Prog. Polym. Sci. 26 (2001) 1233±1285
www.elsevier.com/locate/ppolysci
Hyperbranched polymers: a promising new class of materials
Mitsutoshi Jikei, Masa-aki Kakimoto*
Department of Organic and Polymeric Materials, Tokyo Institute of Technology Meguro-ku, Tokyo 152-8552, Japan
Received 16 June 2000; accepted 25 May 2001
Abstract
Hyperbranched polymers are highly branched macromolecules that are prepared through a one-step polymerization process. Many kinds of hyperbranched polymers have been investigated as novel dendritic macromolecules. The general concepts, syntheses and the properties of hyperbranched polymers are reviewed in this
article. The polymerization reactions are classi®ed into three categories: (1) step-growth polycondensation of ABx
monomers; and (2) self-condensing vinyl polymerization of AB p monomers; (3) multibranching ring-opening
polymerization of latent ABx monomers. Hyperbranched polymers are generally composed of dendritic, linear and
terminal units and a degree of branching (DB) helps to describe their structures. It has been shown that most of the
hyperbranched polymers possess some of the unique properties exhibit dendritic macromolecules, such as low
viscosity, good solubility, and multi-functionality. Owing to multi-functionality, physical properties such as
solubility in solvents and the glass transition temperature can be controlled by the chemical modi®cation of the
end functional groups (endcapping reactions). Applications of designed hyperbranched polymers to speci®c ®elds
are also described. q 2001 Elsevier Science Ltd. All rights reserved.
Keywords: Hyperbranched polymer; Dendrimer; Dendritic macromolecule; Degree of branching; Endcapping reaction
Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1234
1.1. Polymerization of ABx type monomers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1234
1.2. Degree of branching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1236
1.3. General properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1240
1.4. Classi®cation of the polymerization process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1241
2. Step-growth polymerization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1243
2.1. Polyphenylenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1243
2.2. Polyesters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1244
2.3. Polyethers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1251
2.4. Poly(ether ketone)s and poly(ether sulfone)s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1256
2.5. Polyamides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1258
* Corresponding author. Tel.: 181-3-5734-2433; fax: 181-3-5734-2875.
0079-6700/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved.
PII: S0 0 7 9 - 6 7 0 0 ( 0 1 ) 0 0 01 8 - 1
1234
M. Jikei, M-A. Kakimoto / Prog. Polym. Sci. 26 (2001) 1233±1285
2.6. Poly(ether imide)s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1262
2.7. Polyurethanes and polyureas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1264
2.8. Poly(siloxysilane)s and poly(carbosilane)s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1266
2.9. Miscellaneous reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1271
3. Self-condensing vinyl polymerization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1274
3.1. Polystyrenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1274
3.2. Poly(methacrylate)s and poly(acrylate)s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1277
4. Multibranching ring-opening polymerization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1278
4.1. Polyamines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1278
4.2. Polyethers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1279
4.3. Polyesters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1280
5. Concluding remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1282
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1282
1. Introduction
It is well known that the shape of organic molecules is one of the important factors which determines
their properties. During the last 15 years, scientists, especially polymer chemists, have introduced a new
philosophy of `dendritic macromolecules' and prepared globular and spherical molecules in addition to
the more conventional linear ones [1±16]. Dendritic macromolecules are classi®ed into dendrons,
dendrimers and hyperbranched polymers, which are composed of successive branching units. Dendrons
and dendrimers, having a well-controlled size and shape, are usually prepared by multi-step reactions
with tedious isolation and puri®cation procedures. On the other hand, hyperbranched polymers are
prepared by a one-step self-polymerization of ABx type multi-functional monomers. In 1952, Flory
demonstrated that the polymerization of ABx monomers does not exceed the critical gel point determined by statistical calculation [17]. In 1982, Kricheldorf reported the copolymerization of AB and AB2
monomers to form highly branched polyesters [18]. Hyperbranched polymers have lately attracted
considerable attention after Kim and Webster reported the homopolymerization of ABx type monomers
to form hyperbranched polyphenylenes [19]. Many kinds of hyperbranched polymers have been
prepared from ABx type monomers not only by step-growth polymerization but also by chain polymerization. Hyperbranched polymers are potentially promising materials due to their relative ease of
preperation compared to dendrimers. This review article deals with the syntheses, characterizations
and properties of hyperbranched polymers prepared through a one-step polymerization process.
1.1. Polymerization of ABx type monomers
Flory reported a statistical study for the polymerization and the in®nite network formation of multifunctional monomers [17,20,21]. Intramolecular condensations and reactivity changes during the
reaction were neglected for this consideration. The branching coef®cient, a , is de®ned as a probability
that the chain ends in a branching unit, Bf. When a… f 2 1† . 1; in®nite network formation becomes
possible, which is experimentally observed as gelation. That is, the critical branching coef®cient, a c, can
be described as Eq. (1).
ac ˆ 1=… f 2 1†
…1†
M. Jikei, M-A. Kakimoto / Prog. Polym. Sci. 26 (2001) 1233±1285
Nomenclature
List of abbreviations
a
branching coef®cient
ac
critical branching coef®cient
ATRP atom transfer radical polymerization
BINAP 2,2 0 -bis(diphenylphosphino)-1,1 0 -binaphthyl
DB
degree of branching
DBOP diphenyl (2,3-dihydro-2-thioxo-3-benzoxazolyl)phosphonate
DMAc N,N-dimethylacetamide
DMAP 4-(dimethylamino)pyridine
DMF N,N-dimethylformamide
DMSO dimethylsulfoxide
number average degree of polymerization
DPn
weight average degree of polymerization
DPw
DSC differential scanning calorimetry
GPC gel permeation chromatography
LALLS low-angle laser light scattering
MALDI-TOF matrix-assisted laser desorption/
ionization time-of-¯ight
1235
MALLS multi-angle laser light scattering
MBP multi-branching polymerization
MBROP multi-branching ring opening polymerization
number average molecular weight
Mn
weight average molecular weight
Mw
NMP N-methylpyrrolidone
NMR nuclear magnetic resonance
PD
polydispersity
SEC size exclusion chromatography
SCVP self-condensing vinyl polymerization
glass transition temperature
Tg
melting point
Tm
TBDMS tert-Butyldimethylsilyl
TBAC tetra-n-butylammonium chloride
TEMPO 2,2,6,6-tetramethylpiperidinyloxy
TGA thermal gravimetric analysis
THF tetrahydrofuran
TMS trimethylsilyl
VPO vapor pressure osmometry
In the case of the polymerization of bifunctional monomers (A2 and B2) and a trifunctional monomer
(B3), a can be related to the conversion of each functional group (pa and pb), as shown in Eq. (2).
a ˆ p2a r=‰r 2 p2a …1 2 r†Š ˆ rp2b r=‰1 2 rp2b …1 2 r†Š
…r ˆ ‰BŠ=‰AŠ; r ˆ ‰B3 Š=…‰B2 Š 1 ‰B3 Š†† …2†
Herein, r and r represent the ratio of functional groups ([B]/[A]) and the molar ratio of the B3 molecule
against the sum of B2 and B3 molecules, respectively. In the absence of the B2 molecule, r becomes 1 and
Eq. (2) is simpli®ed as Eq. (3).
a ˆ p2a =r ˆ rp2b
…3†
The relationship between pa and a in the case of r ˆ 1:0 and 1.5 is shown in Fig. 1. Since B3 is a
branching unit, a c is equal to 0.5 in this case. In both r cases, a exceeds 0.5 at high conversion of A
function as shown by the dashed lines in Fig. 1. That is, gelation occurs statistically in this range.
In the case of self-polymerization of ABx type monomers, a is related to the conversion pa and pb in
the following equation [17]:
a ˆ pb ˆ pa =… f 2 1† # 1=… f 2 1†
…4†
The maximum conversion of A function is equal to 1, and therefore, a never exceeds a c. In other words,
in®nite network formation which would be observed as gel formation is not allowed statistically in the
case of the self-polymerization of ABx monomers.
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Fig. 1. Relationship between pa and a for the polymerization of A2 and B3 monomers (‰BŠ=‰AŠ ˆ 1:0 (a); 1.5 (b); AB2 monomers
(c)).
The degrees of polymerization and polydispersities of the resulting polymers from ABx monomers are
compared with those for polymers from AB type monomers, as shown in Table 1 [17,22].
The polydispersity denoted by DPw/DPn for polymers from ABx monomers becomes much larger than
that for polymers from AB monomers. For instance, DPw/DPn is calculated to be 26 when p reaches 0.98
in the case of AB2 monomers. It should be pointed out that small molecules, such as a monomer and a
dimer, which are usually removed after the polymerization, are included for the calculation. As a matter
of fact, the polydispersities of hyperbranched polymers determined experimentally are often larger in
comparison with linear polymers, but smaller than the calculated ones. This is a major drawback in the
polymerization of ABx monomers and several attempts to narrower the polydispersity of the resulting
hyperbranched polymers have been reported. These usually involve the slow addition of monomers
during the polymerization [23±25] or polymerizations effected in the presence of core molecules
[24,26±29].
1.2. Degree of branching
A typical structure of a hyperbranched polymer prepared from an AB2 monomer is drawn in Fig. 2.
There are three types of repeating units classi®ed as dendritic, linear and terminal ones depending on the
number of unreacted B functional groups. Although dendrimers and dendrons are composed of dendritic
and terminal units, hyperbranched polymers contain linear units, which possess one unreacted B
function, in addition to dendritic and terminal units. In 1991, FreÂchet described the degree of branching
Table 1
Degree of polymerization and polydispersity of condensation polymers from AB and ABf21 type monomers
Monomer type
AB
ABf 21 a
AB2 a
DPn
DPw
DPw/DPn
1=…1 2 p†
…1 1 p†=…1 2 p†
11p
1=…1 2 p†
…1 2 p2 =…f 2 1††=…1 2 p†2
…1 2 p2 =…f 2 1††=…1 2 p†
1=…1 2 p†
……1 2 p2 †=2†=…1 2 p†2
……1 2 p2 †=2†=…1 2 p†
a
p: conversion of A function.
M. Jikei, M-A. Kakimoto / Prog. Polym. Sci. 26 (2001) 1233±1285
1237
Fig. 2. Schematic architecture of hyperbranched polymers from AB2 monomers.
(DB) as a factor to explain the structure of hyperbranched polymers, as described in Eq. (5) [30]:
Degree of branching …DB† ˆ
D1T
D1T 1L
…5†
Here D is the number of dendritic units, T the number of terminal units and L is the number of linear
units. DB can be used as an indicator in order to compare the structure of hyperbranched polymers with
the corresponding dendrimers and dendrons. Frey has reported on a modi®ed de®nition of DB that is
based on the growth directions, as shown in Eq. (6) [31]
DB ˆ
2D
D1T 2N
ˆ
2D 1 L
D1T 1L2N
…6†
Here N is the number of molecules. Eqs. (5) and (6) give almost the same DBs for hyperbranched polymers with high molecular weights since N in Eq. (6) can be negligible in such
cases. Frey has also pointed out that DB statistically approaches 0.5 in case of the polymerization
of AB2 monomers. Most of the hyperbranched polymers reported in the literature have actually
DBs close to 0.5. It should be noted that hyperbranched polymers possess many isomers even if the DB is
equal to 1, as shown in Fig. 3 [11]. If propagation reactions proceed symmetrically and DB of the
polymer is equal to 1, the architecture of the hyperbranched polymer is eventually the same as that of the
corresponding dendron.
NMR spectroscopy is a powerful tool to determine the DB of hyperbranched polymers. In addition to
1
H NMR, 13C, 15N, 19F and 29Si NMR spectroscopies have all been used to determine the DB for various
hyperbranched polymers. When the polymer is composed of degradable linkages such as esters and
carbonates, the DB can be calculated by the quantitative analyses for the products after degradation
[32,33].
As suggested by Frey, DBs of hyperbranched polymers from the one-pot polymerizations of AB2
monomers tend to be close to 0.5 [31]. There have been several attempts to increase DBs: (1) polymerization of dendrons having prefabricated dendritic units; (2) polymerization of ABx monomers in the
presence of core molecules …Bf †; (3) enhancement of the reactivity of linear units formed during the
polymerization [23].
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Fig. 3. Possible isomeric structures of hyperbranched polyphenylene …DB ˆ 1:0†:
First of all, dendrons composed of ABx type building blocks can be recognized as ABw monomers
containing prefabricated dendritic units. The polymerization of AB4 monomers shown in Fig. 4 gives
hyperbranched polymers with DB values higher than 0.5 when the DB was calculated based on the AB2
units. The DB of polymers from ABw monomers composed of ABx subunits can be calculated statistically by the following equation if intramolecular condensations are negligible and the reactivity of all B
functions is identical during polymerization [23].
DB ˆ
w21
w
x21
…7†
According to Eq. (7), a DB of 0.75 is expected for the polymerization of the AB4 monomer shown in Fig.
4. When the propagation reaction of the AB4 monomers gives only linear units, as shown in Fig. 5, the
DB for the polymer results in 0.67 which is a minimum DB for polymers from AB4 monomers. In 1996,
Hawker reported on the polymerization of AB2, AB3 and AB4 monomers composed of the same subunit
to form hyperbranched poly(ether ketone)s [34]. The DB of the polymer from the AB4 monomer was
increased to 0.71, while the polymerization of the AB2 monomer gave a DB of 0.49. Recently, we have
reported the synthesis of hyperbranched aromatic polyamides from AB2 and AB4 monomers [35]. The
DB was increased to 0.72 by using the AB4 monomer as a starting material although the polymerization
of the AB2 type monomer, 3,5-diaminobenzoic acid, gives a DB of 0.32 due to steric hindrance. It should
be noted that the increase in DB by this strategy is not directly related to the formation of globular
molecules and is mainly due to a decrease of linear units having one unreacted B function.
The second strategy to increase DB values involves the polymerization of ABx monomers in the
presence of core molecules, Bf : Frey has examined the DBs of hyperbranched polymers prepared by the
slow addition of an AB2 monomer to a core molecule Bf or a growing AB2 type hyperbranched polymer
[24]. When the conversion of the A function in the ABx monomer reaches 1, the DB of the resulting
Fig. 4. AB4 type monomer composed of three AB2 units.
M. Jikei, M-A. Kakimoto / Prog. Polym. Sci. 26 (2001) 1233±1285
1239
Fig. 5. Possible structure of the hyperbranched polymer prepared from the AB4 monomer shown in Fig. 4.
hyperbranched polymer increases to DBsa calculated by Eq. (8)
DBsa ˆ
x
2x 2 1
…8†
MuÈller has independently reported the following equation for DBsa of hyperbranched polymers
prepared from an AB2 monomer in the presence of a Bf core molecule [28]. Here i is the degree of
polymerization. A DB value of 0.67 can be expected from both Eqs. (8) and (9) for the polymerization of
AB2 type monomers with this strategy. It is interesting to note that DBsas calculated by Eq. (8) or (9) are
independent of the number of functional groups in a core molecule when the conversion of the monomer
is high.
DBsa ˆ
2…i 2 2f †
2
6 …i @ 1†
3…i 2 f †
3
…9†
The addition of core molecules affects not only the DB, but also the polydispersity of the resulting
hyperbranched polymers. As described in Table 1, the polydispersity of the polymers from the random
polycondensation of AB2 type monomers statistically becomes extremely large. The major reason for the
growth of polydispersity is that the larger propagating molecules have many B functions, and therefore,
grow faster than smaller ones and couple each other easily. The coupling of growing molecules can be
avoided by slow addition strategies. The polydispersity of the polymer prepared by the slow addition of
an AB2 monomer to a Bf core molecule can be estimated from Eq. (10) when the molecular weight of the
polymer is high [24,28,29].
DPw =DPn ˆ 1 1 1=f
…10†
Some experimental data for the polymerization of ABx monomers in the presence of core molecules
have already been reported. Hyperbranched aliphatic polyesters with DB more than 0.8 and polydispersities less than 2 have been prepared by the melt polymerization of an AB2 monomer and a B3
monomer, as shown in Eq. (11) [26,27].
…11†
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M. Jikei, M-A. Kakimoto / Prog. Polym. Sci. 26 (2001) 1233±1285
Moore has reported the preparation of hyperbranched poly(phenylacetylene)s onto a solid-supported
core molecule, as shown in Eq. (12) [36]. The polydispersity of the polymer formed on the support was
about 1.3 that is much lower than that of the polymer prepared by solution polymerization under
identical conditions. It should be noted that the yield of the solid-supported polymer decreased when
the relative amount of the AB2 monomer to the core molecule was increased.
…12†
When the reactivity of the B function is enhanced by the formation of linear units (kL . kT in Eq. (13)),
the amount of dendritic units would be increased during the polymerization of ABx monomers. This is
the third strategy to increase the DB of hyperbranched polymers. It is reported that DB values of 0.8 can
be expected for the polymerization of AB2 monomers when kL is ®ve times larger than kT [23].
…13†
Hyperbranched aliphatic polyamines with high DBs have been prepared by the multibranching ringopening polymerization of a cyclic carbamate initiated by primary amines [37,38]. It was assumed that
kL became larger than kT during the polymerization because the secondary amines in the linear units were
more reactive as a nucleophile than were primary amines in the terminal units.
1.3. General properties
It is well known that the solution viscosities of dendritic macromolecules are lower than those of
conventional linear polymers [39±41]. The low viscosity implies that dendritic macromolecules are less
entangled due to their spherical shapes. The relationship between intrinsic viscosity and molecular
weight is shown schematically in Fig. 6. Dendrimers display an unusual bell-shaped relationship that
results from their regular globular structures. The slope of the plot for hyperbranched polymers is
smaller than that for linear polymers although intrinsic viscosities increase with increasing the molecular
weight of hyperbranched polymers. Generally, a in the Mark±Houwink±Sakurada equation …h ˆ kM a †
lies between 0.5 and 1.0 for randomly coiled linear polymers. It has been reported that many kinds of
hyperbranched polymers possess a values less than 0.5, suggesting a spherical shape for the molecules
in solution. In GPC measurements, the retention volume for hyperbranched polymers tends to be larger
than that for linear polystyrene having the same molecular weight. This also suggests a more compact
form for hyperbranched polymers in solution in comparison with linear polymers.
Most of hyperbranched polymers reported in the literature are amorphous even if the linear
analogs are known as crystalline polymers. For example, hyperbranched poly(ether ketone)s [34] and
M. Jikei, M-A. Kakimoto / Prog. Polym. Sci. 26 (2001) 1233±1285
1241
Fig. 6. Schematic plots for the relationship between log M and log[h ] for macromolecules.
poly(phenylene sul®de) [42] are found to be amorphous. Some polymers containing mesogenic groups
show liquid crystalline phases [43±49].
Hyperbranched polymers prepared from ABx monomers contain many unreacted B functional groups
whose number is theoretically equal to that of the repeating units. Therefore, the nature of the end
functional groups (B) signi®cantly affects the properties of hyperbranched polymers, whereas the
in¯uence of end groups on the properties of linear polymers decreases with increasing the molecular
weight. Moreover, changing the B function can allow for the control of the properties of hyperbranched
polymers, such as the glass transition temperature and solubility in various solvents.
The introduction of dendritic structures signi®cantly improves the organosolubilities. It has been
reported that hyperbranched polyphenylenes [19,50] and hyperbranched aromatic polyamides [51,52]
are soluble in organic solvents even if the linear analogs are almost insoluble due to the rigidity of their
main chains.
Hyperbranched polymers are promising candidates for industrial applications in comparison with
dendrimers because one-step polymerizations are suitable for mass production. The encapsulation of
dye molecules has been investigated for some hyperbranched polymers [34,53] and blends of linear
polymers with hyperbranched polyesters have been reported [54,55]. Intentionally designed hyperbranched polymers have been prepared as novel functional polymers, such as crosslinking agents
[56,57], non-linear optical materials [58±60] and high-spin organic macromolecules [61]. It has been
reported that hyperbranched polymers often show similar properties, such as low viscosities, good
solubilities, thermal properties, and chemical reactivities, as dendrimers. Therefore, hyperbranched
polymers might be an alternative and cost-effective substitute dendrimers under certain conditions.
1.4. Classi®cation of the polymerization process
Many kinds of hyperbranched polymers have been prepared not only by step-growth, but also by chain
polymerizations. In this article, the polymerizations are classi®ed into three categories: (1) polycondensation of ABx monomers; (2) self-condensing vinyl polymerization (SCVP) of AB p monomers;
and (3) ring-opening polymerization of latent ABx monomers.
The propagation reactions applied for the self-polycondensation of AB type monomers are potentially
useful for the synthesis of hyperbranched polymers from ABx type monomers. Many kinds of hyperbranched
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M. Jikei, M-A. Kakimoto / Prog. Polym. Sci. 26 (2001) 1233±1285
polymers can be prepared by one-step self-polycondensation reactions of ABx type monomers, as shown
in Eq. (14).
…14†
AB2 type monomers are often used as starting materials because of the easy preparation of the
monomer, while AB3, AB4 and AB6 monomers have been reported as monomers for hyperbranched
polyesters [34] and polysiloxanes [62]. As discussed by Flory [17], it is impossible statistically to have
gelation caused by in®nite network formation in the self-polymerization of ABx type monomers.
However, gelation may occur experimentally when undesired side reactions are involved in the polymerization process. Side reactions may also make products insoluble in organic solvents. Gel formation
can also be observed when intermolecular interactions like hydrogen bonding of propagating molecules
is strong enough to form three-dimensional networks. Dilution of the monomer in solution polymerization might be effective to avoid gelation caused by intermolecular interactions. On the other hand, side
reactions in the early stage of the polymerization inhibit the formation of polymers having high molecular weights, especially when the A functional group in the ABx molecule is consumed by side reactions
such as ring formation to form cyclic oligomers. It has been reported that ABx monomers composed of
¯exible chain segments, such as long alkyl chains and siloxane chains, often form cyclic oligomers
during the self-polycondensation process [43,62±65].
The second strategy to prepare hyperbranched polymers is termed self-condensing vinyl polymerization (SCVP) de®ned by FreÂchet in 1995 [66]. The applicable monomer denoted by AB p contains an
initiating moiety in addition to a double bond. The initiating moiety is activated, and then, reacted with a
double bond to form a covalent bond and a new active site on the second carbon atom of the double bond,
as shown in Eq. (15).
…15†
The number of activation sites increases proportionally to the propagation reaction in SCVP whereas
two functional groups are always consumed during the self-polycondensation of ABx monomers. If the
new activation sites generated during the polymerization have different reactivities from the activation
site generated from the initiating moiety of the original monomer, the difference dramatically affects the
structure of the resulting polymer. The activated species could be a `living' free radical, an electrophilic
cationic moiety or a carbanion. SCVP of styrenes, methacrylates and acrylates through radical, cationic
or group transfer polymerizations have all been reported.
The third category is ring-opening polymerization of latent ABx type monomers, which was termed
reported `multi-branching polymerization (MBP)' by Suzuki in 1992 [37]. Branching units are generated
during the ring-opening reaction, while the starting AB monomers do not contain branching points. The
polymerization is initiated by the addition of proper initiators to generate active sites, which may allow
M. Jikei, M-A. Kakimoto / Prog. Polym. Sci. 26 (2001) 1233±1285
1243
for control over the molecular weight and molecular weight distribution of the resulting polymers.
Ring-opening polymerizations of cyclic carbamates, epoxides, oxetane, and lactones have all been
reported in literature.
2. Step-growth polymerization
Many kinds of hyperbranched polymers have been prepared by the one-step polycondensation of ABx
type monomers since Kim and Webster reported the synthesis of hyperbranched polyphenylenes from
AB2 monomers in 1990 [19]. This section deals with hyperbranched polymers prepared by step-growth
polymerization. Most of them were prepared from AB2 monomers and tri-substituted aromatic rings are
often used as a branching junction.
2.1. Polyphenylenes
Hyperbranched polyphenylenes can be prepared through aryl±aryl coupling reactions or Diels±Alder
reactions. 1,3,5-Tribromo and trichlorobenzene were used as starting materials for aryl coupling reactions and two reaction paths were employed, as shown in Eq. (16) [19,50].
…16†
A palladium catalyzed polycondensation gave a hyperbranched polyphenylene whose Mn and Mw/Mn
determined by GPC were 32,000 and 1.13, respectively. The polycondensation through the corresponding Grignard agent resulted in the formation of a polymer with a lower molecular weight. In the 1H NMR
spectra, peaks corresponding to aromatic protons became broad, which suggested that the polymer
contained many isomeric structures. Statistically, the ratio of C1 carbon connected with phenyl rings
and C4 carbon located between carbons connected with halogens, described in Fig. 7, becomes 0.25
when the degree of polymerization is larger than 24 and halogens are only located on peripheral phenyl
Fig. 7. Sub-unit structures of hyperbranched polyphenylene.
1244
M. Jikei, M-A. Kakimoto / Prog. Polym. Sci. 26 (2001) 1233±1285
rings. The branching factor based on the deviation from the ratio of C1 and C4 from 0.25 was 0.7 in the
case of the polymerization catalyzed by palladium, implied that the resulting polymer contained onesubstituted linear units, as shown in Fig. 7 [50].
The hyperbranched polyphenylenes were soluble in organic solvents, such as o-dichlorobenzene,
tetrachloroethane and tetrahydrofuran, although the main chain of the polymer was only composed
by rigid aromatic rings. Generally, the thermal properties and the solubilities of hyperbranched polymers
prepared from ABx type monomers are highly dependent on the terminal functional groups in addition to
backbone structure. Therefore, chemical modi®cation of the B function allows for some control over the
properties of hyperbranched polymers. In the case of hyperbranched polyphenylenes, various functional
groups could be introduced in the range 70±80 % by lithiation and subsequent reactions with electrophiles [50,67]. The modi®ed polymers were soluble in diethyl ether when non-polar functions such as
methyl and trimethylsilyl groups were introduced. On the other hand, the polymer became soluble in
water by the formation of micelles when carbon dioxide was used as an electrophile to convert the B
functional group into lithium carboxylate. The polymer modi®ed by non-polar functions also had lower
glass transition temperatures. Possible applications of the hyperbranched polyphenylenes as a core
molecule for the preparation of star polymers and as a melt viscosity modi®er for polystyrene were
proposed.
Hyperbranched polyphenylenes are also prepared by the Diels±Alder cycloaddition of the AB2 type
monomers that have one cyclopentadienone and two triple bonds as a diene and dienophiles, respectively [68]. Polymerization of the AB2 monomers shown in Eq. (17) was carried out in diphenyl ether at
1808C in the presence of tetrabutylammonium ¯uoride (Bu4NF). The weight average molecular weight
of the polymer determined by SEC measurements ranged from 3000 to 107,500 and an 18mer was
detected by MALDI-TOF mass measurements. The resulting hyperbranched polyphenylenes were soluble in toluene and benzene whereas the polymer consisted of extremely dense packing of benzene rings.
It has been reported that the dendritic polyphenylenes are promising candidates to form polycyclic
aromatic hydrocarbons by intramolecular dehydrogenation caused by the dense packing of benzene
rings [69].
…17†
2.2. Polyesters
Many kinds of hyperbranched polyesters have been prepared from 3,5-dihydroxybenzoic acid or 5hydroxyisophthalic acid as a raw material. Some AB2 type monomers are listed in Fig. 8. Hydroxy
groups are usually activated by acetylation or trimethylsilylation in order to use an `A' function. The
M. Jikei, M-A. Kakimoto / Prog. Polym. Sci. 26 (2001) 1233±1285
1245
Fig. 8. AB2 type monomers prepared from 3,5-dihydroxybenzoic acid or 5-hydroxyisophthalic acid.
function `B' represents acetylated, trimethylsilylated or non-activated carboxylic acid groups. Melt
polymerizations in the presence of proper catalysts are often used for the syntheses of all-aromatic
polyesters while solution polymerizations are also reported in some cases.
3,5-Bis(trimethylsiloxy)benzoyl chloride was ®rst used as an AB2 type monomer for the synthesis of
aromatic polyester copolymers by Kricheldorf in 1982 [18], and then, the successful homopolymerization of the AB2 monomer was reported by FreÂchet in 1991 [30], as shown in Eq. (18).
…18†
The bulk polymerization of the AB2 monomer was carried out in the absence or presence of catalysts,
such as trimethylamine hydrochloride and N,N-dimethylformamide (DMF). It was reported that the
purity of the monomer was a critical issue for the melt polymerization [70]. A small amount of impurity
gave an insoluble product even if the reaction temperature was lower than 2008C. Hydroxy-terminated
hyperbranched polyesters were isolated after pouring the solution of the crude product into methanol
to hydrolyze the trimethylsiloxy groups. The Mw of the hyperbranched polyester, determined by
1246
M. Jikei, M-A. Kakimoto / Prog. Polym. Sci. 26 (2001) 1233±1285
Fig. 9. Repeating units of the hyperbranched aromatic polyester.
GPC-MALLS, was about 80,000, which was dependent on the reaction conditions. The resulting
hyperbranched polyesters are composed of three kinds of repeating units: dendritic, linear and
terminal, as shown in Fig. 9. The ratio of each unit could be determined by 1H NMR measurements and the DB calculated by Eq. (5) was 0.55±0.60 [30]. The hydroxy-terminated polymers
were soluble in common organic solvents such as acetone, tetrahydrofuran and DMF, and were
thermally stable up to 4008C with a glass transition temperature (Tg) values of 1978C. Chemical
modi®cation of the hydroxyl groups affected the Tg values of the hyperbranched polyesters. For example,
the Tg dramatically decreased to 68C when the hydroxyl groups were protected by monobenzyl adipic
acid ester [70]
Acetylation of the OH groups instead of trimethylsilylation gives other kinds of AB2 monomers to
prepare hyperbranched polyesters (Eq. (19))
…19†
Higher reaction temperatures were required to achieve high molecular weights in comparison with the
polymerization of 3,5-bis(trimethylsiloxy)benzoyl chloride. The bulk polymerization of 3.5-diacetoxybenzoic acid at 2508C gave a soluble hyperbranched polyester with a Mw . 1,000,000 determined by
SEC [40]. The relationship between intrinsic viscosity and molecular weight of the resulting polyesters
suggested a highly branched structure and the a in Mark±Houwink equation was found to be less than
0.5. Trimethylsilyl 3,5-diacetoxybenzoate gave a soluble product via melt polymerization at 2808C
although insoluble or partially soluble products were isolated from the melt polymerization of 3,5diacetoxybenzoic acid above 2508C [71]. Several kinds of AB2 type monomers having a trimethylsilylated carboxylic acid as an A function and acetylated hydroxyl groups as B functions were reported for
the preparation of hyperbranched poly(ester-amide)s [72,73], poly(ester-imide)s [74], and their copolymers [47,75,76]. Triacetylated gallic acid, 3,4,5-triacetoxybenzoic acid, can be used as an AB3 type
monomer to prepare hyperbranched polyesters and their copolymers. Several copolyesters such as
liquid-crystalline polyesters [48], potentially biodegradable polyesters [77] and poly(ester-amide)s
[78] were prepared by the melt polycondensations of AB3 and AB type monomers. Hyperbranched
polyesters containing coumarin groups were prepared from an acetylated AB2 type monomer [79], as
shown in Eq. (20). The resulting polymers were soluble in chloroform and had Mn in the range
M. Jikei, M-A. Kakimoto / Prog. Polym. Sci. 26 (2001) 1233±1285
1247
2000±50,000. It was reported that the polyesters showed blue light emission in the range 450±492 when
excited at 400 nm.
…20†
5-Acetoxyisophthalic acid is an another type of AB2 monomer to prepare hyperbranched
aromatic polyesters [41]. The product prepared by the melt polymerization of 5-acetoxyisophthalic acid at 2508C was insoluble in organic solvents since intermolecular dehydration between the
carboxylic acid groups occurred during the melt polymerization. Hydrolysis of the crude product
gave a soluble hyperbranched polyester having carboxylic acid groups as an end B function, as
shown in Eq. (21). Since the carboxylic acid terminated hyperbranched polyesters were absorbed
to SEC columns, the carboxylic acid groups were protected with trimethylsilyl groups. The SEC
trace suggested a broad molecular weight distribution. The weight average molecular weight (Mw)
and polydispersity (Mw/Mn) calculated by universal calibration using coupled differential viscometry were 42,900 and 3.69, respectively. The DB determined by 1H NMR spectrum was about
0.5, which was similar to the case for the hyperbranched polyester prepared from 3,5-bis(trimethylsiloxy)benzoate. The carboxyl-terminated hyperbranched polyesters were easily converted to their
ammonium or sodium salts and the salts were soluble in water. The viscosities of the aqueous solutions were dependent on the concentration of the polymer and inorganic salts, similar to conventional
polyelectrolytes.
…21†
The insertion reaction of carbon monoxide catalyzed by palladium can be used for the propagation
reaction to form linear aromatic polyesters. We have recently reported the polycondensation of AB2 and
A2B type monomers via the CO insertion reaction to form hyperbranched aromatic polyesters [80]. 3,5Dibromophenol as an AB2 monomer gave insoluble products which might be due to crosslinking
reactions such as aryl±aryl coupling. On the other hand, soluble products with low molecular weights
were obtained by the polymerization of the A2B type monomers, 5-bromoresorcinol and 5-iodoresorcinol. We assume that side reactions such as aryl±aryl coupling and dehalogenation occurred at the B
function of the monomers, and therefore, insoluble products were isolated from the polymerization of
the AB2 monomer and the polymerization of the A2B monomers was terminated to form products with a
low molecular weight.
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Introduction of alkyl chains into the backbone of monomers causes a decrease in the glass transition
temperature of the resulting hyperbranched polymers, which enables melt polycondensations at lower
temperatures [41]. The ethoxy group was easily introduced by the reaction of dimethyl 5-hydroxyphthalate
and ethylene oxide. The melt polymerization of 5-(2-hydroxyethoxy)isophthalic acid at 1908C in the
presence of an organic tin catalyst proceeded ef®ciently to form a hyperbranched aromatic±aliphatic
polyester, as shown in Eq. (22). Intermolecular dehydration to form anhydrides was avoided since the
reaction temperature was lower than that required for the polymerization of 5-acetoxyisophthalic acid.
The Tg of the hyperbranched polyester was reduced to 1508C in comparison with all aromatic hyperbranched polyesters.
…22†
The melt polymerization of 5-(2-hydroxyethoxy)isophthalic acid dimethyl ester in the presence of a
core molecule, trimethyl 1,3,5-benzenetricarboxylate, was also reported as an approach for the controlled synthesis of hyperbranched polyesters [81].
Hawker reported on the polycondensation of 4,4-(4 0 -hydroxyphenyl)pentanoic acid esters and the
determination of the DB from degradation products [32], as shown in Eq. (23). The resulting polymers
had a DB of 0.49 which was independent of the ester groups in the starting AB2 monomers. 13C labeled
NMR experiments revealed that the extent of cyclization during the polycondensation was less
than 5% [82].
…23†
Aromatic±aliphatic polyesters composed of rigid segments, mesogens, and ¯exible segments
often show liquid crystalline phases. Hyperbranched polyesters containing a long alkyl chain
were prepared by polycondensation of AB2 monomers in the presence of thionyl chloride and
pyridine [46], as shown in Eq. (24). The polyesters with carboxylic acid end groups formed a
nematic phase above their glass transition temperature. It is interesting to note that mesophase
formation was not observed when the carboxylic acid groups were chemically modi®ed to methyl
esters. A cholesteric mesophase was also observed for a hyperbranched polyester containing a biphenyl
unit as a mesogen and an alkyl chain as a ¯exible spacer after the endcapping reaction with a chiral
functional group [45].
…24†
M. Jikei, M-A. Kakimoto / Prog. Polym. Sci. 26 (2001) 1233±1285
1249
Both thermal and solution polycondensation can be applied for the synthesis of hyperbranched
aliphatic polyesters. In 1995, Hult reported the melt polymerization of 2,2-bis(hydroxymethyl)propionic
acid as an AB2 monomer in the presence of a core molecule, 2-ethyl-2-(hydroxymethyl)-1,3-propanediol
[26], as shown in Eq. (25). The polymerization was carried out at 1408C in the presence of p-toluenesulfonic acid as a catalyst. The molecular weight of the resulting polymer depended on the molar ratio of
the AB2 monomer and the core molecule. The deviation from theoretical molecular weight became large
over 10 4. High DB values in the range 0.83±0.96 were achieved for the polymerization in the presence
of the core molecule. The hyperbranched polyesters had Tgs near 408C and good thermal stabilities up to
ca. 3008C.
…25†
Hyperbranched poly(ester amide)s were prepared through esteri®cation reactions with diisopropanolamine and carboxylic anhydrides as raw materials [83]. The poly(ester amide)s are commercially available as tunable and multifunctional materials (Eq. (26)). The ®rst reaction step is
the addition of anhydride to an excess of diisopropanolamine through slow addition technique to
prepare multifunctional monomer units. The functional groups in the intermediates are mainly
hydroxyls and acids, but also amines through a dynamic equilibrium at room temperature. The
subsequent polycondensation proceeds through esteri®cation via oxazolinium ions to form hyperbranched poly(ester amide)s. The poly(ester amide)s can have glass transition temperatures in the
range 10±1008C by changing the ratio of monomers and the type of anhydride used, and can be
easily functionalized with various acids such as unsaturated fatty acids, maleates, fumarates and
methacrylates.
…26†
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M. Jikei, M-A. Kakimoto / Prog. Polym. Sci. 26 (2001) 1233±1285
Combburst-like hyperbranched polymers can be prepared from AB2 type macromonomers
which are prepared by the reaction of oligomeric linear chains and a branching unit such as
3,5-dihydroxybenzoic acid and 2,2 0 -bis(hydroxymethyl)propionic acid. The properties of the
resulting polymers are usually dependent on the linear chains. In 1995, Hawker ®rst reported
the polymerization of AB2 macromonomers to produce hyperbranched poly(ethylene glycol)s
through a transesteri®cation reaction [84], as shown in Eq. (27). The crystallization of ethylene
oxide chains was inhibited by the introduction of the branching unit and the Tg of the polymers
depended on the number of each ethylene oxide unit. The complex formation of a lithium cation
with the ethylene oxide chains of the polymer …n ˆ 5† was con®rmed by the relationship between
Tg21 and the concentration of LiClO4. An ionic conductivity of 10 25 S cm 21 was obtained at 308C for a
concentration of lithium perchlorate of 0.62 molecule per repeat unit. The conductivity approached
10 24 S cm 21 at temperatures above 608C.
…27†
Hedrick reported larger AB2 type macromolecules using of oligo(e-caprolactone)s and 2,2 0 bis(hydroxymethyl)propionic acid as a branching point [85,86], as shown in Fig. 10. Monodispersed oligo(e-caprolactone) chains were prepared by `living' ring-opening polymerization of
e-caprolactone initiated by aluminum benzyloxide or tin octate. The living polymerization system
was terminated with benzylidene-protected 2,2-bis(hydroxymethyl)propionic acid and subsequent
deprotection gave the AB2 macromonomers. The esteri®cation of the AB2 macromonomer was
carried out in the presence of dicyclohexylcarbodiimide and 4-(dimethylamino)pyridinium 4toluenesulfonate as condensation agents. The DB in the hyperbranched polyesters was found to be
0:37 ^ 0:03 and to be independent of the molecular weight of the macromonomer. On the other
hand, the thermal properties of the resulting hyperbranched poly(e-caprolactone)s depended on the
degree of polymerization of the macromonomer.
Multibranching ring-opening polymerization of functionalized lactones also gives hyperbranched
aliphatic polyesters, which will be discussed in Section 4.3.
Hyperbranched aromatic polycarbonates were prepared by the polycondensation of the AB2
type monomer derived from 1,1,1-tris(4 0 -hydroxyphenyl)ethane [33], as shown in Eq. (28). The
polymerization was carried out at 708C in the presence of AgF to precipitate a leaving imidazolide anion as a salt. After hydrolysis of carbonylimidazolide chain ends, phenol-terminated
Fig. 10. AB2 type macromonomers derived from poly(e-caprolactone).
M. Jikei, M-A. Kakimoto / Prog. Polym. Sci. 26 (2001) 1233±1285
1251
hyperbranched polycarbonates were isolated. The hyperbranched polycarbonate was soluble in
common organic solvents such as THF and acetone, and had a DB of 0.53 that was determined by
quantitative analysis of the degradation products of the carbonate bonds by reaction with lithium
aluminum hydride. The hydroxy-terminated polycarbonate was thermally stable up to 3508C and had
a Tg of 2058C.
…28†
2.3. Polyethers
Hyperbranched aromatic polyethers can be prepared through nucleophilic aromatic substitution
reactions of ABx type monomers. Miller and Neenan ®rst reported the syntheses of hyperbranched poly(arylether)s by the self-polycondensation of phenolate monomers in 1993 [87], as
shown in Eq. (29). The resulting polymers are recognized as dendritic analogs of engineering
plastics such as poly(ether ketone) and poly(ether sulfone). Weight average molecular weights of
the resulting polymers determined by SEC versus polystyrene standards were in the range
11,300±134,000, which were dependent on the structures of the spacer groups. The resulting
polymers were soluble in common organic solvents such as chloroform and THF although the
polymers were composed of aromatic rings and short spacer units. Thermal gravimetric analyses
under nitrogen revealed that the hyperbranched polyethers retained over 95 % of their mass up to
5008C and had high thermal stabilities, similar to linear poly(ether ketone)s and poly(ether
sulfone)s. Glass transition temperatures were observed in the range 135±2318C but melting points
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M. Jikei, M-A. Kakimoto / Prog. Polym. Sci. 26 (2001) 1233±1285
were not detected:
…29†
In addition to common electron-withdrawing groups such as sulfone and ketone, certain heterocycles can
activate aryl halides toward aromatic nucleophilic substitution. Hedrick used AB2 type quinoxaline monomers to form poly(aryl ether phenylquinoxaline)s in 1996 [88], as shown in Eq. (30). The monomers were
prepared by the condensation of a substituted benzil and a ¯uoro-substituted o-phenylenediamine. The
polymerization was carried out at 180±2208C in the presence of potassium carbonate in aprotic polar
solvents. The intrinsic viscosity was about 0.5 dl/g for both resulting polymers and the glass transition
temperatures were 255 and 1908C for the polymer prepared from monomer 1 and 2, respectively. Intramolecular ring formation occurred during the polymerization, which was evidenced by the loss of the focal point
group, HF, by MALDI-TOF mass spectoroscopy. It was also reported that organic±inorganic hybrid ®lms
were prepared from the poly(aryl ether phenyl quinoxaline) and poly(silsesquioxane) [89]. The morphology
of the ®lm was in¯uenced dramatically by the nature of the end functional groups of the poly(quinoxaline).
The size scale of phase separation for a ®lm using triethoxysilyl-terminated poly(quinoxaline) was less
than 50 nm and the ®lm was optically transparent. In contrast, spherical hyperbranched `rich' regions on
the order of 1±1.5 mm were observed when hydroxy-terminated poly(quinoxaline) was used.
…30†
M. Jikei, M-A. Kakimoto / Prog. Polym. Sci. 26 (2001) 1233±1285
1253
Aromatic±aliphatic hyperbranched polyethers whose main chains were composed of benzyl ether
linkages were prepared as analogs for dendrimers having the same repeating units [90], as shown in
Eq. (31). The polymerization was carried out in the presence of K2CO3 and crown ether (18-crown-6) in
acetone. It was found that the resulting polymers contained C-alkylated methylene [91] up to 30% of the
polyether structure in addition to O-alkylated benzylic methylene. The Mw of the hyperbranched polyethers, determined by SEC-LALLS, exceeded 10 5, which was 3±5 times higher than the value calibrated
by polystyrene standards. The glass transition temperatures could be controlled from 311 to 3438C by
chemical modi®cation of the phenolic chain ends.
…31†
Hyperbranched polymers containing tetra¯uorinated phenylene units were prepared from the AB2 and
AB4 monomers where A and B denote benzylic hydroxyl group and penta¯uorobenzene, respectively
[92]. Eq. (32) shows the self-polycondensation of the AB2 monomer promoted by the deprotonation of
benzylic alcohol by sodium particles in THF. The polymerization in the presence of sodium particles less
than 1 mm in size resulted in the formation of the polymer with a low molecular weight in high yield. In
contrast, the use of sodium particles larger than 1 mm led to low yields of high-molecular-weight
polymer. Cyclized components were detected by MALDI-TOF mass spectrometry [65]. The resulting
polymers were soluble in common organic solvents, such as THF, DMF, methylene chloride, chloroform, toluene, and acetone, which were independent on the molecular weight of the polymer. The
properties of the resulting polymer could be changed by chemical modi®cation of the penta¯uorobenzene ring via nucleophilic displacement. Contact angle measurements of water and hexadecane on ®lms
of a penta¯uoro-terminated hyperbranched polymer, tri¯uoroethoxy-substituted and 1H,1H,2H,2Hper¯uorodecanoxy-substituted derivatives indicated a high degree of hydrophobicity and lipophobicity.
The 1H,1H,2H,2H-per¯uorodecanoxy-substituted polymer led to a more than two-fold decrease in the
coef®cient of friction and adhesive force [92].
…32†
AB2 type monomers containing two phenolic hydroxyl groups and an alkyl halide were also useful as
the monomers for aromatic±aliphatic hyperbranched polyethers. Fig. 11 shows the AB2 monomers
which give liquid crystalline hyperbranched polyethers [43,44]. The polymerization was carried out
in an o-dichlorobenzene and NaOH aqueous solution (10N) in the presence of tetrabutylammonium
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Fig. 11. AB2 type monomers for liquid crystalline hyperbranched polyethers.
Fig. 12. Schematic representation of the transformation between nematic and isotropic states.
M. Jikei, M-A. Kakimoto / Prog. Polym. Sci. 26 (2001) 1233±1285
1255
hydrogen sulfate. The resulting polymers were isolated after endcapping reactions of phenolic terminal
groups with alkyl halides at the end of the polymerization, which gave soluble hyperbranched polyethers. The molecular weights of the resulting hyperbranched polymers were relatively low when
compared with linear polymers prepared from the corresponding AA and BB monomers since intramolecular cyclization occurred during the polymerization. Some of the hyperbranched polyethers exhibited
an enantiotropic nematic mesophase derived from the anti±gauche isomerization, as shown in Fig. 12.
The temperature range for the nematic mesophase was dependent on the structure of the mesogen and the
end functional groups. A stable nematic phase in the range 50±1328C was observed for the polymer
composed of terphenylene as a mesogen unit and octyl ether as an end group [44]. It was also reported
that the monodendrons, composed of the same repeating unit and prepared by stepwise synthesis, and the
polymers having the monodendrons as a side chain formed a spherical or a cylindrical architecture by
self-assembly [93,94].
Recently, FreÂchet has reported a new approach to prepare hyperbranched poly(hydroxyether)s from
the AB2 monomer composed of two epoxides and one phenolic hydroxyl group, and termed `protontransfer polymerization' [95]. As described in Fig. 13, the polymerization is initiated by the addition of a
base to abstract the phenolic hydrogen and then, nucleophilic addition of the phenolate anion to an
epoxide ring proceeds to generate a dimer containing a secondary alkoxide. The activated dimer does not
propagate directly but instead undergoes a proton exchange reaction with another AB2 monomer to
produce the phenolate anion and a neutral dimer. The much lower pKa (ca. 10) of the phenolic group
relative to that of the secondary alkoxide (pKa , 17) enables the fast proton transfer from the phenolic
proton to the secondary alkoxide. The colorless and soluble polymers were isolated from the polymerization in THF at 70±758C in the presence of KOH as a base. The molecular weight increased with the
Fig. 13. Proton-transfer polymerization of the AB2 monomer.
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M. Jikei, M-A. Kakimoto / Prog. Polym. Sci. 26 (2001) 1233±1285
reaction time up to 206,000 (Mw by GPC-MALLS) and the portionwise addition of the monomer to the
polymerization mixture was effective to reduce the polydispersity to 2.4.
Hyperbranched aliphatic polyethers can be also prepared by multi-branching ring opening polymerization of epoxides or an oxetane as latent ABx monomers, which will be described in Section 4.2.
We have reported the synthesis of a hyperbranched poly(phenylene sul®de) by the self-polycondensation of an AB2 type monomer, methyl 3,5-bis(phenylthio)phenyl sulfoxide. Poly(arylsulfonium cation)
as a precursor was prepared through electrophilic substitution in tri¯uoromethanesulfonic acid, and then,
hyperbranched poly(phenylene sul®de) was isolated by re¯uxing in pyridine [42,96], as described in
Eq. (33). The hyperbranched poly(phenylene sul®de) was amorphous and soluble in organic solvents
like chloroform although linear poly(phenylene sul®de) is known as a crystalline polymer and is
essentially insoluble in most organic solvents at room temperature
…33†
2.4. Poly(ether ketone)s and poly(ether sulfone)s
Miller and Neenan ®rst synthesized hyperbranched poly(ether ketone)s as a kind of hyperbranched
polyether in 1993 [87]. Hawker reported a systematical study for the synthesis of hyperbranched
poly(ether ketone)s by nucleophilic aromatic substitution of ABx type monomers [34]. The monomers
employed are listed in Fig. 14. The self-polymerization was carried out in N-methylpyrrolidone in the
presence of potassium carbonate with the azeotropic distillation of water and toluene. Soluble hyperbranched poly(ether ketone)s were isolated in good yields with the Mn values in the range 20,000±
55,000. The DB for the polymer prepared from the A2B monomer was 0.15 whereas the DB for the
polymer prepared from the AB2 monomer was 0.49. The solubility of the hyperbranched poly(ether
ketone)s was highly dependent on the terminal unit derived from the B functional groups. The
Fig. 14. Multi-functional monomers for hyperbranched poly(etherketone)s.
M. Jikei, M-A. Kakimoto / Prog. Polym. Sci. 26 (2001) 1233±1285
1257
Fig. 15. Hyperbranched poly(etherketone)s endcapped with 1,4-phenylene units.
hydroxy-terminated hyperbranched poly(ether ketone) was soluble in aqueous alkaline solutions. AB3
and AB4 monomers having prefabricated dendritic units gave polymers with a higher DB. It was reported
that the thermal properties of the hyperbranched poly(ether ketone)s were independent on the macromolecular architecture, i.e. the DB, but dependent on the nature of the chain end functional groups. Glass
transition temperature ranged over 97±2908C. The hydroxy- and ¯uoro-terminated hyperbranched
poly(ether ketone)s were stable up to 4808C in nitrogen with the temperature for 10 wt% loss occurring
over 5008C. All of the hyperbranched poly(ether ketone)s were completely amorphous although their
linear analogs have a high degree of crystallinity.
Hyperbranched poly(ether ketone)s with various numbers of 1,4-phenylene units were prepared by the
same reaction strategy [97], as shown in Eq. (34). The resulting polymers were soluble in organic
solvents in the case of n ˆ 1 and 2 whereas the polymers were precipitated in N,N-dimethylacetamide
during the polymerization in the case of n ˆ 3 and 4. The glass transition temperature increased with
increasing the number of phenylene units in the range 190±2408C. Only the hyperbranched poly(ether
ketone) composed of tetraphenylene units showed crystalline character.
…34†
The thermal properties of the poly(ether ketone)s were dramatically changed by the endcapping
reaction by 1,4-phenylene units. The glass transition and melting temperature increased to 315 and
345±3858C, respectively, for the polymer …n ˆ 1† after the endcapping with tetra 1,4-phenylene units
(Fig. 15). That is, the properties of the hyperbranched poly(ether ketone)s were highly dependent on the
nature of end functional groups rather than the number of phenylene units in the main chain.
Electrophilic aromatic substitution is also useful as a propagation reaction for the formation of
hyperbranched poly(ether ketone)s since aromatic rings connected with ether bonds are activated for
electrophiles. The self-polycondensation of AB2 type monomers with carboxylic acid groups, shown in
Fig. 16, was carried out using phosphorus pentoxide/methanesulfonic acid as the condensing agent and
solvent [98,99]. The soluble polymers were isolated and the terminal groups were readily functionalized
to control the glass transition temperature and solubility of the polymers. It was reported that the
Fig. 16. AB2 monomers for hyperbranched poly(etherketone)s through electrophilic aromatic substitution.
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Fig. 17. AB2 monomers for hyperbranched poly(ether sulfone)s.
ammonium salt of the hyperbranched poly(ether ketone) was soluble in water and behaved as a
unimolecular micelle.
Hyperbranched poly(ether sulfone)s can be synthesized through nucleophilic aromatic substitution,
similar to the case for poly(ether ketone)s. Hay reported the self-polycondensation of AB2 monomers,
shown in Fig. 17, which possess aryl ¯uoride or chloride groups activated by a sulfone moiety as an
electron-withdrawing group [100±102]. The polymerization was carried out in DMAc at 150±1708C in
the presence of CsCO3 and Mg(OH)2 to generate the phenolate ion in situ. The halogen-terminated
hyperbranched poly(ether sulfone)s were soluble in organic solvents such as THF, chloroform and N,Ndimethylacetamide, and were thermally stable in comparison with the hydroxy-terminated ones. Glass
transition temperatures were in the range 226±2778C, depending on the structures of the end functional
groups. MALDI-TOF mass measurements revealed that an intramolecular cyclization was competing
with the polymerization reaction, while SEC measurements suggested the formation of hyperbranched
poly(ether sulfone)s with a Mw over 10 4.
2.5. Polyamides
Generally, there are two reaction routes to synthesize wholly aromatic polyamides: (1) low temperature polycondensation reactions of amines and acid chlorides; (2) direct polycondensation between
amines and carboxylic acids in the presence of condensation agents. Kim ®rst reported the synthesis
of hyperbranched aromatic polyamides by the low temperature polycondensation of AB2 and A2B
monomers shown in Fig. 18 [51]. In an amide solvent, the hydrochloride was neutralized to give the
free amine, allowing self-condensation to take place. The GPC analysis of resulting hyperbranched
polyamides in a DMAc/LiBr/H3PO4/THF mixture showed that the molecular weight ranged between
24,000 and 46,000 with polydispersities of 2.0±3.2, based on polystyrene standards. Both carboxyl- and
amino-terminated polyamides were soluble in amide solvents such as DMF, NMP and DMAc, while the
amino-terminated polyamide was soluble in water because the amino groups were converted to their
hydrochloride salt during the polymerization. NMP solutions containing more than 40 wt% of the
carboxyl-terminated hyperbranched polyamide exhibited a nematic liquid crystalline phase and did
Fig. 18. AB2 monomers for low temperature polycondensation to form hyperbranched aromatic polyamides.
M. Jikei, M-A. Kakimoto / Prog. Polym. Sci. 26 (2001) 1233±1285
1259
Fig. 19. AB2 monomers for direct polycondensation to form hyperbranched aromatic polyamides.
not lose birefringence up to 1508C. On the other hand, the amino-terminated polyamides did not show
birefringence.
The direct polycondensation by using condensation agents to activate carboxylic acid groups in situ is
the other attractive route to synthesize hyperbranched polyamides since carboxylic acid chlorides are
highly reactive, and therefore, sensitive to moisture. Some of AB2 monomers for hyperbranched
aromatic polyamides are listed in Fig. 19. Triphenyl phosphite/pyridine and diphenyl (2,3-dihydro-2thioxo-3-benzoxazolyl)phosphonate have been reported as ef®cient condensation agents for the polymerization [52,103±106]. Polymerization in low concentrations of the monomer can be effective for
avoiding gelation when the intermolecular interactions between amide bonds are strong enough to form
gels during polymerization. The resulting hyperbranched polyamides were soluble in amide solvents and
their solubilities and thermal properties were dependent on the nature of end functional groups. In the
case of polymerization of 3,5-bis(4-aminophenoxy)benzoic acid, a glass transition temperature of 2008C
for the original polymer was lowered to 1648C by the endcapping reaction of amino groups with
heptanoyl chloride [104].
We have recently reported the direct polycondensation of AB2, AB4 and AB8 monomers, shown in
Fig. 20, to form hyperbranched aromatic polyamides with various DBs [35]. The DB of the polymer
prepared from the AB2 monomer was 0.32, which was much lower than the statistical expectation of 0.5.
The low DB might be caused by the steric hindrance during the polymerization. The polymerization of
AB4 and AB8 monomers resulted in the formation of the polymer with higher DBs (0.72 for the AB4 and
0.84 for the AB8 monomers) since the monomers contained prefabricated dendritic units. The DBs could
be also controlled by the copolymerization of the AB2 and AB4 monomers. The solubilities and thermal
properties of the hyperbranched polyamides were not in¯uenced by different DBs in this range.
Although thermal polymerization is a well-known process for obtaining aliphatic high polyamides
with high molecular weights, it is generally dif®cult to obtain aromatic polyamides having high molecular
weights by the molten polycondensation method. However, melt polycondensation is applicable for
Fig. 20. AB2, AB4 and AB8 monomers composed of the same building block for hyperbranched aromatic polyamides.
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some AB2 type monomers to prepare hyperbranched aromatic polyamides with a reasonably high
molecular weight [103], as shown in Eq. (35). The thermal polymerization of 3,5-bis(4-aminophenoxy)benzoic acid at 2358C proceeded in a stable molten state and gave a glassy solid after
cooling to room temperature. Pouring a DMF solution of the glassy product into methanol
containing lithium chloride precipitated a white polymer. The weight average molecular weight
(Mw) and molecular weight distribution (Mw/Mn) determined by GPC-MALLS was 74,600 and
2.6, respectively, which were equivalent to the values for the polymer prepared by direct polycondensation. Thermal polymerization of the corresponding AB monomer, 3-(4-aminophenoxy)benzoic acid, failed because the melt phase advanced to a nontransparent solid quickly. The end
functional groups in the product from the AB monomer (amine and carboxylic acid) were clearly
observed by spectroscopic measurements.
…35†
Statistically, the condensation of difunctional (A2) and trifunctional (B3) compounds results in
gelation over a certain conversion of functional groups. On the other hand, the polymerization of
ABx monomers gives soluble hyperbranched polymers without gelation. We found that soluble
hyperbranched polyamides could be isolated from the polymerization of aromatic diamines (A2
monomer) and trimesic acid (B3 monomer) [107], as shown in Fig. 21. The potential advantage
of the A2 and B3 polymerization lies on the use of commercially available monomers to prepare
hyperbranched polyamides. The reactivity of the amino groups in the original diamine decreases after
one amino group is reacted to form an amide bond, which is suitable for the formation of AB2 type
intermediates. Reaction conditions such as the reaction temperature, concentration of monomers and the
amount of condensation agents and inorganic salts should be optimized in order to avoid gelation. The
direct polycondensation of p-phenylene diamine and trimesic acid at 808C in the presence of triphenyl
phophate and pyridine proceeded without gelation to form a hyperbranched aromatic polyamide, as
shown in Eq. (36). The resulting polyamide was soluble in aprotic polar solvents and contained large
amounts of unreacted carboxylic acid groups. The inherent viscosity of the polyamide in DMF at 308C
was 0.8 dl/g, which is much higher than that of hyperbranched polymers prepared from ABx
Fig. 21. Direct polycondensation of aromatic diamines (A2) and trimesic acid (B3).
M. Jikei, M-A. Kakimoto / Prog. Polym. Sci. 26 (2001) 1233±1285
1261
type monomers:
…36†
Palladium catalyzed CO insertion was also attempted for use as a propagation reaction to
prepare a hyperbranched aromatic polyamide [108]. Unfortunately, the polymerization of 3,5dibromoaniline resulted in the formation of an insoluble product, which might be caused by undesired
side reactions. The polymerization with a core molecule, tetraphenyladamantane, gave a partially
soluble product.
Hyperbranched poly(ether amide)s were prepared through nucleophilic ring opening polymerization
of 2-oxazoline-containing AB2 monomer [109], as shown in Eq. (37). The polymerization of 2-(3,5dihydroxyphenyl)-1,3-oxazoline in N-methylcaprolactam resulted in a well-de®ned and fully soluble
hyperbranched polymer with an ether amide structure. The number average molecular weight determined by GPC based on polystyrene standards was 47,500 and MALDI-TOF mass measurements
indicated that the polymer was composed of repeating unit of 179.3 Da, which was in agreement
with the proposed structure. Glass transition temperature and decomposition onset temperature for
the poly(ether amide) were 176 and 3308C, respectively. The hydrolysis of some oxazoline groups as
a side reaction was observed during the polymerization when the process was carried out in tetramethylene sulfone or in bulk
…37†
A series of AB2 aminoacrylate hydrochloride monomers gave hyperbranched poly(amidoamine)s
through Michael addition chemistry [110], as shown in Eq. (38). Heating the hydrochloride salts resulted
in the consumption of the acryloyl group and the formation of new methylene units. The polymer had the
same elemental composition as the monomer used, which indicated that no hydrogen chloride was lost
during the polymerization. On the other hand, polymerization of the free amine like N-acryloyl-1,6diaminohexane gave an insoluble product. It was presumed that the propagation reaction involved
addition of the free amine to the acryloyl unit since the alkyl ammonium ions were insuf®cient nucleophiles for the Michael addition. The formation of the proposed structure and intramolecular cyclization
during the polymerization was ruled out by MALDI-TOF mass analysis. The number of methylene units
affected both the reactivity of the monomer and the properties of the resulting polymers. N-Acryloyl-1,2diaminoethane hydrochloride …n ˆ 2† showed the high reactivity for the polymerization and gave a
polymer with a DB greater than 0.9. The glass transition temperature of the polymers increased with
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M. Jikei, M-A. Kakimoto / Prog. Polym. Sci. 26 (2001) 1233±1285
increasing internal CH2 units governed by a pronounced odd/even effect:
…38†
The copolymerization of the AB2 and Bn molecules allows for the control of the molecular weight of
the hyperbranched poly(amidoamine). Especially, the polymerization of N-acryloyl-1,2-diaminoethane
hydrochloride …n ˆ 2† in the presence of a B6 core molecule, tris-(2-aminoethylamine), gave an analog
for the hydrochloride salt of PAMAM dendrimers. The relationship between the molecular weight,
calculated from the monomer to the core ratio, and intrinsic viscosity of the copolymer is described
in Fig. 22. A trend of decreasing intrinsic viscosity with increasing molecular weight was observed over
a certain molecular weight, similar to the case for PAMAM dendrimers, while the `one-pot' synthesis
gives rise to the distribution in both molecular weight and structure.
2.6. Poly(ether imide)s
Aromatic polyimides known are important high-performance polymers due to their excellent thermal
Fig. 22. Relationship between molecular weight and intrinsic viscosity of the copolymer prepared from N-acryloyl-1,2diaminoethane (AB2) and tris-(2-aminoethylamine) (B6).
M. Jikei, M-A. Kakimoto / Prog. Polym. Sci. 26 (2001) 1233±1285
1263
and chemical stabilities. Especially, poly(ether imide)s are widely used in many ®elds including microelectronics, the aerospace industry, and for engineering parts employed in severe conditions. There are two
strategies to prepare poly(ether imide)s: (1) ether bond formation of the monomer containing an imide ring;
(2) imide ring formation through amic acid precursors. Moore synthesized a hyperbranched aromatic
poly(ether imide) using the ®rst approach from a protected AB2 monomer [111], as shown in Eq. (39).
…39†
The self-polycondensation was carried out through the etheri®cation of silylated phenol and aryl ¯uoride
in diphenylsulfone at 2408C in the presence of cesium ¯uoride. The tert-butyldimethylsilyl (TBDMS)
group was used to protect the hydroxyl groups since TBDMS was less labile than the trimethylsilyl
groups, and therefore, allowed thorough puri®cation of the monomer by recrystallization. TBDMS
¯uoride evolution was observed within seconds after melting of the monomer. The DB of the hyperbranched poly(ether imide) determined by 1H NMR measurements was 0.66, which was in agreement
with the DB estimated from a model compound study shown in Fig. 23. The Mns determined by GPC in
THF and NMP containing lithium bromide were 19,200 and 9300, respectively. 1H NMR spectrum of
the polymer also gave the Mn of 6300 although the value might be unreliable due to the high extent of
reaction. The hyperbranched poly(ether imide) was soluble in common organic solvents. The temperature for 10 wt% loss in air was 5308C, which implies that the polymer is one of the most thermally stable
hyperbranched polymers prepared to date.
We have recently reported the synthesis of hyperbranched poly(ether imide)s through the corresponding poly(amic acid) ester precursor [112], as shown in Eq. (40). Polymerization of the AB2 monomer
prepared from 3,5-dimethoxyphenol, 4-¯uoronitrobenzene and 4-nitrophthalonitrile proceeded in the
presence of diphenyl (2,3-dihydro-2-thioxo-3-benzoxazolyl)phosphonate (DBOP) and triethylamine at
room temperature. Subsequent imidization of the resulting precursor gave a hyperbranched poly(ether
imide) quantitatively. Chemical imidization in the presence of acetic anhydride and pyridine resulted in
Fig. 23. Model reaction and products to estimate the DB for the hyperbranched poly(ether imide).
1264
M. Jikei, M-A. Kakimoto / Prog. Polym. Sci. 26 (2001) 1233±1285
the formation of the soluble polyimide endcapped with an acetamide group. The hyperbranched polyimide had a DB of 0.48 and a Mw of 188,000 determined by GPC MALLS. DSC and TGA measurements
revealed a Tg of 1938C and a 10 wt% loss temperature of 4708C in air. This polymer was soluble in
aprotic polar solvents and had a low inherent viscosity, similar to other hyperbranched polymers.
…40†
2.7. Polyurethanes and polyureas
Generally, urethane and urea linkages are formed from condensation reactions between isocyanates
and alcohols or amines. Isocyanate groups in the self-condensing monomers must be protected since
they are highly reactive against nucleophiles. Deprotection is usually carried out in situ during the
polymerization process.
Aryl±aryl carbamate groups are useful for the protection of aromatic isocyanates since the carbamate
bonds are easily cleaved by heating. Hyperbranched polyurethanes were prepared from the AB2 type
monomers which had one alkyl hydroxyl group and two protected isocyanate functions [113]. The
polymerization proceeded in re¯uxing THF in the presence of dibutyltin dilaurate as a catalyst, as
shown in Eq. (41).
…41†
M. Jikei, M-A. Kakimoto / Prog. Polym. Sci. 26 (2001) 1233±1285
1265
The soluble polymer with a Mw of 34,000 was isolated from the polymerization with a monomer
concentration of 1 mol/l. Further increases in the concentration resulted in the formation of crosslinked
networks which swelled in THF, DMF, and DMSO. The crosslinked network might be caused by side
reactions of isocyanate groups such as dimerization and trimerization. In order to avoid network formation, the polymerization of the AB2 monomer was carried out in the presence of an aliphatic alcohol
which gave rise to end-capping reactions during the polymerization. The endcapped polyurethanes were
thermally stable up to 2008C, whereas the polymers isolated without the endcapping reactions were not
stable above 1208C. Glass transition temperatures of the polymers ranged from 119 to 578C, depending
on the end functional groups. All of the endcapped polymers were soluble in THF, DMF and DMSO,
with the decyl-capped polymer also being soluble in diethyl ether±chloroform mixture.
Carbonyl azides are known as synthons (protective functional groups) for isocyanates and AB2 type
monomers containing carbonyl azide groups have been reported as monomers for self-polycondensation
reactions to form hyperbranched polyurethanes and hyperbranched polyureas. Fig. 24 shows the AB2
monomers for hyperbranched polyurethanes. Wholly aromatic hyperbranched polyurethanes were
prepared by the self-polycondensation of the AB2 and A2B monomers [114,115]. Although both the
AB2 monomer and the resulting polymer are insoluble in toluene, a polymerization in re¯uxing toluene,
considered as a dispersion polymerization, resulted in the formation of hyperbranched polyurethanes in
high yields. The Mw determined by GPC based on polystyrene standards was 9100 and the DB determined by 1H NMR was 0.59. The polymerization of 5-hydroxyisophthaloyl azide as an A2B monomer
gave a carbamate-terminated polyurethane after an endcapping reaction with ethanol before isolation.
The resulting polyurethane was found to be insoluble in most of organic solvents, which was probably
due to the side reactions of the isocyanate groups. Soluble polymers resulted when the polymerization of
the A2B monomer was carried out in the presence of an equivalent amount of an alcohol. Melt polymerization was applicable for the AB2 monomer containing oligoethyleneoxy segments. IR and 1H
NMR spectra indicated the complete removal of azide groups and the formation of the carbamate groups.
The inherent viscosities of the hyperbranched polyurethanes increased with the increasing length of the
oxyethylene segments and ranged from 0.20 to 0.30 dl/g in DMSO. Glass transition temperatures
decreased remarkably with the introduction of oxyethylene units in the range 106±138C. The oxyethylene units improved thermal stabilities by about 1008C in comparison with the corresponding wholly
aromatic polyurethane.
A hyperbranched polyurea was prepared by the self-polycondensation of the AB2 monomer containing carbonyl azide and amino groups [116], as shown in Eq. (42). The thermal decomposition of 3,5diaminobenzoyl azide in NMP at 1108C generated the corresponding 3,5-diaminophenyl isocyanate in
situ. Rapid evolution of N2 was observed during the initial stage of the reaction and subsequent heating
resulted in the formation of a wholly aromatic hyperbranched polyurea in high yield. The IR spectrum
indicated the absence of any residual azide or isocyanate and showed the formation of urea linkages. The
Fig. 24. AB2 monomers containing carbonyl azides for hyperbranched polyurethanes.
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DB determined by the 1H NMR spectrum was found to be 0.55. The Mw and Mw/Mn determined by GPC
based on polystyrene standards were 19,500 and 1.56, respectively. The wholly aromatic hyperbranched
polyurea was soluble in organic solvents such as DMSO, NMP and DMF, whereas the linear analogs
were essentially insoluble in those solvents.
…42†
2.8. Poly(siloxysilane)s and poly(carbosilane)s
Hydrosilylation between a double bond and a Si±H group is a useful propagation reaction to prepare
hyperbranched polysilanes. Intramolecular cyclization often competes with the propagation reaction
since the main chains are composed of ¯exible segments. When the monomers contain Si±O linkages,
the resulting polymers are called poly(siloxysilane)s. Several ABx monomers have been reported in the
literature, as shown in Fig. 25.
Mathias ®rst reported a hyperbranched poly(siloxysilane) through the hydrosilylation of AB3-I [117],
as shown in Eq. (43). The polymerization was carried out in the mixture of acetonitrile and ether under
nitrogen at 528C in the presence of hydrogen hexachloroplatinate hydrate as a catalyst. IR and 1H NMR
Fig. 25. ABx monomers for hyperbranched poly(siloxysilane)s.
M. Jikei, M-A. Kakimoto / Prog. Polym. Sci. 26 (2001) 1233±1285
1267
spectra of the resulting polymer indicated the consumption of vinyl groups and presence of Si±H groups
located at the chain ends. SEC measurements in THF gave a Mw of 19,000 based on polystyrene
standards. It was pointed out that an intramolecular cyclization to form a six-membered cyclic product
also occurred during the polymerization of AB3-I.
…43†
Rubinsztajn reported the self-polycondensation of AB3-II, AB3-III and AB3-IV shown in Fig. 25
[118,119]. The possible ®ve-membered cyclic products from AB3-II have a high ring strain energy,
about 10 kcal/mol more than the six-membered ones from AB3-I, which favors the intermolecular
reaction leading to the hyperbranched poly(siloxysilane). The polymerization of AB3-II and AB3-III
proceeded in the presence of a heterogeneous platinum catalyst to form hyperbranched poly(siloxisilane)s in good yield. The slow addition of AB3-II afforded the polymer with a high molecular weight
…Mw ˆ 53; 000† and a broad molecular weight distribution …Mw =Mn ˆ 5:6†: 13C NMR measurements
revealed that a-addition to the double bond occurred during the polymerization in 10±20% in addition to
b-addition. The reactivity of AB3-III for the polymerization was much lower than that of AB3-II, which
might be caused by the chelating ability of double bonds in AB3-III to platinum. It is known that siliconhydride moieties are thermally unstable and easily undergo thermal rearrangement. The endcapping
reaction with vinyltrimethylsilane signi®cantly improved the thermal degradation temperature of the
hyperbranched poly(siloxisilane)s. The degradation onset temperature for the polymer prepared from
AB3-II after the endcapping reaction was 401.58C. The polymerization of AB3-IV, the aromatic analog
of AB3-II, in the presence of platinum catalyst was carried out in the same manner. 29Si NMR measurements suggested that the ratio (b /a ) of silicon hydride to vinyl group was 60/40. The hyperbranched
poly(siloxysilane) was soluble in common organic solvents and the Mw and Mw/Mn values determined by
SEC based on polystyrene standards, were 9800 and 1.6, respectively. The glass transition temperature
of the polymer from AB3-IV …Tg ˆ 2588C† was substantially higher than the Tg of the polymer from
AB3-II …Tg ˆ 21008C†:
FreÂchet reported the polymerization of the AB2, AB4 and AB6 monomers, shown in Fig. 25, to form
hyperbranched poly(siloxysilane)s with silicone hydride terminal groups and subsequent endcapped
polymers [62]. The polymerization was carried out at 508C in the presence of platinum-on-active
charcoal as a catalyst. For the polymer from the AB2 monomer, the relative integration of a and b
signals in the 1H NMR spectrum indicated that the a addition occurred in ca. 30%. The resulting
polymers were soluble in organic solvents such as hexane, halogenated alkanes and ethers, but were
insoluble in polar solvents, such as methanol or acetonitrile. The Mw determined by GPC ranged from
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M. Jikei, M-A. Kakimoto / Prog. Polym. Sci. 26 (2001) 1233±1285
5800 to 8900 and was slightly increased by the addition of fresh monomer to the polymerization mixture.
According to 1H NMR, IR spectra and GPC traces, the polymerization process was accompanied by
signi®cant intramolecular cyclization reactions in all cases. Progressive slow addition of the AB2 into the
polymerization mixture resulted in a signi®cant increase in molecular weight up to 86,000. The molecular weight and the polydispersity could be controlled by changing the rate of addition or the amount of
monomer fed. The chemical modi®cation of the silicone hydride groups allows for some control in the
properties of the hyperbranched poly(siloxysilane)s. The endcapping reaction was carried out by hydrosilylation with a variety of vinyl compounds. The glass transition temperature ranged from 2126 to
228C, depending on the nature of the end functional groups.
Hyperbranched polycarbosilanes which do not contain siloxane linkages can be prepared by the
hydrosilylation of ABx monomers, similar to the case of hyperbranched poly(siloxysilane)s. The
triallylsilane, readily prepared from the Grignard reaction of allyl bromide with trichlorosilane,
was allowed to polymerize in the presence of a platinum catalyst in the bulk [120,121], as shown
in Eq. (44).
…44†
The complete conversion of the Si±H functionality was con®rmed by 1H NMR spectroscopy and the DB
could be deduced from 29Si NMR spectroscopy. The possible repeating units of the poly(carbosilane),
dendritic (D), semidendritic (sD), linear (L) and terminal (T ), are described in Fig. 26 and an expression
for the DB of the polymers from AB3 monomers is shown in Eq. (45). The integration of each resonance
in the 29Si NMR spectrum gave a DB of 0:48 ^ 0:05; which agreed well with that derived from a
statistical consideration …DB ˆ 0:44† [31,121].
DB ˆ
2D 1 sD
2=3…3D 1 2sD 1 L†
…45†
GPC trace of the hyperbranched poly(carbosilane) showed a trimodal molecular weight distribution.
Each molecular weight based on polystyrene standards corresponded to 500,000, 80,000 and 1000,
respectively. All of the fractions were soluble in common organic solvents. The addition of an alkene
as the B1 molecule allowed for control over the molecular weight and molecular weight distribution
of the poly(carbosilane). 2-(10-Decen-1-yl)-1,3-oxazoline, which was inert under the conditions of
Fig. 26. Possible repeating units of the hyperbranched poly(carbosilane).
M. Jikei, M-A. Kakimoto / Prog. Polym. Sci. 26 (2001) 1233±1285
1269
Fig. 27. Schematic representation of macromolecular architectures prepared from the oxazoline-anchored hyperbranched
poly(carbosilane) as a macromonomer.
hydrosilylation and possessed a reactive functionality, was chosen as a B1 molecule. According to the
GPC traces, an increasing amount of the oxazoline decreased the degree of polymerization as well as the
polydispersity of the ®nal product. The oxazoline-anchored hyperbranched poly(carbosilane) can be
considered as a macromonomer for further chemical reactions. The coupling with a core molecule, ringopening polymerization of the oxazoline group, copolymerization with 2-phenyl-1,3-oxazoline [122]
and the chemical modi®cation of vinyl groups [123] resulted in the formation of various macromolecular
architectures, as described in Fig. 27.
Recently, hydrosilylations of methyldivinylsilane, methyldiallylsilane, triallysilane, and methyldiundecenylsilane, listed in Fig. 28, were carried out in order to investigate the in¯uence of the monomer
structure on the formation of hyperbranched poly(carbosilane)s [124]. Methyldivinylsilane, methyldiallylsilane, and triallysilane did not yield high molecular weight polymers when the reaction mixture
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M. Jikei, M-A. Kakimoto / Prog. Polym. Sci. 26 (2001) 1233±1285
Fig. 28. ABx monomers for hyperbranched poly(carbosilane)s.
was diluted by a solvent. The bulk polymerization of the short-chain monomers often resulted in the
formation of crosslinked products, which might be caused by the rearrangement of the terminal double
bonds and reductive elimination reactions to form disilanes. The crosslinking reactions were avoided by
the reaction temperature and catalyst concentration being kept low. High molecular weight polymers
were readily obtained by the polymerization of methyldiundecenylsilane where the distance between the
double bond and silicon atom was essentially enlarged. The molecular mass of the hyperbranched
poly(carbosilane) was controlled by subsequent addition of further monomer to the polymerization
mixture.
Hyperbranched poly(carbosilarylene)s were also synthesized through the same strategy as aliphatic
poly(carbosilane)s [125]. The AB3 monomers examined are listed in Fig. 29. The glass transition
temperatures of the hyperbranched poly(carbosilarylene)s depended on the structures of the starting
monomers and ranged from 2458C (AB3Ar-III) to 128C (AB3Ar-I). The DB of these polymers determined by quantitative 29Si NMR measurements was found to be close to the statistical calculation of
0.44.
The synthesis and degradation behavior of poly(alkoxysilane)s was investigated since alkoxysilane
groups can be easily hydrolyzed under acidic conditions [126]. Polymerization of the AB2 type monomer
in the presence of platinum catalyst in hexane or Co2(CO)8 in bulk resulted in the formation of soluble
polymers, as shown in Eq. (46). The GPC diagram of the resulting polymers suggested a broad molecular
weight distribution, as is typical for hyperbranched polymers. Degradation of the polymer in THF/
methanol mixture was completed after 40 h of stirring under re¯ux to form mono(dialkoxymethylalkylsilane)s.
…46†
Fig. 29. AB3 monomers for hyperbranched poly(carbosilarylene)s.
M. Jikei, M-A. Kakimoto / Prog. Polym. Sci. 26 (2001) 1233±1285
1271
2.9. Miscellaneous reactions
Methylene groups activated by electron-withdrawing groups can give rise to a difunctional
systems. It was actually reported that the polycondensation of phenylacetonitrile and 1,4bis(chloromethyl)benzene proceeded to form linear polymers [127]. Recently, AB2 type monomers which have an activated methylene and a chloromethylene group were reported as starting
materials to form hyperbranched polymers [128,129]. The polymerization of p-(chloromethyl)phenylacetonitrile proceeded in DMSO in the presence of NaOH aqueous solution. The activated
methylene units are converted to branching junctions during the polymerization. Polymerization
in the presence of tetra-n-butylammonium chloride (TBAC) as a catalyst resulted in gelation
although the molecular weight of the polymer prepared without catalysts was not high enough.
The polymerization of 4-(4 0 -chloromethylbenzyloxy)phenylacetonitrile proceeded in a DMSONaOH(aq) medium in the presence of TBAC without gelation in high yield, as shown in Eq.
(47). The Mn determined by 1H NMR spectroscopy was 9100 and GPC measurement suggested a
broad molecular weight distribution. The addition of monofunctional monomers, such as methoxybenzyl chloride, was used to control the molecular weight and molecular weight distribution.
The resulting hyperbranched polymers were soluble in organic solvents, such as DMF, DMSO
and THF.
…47†
Cyanoacetic acid and its methyl ester are also known as activated methylene compounds.
The condensation reaction with aromatic aldehydes in the presence of a base results in the
formation of 2-cyano-2-carbonylvinyl groups (Knoevenagel condensation). The hyperbranched
polyesters containing carbazole substituted with two electron acceptor groups were prepared
through Knoevenagel condensation and ester exchange condensation to form new nonlinear
optical materials [58,59], as shown in Eq. (48). A one-pot polycondensation of the carbazole
and cyanoacetic acid or its methyl ester proceeded in two steps in the presence of 4(dimethylamino)pyridine as a catalyst. The ®rst step was carried out in THF solution,
presumably to form AB2 type molecules. This was followed by a solid stage polycondensation to yield the hyperbranched polymers. The ester exchange reaction of the isolated AB2
monomer (3,6-bis(2-cyano-2-methoxycarbonylvinyl)-9-(11-hydroxyundecyl)carbazole) prepared
by the Knoevenagel condensation resulted in the formation of insoluble products. IR and
1
H NMR spectra supported the formation of the proposed structure. In addition, esteri®cation
by cyanoacetate at the focal point was also observed. This might compete with the propagation reactions. The Mn and Mw/Mn of the polymer from the carbazole and cyanoacetic acid
1272
M. Jikei, M-A. Kakimoto / Prog. Polym. Sci. 26 (2001) 1233±1285
were 19,000 and 3.0, respectively. The hyperbranched polymers were soluble in chloroform
and glass transition temperatures ranged from 83 to 988C.
…48†
Hyperbranched polymers composed of electron-donating and electron-withdrawing groups to
generate a large dipole moment can be new three-dimensional materials for nonlinear optics. The
hyperbranched polyesters containing carbazole moieties exhibited reasonably high second harmonic
generation ef®ciencies …d33 ˆ 7 pm=V† after electric corona poling at about 1008C. The application of
the hyperbranched polymers for electroluminescent devices was also examined [130].
Polycondensations catalyzed by transition metal complexes have been investigated not only for
conventional linear polymers, but also for novel hyperbranched polymers. The polycondensation of the
AB2 monomer which possess one aryl ethynyl and two aryl iodide units gives hyperbranched poly(ethynylene)s through palladium catalyzed carbon±carbon coupling reactions [131], as shown in Eq. (49). The
hyperbranched poly(ethynylene)s were soluble in organic solvents and the Mw determined by GPC ranged
from 6000 to 10,000 with a polydispersity (Mw/Mn) of 1.6±3.0. The spectroscopic measurements indicated
that the polymer contained diacetylenic moieties (ca. 25%) formed by the oxidative coupling of two aryl
ethynyl units in addition to the desired aryl ethynylene units.
…49†
M. Jikei, M-A. Kakimoto / Prog. Polym. Sci. 26 (2001) 1233±1285
1273
The self-polycondensation of styrenes containing aryl-halogen units, catalyzed by palladium, afforded
poly(phenylene vinylene)s. Nishide reported the polymerization of the AB2 monomer derived from
2,6-dibromostyrene to form hyperbranched poly(phenylene vinylene) [61], as described in Eq. (50).
The resulting polymer was soluble in organic solvents and the Mw and Mw/Mn were 13,000 and 1.5,
respectively. Deprotection of the acetyl ester and subsequent oxidation of the phenoxyl group resulted in
the formation of poly(phenoxyl radicals).
…50†
Palladium-catalyzed C±N coupling reactions were applied as propagation reactions to form hyperbranched polyanilines [132]. The polymerization of 3,5-dibromoaniline proceeded in the presence
of a palladium catalyst, a ligand, and a base, as shown in Eq. (51). 2,2 0 -Bis(diphenylphosphino)-1,1 0 binaphthyl (BINAP) was an effective ligand for the formation of diphenylamine backbone. The resulting
hyperbranched polyaniline was soluble in organic solvents and the Mw determined by GPC in THF was
7000.
…51†
Hyperbranched poly(triphenylamine) was prepared through aryl±aryl coupling of a Grignard
reagent in the presence of a nickel compound [133], as shown in Eq. (52). The resulting polymer
had an average molecular weight of 4000 and was soluble in organic solvents, such as THF and
chloroform. The polymer showed a new p±p p transition band at 360 nm, possibly brought about
by the conjugation between biphenylene units and nitrogen atoms. Here, the introduction of
branching units could be an effective and alternative way for improving the solubility of some conjugated polymers.
…52†
1274
M. Jikei, M-A. Kakimoto / Prog. Polym. Sci. 26 (2001) 1233±1285
Fig. 30. Acetophenone derivatives as AB2 monomers for ruthenium catalyzed polymerization.
Since the acetyl group activates both of the ortho C±H bonds for the addition reaction to ole®ns
catalyzed by ruthenium complexes, acetophenone derivatives should be suitable AB2 monomers for
the corresponding hyperbranched polymers. Weber has reported the polymerization of several acetophenone derivatives catalyzed by ruthenium complex [134±136], as shown in Fig. 30 and Eq. (53). The
ruthenium hydride complex ([Ph3P]3RuH2CO) and its coordinately unsaturated complex prepared by the
hydrogenation of styrene were found to be effective for the polymerization. Both Markovnikov and
anti-Markovnikov orientation were observed as a component of the hyperbranched polymers and the
ratio of composition was dependent on the reaction conditions and monomer structures. The hyperbranched polymer prepared from 4-acetylstyrene was soluble in THF and toluene but insoluble in
methanol.
…53†
3. Self-condensing vinyl polymerization
Monomers containing one vinyl group and one initiating moiety (AB p monomers) give hyperbranched polymers through self-condensing vinyl polymerization (SCVP). The activated species
could be a radical, cation, or a carbanion. Living/controlled polymerization systems are preferred
in order to avoid crosslinking reactions and gelation caused by chain transfer or dimerization
reactions.
3.1. Polystyrenes
Cationic polymerization and radical polymerizations have been reported for SCVP of styrene
M. Jikei, M-A. Kakimoto / Prog. Polym. Sci. 26 (2001) 1233±1285
1275
derivatives. Hyperbranched polystyrene was ®rst reported via the cationic polymerization of 3-(1-chloroethyl)styrene in the presence of SnCl4 and tetrabutylammonium bromide [66], as shown in Eq. (54).
…54†
The polymerization was carried out in dry dichloromethane under nitrogen between 215 and 2208C.
The combination of the Lewis acid and the ammonium salt is known as a living/controlled system for the
polymerization of styrenes. It was reported that the time-dependence pro®le of the molecular weights
resembled that of a typical polycondensation; an initial period of slow chain growth was followed by
a period of exponential increase of the molecular weight. SEC chromatograms suggested that the
coupling of growing oligomers was a major reaction pathway even after 5 min of reaction. The proposed
Fig. 31. Self-condensing vinyl polymerization of 3-(1-chloroethyl)styrene catalyzed by SnCl4.
1276
M. Jikei, M-A. Kakimoto / Prog. Polym. Sci. 26 (2001) 1233±1285
mechanism for SCVP of the AB p monomer is drawn in Fig. 31. The addition of approximately equimolar
amounts of the SnCl4 to the AB p monomer in relatively high concentration resulted in the formation of
high molecular weight polymers. The Mw determined by GPC with universal calibration ranged from
660,000 to 17,000, depending on the reaction conditions. The polydispersity (Mw/Mn) was large in the
range 2.9±9.8. The intrinsic viscosity of the hyperbranched polystyrene was lower than that of linear
polystyrenes, similar to hyperbranched polymers prepared by self-polycondensation of ABx monomers.
The shape factor, a , in the Mark±Houwink±Sakurada equation was 0.430, which was much smaller than
the value (0.702) for linear polystyrene.
Hyperbranched and star polystyrenes were prepared through nitroxide-mediated living free radical
polymerization [137]. The AB p monomer employed contains a polymerizable styrene group and an
initiating/propagating moiety consisting of a nitroxide linked to a substituted benzylic carbon atom
(Eq. (55)). Thermal degradation of the AB p monomer to generate a propagating benzylic radical and
an inactive TEMPO (2,2,6,6-tetramethylpiperidinyloxy) radical is a reversible system. The polymerization was conducted using bulk polymerization conditions at 1308C under argon. No insoluble or crosslinked material was observed during the polymerization. The Mw and Mw/Mn determined by SEC were
6000 and 1.40, respectively. The hyperbranched polystyrene can be used as a multi-functional initiator
for living free radical polymerization to prepare multi-arm star polymers. The bulk polymerization of the
hyperbranched polystyrene and styrene at 1308C gave a soluble hyperstar polymer in good yield. The Mw
and Mw/Mn of the star polymer were 300,000 and 4.35, respectively.
…55†
Transition metal mediated living free-radical polymerization, referred to as atom transfer radical
polymerization (ATRP), can be applied for the synthesis of hyperbranched polystyrenes. Matyjaszewski
reported the polymerization of 4-(chloromethyl)styrene in the presence of Cu(I) and 2,2 0 -bipyridyl
[138]. The chlorine atom at the benzylic position is reversibly abstracted by Cu(I) to form Cu(II)Cl
and a benzyl radical capable of initiating the polymerization, as shown in Eq. (56). Both the bulk and
solution polymerization of the AB p monomer gave a hyperbranched polystyrene in high yield. 1H NMR
indicated the remaining double bond and the Mn determined by the 1H NMR signals ranged from 1900 to
6280. It should be pointed out that the reactivity of the secondary benzylic radicals generated during the
polymerization is not equal to that of the primary benzylic radicals from the AB p monomer. This unequal
reactivity dramatically affects the structure of the resulting polymers. It was reported that an almost
linear polymer was obtained for the polymerization with a catalyst to monomer ratio of 0.01, while a
high catalyst to monomer ratio resulted in the formation of a branched structure [139]. Gelation or
crosslinking, probably caused by radical±radical coupling, occurred when the concentration of radicals
was relatively high.
…56†
M. Jikei, M-A. Kakimoto / Prog. Polym. Sci. 26 (2001) 1233±1285
1277
Fig. 32. AB p monomers for SCVP to form hyperbranched poly(acrylate)s and poly(methacrylate)s.
3.2. Poly(methacrylate)s and poly(acrylate)s
Hyperbranched poly(acrylate)s and poly(methacrylate)s have been synthesized by the polymerization
of AB p monomers through ATRP in the presence of Cu(I) and a ligand [140±143]. The monomers
employed are listed in Fig. 32. The bulk polymerization of BPEA was carried out in the presence of
copper(I) bromide, copper (II) bromide and 4,4 0 -di-tert-butyl-2,2 0 -bipyridine at 1008C. Residual double
bonds in the resulting polymers were observed by 1H NMR spectroscopy and the Mn calculated by the
NMR signals was 25,400.
MuÈller has discussed the structure of the hyperbranched polymers prepared through the SCVP process
[144±146], as shown in Fig. 33. Since the propagating oligomers contain two activation center, A p and
B p, the ratio of kA and kB dramatically affects both the DB and polydispersity of the resulting hyperbranched polymers. In addition, there are two types of linear units, Lc and Lv, in the polymer backbone in
contrast with the polymer prepared by the self-polycondensation. It was demonstrated statistically that a
maximum value of DB (0.5) could be obtained when r (kA/kB) was equal to 2.59 and linear polymers
resembling a polycondensate would be formed when kA is much larger than kB. In the case of polymerization of BPEA, the polymer with a DB of 0.49 was obtained when 4,4 0 -di-tert-butyl-2,2 0 -bipyridine was used as a ligand for the polymerization at 508C. On the other hand, an essentially linear
polymer was isolated when the polymerization was carried out in the presence of 2,2 0 -bipyridyl at 508C.
An hyperbranched poly(methacrylate) was also prepared by group transfer polymerization of an AB p
monomer containing a silyl ether [147,148]. The polymerization of 2-(2-methyl-1-triethylsiloxy-1propenyloxy)ethyl methacrylate proceeded in THF at 2508C in the presence of tetrabutylammonium
bibenzoate as a nucleophilic catalyst, as shown in Eq. (57). The degree of polymerization reached a
Fig. 33. Schematic representation for the propagation reaction and the structure of the polymer for SCVP. A, A p and B p denote a
double bond, an activating center generated during the reaction, and an original activating center derived from the monomer,
respectively. a and b denote inactive function produced by the condensation of A p and A or A and B p.
1278
M. Jikei, M-A. Kakimoto / Prog. Polym. Sci. 26 (2001) 1233±1285
maximum at short reaction time (4 min) and then decreased rapidly. The `back-biting' process,
an intermolecular Claisen condensation, probably caused the decrease. The polymer isolated
from the polymerization at 2508C for 32 min had a Mw of 55,500 and a Mw/Mn of 34. The
compact nature of the hyperbranched poly(methacrylate) was evidenced by the low value of
the Mark±Houwink exponent (0.32). It was also reported that adding freshly distilled methyl
methacrylate into the active polymer solution …Mw ˆ 1000† resulted in the formation of star
shaped PMMA.
…57†
4. Multibranching ring-opening polymerization
The third strategy to produce hyperbranched polymers is called multibranching ring-opening
polymerization (MBP or MBROP). In 1992, Suzuki ®rst reported the MBP of cyclic carbamate
catalyzed by palladium to produce hyperbranched polyamines [37]. The conceptional scheme is
shown in Fig. 34. The monomer itself does not contain a branching point and branching points are
generated through the propagation reaction. Therefore, the monomer can be recognized as a latent
ABx monomer. The polymerization is promoted by the addition of proper initiators to the latent ABx
monomer. MBROPs of cyclic carbamates, epoxide, oxetanes and caprolacrones have all been
reported to date.
4.1. Polyamines
Hyperbranched polyamines consisting of primary, secondary and tertiary amino moieties were
prepared by palladium-catalyzed ring opening polymerization [37,38], as shown in Eq. (58). The
polymerization of the cyclic monomers was initiated by the addition of benzylamine and
proceeded at room temperature with the evolution of CO2. It was reported that the p±allylpalladium complex was the key intermediate for the polymerization. NMR spectroscopy indicated that
the polymers contained not only primary and tertiary but also secondary amino moieties as linear
units. The unit ratio of the tertiary amino moiety to the sum of tertiary and secondary amino
groups was 0.81 for the polymer prepared from AB-II in DMSO. The molecular weight increased
Fig. 34. Conceptional scheme for multibranching ring-opening polymerization.
M. Jikei, M-A. Kakimoto / Prog. Polym. Sci. 26 (2001) 1233±1285
1279
with decreasing the amount of the initiator used, whereas precipitation during the polymerization
of AB-II restricted the molecular weight up to ca. 3000.
…58†
4.2. Polyethers
Glycidol contains one epoxide and one hydroxyl group, and therefore, represents a latent AB2
monomer for MBP. Hyperbranched aliphatic polyethers were prepared by anionic ring-opening polymerization of glycidol [25], as shown in Eq. (59).
…59†
Partially deprotonated (10%) 1,1,1-tris(hydroxymethyl)propane was used as an initiator in order to
control the concentration of active sites (alkoxides) during the polymerization. The polymerization
was carried out under slow monomer addition to the reaction mixture in order to minimizing possible
side reactions like intramolecular cyclization. Due to the unsymmetrical structure of glycidol, the
resulting hyperbranched polymers are composed of four structural units, as shown in Fig. 35. Inverse
gated 13C NMR spectra allowed the authors to calculate not only the DB based on Eq. (60) but also the
degree of polymerization (DPn). The DB ranged from 0.53 to 0.59 and the DPn varied from 15 to 83,
depending on the monomer/initiator ratios. The Mw/Mn determined by GPC was in the range 1.13±1.47,
Fig. 35. Possible repeating units for hyperbranched polyglycerol.
1280
M. Jikei, M-A. Kakimoto / Prog. Polym. Sci. 26 (2001) 1233±1285
Fig. 36. Schematic representation of liquid crystalline hyperbranched polymers with mesogenic end groups.
which suggested the polymerization to be well controlled to give a narrow polydispersity:
DB ˆ
2D
2D 1 L13 1 L14
…60†
Frey has demonstrated the versatility of the polymerization of glycidol [149]. Copolymerization with
propylene oxide allowed controlling the polarity of the highly hydrophilic nature of polyglycerol and
glass transition temperature from 237 to 2718C [150]. End group functionalization with mesogens
afforded liquid crystalline hyperbranched polymers with mesogenic end groups [49], as described in
Fig. 36. Amphiphilic star polymers prepared by the functionalization of polyglycerols with fatty acids
were investigated as molecular nanocapsules [53].
Hyperbranched aliphatic polyethers were also prepared by the cationic ring-opening polymerization
of oxetane derivatives [151]. The polymerization of 3-ethyl-3-(hydroxymethyl)oxetane was carried out
in bulk at 1208C in the presence of benzyltetramethylenesulfonium hexa¯uoroantimonate as a thermal
initiator (Eq. (61)). 1H NMR indicated the consumption of the oxetane ring and the DB determined by
13
C NMR was found to be 0.41 (FreÂchet's equation). The Mn determined by SEC and MALDI-TOF mass
measurements were 4170 and 3762, respectively. It was also reported that the hyperbranched polyether
could be used as a macroinitiator for the ring-opening polymerization of e-caprolactone
…61†
4.3. Polyesters
Lactones containing hydroxyl groups as an initiating moiety can be recognized as latent AB type
monomers to give hydroxy-terminated hyperbranched aliphatic polyesters. The lactone monomers
reported to date are described in Fig. 37 [152,153].
The bulk polymerization of lactone-1 was carried out at 1108C in the presence of a catalytic amount of
stannous octoate [152], as shown in Eq. (62). The hyperbranched polyester was isolated as a clear
M. Jikei, M-A. Kakimoto / Prog. Polym. Sci. 26 (2001) 1233±1285
1281
Fig. 37. Lactone monomers for MBROP.
viscous liquid with a Mw in the range 65,000±85,000 and a PD of ca. 3.2 determined by SEC based on
polystyrene standards. Lower catalyst concentrations tend to give polymers with higher molecular
weights. The proposed structure was con®rmed by 1H, 13C NMR and MALDI-TOF MS spectroscopy
and the DB determined by 13C NMR spectrum was 0.50. The solubility of the hyperbranched polyesters
was highly dependent on the structure of end functional groups, similar to other hyperbranched polymers. The hydroxy-terminated hyperbranched polyesters were soluble in common organic solvents, such
as DMSO, DMF, methanol, but were insoluble in THF, CH2Cl2 and CHCl3. The hyperbranched polyesters endcapped with acetyl esters became soluble in THF, CH2Cl2 and CHCl3, but were no longer
soluble in methanol.
…62†
Hedrick reported the ring-opening polymerization of the other lactone monomer (lactone-2) and its
copolymerization with e-caprolactone [153]. The lactone-2 is composed of caprolactone ring and
bis(hydroxymethyl) groups derived from 2,2 0 -bis(hydroxymethyl)propionic acid which is known as
an effective initiators for the ring-opening polymerization of lactides and lactones in the presence of
stannous octoate. The self-polymerization of the lactone-2 proceeded in bulk at 1108C to form the
corresponding hyperbranched aliphatic polyester. Moderate molecular weights (3000±8000) with
broad polydispersities (2.3±2.8) were obtained with the polymerization with or without e-caprolactone.
The DB determined by 1H NMR spectrum was 0.5, similar to the case of lactone-1.
1282
M. Jikei, M-A. Kakimoto / Prog. Polym. Sci. 26 (2001) 1233±1285
5. Concluding remarks
In this decade, various hyperbranched polymers have been prepared from not only ABx type but also
AB p and latent ABx monomers. Although hyperbranched polymers contain linear units as insuf®cient
branching, many properties of dendritic macromolecules, such as good solubility, low viscosity and
multi-functionality at end groups, are generally inherited. It has been reported for many hyperbranched
polymers that properties can be often controlled by chemical modi®cation of end functional groups.
From the synthetic point of view, two novel approaches, SCVP and MBROP, have been reported in
addition of self-polycondensation of ABx monomers. SCVP has opened the door for hyperbranched
vinyl polymers and using external initiators has brought a possibility for molecular weight control of
hyperbranched polymers. Slow addition of the monomer to the polymerization mixture is commonly
effective for increasing the DB and narrowing the polydispersity of hyperbranched polymers. It is clear
that the one-step polymerization process to form hyperbranched polymers is suitable for large-scale
production in comparison with the multi-step reactions required to prepare dendrimers. Although only a
few papers have discussed the various applications of hyperbranched polymers, there are potentially
broad application ®elds, such as coatings, resins, and additives. Tailor-made properties can be achieved
by endcapping reactions on hyperbranched polymers. While linear and partially branched polymers are
commonly used in industry hyperbranched polymers are likely to be adopted to some speci®c ®elds in
the near future due to their unique and attractive properties.
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