polyamide
with monomers 22, 23 and 24 as raw
materials in the presence of triphenyl phosphite
and pyridine as condensation agents through
consideration of the polymerization conditions
(Scheme 1).
When p-phenylene diamine (monomer 22)
reacted with trimesic acid (monomer 24) with a
feed ratio of A2 to B3 of 1/1 at 80 8C for 3 h with
a total monomer concentration of 0.21 mol/l
(3.3 wt%), no gelation was observed for the
polymerization. When the total monomer concentration in N-methyl-2-pyrrolidone (NMP) was
increased to 0.31 mol/l or 4.9 wt%, cross-linking
occurred in 2 h at the same reaction conditions.
Addition of LiCl can accelerate the polymerization.
In the absence of LiCl, three equivalents of
triphenyl phosphite to monomer 24 were required
to gain a high inherent viscosity sample. In the
presence of LiCl, only two equivalents of triphenyl
phosphite were needed to obtain a polymer with a
high viscosity. If monomer 23, instead of 22, was
used to react with 24, soluble hyperbranched
aromatic polyamide was successfully achieved
with three equivalents of triphenyl phosphite. If
the amount of triphenyl phosphite was decreased to
two equivalents, gelation occurred when the reaction was performed with LiCl even if the total
monomer concentration was decreased from 0.21 to
0.168 mol/l. So, gelation occurred more easily
for the polymerization of 23 and 24 than that of
22 and 24, which may be attributed to the higher
DB or the higher percentage of trisubstituted 24
units in the resulting polymer formed from 23 and
24. Gel formation was observed within 10 –20 min,
and soluble hyperbranched polyamide was not
obtained when the feed ratio of A2 to B3 was set
to 3/2.
A powdery polymer was obtained by pouring the
reaction mixture into methanol containing 12N
aqueous HCl. The hyperbranched polyamides HP1
and HP2 aggregated in N,N-dimethylforamide
(DMF) even when LiBr was added to the solution
system. The resultant hyperbranched polymer with
carboxylic acid terminal groups was end-capped
with p-anisidine in order to avoid the aggregation
effect in the measurement of molecular weight.
192
C. Gao, D. Yan / Prog. Polym. Sci. 29 (2004) 183–275
The weight-average molecular weights Mw of the
end-capped samples HP3 and HP4 as determined by
SEC measurements with a laser light-scattering
detector for solutions in DMF containing LiBr
(0.01 mol/l) were 285,000 and 86,700, respectively.
The polydispersity indices for HP3 and HP4 were
5.26 and 4.93, respectively.
Fréchet and coworkers [160] independently applied
the ‘A2 þ B3’ approach to synthesize hyperbranched
polyether employing 1,2,7,8-diepoxyoctane (25)
Table 5
The A2- and B3-type monomers used in ‘A2 þ B3’ methodology
A2
B3
Reference
[159]
[160]
[172]
[173,174]
[175]
[176]
[177]
(continued on next page)
C. Gao, D. Yan / Prog. Polym. Sci. 29 (2004) 183–275
193
Table 5 (continued)
A2
B3
Reference
[178]
[181]
as the A2 monomer and 1,1,1-tris(hydroxymethyl)ethane (26) as the B3 monomer. The initiation
and propagation steps are schematically shown in
Scheme 2. Tetra-n-butylammonium chloride (Bu4NCl,
5 mol% based on 25) was adopted as the nucleophilic
catalyst. Nucleophilic attack of the chloride ion on an
epoxide of 25 at the less-hindered terminal carbon led
to the formation of secondary alkoxide 25 p. Equilibrium between primary and secondary alkoxides was
established through rapid proton exchange, nucleophilic attack of primary alkoxides on the epoxide rings
resulted in the propagation of the formed oligomers,
and finally the formation of aliphatic hyperbranched
polyether epoxy HP5. As the feed ratio of 25 to 26
varied from 1.5 to 3, the resulting hyperbranched
polyether contained two types of terminal units (T),
one sort of dendritic unit (D), and linear ones (L) as
shown in Scheme 2.
Polymerization rates between 25 and 26 were too
slow to be practical in THF solution; the propagation
rates could be improved significantly via
bulk polymerization at temperatures above 100 8C.
Gelation can be avoided by removing the reaction
vessel from the heating bath prior to the critical point
of gelation. By increasing the feed ratio of 25 to 26,
the epoxy content of the isolated polymer
product increased, and the percentage of trisubstituted
B3 units in the resulting polymer also increased.
However, the PDI of the polyether increased with the
increase of molecular weight (, 1.5 – 1.8 at Mw ,
1000 and , 5.0 at Mw , 7000).
Aliphatic hyperbranched polyethers are viscous
liquids with glass-transition temperatures ðTg sÞ
below room temperature (about 2 20 8C for the
product with Mw , 8000) [172]. The decomposition
temperature ðTd Þ of a polymer with a Mw of about
10,000 is about 305 8C. Although HP5 is isolable by
fractionation into water, it is still a hydrophilic
material, as demonstrated by a contact-angle goniometry value of 42– 448 for a drop of water on spincoated films of HP5 on silicon wafers. After the
hyperbranched polymer was modified with hydrophobic chains (Scheme 2), the obtained products
HP6, HP7 and HP8 increased hydrophobicity over
194
C. Gao, D. Yan / Prog. Polym. Sci. 29 (2004) 183–275
Scheme 1.
the hydroxy-rich precursors, as demonstrated by
water contact angles that were 20– 308 greater than
that for HP5.
The direct polymerization of 1,4-butanediol (27)
and triepoxide (28) under a variety of reaction
conditions using Bu4NCl as the catalyst was also
explored by Fréchet et al. [172]. Again, the reaction
was very slow in THF solution. In bulk polymerization, the reaction was much faster, and gelation
occurred after 30 h at 75 8C. Analysis by SEC
showed that the PDI of the soluble polymers
isolated before the critical point was extremely
broad ðMw =Mn . 20Þ: Unfortunately, the product
could not be separated from either the catalyst or the
residual monomers by fractionation, so it was
difficult to fully characterize the material.
Through the formation of a Schiff-base between
dialdehyde (29) and triamine (30), Dai et al. prepared
hyperbranched polyconjugated conducting polymers
which showed relatively strong fluorescence emission
and high conductivities (ca. 1023 S/cm) after doping
[173,174]. Hyperbranched conjugated poly(1,3,5trisvinylic)benzene was synthesized via a Wittig
reaction between 31 and 32 [175], discussed in detail
in Section 5.
Tanaka and coworkers [176] prepared a new
conjugated alternate copolymer (HP9) consisting of
triphenylamine and phenylene units (Scheme 3) by
palladium catalyzed Suzuki coupling of an A2
monomer, benzene-1,4-diboronic acid (33), and a
B3 monomer, tris( p-bromophenyl)amine (34). The
light-yellow powder polymer was isolated after
evaporating the solvent and then pouring the crude
product into acetone. The resulting conjugated
hyperbranched polymer had an average molecular
weight of 5400 and was soluble in organic solvents
such as chloroform and THF. The conjugated
polymer had an absorption band at 365 nm and
an emission peak at 430 nm. The cyclic voltammogram of the polymer film deposited on a platinum
plate displayed reversible p-type doping and
undoping, with a color change between brownyellow (undoped) and green (doped). By using the
reference electrode of Ag wire and the supporting
electrolyte of a 0.1 mol/l Bu4NBF4 solution in
propylene carbonate, an anodic and cathodic peaks
were found at 1.2 and 1.0 V, respectively. In the
neutral state the polymer film was brown-yellow,
while in the doped state it turned green; 0.47
electron per monomeric units was found during the
redox cycle. This conjugated hyperbranched polymer could potentially be used as a charge storage
material, such as an electrode for secondary
batteries.
Fang et al. successfully fabricated wholly aromatic hyperbranched polyimides by direct polycondensation of commercially available dianhydrides 35
[2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride], 36 (3,30 ,4,40 -diphenylsulfonetetracarboxylic dianhydride) and 37 (pyromellitic
anhydride) and triamine 38, employing a two-step
C. Gao, D. Yan / Prog. Polym. Sci. 29 (2004) 183–275
195
Scheme 2.
polymerization method [177]. First, the hyperbranched polyamic acid precursors were prepared
by reaction of the dianhydrides and 38 in N; Ndimethylacetamide (DMAc). Then, chemical or
thermal imidization of the precursors gave rise to
the final hyperbranched polyimides (Scheme 4).
Reaction conditions, such as the monomer feed
ratio, monomer addition manner, and concentration
strongly influenced the polymerization results. A
hyperbranched polyimide with amino terminal
groups was obtained when a dianhydride solution
was added to the solution of 38 with a 1/1 molar
196
C. Gao, D. Yan / Prog. Polym. Sci. 29 (2004) 183–275
Scheme 3.
feed ratio of A2 to B3 (manner 1). In comparison,
the addition of 38 to a dianhydride solution with a
2/1 molar feed ratio of A2 to B3 (manner 2) yielded
the anhydride-terminated hyperbranched polyimide.
The concentration of the added dianhydride or
triamine approached zero during addition to avoid
a high local concentration of added monomer, which
could cause immediate gelation. Higher concentration of the feed monomers also led to crosslinking. The total feed monomer content should be
kept below 0.2 mol/l for method 1 and 0.075 mol/l
for method 2. 1H NMR spectra showed that the DB
for the amine-terminated HP10, HP11, and HP12
were 0.64, 0.72 and 0.68, respectively. The
anhydride-terminated HP13, HP14, HP15 were
fully branched ðDB ¼ 1Þ due to the relatively high
content and high reactivity of the anhydride
groups. SEC determination revealed that the hyperbranched polyimides HP10 and HP13 had moderate
number-averaged molecular weights (6400 for HP10
and 8400 for HP13) and broad molecular weight
distributions (5.8 for HP10 and 18 for HP13). No Tg
was observed for HP12 and HP15 using differential
scanning calorimetry (DSC); the Tg ranged from 339
to 295 8C for other four hyperbranched polyimides.
The amine-terminated polyimides showed a higher
Tg than the corresponding anhydride-terminated
ones, owing to their lower DBs and stronger
macromolecular interaction resulting from the
hydrogen bonds between amino groups. Thermogravimetric analysis (TGA) showed that the hyperbranched polyimides had a 5% weight loss at 450 8C
in N2.
HP10 and HP13 based on the monomer 35 showed
good solubility in strong polar solvents such as
DMAc, NMP, and dimethyl sulfoxide (DMSO), and
poor solubility in tetrahydrofuran (THF), acetone and
1,4-dioxane. However, the hyperbranched polyimides
C. Gao, D. Yan / Prog. Polym. Sci. 29 (2004) 183–275
Scheme 4.
197
198
C. Gao, D. Yan / Prog. Polym. Sci. 29 (2004) 183–275
based on monomers 36 and 37 were insoluble in all of
the aforementioned solvents.
To improve the solubility of aromatic hyperbranched polyimides, a new triamine [1,3,5-tris(4aminophenoxy)benzene, 41] was introduced to react
with conventional dianhydrides such as 35, 39 (4,40 oxydiphthalic anhydride) and 40 (3,30 ,4,40 -benzophenonetetracarboxylic dianhydride) [178 – 180].
The resulting hyperbranched polyimides exhibited
excellent solubility in strong polar solvents such as
DMAc, DMF, DMSO, NMP and m-cresol. It was
found that the polyimides made from 35 and 41
were soluble, even in common low-boiling-point
solvents such as THF, acetone and chloroform.
Photosensitive hyperbranched polyimide was prepared by reaction of cinnamoyl chloride with
phenol-terminated hyperbranched polyimide generated from the end capping of the anhydrideterminated hyperbranched poly(amic acid)
precursor of 35 and 41 with 4-aminophenol
during the course of polymerization. Highly
resolved patterns with a line width of 10 mm were
obtained via photolithography of the photosensitive
polyimide [179].
Two novel B3 monomers [tri(phthalic anhydride), 42] and [tri(phthalic acid methyl ester),
43] were synthesized and used to polycondense
with 1,4-phenylenediamine (22) in a feed mole
ratio of 1/1, resulting in hyperbranched polyimides
(Scheme 5) [181]. The polymerization of 42 and 22
was termed method A, and that of 43 and 22
termed method B. In method A, direct reaction of
42 and 22 in the solution of DMAc afforded the
hyperbranched poly(amic acid) precursor. Further
chemical imidization of the poly(amic acid) in the
presence of pyridine and acetic anhydride (Ac2O)
gave the anhydride-terminated hyperbranched polyimide HP16. If the poly(amic acid) precursor was
end-capped with p-toluidine, and then chemical
imidization of the end-capped precursor occurred,
toluidine end-capped hyperbranched polyimide
HP17 was obtained. As shown in Scheme 5,
HP16 and HP17 can also be prepared via method
B through chemical imidization of poly(amic acid
methyl ester) and toluidine end-capped poly(amic
acid methyl ester) precursors, respectively. Polymerization by method A often generated a gel due to
high reactivity between anhydride and amino
groups, so the reaction temperature was set at
0 8C. In method B gelation was effectively avoided
by using diphenyl(2,3-dihydro-2-thioxo-3-benzoxazolyl)phosphonate (DBOP) as a condensation agent
under room temperature (RT) at a concentration
lower than 0.1 g/l.
It is interesting that the polymers made by
methods A and B exhibit different properties from
each other, even though they have the same
structure. The precursor and hyperbranched polyimides made by method A are hardly soluble in
organic solvent at room temperature, but are soluble
upon heating in strong polar solvents, and the
solution remains homogeneous when cooled to room
temperature. By comparison, the precursor and the
hyperbranched polyimides made by method B are
soluble in DMAc, DMF, DMSO and NMP at room
temperature. Furthermore, the HP17 made by
method A has a high Mw (3.02 £ 105), a broad
PDI (23) and a low inherent viscosity
(hinh ¼ 0:28 dl=g in DMAc at 30 8C). The HP17
with a Mw of 1.25 £ 105 and a PDI of 2.63 made by
method B has a much higher inherent viscosity
ðhinh ¼ 0:97 dl=gÞ: The authors [181] explained this
phenomena, saying that the high molecular weight
component of HP17 made by method A was
actually a slightly cross-linked microgel formed at
the precursor formation stage.
The resulting hyperbranched polyimides have DBs
of 0.52 – 0.56 determined by 1H NMR, and Tg s of
212– 236 8C as measured by DSC. Self-supporting
films were successfully fabricated by casting the
DMAc solutions of toluidine end-capped hyperbranched poly(amic acid) and poly(amic acid methyl
ester) precursors onto glass plates upon heating.
Dynamic mechanical analysis (DMA) showed that
the storage modulus of HP17 films by method B
ranged from 3.1 to 4.0 GPa, similar to that of
their linear analogs. A TGA analysis revealed that
the 5% weight loss temperature of the films
approached 500 8C.
3. Couple-monomer methodology
Although the A 2 þ B3 approach to hyperbranched polymers holds some merit over the
conventional AB2 polycondensation approach, such
C. Gao, D. Yan / Prog. Polym. Sci. 29 (2004) 183–275
Scheme 5.
199
200
C. Gao, D. Yan / Prog. Polym. Sci. 29 (2004) 183–275
as facile preparation and commercial availability of
monomers, it still exhibits the major problem
of uncontrollable gelation, especially under
conditions of relatively high monomer concentration
and high reaction temperature. To avoid crosslinking, the reaction must be performed under low
monomer concentration or slow monomer
addition, or the polymerization must be stopped
prior to the critical point of gelation. This strongly
limits the wide application of the A 2 þ B 3
approach in large-scale manufacture of hyperbranched polymers. A new strategy with the
advantages of commercial availability of monomers
and lack of incidence of gelation is eagerly
anticipated.
3.1. General description of CMM
Flory’s gelation theory for the ideal polymerization of A2 and B3 monomers is based on three
assumptions: (1) equal reactivity of all A or B
groups at any given stage of the reaction; (2) no side
reactions, and the reaction restricted to the condensation of A and B groups; (3) no intramolecular
cyclization and chain termination in the process
[182]. If the first assumption is invalid gelation
should be avoided, leading to soluble hyperbranched
polymers with high molar mass. The new strategy
based on the non-equal reactivity of functional
groups in specific monomer pairs was invented
independently by Yan and Gao [161,162], and DSM
Research [165,166], and further developed and
extended by Gao and Yan. Because the two sorts
of raw monomers would preferentially generate one
type of ABn intermediate in situ in the initial stage
of polymerization, to produce hyperbranched macromolecules without gelation, we call the new strategy
for preparation of hyperbranched polymers CMM or
couple-monomer strategy.
Choosing a suitable monomer pair is the most
important step for the molecular design of a
hyperbranched polymer using CMM. The basic
principle of CMM is shown in Fig. 3. In monomer
AA0 , if A is identical to A0 , and B is equal to B0 , CMM
degenerates into the ‘A2 þ B3’ polymerization. It is
anticipated that cross-linking will not occur if an
asymmetric monomer AA0 or B0 B2 is used. If A0 has a
higher reactivity than A, CMM affords ‘AA0 þ B3’
polymerization; and if B0 is more active than B, CMM
presents ‘A2 þ B0 B2’ and ‘A2 þ CB2’ polymerization
systems. When both A and B groups are different from
A0 and B0 groups, CMM gives an ‘AC þ DB2’
Fig. 3. ‘AA0 þ B0 B2’ approach to hyperbranched polymers as a typical example for the basic principle description of CMM.
C. Gao, D. Yan / Prog. Polym. Sci. 29 (2004) 183–275
polymerization. In all of the designed polymerization
systems, AB2 intermediates will be generated in situ,
and further reaction of the AB2 species will result in
hyperbranched macromolecules. For example, if the
reactivity of the B0 group in the B0 B2 monomer is
greater than that of the two B groups, AB 2
intermediates can be formed in situ during the initial
reaction. Further self-polycondensation of the formed
AB2 species will give rise to hyperbranched polymers
without gelation. This reaction process is shown in
route 1 of Fig. 3. On the other hand, the molecule
containing four B groups should be generated if the
formed AB2 further reacts with a B0 B2 molecule due
to the higher reactivity of B0 group. The species B4
can play the role of a core molecule in the preparation
of hyperbranched polymers, and leads to the narrower
molecular weight distribution. Further polymerization
of AB2 and B4 generates a hyperbranched macromolecule with a core (route 2).
The molecular weight and terminal functional
groups of the resulting hyperbranched polymers can
be adjusted by the feed ratio of the two monomers.
Higher molecular weight product can be obtained if
the molar feed ratio of AA0 to B0 B2 is adjusted from
1/1 to 2/1. When the feed ratio is lower than 1/1, lower
molecular weight products with B groups can be
fabricated. Products with A groups can be synthesized
when the feed ratio is greater than 2/1. If the feed ratio
is in the range of 1/1 and 2/1, hyperbranched
macromolecules with both A and B functional groups
can be synthesized.
CMM is essentially different from the conventional
method involving polycondensation of AB2 monomers in the presence of multifunctional core moieties
(ABn þ B m approach). Compared with the
‘ABn þ Bm’ method, CMM exhibits three characteristics. First, the ABn and Bm compounds are generated
in situ from two or more monomers, which can be
commercially available. Second, no clear borderline
exists between the formation of ABn and its
polymerization. The polymerization reaction generally occurs as soon as the formation of an ABn
molecule occurs, although the molecule only appears
under special conditions. So, pure ABn or Bm
compounds formed in situ are not separated. Although
the reactivity between A and B0 groups is higher than
between A and B groups, hyperbranched polymers are
prepared in a single step from an A2-type difunctional
201
monomer and B0 B2-type trifunctional monomer.
Finally, CMM can be used to design and develop
novel hyperbranched polymers with new architectures
and structures. Through the new strategy, series of
hyperbranched polymers with alternating different
units, for example ab0 and ab units, can be easily
fabricated. On the contrary, it is very difficult to
acquire hyperbranched polymers with the same
structure through the conventional ‘AB2 þ Bm’
approach or polycondensation of AB2 monomers.
3.2. ‘A2 þ BB20 ’ approach to hyperbranched poly
(sulfone amine)s and poly(ester amine)s
The polymerization of an A2 type of difunctional
monomer and BB20 type of trifunctional monomer to
synthesize hyperbranched polymers is termed the
‘A2 þ BB20 ’ approach [161,162,183]. The BB20 monomer contains one B functional group and two B0
functional groups. Both the B and B0 groups can react
with the A group, but the reactivities of B and B0
groups are different from each other. The difference in
reactivities may be attributed to the differences in the
chemical environment or chemical structures of the
two functional groups. If the reaction between B and
A is much faster than that between B0 and A, an AabB20 intermediate will be predominantly formed at the
initial stage of the polymerization. This intermediate
can be regarded as a new kind of AB20 monomer.
Subsequently, a core molecules with four B0 groups
will be formed, with further reaction leading to
hyperbranched macromolecules. Through this
approach, a series of hyperbranched poly(sulfoneamine)s and poly(ester amine)s were successfully
prepared without any catalysts. Scheme 6 shows the
reaction mechanism and the selected monomers.
In the ‘A2 þ BB20 ’ approach, vinyl group is the A
group, and the active hydrogen of the secondary
amino group and the hydrogen of the primary amino
group are the B and B0 functions, respectively.
Because of the influence of the electron-donating
group attached to the secondary-amino group, B is
more active than B0 in the nucleophilic addition,
which has been demonstrated by monitoring the
reaction process with in situ Fourier transform
infrared (FTIR) spectroscopy.
Polymerizations of 44 (divinyl sulfone) and 51 [1(2-aminoethyl)piperazine] were carried out in either
202
C. Gao, D. Yan / Prog. Polym. Sci. 29 (2004) 183–275
Scheme 6.
water or organic solvents such as chloroform and
DMF under mild temperature (20 – 60 8C) [161].
When the feed ratio of 44 to 51 was 1/1, no gelation
was observed for a total monomer concentration
below 7.0 mol/l, and in situ FTIR measurements
showed that the reaction was completed after
approximately 5 h. A higher concentration results in
extremely fast reaction times, and a very high local
temperature, resulting in loss of control of the
polymerization. Solid state product rapidly appeared.
The solids were generally soluble in aqueous HCl
solution, although sometimes they were found to be
gel particles, insoluble in any solvents tested. When
the feed ratio of 44 to 51 was set at 3/2, gelation was
not found when the reaction mixture was kept in
solution. The resulting hyperbranched poly(sulfoneamine)s contain large amounts of amino and vinyl
groups (Scheme 7), which further reacted and finally
led to cross-linking after the solvent was removed
from the system. If the terminal amino groups were
protected with hydrochloride acid or the vinyl groups
were end-capped with added amines in the solution,
no gelation was observed even though the product was
separated from the solution. The resultant solution,
including the hyperbranched polymers, can be
preserved for a long time (at least one year), and
used as material for other functional macromolecular
building blocks.
Hyperbranched poly(sulfone amine)s were also
successfully prepared from 44 and 52. Monomer 53
(N-ethylethylenediamine) and 44 reacted in either
water or organic solvents such as chloroform, DMAc,
DMF, and NMP to yield hyperbranched polymers
[183 –185]. Similar to the reaction between 44 and 51,
no gelation was observed when the feed ratio of 44 to
53 was 1/1. The resulting hyperbranched polyamines
were characterized with 1H NMR and FTIR. Amino
groups were clearly detected, while no signals of the
carbon –carbon double bonds could be measured,
which suggested that the vinyl groups had completely
reacted with the amino groups, so that the resulting
polymers had terminal amino groups.
For reaction with a 3/2 feed ratio, the polymerization
medium dramatically influenced the results. If the
reaction was performed in organic solvents, no gelation
occurred, even after 30 days. The hydrochloride of
C. Gao, D. Yan / Prog. Polym. Sci. 29 (2004) 183–275
203
Scheme 7.
the product was highly soluble in water, and the sample
end-capped with benzoyl chloride was soluble in
organic solvents such as chloroform, DMF and NMP.
If water was used as the reaction medium, the reaction
solution turned into a turbid heterogeneous mixture at
around 15 –20 min, and cross-linking was observed at
12 –15 h. The turbid mixture turned transparent in the
aqueous HCl, indicating that no networks formed in the
initial stage. This phenomenon, termed ‘pseudogelation’, may be caused by the interactions between
water and the oligomers formed from 44 and 53.
The reaction between 44 and 54 (N-methyl-1,3propanediamine) is very similar to that between 44
and 53 [186]. No cross-linking was found in organic
solvent for monomer feed ratios of 1/1 or 3/2.
The ‘pseudo-gelation’ phenomenon appeared at
12– 18 min, for polymerizations carried out in water,
and gelation occurred at 15– 20 h for a reaction with a
feed ratio of 3/2.
For the reaction systems described above
(44 þ 51, 44 þ 53, and 44 þ 54), product can be
obtained by two means: (1) pouring the reaction
solution into the mixture of concentrated aqueous HCl
and a precipitation agent such as acetone, methanol or
ethyl ether to obtain protonated hyperbranched
polycations; and (2) pouring the reaction solution
into ethyl ether and drying the precipitate under
vacuum at room temperature to afford hyperbranched
polymers with copious active amino groups. SEC
measurements with water as the solvent and PEO as
standards showed that the resulting hyperbranched
poly(sulfone amine)s had moderate apparent Mn s
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C. Gao, D. Yan / Prog. Polym. Sci. 29 (2004) 183–275
(20,000– 40,000) and very narrow PDIs (1.2 – 2.5),
which was in agreement with the in situ formation of
the core molecules in the reaction system. A peak in
the mass spectrum of the sample taken from the
reaction system at initial reaction moment corresponded to the core molecules.
The dendritic units (D), linear units (L), and
terminal units (T) of the hyperbranched poly(sulfone
amine)s in a 1/1 feed ratio are the tertiary
amino, secondary amino and primary amino groups,
respectively (Scheme 7). The terminal units of the
polymers in a 3/2 feed ratio contain primary amino
and vinyl groups. The DBs of the resulting hyperbranched polymers can be determined from the
corresponding 1H NMR spectrum. Theoretically, a
hyperbranched polymer prepared by polycondensation of an AB2 monomer has a maximum DB of 0.5 if
the reactivity of the two B groups is the same [127,
187,188]. Because of the higher reactivity of the
secondary amino group over the primary amino
Scheme 8.
C. Gao, D. Yan / Prog. Polym. Sci. 29 (2004) 183–275
group, the DB of the prepared hyperbranched
poly(sulfone amine)s ranged from 0.51 to 0.75. The
DB of the product generated from a 3/2 feed ratio is
higher than that with 1/1 feed ratio. By comparison,
the DBs of the hyperbranched polyamidoamines made
from AB2 monomers containing vinyl and primary
amino groups are also much higher than 0.5 [96,97].
The commercially available cross-linking agent,
ethylene diacrylate (monomer 45), can be used as a
raw material to synthesize water-soluble hyperbranched poly(ester amine)s via a Michael addition
(Scheme 8) [189]. In situ FTIR measurements showed
that the secondary amino groups disappeared at
approximately 3 h, and that the reaction was completed after 72 h in a dilute solution. In the mass
spectrum of the reaction mixture taken from the
polymerization system of 45 and 51 at 3 h, the AB20
intermediate ðm ¼ 299:4Þ and its dimer (598.7) and
trimer (898.1) can be found as peaks at m=z ¼ 300:4;
599.8 and 899.2, respectively. Propagation occurred
as soon as the formation of AB20 occurred although the
reactivity between A and B0 is not as high as that
between A and B, so it was hard to separate the pure
AB20 species from the reaction mixture.
The initial molar ratio of 45 to 51, 52, 53 or 54 was
set at 1/1 and 3/2. Relatively high molecular weight
products were prepared in organic solvents. Chloroform was a good solvent for the reaction in which
the highest molecular weight poly(ester amine) was
obtained. When the monomers were added to water
for the reaction, a heterogeneous mixture appeared at
first due to the poor solubility of 45 in water. After
several minutes a homogeneous solution appeared.
This phenomenon indicates that the Michael addition
of amino groups to vinyl groups can occur smoothly in
water, resulting in oligomers soluble in water.
However, hyperbranched poly(ester amine)s with
high molar masses were not obtained in water,
attributed to degradation of the ester bonds.
Reaction conditions such as temperature and
monomer concentration have a strong effect on the
Michael addition polymerization. No gelation
occurred for polymerization with a 1/1 feed ratio.
When the feed ratio was increased to 3/2, crosslinking was not yet observed in a dilute solution at low
temperature. When the reaction temperature was
lower than 40 8C, gelation was not observed after 30
days. At higher temperatures, networks formed
205
depending on the monomer concentration: if the
feed volume ratio of the two monomers to solvent was
greater than 2/1, gelation occurred quickly; while gel
did not appeared if the volume ratio was lower than
1/2. HCl usually helps the solubilization and protection of amine-containing compounds, and soluble and
stable hyperbranched poly(ester amine)s with terminal amino and vinyl groups can be obtained by
precipitating the reaction mixture with acetone and
aqueous HCl.
The Mw s for hyperbranched poly(ester amine)s
measured by SEC with water as the eluent and PEO as
standards ranged from 15,000 to 25,000. Again, their
molecular weight distributions were very narrow
(1.28 – 1.40). The DBs of the polymers were also
higher than 0.5, approaching 0.58 –0.75.
If monomers 47, 48 or 49, instead of 45, are
adopted to react with 51, 52, 53, or 54, hyperbranched
poly(ester amine)s containing ethylene oxide units in
their frameworks can be produced according to the
same reaction conditions and procedure as
the preparation of HP20, HP21, HP22 and HP23.
The number average degree of polymerization (DPn)
and the molecular weight of the resulting product
depend on monomer purity.
All of the hyperbranched poly(sulfone amine)s and
poly(ester amine)s mentioned above are highly
soluble in water and certain organic solvents such as
chloroform, DMF, DMAc, NMP, DMSO, and are
poorly soluble in THF.
The steric and electron-accepting effects of the
adjacent groups influence the reactivity of the vinyl
groups. For example, only oligomers were obtained
through the use of monomer 46, instead of 45,
reaction with the BB20 monomers. Monomer 50 (1,4divinylbenzene) is also often used as a cross-linking
agent. Using lithium amide catalyzed anionic polyaddition of 50 to the compounds with two secondary
amino groups, Tsuruta and coworkers [190,191]
prepared linear polyamines with Mns of 1000 –5000.
Monomer 50 cannot react with amino groups without
a catalyst. In the presence of a strong base such as
lithium alkylamide, only branched products with Mn s
below 8000 were obtained in our laboratory.
In summary, the control of the structures and units
with suitable monomers, the simplicity of polymerization process, and the efficiency in avoiding gelation
would make the ‘A2 þ BB20 ’ approach, and the new
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C. Gao, D. Yan / Prog. Polym. Sci. 29 (2004) 183–275
hyperbranched polycations with many terminal amino
groups prepared via this method, very attractive in the
large-scale manufacture and application of polymeric
materials.
3.3. ‘A2 þ CBn’ approach to hyperbranched
poly(urea urethane)s
Linear polyurethanes and polyureas are now
widely used in rubber, plastics, fiber, and coatings.
It is well known that polyurethanes or polyureas can
be prepared by polycondensation of diisocyanates
with dihydroxyl compounds or diamines. Because of
the high reactivity of isocyanato groups ( –NyCyO)
against the nucleophile, it is hard to obtain an AB2
monomer with an isocyanato group. Therefore,
hyperbranched polyurethanes or polyureas cannot be
directly synthesized from AB2 monomers. Generally,
an AB2 monomer containing a functional group such
as a carbonyl azide that can be transferred in situ into
the isocyanato group is synthesized in advance, and
then polymerization of the formed monomers leads to
hyperbranched polyurethanes or polyureas [88 – 91].
However, it is very difficult to apply the prepared
hyperbranched polyurethanes or polyureas owing to
the complexity of the synthesis and the purification
procedures.
Through the ‘A 2 þ CB n’ approach, hyperbranched poly(urea urethane)s can be easily
prepared by direct polymerization of commercially
available diisocyanates and multi-hydroxyl amines
(Scheme 9). A2 monomers containing two isocyanato groups are industrial chemicals that have been
widely used in the manufacture of linear polyurethanes and polyureas. A CBn monomer has one
amino group and n hydroxyl groups. The fast
reaction between the A group and the C group in
CBn affords ABn and Bn –Bn intermediates at mild
temperature. Further reactions between isocyanato
and hydroxyl groups at higher temperature give rise
to hyperbranched polymers with alternating ureido
and urethano units [192 – 194].
This reaction process has been monitored with in
situ FTIR spectroscopy. The reaction between monomer 57 and 62 was selected as a typical example. It
was found that the carbonyl groups (CvO) of the
ureido units (– HNCONH –) first rapidly appeared as a
peak at 1632 cm21, with a significant decrease in the
absorption band of the isocyanato group at
2261.6 cm21 during the initial minute. The peak at
1632 cm21 then stayed constant, and the carbonyl
groups of the urethano units ( –HNCOO – ) appeared
as a new peak at 1705 cm21, and gradually increased
with decrease of the absorption peak of the isocyanato
groups. After approximately 200 h, the absorption
band of the isocyanato groups disappeared from the
spectrum, the hydroxyl group absorption still clearly
observed. This data shows that the one isocyanato
group of 57 quickly reacted with the amino group of
62, resulting in the formation of intermediate 67. Then
self-condensation of intermediate 67 along with other
formed species generated the hyperbranched macromolecules HP24 with ureido and urethano units
(Scheme 10). Similar results were obtained for other
reaction systems of ‘A2 þ CBn’.
The peaks of the ABn and Bn – Bn species formed
during the initial reaction period were also detected
with mass spectroscopy. For example, the molecular
ion peaks 67 ðm ¼ 327:4Þ and 72 ðm ¼ 432:6Þ were
found as two peaks at m=z ¼ 328:3 and 433.4,
respectively. Their corresponding m þ Naþ ion
peaks were also observed at m=z ¼ 350:3 and 455.4,
respectively. The mass spectrum measurements for
Scheme 9.
C. Gao, D. Yan / Prog. Polym. Sci. 29 (2004) 183–275
207
Scheme 10.
other reaction systems indicated that the ABn and Bn –
Bn intermediates did form in the initial stage.
Reaction conditions such as temperature, concentration, and monomer feed ratio strongly influenced
the reaction. The temperature during the initial several
hours should be kept below 25 8C, otherwise,
insoluble product was generated. Gelation was barely
observed for the polymerization in a feed ratio of 1/1
(A2/CBn) at given conditions. For the reaction with a
3/2 feed ratio (A 2/CB n), cross-linking mainly
208
C. Gao, D. Yan / Prog. Polym. Sci. 29 (2004) 183–275
depended on the temperature and concentration.
Gelation occurred within several minutes in the
conditions of high concentration (1.0 mol/l) and
high temperature (60 –80 8C), whereas at low concentration (0.15 mol/l) and low temperature (20 8C)
gelation was not observed. SEC with 0.05 M LiCl in
DMAc eluent at 70 8C showed that the resulting
hyperbranched poly(urea urethane)s had a moderate
Mws ranged from 18,000 to 130,000, and relatively
narrow PDIs with typical values of 1.7 – 3.5.
The DBs of the prepared hyperbranched polymers
were determined with 13C NMR spectroscopy. For the
polymers made from A2 and CB2, there are two
hydroxyl groups in the terminal units, one hydroxyl
group in the linear units, and no hydroxyl group in the
dendritic units. Scheme 11 shows the three repeating
units of the polymer derived from the reaction
between A2 and 64. The terminal ((C H2OH)2) and
linear (C H2OH) units can be assigned independently
in the 13C NMR spectrum, so the DB can be calculated
with Eq. (1)
DB ¼ ðD þ TÞ=ðD þ T þ LÞ
¼ ðT 2 1 þ TÞ=ðT 2 1 þ T þ LÞ
< 2T=ð2T þ LÞ ¼ 1=ð1 þ L=2TÞ
ð1Þ
DBs calculated from Eq. (1) ranged from 0.42
to 0.56.
Scheme 12 shows the structure of the hyperbranched polymers made from A2 and CB3 (65). It
was found that there are two sorts of branched units
(Scheme 13), so the DBs were much higher than 0.5,
approaching 0.7– 0.74.
Although the reactivity of the primary hydroxyl
group is greater than that of the secondary hydroxyl
group, the poly(urea urethane)s made from A2 and
CB5 (66) are also highly branched (Scheme 14). In
their corresponding DEPT-135 13C NMR spectra,
the ratio of the residual primary hydroxyl group
(CH2OH) in the resulting polymer to the primary
hydroxyl group (CH2O – ) is higher than 50%. Therefore, it can be deduced that the secondary hydroxyl
groups of 66 also reacted with isocyanato groups, and
highly branched polymers containing a high density
of hydroxyl groups were formed. Tg s of the obtained
hyperbranched poly(urea urethane)s measured by
DSC were 25 –50 8C. The hyperbranched poly(urea
urethane)s are soluble in polar organic solvents, such
as DMF, DMAc, DMSO and NMP.
3.4. ‘AB þ CDn/Cn’ approach to hyperbranched
poly(amine ester)s and poly(amide amine)s
3.4.1. ‘AB þ CDn approach’ to hyperbranched
poly(amine ester)s
The monomer AB contains one A and one B
functional group, which cannot react with each other.
Monomer CDn has one C and n D functional groups,
and C cannot react with D. The reaction between B
and C is more easily conducted than that between
A and D. At mild temperatures, the B group reacts
with the C group to form an ADn intermediate.
Hyperbranched macromolecules can be produced by
self-condensation of ADn under stronger conditions
(Scheme 15). If the methyloxy carbonyl (CH3OCyO),
the acrylate group (CH2yCHCO), the secondary
amino group (HN –), or the hydroxyl (– OH) moiety
are selected as A, B, C and D groups, respectively,
water-soluble hyperbranched polyesters with
tertiary amino units in their backbones can be
prepared [195,196].
Some side reactions may occur between AB and
CDn reactions during the initial stage. For example, at
least three intermediates 78, 79 and 80 may be
generated in the reactions of 77 and 62 as shown in
Scheme 16. The reactions between A and C or D can
be suppressed by using methanol as solvent, since
methanol would be generated accompanying with
Scheme 11.
C. Gao, D. Yan / Prog. Polym. Sci. 29 (2004) 183–275
209
Scheme 12.
formation of 79 or 80. Measurements of in situ 1H
NMR with diethylamine and 1,4-butanediol as model
compounds confirmed that the reaction between the
methyloxy carbonyl and the amino or hydroxyl group
was negligible under mild conditions.
The reaction temperature during the initial stage is
very important in the preparation of a high molecular
weight product. In the mass spectrum of the sample
taken from the reaction system at room temperature,
the molecular ion peak of 78 ðm ¼ 191:2Þ appeared at
m=z ¼ 192:3; and the peak of 78 coupled with a Naþ
appeared at m=z ¼ 214:2: The molecular ion peaks of
the dimer (M2Hþ), trimer ((M3Hþ), and tetramer
(M4Hþ) of 78 were found at m=z ¼ 351:5; 510:5; and
691.7, respectively. No peaks attributed to 79 and 80
were detected. As a comparison, the peaks of 79 and
210
C. Gao, D. Yan / Prog. Polym. Sci. 29 (2004) 183–275
Scheme 13.
80 did appear in the mass spectrum on increase of the
reaction temperature to 60 8C during the initial stage.
The existence of 79 and 80 may lead to cross-linking
in the next reaction stages, so it is better to keep the
temperature in the initial period below 40 8C for the
polymerization of AB and CDn monomers.
To improve the conversion ratio of 62 or 77 and
shorten the reaction time of the initial stage, the feed
ratio of AB to CDn is a little higher than 1/1 (typically
1.05 –1.1/1). The residual 77 was removed from the
reaction system by reduce-pressure distillation on a
revolving-distillation apparatus. After the residual
MA and the solvent (methanol) were removed, an
amount of catalyst (0.5 g per mole of CDn) was added
to the reaction system. Under vigorous revolving and
vacuum distillation, the mixture was kept at 60 8C for
1 h, 100 8C for 2 h, 120 8C for 2 h, and 135 – 150 8C
for 2 h. After reprecipitation of the raw product from
DMSO into acetone, hyperbranched poly(amine ester)
with a yield of 75 –80% was obtained.
Compared with zinc acetate anhydrous [Zn(CH3CO2)2], tetrabutyl titanate [Ti(C4H9O)4] exhibited
higher catalytic efficiency for the polymerization. For
the same amount of catalyst, the molecular weight of
the product catalyzed with Ti(C4H9O)4 was higher
than that catalyzed with Zn(CH3CO2)2. Nevertheless,
insoluble product was formed if the amount of
Ti(C4H9O)4 was too high (. 2 g per mole of CDn).
The structures of the hyperbranched poly(amine
ester)s made from 77 and 62 or 66 are shown in
Schemes 17 and 18, respectively. Depending on the
reaction temperature and the catalyst, Mn of HP29
ranged from 10,000 to 260,000, and that of HP30
ranged from 38,000 to 66,000, with a narrow
molecular weight distribution. The molecular weight
of HP29 was generally greater than that of HP30
under the same reaction conditions. Inter- or intramolecular cyclization might have caused the lower
DPn of HP30 because of the high density of hydroxyl
groups in its repeating unit.
The terminal and linear units of HP29 can be
independently detected in a quantitative 13C NMR
spectrum. The DB of HP29 calculated with Eq. (1)
was slightly higher than 0.5 (0.52 –0.56). Characterization by a quantitative 13C NMR spectrum showed
that HP30 was also a highly branched polymer,
although its DB cannot be accurately calculated due to
its complex structure.
There is one ester bond and one tertiary amino group
in each repeating unit of HP29 or HP30. Such a
structure is very attractive in the field of drug-delivery.
The hyperbranched polymers are degradable in water
because of their ester bonds. Owing to the introduction
of amino groups, the resulting polymers are easily
soluble in water. Furthermore, the acid groups formed
in the hydrolysis neutralize the amino groups. So, the
pH of the aqueous system of HP29 or HP30 can be selfadjusted to about 7, which is an important parameter
for a good drug-delivery material [197].
TGA measurements showed that the 5% weightloss temperature for HP29 and HP30 is above 200 8C,
and the 10% weight-loss temperature is above 250 8C.
In the DSC curves, Tg s of about 10 – 20 8C were found
for HP29 and HP30, but the transition was not as
strong as that of general linear polyesters.
A methoxycarbonyl-terminated hyperbranched
polymers could be synthesized by the reaction of
methyl acrylate (77) with a compound containing a
hydroxyl and one primary amino or two secondary
amino groups (Scheme 19). The feed ratio and the
catalyst have a dramatic influence on the polymerization. Gelation occurred for a ratio of 77 to
ethanolamine (82) lower than 2.1/1. Cross-linking
was not observed for a ratio higher than 2.3/1. In the
presence of Ti(C4H9O)4, the polymerization occurred
faster, and a higher molecular weight product was
approached under the same conditions. SEC measurements with polystyrene as standards showed that
the methoxycarbonyl-terminated hyperbranched
poly(amine ester)s had apparent Mn s of 35,000 –
70,000. The onset decomposition temperature ðTd Þ of
HP31 was 275 –290 8C.
C. Gao, D. Yan / Prog. Polym. Sci. 29 (2004) 183–275
211
Scheme 14.
3.4.2. ‘AB þ Cn’ approach to hyperbranched
poly(amide amine)s
Polyamines (C n monomers) react with the
aforementioned AB monomer to yield aliphatic
hyperbranched poly(amide amine)s (Scheme 20).
The ACn or ACn21 intermediate should be formed
in situ at room temperature for a feed ratio of AB to Cn
of 1/1. Hyperbranched polyamines could then be
212
C. Gao, D. Yan / Prog. Polym. Sci. 29 (2004) 183–275
Scheme 15.
prepared by self-condensation of the intermediates at
higher temperatures and in vacuum. Interestingly, the
feed ratio of AB to Cn can be adjusted over a relatively
wide range from 1/1 to n=1; resulting in hyperbranched polymers with various terminal groups. Due
to their different functional groups, the hyperbranched
polymers exhibit different properties. Therefore, the
structure and property of the hyperbranched polymers
can be controlled through the feed ratio of AB to Cn
[198,199].
The reaction conditions between AB and Cn are
very similar to those between AB and CDn. Methyl
acrylate (77) was added dropwise to the amine/methanol solution. The reaction mixture was kept at room
temperature for 24– 48 h, and then the solvent was
removed from the reaction system under reduced
pressure at 60 8C on a rotary evaporator. Under
vigorous revolving and vacuum distillation, the
mixture was kept at 60 8C for 1 h, 100 8C for 2 h,
120 8C for 2 –4 h, and finally 135 –150 8C for 2– 4 h.
Pale yellow viscous solid hyperbranched polymers
were produced. Since no extra reagents such
as catalyst and condensation agent were added to
the reaction mixture, it was not necessary to separate
the raw product by reprecipitation, so the yield was
very high (. 95%).
The Michael addition of 84 to 77 affords 91.
Intermediate 91 can be regarded as an asymmetrical
AC2 monomer that can further self-condense to give
rise to hyperbranched polymer HP32 (Scheme 21).
There are two kinds of linear repeating units in HP32.
The reaction between 77 and 85 would result
in a symmetrical AC2 intermediate (92) and an
asymmetrical AC3 species so the hyperbranched
polymer HP33 contains the hybrid structures of 92
and 93 (Scheme 22). The reaction of monomer 86, 87
or 88 with 77 yields more ACn or ACn21 species.
Thus, the corresponding hyperbranched polymers
have a more complex structure, and more types of
repeating units can be found. Both 89 and 90 have
three primary amino groups. Three asymmetrical AC3
intermediates (94, 95, 96) occurred when 89 was
employed to react with 77 (Scheme 23), and then
hyperbranched polymers with hybrid structures were
generated. Because of the symmetrical structure of 90,
only one AC3 species (97) was formed. The resulting
hyperbranched polymers HP34 have amido and
tertiary amino units in their backbones and many
primary amino groups in their terminal chains
(Scheme 24).
Variation of the AB to Cn feed ratio results in a
change in the structure and properties of the products.
For example, the solubility of the hyperbranched
polymer made from 77 and 85 decreases with an
increase in the feed ratio of 77 to 85.
All of the hyperbranched poly(amide amine)s
with a feed ratio of 1/1 described above are highly
soluble in water and polar solvents such as DMF,
DMAc, DMSO and NMP. The water-soluble
cationic hyperbranched polyelectrolytes may be
useful in the pigments, coatings, and gene delivery
or transport fields.
3.5. ‘Ap þ CB2/Bn’ approach to hyperbranched
poly(ester amide)s and polyesters
Scheme 16.
3.5.1. ‘Ap þ CB2’ approach to hyperbranched
poly(ester amide)s
DSM research first reported the ‘Ap þ CB2’
approach [165,166]. In their presentation, Ap and
C. Gao, D. Yan / Prog. Polym. Sci. 29 (2004) 183–275
213
Scheme 17.
CB2 were denoted as Aa and bB2, respectively [166].
They used 1,2-cyclohexane dicarboxylic anhydride as
the Aa monomer, and di-2-propanolamine as the bB2
source. Recently, Gao and Yan [200,201] developed
the ‘Ap þ CB2’ approach, using dicarboxylic anhydride compounds as Ap. An Ap monomer contains two
latent functional groups; one A group will be formed
after the reaction of Ap with the CB2 group, giving rise
to an AB2 intermediate. Self-polycondensation of the
intermediate results in the hyperbranched macromolecule (Scheme 25).
The reaction between 98 and 63 has been investigated using electrospray ionization (ESI) and matrixassisted laser desorption ionization/time-of-flight
(MALDI-TOF) mass spectrometry, and size exclusion
chromatography (SEC) in combination with on-line
differential viscosimetry detection (SEC-DV) [166,
202,203]. The exothermal reaction of 98 and 63 normally leads to a species (102) having one carboxylic
acid group and two (2-hydroxypropyl)amide groups.
Ester bonds were formed through an intermediate
oxazolinium-carboxylate ion pair, and hyperbranched
macromolecules were generated under reduced pressure (5 mbar) at around 180 8C, without a catalyst
(Scheme 26).
The hyperbranched poly(ester amide) HP35 was
then modified in situ with 1-dodecanoic acid to obtain
hyperbranched resins with terminal long aliphatic
chains. Other hyperbranched poly(ester amide)s series
can be prepared by direct polymerization of 99, 100,
or 101 with 62 or 63 with similar reaction routes and
conditions. The Mn s of the hyperbranched poly(ester
amide)s detected with SEC-DV by using polystyrene
as standards and dichloromethane as solvent and
eluent were very low (680 – 6000) [166,202,203].
3.5.2. ‘Ap þ Bn’ approach to hyperbranched
polyesters
Polyhydric alcohols (Bn) can directly polymerize
with the aforementioned dicarboxylic anhydrides,
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C. Gao, D. Yan / Prog. Polym. Sci. 29 (2004) 183–275
Scheme 18.
forming hyperbranched polyesters (Scheme 27). The
reaction between 99 and 104 had been investigated by
Kienle et al. as early as 1929 [35]. However, only lowmolecular-weight product or insoluble materials were
obtained at that time. To obtain soluble highmolecular-weight hyperbranched polymers, it was
necessary to re-investigate the reactions between
dicarboxylic anhydrides and polyhydric alcohols.
Because each hydroxyl group of Bn has almost the
same reactivity, gelation easily occurred in
the reaction. It is crucial to remove water from the
reaction system during the initial stage to avoid
network formation. The existence of water in the
monomers or solvents leads to the ring opening of
the anhydrides, giving difunctional A2 molecules and
leading to insoluble product within several hours.
Two routes were employed in the polymerization.
Scheme 28 describes a typical example. Route A
represents direct polymerization of the in situ
formed AB2 intermediate possessing a carboxylic
C. Gao, D. Yan / Prog. Polym. Sci. 29 (2004) 183–275
215
Scheme 19.
acid group and two hydroxyl groups (107). In route
B, the AB2 species was modified with methanol in
advance to afford the new monomer 108, and
then hyperbranched polyesters were prepared by
polymerization of the modified AB2. Only moderate
molecular weight product was obtained
(Mn ¼ 5000– 20,000) via route A, even through
the reaction was performed under reduced pressure
at 140 –150 8C for 10 – 20 h. The color of the raw
product was very deep, indicating that carbonization
Scheme 20.
216
C. Gao, D. Yan / Prog. Polym. Sci. 29 (2004) 183–275
Scheme 21.
Scheme 22.
C. Gao, D. Yan / Prog. Polym. Sci. 29 (2004) 183–275
Scheme 23.
Scheme 24.
217
218
C. Gao, D. Yan / Prog. Polym. Sci. 29 (2004) 183–275
Scheme 25.
or oxidization occurred in part of the polymers. As a
comparison, the products made through route B had
a high Mn s (30,000 – 120,000) and an almost white
color [200,201].
The hyperbranched polyesters made from 103 and
polyhydric alcohols have carbon – carbon double
bonds in every repeating unit, so the cross-linked
membrane can be prepared by UV light irradiation.
The polymerization of 106 with anhydrides is
relatively difficult because of its poor solubility in
organic solvents; only low molecular weight product
was acquired.
Scheme 26.
C. Gao, D. Yan / Prog. Polym. Sci. 29 (2004) 183–275
219
3.6. ‘AAp/Ap2 þ B2’ approach to hyperbranched
polyesters
Scheme 27.
The AAp monomer contains one A and one Ap group,
and Ap2 contains two Apgroups. As claimed above, one
Ap is a latent A2. In the ‘AAp þ B2’ approach, the
reaction between Ap and B groups generates an A2B
intermediate which can be self-polycondensed to attain
hyperbranched polymer [204,205]. Scheme 29 shows
the reported AAp and B2 monomers. Benzene-1,2,4tricarboxylic acid-1,2-anhydride (109) has a carboxylic
Scheme 28.
220
C. Gao, D. Yan / Prog. Polym. Sci. 29 (2004) 183–275
Scheme 29.
acid group (A) and an anhydride group (Ap). Because of
the much higher reactivity of anhydride group over the
carboxylic acid group in the condensation, a species
with two carboxylic acid groups and one
hydroxyl group can be presented. Then hyperbranched
polyesters can be prepared smoothly according to the
conventional polymerization conditions and process.
Similar to the reaction of AAp and B2, the reaction of one
of the Ap of Ap2 with one of the B groups in B2
affords the species AApB containing a carboxylic acid
group, one anhydride group and one hydroxyl group.
Self-condensation of two AApB gives the A3B
species. Hyperbranched macromolecules can also be
prepared by further polycondensation of the A3B
molecules. The Ap2 monomers used are also given
in Scheme 29.
A typical example for the ‘AAp þ B2’ approach
is described in Scheme 30. The first step was
conducted at room temperature in chloroform. Two
intermediates (115 and 116) were formed. Hyperbranched polymers were prepared through two
routes. In route A, direct polymerization of the
species 115 and 116 led to the carboxylic acidterminated hyperbranched polyester HP37. Polymer
HP37 contains two types of linear units (L1 and L2)
and two types of terminal units (T1 and T2). In
route B, A2B intermediates 115 and 116 reacted
with methanol, generating species 117 and 118,
respectively. Further polycondensation of 117 and
118 gave rise to ester-terminated hyperbranched
macromolecules HP38. Again, the products synthesized using route B exhibited a higher molecular
weight than those synthesized using route A. The
former products have Mn s of 20,000 –120,000, while
the latter products have Mn s of 3000 –15,000. The
hyperbranched polyesters HP37 and HP38 were
further end-capped with 1-butanol, resulting in
HP39. It is well known that n-butyl phthalate is a
good plasticizer for polyvinyl chloride (PVC).
However, the low molar mass n-butyl phthalate
often releases from the PVC, which strongly
influences the quality of the plasticized product.
HP39 has a similar structure of n-butyl phthalate
and a much higher molecular weight. Furthermore,
the melting viscosity of hyperbranched polymers is
very low. It is expected that in the future HP39 will
become a better plasticizer for PVC or other
commercial plastics.
The reaction conditions and processes for the
reaction between dianhydrides and dihydroxy alcohols are the same as those employed in the reaction of
AAp and B2. Two routes were applied to prepare
hyperbranched polyesters (Scheme 31). Route B was
more efficient than route A for preparing higher
C. Gao, D. Yan / Prog. Polym. Sci. 29 (2004) 183–275
221
Scheme 30.
molecular weight hyperbranched products. Through
route B, hyperbranched polyesters with Mn s ranging
from 15,000 to 100,000 can easily be synthesized,
according to the feed ratios. On the contrary, only
relatively lower molecular weight products (3000 –
12,000) were made from route A. On the other hand,
almost linear oligomers appeared during the beginning reaction period owing to the higher reactivity of
anhydride groups over carboxylic acid groups. Then
branched molecules were formed because the content
of carboxylic acid group is very high. However, the
DBs of the obtained products were lower than those
made from route B.
If one hydroxyl group of B2 is replaced by a
secondary amino group, water-soluble hyperbranched poly(ester amide)s can be prepared
(Scheme 32) [206,207]. Direct polymerization of
benzene-1,2,4-tricarboxylic acid-1,2-anhydride (109)
with N-(2-hydroxyethyl)piperazine (127) failed to
produce macromolecular products although the
reaction occurred at a high temperature (160 8C) in
the presence of catalyst. When 127 was added to a
solution of 109 in DMSO, a precipitate was rapidly
observed, caused by the interaction between acid
and amino groups. To dissociate the aggregation
species, an organic base such as triethylamine was
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C. Gao, D. Yan / Prog. Polym. Sci. 29 (2004) 183–275
Scheme 31.
introduced to the reaction system. However, a
powdery precipitate appeared again during polymerization after heating under reduced pressure.
Therefore, the carboxylic acid was modified with
methanol to obtain the intermediate 130 or 131.
Under mild conditions and in the presence of
catalyst such as phosphoric acid and tetrabutyl
titanate (Ti(C 4H9O)4), hyperbranched polymers
were prepared, with moderate Mn (25,000 –50,000)
and narrow PDI (1.4 – 2.5). Although the acid groups
were end-capped with methyloxy moieties, the
prepared hyperbranched poly(ester amide)s were
highly soluble in water because of their tertiary
amino and amido units.
C. Gao, D. Yan / Prog. Polym. Sci. 29 (2004) 183–275
223
Scheme 32.
3.7. ‘A2 þ B2 þ BB20 ’ approach to highly branched
copolymers
CMM can be extended to prepare highly branched
copolymers with different DBs by copolymerization
of suitable difunctional monomers with the monomer
pairs described above. The DBs and some thermal and
mechanical properties can be controlled by the feed
ratio of the monomers. Here, we selected the
‘A2 þ B2 þ BB20 ’ approach as an example to recount
how to control the architecture and properties of the
resulting polymers in copolymerization [163,189,
208 –210].
In the ‘A2 þ B2 þ BB20 ’ approach, the A2 and BB20
monomers are the same as those of ‘A2 þ BB20 ’
approach, and B2 is the molecule with two secondary
amino groups. During polymerization, the rapid
reaction between the A and B groups results in the
formation of an AB20 -type intermediate. The linear
units formed from the reaction of A and B2 also are
embedded in the AB20 species, which indicated that the
segment between two branching points and the
number of linear units can be adjusted by the feed
ratio of B2 to BB20 . On the other hand, the B40 species
will also be generated. Highly branched copolymers
with various linear units will be obtained from the
further self-condensation of the intermediates
(Scheme 33).
The feed ratio of A2 to the total B2 and BB20 was set
as 1/1, and the ratio of B2 to BB20 ðnÞ was varied. For
the reaction of 44 with B2 and BB20 , the reaction
conditions such as temperature, time, concentration
and solvent influence on the polymerization [163]. A
homogeneous solution was observed throughout
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C. Gao, D. Yan / Prog. Polym. Sci. 29 (2004) 183–275
Scheme 33.
the reactions carried out in strong polar organic
solvents such as DMF, DMAc, DMSO and NMP.
Nevertheless, a precipitate was found for B2 to BB20
ratios greater than 3/1 if water or chloroform was
utilized as the solvent. The phenomenon was
attributed to the crystallization of the formed
macromolecules rather than the appearance of a gel.
In fact, the precipitate was soluble in polar solvents
such as DMF and DMSO, and the hydrochloride of the
polymers was highly soluble in water. No gelation
was observed even through at a very high concentration of the total monomers (1 g per 1 ml of solvent)
and at a high temperature (80 8C). The incidence of
cross-linking can be easily avoided under general
reaction conditions.
The polymerization procedure was investigated
with in situ FTIR spectroscopy. The reaction between
secondary amino groups and vinyl ones was surprisingly fast, and completed within 1 min. The whole
polymerization only took 6 – 10 h even at room
temperature in a diluted solution. So highly branched
macromolecules can be approached within several
hours. The propagation of the molecules during the
initial short period can be detected clearly from
the corresponding mass spectrum. The dominant
reactions were well in agreement with the prediction.
Scheme 34 displays the dominant reaction pathways
at initial stage for the polymerization of 44 with 51
and 132 in a 1/1 ratio.
The structures of the highly branched copoly(sulfone amine)s made from A2, B2 and BB20 monomers
are shown in Schemes 35 and 36. Every polymer
contains two sorts of linear units (L1 and L2), one sort
of dendritic unit and one sort of terminal unit. The DB
can be calculated by Eq. (2)
DB ¼ ðD þ TÞ=ðD þ T þ L1 þ L2Þ
ð2Þ
For a copolymer made from A2, B2 and BB20 , D,
T and L2 depend on the reaction between A and B0 .
Therefore, DB can be controlled through the value
of L1. When L1 is zero (no B2 monomer is added),
DB is that for a polymer made from A2 and BB20 .
When L1 becomes a large value (large feed ratio of
B2 to BB20 ), DB approaches zero. Thus, DB of the
copolymer can be adjusted at least from 0.5 to 0 by
the feed ratio of B2 to BB20 . On the other hand, each
repeating unit has a sulfone bond resulting from A2
monomer. So the total units are equal to the number
of sulfone units (S), and DB can be obtained by the
following equation:
DB ¼ ðD þ TÞ=S < 2T=S
ð3Þ
The dendritic units (D) and terminal units (T) are
tertiary amino formed from the reaction between A
and B and primary amino groups. The methylene
attached to primary amino groups (CH2NH2) and the
methylene linked to sulfone bonds (CH2SO2CH2)
can be easily detected independently from the
corresponding 1H NMR spectrum. So DB of each
branched copolymer is conveniently available.
When the feed ratio of B2 to BB20 is changed from
1/5 to 5/1, the DB is varied from around 0.5 to 0.1.
The dependence of inherent viscosity on DB is also
interesting for the branched copolymers with different
DBs. Fig. 4 gives a typical relationship between DB
and hinh for the branched copoly(sulfone amine)s
C. Gao, D. Yan / Prog. Polym. Sci. 29 (2004) 183–275
225
Scheme 34.
ða ¼ hinh =Mn Þ: In order to clearly reflect the increased
segment density on increased DB, Mn can be used as a
normalizing factor. With the same molecular weight,
hinh decreases slightly with increasing DB at first, and
then decreases dramatically after DB equals around
0.3. The results suggest that the segment density of a
branched polymer increases slightly, and then
increases significantly with the increase of DB after
a critical point. The studies for other copoly(sulfone
amine)s show similar behavior, and the critical DB is
0.3– 0.35.
The DB also influences the crystallization of a
branched polymer. For the branched copoly(sulfone
amine)s containing piperazine units, one sort of linear
unit is the same. When the feed ratio of B2 to BB20 is
equal to 3/1 ðn ¼ 3Þ; crystallization and melting peaks
can be observed in the DSC curves of copolymers
made from 44, 51 (or 53) and 132 [163,208]. By
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C. Gao, D. Yan / Prog. Polym. Sci. 29 (2004) 183–275
Scheme 36.
Scheme 35.
comparison, crystallization and melting peaks were
found for the copolymer made from 44, 54 and 132
with n ¼ 2 [209]. The copolymer side groups can alter
their thermal properties. The side group for
the copolymer made from 44, 54 and 132 is methyl
( – CH3), while that for the copolymer made from 44,
53 and 132 is ethyl ( –CH2CH3). The steric effect of
ethyl is stronger than that of methyl in crystallization,
so the macromolecules with the units of 53 are more
difficult to crystallize than those with the units of 54.
Although there are no side groups in the unit of 51, no
crystallization was detected for the sample with n ¼ 2
because the ring of piperazine is more rigid than the
carbon – carbon single bond.
The crystallization is influenced not only by the
side groups of a branched copolymer, but also by
the stiffness of its main chain. Double-melting
endotherms were observed for the copolymers with
units of 133 [209,210]. Compared with the
copolymer made from 44, 132 and 53, the
copolymers made from 44, 133 and 53 are difficult
to crystallize, and no crystallization appeared for
the sample with n ¼ 3: Copolymers including units
of 133 are rather slow to crystallize, and no
crystallization peak can be observed at a normal
cooling rate (10 – 20 8C/min). Furthermore, Tg s were
not clearly for the copolymers found in DSC
measurements. The Td s for the copolymers in
nitrogen are about 285 –320 8C, with the Td for a
crystalline sample higher 10– 20 8C than that of an
amorphous one.
Branched copoly(ester amine)s with various DBs
could be synthesized using monomer 45 to react with
B2 and BB20 monomers [189]. Similar phenomenon for
the relationship of DB and hinh was observed for the
copoly(ester amine)s. Because the copolymers were
C. Gao, D. Yan / Prog. Polym. Sci. 29 (2004) 183–275
227
architecture and the large number of terminal
functional groups of hyperbranched macromolecules,
five modification manners have been developed: (1)
end-capping with short chains or organic molecules;
(2) terminal grafting via living polymerization; (3)
growing hyperbranched polymers on the surface, or
grafting from/onto the surface; (4) hypergrafting to
obtain hyperbranched polymers with a linear macromolecular core; (5) blending or crosslinking. The first
four methods will be discussed in this section, and the
fifth one will be discussed in Section 5.
4.1. End-capping
Fig. 4. Ratio of inherent viscosity to number-average molecular
weight ðaÞ as a function of degree of branching for the copolymer
made from divinyl sulfone (44), piperazine (132) and 1-(2aminoethyl)piperazine (51).
obtained as the hydrochloride, all of the samples were
amorphous.
As illustrated above, CMM exhibits generality,
simplicity, controllability, versatility and availability
in the preparation of hyperbranched polymers.
Although many new hyperbranched polymers have
been produced via the novel approaches aforementioned, CMM is still developing, and more hyperbranched polymers with fundamental application
potentials will be found through this novel
methodology.
4. Modification of hyperbranched polymers
The properties of hyperbranched polymers are
often affected by the nature of the backbone and the
chain end functional groups, degree of branching,
chain length between branching points, and the
molecular weight distribution. Hyperbranched polymers are often modified to tailor their properties for a
specialized purpose. Based on the highly branched
The large number of functional end groups
attached to the linear and terminal units of hyperbranched polymers can be conveniently end-capped
with small organic molecules. In the end-capping
process three major purposes are emphasized: (1) to
exclude the influence of some functional groups on
the measurement of molecular weight; (2) to investigate the effect of terminal groups on the properties of
hyperbranched polymers; (3) to fabricate novel
functional polymeric materials.
Many experiments demonstrated that the nature of
terminal functional groups strongly influenced Tg ;
solubility and even Td of a hyperbranched polymer.
Kim et al. [48] investigated the influence of end
groups on the Tg of hyperbranched polyphenylene
(Scheme 37), and found that the Tg can be varied over
a wide range, from 96 8C for the polymer with a-vinyl
phenyl end groups to 223 8C for the polymer with panisol end groups although the modified hyperbranched polymers have the same backbone.
Wooley et al. [56] reproducibly prepared hyperbranched aromatic polyester by self-polycondensation
of 3,5-bis(trimethylsiloxy)benzoyl chloride (138) in the
presence of catalytic amounts of DMF or triethylamine
hydrochloride. Then a variety of different functional
groups were introduced into the hyperbranched macromolecules by modification of their phenolic groups
Scheme 37.
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C. Gao, D. Yan / Prog. Polym. Sci. 29 (2004) 183–275
Scheme 38.
(Scheme 38). Tg of the hyperbranched macromolecules
modified with monobenzylester of adipic acid is only
6 8C, whereas it is as high as 197 8C with terminal
hydroxyl groups. The flexible alkyl units distributed
throughout the macromolecular structure may play the
role of plasticizers for the aromatic polyesters, reducing
Tg in comparison with that for samples with relatively
shorter end chains. Furthermore, the solubility was also
affected dramatically by the functionality of the chain
ends. The phenolic terminated polyesters are only
soluble in polar solvents, such as DMSO, NMP,
methanol and THF, while the end-capped products are
also soluble in non-polar solvents such as chloroform,
dichloromethane and toluene.
Surprisingly, the effect of phenolic groups on the
Tg for hyperbranched aromatic polyethers made from
monomer 139 is not so great as that for the polyesters
mentioned above [50], and Tg of the hyperbranched
polyether with hydroxyl terminated groups is only
38 8C (Scheme 39). It seems that several factors such
as the rigidity, polarity, length, and steric hindrance of
the terminal functional groups can influence Tg of the
resulting polymers.
For the hyperbranched poly(ether ketone)s made
from monomer 140 or 141, Tg increased by nearly
200 8C on going from an octyloxy terminal group to a p(carboxy)phenoxy terminal group (Scheme 40) [84].
Another feature for the hyperbranched poly(ether
ketone)s is that their thermal properties, for example
Tg ; can be essentially independent of their macromolecular architecture (DB), illustrated by a Tg for a
polymer with a DB of 0.49 that is almost equal to that for
a similar polymer with a DB of 0.15. Similarly, the
fluoro-terminated hyperbranched poly(ether ketones)
made from AB2, AB3, and AB4 monomers differ in DB
(0.49, 0.39, 0.71, respectively), but have the same Tg
(162 8C).
Few linear polyetherketones are soluble, they
crystallize easily. When compared to their linear
analogs, it is found that the solubility of a hyperbranched polymer is dependent on both the nature of
terminal functional groups and its highly branched
architecture. The polymer with polar end groups is
soluble in polar solvents, and the non-polar-terminated polymer is soluble in apolar solvents. The
fluoro-terminated sample is only sparingly soluble in
DMF and totally insoluble in DMSO and aqueous
solutions, while the phenolic-terminated polymer
is soluble in DMF, DMSO, and aqueous K2CO3 or
KOH solutions. In addition, the former is highly
soluble in relatively weaker polar or non-polar
solvents, such as chloroform and 1,2-dichloroethane,
whereas the latter is totally insoluble in these.
Shu and coworkers [87] prepared carboxylic acidterminated hyperbranched poly(ether ketone) from
5-phenoxyisophthalic acid (142), using phosphorus
pentoxide/methanesulfonic acid in a weight ratio of
1:12 (PPMA) as condensing agent and solvent.
Chemical modification of the carboxylic chain ends
led to hyperbranched poly(ether ketones) containing a
variety of different functional groups. DSC measurements showed that Tg of the hyperbranched poly(ether
ketone)s were strongly related to the chain end groups,
with Tg increasing with increasing the polarity of the
end groups (Scheme 41). Compared with the carboxylic acid-terminated polymer or its ammonium
derivative (Tg of 226 and 236 8C, respectively), Tg s of
polymers linked less polar terminal groups such as ester
Scheme 39.
C. Gao, D. Yan / Prog. Polym. Sci. 29 (2004) 183–275
229
Scheme 40.
and ketone groups are lower than by about 100 8C. The
solubility of the modified hyperbranched poly(ether
ketone) made from 142 is similar to that for polymers
made from 140 or 141. The non-polar groupterminated hyperbranched polymers are soluble in
non-polar solvents, such as chloroform and dichloromethane, and the polar group-terminated polymers
are soluble in polar solvents such as DMF and NMP;
the ammonium derivative is even soluble in water.
Shu and coworkers [211] synthesized an ABB0
monomer (143) containing a pair of phenolic groups
and an aryl fluoride, activated toward displacement by
the attached oxazole ring. Nucleophilic substitution of
the fluoride with the phenolic groups resulted in the
formation of ether linkage, and subsequently the
hyperbranched poly(aryl ether oxazole). The endcapping approach was used to chemically modify the
phenolic terminal groups (Scheme 42). As in the
preceding, Tg determined by DSC was very dependent
on the nature of the chain ends, increasing with
increasing end group polarity, e.g. Tg of the phenolicterminated sample is as high as 247 8C, whereas Tg of
the sample modified with 1-hexanol is only 119 8C. In
addition, the data summarized in Scheme 42 also
showed that Tg decreases with increasing length of the
end chain for samples with same sort of end groups,
and Tg of a sample with ester end chains is higher than
Tg of a polymer with ether end chains, for comparable
end chains length. Similar results are also observed
for hyperbranched aromatic poly(ether imide)s made
from N-{4-[1,1-di(4-hydroxyphenyl)ethyl]phenyl}-4fluorophthalimide (144) (Scheme 43) [212].
As indicated in the preceding, the thermal and
mechanical characters of hyperbranched polymers
can be conveniently manipulated via the end capping
method on the basis of the relationship between
terminal groups and properties. Generally, alkyl- and
ether groups-terminated hyperbranched polymers will
have relatively low Tg values, low melting viscosity
and low mechanical strength, and carboxylic acid- and
hydroxyl-terminated hyperbranched ones have relatively high Tg values, high melting viscosity and high
mechanical strength. Kim and coworkers [213]
synthesized hyperbranched poly(ether ketone) analogs with heterocyclic triazine moiety by polymerization of monomer 145, and then modified the polymer
with flexible moieties (Scheme 44). The phenolicterminated hyperbranched polymer has a Tg of
264 8C, and the Tg of the methoxy-terminated
polymer is only slightly lower (260 8C). The incorporation of longer flexible moieties such as di- or
tetraethyleneoxy and stearyl units at the chain ends
Scheme 41.
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C. Gao, D. Yan / Prog. Polym. Sci. 29 (2004) 183–275
Scheme 42.
greatly lowered the Tg values (169, 149 and 108 8C,
respectively). On the other hand, because the di- and
tetraethyleneoxy-terminated hyperbranched polymers
consist of a hydrophobic core and hydrophilic surface,
they may exhibit amphiphilic characteristics in an
aqueous phase. The investigation of self-aggregation
of the amphiphilic core – shell hyperbranched polymers using a pyrene fluorescence probe showed that
the critical aggregation concentration (CAC) of the
tetraethyleneoxy-terminated hyperbranched polymer
was 12.6 mg/l, much lower than that of low molecular
weight surfactants, e.g. 2.3 g/l for sodium dodecyl
sulfate (SDS) in water [213].
The nature of end groups has also significant
effect on the thermal properties of the hyperbranched poly(siloxysilanes) with flexible backbones. Miravet et al. [100] synthesized
hyperbranched poly(siloxysilanes) by polyhydrosilation of methylvinylbis(methylsiloxy)silane (146).
End capping of the terminal silicon hydride groups
by hydrosilation with a variety of reagents gave
a diversity of terminal modified hyperbranched
poly(siloxysilane) materials (Scheme 45). Because
of the conformation freedom of the carbosiloxane
skeleton, the SiH terminated polymer made from 146
exhibits a very low Tg (2 106 8C). After end capping
a SiH-terminated sample with di-tert-butylphenol
units, Tg increased significantly, by about 74 –
104 8C. Phenyl ether- and chloromethylbenzeneterminated polymers have intermediate Tg values
that are also greatly higher than that of the starting
SiH-terminated polymer. Conversely, the Tg values
of polymers having flexible triethylene glycol type
end chains are only slightly higher than that of
starting hyperbranched poly(siloxysilane) with SiH
end groups.
The influence of terminal alkyl groups on the
thermal and mechanical properties of aromatic
hyperbranched polyesters was also studied by
introducing alkyl chains with different length into
the end groups of hyperbranched polyester based on
3,5-dihydroxybenzoic acyl [214]. The degree of
modification was varied from 11 to 100%. The Tg
first decreased with increasing length of the alkyl
chain, but then did not decrease with further
increasing chain length, perhaps owing to the
possibility of crystallization. Thus, increase of the
degree of modification led to a pronounced decrease
of Tg, and an increase of the side chain crystallization for long alkyl chain modification.
Scheme 43.
C. Gao, D. Yan / Prog. Polym. Sci. 29 (2004) 183–275
231
Scheme 44.
The measurements showed that modification with
C12 chains caused a pronounced decrease in the
complex viscosity and the moduli.
By contrast with the preceding, it must be
recognized that there are limits on the extent to
which the thermal properties for aliphatic hyperbranched polyether polyols may be tailored by end
group modification. DSC measurements for the
hyperbranched
polyglycerols
(DPn ¼ 23 – 83;
Mw =Mn ¼ 1:2 – 1:5) as well as their propoxylated
derivatives esterified to different extents (23 –100%)
with various carboxylic acids (C2 to C18, benzoic,
biphenylcarboxylic, and benzoic acid) indicate that Tg
of these hyperbranched polymers is mainly controlled
by two factors: (1) the extent of hydrogen bonding,
and (2) the formation of order structure (mesophases
and crystallization). The Tg values of the esterified
polyether polyols are in the range 2 58 to 2 22 8C.
The difference ðDTg Þ in Tg value between the original
and derivatized samples is only 2 9 to 34 K [215].
Gedde and coworkers [216] end capped the
hyperbranched aliphatic polyester originating from
2,2-dimethylolpropionic acid (147) with 1,1,1-tri(hydroxymethyl)propane (105) as core, affording three
hyperbranched polyesters with the same backbone
structures, but different terminal groups (acetate,
benzoate or hydroxyl groups) (Scheme 46). The DTg
(33 K measured with DSC) between the hydroxyterminated polymer and acetate-terminated samples
was less than had been anticipated.
On the other hand, end capping the hyperbranched polymers with specific compounds is an
efficient approach to fabricate novel functional
polymer materials. Frey et al. [137,217] synthesized
Scheme 45.
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C. Gao, D. Yan / Prog. Polym. Sci. 29 (2004) 183–275
Scheme 46.
hyperbranched polyether polyols by ring-opening
multibranching polymerization (ROMBP) of glycidol (19), using 1,1,1-tri(hydroxymethyl)propane
(105) as core initiator. Then the hyperbranched
polyglycidols were end capped with acid 148 or 149
in the presence of diisopropylcarbodiimide (DIPC)
as condensing agent, affording hyperbranched polyether with mesogenic end groups (Scheme 47)
[218]. The degree of functionalization of the
hyperbranched polyglycerols achieved 88% for the
lower molecular weight sample ðMn ¼ 1500 g=molÞ
and 73 – 79% for the higher molecular weight
samples ðMn ¼ 3500 g=molÞ: SEC measurements
showed narrow molecular weight distributions for
all the three samples ðMw =Mn , 1:2Þ: DSC, polarizing microscopy, and wide-angle X-ray scattering
(WAXS) were used to investigate the liquid crystalline properties of the three samples. A dramatic
increase of Tg (40 – 50 8C) was observed for the
three samples. Apparently, molecular weight did not
strongly influence the formation of the mesophase.
The longer alkyl-terminated sample exhibits a much
higher transition enthalpy, indicating the presence of
more highly ordered smectic clusters.
Salazar et al. [219] synthesized hyperbranched
amino-terminated polyglycidols by end-modification
of terminal hydroxyls with tolylsulfonyl chloride
(TsCl), followed by nucleophilic substitution of the
tolylsulfonyl groups with secondary aliphatic
amines (Scheme 48). The hyperbranched aminoterminated polymers obtained can be used as
macromolecular ligands in an oxidative coupling
reaction of phenylacetylene. It is found that the
amino-terminated polyglycidols – CuCl complexes
are more effective catalysts for the oxidative
coupling reaction than the reference monomeric
tertiary amines-CuCl, while less effective than
the most efficient N,N,N0 ,N0 -tetramethylethylenediamine – CuCl complexes. The performance improvement for the hyperbranched polymeric ligands may
Scheme 47.
C. Gao, D. Yan / Prog. Polym. Sci. 29 (2004) 183–275
233
Scheme 48.
be attributed to two factors: (1) the better
complexation abilities, and (2) local increase of
the reagent concentration.
Bergenudd et al. [220] end modified the
commercially available hyperbranched polymer
Boltorne with 3-(3,5-di-tert-butyl-4-hydroxy-phenyl)-propionic acid, resulting in hyperbranched
polymeric antioxidants. Then the antioxidants
were evaluated in squalane, and in polypropylene
(PP) films, to determine their oxidation induction
time. The hyperbranched antioxidants are more
effective than the commercial antioxidant Irganox
1010 in the liquid compound squalane, but inferior
in PP. Low mobility in combination with low
solubility may cause the lower efficiency in PP.
This result suggests that the macromolecular
antioxidants would be suitable as low volatile
stabilizers for liquid systems such as oils and
lubricants.
Wooley et al. [221] prepared a hyperbranched
polyfluorinated benzyl ether polymer (Scheme 49), by
deprotonation of benzylic alcohol and nucleophilic
substitution of p-fluorines of the two pentafluorophenyl groups attached to 3,5-bis[(pentafluorobenzyl)oxy]benzyl alcohol (150). Then fluoroalkyl groups were
introduced into the polymer by chemical modification
Scheme 49.
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C. Gao, D. Yan / Prog. Polym. Sci. 29 (2004) 183–275
of its p-fluorines with lithium trifluoroethoxide
and lithium 1H; 1H; 2H; 2H-perfluorodecanoxide.
Additionally, an X-ray opaque derivative was
obtained by end capping the pentafluorophenylterminated hyperbranched polymer with p-iodophenol. Contact angle measurements of water and
hexadecane on the films of fluoroalkyl-modified
polymers indicate a high degree of hydrophobicity
and lipophobicity. Interestingly, essentially equivalent hydrophobicity was obtained with 36 or 100%
1H; 1H; 2H; 2H-perfluorodecanoxy-substitution, with
only a 58 difference in the lipophobicity between the
former and the latter. Atomic force microscopy
(AFM) was applied to investigate the surface
morphologies and properties of the polymer films.
Tapping mode AFM images showed phase separation
in the partially 1H; 1H; 2H; 2H-perfluorodecanoxysubstituted polymeric material. More than a twofold decrease in the coefficient of friction and
adhesive force for the 1H; 1H; 2H; 2H-perfluorodecanoxy-modified polymer was detected with lateral
force AFM using a silicon nitride probe.
An increase in the content of heavy atoms such as
halogens would decrease the absorption in the near
infrared (NIR), suggesting that the halogenated
hyperbranched polymers may be utilized as photonic
compounds. A good optical waveguide material also
requires the combination of low optical loss, controlled refractive index, good thermal and mechanical
properties, and ease of processing. On the basis of the
high fluorine content of the AB2 monomer 3,5diphentafluorophenyl-1-(hydroxy)benzene (151) [51,
222], fluorinated hyperbranched polymer and its endmodified products have been developed,
potentially useful for optical waveguide applications
(Scheme 50) [223,224]. Very low losses in the NIR,
0.1 dB/cm at 1550 nm, were demonstrated for the
hyperbranched polymer made from 151, because of its
amorphous structure and low C – H bond content.
Depending on the type and amount of the terminal
functional groups of the resulting polymers, the
refractive indices at 633 nm can be varied from
1.450 to 1.638, which matches the index of materials
involved in optical devices, and thus minimizes
backscattering at interconnections. The Tg of the
fluorinated hyperbranched polymer and its derivatives
ranged from 91 to 140 8C, and the temperatures of
initial decomposition ranged from 335 to 430 8C. A
highly crosslinked polymer was achieved by cationic
photoinitiating the fluorinated p-hydroxystyrene-terminated hyperbranched polymer.
Radical addition reaction can be used to functionalize the terminal groups of hyperbranched polymers.
Matyjaszewski and coworkers [113 – 116] prepared
Scheme 50.
C. Gao, D. Yan / Prog. Polym. Sci. 29 (2004) 183–275
235
Scheme 51.
a hyperbranched polyacrylate by ATRP of 2-2(bromopropionyloxy)ethyl acrylate (BPEA), followed
by addition of 1,2-epoxy-5-hexene to the polymer
chain end, resulting in epoxy-terminated functional
hyperbranched polymer (Scheme 51) [225].
In addition, a hyperbranched polymer with azo
chromophores end groups was prepared by modification of a hydroxyl-terminated hyperbranched
polyester with ethyl 4-{40 -[N,N-di(hydroxyethyl)aminobutoxy]phenylazo}benzoate [226]. Depending on
the difference between interior (closer to the focal
unit) and periphery (distant from the focal unit)
hydroxyl groups in the hyperbranched polyglycerol,
core – shell-type hyperbranched polyglycerols were
fabricated by selective chemical differentiation [227].
Fluorescent chromophores were introduced into the
periphery of hyperbranched polymers via end-capping, affording fluorescent hyperbranched polymeric
materials [228 – 230].
4.2. Terminal grafting
Terminal grafting can also be called ‘grafting
from’. Grafting polymers from macromolecular
initiators prepared by modification of the functional
groups of hyperbranched polymers affords core –shell
multi-arm star polymers or hyperstars. Some properties, such as polarity, solubility and flexibility of the
hyperbranched scaffolds, can be conveniently tailored
through terminal grafting modification. Three polymerization methods (e.g. anionic, cationic and living/controlled radical polymerizations) have been
adopted to fabricate the hyperstars via reaction
processes including macromolecular initiator-first
and in situ grafting. To date, most of the hyperbranched cores used are polyols.
Through a two-step, one-pot approach, Frey et al.
[138] obtained short poly(propylene oxide) (PPO)
multi-arm star polymers by anionic ring-opening
multibranching polymerization (ROMBP) of glycidol,
followed by anionic polymerization of propylene
oxide (Scheme 52). The resulting polyether polyols
containing up to five propylene oxide units per end
group showed apparent Mn values in the range 5000 –
12,000 g/mol and narrow polydispersity ðPDI , 1:7Þ
by using DMF as eluent and poly(propylene glycol)s
with Mn in the range 1000– 12,000 as samples for
calibration. The polarity of the highly hydrophilic
Scheme 52.
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C. Gao, D. Yan / Prog. Polym. Sci. 29 (2004) 183–275
polyglycerols (PG) can be varied through this
approach. Depending on the length of the propylene
oxide segments, Tg of the grafted polymers varied
from 2 37 to 2 71 8C.
Through a macromolecular initiator-first approach,
Lutz et al. [231] synthesized poly(ethylene oxide)
(PEO) multi-arm star polymers by employing hyperbranched PG and PG modified with short oligo(propylene
oxide)
segments
(PG-b-PPO)
as
polyfunctional initiators for the living anionic ring
opening polymerization of ethylene oxide (EO) in the
presence of diphenylmethylpotassium (DPMP)
(Scheme 52). In the case of PG without modification,
aggregation would occur after metalation, leading to
inefficient initiation and propagation. By comparison,
hydroxyfunctional PEO multi-arm star polymers (PGb-PPO-b-PEO) with Mn values in the range 34,000–
95,000 g/mol, and arm numbers in the range 26 – 55
were prepared smoothly with PG-b-PPO as macromolecular initiators in a core-first strategy.
The polydispersity ðMw =Mn Þ of the three blocked
star polymers was lower than 1.5. Measurements of 1H
and 13C NMR demonstrated completely conversion
of the end groups of the propylene oxidemodified initiator. Star polymers with higher molecular weights (Mn ¼ 180,000) can be obtained by
reinitiating multi-arm PEO stars. The arm length has
considerable influence on the thermal properties of the
prepared stars. Raising the length of the PEO chains
from 17 to 39 units results in an increase of the
melting temperature ðTm Þ from 33 to 52 8C, and an
increase of the melting enthalpies ðDHm Þ from around
88 to 133 J/g. These trends may be attributed to a
gradually lowered influence of the hydroxyl end
groups and the amorphous PG core on the thermal
properties of the stars with increasing arm length.
Thus, it is possible to tailor the thermal properties of
the multi-arm star polymers by the variation of the arm
length and functionality.
Poly(1-caprolactone) multi-arm star block copolymers (PG-b-PPO-b-PCL) have been prepared based
on the propoxylated hyperbranched polyglycerol (PGb-PPO), with stannous 2-ethylhexanote (Sn(Oct)2) as
the catalyst [232]. With increasing ratio of monomer
to initiator, the length of the caprolactone arms
increased from 10 to 50 units, and the corresponding
apparent Mn values (SEC measurement was
carried out at 35 8C with chloroform as eluent and
polystyrene standards for calibration) of the hyperstars increased from 75,200 to 210,000, with narrow
PDI of the block copolymers ðMw =Mn ¼ 1:16 – 1:33Þ:
These hyperstars are very attractive in the biomedical
applications due to the biodegradability of the PCL
arms and the biocompatible core [233].
ATRP have also been utilized to prepare multi-arm
star polymers with PG core via the terminal grafting
strategy. Maier et al. [234] synthesized a hyperbranched macromolecular initiator for ATRP by
modification of PG with 2-bromoisobutyryl bromide,
then polymerized methyl acrylate (MA) in the
presence of the initiator and CuBr/pentamethyldiethylenetriamine (PMDETA) affording a multi-arm block
copolymers with polyether core and PMA arms
(Scheme 52). The length of PMA arms can be
controlled through monomer/initiator ratio and conversion. No micro-phase separation of PG core and
PMA arms occurred as demonstrated with the DSCmeasurements (only one Tg for each hyperstar).
Hyperbranched polyether polyols made from 3alkyl-3-(hydroxymethyl)oxetane have also been
widely applied as core molecules in the preparation
of multi-arm star block copolymers. Similar to the
route employed by Frey et al. [234], Carlmark et al.
[235] synthesized a hyperbranched multifunctional
initiator with approximately 25 initiating points for
ATRP by reacting hyperbranched poly[3-ethyl-3(hydroxymethyl)oxetane] (PEHO) with 2-bromo-isobutyrylbromide (Scheme 53). Then the hyperstars
with PEHO core and PMA arms (PEHO-b-PMA)
were prepared by ATRP of MA in the presence of the
macromolecular initiator and CuBr/tri(2-(dimethylamino)ethyl)amine (Me6-TREN) in ethyl acetate at
room temperature. Because of the inter- and intramolecular reactions, gel would be easily formed in the
polymerization. In order to prevent crosslinking, the
concentration of propagating radicals in the system
must be kept low by dilution with solvents, stopping
the reaction after low conversion of the monomer, or
decreasing the amount of catalyst used. In the case
described above, the initiating sites-to-catalyst could
not exceed 1:0.1, otherwise, either gelation would
occur or the polymerization went out of control. The
most successful reactions were conducted with a ratio
of 1:0.05. Different from the thermal properties of PGb-PMA, two glass transitions were observed from the
DSC curves of the PEHO-b-PMA hyperstars,
C. Gao, D. Yan / Prog. Polym. Sci. 29 (2004) 183–275
237
Scheme 53.
suggested that phase-separation existed in the latter
block copolymers. The lower Tg at around 2 40 8C
originated from PEHO cores is hardly affected by the
length of PMA arms, while the higher one assigned to
PMA arms increased with increasing PMA chain
length.
Yan and coworkers [236] synthesized hyperbranched poly[3-ethyl-3-(hydroxymethyl)oxetane]block-poly(2-dimethylaminoethyl methacrylate)
(PEHO-b-PDMA) via oxyanionic polymerization.
Potassium alcoholate macroinitiators with high initiating efficiency was obtained by reaction of the
hydroxyl groups of PEHO with KH. It was found that
lower critical solution temperature of PEHO-bPDMA decreased with either increasing DMA chain
length or increasing pH of the solution.
Hou et al. [237] obtained hyperstars with hyperbranched poly(3-methyl-3-(hydroxymethyl)oxetane)
(PMHO) core and short polytetrahydrofuran (PTHF)
arms via an in situ grafting approach (Scheme 53). The
cationic ring-opening polymerization of 3-methyl-3(hydroxymethyl)oxetane (MHO) was initiated with
BF3·O(CH2CH3)2. Because the rate constant for the
addition reaction between terminal units and MHO is
much lower than that of MHO homopolymerization,
the first stage polymerization in THF solution
produced PMHO, with copolymerization with THF
after the MHO was almost completely reacted. The
results were confirmed as compared with those
obtained using a two-step copolymerization, i.e.
homopolymerization of MHO and then addition of
suitable amount of THF to the reaction system to effect
the copolymerization. The 1H NMR measurements
showed that the average degree of polymerization of
THF arms increased from 0.41 to 3.86 on increasing the
feed ratio of THF to MHO from 0.5:1 to 6:1.
Commercially available aliphatic hyperbranched
polyester with 32 hydroxyl groups (Boltorne-H30)
was used as the core by Hult et al. [238] to prepare
PCL multi-arm star copolymers in the presence of
Sn(Oct)2. Rheological measurements of zero shear
rate viscosity, h0 ; showed that the star polyesters had
a significantly lower h0 than linear PCL. Dynamic
mechanical data indicated that the films made from
the shorter-armed resins were amorphous after curing,
while those from longer-armed resins were crystalline. Another multi-arm star copolymer with hyperbranched poly[2,2-bis(hydroxymethyl)propionic
acid] core and PCL arms was obtained via a similar
route [239]. As a reference material, PCL multi-arm
star copolyesters initiated from a dendrimer were also
prepared. Comparison of the results obtained for
dendrimeric and hyperbranched initiators showed that
the reactivity of the terminal hydroxyl groups in the
dendrimer was considerably higher than in the
hyperbranched polymer.
Amphiphilic multi-arm star polymers with a
hydrophobic hyperbranched core and hydrophilic
graft arms have been reported by Weberskirch et al.
[240]. Modification of the hyperbranched polymer
derived from 4,4-bis(40 -hydroxyphenyl)valeric acid
with 3-(chloromethyl)benzoyl chloride gave rise to
hyperbranched macroinitiator, followed by cationic
ring-opening polymerization of 2-methyl-2-oxazoline
238
C. Gao, D. Yan / Prog. Polym. Sci. 29 (2004) 183–275
affording the core –shell amphiphiles with up to 12
arms. Single star molecules ranging from 22 to 50 nm
in diameter were detected with photon correlation
spectroscopy measured in methanol and chloroform.
4.3. Surface growing
The method to modify the specific surface or
interface with hyperbranched macromolecules or to
graft hyperbranched polymers onto the surface is
denoted as ‘surface growing’ in this paper. The
grafted hyperbranched polymers can be regarded as
so-called polymer brushes, which are typically
anchored to a surface by one end of the polymer
chain, such that the polymer can extend away from
the surface [241]. Surface growing is an efficient
strategy to fabricate inorganic/organic-hyperbranched polymer hybrid materials and functional
devices or improve the properties of surface
objects.
Four routes have been developed in these
modifications and functionalizations, as shown in
Fig. 5. Route 1 can be called the graft-on-graft
approach. The reaction procedure is similar to the
step-by-step synthesis methodology of a dendrimer.
Functional groups (D) are firstly introduced onto the
surface, followed by reaction of AB2 monomers or
multifunctional polymers with the D groups and
then reaction of CD monomers with the B groups
attached (in some examples, the addition order of
AB2 or CD is reversed). Repeating the reaction
between AB2 and CD grows hyperbranched
polymers with numerous functional groups onto
the surface. Route 2 is a ‘grafting from’ technique
of surface initiating polymerization in which the
initiating functional groups are incorporated onto the
surface and then initiate the polymerization of AB2
or latent AB2 (ABB0 ) monomers. In route 3, a
‘grafting to’ approach is used, whereby prepared
hyperbranched polymers are covalently linked to a
prepared surface. Route 4 called ‘surface adsorption’, hyperbranched macromolecules are directly
assembled or adsorbed onto the surface. Notably,
routes 1-3 are chemical methods, while route 4 is a
physical method. Therefore, both chemical and
physical approaches can be used to graft hyperbranched polymers onto the surface.
A series of solid matrixes such as gold (Au),
silica (SiO2), silicon wafer (Si), aluminum (Al),
porous alumina (Al2O3), C60, carbon black, polyethylene (PE), polypropylene (PP), and chitosan
powder (CP) have been adopted as supporting
substrates for the functionalization of hyperbranched
polymers. Some hyperbranched polymers grown on
the surface and their grafted approaches are listed in
Table 6.
4.3.1. On the surface of Au
Carboxylic acid functional groups are introduced
on a gold surface or the surface of gold-coated porous
alumina via the attachment of mercaptoundecanoic
acid (MUA) to the substrate, followed by the
conversion of the carboxylates to mixed anhydrides.
Then amino-terminated poly(tert-butyl acrylate)
Fig. 5. The surface growing routes for hyperbranched polymers.
C. Gao, D. Yan / Prog. Polym. Sci. 29 (2004) 183–275
239
Table 6
Growing hyperbranched building blocks on the surfaces
Surface
Introduced surface group
Major building units or monomers
Approach
Reference
Au
–COOH
ClCO2CH2CH3 þ PAA
graft-on-graft
[241–249]
SiO-2
–NyN –
–OCO2O–
–NH2
–NH2
–NH2
–CO2CBr(CH3)2
–
MMA þ MOI
CH2yCH-R
MA þ H2N(CH2)nNH2
MA þ H2N(CH2)2NH2
Aziridine
BPEA
PEI
graft-on-graft
graft-on-graft
graft-on-graft
graft-on-graft
Grafting from
Grafting from
Adsorption
[250]
[251]
[252]
[253]
[254]
[255]
[256]
Si
–OH
–OH
–NH2
–CO2CBr(CH3)2
Hyperbranched EHBP
HBP3/HBP4
Aziridine
BPEA, BIEM
Grafting to
Adsorption
Grafting from
Grafting from
[257,258]
[259]
[254]
[260]
PE
–COOH
–NH2
ClCO2CH2CH3 þ PAA
MA þ H2N(CH2)2NH2
graft-on-graft
graft-on-graft
[261–267]
[268]
PP
–COOH
–
ClCO2CH2CH3 þ PAA
HBP-Boltorn E2
graft-on-graft
Grafting to
[269]
[270]
CP
–NH2
MA þ H2N(CH2)2NH2
graft-on-graft
[271]
(PTBA) is grafted on the surface through the
attachment sites of the mixed anhydrides. Hydrolysis
of the tert-butyl groups to carboxylic acid generates a
poly(acrylic acid) (PAA) layer. Repeating the steps of
the formation of mixed anhydrides, grafting of PTBA
and hydrolysis of tert-butyl groups affords multigrafting hyperbranched PAA layers [241 –249]. A
typical procedure is described in Fig. 6. In order to
change the surface properties, the carboxylic acid
groups in PAA are further modified with other
molecules such as H2NCH2(CF2)6CF3 [242,245,
247], and molecules containing pyrene, ferrocene,
poly(ethylene glycol), 15-crown-5,3,4,9,10-perylenetetracarboxylic diimide, etc. [244].
4.3.2. On the surface of SiO2
The silica surface is also widely grafted with
hyperbranched polymers. Tsubokawa et al. [250]
achieved surface-grafting polymers with azo pendant
groups by reaction between 4,40 -azobis(4-cyanopentanoic acid) (ACPA) and isocyanate groups of grafted
polymer on a silica surface. The azo groups can
initiate the copolymerization of methyl methacrylate
(MMA) with 2-methacryloyloxyethyl isocyanate
(MOI). The isocyanato groups in PMOI can be
modified with ACPA, which can initiate copolymerization of MMA and MOI. Thus, hyperbranched
copolymers can grow on the silica surface via the
graft-on-graft approach. Polymer chains possessing
peroxycarbonate groups may be obtained by copolymerization of tert-butylperoxy-2-methacryloyloxyethylcarbonate with other vinyl monomers. Similar
to the azo groups, peroxycarbonate groups can initiate
the polymerization of vinyl monomers to give
hyperbranched polymers anchored to the silica surface [251].
Reaction of terminal diamine-type polyoxyethylene with epoxy groups previously introduced onto
the silica surface through 3-glycidoxypropyltrimethoxysilane treatment leads to a modified surface
having numerous amino groups. By repeating
Michael addition of MA to amino groups followed
by amidation of the ester moieties with diamines
(which is similar to the synthesis of polyamidoamine dendrimer), hyperbranched polyamidoamines
are grafted onto the silica surface [252,253].
Aminosilylated surface can also directly initiate
the ring-opening polymerization of aziridine to form
hyperbranched poly(ethyleneimine) (PEI) on a solid
support [254].
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C. Gao, D. Yan / Prog. Polym. Sci. 29 (2004) 183–275
Fig. 6. The general procedure for grafting hyperbranched poly(acrylic acid) layers on the gold surface by graft-on-graft approach.
Müller and coworkers [255] obtained hyperbranched polymer – silica hybrid nanoparticles by
SCVP via ATRP of BPEA from silica surface linked
a-bromoester initiator layer. Mészáros et al. [256]
investigated the direct adsorption on silica wafers of
hyperbranched PEI with reflectometry, and showed
that significant charge reversal and shift in the
isoelectric point of silica wafer occurred because of
the adsorption of PEI.
4.3.3. On the surface of Si
A molecular surface coating with a thickness of
4.5 nm has been fabricated by reaction of epoxyfunctionalized hyperbranched polyesters (EHBP)
with the hydroxyl groups of silicon wafers via a
grafting to approach [257,258]. Around 3 – 4 epoxy
groups per molecule attached to the uppermost
surface layer provided residual functionality sufficient
to graft another polymer layer. It was found that the
grafted hyperbranched polymeric layers are very
robust and can sustain high compression and shear
stresses, while possessing high elasticity [257]. The
uniform polymer layers were homogeneous on a
nanoscale, without signs of the microphase separation
[258].
The commercially available Boltorne hyperbranched polyesters with third and fourth generations (HBP3 and HBP4) can be directly adsorbed
on a bare silicon surface; the adsorption amount of
lower generation HBP3 is higher than that of HBP4
[259]. In a HBP3 adsorbed layer, surface coverage
is observed with thickness ranging from less than
1 nm to about 3 nm. For HBP4 located on the
surface, a stable, close-to-spherical shape with a
diameter of 2.5 nm is found throughout the whole
coverage, including both dense monolayers and
isolated molecules. This differing surface
behavior might be caused by the different molecular
flexibility and constrained mobility between HBP3
and HBP4.
C. Gao, D. Yan / Prog. Polym. Sci. 29 (2004) 183–275
Similar to the process for growth of hyperbranched
PEI and hyperbranched poly(BPEA) on the silica
surface [254,255], hyperbranched PEI [254] and
hyperbranched polyesters [260] have been successfully anchored to the silicon surface by ring-opening
polymerization of aziridine and self-condensing
ATRP of BPEA and 2-(2-bromoisobutyryloxy)ethyl
methyacrylate (BIEM), respectively, via grafting
from approach.
4.3.4. On the surface of PE
Bergbreiter and coworkers [261 –267] have grafted
hyperbranched PAA onto the PE surface and modified
the pendant carboxylic acid groups of PAA with
fluorescent chromophores and hydrophobic molecules
by extending the chemistry employed in the hyperbranched grafting on Au-coated silicon wafers
described above [241 –249]. Oxidized PE films with
carboxylic acid functional groups are obtained by
oxidation with CrO3 – H2SO4. Then amino-terminated
poly(tert-butyl acrylate) is attached to the oxidized PE
through amidation, so that subsequent hydrolysis of
the tert-butyl esters affords a PE film with one layer of
PAA grafts. Repetition of the process through 2– 4
more cycles can produce heavily grafted PE films.
The resulting hyperbranched grafts exhibit
higher efficiency as platforms for further ionic
modification of PE by comparison with the simple
oxidized PE [261].
Amidation or esterification of ultrathin hyperbranched PAA grafts with thiophenes having amino
or hydroxyl groups generated thiophene units-contained interfaces that could be applied as substrates for
an oxidative polymerization method with FeCl3 as an
oxidant [265]. A conjugated fluorescence 2,5-coupled
oligothiophene has been fabricated within the hyperbranched grafts, as demonstrated with its yellow –
green light emitted under UV irradiation. The
hyperbranched PAA grafts can also be used as
substrates for a mild hydrogen-bond-based grafting
method [266].
Nakada et al. [268] introduced amino groups to the
unsaturated pendant and bridge sites of a PE
substrate, followed by application of poly(amidoamine) dendrimer synthesis methodology (repeating
the steps of 2-methoxycarbonylethylation and 2aminoethylamidation), resulting in ultrathin hyperbranched poly(amidoamine) grafts. The amino
241
amplified PE films showed anionic exchange capacity
and could adsorb acid dye.
4.3.5. On the other surfaces
The techniques previously adopted with PE surface
can be extended to modify PP by hyperbranched
grafting with PAA grafts [269]. Hydrophilic and
hydrophobic surfaces were obtained by treatment of
the as-prepared PAA-grafted PP with alkali, and with
ethyl chloroformate followed by pentadecylfluorooctylamine. The adhesion of the surface increases from
2 J/m2 for unmodified PP to 29 J/m2 for the PP with
five stages of hyperbranched grafting. Epoxy-terminated hyperbranched polyesters (Boltorne-E2) were
grafted on the PP surface modified with maleic
anhydride (MAH) by melt blending in a twin-screw
extruder [270].
In addition, hyperbranched polyamidoamine was
grafted onto the surface of CP by repeating two
processes including Michael addition of MA to
surface amino groups and amidation of the resulting
esters with ethylenediamine [271]. C60-polystyrene
hyperbranched macromolecular thin films were also
reported [272].
4.4. Hypergrafting
Hypergrafting represents grafting hyperbranched
macromolecules to a multifunctional polymeric core,
and the resulting hybrid material is called a ‘hypergrafted polymer’ in this article. If the core used is a
linear polymer, a new sort of comb-like polymer
cylinder will be formed. As shown in Fig. 7, four
routes may be applied to obtain the hypergrafted
polymers: (1) graft-on-graft from a polymeric core
with functional side groups; (2) polymerization of
hyperbranched macromonomers; (3) polymerization
of AB2 or latent AB2 monomers initiated from
macromolecules; (4) direct grafting hyperbranched
molecules onto a polymer.
Using the graft-on-graft approach, Fréchet et al.
prepared hypergrafted polystyrene (PS) and its copolymers [107]. A linear backbone incorporating latent
ATRP initiating sites was first synthesized by nitroxide
mediated ‘living’ radical polymerization of PS and
chloromethylstyrene (CPS), followed by initiating
ATRP of CPS in the presence of Cu(I)Cl/2,20 bipyridine complex, giving rise to PS cylinders.
242
C. Gao, D. Yan / Prog. Polym. Sci. 29 (2004) 183–275
Fig. 7. Possible approaches to hypergrafted polymers.
The active side points of CPS units were used to further
initiate the atom transfer polymerization of other vinyl
monomers, affording hypergrafted polymers.
Poly(allylamine) (PAAM) grafted chain-pendant
hyperbranched PEI has been prepared by an in situ
reaction of PAAM with 2-chloroethylamine hydrochloride (CAH) in the presence of sodium hydroxide
[273]. The as-prepared polymers with abundant amino
groups can react with CAH to produce hypergrafted
polymers with multi-layers of PEI side chains. Potentiometric titration and ultraviolet – visible (UV –Vis)
spectroscopy showed that the hypergrafted polyamines
can act as multidentate ligands for ions and form stable
complexes with Cu2þ, with about three EI dents per ion.
Frey et al. [274] reported polyethyleneimines
with apolar hyperbranched carbosilane side chains,
prepared by ring-opening polymerization of oxazoline-based hyperbranched macromonomers earlier
synthesized. The hypergrafted polymers can form
superstructures in solution and in bulk.
Knauss et al. [275] obtained branched PS macromonomers by slow addition of a coupling agent
(4-(chlorodimethylsilyl)styrene or CPS) to living
polystyryllithium. Then, hypergrafted copolymers
were produced by copolymerization of the macromonomer with PS or MMA.
Although the literature published on hypergrafted
polymers is limited, this new class of polymers is
likely to receive more and more attention and be
widely explored and used in a variety of potential
application areas such as supramolecular chemistry,
biomaterials, nanotechnology and molecule-devices
in the near future.
5. Application of hyperbranched polymers
5.1. Conjugated functional materials: optical,
electronic and magnetic properties
Because of their good solubility and excellent
processibility, three-dimensional hyperbranched
conjugated polymers are attracting more and more
attention as novel optical, electronic and magnetic
materials. Linear conducting polymers are usually all
para linked, while the hyperbranched polymers are
almost always meta connected and have a far more
limited conjugation. Consequently, very different
properties are found for the hyperbranched polymers
than their linear counterparts.
5.1.1. Hyperbranched poly(phenylenevinylene)
Lin et al. [175] synthesized hyperbranched
poly(3,5-bisvinylic benzene)s (HP42 or HP43) with
a relatively low yield and low molecular weight via
Gilch or Wittig reactions (Scheme 54). The SEC
measurement showed that Mn of the product made
from route A was 1934 with a PDI of 1.13, and the Mn
value of the sample produced from route B was even
smaller. The resulting hyperbranched polymers are
soluble in common organic solvents such as THF,
chloroform, dichloromethane and ethyl acetate. The
optical properties of HP42 are listed in Table 7.
The absorption maxima ðlab Þ determined with UV –
Vis spectroscopy are 290 and 310 nm with a molar
absorption coefficient in the order of 104, and the
fluorescence emission maximum ðlem Þ in dichloride
methane solution monitored occur for 359 and 369 nm
C. Gao, D. Yan / Prog. Polym. Sci. 29 (2004) 183–275
243
Scheme 54.
with a fluorescence lifetime ðtÞ of 0.6– 8 ns and a
quantum yield ðFf Þ of 36% for the excitation
maximum ðlex Þ at 305 and 325 nm. The incorporation
of the m-phenylene group decrease the effective
conjugation length and increase the bandgap, so
both the absorption and emission bands are blue
shifted in comparison with the widely studied linear
poly(p-phenylenevinylene) (PPV). The solid film of
HP42 fabricated by spin coating technique exhibits an
emission maximum ðlem;s Þ at 446 nm as excited at its
solid absorption maximum ðlab;s Þ; about 80 nm redshifted with respect to the value in solution. Enhanced
inter-chain interaction in the solid film may cause the
dramatic difference.
Table 7
Photophysical results of some hyperbranched conjugated polymers
No.
HP42
HP44
HP45
HP46
HP47
HP48
HP49
HP50
HP51
HP52
160
161
HP53
HP54
HP55
HP56
HP57
HP58
HP59
HP62
HP63
a
PDI
lab (nm)
1930
2810
1.13
2.15
2170
2870
2840
40,180
2.54
4.1
4.3
1.76
289, 310
323, 405
315, 397
326, 406
330, 409
312, 396
317, 402
330, 405
332, 405
Mn
1077
2710
9080
4940
4770
4030
6240
28,000
2500
1.44
1.58
2.24
5.78
3.4
1.58
2.15
2.5
1.7
399
383
284
lex (nm)
lem (nm)
lab;s (nm)
lem;s (nm)
Ff (%)a
305, 325
329, 402
306, 390
319, 405
322, 409
310, 382
307, 389
388
332, 402
446
501, 529
500
508
505
492
470
510
510
36
26
47
100
62
69
90
24
49
420
350
350
285
359, 369
457
453, 480
453, 480
453, 480
462, 479
453, 480
461, 490
461, 490
500
473
499
450
309
343
334
351
264, 419
400
404
398
397
400
399
398
484
533
The fluorescence quantum efficiency was recorded with different standards.
0.1
49
9
31
20
10
244
C. Gao, D. Yan / Prog. Polym. Sci. 29 (2004) 183–275
Scheme 55.
Hyperbranched PPV derivatives with various of
2,5-aromatic substituted alkyl groups and terminal
groups were prepared by direct polycondensation of A2
monomer 31 (2,5-diR1O-1,4-bis(bromotriphenylphosphinomethyl)benzene) or 154 (2,5-dimethoxy-1,4bis(chlorotriphenylphosphinomethyl)benzene) with
B3 monomer 32 (1,3,5-benzenetricarbaldehyde) in
the presence of strong base through a Wittig reaction,
followed by end group modification of the hyperbranched polymers (Scheme 55). Their photophysical
properties were investigated in detail in solution and
solid films [276 –280]. As shown in Table 7, all the
alkyl-substituted products showed a red shift in both
the absorption and emission peaks. No significant
variations in absorption and emission maxima were
detected on increasing the length of the side alkyl
groups, while marked difference was observed in their
fluorescence quantum efficiency, with the sample with
C5-alkyl exhibiting the highest Ff : On the other hand,
the terminal groups of the resulting hyperbranched
polymers showed slight or moderate influence on their
absorption and emission maxima, but great influence
on the photoluminescence efficiency.
HP49, HP50 and HP51 were used as the active
layer in electroluminescent devices. The turn-on
voltages of the single- and double-layer light emitting
diodes (LEDs) made from HP51 are approximately
6 V, and that of light emitting electrochemical cell
with HP50 as active polymers is 3 V [280]. Significant
enhancement in the device photoluminescent performance was observed by dispersing HP49 in a linear
poly(9-vinylcarbazole) (PVK) matrix, owing to
energy transfer between PVK and HP49. For the
LED with PVK : HP49 ¼ 5 : 1 as the emitting layer,
bright blue emission up to 810 cd/cm2 was achieved at
a bias of 24 V, and the maximum transfer efficiency of
current density to brightness of the blend polymer
devices is as high as 0.27 cd/A, an order of magnitude
greater than that of HP49-based single layer devices
(0.025 cd/A) [278].
5.1.2. Hyperbranched poly(b,b-dibromo-4ethynylstyrene)
Fully conjugated hyperbranched polymers and
oligomers of b,b-dibromo-4-ethynylstyrene were
prepared via a Heck reaction (Schemes 56 and 57).
In the presence of triphenylphosphine (TPP), CuI and
bis(triphenylphosphine)palladium(II) dichloride,
polycondensation of 155 yielded HP52 with a Mw of
70,722 and a PDI of 1.76 [281]. HP52 showed an
emission maximum at 500 nm and a shoulder at
460 nm upon excitation at 420 nm. Similarly, blue
emission with a peak of 440 –500 nm was found for
the oligomers of 155 synthesized by stepwise growth
when excited at 350 nm. Interestingly, all of the
oligomers except 161 exhibited two peaks in their
corresponding excitation spectrum [282]. The broader
and lower peak close to the absorption peak can be
attributed to emission from S1 state, but the origin for
the sharper and higher one located very close to the
emission peak is not yet clear.
5.1.3. Hyperbranched poly(arylene/1,1-vinylene)s
Dihydridocarbonyltris(triphenylphosphine)ruthenium [RuH2CO(PPh3)3] catalyzed self-polycondensation of 4-((trimethylsilyl)ethynyl)acetophenone (162)
resulted in a hyperbranched polymer with regularly
alternating arylene and 1,1-vinylene units (HP53) by
C. Gao, D. Yan / Prog. Polym. Sci. 29 (2004) 183–275
245
Scheme 56.
catalytic addition of the ortho C – H bonds of
acetophenone group to the triple bond of the
trimethylsiylethynyl group (Scheme 58). The Mn
measured with SEC and 1H NMR for HP53 is 1077
and 2400, respectively. HP53 exhibited an absorption
peak at 284 nm and fluorescence emission maximum
at 450 nm with a Ff of 0.1% [98].
5.1.4. Hyperbranched polyarylenes
Hyperbranched polyphenylenes were first reported
as novel hyperbranched polymeric materials [46 – 49].
Suzuki polycondensation of AB2 monomer 3,5dibromophenylboronic acid or polymerization of
3,5-dihalophenyl Grignard reagents gave hyperbranched polyphenylenes (Scheme 59). The polymers
obtained from the Suzuki Pd(0) catalyzed aryl –aryl
coupling reaction had Mw s of 5000– 35,000 with PDIs
less than 1.5. The resulting hyperbranched polyphenylenes are thermally stable to 550 8C and soluble in
many organic solvents such as THF, tetrachloroethane
and o-dichlorobenzene although their backbones
consist only of rigid aromatic rings. Research on
Scheme 57.
246
C. Gao, D. Yan / Prog. Polym. Sci. 29 (2004) 183–275
Scheme 58.
the potential photophysical properties of the resulting
polymers was not reported.
Hyperbranched polyphenylenes with a higher density of benzene rings in one unit can be prepared by
Diels-Alder self-[2 þ 4]cycloaddition of AB2 monomers, ethynyl-, propynyl and phenylethynyl-substituted tetraphenylcyclopentadienones, in the presence
of tetrabutylammonium fluoride (Bu4NF) at 180 8C
(Scheme 60). The Mw s of the resulting hyperbranched
polymers determined by SEC with PS as standards
ranged from 3000 to 107,455 with PDIs of 1.66 – 6.85
[283]. It is notable that polymers with such a high, dense
packing of benzene rings are soluble in toluene or
benzene. Their potential application as the polycyclic
aromatic hydrocarbons has been proposed [284].
Tang et al. [285] successfully prepared soluble
hyperbranched polyarylenes containing various side
groups as shown in Scheme 61 by copolycyclotrimerization of compounds with double triple bonds and
monomers with a single triple bond. The polymerization can be carried out at lower temperature (65 8C)
if UV irradiation is employed to activate the Co
catalyst. The temperature for 5% weight loss of each
polyarylene is higher than 400 8C. The results and
photophysical data of the resulting hyperbranched
polyarylenes are summarized in Table 7. It is found
that the emission properties of the polymers can be
tuned by changing the comonomer pairs [286]. The
polymer made from 163a and 164b exhibits the
highest fluorescence quantum yield (49%) with 9,10diphenylanthracene as the standard. The emission
from the polymer with long side chains (HP56) is very
weak ðFf ¼ 9%Þ; possibly caused by self-quenching
due to absorption in the same spectral region. Because
of their excellent optical limiting properties, good
thermal stability and processing advantages, these
hyperbranched polyarylenes may be useful in the field
of functional materials and devices.
Scheme 59.
Scheme 60.
5.1.5. Hyperbranched non-linear optical polymers
Compounds possessing donor and acceptor
chromophores may exhibit non-linear optical
(NLO) properties. The NLO chromophores can be
C. Gao, D. Yan / Prog. Polym. Sci. 29 (2004) 183–275
247
Scheme 61.
aligned as shoulder-to-shoulder, head-to-tail, headto-head, and random. Hyperbranched polymer with
4-(2-cyano-2-methoxy-carbonylvinyl)aniline as a
second-order NLO chromophore was synthesized
by Zhang et al. [287] via a one-pot Knoevenagel
polycondensation of 4-formyl-N,N-di(2-hydroxyethyl)aniline (165) and cyanoacetic acid with
N,N-dicylohexylcarbodiimide (DCC) as a water
acceptor and 4-(dimethylamino)pyridine (DMAP)
as a base (Scheme 62). The resulting hyperbranched polymer was soluble in polar solvents
such as DMF and DMSO, with a Tg of 86 8C, a Td
of 330 8C in air and a hinh of 0.27 dl/g in DMSO at
30 8C. Each branched chain of HP60 looks like a
head-to-tail main chain backbone, so NLO properties are reasonably anticipated for HP60. The
second harmonic generation measurements indicated that the second harmonic coefficient ðd33 Þ for
HP60 was about 2.8 pm/V with a Y-cut quartz
crystal plate as a reference ðd11 ¼ 0:5 pm=VÞ:
Carbazole moieties were incorporated into the
backbones of hyperbranched polymer starting from
3,6-diformyl-9-(11-hydroxyundecyl)carbazole (166)
and cyanoacetic acid (Scheme 63) via a method
similar to that employed in the synthesis of HP60.
The d33 of the obtained hyperbranched polymer
HP61 was found to be higher than that of HP60,
reaching 7 pm/V. However, the d33 value is still
not large in comparison with linear main-chain
polymer containing the similar NLO moieties,
attributed to the decreased alignment of the NLO
moieties in the hyperbranched macromolecules
[288]. Light emitting devices consisting of poly(9tetradecanyl-3,6-dibutadiynyl-carbazole) as a hole
transport layer and HP61 as an electron transport
and emitting layer have been fabricated. Strong
Scheme 62.
248
C. Gao, D. Yan / Prog. Polym. Sci. 29 (2004) 183–275
Scheme 63.
emission with the maximum at 480 nm was
observed for the double-layered devices.
5.1.6. Hyperbranched coumarin-containing polymers
Coumarines (2H-1-benzopyran-2-ones) are widely
used as laser dyes and light emitters because of their
good photostability and high quantum yield of
photoluminescence (PL). Hyperbranched coumarincontaining polymer (HP62) and copolymers have been
prepared by polycondensation of (6-(3-carboxy)coumarinyl)diacetylhydroquinone (167) and by copolymerization of 167 with m-acetoxybenzoic acid in
different feed ratios. The polymerization was carried
out at 200 8C for 1 h under nitrogen flow, followed by
reaction at 330 8C for 1 h more under reduced pressure
(0.1 mm Hg), as shown in Scheme 64 [289]. Significantly, the measurements of 1H NMR suggested
that the resulting hyperbranched polymer and copolymers had DBs close to 100%, which might result from
the high temperatures of the reaction. Under these
extreme conditions, the high mobility of the macromolecular chains can overcome steric hindrances,
enhancing the reaction between acetoxyl and carboxylic acid groups. The coumarin-containing polymers showed Mn s ranging from 21,000 to 54,000 with
PDIs of 2.3– 2.7, and Tg s of 230 –160 8C. The Tg
values of the resulting polymers increased on increasing the feed ratio of monomer 167. Fluorescence
observations showed that all polymers were blue
emitters, in the range 484– 497 nm. Measurements of
current density – voltage relationships for the
single layer electroluminescence (EL) devices with
the coumarin-containing polymers as lightemitting material revealed that the higher the
content of coumarin units in polymer, the lower turn
on voltage.
5.1.7. Hyperbranched polythiophenes
As shown in Scheme 65, light-harvesting
hyperbranched polythiophene was prepared by
Scheme 64.
C. Gao, D. Yan / Prog. Polym. Sci. 29 (2004) 183–275
249
Scheme 65.
polycondensation of a Grignard reagent 169 with
Ni(bis(diphenylphosphino)propane)Cl2 as the catalyst. The unsymmetrical AB2 monomer 169 was
synthesized by treatment of 2,3-dibromothiophene
(168) with lithium diisopropylamide (LDA) followed by MgBr2 [290]. Determination of SEC with
PS as standard and THF as the eluent gave Mn and
PDI of the as-prepared hyperbranched polythiophene as 3400 and 1.3, respectively. It was found
that the solubility of bromo-terminated product was
low, and introduction of the alkyl groups onto the
periphery of the polymer improved its solubility. In
the structure of end-capped polymer HP63
(Mn ¼ 2500; PDI ¼ 1:7), only 2,3 or 2,5-linked
thiophene units are conjugated. So a gradient of
conjugation lengths exists in a single hyperbranched macromolecule, resulting in a lightharvesting effect, and thus enhanced light emission
or intramolecular energy transfer from the less
conjugated units to the longest conjugation fragment. Furthermore, the conjugation length increases
on increasing the molecular weight of the resulting
polymer. A broad absorption band in the range of
220 –600 nm appears in the UV spectrum of HP63,
consistent with the gradient conjugation structure of
the polymer. The emission maximum of HP63 is
533 nm for excitation at 404 nm. Compared with
their linear analogs, the hyperbranched polythiophenes have a much broader distribution of
conjugation lengths, which makes them potentially
better light-absorbing materials for efficient photovoltaic as well as light emitting devices.
In addition to the p –p and p –p conjugation
systems described above, a new structural class of
s– p conjugated hyperbranched poly(2,5-silylthiophenes) has been reported [291]. As outlined in
Scheme 66, an AB3 type monomer, 2-bromo-5trimethoxysiylthiophene (170), was firstly synthesized, followed by addition of 170 to magnesium
turnings in THF, resulting in rapid formation of the
Grignard reagent. In situ polymerization
of the Grignard reagent gave hyperbranched
poly(2,5-silylthiophenes) (HP64). Hyperbranched
polythiophenes with different terminal groups were
obtained by reaction of HP64 with the appropriate
nucleophilic reagents. The resulting hyperbranched
polymers have Mn s of 4460 – 6580 determined by
SEC and DB of 0.46 calculated from 1H NMR
spectrum. UV –Vis measurements indicated that a
high degree of conjugation through the silicon atoms
existed in the polymer backbones, demonstrated by
dramatic red-shift of the p – pp transitions in the
polymers (around 250 nm) in comparison to thiophene (230 nm). In addition, charge transfer from
Scheme 66.
250
C. Gao, D. Yan / Prog. Polym. Sci. 29 (2004) 183–275
thiophene to silicon was detected for the polymers,
according to their broad absorptions in the 300–
320 nm regions.
self-condensation of monomer 173 or 174 failed,
and polymerization of monomer 175 gave insoluble
product [294].
5.1.8. Hyperbranched polyanilines
Linear polyanilines (PAs) are well known and
have been widely studied as a conductive and
magnetic polymeric materials. However, the linear
polymers and their crosslinked products are hardly
soluble in solvent, leading to poor processibility.
Soluble PAs can be approached by the introduction
of dendritic units into the polymer backbones. To
date, soluble hyperbranched polyanilines and their
derivatives have been successfully prepared. The
AB2 monomers used to synthesize hyperbranched
PAs are shown in Scheme 67. The polymerization
of 3,5-dibromoaniline (171) was performed in THF
at 90 8C in the presence of 2,20 -bis(diphenylphosphino)-1,10 -binaphthyl (BINAP) ligand and 1.1
equivalents of sodium tert-butoxide (tert-BuONa)
per amine [292]. The Mw and PDI determined with
SEC in THF for the hyperbranched PA were 7000
and 3.2, respectively. The hyperbranched PA
showed magnetic properties similar or superior to
that of its linear analogs.
Hyperbranched poly(triphenylamine) was synthesized through aryl –aryl coupling of the AB2 type
Grignard reagent 172 [293]. The resulting hyperbranched polymer was soluble in common organic
solvents such as THF and chloroform, and exhibited a
new p – pp transition absorption band at 360 nm,
possibly resulting from the conjugation between
biphenylene units and nitrogen atoms.
Hyperbranched polymers with alternating phenylene sulfide and phenyleneamine units have
been designed, but attempts to obtain them by
5.2. Polymer electrolytes
Scheme 67.
A solid polymeric electrolyte should meet the
requirements of (1) being amorphous, (2) having a
high solvating power for appropriate ions, (3) good
ion transport, and (4) electrochemical stability. It is
well known that oligo(ethylene glycol) segments
satisfy the last three requirements, and hyperbranched
polymers are usually amorphous. Thus, hyperbranched macromolecules possessing ethylene glycol
(EG) chains have been designed, prepared and used as
novel polymeric electrolytes or ion-conducting elastomers. Table 8 lists the monomers used to synthesize
hyperbranched polymeric electrolytes and the properties of the resulting materials.
Hawker and coworkers [295] first prepared hyperbranched poly(ether ester)s containing linear EG units
of varying lengths in good yields with high molecular
weights ðMw ¼ 50; 000 – 81; 000Þ by polymerization
of AB2 monomers (176, 177, 178) with EG segments.
The properties of the hyperbranched polymer derived
from hexaethylene glycol ðn ¼ 5Þ-lithium perchlorate
complexes were examined as polymeric electrolytes.
It is found that the Tg values of the complexes obeyed
the Di Mario equation for general polymeric electrolytes [295]:
Tg 2 Tg0
ac
¼
1 2 ac
Tg0
ð4Þ
In Eq. (4), Tg is the glass transition temperature of
the polymer –lithium perchlorate complex, Tg0 is the
glass transition temperature of the salt-free polymer,
and c is the mole ratio of LiClO4 to repeat unit of
polymer, a is a constant for a specific polymeric
electrolyte. DSC measurements showed that a was
equal to 0.049 for the hyperbranched macromoleculeLiClO4 complex. The ionic conductivity (IC) of the
hyperbranched polymeric electrolytes increases with
both increasing temperature and concentration of Liþ,
with a value of 7 £ 1025 S cm21 being observed at
333 K for [Liþ] ¼ 0.62 mol per mole repeat EG unit.
Itoh et al. [296] synthesized the AB2 monomers
reported by Hawker via a different chemistry. Bulk
C. Gao, D. Yan / Prog. Polym. Sci. 29 (2004) 183–275
251
Table 8
The monomers and salts used in the preparation of ion-conducting materials.
Monomer
Salt
Tg (K)
IC £ 105 (S cm21)
Reference
LiClO4
255 ðn ¼ 1Þ; 268 ðn ¼ 2Þ;
234 ðn ¼ 5Þ
7.0 (333 K),
1.0 (303 K)
[295]
LiCF3SO3,
LiN(CF3SO2)2
276 ðn ¼ 1Þ;
259 ðn ¼ 2Þ
4.17 (353 K),
0.05 (303 K)
[296–301]
LiClO4
1.74 (333 K),
0.14 (298 K)
[302]
LiClO4
63.0 (333 K),
0.66 (293 K)
[303]
polymerization of 176 or 177 in the presence of a
catalytic amount of dibutyltin diacetate at 165 8C
under nitrogen gave the hyperbranched poly(ether
ester) with Mn of 8800 (made from 176) or 21,000
(made from 177). Then the hydroxyl-terminated
hyperbranched polymers were acetylated with acetyl
chloride in the presence of triethylamine in dichloromethane at ambient temperature, providing terminalacetylated hyperbranched poly(ethylene glycol)
derivatives containing diethylene and triethylene
glycols and 3,5-dioxybenzoate branching units with
Tg values of 276 and 259 K, respectively. Again, no
evidence of melting transitions was observed in the
DSC traces of the acetylated hyperbranched polymers. The IC of the polymeric electrolytes is not only
affected by the testing temperature, salt concentration
and the chain length of EG, but it is also affected by
the sort of salt. Under the same conditions, the
polymer with LiN(CF3SO2)2 salt exhibits lower Tg
and somewhat higher conductivity compared with that
using LiCF3SO3, attributed to the greater mobility of
the polymer chains with the LiN(CF3SO2)2 salt
because the large and flexible (CF3SO2)2N2 ion can
act as a plasticizer for the polymer matrix. For the
hyperbranched polymer with triethylene glycolsLiN(CF3SO2)2 system, the IC is 4.9 £ 1027 S cm21
at 30 8C and increases to 4.17 £ 1025 S cm21 at 80 8C
with [Liþ]/EG of 1.2.
The influences of the terminal groups, the length of
the EG unit, the composites of the linear polymeric
electrolytes blended, and the addition of filler on the
properties of the hyperbranched polymeric electrolytes were systematically investigated by Itoh and
coworkers [297 – 301]. By compared with hyperbranched poly[bis(diethylene glycol)benzoate]
capped with acetyl groups (HP65 in Scheme 68),
hyperbranched poly[bis(diethylene glycol)benzoate]
capped with 3,5-bis[(30 ,60 ,90 -trioxodecyl)oxy]benzoyl
groups (HP66) exhibited higher ionic conductivity
(7 £ 1024 S cm21 at 80 8C and 1 £ 1026 S cm21 at
30 8C) in the salt of LiN(CF3SO2)2 [297]. The
hyperbranched HP66 electrolyte showed an electrochemical stability window of 4.2 V at 70 8C and was
stable until 300 8C. Increasing the length of EG
252
C. Gao, D. Yan / Prog. Polym. Sci. 29 (2004) 183–275
Scheme 68.
segments in the terminal-acetylated macromolecular
unit to 6 ðn ¼ 5Þ; resulted in a hyperbranched polymer
electrolyte with LiN(CF3SO2)2 had a maximum
conductivity of 9 £ 1025 S cm21 at 80 8C with a
[Liþ]/[repeat unit] ratio of 0.6. Interestingly, addition
of hyperbranched HP65 to the system of linear PEO
and LiN(CF3SO2)2 salt can also improve the lithium
ionic conductivity and lithium/electrolyte interfacial
performance. The blend-based polymer electrolyte
with a HP65/PEO ratio of 20/80 in wt% and
[LiN(CF3SO2)2]0.125 showed a extremely high ionic
conductivity (8.1 £ 1024 S cm 21 at 80 8C and
3.8 £ 1025 S cm21 at 30 8C), with an electrochemical
stability of about 4.9 V [298,299].
Composite polymer electrolytes based on PEO,
HP65, BaTiO3 (as a ceramic filler), and LiN(CF3SO2)2
salt were fabricated, and investigated as the electrolyte
for all solid-state lithium polymer batteries. The
optimized composite polymer electrolyte, [(PEO20 wt% HP65)12(LiN(CF3SO2)2)] –10 wt% BaTiO3,
in which PEO with a Mn of 600,000, HP65 with a Mn of
15,000 and BaTiO3 with a particle size of 0.5 mm were
employed, showed ionic conductivities as high as
5.2 £ 1023 S cm21 at 80 8C and 2.6 £ 1024 S cm21 at
30 8C, exhibited an electrochemical stability window
of 4.0 V, and was stable to 312 8C in air [300]. In
addition, another kind of composite polymer electrolytes with HP65, an inset ceramic filler such as aLiAlO2 or g-LiAlO2, and LiN(CF3SO2)2 salt were
prepared via solvent casting method. It was observed
that both nanosized fillers improved the mechanical
properties, ionic conductivities and lithium ion transference numbers of the hyperbranched polymer
electrolytes. The a-LiAlO2 filler was more effective
for improving the electrochemical compatibility with
a lithium metal electrode, while the g-LiAlO2 filler
was more efficient in enhancing the mechanical
properties of the hyperbranched polymer-based electrolytes [301].
Hong et al. [302] synthesized hyperbranched
polyurethanes with EG units by polycondensation
of AB2 monomers 3,5-bis(20 -hydroxy ethyleneoxy)benzoyl azide (179) and 3,5-bis[(50 -hydroxy-30 oxopentyl)oxy]benzoyl azide (180), and prepared
composite polymer electrolytes using hyperbranched polyurethanes, linear polyurethanes and
LiClO4 as raw materials. The IC of the composite
electrolytes could approach 1.35 £ 1026 S cm21 at
25 8C and 1.74 £ 1025 S cm21 at 60 8C for a ratio
of [Liþ]/[EG] (c) equal to 1/4.
The hyperbranched poly(glycidol) (PG) made
from monomer 19 was also applied as a
substrate for the composite polymer electrolyte
with LiClO 4 salt [303]. The IC of the
composite reached 6.6 £ 1026 S cm21 at 20 8C
and 6.3 £ 1024 S cm21 at 60 8C.
Nishimoto et al. [304] prepared ethylene oxidebased network polymer electrolytes with hyperbranched ether side chains. The highest IC of the
electrolytes approached 1 £ 1024 S cm21 at 30 8C
and 1 £ 1023 S cm21 at 80 8C with lithium bis(trifluomethylsulfonyl)imide (LiTFSI) as the salt.
5.3. Nanomaterials and biomaterials
5.3.1. Nanomaterials
Hyperbranched polymers and their substitutes can
be used as nanomaterials for host –guest encapsulation
and the fabrication of organic – inorganic hybrids, and
even directly used as nanoreactors for some reactions.
Amphiphilic hyperbranched polyglycerols (PGs)
with a hydrophilic core and a hydrophobic shell were
prepared by partial esterification of the pre-synthesized
hyperbranched PG with a fatty acid such as palmitoyl
C. Gao, D. Yan / Prog. Polym. Sci. 29 (2004) 183–275
253
Scheme 69.
chloride in the presence of pyridine, as shown in
Scheme 69 [305]. Water-soluble dyes such as Congo
red, bromophenol blue and rose Bengal can be
encapsulated irreversibly into the amphiphilic core –
shell nanocapsules via phase transfer from the water
phase to the organic phase (CHCl 3). UV – Vis
measurements for the encapsulated nanoobjects
revealed that the dye was quantitatively extracted
from the aqueous layer into CHCl3 if the dye
concentration was lower than the saturation concentration. The average encapsulation maximum
depended mainly on two factors: the molecular weight
of the hyperbranched core and the length of the alkyl
chains attached to the polymer scaffold. Interestingly,
the degree of substitution of terminal groups had only a
small influence on the encapsulation maximum. By
variation of the factors mentioned above, the encapsulation maximum can be adjusted in the range of 0.7–
2.7 dye molecules per polymer molecule. Although the
encapsulation is irreversible, some chemical reactions
can still take place in the encapsulated core in the
organic phase. Comparison of these results with those
obtained from the linear analogs suggests that the
hyperbranched topology plays a crucial role in the
supramolecular encapsulation [306].
Encapsulation of sulfonated pincer-platinum(II)
complexes into amphiphilic nanocapsules was also
realized [307]. The incorporated Pt(II) complexes
showed catalytic activity in a double Michael addition
for homogeneous catalysis. Furthermore, PdCl2 or
Pd(OAc)2 were successfully encapsulated into nanocapsules based on hyperbranched PG [308]. Stable
solutions of Pd colloids were prepared through
reduction of Pd(II) in organic solution. The metal
particle size increased with increasing the molecular
weight of the hyperbranched polymer. The metal
colloids were catalytically active.
Compositions of hyperbranched PPVs containing
different side alkoxy groups with nanosized cadmium
sulfide (CdS) were prepared and investigated using
static and dynamic fluorescence spectroscopies and
AFM [309]. Electrostatic force plays a key role in the
behavior of these compositions, with the interaction
between PPV and CdS particles controlled by the
chain length of side alkoxy groups. The presence of
hyperbranched PPV can efficiently decrease the selfaggregation of nanosized CdS particles.
Hyperbranched polymer layered silicate nanocomposites were made using a hydroxy-terminated hyperbranched polyester (HP67 in Scheme 70), prepared by
condensation of 2,2-bis-hydroxymethyl propionic acid
(bis-MPA) with a tetrafunctional ethoxylated pentaerythritol core as the organic phase and sodium
montmorillonite (NaþMMT) as inorganic silicate
layers in deionized water [310]. WAXS measurements
showed that the silicate layer basal spacing could be
increased from 2.5 –2.8 nm to 3.6– 3.9 nm by increasing the molecular weight of the hyperbranched
polyester (1.06 nm for the as-received NaþMMT).
Significantly, the nanocomposites can be redispersed in
organic solvent and that the functional groups in
hyperbranched polymers remain active, as demonstrated by the reaction of the composites with methyl
diphenyl diisocyanate to produce a polyurethane network. The initial exfoliated state of the silicate layers
did not change on incorporation of the polyurethane.
Hedrick et al. [311] prepared inorganic –organic
hybrid materials from NMP solutions of
254
C. Gao, D. Yan / Prog. Polym. Sci. 29 (2004) 183–275
Scheme 70.
a precondensed poly(silsesquioxane) (181 in Scheme
70) and the relevant hyperbranched polymers (HP68
and HP69 in Scheme 70) containing 4% water and 2%
base. In the hybrids, the content of hyperbranched
macromolecules was maintained at about 20 wt%. The
thermal stabilities of the composites were comparable
to that of the pure inorganic material 181. On the other
hand, the nature of the terminal groups of the
hyperbranched polymers had a strong effect on the
phase separation and morphology of the hybrids. For
example, the size scale of phase separated domains was
less than 50 nm for trimethoxysilyl-terminated HP69,
while that for phenol-terminated polymer was in the
order of 1 –1.5 mm.
Mesoporous magnetoceramic materials were successfully prepared using a hyperbranched organometallic poly[1,1 0 -ferrocenylene(n-alkyl)silyne] as
precursors (such as HP70 in Scheme 70) [312,313].
It was found that (1) the ceramization yield
increased with decreasing alkyl chain length of
the hyperbranched polymer precursors; (2) the
hyperbranched polysilynes are superior to their linear
analogs with respect to the pyrolytic conversion to
inorganic networks and the retention of elemental iron
in the ceramic materials; (3) a three-dimensional
mesoporous network of nanoclusters can be fabricated
with the simultaneous evaporation of volatile organics
and agglomeration of inorganic elements in
the pyrolysis of HP70; (4) the resulting iron silicide
nanocrystals made from HP70 exhibit a high magnetizability and a negligibly small hysteresis loss.
Copolymerization of 1-caprolactone with bis(hydroxymethyl)-substituted 1-caprolactone monomer 21
in a feed ratio of 4/1 in the presence of Sn(Oct)2
gave a hyperbranched aliphatic copolyester with a
DB of 0.15 and a molecular weight of 8000 g/mol
ðPDI ¼ 2:81Þ: The hyperbranched polyesters were
observed to be superior materials for templating
nanostructures in organosilicates, and a porous
organosilicate film derived from a hybrid containing
30 wt% polyester with an average pore size of 200 Å
had been prepared [314]. As novel templating agents,
hyperbranched polyesters showed several merits, such
as higher solids content solutions, higher polymer
compositions in solution and better film quality than
obtained with linear and star-shaped polyesters.
Sol – gel processing of both linear and hyperbranched alkoxy-substituted polycarbosilanes of
the type ‘[Si(OR)2CH2]n (R ¼ Me or Et)’ led to
cross-linked polycarbosilane/siloxane hybrid polymers [315]. The molecular structure of these hybrid
gels and their pyrolysis to silicon oxycarbide ceramics
has been studied.
Hyperbranched polymers can also be utilized in
nanoimprint lithography [316]. A fascinating application of the nanoimprint lithography technique is to
C. Gao, D. Yan / Prog. Polym. Sci. 29 (2004) 183–275
fabricate so-called quantum magnetic disks, comprising single magnetic domain bits surrounded by a nonmagnetic material. Lebibet al. [316,317] employed the
commercially available hyperbranched poly(amide
ester) Hybrane, prepared by polycondensation of cyclic
anhydrides with diisopropanolamine as the top imaging resist to control the critical dimension in trilayer
nanoimprint lithography. Dot arrays of 100 nm with a
critical dimension control accuracy better than 10 nm
were fabricated. Magnetic Co dot arrays with different
diameters and thickness were made with the same
mold. Compared with the frequently used PMMA, the
hyperbranched polymer showed higher etch resistance.
Recently, the technique has been applied at room
temperature and low-pressure, using Hybrane HS2550,
a semi-crystalline hyperbranched resist polymer, and a
high-density pattern replication with graftings of
75 nm line and spacing has been realized [317].
Hyperbranched polyesters derived from bis-MPA
and 4,4-bis(hydroxyphenyl)valeric acid have been
255
used as the matrix polymers for molecular imprinting
to cope with the major drawbacks resulted from
the conventional crosslinked imprinted polymers
[318].
5.3.2. Biomaterials
Because low-cost and well-defined hyperbranched
polymers with multifunctional terminal groups and
narrow polydispersity are available by improving
synthesis methodology and techniques, hyperbranched
polymers are receiving increasing attention in biomaterials application [319]. In this field, hyperbranched
polymers can play two main roles: biocarriers and
biodegradable materials. As carriers, hyperbranched
macromolecules can offer their interior or peripheral
functional groups to covalently fix bioobjects, or
depending on their core –shell architecture, to sequester guest molecules. Some monomers used to prepare
hyperbranched polymeric biomaterials are summarized in Scheme 71.
Scheme 71.
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C. Gao, D. Yan / Prog. Polym. Sci. 29 (2004) 183–275
Hyperbranched aromatic polyamides as potential
supports for protein immobilization have been
prepared by self-condensation of AB2 monomers
182 and 183, or by polymerization of reactant pairs
(A2 þ B3, A2 þ B4 or A3 þ B3, with the corresponding monomers being 22, 24, 184, 185, 186, 187,
respectively) [320]. A rigid structure of the
resulting hyperbranched polyamides is required for
their use as enzyme-supporting materials, particularly
in biotechnological processes with high flow rates.
Using 1-ethyl-3-(30 -dimethylaminopropyl)carbodiimide hydrochloride as a coupling agent, a-amylase
was covalently attached to carboxylic acid-terminated
hyperbranched polymers. Evaluation of enzyme
activity revealed that the hyperbranched samples
with larger ‘holes’ in the structure or higher distance
between the trifunctional branching points (e.g. the
polymers made from 22 þ 186, or 184 þ 187) had a
greater affinity for the substrate and lower enzymatic
activity, while the samples with more compact
structure (e.g. the polymers made from 22 þ 24, or
22 þ 185) showed a weaker substrate affinity and
higher catalytic efficiency. Significantly, high bioactivity retention was found for all the samples analyzed
three and six months after the binding reaction.
Although a decrease of the enzymatic activity was
observed for both the immobilized and the free
enzyme at pH 4.7 and 9.0, the residual activity of
the former was considerably higher than that of the
latter at pH 9.0. Therefore, hyperbranched polyamides
are suitable potential supports for protein immobilization, and may open new perspectives for the
development of enzyme-based bioobjects with tuned
binding affinity, structural stability and catalytic
capability.
Steroidal ketones have been covalently linked to
the periphery hydroxyl groups on a hyperbranched PG
polymer support [319].
Park et al. [321] synthesized a hyperbranched
polymer having interior tertiary amino groups and
biodegradable units by bulk polymerization of
monomer 83 in the presence of core moiety 188.
The resulting hyperbranched poly(amine ester)s
showed Mn s of 9100 – 11,400 determined from
SEC with PS standard and DBs of 0.58 – 0.62
calculated from 13C NMR spectra. Then the methyl
ester-terminated product was modified with benzyl
N-(2-hydroxyethyl)carbamate, followed by catalytic
hydrogenolysis with H 2 over Pd/C, affording
primary amino-terminated hyperbranched polymer.
It was observed that the hyperbranched poly(amine
ester) with terminal amino groups is minimally
toxic and shows relatively high transfection
efficiency for DNA.
Oligosaccharides with branched architecture are
found at natural cell surfaces and in intercellular
systems. Chemical synthesis of hyperbranched aminopolysaccharides has been achieved by acid-catalyzed polycondensation of AB2 saccharide monomer
189, with 10-camphorsulfonic acid as a catalyst [322].
The as-prepared hyperbranched polysaccharide was
soluble in highly polar organic solvents such as DMF,
DMSO and pyridine, but insoluble in the organic
solvents of low polarity or water. The Mn of the
polymer was 6300 detected with SEC with PS as
standard and DMF as eluent. Interestingly, DB of the
resulting polymer calculated from 1H NMR was found
to approach 1. So almost perfect-branched hyperbranched polymer was obtained.
Biodegradable hyperbranched polyesters were
prepared through one-pot copolymerization of L lactide (190) with DL -mevalonolactone (191), using
tin 2-ethylhexanoate as a catalyst [323], or polycondensation of AB2 macromonomer 193 as well as
copolymerization of 193 with other macromonomers
based on monomer 190, 192 or 194 [324]. Polymerization of 193 in the presence of core moiety 195
gave biodegradable hyperbranched elastomers with a
dendrimer-like structure [324,325].
5.4. Supramolecular chemistry
Supramolecules represent compounds beyond
molecules, with an order organized by non-covalent
interactions. As novel building blocks, hyperbranched
macromolecules might play important roles in the
following supramolecular areas: (1) self-assembly
films and layers; (2) core – shell amphiphiles; (3) selfassociation objects; (4) macroscopic molecular
self-assembly; (5) liquid-crystalline materials; (6)
host –guest encapsulation.
An aldehyde-terminated hyperbranched PPV made
from monomers 31 and 32 was used as a selfassembling building material [326,327]. The structure
of the hyperbranched PPV (HP71) is shown in
Scheme 72. The dehydration between aldehyde
C. Gao, D. Yan / Prog. Polym. Sci. 29 (2004) 183–275
Scheme 72.
groups of HP71 and alcohol groups of a pretreated
quartz slide in the presence of DCC resulted in
covalently linked films on the substrate. Then ordered
homogeneous self-assembled films were obtained in
situ by immersing the modified substrate in a chloroform solution of HP71. The morphology and aggregated grain size of the self-assembled objects
examined by AFM depended on the immersion time
and the HP71 concentration. For a concentration of
3.3 £ 1025 M, the average grain size decreased from
143 to 50 nm when the immersion time was prolonged
from 1 day to 3 days. Investigations using AFM and
spectroscopy techniques showed that the self-assembly film linked through covalent bond was more
homogenous and stable than the spin-coated film.
Excimers of HP71 were observed for the selfassembled film, indicating strong interactions
between the excited state of the hyperbranched
macromolecules in the solid state.
Tang and coworkers [328 –330] prepared hyperbranched polyester with carboxylic end groups by
polycondensation of 5-hydroxyethoxyisophthalic acid
(196) with Zn(OAc)2 as a catalyst (Scheme 73). Selfassembled films were successfully formed via an
electrostatic layer-by-layer adsorption approach
[331], from both aqueous solution and water – THF
257
mixture, using poly(diallyldimethylammonium chloride) as a polycation and HP72 as a polyanion with a
freshly treated quartz slide as the substrate [328]. The
self-assembly procedure was monitored by UV –Vis
spectroscopy and contact angle measurements. A
linear relationship was observed between the absorbance at 215 nm and the number of bilayers,
suggesting the same thickness for each assembled
bilayer. After four dipping cycles, the contact angle of
the surface increased from around 10 –20 to 558,
indicating a dramatic change of the surface
hydrophilicity.
Pyrene-labeled hyperbranched amphiphiles with
hydrophilic core and hydrophobic shell were prepared
by reaction of hyperbranched poly(sulfone amine),
HP18, with 4-(1-pyrene)butyryl chloride (197), and
then with palmitoyl chloride in the presence of
triethylamine, as shown in Scheme 74 [332]. Fluorescence characterization exhibited that the core – shell
amphiphiles could self-associate to form micelles in
aqueous solution, with the excimer-to-monomer
intensity ratio ðIE =IM Þ increasing with increasing
attached palmitoyl units. The critical association
concentration (CAC) for the hyperbranched amphiphiles is 2-5 g/l at pH , 7, or 2 –3 g/l in a basic
solution ðpH ¼ 8:5Þ: Moreover, the hyperbranched
poly(sulfone amine)s without the hydrophobic shell
can associate in aqueous solution through hydrogen
bonds, as monitored with both fluorescence label and
probe techniques [333,334]. Significantly, the selfassociation ability of the hyperbranched polymers
depends on their DBs, with the tendency for
association in increasing with the DB [333]. This
phenomenon may be partially attributed to the
different architecture of hyperbranched poly(sulfone
amine)s with various DBs, and partially to different
inter- and intra-molecular hydrogen bonds in the
hyperbranched macromolecules.
Scheme 73.
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C. Gao, D. Yan / Prog. Polym. Sci. 29 (2004) 183–275
Scheme 74.
Pyrene-labeled hyperbranched poly(ethylene
imine) (PEI) containing different amounts of pyrene
per polymer was prepared by reaction of PEI with 1pyrenecarboxaldehyde [335]. The interaction of
the as-prepared pyrene-labeled PEI with the anionic
surfactant SDS in aqueous solution was also studied
by steady-state fluorescence [336]. The monomer
emission of pyrene increased with increasing SDS
concentration, indicating that the SDS molecules
protect pyrene fluorescence from quenching by
forming a polymer-bound micelles at the sites of the
pyrene groups.
An example of a non-covalent hyperbranched
polymer formed by self-assembly of AB2 monomer
via coordination interactions is given in Scheme 75,
2
where the counter anion X2 is BF2
4 , ClO4 , CF3SO3,
2
2
p-MeC6H4SO3 or BPh4 . Removal of acetonitrile
through repeated evaporation and redissolving the
residue in nitromethane leads to intermolecular
coordination between cyano groups and Pd, forming
hyperbranched coordination polymer HP73 [337].
Quasi-elastic light-scattering, AFM and transmission
electron microscopy revealed that the size of the
assembled hyperbranched spheres changed from 100
to 400 nm with variation of the building block
structure and/or the counter anions.
Both thermotropic liquid crystalline hyperbranched polyesters [338 – 341] or polyethers [52,
53] and lyotropic liquid crystalline hyperbranched
polyamides [79 – 82,159,342] made from AB n
monomers or specific monomer pairs have been
achieved, and their supramolecular behaviors have
been studied. Scheme 76 listed some AB2 monomers used to synthesize thermotropic liquid crystalline hyperbranched polymers.
Because of the three-dimensional character of
hyperbranched macromolecules, some guest molecules can be encapsulated into their interior
cavities. Therefore, hyperbranched polymers can
be applied in the supramolecular encapsulation
Scheme 75.
C. Gao, D. Yan / Prog. Polym. Sci. 29 (2004) 183–275
259
Scheme 76.
as host macromolecular boxes. The related contents
have been mentioned in the ‘nanomaterials’ section
of this article.
5.5. Patterning of hyperbranched polymer films
Patterning of polymer films at micron or even
submicron resolution is of critical technological
importance in the microelectronics industry [343].
Because hyperbranched polymers have many functional groups, moieties with interesting optical,
electrochemical, biological, and mechanical properties
can be incorporated into the hyperbranched polymer
films. Thus, patterning of hyperbranched polymer films
on the substrates is receiving more and more attention.
Related techniques including template-based approach
and photolithography have been developed by Crooks
and coworkers [343 –346]. The templates consist of
self-assembled monolayers fabricated by micro-contact printing (mCP) [347,348], while photolithographic
patterning relies on a photomask.
Two typical approaches to pattern hyperbranched
polymer films on a substrate are outlined in Fig. 8. The
mCP (approach A) consists of three key steps: (1)
applying CH3(CH2)15SH (C16SH) to the poly(dimethylsiloxane) (PDMS) stamp, followed by patterning of the Au substrate by manual application of the
stamp to the surface for a short time; (2) exposing
the C16SH-patterned substrate to a 1 mM ethanolic
solution of HOOC(CH2)15SH (MUA) for 1 min; (3)
activating the carboxylic acid groups of the MUApatterned portions by a reaction with a mixed anhydride
and then a,v-diamino-terminated poly(tert-butyl acrylate) (H2NR-PTBA-RNH2), subsequent hydrolyzing
of the grafted PTBA layer with MeSO3H to afford the
first layer of PAA (1-PAA), and yielding a hyperbranched PAA films after two more cycles of
activation, grafting and hydrolysis. As a consequence,
the resulting polymer pattern is a negative image of the
stamp. On an inorganic substrate such as Au, polymer
layers with a thickness of 25 nm and critical lateral
dimensions in the order of 1 mm can be patterned
through the mCP approach. The method has been
extended to pattern hyperbranched polymer films on
Fig. 8. Two approaches to patterning of hyperbranched polymer
films.
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C. Gao, D. Yan / Prog. Polym. Sci. 29 (2004) 183–275
organic substrate such as PE [345]. Significantly, the
fidelity of the underlying hyperbranched PAA patterns
is still maintained after grafting PEG on the PAA films
[347,348]. As pointed out by Crooks [343], however,
this approach has some disadvantages although it is a
fast, and flexible method for patterning polymer
surface. For example, the performance of multiple
patterning steps on a single substrate is very difficult,
and it is inconvenient to prepare a patterned surface
having two sorts of polymers with the mCP.
A method based on photolithography was developed to resolve the problems arising from the
template-based approach. For example, patterning of
multiple fluorescent dyes on hyperbranched PAA thin
films was achieved using photoacid-based lithography
(approach B) via three steps [349]: (1) covalent
grafting of hyperbranched PTBA thin films on a Au
substrate and then overcoating the polymer films with
a layer of photoacid, (2) utilization photolithography
to generate acid in the exposed region and catalyzed
hydrolysis of PTBA to form PAA, and (3) chemical
modification of the PAA regions with dyes.
The patterns fabricated can serve as templates to
segregate viable mammalian and bacterial cells.
Consequently, preparation of more complex structures by the patterning approaches is anticipated, such
as ensembles of organized tissues, including nerves
and blood vessels.
5.6. Coatings
Hyperbranched polymers have been used as the
base for various coating resins, such as powder
coatings [350,351], high solid coatings [352], flame
retardant coatings [353] and barrier coatings for
flexible packaging [354], depending on their high
solubility, low viscosity and abundant functional
groups.
The most widely studied hyperbranched polymers
in the field of coatings are the two products
commercially available at the time of this review:
Boltorne (hyperbranched aliphatic polyesters) [350,
352,354 –358] and Hybranee (hyperbranched polyester-amides) [351,359]. For the purpose of UV-curing,
the hyperbranched polymers are generally end-capped
with methacrylate or acrylate groups [352 – 358,360].
Resins based on hyperbranched polymers have lower
viscosities and higher curing rates than those of linear
unsaturated polymers.
5.7. Modifiers and additives
Based on their unique properties, hyperbranched
polymers can be applied as tougheners for thermosets [361 –369], curing, cross-linking or adhesive
agents [370 – 372], dye-receptive additives for
polyolefins [373,374], compatilizers [375], dispersers [376], processing aids, and rheology modifiers
or blend components [48,377 –383]. Table 9 lists
some hyperbranched polymers and their
applications.
The Boltorne three-generation hyperbranched
polyesters can act as outstanding tougheners in
epoxy matrix composites, and they can induce more
than a two-fold increase in the critical strain energy
release rate GIC for both low and highly cross-linked
matrices, without affecting either the viscosity of the
uncured resins or the thermomechanical properties of
the cured material. Until now, no other modifiers
could meet the challenges of property improvement
without loss of the general performance of the resin
system [361]. A quantitative thermodynamic theory
model for the prediction of phase equilibrium
behavior of thermoset-reactive modifier polymer
blends has also been suggested [366,384]. However,
the Boltorne hyperbranched polyesters cannot provide more efficient toughness for bismaleimide (BMI)
thermosets than a linear low molar mass thermoplastic
[363,364].
As early as 1992, Kim and Webster [48] had
pointed out that hyperbranched polymers could be
useful as polymer rheology control agents, and the
melt viscosity of PS blend decreased dramatically
after addition of a trace amount (about 5%) of bromoterminated hyperbranched polyphenylene to the PS
blending system, without affecting the thermal
stability of PS. Mulkern et al. [379] reported that
Boltorne-G4 is an excellent processing additive for
PS. The hyperbranched polyesters behave as lubricants during processing, and as self-compatibilizing
toughening agents in the final formation of the blend.
A considerable drop in the blend viscosity of PS or
styrene maleic anhydride (SMA) copolymer was
immediately observed in addition of the hyperbranched polymer modifier.
C. Gao, D. Yan / Prog. Polym. Sci. 29 (2004) 183–275
261
Table 9
The hyperbranched polymers used as modifiers and additives
Hyperbranched polymer
Role
Blended compound
References
Boltorne
Toughener
Toughener
Toughener
Modifier
Modifier
Processing aid
Compatilizer
Curing additive
Epoxy resins
Bismaleimide
Vinylester –urethane hybrid
PS/PS–SMA
Diglycidyl ether of bisphenol-A
LLDPE
PP/PA-6 blends
[361,367]
[363,364]
[368,369]
[379]
[381]
[377,378]
[375]
[370]
Processing aid
Dyeing additive
Dye carrier
PET
PP
PP/PE
[380]
[373]
[374]
Disperser
Carbon nanotube
[376]
Processing aid
PC
[382]
Processing aid
bis(maleimide)
[383]
Hybrane PS2550
In the tubular film blowing process of linear lowdensity polyethylene (LLDPE), Boltorne-H30 hyperbranched polyester successfully acted as a processing
aid, and sharkskin was eliminated with no significant
influence on other physical properties of the LLDPE
films [378]. In processing, the hyperbranched polymer
has a tendency to migrate to the surface, leading to a
hyperbranched polymer-rich surface, which creates
the opportunity to tailor the surface properties for
various potential applications.
PP is a versatile and widely used polymeric
material. However, PP does not accept dyes, owing
to its non-polar structure as well as its high crystallinity, resulting from the high stereo-regularity of the
PP. The dyeability of PP with CI. Disperse Blue 56
can be markedly enhanced through the incorporation
of hyperbranched macromolecules such as the
Hybrance PS2550 (hyperbranched polyester-amide)
into PP prior to fiber spinning [373]. The amphiphilic
hyperbranched aromatic polyester prepared by polycondensation of 3,5-dihydroxybenzoic acid followed
by end group modification with dodecanoyl chloride
can also be used as a dye carrier in blends of PP or
high-density polyethylene (HDPE), with similar
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C. Gao, D. Yan / Prog. Polym. Sci. 29 (2004) 183–275
dynamic– mechanical behavior of the blends compared to that of the pure polyolefins [374].
To understand the physical properties of hyperbranched macromolecular blends, the melt and
solution rheological behaviors of polymer blends
containing hyperbranched polymers with different
molecular weights or generations were investigated
[385 – 388]. Study of the rheological features of melt
processed hyperbranched polyesters (Boltorn Hx
series dendritic polymers) and their blends
indicated that the lower generation samples showed
shear-thinning behavior, while the higher generation
ones exhibited Newtonian behavior. Blends consisting of two hyperbranched polyesters showed
only Newtonian characteristics in both steady
shear and oscillatory shear conditions if at
least one of the components behaved as a Newtonian
fluid [385].
5.8. Other applications
Applications of hyperbranched polymers for gas
and solution separation as well as chemical sensors
have been suggested.
Hyperbranched polyimides were used as gas
separation materials [389]. The synthesis route and
chemical structure of the hyperbranched polyimides
used are shown in Scheme 4. By controlling the
monomer feed ratio and manner of the monomer
addition, both amine-terminated and dianhydrideterminated polyimides were obtained. Reaction of
the terminal groups with a difunctional crosslinking
agent afforded hyperbranched polyimide films. The
gas permeability coefficients of the as-fabricated films
were mainly affected by three factors: (1) the nature
of terminal groups of hyperbranched polymer; (2)
the crosslinking density of the film; (3) the ‘flexibility’
of crosslinking agent. The amino terminal
groups, lower crosslinking density and ‘rigid’ crosslinking agent (e.g. terephthaldehyde, TPA) were
favorable to improved gas permeability coefficients.
A better separation performance was obtained for the
TPA-crosslinked hyperbranched polyimide membranes than for their linear analogs or many other
linear polymeric membranes. For example, one
amine-terminated
hyperbranched
polyimide
membrane crosslinked with TPA exhibited fairly
high CO2/N2 separation performance and good
selectivity, i.e. PCO2 ¼ 65 barrier and PCO2 =PN2 ¼ 30
at 1 atm and 35 8C.
Hyperbranched PG, hyperbranched polyester Boltorne-H20, hyperbranched poly(ester amide)
Hybrane H1500, made from 1,2-cyclohexanedicarboxylic anhydride and diisopropanolamine, and
hyperbranched poly(ester amide) Hybrane S1200,
made from succinic anhydride and diisopropanolamine, are potentially useful in extractive distillation
and solvent extraction [390,391]. It was found that
hyperbranched polyglycerol and Hybrane S1200 had
remarkable separation efficiencies for the ethanol –
water azeotropic mixture.
Melt polycondensation of AB2 monomers 3,5trimethylsiloxyl benzoyl chloride, 3,5-diacetoxy benzoic acid, and 5-acetoxyphthalic acid resulted in three
hyperbranched polyesters with the same backbones
and different end groups (e.g. OH, COOH, and OAc).
The hyperbranched polyesters proved to be promising
materials for chemical sensors, able to calibrate and
discriminate a series of alcohols and different
halogenated hydrocarbons [392]. In addition, Bergbreiter and Crooks [393] reported a polymeric
‘molecular filter’ for chemical sensors, consisting of
hyperbranched PAA films and b-cyclodextrin
receptors.
6. Concluding remarks
In spite of the fact that highly branched topologies
and patterns have been widely observed in both
abiotic system and the biological world for tens of
billions years, intentionally synthesized hyperbranched polymers still represent a new kind of
material with promising, attractive properties and
potential applications. Until now, hyperbranched
macromolecules could be approached through SMM
and DMM. In SMM, one type of ABn or latent ABn
molecules is adopted as the raw monomers. As a
consequence, hyperbranched polymers are available
via polycondensation of ABn monomers, SCVP,
SCROP, or PTP. On the other hand, hyperbranched
macromolecules also can be prepared by polycondensation of two types of A2 and B3 monomers,
‘A2 þ B3’ methodology, or by polymerization of
unsymmetrical monomer pairs based on in situ
formation of ABn intermediates, CMM. Through
C. Gao, D. Yan / Prog. Polym. Sci. 29 (2004) 183–275
CMM, hyperbranched polymers with alternating
specific bonds in the frameworks can be conveniently
produced in a large scale, without gelation, opening an
avenue for the functionalization and application of
hyperbranched polymers.
A hyperbranched polymer can be modified and
functionalized from its core to periphery by end
capping, terminal grafting, surface growing, hypergrafting and hybrid blending, achieving tailor-made
properties and complex structures. Hyperbranched
polymers and their derivatives are potentially useful
in the areas of supramolecular chemistry, nanoscience
and technology, biomaterials, polymer electrolytes,
coatings, additives, optical and electronic materials,
and so forth. Because the properties of hyperbranched
polymers such as solubility, polarity, capacity,
crystallinity, chain entanglement, melt and solution
viscosity or rheology, thermal stability as well as
rigidity (glass transition temperature) can be tailored,
more and more fascinating materials and devices
based on hyperbranched polymers will be successfully developed and fabricated in the future.
However, one should recognize that extensive
research of hyperbranched polymers, particularly the
aspect of functionalization and application, is still in
its infancy, and the main object to apply hyperbranched polymers in industry fields is still a dream.
Thus, the opportunities for development space are
unlimited.
From our point of view, the following directions
deserve attention in the near future: (1) development
of new synthesis strategies and approaches based on
low cost for the whole system and good control of
the process and product; (2) preparation of novel
highly branched polymeric materials with hybrid
architectures via covalent linking or supramolecular
self-assembling (e.g. to obtain linear-hyperbranched,
hyperbranched – hyperbranched, hyperbranched –
polymer brush, or hyperbranched –dendritic hybrid
materials); (3) tailoring the properties of hyperbranched macromolecules by living polymerization,
inorganic – organic blending, molecular encapsulation, and other methods and techniques; (4) the
opening of new interdisciplinary application fields for
hyperbranched polymers, especially fields combining
hyperbranched polymers with nanotechnology,
micro-device technology, biology, supramolecular
chemistry, or single-molecule science.
263
Acknowledgements
This work was supported by the National Natural
Science Foundation of China (Nos 50233030,
20274024 and 20304007). The authors are much
indebted to Prof. Shoukuan Fu of Fudan University
(People’s Republic of China) for constructive discussions and suggestions and Prof. Dr E. Goethals of
Universiteit Gent (Belgium) for providing reference
38 and helpful discussions.
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