Progress in Polymer Science 34 (2009) 317–350
Contents lists available at ScienceDirect
Progress in Polymer Science
journal homepage: www.elsevier.com/locate/ppolysci
Synthesis of functional polymers with controlled architecture by CRP
of monomers in the presence of cross-linkers: From stars to gels
Haifeng Gao, Krzysztof Matyjaszewski ∗
Department of Chemistry, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, PA 15213, United States
a r t i c l e
i n f o
Article history:
Received 8 October 2008
Received in revised form 6 January 2009
Accepted 6 January 2009
Available online 7 February 2009
Keywords:
Controlled/“living” radical polymerization
(CRP)
Atom transfer radical polymerization
(ATRP)
Cross-linker
Star polymer
Gels
Click reactions
a b s t r a c t
Recent developments in the synthesis of functional polymers with controlled architecture
and site-specific functionality via applying controlled radical polymerization (CRP) techniques are reviewed. Particular emphasis is placed on the strategy of employing divinyl
cross-linkers to introduce branching points into polymer chains during the copolymerization procedures. By rational selection of initiator, monomer and divinyl cross-linker and
their polymerization sequence, star-like polymers with a cross-linked core but various arm
compositions and site-specific functionalities are formed. In contrast, concurrent copolymerization of both monomer and cross-linker generates “randomly” branched polymers
or gels. As compared to the conventional radical copolymerization procedures, the copolymerization of cross-linker in CRP processes shows retarded gelation behavior and produced
branched polymers and/or gels with more homogeneous structure and preserved chain-end
functionality. This is because of the fast initiation and quick reversible deactivation reactions in CRPs. Progress related to other synthetic strategies to introduce branching points
in polymer chains is also discussed, including the use of a multifunctional initiator, the use
of a multifunctional coupling agent (MCA) by click reactions, and the use of an AB* inimer.
© 2009 Elsevier Ltd. All rights reserved.
Contents
1.
2.
3.
Introduction and scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
General features of various CRP techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Synthesis of star polymers with a cross-linked core by sequential polymerization of monomer and cross-linker via
CRP techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.1.
Synthesis of star polymers by the “arm-first” method: polymerization of monomer before the addition of cross-linker
3.1.1.
Synthesis of star polymers with a cross-linked core by ATRP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.1.2.
Synthesis of star polymers with a cross-linked core by NMP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.1.3.
Synthesis of star polymers with a cross-linked core by RAFT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.1.4.
Synthesis of star polymers with a cross-linked core by other CRP techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.1.5.
Synthesis of miktoarm star copolymers by the “in-out” method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.1.6.
Synthesis of miktoarm star copolymers with a cross-linked core by a general “arm-first” method . . . . . . . . . . . .
∗ Corresponding author at: Department of Chemistry, Carnegie Mellon University, J.C. Warner University Professor of Natural Sciences, 4400 Fifth
Avenue, Pittsburgh, PA 15213, United States. Tel.: +1 412 268 3209; fax: +1 412 268 6897.
E-mail address: [email protected] (K. Matyjaszewski).
0079-6700/$ – see front matter © 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.progpolymsci.2009.01.001
318
319
321
322
322
325
325
325
326
326
318
H. Gao, K. Matyjaszewski / Progress in Polymer Science 34 (2009) 317–350
3.2.
4.
5.
6.
Synthesis of star polymers with a cross-linked core by a novel “core-first” method: polymerization of cross-linker before
the addition of monomer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 327
3.3.
Applications for star and miktoarm star copolymers with cross-linked cores (nanogels) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 328
3.3.1.
Application of star polymers with a cross-linked core as nanocontainers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 329
3.3.2.
Application of charged star polymers as building blocks for a multilayer film . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 330
Synthesis of branched polymers and/or gels by copolymerization of monomer and cross-linker via CRP techniques . . . . . . . . . . 330
4.1.
Flory–Stockmayer’s statistical theory: prediction of the gel point . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 331
4.2.
Differences in the gelation process between RP and CRP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 332
4.3.
Synthesis of branched polymers and gels via CRP techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 332
4.4.
CRP of cross-linker in dispersed media: synthesis of cross-linked microgels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 334
4.5.
Avoiding gelation in conventional RP by using chain transfer agent . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 334
Other strategies for introduction of branching points . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 335
5.1.
Introduction of branching points using an inimer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 335
5.2.
Introduction of branching points using multifunctional initiators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 336
5.2.1.
Synthesis of star and miktoarm star polymers by the “core-first” method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 336
5.2.2.
Synthesis of molecular brushes by the “grafting-from” method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 337
5.3.
Introduction of branching points using multifunctional coupling agents (MCAs) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 338
5.3.1.
Synthesis of star polymers by the “coupling-onto” method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 338
5.3.2.
Synthesis of brush/grafted copolymers by the “grafting-onto” method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 339
5.3.3.
Synthesis of model networks by using MCA in CRPs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 341
5.4.
Synthesis of brush/grafted copolymers using the “grafting-through” method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 341
Summary and outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 341
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 342
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 342
1. Introduction and scope
Since the mid-1990s, the field of polymer chemistry
has witnessed the explosive development of a number of procedures for conducting a controlled/“living”
radical polymerization (CRP) [1,2]. CRP allows the synthesis of various types of functional polymeric materials and provides the capability of designing polymers with controlled molecular weight and molecular weight distribution (MWD), in addition to controlled chemical composition, chain-sequence distribution,
site-specific functionality and predeterminable topology
[3–5].
One major effort, in the area of polymer topology,
focuses on the synthesis of well-defined macromolecules
with precisely controlled architecture by incorporating
site-specific branching points and functionalities [6]. CRP
techniques allow branching points to be introduced into a
macromolecule by three different strategies using: multifunctional initiators, multifunctional coupling agents and
multivinyl cross-linkers (especially divinyl cross-linkers).
Depending on the functionality, the number and relative arrangement of the branching points within the
macromolecule, polymers having a branched architecture
can be further classified as: star polymers, molecular
brushes/grafted polymers, randomly branched polymers,
hyperbranched polymers, dendrimers and gels (Scheme 1)
[7,8]. For example, an ideal star polymer contains an
n-functional branching point at the central core and n
emanating arms. Molecular brushes and/or densely grafted
copolymers contain many 3- or 4-functional branching
points, distributed along a linear backbone and, consequently, hundreds of side chains [9,10]. In randomly
branched polymers, the branching units are statistically distributed throughout the macromolecule, similar
to the structure of an insoluble gel, although the lat-
ter is a macroscopic network with “infinite” molecular
weight.
In this review, we will discuss strategies recently
developed for the synthesis of branched polymers with
controlled architecture and site-specific functionality.
The review will focus on the approach of employing divinyl cross-linkers to introduce branching points
(cross-linkages) into polymer chains during various controlled radical copolymerization (CRcP) procedures. Other
synthetic strategies, commonly used to introduce branching points in polymer chains during CRPs, include the
use of multifunctional initiator, multifunctional coupling
agent (MCA) or an AB* inimer (containing a double
bond A and initiator fragment B* in one molecule)
[11]. They will be briefly discussed in a separate section.
Rational selection of functional initiators, monomers
and/or divinyl cross-linkers for the copolymerization,
allows incorporation of a variety of functionalities into
the copolymer and the preparation of materials with
predetermined properties, such as degradability, biocompatibility and environmental sensitivity. The structure
of the copolymers can be varied by simply changing
the sequence of the polymerization of the monomer
and cross-linker. They will include soluble star-like polymers with a cross-linked core and linear radiating
arms, highly branched copolymers, as well as insoluble gels. Star-like polymers with a cross-linked core
are formed either when the monomer is polymerized
prior to addition of cross-linker, or if polymerization
of monomer occurs after cross-linker. Each approach
results in the formation of a star, but with different sitespecific functionality depending on the time of addition.
Concurrent copolymerization of monomer and crosslinker generates “randomly” branched polymers or gels
(Scheme 2).
H. Gao, K. Matyjaszewski / Progress in Polymer Science 34 (2009) 317–350
Nomenclature
AA
acrylic acid
ARGET activators regenerated by electron transfer
ATRcP
atom transfer radical copolymerization
ATRP
atom transfer radical polymerization
BA
n-butyl acrylate
tBA
tert-butyl acrylate
BMA
n-butyl methacrylate
bpy
2,2 -bipyridine
BiBEMI N-[2-(2-bromoisobutyryloxy)ethyl]maleimide
Conv
conversion
CRcP
controlled radical copolymerization
CRP
controlled/“living” radical polymerization
Cu
copper
DA
Diel–Alder
DMA
N,N-dimethylacrylamide
DMAEMA 2-(dimethylamino)ethyl methacrylate
DMF
N,N-dimethylformamide
DOTA
1,4,7,10-tetraazacyclododecane-1,4,7tris(acetic acid-t-butyl ester)-10-acetic
acid
DP
degree of polymerization
DB
degree of branching
DVB
divinylbenzene
EBrP
ethyl 2-bromopropionate
EGDA
ethylene glycol diacrylate
EGDMA ethylene glycol dimethacrylate
F–S
Flory–Stockmayer
GPC
gel permeation chromatography
HEMA 2-hydroxyethyl methacrylate
kact
activation rate constant in ATRP
deactivation rate constant in ATRP
kdeact
kd
dissociation rate constant
kexchange degenerative chain transfer rate constant
kp
propagation (polymerization) rate constant
termination rate coefficient
kt
LbL
layer-by-layer
LCCC
liquid chromatography under critical conditions
M
monomer
MA
methyl acrylate
MCA
multifunctional coupling agent
Me6 TREN tris[2-(N,N-dimethylamino)ethyl]amine
MI
macroinitiator
MM
macromonomer
MMA
methyl methacrylate
Mn
number-average molecular weight
Mw
weight-average molecular weight
MWD
molecular weight distribution
NAS
N-acryloxysuccinimide
NMP
nitroxide mediated polymerization
NMR
nuclear magnetic resonance
[PC]
primary chain concentration
PCL
poly(␧-caprolactone)
PDMS
poly(dimethylsiloxane)
PEO
poly(ethylene oxide)
PEOMA poly(ethylene
oxide)
methyl
ether
methacrylate
PMDETA
PPh3
PRE
P-X
Py-Br
RAFT
RcP
ROP
RP
Ru
R-X
SCVCP
SCVP
SS
St
TEMPO
UCST
X
319
N,N,N ,N ,N -pentamethyldiethylenetriamine
triphenylphosphine
persistent radical effect
halogen-terminated polymer chain
1-pyrenebutyl 2-bromoisobutyrate
reversible addition-fragmentation chain
transfer
radical copolymerization
ring-opening polymerization
conventional radical polymerization
ruthenium
alkyl halide
self-condensing vinyl copolymerization
self-condensing vinyl polymerization
bis(methacryloyloxyethyl) disulfide
styrene
tetramethyl piperidyl-N-oxyl
upper critical solution temperature
cross-linker
2. General features of various CRP techniques
In the current market, nearly 50% of all commercial
synthetic polymers are produced via conventional radical
polymerization (RP) processes [8,12]. The widespread use
of RP for polymer synthesis is largely due to its versatility, synthetic ease, and compatibility with a wide variety
of functional groups, coupled with its tolerance to water
and protic media. As any chain-growth polymerization,
RP comprises four elementary reactions: initiation, propagation, transfer and termination. In the absence of any
mediating reagent, the radicals are usually generated via
thermal decomposition of initiators and quickly polymerize
vinyl monomers, via a chain-building propagation reaction. It is followed by bimolecular radical–radical coupling
or disproportionation termination and transfer reactions
[13]. The slow and continuous initiation process used in
a conventional RP results in formation of polymers with
a broad MWD and does not provide a means to control
molecular structure. Moreover, the continuous termination
reactions in RP lead to nearly all of the polymer chains
being “dead” at any given instant, i.e., without capability for further chain extension. Therefore, in conventional
Scheme 1. Illustration of branched polymers with various topologies.
320
H. Gao, K. Matyjaszewski / Progress in Polymer Science 34 (2009) 317–350
Scheme 2. Influence of the sequence of addition and incorporation of cross-linker (X) and monomer on polymer architectures during atom transfer radical
copolymerization (ATRcP) of monomer (M) and cross-linker using R-Br as initiator.
RP it is essentially impossible to prepare polymers with
predetermined molecular weight and/or polymers with
well-defined microstructures, such as block copolymers
and gradient copolymers.
On the other hand, since the introduction of the concept of “living” polymers in the 1950s [14], well-defined
polymers with uniform size, desired functionality and various architectures have been prepared using living ionic
polymerization techniques. However, ionic polymerization techniques [2,7,15], such as anionic and cationic, are
not suitable for the (co)polymerization of a wide range
of functional vinyl monomers, mainly due to the incompatibility of the growing polymer chain ends (anions or
cations) with numerous functional groups and certain families of monomers. In addition, these ionic polymerization
techniques require stringent reaction conditions, such as
ultrapure regents and complete exclusion of water and air
[16].
Synthetic polymer chemists sought to develop new
polymerization strategies to overcome these limitations
and combine the chemical robustness of conventional RP
with a high level of control over polymer composition and
architecture. Consequently, the field of CRP has witnessed
an explosive growth in terms of both synthetic possibilities
and mechanistic understanding [1,2].
All CRPs proceed through the same radical mechanism
and the same radical intermediates as conventional RP.
They exhibit similar chemo-, regio- and stereo-selectivities,
and can copolymerize a similar range of monomers [2].
However, in contrast to conventional RP, the fundamental features of CRPs include fast initiation and a dynamic
equilibrium between a low concentration of propagating radicals and a large amount of dormant reactivatable
species. This concept originates from “living” cationic
polymerization and minimizes the contribution of chain
breaking reactions and provides structural control over the
polymers.
In CRP, the fast initiation reactions, relative to propagation reactions, result in all polymer chains undergoing
initiation at approximately the same time and a nearly
constant number of chains growing throughout the polymerization, which enables control over chain architecture.
The dynamic equilibrium, a fast exchange reaction between
a low concentration of propagating radicals and a large
amount of dormant species, leads to a fast deactivation of
growing radicals before less than a few monomer units are
added to the chain end. The lifetime of growing chains is
extended from ∼1 s in conventional RP to more than 1 h in
CRP due to the intermittent reversible activation of the dor-
mant species. The proportion of terminated chains in CRP
is much lower than in RP (≤10% vs. ∼100%), which ultimately enables control over chain-end functionality and
chain architecture [2].
Three different mechanisms of intermittent activation are employed in CRPs. They include: dissociation–
combination, represented by nitroxide mediated polymerization (NMP) [17,18] or organometallic radical polymerization [19,20]; catalytic atom (group) transfer, represented
by atom transfer radical polymerization (ATRP) [21–25];
and degenerative chain transfer, represented by iodine
mediated polymerization [26–29] or reversible additionfragmentation chain transfer (RAFT) [30–32] polymerization. During these CRP processes, the active radicals either
undergo a reversible activation/deactivation process (i.e.,
NMP and ATRP), or participate in a degenerative transfer
reaction (e.g., RAFT) to assure simultaneous growth of all
chains.
In NMP process, the dormant species is cleaved by a
thermal or photochemical stimulus to produce the stable
free radical and the active propagating radical (Scheme 3A).
The ATRP process is kinetically similar to NMP, except
that the activation process includes the participation of
both a dormant species and a catalyst-based activator, a
lower oxidation state metal complex. The higher oxidation
state metal complex formed in this activation procedure
functions as the deactivator (Scheme 3B). Both NMP and
ATRP are controlled by the persistent radical effect (PRE)
[33–35], which describes the procedure for self-regulation
of the concentration of active radicals. In other words,
every radical–radical termination leads to an irreversible
accumulation of deactivator, which shifts the equilibrium
towards the dormant species and consequently decreases
the probability of termination reactions. The persistent radicals can be deliberately added to the reaction to increase
the initiation efficiency and reduce the termination reactions that occurred during the initial nonstationary stage.
The difference between NMP and ATRP is that stoichiometric amount of mediating agent (e.g., nitroxide) is required to
cap all dormant chains in the NMP system. In contrast, the
amount of transition metal catalyst added to an ATRP can
be sub-stoichiometric because the catalytic process in ATRP
employs an atom (or group) as capping group, which transfers between growing chains and a redox active catalyst
[36–41].
The RAFT process is a degenerative chain transfer reaction and is not based on the PRE (Scheme 3C). Its overall
kinetics and polymerization rate resemble a conventional
RP process with slow initiation and fast termination reac-
H. Gao, K. Matyjaszewski / Progress in Polymer Science 34 (2009) 317–350
321
Scheme 3. General schemes of NMP, ATRP and RAFT polymerization processes.
tions [34]. However the chain transfer agent employed to
provide control, such as dithioester or xanthate, is present
at a much higher concentration than the radical initiator
and quickly exchanges a group/atom among all growing
chains. Thus, the transfer agent plays the role of the dormant species to provide control over molecular weights and
polydispersity.
The ability of CRP techniques to control molecular
weight and polydispersity and to provide access to welldefined molecular architecture, originates from both fast
initiation of all chains and limitation of the chain growth
during each activation cycle to a level where the contribution of chain breaking reactions is negligible. Since the
invention of these various CRP techniques, they have been
constantly improved and applied to the preparation of welldefined polymers with controlled chemical compositions,
molecular weights and MWDs, chain-sequence distributions, functionalities and topologies.
3. Synthesis of star polymers with a cross-linked
core by sequential polymerization of monomer and
cross-linker via CRP techniques
Among the spectrum of polymers with complex architectures, star polymers provide one of the simplest
arrangement of linked chains, because a structurally welldefined star polymer contains only one central branching
point, the core of the star, and multiple emanating
arms. The study of star polymers provides an insight
to understand the fundamental structure–property relationship of branched polymeric chains in solution and/or
melt state [42,43]. Moreover, star polymers possess several distinct features, including globular shape, core–shell
microstructure with multiple radiating arms and chain-end
functionalities. In many cases, star polymers can be considered to be analogs of dendrimers. Star polymers have a
wide range of potential applications in drug delivery, membranes, coatings and lithography [44–47], but with much
lower synthetic cost than dendrimers.
Based on the chemical compositions of the arm
species, star polymers can be classified into two categories (Scheme 4): homoarm (or regular) star polymer and
miktoarm (or heteroarm) star copolymer [7,48]. Homoarm
star polymers consist of a symmetric structure comprising
radiating arms with similar molecular weight and identical chemical composition. In contrast, a miktoarm star
molecule contains two or more arm species with different
chemical compositions and/or molecular weights [48]. Star
polymers are usually synthesized via one of three common
strategies: “core-first” by growing arms from a multifunctional initiator (Section 5.2.1); “coupling-onto” by attaching
linear arm precursors onto a multifunctional core (Section
5.3.1) and the “arm-first” method by cross-linking preformed linear arm precursors using a divinyl compound.
These three synthetic strategies are different from each
other based on the sequence of the formation of the core
and the arms.
Scheme 4. Categories of star polymers.
322
H. Gao, K. Matyjaszewski / Progress in Polymer Science 34 (2009) 317–350
Scheme 5. Synthesis of star polymers with a cross-linked core via ATRP using the “arm-first” method.
3.1. Synthesis of star polymers by the “arm-first” method:
polymerization of monomer before the addition of
cross-linker
The three most popular CRP techniques, e.g., ATRP,
NMP and RAFT polymerization, have been applied to
the synthesis of star polymers with a cross-linked core
by cross-linking reactive linear chains using a divinyl
cross-linker. Since the formation of linear chains (arm
precursors) by polymerization of monovinyl monomer is
essentially complete before the formation of the crosslinked core via polymerization of cross-linker, this method
is strategically termed as the “arm-first” route for the
synthesis of star polymers. The “arm-first” method was
first developed in the context of anionic polymerization [49,50]. This approach has been later extensively
employed using different CRP methods for the synthesis of various functional star polymers, because of the
easy experimental setup and broad range of monomers in
CRP.
The addition of a cross-linker to a solution containing
linear macroinitiator (MI) with reactivatable chain-end initiating site initially generates pendant vinyl groups during
the polymerization of cross-linker from the linear chain.
The highly cross-linked core is formed through intermolecular reactions between the chain-end radicals and the
pendant double bonds. It produces a star polymer with a
statistical distribution of the number of incorporated arms
(Scheme 5). Furthermore, star–star coupling reactions concomitantly occur, increasing the star molecular weights and
leading to a broader MWD for the obtained star molecules.
The average number of arms attached to a star core depends
on several experimental parameters, including the degree
of polymerization (DP) and composition of the arm precursor, the chemical nature of cross-linker, the amount and
the addition moment of cross-linker. Incomplete incorporation of linear arm precursors into the formed star is a
common problem in this “arm-first” method, which could
be explained by the loss of chain-end initiating sites or a
buildup of steric hindrance around the core, as the coupling
reactions proceed [51].
3.1.1. Synthesis of star polymers with a cross-linked core
by ATRP
3.1.1.1. Linear macroinitiator as arm precursor (MI method).
The first synthesis of star polymers with a cross-linked core
by ATRP was reported in 1999. Polystyrene-based linear MIs
containing bromine chain-end functionality were crosslinked by using various divinyl cross-linkers in anisole at
110 ◦ C [52]. The structure of the resulting star polymer could
be denoted as (polySt)n -polyX, where polyX represents the
core of the star polymer and n is the average number of
polySt arms per star molecule. The use of divinylbenzene
(DVB, 1 in Scheme 6) led to the formation of star polymers
with the best controlled structure, as compared to other
cross-linkers, such as ethylene glycol diacrylate (EGDA, 2
in Scheme 6) and ethylene glycol dimethacrylate (EGDMA,
3 in Scheme 6). A molar ratio of DVB to polySt MI between
5 and 15 was found optimal for the formation of stars with
fairly high star yield but the star product was contaminated with residual linear chains and exhibited broad MWD
due to star–star coupling reactions. Following a similar
route, chain-end functionalized (polytBA)n -polyDVB star
polymers were synthesized through the use of functional
ATRP initiators for the synthesis of linear poly(tert-butyl
acrylate) MIs [53]. Various functional groups, e.g., epoxy,
amino, cyano or bromo, were introduced into the chain end
of each arm, the periphery of the formed star. The prepared
(polytBA)n -polyDVB star polymers could subsequently be
functionalized by hydrolysis of the tert-butyl groups to prepare (polyAA)n -polyDVB stars with polyelectrolyte arms
(AA: acrylic acid) [54].
Instead of isolating and purifying the linear polytBA
MIs, cross-linker could be added to the polymerizing system at certain tBA conversion to produce higher yield star
polymers with a cross-linked core in a one-pot reaction
(Scheme 7) [55]. The timing of addition of the subsequent DVB at different tBA conversions significantly
affected the structure of star polymers formed in these
reactions. For instance, by keeping the initial molar ratio
of [tBA]0 /[EBrP]0 = 50/1 constant, earlier addition of DVB
resulted in formation of a shorter polytBA arm precursor and consequently more tBA monomer remained for
copolymerization with DVB. This produced star polymers
with looser core, more arms per star molecule, and broader
MWD.
Sawamoto also applied the “arm-first” approach to the
synthesis of star polymers containing polymethacrylate
arms using ruthenium (Ru)-based catalyst complexes and
various divinyl cross-linkers, such as dimethacrylate- and
dimethacrylamide-based compounds (4–9 in Scheme 6)
[56–58]. In addition to investigating the influence of experimental parameters on the structures of the star polymers,
they reported the synthesis of functionalized star polymers
by introducing various functionalities into the star core
[59,60] and the star periphery [61].
After this pioneering work on the synthesis of star
polymers with a cross-linked core by using ATRP, recent
developments in this area mainly focused on two aspects
[62–64]: (1) exploration of new synthetic methodologies
to achieve better structural control over the topology of the
star polymers and (2) introduction of various site-specific
H. Gao, K. Matyjaszewski / Progress in Polymer Science 34 (2009) 317–350
323
Scheme 6. Various divinyl cross-linkers used for star synthesis in CRPs.
functionalities (e.g., targeting, imaging and biocompatible groups) into the star core, arm and periphery. For
example, use of a functional cross-linker and/or functional
comonomer in the star core formation step demonstrated
the ease of successfully encapsulating functional groups
into the star core, such as fullerene [65], and fluorophore
(10 in Scheme 6) [66,67]. Core degradable star polymers
were synthesized by using degradable cross-linker containing disulfide group, 11 in Scheme 6 [68], acetal group, 12
in Scheme 6 [69,70], or siloxane group, 13 in Scheme 6
[71], as the linker. When a functional initiator was used
to synthesize the linear MI, various types of functionalities were introduced onto the star periphery. Examples
include dendron groups [72], benzophenone groups [73],
and oligomeric poly(ethylene oxide) (PEO) [74].
Moreover, the choice of functional monomers provides an unparallel number of options to introduce a
variety of functional groups into the star arms, which
can tune the star property to satisfy the requirements
of many specific applications. Polyester-based polymers
have attracted significant attention because of the facile
hydrolytic degradation of the ester linkage. In particular,
poly(␧-caprolactone) (PCL) is a biodegradable and biocompatible material with the degraded product being capable
of absorption by the body with minimal tissue reaction
[75]. The incorporation of PCL arms into star polymers with
324
H. Gao, K. Matyjaszewski / Progress in Polymer Science 34 (2009) 317–350
Scheme 7. Synthesis of (polytBA)n -poly(DVB-co-tBA) star polymers via
ATRP using the “arm-first” method in a one-pot process [55]. Reproduced
with permission from American Chemical Society.
a cross-linked core has been achieved through chain-end
functionalization of linear PCL with an alkyl halide group,
followed by chain extension with divinyl cross-linker using
ATRP [76,77]. The alkyl halide functionalized PCL MI could
be alternatively synthesized by using a halogen-containing
alcohol to initiate the anionic ring-opening polymerization
(ROP) of ␧-caprolactone monomer [78].
It is interesting to note that cross-linking a monofunctional linear MI generates star polymers with a cross-linked
core, while cross-linking a difunctional linear MI could
produce a dumbbell-structured nanoobject [79] or model
network [80,81], depending on the solvent quality and the
ratio of monovinyl monomer to divinyl cross-linker during
the core formation process.
3.1.1.2. Linear macromonomer as arm precursor (MM
method). A major drawback to star synthesis using linear
MI as the arm precursor is that the star polymers usually have a broad MWD due to the significant level of
star–star coupling reactions. Furthermore, caution has to
be taken in order to avoid macroscopic gelation when too
much star–star coupling occurs. Star–star coupling reactions can be decreased by using less divinyl cross-linker,
e.g., lower molar ratio of cross-linker to arm precursor,
and/or conducting the reaction under dilute solution conditions, although the molecular weight and the yield of
the obtained star molecules decrease significantly [52,58].
Moreover, the final star product formed via cross-linking
the linear MIs is often contaminated by the presence of
residual unincorporated linear polymers. This requires an
extra purification step in order to obtain a star polymer with
higher purity and narrower MWD.
As noted above, star–star coupling reactions can occur
via two possible routes: a radical–radical reaction or a
radical-pendant vinyl group reaction between two star
molecules. Since both reactions require the participation of
radicals within the star core, a rational experimental design
to decrease the molar ratio of initiating sites to arms per star
could reduce the star–star coupling process and increase
star uniformity. However, when linear MIs are used as arm
precursors, both initiating sites (dormant form of radicals)
and arms in the star molecule originate from the MIs, resulting, by default, in an identical number of arms and initiation
sites in each star [55,82].
A recently developed strategy used linear MM, instead
of MI, as the arm precursor for the synthesis of lowpolydispersity star polymers [83]. The biggest advantage
of using a linear MM is that the number of initiating sites and arms can be independently controlled,
since they are derived separately from the initiator
and the MM. The incorporation of linear MM into
the star molecule only increases the averaged number of arms per star, rather than changing the number
of initiating sites. The number of initiating sites in
the star core could be decreased, simply by decreasing the molar ratio of low-molar-mass initiator to MM.
This effectively limits the extent of star–star coupling
reactions and results in star polymers with low polydispersity.
When star polymers are formed via copolymerization
of linear MM and divinyl cross-linker using a lowmolar-mass ATRP initiator, the residue of the initiator
is incorporated into the star core segment. Therefore,
different functional groups could be readily introduced
into the star core through use of functional ATRP initiators (Scheme 8) [84]. Compared to the strategy of
using a functional comonomer to introduce star core
functionality in the MI method, the use of functional initiator does not lead to a higher polydispersity of the
obtained star polymers. For instance, a pyrene-containing
ATRP initiator (Py-Br) was used for the copolymerization of poly(n-butyl acrylate) (polyBA) MM and DVB.
The core-functionalized star polymers showed strong UV
absorption between 330 and 360 nm and high pyrene
encapsulation efficiency (ca. 80%). When PEO methyl ether
methacrylate MM was used for star synthesis via ATRcP
with EGDMA, amphiphilic (PEO)n -polyEGDMA(pyrene)
star polymers with a hydrophobic core and hydrophilic
PEO arms were synthesized. The functional stars showed
high solubility in water and strong UV absorption due
to the incorporation of pyrene groups into the star
core.
The incorporation of linear MMs into the preformed star
polymer increased the star yield but kept a low polydispersity of the resulted stars. The star polymer continued to
grow until the core was fully covered by the linear arms and
reached a steric saturation state, when further star growth
stopped. Addition of another batch of cross-linker and ATRP
initiator at this stage introduced more pendant vinyl groups
and initiating sites to the star core, expanding its size and
functionality. This expansion decreased core congestion
and made further incorporation of linear chain into star
polymer possible. With appropriate amounts of additional
cross-linker and ATRP initiator, it is possible to conduct
star-linear MM reactions with limited star–star reactions.
Therefore, the newly added cross-linker and ATRP initiator
increased the star yield and star molecular weight while
avoiding broadening of MWD. Star-linear MM reactions
stopped when the star polymer reached its new saturated
size, but the addition of a second batch of cross-linker
and ATRP initiator expanded the core and allowed further
star growth. This process could be repeated until the star
yield essentially reaches 100% incorporation of the initially
added MM (Fig. 1) [83].
This novel MM method could be extended as a general method to conventional RP [85,86] and other CRP
techniques for the synthesis of star polymers with high
star yield, high molecular weight and low polydispersity, although the synthesis of linear MMs [87–89] is not
as straightforward or as easy as the synthesis of linear
MIs.
H. Gao, K. Matyjaszewski / Progress in Polymer Science 34 (2009) 317–350
325
Scheme 8. Synthesis of core-functionalized star polymers via ATRP using MM method [84]. Reproduced with permission from American Chemical Society.
3.1.2. Synthesis of star polymers with a cross-linked core
by NMP
Stable free radical polymerization, specifically NMP,
was also applied to the synthesis of star polymers with
a cross-linked core by the “arm-first” approach. Solomon
and coworkers [90] first reported the synthesis of poly(4tert-butylstyrene) star polymers by employing tetramethyl
piperidyl-N-oxyl (TEMPO) as the persistent stable radical
to mediate the NMP of 4-tert-butylstyrene and subsequent
cross-linking reaction using DVB as cross-linker. In the early
stages of NMP, linear polySt MI was most frequently used
for the synthesis of (polySt)n -polyDVB star polymers with
a cross-linked core due to the lack of powerful nitroxide
mediating radicals for the polymerization of acrylate and
methacrylate monomers. However, the optional ratios of
polySt MI to DVB for obtaining high star yield significantly
varied in different reports due to their applied conditions.
Pasquale and Long [91] observed that in order to obtain efficient star formation (star yield ∼ 70%, Mw /Mn ∼ 3.0) with a
polySt MI (Mn = 19,300 g/mol) and m-xylene as solvent, a
high molar ratio of [DVB]0 /[polySt MI]0 = 68 (1/2 by weight)
was required. In contrast, Hadjichristidis and coworkers
[92] employed a much lower molar ratio of [DVB]0 /[polySt
MI]0 = 13 with a polySt MI (Mn = 10,000 g/mol) to obtain
a (polySt)n -polyDVB star polymer in 75% star yield and
polydispersity Mw /Mn = 1.56 by using benzene as solvent at 125 ◦ C. Use of a glycol-conjugated TEMPO-based
Fig. 1. Influence of several-step addition of DVB and EBrP on GPC
traces of (polyBA)n -polyDVB star polymers in the MM method; experimental conditions: [polyBA MM (DP = 42)]0 /[EBrP]0 /[DVB]0 /[CuBr]0 /
[Me6 TREN]0 = 1/(0.07 + 0.07 × 4)/(3 + 1 × 4)/0.2/0.2, [MM]0 = 0.06 M, in
anisole at 80 ◦ C [83]. Reproduced with permission from American Chemical
Society.
alkoxyamine or St-derived functional comonomer synthesized polySt star polymers carrying functionalities in the
core [93] and at the periphery [94], respectively.
Initially, the strong covalent bond in a TEMPO-based
alkoxyamine impeded the structural control of the star
polymers and limited composition of the arms to polySt
and its derivatives. With the development of a more active
second-generation ␣-hydrido-based alkoxyamine, Hawker
expanded utility of NMP to allow the synthesis of a series
of star polymers with a cross-linked core and a variety of
arm compositions by using different monomers, including
St, acrylate, vinyl pyridine, methacrylate and acrylamide
[95,96]. Furthermore, they used combinatorial techniques
for high throughput star synthesis and screened the key
experimental parameters to optimize control over star
structures. Functional groups could be introduced onto
star periphery and along the arms by using functionalized
alkoxyamines and functional monomers, respectively.
3.1.3. Synthesis of star polymers with a cross-linked core
by RAFT
Compared to the broad application of ATRP and NMP for
the synthesis of functional star polymers using the “armfirst” method, only limited success has been obtained with
RAFT polymerizations. Moad first proposed the possibility of using the “arm-first” method in RAFT polymerization
for the synthesis of star polymers with a cross-linked core
[97]. The first experimental proof of the synthesis of star
polymers with a cross-linked core by RAFT was reported
by Davis and coworkers [98] although the synthesized
(polySt)n -polyDVB star polymers were poorly controlled
with low star yield and high polydispersity. Zheng and
Pan [99] reported the synthesis of (polySt)n -polyDVB star
polymers containing a cross-linked nodule by using benzyl dithiobenzoate as RAFT agent. The use of a comonomer
during the core formation process and the appropriate
selection of solvent, could favor micelle formation during
the cross-linking of linear MIs, which improved both star
formation and star yield [100,101].
3.1.4. Synthesis of star polymers with a cross-linked core
by other CRP techniques
Compared to the success with the three most robust CRP
techniques, e.g., ATRP, NMP and RAFT, the other types of CRP
techniques are less successful in star polymer synthesis.
Iniferter [102,103], degenerative chain transfer mediated
by iodine [26–29,104] or other groups [105–107], and transition metal-mediated stable free radical polymerization
[19,20,108], are still mainly focused on optimizing polymer-
326
H. Gao, K. Matyjaszewski / Progress in Polymer Science 34 (2009) 317–350
ization conditions to achieve the synthesis of linear block
copolymers with various monomers, well-defined structure and high chain-end functionality. Ishizu et al. [109]
reported the synthesis of star block copolymers with a
cross-linked core using dithiocarbamate iniferter under UV
irradiation. The arm composition could be polyacrylate or
polymethacrylate. By varying the molar ratio of linear MI
to EGDMA cross-linker, the highest star yield obtained was
lower than 60%. The low star yield was possibly due to
the slow and incomplete initiation of alkyl dithiocarbamate
under UV irradiation and loss of chain-end functionality
via the potential dimerization of dithiocarbamyl mediating
radicals.
3.1.5. Synthesis of miktoarm star copolymers by the
“in-out” method
Miktoarm star copolymers contain two or more arm
species with different chemical compositions and/or
molecular weights connected to one central core [48].
Therefore, miktoarm star molecule combines the features
of segmented block copolymer, such as microphase separation and self-assembly capacity, and the features of globular
shape and multiple dangling chains within the concept
of star polymer. Miktoarm star copolymers have shown
interesting properties, such as microphase separations in
bulk [110–112], in solution [113,114], at interfaces [115],
and segregated compartmentalization for guest molecule
encapsulation [113]. However, compared to the synthesis
of homoarm star polymers, the synthesis of miktoarm star
copolymers is more difficult and consequently their preparation is described in fewer reports.
Among the methodologies developed for the synthesis
of miktoarm star copolymers using CRP techniques, the “inout” method [68,76,92] represents an important strategy
for the synthesis of miktoarm star copolymers that contain
two kinds of arms with different chemical compositions
tethered onto one cross-linked core. Star polymers, synthesized by the “arm-first” method, preserve the dormant
initiating sites in the cross-linked core and can be used as
multifunctional star MIs to initiate the polymerization of
another monomer and form miktoarm star copolymers. The
word “in” refers to the “arm-first” method for formation of
the star MI and the word “out” represents the subsequent
growth of the second generation of arms from the multifunctional star core (Scheme 9). NMP and ATRP techniques
were applied to the synthesis of miktoarm stars using the
“in-out” method. Based on the sequence of the arms and
core formation, the obtained miktoarm star copolymer is
coded as (polyA)n -polyX-(polyB)p , where polyX represents
the cross-linked core of the miktoarm star polymer; n and
p are the average number of polyA and polyB arms per star
molecule, respectively.
The “in-out” strategy was first developed for miktoarm star synthesis using anionic polymerization. In that
case, the number of the second generation of arms was
assumed to be the same as that of the first arms because
the electrostatic repulsions between two carbanions prevent termination by coupling [48]. When this “in-out”
method was extended to CRPs, the radical–radical coupling becomes an inevitable side reaction, decreasing
the number of effective initiating sites in the star MI.
Moreover, the preserved initiating sites (alkyl halide or
alkoxyamine) in the star MI are distributed throughout
the highly cross-linked core, surrounded by multiple arms.
Due to the congested environment, not all of the initiating
sites embedded in the core are accessible to the catalyst
and monomer and can participate in the formation of the
second generation of arms. This limited accessibility of
initiating sites is often encountered during CRP using a
multifunctional MI with a congested structure [116]. This
results in an incomplete initiation efficiency of the star MI
during the synthesis of miktoarm star copolymers using the
“in-out” method.
The initiation efficiency of the star MI was quantitatively determined by two different methods. The first
one employed a disulfide-containing cross-linker (SS), 11
in Scheme 6, for the synthesis of (polyMMA)n -polySS(polyBA)p miktoarm star copolymers containing a cleavable
core [68]. The initiation efficiency of the star MI was determined via gel permeation chromatography (GPC) analysis
of the cleaved product of the miktoarm star copolymer,
which was a mixture of a linear triblock copolymer and a
linear diblock copolymer. The result indicated that only ca.
20% of the initiating sites in the (polyMMA)n -polySS star
MIs participated in the formation of the second generation
of polyBA polymer arms.
The second method employed a general kinetic analysis for determination of the initiation efficiency of the
(polytBA)n -poly(DVB-co-tBA) star MIs. It compared the relative rates of monomer consumption during the formation
of the second generation of arms to a control ATRP reaction under the same experimental conditions, except for
the use of a small monofunctional initiator [117]. Three factors, including the arm length of the star MI, the structural
compactness of the star MI and the chemical compatibility
of the two kinds of arms on one miktoarm star molecule,
showed a significant effect on the initiation efficiency of
the star MI. It was found that the initiation efficiency of the
star MI decreased with increasing arm length and structural
compactness. When the two arms had the same chemical composition, the star MI had the highest initiation
efficiency (∼54%), indicating the number of the second generation of arms was as high as half of the number of the first
arms [117].
Miktoarm star copolymers, synthesized by the “in-out”
method using CRP, comprise of various arm compositions including polySt, polyacrylate and polymethacrylate,
depending on the choice of monomers. The composition of
the first generation of arms, formed before the formation of
the core of the star MI, can be expanded to include polyether
and/or polyester, such as PEO [118] and PCL [76,77], synthesized by ROP and subsequent chain-end modification to
introduce CRP initiating sites. The incorporation of these
arms into the final miktoarm star could either improve
biocompatibility or provide a degradable feature to the
miktoarm star product and expands their potential applications to include drug delivery vehicles [64].
3.1.6. Synthesis of miktoarm star copolymers with a
cross-linked core by a general “arm-first” method
As discussed above, although the “in-out” route represents an important methodology for the synthesis of
H. Gao, K. Matyjaszewski / Progress in Polymer Science 34 (2009) 317–350
327
Scheme 9. Illustration of the “in-out” method for synthesis of miktoarm star copolymer.
miktoarm star copolymers containing two kinds of arms
with different chemical compositions, the number of the
second generation of arms was always lower than the number of the first arms as a consequence of the incomplete
initiation of the star MI. Moreover, it is conceptually impossible to synthesize a miktoarm star copolymer containing
more than two kinds of arms using the “in-out” method.
A recent report directly applied the traditional “armfirst” method for the synthesis of miktoarm star copolymers
via one-pot cross-linking of a mixture of linear MIs [119].
The composition of the arms in the miktoarm star product
was systematically varied by simply changing the molar
ratios of the initial MIs. An illustrative ATRP experiment
includes a cross-linking reaction of two types of linear
MIs, polyBA-Br MI and poly(methyl acrylate)-Br (polyMABr) MI, using DVB as cross-linker. It produced a miktoarm
star product with high molecular weight and high star
yield. During this process, the two arms were expected
to be concurrently incorporated into one miktoarm star
molecule (Scheme 10). Thus, the produced star polymer is
coded as (polyBA)n -(polyMA)p -polyDVB, which is different
from the miktoarm star formed by the “in-out” method,
(polyA)n -polyX-(polyB)p , where the two types of arms are
sequentially incorporated into the miktoarm star molecule.
It was critical to determine that the star product from
the one-pot cross-linking of a mixture of arm precursors was a miktoarm star copolymer, containing two or
more arm species in one molecule, rather than a mixture of homoarm stars. Traditional GPC separates polymers
based on hydrodynamic volume, which cannot differentiate miktoarm stars from corresponding homoarm stars
with similar size (Fig. 2). In order to determine the distribution of the two kinds of arms in the miktoarm star
copolymer, a liquid chromatography technique, which separates polymers based on their chemical compositions,
was employed. For instance, the liquid chromatography
Scheme 10. Synthesis of miktoarm star copolymers by cross-linking two
kinds of linear MIs via the “arm-first” method using ATRP [119]. Reproduced
with permission from American Chemical Society.
under the critical conditions (LCCC) of polyMA homopolymer [120,121] was applied to the characterization of the
(polyBA)n -(polyMA)p -polyDVB miktoarm star product and
the two corresponding homoarm stars, (polyBA)n -polyDVB
and (polyMA)n -polyDVB. The elution chromatogram of the
miktoarm star product showed one elution peak between
the peaks of the two homoarm stars, indicating the absence
of homoarm star contaminants in the miktoarm star
copolymer.
Similar to the successful synthesis of low-polydispersity
star polymers using linear MM as the arm precursor
[83], cross-linking a mixture of linear MMs using a lowmolar-mass initiator, such as an alkyl halide, produced
low-polydispersity miktoarm star copolymers with high
star yield. A lower molar ratio of initiator to total MMs
decreased the number of initiating sites in the star core,
which effectively limited the extent of star–star coupling
reactions [122]. Addition of extra cross-linker and initiator increased the molecular weight and the yield of the
miktoarm star molecules while preserving their low polydispersity.
3.2. Synthesis of star polymers with a cross-linked core
by a novel “core-first” method: polymerization of
cross-linker before the addition of monomer
The “arm-first” method for the synthesis of star and
miktoarm star copolymers requires the formation of linear
arm precursors, either MI or MM, prior to the addition of
a divinyl cross-linker. In other words, the polymerization
of monovinyl monomer from a monofunctional initiator,
forming the arm precursor, is before the polymerization of
divinyl cross-linker, which forms the cross-linked core. An
alternative strategy for the synthesis of star polymers with a
cross-linked core is polymerization of the cross-linker prior
to addition of the monovinyl monomer (Scheme 11). The
homopolymerization of a divinyl cross-linker (e.g., EGDA)
by using ATRP under dilute conditions produced multifunctional nanogels containing a statistical distribution of
initiating sites. A monovinyl monomer (M1 ) was added to
the system, at high conversion of cross-linker, and was polymerized from the multifunctional nanogel MI to form the
polyX-(polyM1 -Br)n star polymer, where n is the average
number of polyM1 arms per star molecule. Since a highly
cross-linked core is formed by homopolymerization of the
cross-linker before the growth of emanating arms, this synthetic strategy belongs to the “core-first” method of star
synthesis [123,124].
328
H. Gao, K. Matyjaszewski / Progress in Polymer Science 34 (2009) 317–350
Fig. 2. (A) GPC traces and (B) LCCC chromatograms of (polyBA)n -polyDVB, (polyMA)n -polyDVB homoarm stars and (polyBA)n -(polyMA)p -polyDVB miktoarm
star copolymers; LCCC conditions for polyMA homopolymers: two sets of silica columns with pore size of 100 and 300 Å, respectively, mobile phase as
2-butanone/cyclohexane (86/14 by volume) with flow rate as 0.5 mL/min at 32 ◦ C [119]. Reproduced with permission from American Chemical Society.
The molecular weight of the star polymers increased
with the extent of polymerization of monovinyl monomer
from the cross-linked core. At the same time, a small
amount of linear polymers coexisted with the star polymer that originated from unincorporated monofunctional
initiating sites. The free linear chains served as an internal standard for calculation of the average number of arms
per star molecule: Narm = Mw,star /Mw,arm , where Mw,star and
Mw,arm are the weight-average molecular weights of star
polymer and linear reference chain, respectively. Due to
the broad size distribution of the multifunctional nanogel
cores, the apparent polydispersity of the star polymers
determined by GPC was around Mw /Mn = 2.0. The star polymers synthesized by this new “core-first” method had a
similar structural compactness to those from the traditional “arm-first” method. However, in contrast to the star
polymers formed by the “arm-first” method, which contained dormant initiating sites in the star core, the star
polymers produced by the new “core-first” method preserved the initiating sites at the chain ends, the periphery
of the star. Therefore, the chain extension of the star MI
by polymerization of a second monomer (M2 ) formed star
block copolymers, polyX-(polyM1 -b-polyM2 )n (Scheme 11)
[125].
A novel one-pot approach was also reported for the synthesis of star-like polymers with a highly branched core
by self-condensing vinyl copolymerization (SCVCP) [11]
of N-[2-(2-bromoisobutyryloxy)ethyl]maleimide (BiBEMI)
inimer with a large excess of St using ATRP [126]. Due to
the alternating copolymerization behavior of maleimide
and St [127], the BiBEMI inimer was quickly consumed
via an alternating copolymerization with St and formed
a hyperbranched core containing many alkyl bromide initiating sites. It further served as a multifunctional MI for
homopolymerization of the excessive St to produce a star
polymer (Scheme 12). The broad distribution of the hyperbranched core resulted in a less structure-controlled star
polymer (multimodal distribution). This might require an
extra step of purification to remove the linear polymers.
The feature of alternating copolymerization between
maleimide and St, was also utilized for a one-pot copolymerization of bismaleimide cross-linker and an excess
of St using a low-molar-mass-ATRP initiator, producing
polySt star polymers containing a highly branched core
[128]. Similarly, the iniferter technique was applied for a
one-pot copolymerization of dithiocarbamate-containing
St-based inimer, stoichiometric amount of bismaleimide
cross-linker and an excess of MMA monomer, resulting in a
polyMMA star polymer with a cross-linked core [129]. The
dithiocarbamate-capped polyMMA stars could be used for
arm extension by ATRP of n-butyl methacrylate (BMA) in
the presence of CuCl/bipyridine (bpy) catalyst to produce
star block copolymers.
3.3. Applications for star and miktoarm star copolymers
with cross-linked cores (nanogels)
The development of various robust radical-based synthetic methodologies facilitates the large-scale production
of star polymers with a cross-linked core at a relatively low
cost. It also paves the way for material scientists to study
their physical properties in solution and bulk [54,130] and
Scheme 11. Synthesis of star and star block copolymers by a novel “core-first” method: polymerization of cross-linker before the addition of monomer
[125]. Reproduced with permission from American Chemical Society.
H. Gao, K. Matyjaszewski / Progress in Polymer Science 34 (2009) 317–350
329
Scheme 12. Synthesis of star polymers in a one-pot reaction by ATRP of N-[2-(2-bromoisobutyryloxy)ethyl]maleimide and styrene [126]. Reproduced with
permission from American Chemical Society.
to explore their utility as functional nanomaterials in different fields. The star polymers with a cross-linked core have
a compact structure and globular shape, which resembles
a core/shell nanogel containing a cross-linked core, multiple radiating arms (shell) and chain-end (surface) groups
with a broad choice over composition, and site-specific
functionality. The incorporation of arms with different
compositions onto one cross-linked core, a miktoarm star,
further provides segmented structure to this novel nanoobject. All these structural features suggest that star polymers
with a cross-linked core are promising nanomaterials in a
variety of fields [6,64], such as nanocontainers for encapsulation of guest molecules or self-assembly into multilayer
porous films.
3.3.1. Application of star polymers with a cross-linked
core as nanocontainers
Star polymers with a cross-linked core and segmented
arms resemble a three-dimensional core/shell nanostructure, which can be used as a carrier, or container for
different guest cargos, such as catalysts, drugs, dyes,
fragrances and imaging radioactive atoms. Sawamoto
and coworkers [60,131] reported the ATRP synthesis of
(polyMMA)n -polyEGDMA star polymers containing Ru(II)triphenylphosphine complex within the cross-linked core.
Scheme 13. Synthesis of core-functionalized star polymers bearing Ru(II)
complex in the core [60]. Reproduced with permission from American Chemical Society.
A phosphine-containing functional comonomer was used
during the core formation process, which served as a ligating site to in situ encapsulate the Ru(II) atoms into
the cross-linked core via ligand exchange (Scheme 13).
These core-functionalized star polymers were regarded as
homogeneous/heterogeneous hybrid polymer-supported
catalysts and contained hundreds of Ru(II) atoms per
molecule. The star polymers were subsequently used to
catalyze the homogeneous oxidation of sec-alcohols to
ketones, showing a comparable catalysis efficiency to the
small molecule catalyst, RuCl2 (PPh3 )3 [60,132]. When linear block copolymer containing both PEO methyl ether
methacrylate (PEOMA, Mn ∼ 475 g/mol) and MMA units
was employed as the arm precursor, the obtained star
polymers showed amphiphilic solubility (soluble in alcohol and water) and an upper critical solution temperature
(UCST ∼ 30 ◦ C) in 2-propanol [133].
Hawker and coworkers [47] introduced a 64 Cu-chelating
group into the inner shell segment of core/shell star block
copolymers via reaction of 1,4,7,10-tetraazacyclododecane1,4,7-tris(acetic
acid-t-butyl
ester)-10-acetic
acid
(DOTA) with the succinimide reactive groups in the
inner shell blocks. The functional star block polymers
were synthesized by cross-linking a linear PEO-bpoly(DMA-co-NAS) (DMA: N,N-dimethylacrylamide, NAS:
N-acryloxysuccinimide) block MI via NMP of DVB. After
introducing the radioactive 64 Cu nuclei into the inner
shell of the star, these functionalized star polymers with
PEO protecting shells were used as in vivo carriers for
positron emission tomography imaging with promising
bio-distribution.
Hawker and Fréchet [44] also demonstrated the application of core-functionalized star polymers in catalysis.
The soluble star polymers served as scaffolds to encapsulate and isolate a functional catalyst within the interior
of the cross-linked core. Such a site-isolation effect provided by the star polymer allowed a multistep cascade
reaction using chemically incompatible catalysts to occur
in one reactor. For instance, they used the core cross-linked
polySt star polymers synthesized by NMP to encapsulate incompatible acid and base catalysts into separate
star cores and achieved a two-step sequential reaction
involving acid-catalyzed acetal hydrolysis followed by
the amine-catalyzed Baylis–Hillman reaction in one pot
(Scheme 14) [44]. Recently, Fréchet and coworkers [134]
further explored the use of these non-penetrating star
polymers to combine iminium, enamine, and hydrogenbond catalysts in one pot for asymmetric reactions that
generated cascade products with more than one chiral center.
330
H. Gao, K. Matyjaszewski / Progress in Polymer Science 34 (2009) 317–350
Scheme 14. Application of acid- and base-containing star polymers with non-interpenetrating cross-linked cores in a one-pot cascade reaction.
In addition to the application of the star polymers with
a cross-linked core as containers to protect and isolate
the fragile imaging atoms and catalysts from environment, star polymers with segmented arm structure can
also be used as a single molecular template for the synthesis of inorganic nanoparticles [135,136]. Compared to
the self-organized templates from organic surfactants and
amphiphilic block copolymers, such as micelles and vesicles, the globular star polymers with covalently stabilized
core and segmented structures are more stable. These star
templates can effectively protect metallic nanoparticles
from coagulation even under harsh conditions, e.g., elevated temperature, high ionic strength and dilution [137].
For instance, ATRP was used for the synthesis of core
cross-linked star polymers with block-copolymer arms,
(polyMMA-b-polytBA)n -polyDVB. Subsequent hydrolysis
of the polytBA block resulted in formation of an amphiphilic
star polymer with hydrophobic polyMMA outer shell and
hydrophilic polyAA inner shell. This star polymer was used
as a unimolecular container to coordinate Ag+ cations at the
inner shell, which were in situ reduced into Ag(0) clusters
in the star container [136].
3.3.2. Application of charged star polymers as building
blocks for a multilayer film
Layer-by-layer (LbL) assembly has been widely
employed as a simple and versatile method for construction of controlled nanostructures on a surface [138,139]. It
allows the creation of highly tunable, functional thin films
with nano-level control over the structure, composition,
and properties [140]. A wide variety of materials have
been explored as active building blocks for LbL assembly
beyond simple linear polyelectrolytes, including inorganic
nanoparticles, polymeric micelles, dendrimers, carbon
nanotubes, and biological molecules. The incorporation
of a broader range of materials based on the various
intermolecular interactions has expanded the potential
applications of LbL assembly ranging from energy and
electrochemical devices to drug delivery platforms.
Star polymers with a cross-linked core represent a
soft nanomaterial with controlled dimension and high
local densities of functional groups. The integration of
star polymers into polymeric thin films on surfaces has
been demonstrated by a few groups. Xu and Chen [141]
reported a multilayer formation based on the hydrogenbonding interaction between poly(vinylpyrrolidone) linear chains and a polyAA star polymer containing a
poly(methylsilsesquioxane) core. The film thickness was
found to be a linear function of the number of bilayers and the average increase in thickness per bilayer was
28.3 nm. Similarly, Caruso and Qiao [142] showed the incorporation of (polyAA)n -polyDVB star polymers within LbL
assembled polymeric multilayers and their pH-responsive
surface properties. Recently, Hammond and Matyjaszewski
[143] assembled ultrathin multilayer films exclusively
based on the sequential LbL deposition of two types
of star polymers with oppositely charged polyelectrolyte
arms and hydrophobic cores: poly[2-(dimethylamino)ethyl
methacrylate] (polyDMAEMA) star and polyAA star. The
star polymers were synthesized via ATRP using the onepot “arm-first” method. The resulting star/star multilayer
films displayed non-uniform and nanoporous structures,
and showed a morphological rearrangement in response
to environmental pH-stimulus, which was probably due to
the changes in ionization of the polyelectrolytes and the
distinctive geometry of star polymers.
4. Synthesis of branched polymers and/or gels by
copolymerization of monomer and cross-linker via
CRP techniques
As discussed in Section 3, the sequential polymerization of monomer and cross-linker, resulting from two-batch
additions using CRP techniques, produces stars and/or
miktoarm star copolymers with a cross-linked core. The
chemical compositions and site-specific functionalities are
predetermined and variable, depending on the selected
monomers, cross-linkers, initiators, the timing of addition
and the duration of the reaction. In contrast, the one-batch
radical copolymerization (RcP) of a monomer and a crosslinker produces branched polymers and gels. The gel point
and the final copolymer structures are determined by various parameters, including the polymerization technique,
the molar ratios of reagents, the instantaneous concen-
H. Gao, K. Matyjaszewski / Progress in Polymer Science 34 (2009) 317–350
trations of reagents, and the final conversion. Although
conventional RP techniques have been broadly used for
more than 70 years for the synthesis of branched polymers
and gels via copolymerization of a monovinyl monomer
and a divinyl cross-linker, the recently developed CRP techniques provide fundamental differences on the ability to
manipulate the structures of branched polymers and gels.
(1)
where p is the extent of reaction, or conversion of double bonds, is the fraction of all double bonds residing on
divinyl molecules in the initial mixture. DPw is the weightaverage DP of the linear primary chains, which would be
resulted if all cross-linkages in the network at the gel point
were cut. When DPw 1 and the polydispersity of the
primary chain (DPw /DPn ) is considered [145], Eq. (1) is
expressed as:
DPw
=1
DPn
(2)
in which the product of pDPn is the number-average number of cross-linking unit per primary chain ( = pDPn ),
termed as cross-linking index by Flory [146]. Since
=
2[X]0
[M]0 + 2[X]0
[PC]t
p
(4)
DPw
=p
DPn
2[X]0
[M]0 + 2[X]0
[M] + 2[X] DP
w
0
0
[PC]t
p
DPn
=1
which can be further rewritten as:
In general, the RcP of a monovinyl monomer and a
divinyl cross-linker generates polymers with a branched
structure. The cross-linkage, provided by a cross-linker
with both vinyl groups reacted, is formed in the polymer chains via reaction of pendant vinyl group with the
propagating chain-end radicals either intermolecularly or
intramolecularly. The molecular weight and/or size of
the branched polymers increase exponentially with the
progress of intermolecular cross-linking reactions, and
finally reach an “infinite” value with the formation of a polymeric network (gel). The transition from sol to gel is defined
as the “gel point”.
To predict the theoretical gel point, Flory outlined a
statistical method based on the mean-field theory for
determining the critical extent of reaction when gelation occurred [144]. Stockmayer further expanded Flory’s
approach to the addition polymerization of monomer and
divinyl cross-linker and obtained a general expression, Eq.
(1), for predicting the gel point [145]. The Flory–Stockmayer
(F–S) theory was established based on an ideal polymer
network with two assumptions. First, all vinyl groups,
from monovinyl monomer, divinyl cross-linker and pendant double bonds, have the same reactivity. Second,
intramolecular cyclization reactions, which consume pendant vinyl groups but do not contribute to an increase in the
molecular weight of the branched polymer, are neglected.
Eq. (1) predicts that the critical gel point for the formation of an “infinite” polymer network is reached when
the weight-average number of cross-linking unit () per
primary chain equals unity.
c = pDPn
[M] + 2[X] 0
0
where [M]0 , [X]0 are the initial concentrations of monomer
and divinyl cross-linker, respectively, and [PC]t is the concentration of primary chains in the system at any time t.
Substitution of Eqs. (3) and (4) into Eq. (2) leads to
c =
4.1. Flory–Stockmayer’s statistical theory: prediction of
the gel point
c = p(DPw − 1) = 1
DPn =
331
(3)
c =
2[X]0 DPw
DPw
=p2
=1
DPn
[PC]t DPn
(5)
A direct explanation of Eq. (5) indicates that the numberaverage cross-linking units per primary chain () equals the
weight-average cross-linking units per primary chain ()
in a system when the primary chain is monodisperse. For
a copolymerization system with the most probable distribution (DPw /DPn = 2) of the primary chains, c = 0.5 is the
critical value required to reach the theoretical gel point. At
the moment of gelation, the critical conversion of double
bonds pc equals
pc =
[PC]t
1
2[X]0 DPw /DPn
(6)
which indicates that the theoretical gel point based on the
conversion of double bonds is determined by the initial
amount of divinyl cross-linker, the instantaneous concentration of primary chain and its polydispersity.
For the RcP of monomer and divinyl cross-linker, the
reaction of the free cross-linker incorporates a pendant
vinyl group into the polymer chain. The reaction of the
resulting pendant vinyl group with a propagating radical
generates the cross-linkage and introduces two crosslinking units into the connected chains. If all vinyl species
are assumed to have the same reactivity, at the conversion of monomer p, the conversion of divinyl cross-linker
should be convX = (2p − p2 ) and the fraction of cross-linkage
vs. the initially added cross-linker is p2 because each
cross-linker contains two vinyl bonds. At the gel point,
the averaged number of cross-linkage per primary chain
will be p2c ([X]0 /[PC]t ) and the averaged number of crosslinking unit per primary chain c = 2p2c ([X]0 /[PC]t ), which
is in an agreement with Eq. (5). Therefore, for a RcP of
monomer and divinyl cross-linker, the critical number for
cross-linkage per primary chain ( c /2) should equal 0.5 and
0.25 for monodisperse primary chains and most probable
distributed primary chains, respectively. In other words, a
ratio of one cross-linkage to four primary chains is sufficient for the onset of theoretical gelation when a primary
chain with polydispersity, DPw /DPn = 2, is used [144].
The theoretical gel point based on the F–S theory
provides important guidelines for experimental designs
seeking to obtain branched polymers and gels, although the
assumptions, particularly the “no intramolecular cyclization” assumption, are not completely valid during the
experiments [147–150]. In some literature references, the
intramolecular cyclization reaction was further categorized into two reactions: primary cyclization and secondary
cyclization. The former cyclization reaction represents the
332
H. Gao, K. Matyjaszewski / Progress in Polymer Science 34 (2009) 317–350
cycle formed within one primary chain and the secondary
cyclization occurs between two connected primary chains
[151,152].
When soluble branched polymers are targeted during
the RcP of monomer and cross-linker, the reaction should
be stopped before the critical gel point (pc ) in order to
exclusively obtain soluble sols. Based on Eq. (6), several
strategies are applicable in order to delay the gelation in
a system and push gelation to higher conversion, including: (1) increasing the initial primary chain concentration,
e.g., decreasing the primary chain length, by using more
initiator or chain transfer agent; (2) using less cross-linker;
(3) stopping the reaction at lower monomer conversion.
Furthermore, intramolecular cyclization reactions could be
enhanced by performing the copolymerization in a dilute
condition, in a selective solvent and/or in a confined space,
e.g., emulsion, which usually delays and prevents macroscopic gelation and produces microgels.
4.2. Differences in the gelation process between RP and
CRP
Highly branched polymers and/or gels with inhomogeneous structures are formed during most conventional
RP reactions due to the intrinsic limitations of the RP
method, including slow initiation, fast chain propagation,
and exclusive radical termination reactions [150,152–155].
Due to the slow initiation, primary radicals are slowly but
continuously generated in the system, resulting in a very
dilute polymer solution at the beginning (∼␮M). On the
other hand, the generated primary radicals quickly propagate within a time scale of seconds before the occurrence
of a termination reaction to permanently lose the chainend functionality. The DPn of primary chains could exceed
DPn ∼ 103 at the beginning of polymerization, which is not
merely determined by the conversion and the initial molar
ratio of double bonds to initiator, but more importantly,
dependent on the kd (dissociation rate constant of thermal
initiator), kp (propagation rate constant, kp ∼ 103 s−1 ) and
kt (termination rate coefficient) values. Based on Eq. (6),
gelation in a conventional RP reaction should occur immediately at very low conversion, e.g., pc = 0.16%, under bulk
conditions ([M]0 = 10 M) with 1 mol% of cross-linker in the
initial formula and DPw /DPn = 2 for the primary chains.
However, the measured experimental gel point based on
monomer conversion is typically 1 or 2 orders of magnitude larger than the predicted value. This is mainly due to
an excluded volume effect of polymer chains and a significant contribution of intramolecular cyclization reactions
(primary and secondary cyclizations) [150,152,156]. At the
beginning of the copolymerization, the polymer chains
formed in the reaction seldom overlap with each other
because of the extremely low polymer concentration. Consequently, most of the pendant vinyl groups are consumed
via intramolecular cyclization reactions, producing a lessswollen nanogel with a highly cross-linked nanodomain
(Scheme 15). As the reaction proceeds, the number of these
nanogels increases and radicals generated later in the reaction connect these preformed overlapped nanogels into
larger molecules and form a heterogeneous network.
In contrast to the conventional RP technique, the
recently developed CRP techniques have several advantages when targeting the preparation of more homogeneous polymer networks, due to the fast initiation and
quick reversible deactivation reactions. The fast initiation reactions, relative to propagation reactions, result
in a quick conversion of all initiators into primary
chains and a nearly constant number of growing primary chains throughout the polymerization (Scheme 15).
Therefore, the concentration of primary chains in a well
controlled CRP system is similar to the concentration
of added initiators throughout the polymerization (e.g.,
[PC]t ∼ [Initiator]0 ∼ mM) [2,34]. Compared to the slow initiation in conventional RP, the fast initiation in CRPs implies
that the theoretical gel point based on Eq. (6) is now
delayed to pc = 5.0% for a bulk system ([M]0 = 10 M, 1 mol%
cross-linker, [Initiator]0 = 1 mM) with DPw /DPn = 2 for the
primary chains. When the primary chains are monodisperse (DPw /DPn = 1.0), the value of pc would be pc = 7.1%.
This calculated result clearly explains the retarded gelation
observed in CRP process due to the feature of fast initiation
reaction.
Moreover, the differences between the fast propagation
of a polymer chain in conventional RP and the dynamic
equilibrium in CRP, established between a low concentration of active propagating chains and a large number
of “dormant” chains, ensure that only a few monomer
units are incorporated into the polymer chains in every
activation/deactivation cycle. During the long “dormant”
period, the polymer chains cannot propagate, but can diffuse and relax, which results in the probability of reaction
of each vinyl species: monomer, cross-linker or pendant
vinyl group, is statistically determined by their concentration. Thus, the application of CRP techniques [157–177] to
copolymerization of monomer and cross-linker results in
a more homogeneous incorporation of branching points
into the soluble branched copolymers and insoluble gels,
as compared to polymers synthesized by RP methods
from similar concentrations of comonomers. Moreover, the
chain-end functionalities are preserved in the branched
polymers and/or gels synthesized by CRP, which can be further used for chain-end modification and chain extension
reactions [161,167].
4.3. Synthesis of branched polymers and gels via CRP
techniques
Ide and Fukuda [157,158] first reported the CRcP of
St and 4,4 -divinylbiphenyl (14 in Scheme 6) in bulk by
using polySt-TEMPO alkoxyamine as initiator. Owing to
the controlled character of the system, the polymerization proceeded via a slow and simultaneous growth of a
nearly constant number of primary chains. The pendant
vinyl group showed a similar reactivity to the free monomer
St and the cross-linking reaction progressed in a highly
homogeneous manner. The gels produced in this controlled
copolymerization showed remarkable differences from the
gels prepared via RcP process, including retarded gelation kinetics, steady increase of gel fraction after gelation
and higher swelling ratios of gels. The critical number of
cross-linkage per primary chain to reach the experimental
H. Gao, K. Matyjaszewski / Progress in Polymer Science 34 (2009) 317–350
333
Scheme 15. Different gelation processes between RP and CRP.
gelation ( c /2(expt) ∼ 1) was twice higher than the value
from F–S theory, indicating 50% of the cross-linker was
consumed by cyclization even under bulk polymerization
conditions.
Several research groups also exploited ATRP [159,160,
169,171] and RAFT copolymerization [163,167,173] of crosslinker, in order to study gelation kinetics and/or to
synthesize branched polymers and gels with better controlled structures. Compared to the conventional RcP
system at the same concentration and the same molar
ratios of reagents, the CRcP of monomer and divinyl crosslinker showed delayed gelation and consequently allowed
the synthesis of soluble branched polymers even at complete conversion. When the concentration of monomers
was high, such as bulk conditions, selecting the correct
initial molar ratio of divinyl cross-linker to initiator was
crucial in order to prevent experimental gelation and obtain
soluble branched polymers. In a CRP system with high
initiation efficiency and good control over the polydispersity of primary chains (low Mw /Mn ), no gelation was
observed when the initial molar ratio of cross-linker to initiator was less than 1, even under bulk conditions with
complete conversion [160,169–171]. This result indicates
that at least half of the cross-linker was consumed via
intramolecular cyclization (primary and secondary cyclizations). However, the twofold deviation of the experimental
gel point in the CRP systems, compared to the value from
F–S calculation, is strikingly smaller than the situation in
the RP system, which typically displays a 1 or 2 order of
magnitude difference. Among the different CRP techniques,
the RAFT copolymerization of monomer and cross-linker
resulted in a greater extent of intramolecular cyclization
and showed a higher discrepancy from the ideal branching
process than the ATRcP system under similar conditions
[163,173]. This is plausibly due to the fundamental differ-
ence in initiation/activation mechanism between ATRP and
RAFT techniques.
The extent of cyclization reactions occurring during the
CRcP of a cross-linker could be controlled by adjusting the
initial concentration of reagents. For instance, during the
ATRcP of MA and EGDA at fixed molar ratios of monomer,
cross-linker and initiator, simply diluting the system, via
addition of more solvent, dramatically postponed, or even
prevented, the experimental gel point at higher monomer
conversion, i.e., longer reaction time, than the value based
on F–S theory [175]. Since the experimental gel point under
Fig. 3. Reciprocal plot of experimental gel point against 1/[MA]0
during ATRcP of MA and EGDA under different MA concentrations; experimental condition: [MA]0 /[EGDA]0 /[R-Br]0 /[CuBr]0 /[CuBr2 ]
◦
0 /[PMDETA]0 = 50/10/1/0.45/0.05/0.5, in DMF at 60 C; the experimental
gel point was the moment when the reaction fluid lost its mobility when
held in an upside down position for 10 s.
334
H. Gao, K. Matyjaszewski / Progress in Polymer Science 34 (2009) 317–350
bulk conditions was still twice higher than the F–S value, a
reciprocal method could be applied to plot the experimental gel points based on monomer conversion against 1/[M]0 .
Extrapolation of this curve to zero provides the imaginary
experimental gel point at infinite concentration (Fig. 3).
When the molar ratios of monomer, cross-linker and initiator were [MA]0 /[EGDA]0 /[R-Br]0 = 50/10/1, the gel point at
infinite concentration would occur at 26.0% of MA conversion. This value was very close to the calculated gel point,
22.3% MA conversion, based on the prediction provided by
F–S theory.
Armes used bis(2-iodoethoxy)ethane as a branching
agent for post-polymerization quaternization of monodisperse polyDMAEMA prepolymer containing a reactive
tertiary amine group on every monomer unit [177]. They
explored the propensity for intermolecular branching
reactions and intramolecular cyclization reactions under
different concentrations. No experimental gelation was
observed even when a molar ratio of di-iodide to polyDMAEMA prepolymer equaled to 2, under high polymer
concentration condition (47%, w/v in solution), which
is much higher than the critical overlap concentration
(c* = 10%, w/v). This result indicated that intramolecular
cyclization occurred to a greater degree in the quaternization reaction than during the copolymerization of
monomer and cross-linker under the same monomer unit
concentration. This was probably due to the large molar
ratio of tertiary amine to pendant iodide during the
quaternization reaction, which facilitated intramolecular
cyclization.
In addition to the concentration effect, several other
parameters, such as the reactivity of cross-linker compared to monomer, the initiation efficiency of initiator, the
polydispersity of the primary chains and the solvent quality, can also affect the experimental gel points during the
CRcP of monomer and divinyl cross-linker. For instance,
the ATRcP of MA and dimethacrylate cross-linker, e.g.,
EGDMA, resulted in a different gelation behavior than the
model copolymerization of MA and EGDA with equivalent
reactivity for all acrylate groups [178]. The higher reactivity of methacrylate groups present in EGDMA resulted in
faster consumption of cross-linker and quicker generation
of branching points into polymer chains, an acceleration
effect. On the other hand, the branched polymer resembled
a star-like structure, containing a gradient distribution of
pendant methacrylate groups and branching points from
the inner core to outer shell. In other words, the pendant vinyl groups, located in the more densely cross-linked
core, were isolated by the surrounded shell, which reduced
the probability for intermolecular reactions and retarded
gelation, a deceleration effect. These two effects intercorrelated with each other, depending on the amount of
added EGDMA, significantly affected the experimental gel
points of the poly(MA-co-EGDMA) system. When more
EGDMA was added, the acceleration effect became dominant, which led to a faster gelation at lower MA conversion,
compared to the ATRcP using the same molar amount of
EGDA (Scheme 16).
In addition, the use of hydrophilic monomers and
cross-linkers in CRcP provides new strategies to obtain
water-soluble branched polymers and hydrogels with well
controlled structure, highly preserved chain-end functionality [161,169] and/or fast response upon environmental
stimulus [179].
4.4. CRP of cross-linker in dispersed media: synthesis of
cross-linked microgels
Radical (co)polymerization can be conducted in various dispersed systems, e.g., emulsion, miniemulsion,
microemulsion, precipitation, dispersion, and suspension
polymerizations [180]. Water is predominantly used as the
continuous phase, oil in water (O/W) system, although
the inverse situation with oil as continuous phase, water
in oil (W/O) system, has also applied to the synthesis of
hydrophilic latexes. When a divinyl cross-linker is used
as a comonomer, the RcP in the heterogeneous media
becomes a standard method for the synthesis of crosslinked microparticles with dimensions varying from tens
of nanometers to hundreds of microns. Based on the synthetic procedures and the dispersed media used, these
microparticles vary significantly in terms of size, composition, surface functionality and morphology, and are broadly
used in traditional industries, such as coating, adhesives,
rubbers, and emerging new markets, including medical and
biological detection and optical imaging probes [181].
Since the invention of CRP techniques, tremendous
efforts have been devoted to understanding the fundamentals required for application of various CRP techniques
to dispersed media, both O/W system and W/O system.
Due to the multicomponent nature of CRPs, as well as the
complex nature of heterogeneous media, the initial challenges of conducting a CRP in a dispersed medium included
decreased colloidal stability, wide particle size distribution,
loss of control over polymerization, and low initiation efficiency. However, remarkable progress has been achieved in
the past few years and several recently published reviews
have comprehensively covered the recent development of
true CRPs in dispersed systems [182–188].
Compared to the conventional RcP of cross-linker in
the heterogeneous media, the CRcP of cross-linker produces cross-linked microparticles with controlled structure
and preserved chain-end initiating sites. The latter feature
enables further chain extension by polymerizing another
monomer from the microgels to functionalize the microgel
surface and produce amphiphilic hairy particles.
4.5. Avoiding gelation in conventional RP by using chain
transfer agent
As discussed in Section 4.1, the intrinsic limitations in
conventional RP technique, such as slow initiation, result
in gelation at low monomer conversion and gels with a
heterogeneous structure. In order to effectively postpone
and even prevent the macroscopic gelation in the RP system, Sherrington developed a practical strategy for the
synthesis of soluble branched polymers, the “Strathclyde
route”, which uses a large amount of radical chain transfer
agent, such as a mercaptan, during the RcP of monomer and
cross-linker [189–191]. The number and the kinetic length
of the primary chains were controlled by the amount of
chain transfer agent used, which was usually present in
H. Gao, K. Matyjaszewski / Progress in Polymer Science 34 (2009) 317–350
335
Scheme 16. Effect of cross-linker reactivity on experimental gel points [178]. Reproduced with permission from American Chemical Society.
significant molar excess relative to the free radical initiator in order to reduce the primary chain length and retard
branching and gelation [189,192,193]. This situation is very
similar to that arising in RAFT polymerization, although the
re-activation of the “dead” polymer chains is not possible
in the “Strathclyde route”. Similarly, Sato et al. [194,195]
reported a method employing a high concentration of azobased thermal initiator during the homopolymerization of
divinyl adipate. The high concentration of initiator resulted
in an increased rate of termination reactions and fast chain
transfer to initiator. Thus a large number of primary chains
were introduced into the reaction at early stage of polymerization, which decreased the length of primary chains and
produced soluble branched polymers.
5. Other strategies for introduction of branching
points
Several other strategies have also been extensively
employed for the synthesis of polymers with branched
architectures, including: (1) the use of multifunctional initiators to introduce branching points into the polymer
chains before the polymerization of monovinyl monomer,
(2) the use of multifunctional coupling agents to introduce branching points after the formation of arms or side
chains, and (3) the use of AB* inimer (initiator–monomer
containing double bond A and initiator fragment B* in one
molecule) to introduce “T” shaped branching points into
the (hyper)branched polymers.
5.1. Introduction of branching points using an inimer
As discussed in Section 4, branched polymers and
gels are produced during the CRcP of monomers and
divinyl cross-linkers. The pendant vinyl groups in the polymer chains are generated by the incorporation of free
cross-linker and consumed via their reaction with propagating radicals to produce an “X” shaped cross-linkage,
from which four polymer chains radiate out. Due to the
adjustable molar ratio of cross-linker to initiator and the
statistical distribution of cross-linkage among the primary
chains, gelation occurs in the system when the average
number of cross-linkage per primary chain reaches a critical value. In other words, the reaction has to be stopped
before the gel point if soluble branched polymers are targeted.
A recently developed comparable reaction is the chaingrowth self-condensing vinyl polymerization (SCVP) of an
AB* inimer [11], which provides a convenient, one-pot
synthesis of hyperbranched polymers by using controlled
polymerization techniques. During the polymerization, the
fragment B* can be converted to an active initiating group
by an external stimulus and react with the chain-end vinyl
groups to generate a hyperbranched polymer with “T”
shaped cross-linkage, with three radiating polymer chains.
In an ideal case without any chain transfer and termination
reactions, the number of initiating sites in a hyperbranched
molecule is equal to its DP, while the number of pendant
vinyl group is only one (Scheme 17). Moreover, every primary chain contains one cross-linkage or two cross-linking
units, and there is no distribution of cross-linkage among
different chains. Therefore, no gelation occurs during the
SCVP reaction even at complete conversion.
The SCVP of an inimer is very similar to the polycondensation reactions involving trifunctional AB2 monomers.
In both reactions, degree of branching (DB) ≈ 0.5 can be
reached at high conversion and the polydispersities are predicted to be very high Mw /Mn = DPn [196,197]. A variety of
hyperbranched polymers, such as polySt and its derivatives,
polyacrylates, polymethacrylates and poly(vinyl ether)s,
have been synthesized using living anionic [198], cationic
[11,199], ATRP [200–203], NMP [204], and RAFT [205]
techniques. A similar strategy was also applied to selfcondensing ring-opening polymerization of cyclic inimers
for successful synthesis of hyperbranched polyamines,
polyethers and polyesters [206–208]. Hyperbranched polymers with narrower MWD and higher DB were obtained by
combining the use of a multifunctional initiator and slow
addition of the inimer to the polymerization [206].
When a monovinyl monomer, M, was copolymerized
with AB* inimer, the SCVP technique was extended to
SCVCP, which could produce hyperbranched polymers with
various functionalities, controlled polydispersity and DB.
Different types of functional groups can be incorporated
into the hyperbranched polymers, depending on the chemical nature of the comonomer. The chain architecture and
the value of DB can be easily controlled by a suitable choice
of the ratio of comonomer to inimer (ˇ = [M]0 /[AB*]0 ) in the
336
H. Gao, K. Matyjaszewski / Progress in Polymer Science 34 (2009) 317–350
Scheme 17. Synthesis of hyperbranched polymers via SCVP of AB* inimer.
feed [209]. A variety of hyperbranched polymers have been
synthesized by using CRP techniques for SCVCP. When a
hydrophilic comonomer, or its precursor was used, watersoluble hyperbranched polymers were obtained [210,211].
A derivative of the SCVCP of an inimer and comonomer
is the homopolymerization of a linear macroinimer, an
A-polyM-B* molecule. The macroinimer was synthesized
by homopolymerization of a monomer, M, followed by
chain-end modification to introduce the double bond at ␣end, which resulted in a well controlled structure for the
macroinimer [212,213]. The theoretical calculation indicated that when a monodisperse macroinimer with DP = ˇ
was used, the MWD of the obtained hyperbranched polymer with a given Mn value could be (ˇ + 1) times lower than
that from the SCVP of an AB* inimer [214].
5.2. Introduction of branching points using
multifunctional initiators
The advantageous features of CRP reactions include fast
initiation, concurrent growth of all polymer chains and
suppressed fraction of terminated chains which together
allow the use of multifunctional initiators for the synthesis
of polymers with branched architecture. Typical examples
include the synthesis of star polymers via the “core-first”
method and the synthesis of brush/grafted copolymers via
the “grafting-from” method.
5.2.1. Synthesis of star and miktoarm star polymers by
the “core-first” method
The “core-first” method, the use of a multifunctional
initiator (core), was first employed for the synthesis of
star polymers after the development of various CRP tech-
niques, [215–229]. The star core could be an atom, a small
molecule, or a macromolecule (Scheme 18). The number of
arms per star polymer is determined by the number of initiating functionalities on the multifunctional initiator when
the initiation efficiency of the multifunctional initiator is
quantitative. The very early examples exclusively employed
multifunctional small molecules as the initiator for the
synthesis of well-defined star polymers with the number
of arms less than 10. Subsequently, methods were developed for star synthesis using multifunctional polymers as
initiators, which included dendrimers [230–233], hyperbranched polymers [234–237], and inorganic nanoparticles
[238–240]. Consequently, the star-like polymer contained a
statistical distribution of tens to hundreds of arms per star.
The star polymers synthesized by the “core-first”
method preserved the initiating site at the chain end,
which can be further used for chain extension with a second monomer to form star block copolymers (Scheme 18)
[221,222,241–243]. The use of functional monomers and
multifunctional initiators allowed different types of functionalities to be incorporated into the star core, arm and star
periphery, which led to a number of applications of these
functional star polymers in catalysis [244], as unimolecular
templates for inorganic nanoparticles [245–247], sacrificial templates for pores [248], and drug delivery vehicles
[45,46,249].
It is interesting to note that the RAFT technique was
recently applied for star synthesis using a multifunctional
mediating agent. The core could be linked to the arms
either via the leaving R-group (R-RAFT) [226,228,229] or
via the stabilizing Z-group (Z-RAFT) [250–254]. The RRAFT approach is similar to the standard “core-first” route
applied in ATRP and NMP methods, in which the arms grow
H. Gao, K. Matyjaszewski / Progress in Polymer Science 34 (2009) 317–350
337
Scheme 18. Synthesis of star polymers by using the “core-first” method.
in a divergent manner from the core and termination of
two star polymers results in formation of star–star coupling
product. In contrast, the radical mediating functionality
(such as dithioester or xanthate) does not fragment from
the star core in the Z-RAFT method. Dormant chains or
arms linked to the star core are released for propagation
when another propagating chain couples with the star core
in a reversible chain transfer reaction. Thus, this Z-RAFT
method for star synthesis is more like a “coupling-onto”
synthetic category (Section 5.3.1), rather than the typical
“core-first” method. Only linear–linear radical termination
reactions occur in the Z-RAFT method, although the chain
transfer reaction to polymer arms during the polymerization of acrylates can generate star-based radicals [254]. The
main synthetic challenge in the Z-RAFT approach is that the
steric congestion builds up around the core as the molecular weight of the arm and the number of arms increase
[255], which will be discussed in Section 5.3.1.
Miktoarm star copolymers containing two or more arm
species could also be synthesized by using a multifunctional miktoinitiator containing several initiating species
with different initiation mechanisms, especially by combining various CRPs and ROP technique [15,241,256–260].
Different types of arms grow sequentially from the corresponding initiating sites via polymerization of various
monomer species using different polymerization methods.
The numbers and species of the arms in the miktoarm star
copolymer are determined by the numbers and species of
the initiating sites on the miktoinitiator. Chain-end modification of a preformed linear MI is an alternative strategy
for the synthesis of miktoinitiators [261–265]. A branching point containing multiple initiating sites is introduced
into the chain end of linear polymer, which is subsequently
used as a MI for polymerization of another monomer to
grow a second compositionally different kind of arms from
the branching point.
5.2.2. Synthesis of molecular brushes by the
“grafting-from” method
Molecular brushes, or bottle brush macromolecules
due to their appearance, are unique polymer molecules
whose conformation and physical properties are controlled
by steric repulsion of densely grafted side chains [9,10].
Molecular brushes are usually synthesized via one of three
strategies: “grafting-from”, “grafting-onto” (Section 5.3.2),
and “grafting-through” (Section 5.4). The “grafting-from”
strategy is applied to the synthesis of molecular brushes
starting with the preparation of a backbone polymer. The
linear backbone precursor represents a multifunctional MI
with a predetermined number of initiation sites that are
subsequently used to initiate the polymerization of side
chains. The MI can be prepared by direct polymerization
of a functional monomer containing initiating side groups
[266–268], or by first preparing a precursor that is subsequently functionalized to attach initiating moieties to
the backbone [269,270]. During the polymerization, all side
chains within one molecular brush grow in a gradual and
simultaneous pace, which alleviates concerns over steric
congestion and leads to successful synthesis of densely
grafted polymers, essentially one side chain per backbone
unit [10]. CRP techniques are suitable for molecular brush
synthesis since they facilitate a low instantaneous concentration of radical species, thereby limiting intermolecular
and intramolecular termination events. This is especially
important for brush polymerizations due to the high local
concentration of chains that exist in the vicinity of the
backbone polymer. By far grafting side chains from the
multifunctional backbone has mainly been achieved by
use of ATRP because of the relative ease of preparing the
multifunctional MI. The monomer species selected for the
growth of the tethered side chains included St [269–271],
acrylate [269–271], methacrylate [272–274], acrylamide
[273], and acrylonitrile [275]. A potential concern associated with the “grafting-from” method is reduced levels of
control over side-chain length and grafting density, both of
which depend on the initiation efficiency of the multifunctional backbone MI [116].
Molecular brushes synthesized by the “grafting-from”
route preserve the functional initiating sites at the sidechain ends, which can be further used for chain extension to
produce core–shell brush molecules with block-copolymer
side chains [269–271,276,277]. When the side chain is
an amphiphilic block copolymer, the cylindrical polymer
brush resembles a unimolecular micelle of cylindrical
shape, which can be used as a unimolecular template for
the synthesis of inorganic nanorods [278–280]. Moreover,
338
H. Gao, K. Matyjaszewski / Progress in Polymer Science 34 (2009) 317–350
these cylindrical micelles can respond to environmental
stimuli (e.g., solvent, pH, and temperature) in an anisotropic
manner and show unique solution properties.
Other examples of molecular brushes with complex
architectures include a cylindrical brush with gradient
grafting density [281,282], a star-like brush [283], and a
hetero-grafted brush containing two or more side chains
with different compositions [268,277]. Molecular brushes
with a high density of side chains were also synthesized using the “grafting-from” strategy, including a doubly
grafted brush containing two side chains per backbone
monomer unit [284], and a dendronized brush containing
four linear side chains per backbone unit [285].
When two chemically different side chains were incorporated into one brush molecule, the side chains can have a
mixed structure or segregate into two different hemicylinders, depending on the distribution of the side chains along
the backbone, their interaction parameters, as well as the
nature of the solvent. This latter type of macromolecule has
been named a Janus cylinder [286] or proto-type brushes
[287] because of the incompatibility of the different side
chains.
5.3. Introduction of branching points using
multifunctional coupling agents (MCAs)
Compared to the use of multifunctional initiator, from
which the arms or side chains start to grow, the use of multifunctional coupling agent represents a different synthetic
concept to introduce a branching point into a macromolecule via the coupling reaction between a MCA and
linear polymers with a reactive group at the chain end.
Depending on the number of functional groups and the
shape of the MCA, this strategy is typically used for the synthesis of star polymers via the “coupling-onto” method and
the synthesis of brush/grafted polymers via the “graftingonto” method. In the latter case, the MCA is a linear polymer
chain containing hundreds of coupling side groups.
5.3.1. Synthesis of star polymers by the “coupling-onto”
method
The “coupling-onto” method was developed and widely
used in anionic polymerization procedures for the synthesis of well-defined star and miktoarm star copolymers
with various arm compositions and arm numbers, up to
128 [288]. The living polymer chain with a terminal carbanion chain end is deactivated via coupling with a MCA,
such as multifunctional chlorosilane or chloromethyl benzene. When an excess amount of linear chains are used,
the coupling efficiency is nearly quantitative. The functionality of the MCA determines the number of arms in the
resulting star molecule. However, the direct application of
this “coupling-onto” method in CRPs for the synthesis of
star polymers was not straightforward. The main problem
encountered was a non-selective and slow coupling reaction between the polymer radicals and the MCA, which was
further complicated by the high steric congestion around
the star core. For example, during the Z-RAFT approach for
the synthesis of star polymers (Section 5.2.1), the experimental molecular weight gradually deviated from the
expected values with increasing arm molecular weight
[250,253,255]. Since the reactive center (radical mediating
functionality) sits close to the star core, the degree of control over the RAFT process is speculated to be reduced due to
steric shielding from the surrounded arms, which decreases
the coupling, i.e., chain transfer, efficiency between the
propagating radicals and Z core [255,289]. The shielding
effect could be alleviated by expanding the Z core moiety
and become more significant with increase of arm length
and targeted number of arms. Moreover, it is difficult to
use an excess of linear chains, i.e., through the addition of
more free radical initiator, in the Z-RAFT method because of
the significant degree of linear–linear radical coupling reactions in the system. This side reaction was largely avoided
in the anionic polymerization procedure due to the electrostatic repulsion between two anion chain ends.
Compared to the Z-RAFT method, which generates the
star polymers during the polymerization via degenerative
chain transfer reactions between propagating linear chains
and multifunctional Z cores, an alternative strategy for
the synthesis of star polymers employing CRP methods
includes conducting post-polymerization coupling reactions between chain-end functional polymers with a MCA
containing complementary functionality. The involved coupling reaction could further include secondary interactions,
such as hydrogen bonding [290], coordination [291], and
ionic interaction [292,293]. Two requirements should be
satisfied in order to successfully prepare star molecules
using this approach: high chain-end functionality of the linear arm precursor and selection of a highly efficient organic
coupling reaction.
Among the various CRP techniques, ATRP is particularly
attractive for the synthesis of polymers with high chainend functionality [4]. For example, selection of a functional
ATRP initiator allows the incorporation of functionality at
the ␣-end of a polymer chain. At the same time, the ␻end retains a terminal halogen atom(s) that can be readily
converted into various desired functional chain-end groups
through appropriate chemical transformations, especially
nucleophilic substitution reactions.
Among the various efficient coupling reactions, “click
reactions”, a term coined by Sharpless and coworkers [294],
have gained a great deal of attention due to their high
specificity and quantitative yields in the presence of most
functional groups. The most popular click reaction is the
copper(I)-catalyzed Huisgen dipolar cycloaddition reaction
between an azide and an alkyne, leading to formation of a
1,2,3-triazole [295–297]. Recent publications on this click
reaction [298–302] indicate that it is a versatile method
for the synthesis of functional polymers [303–310], bioconjugated polymers [309,311,312], and polymers with
complex topologies [313–323].
The first synthesis of star polymers using the combination of ATRP and click reactions was accomplished by
coupling azide-terminated polySt with compounds bearing
multiple alkyne functionalities (Scheme 19). The coupling
agents included a di-, tri-, and tetra-alkyne, to which azideterminated polySt was attached with 95, 90, and 83%
efficiency, respectively [320]. All coupling reactions were
complete within 3 h. The presence of small amount of Cu(0)
enhanced the coupling efficiency, and the highest product
yield was obtained when the molar ratio of azide/alkyne
H. Gao, K. Matyjaszewski / Progress in Polymer Science 34 (2009) 317–350
339
Scheme 19. Schematic illustration of synthesis of linear, 3-arm and 4-arm polySt star polymers by combination of ATRP and click reactions [320]. Reproduced
with permission from American Chemical Society.
groups was close to unity. However, the efficiency of the
click reaction decreased with increasing molecular weight
of the arm precursors, which was probably due to higher
steric congestion and less precisely balanced stoichiometry between azide and alkyne groups. Nevertheless this
“coupling-onto” strategy has proven a versatile method for
preparation of numerous star polymers with high star yield.
Similar methodologies were utilized shortly thereafter
for successful synthesis of star-shaped polymers with various arm numbers and chemical compositions [324–326],
including the synthesis of PCL star polymers by coupling of alkyne-terminated linear PCL precursor with
a heptaazide-containing ␤-cyclodextrin [324]. Three-arm
(polySt-b-PEO)3 star block copolymers were also synthesized via click coupling of (polySt-N3 )3 star polymer with
alkyne-terminated PEO precursor [326].
Monteiro reported the synthesis of 3-arm miktoarm
star polymers and the first-generation of polymeric miktodendrimers comprised of polySt, polytBA and polyMA
arms, by combining ATRP and click chemistry [327]. The
miktoarm stars were prepared by first coupling azideterminated polymer to a large excess of tripropargylamine,
followed by addition of the purified dialkyne-containing
linear polymer to a solution of a different azide-terminated
polymer. Other groups have combined different CRP
techniques with click reactions and synthesized various miktoarm star copolymers containing 3–5 arms with
different molecular weights and chemical compositions
[328–333]. For instance, Tunca prepared ABC miktoarm star
copolymers by a combination of ATRP, NMP, and click chemistry [328]. A trifunctional miktoinitiator was synthesized
that contained an ATRP initiating moiety, an alkoxyamine,
and an alkyne group. This initiator was sequentially used for
the ATRP of MMA, the NMP of St, and followed by click coupling with azide-terminated polytBA or PEO (Scheme 20).
In addition to the broad application of the Cu(I)catalyzed cycloaddition reaction between an azide and
an alkyne, other types of highly efficient click reactions, such as Diel–Alder (DA) reactions, were also used
for the synthesis of star and miktoarm star copolymers
[334–336]. The efficiency of a DA coupling reaction was
found to be comparable to the Cu(I)-catalyzed azide–alkyne
cycloaddition coupling reaction. For example, the yield
of (polytBA)3 star polymers reached 93% via coupling
reaction between furan protected maleimide terminated
polytBA linear chains and trianthracene functionalized
MCA [334].
5.3.2. Synthesis of brush/grafted copolymers by the
“grafting-onto” method
Molecular brushes or densely grafted copolymers can
also be synthesized via the coupling reactions between
end-functional linear chains with a polymer backbone
precursor. Since the MCA used for brush synthesis is
usually a linear polymer chain containing hundreds of
functional side groups, this strategy is usually termed as
the “grafting-onto” method, to differentiate it from the
previous “coupling-onto” method (Section 5.3.1) for star
synthesis when a small MCA is used. Compared to the
“grafting-from” method (Section 5.2.2), an advantage of
the “grafting-onto” route is that both the backbone and
side chains are prepared separately with a better controlled structure. Due to the large number of side chains
per backbone, the steric congestion among the side chains
is expected to be more severe than the star polymers.
Therefore, an excess of side chains is usually employed to
drive the grafting reaction to higher conversion, which conversely introduces increased difficulty during purification
of the brush polymer and removal of the unreacted side
chains.
340
H. Gao, K. Matyjaszewski / Progress in Polymer Science 34 (2009) 317–350
Scheme 20. Synthesis of ABC miktoarm star copolymers by combination of ATRP, NMP and click reactions [328]. Reproduced with permission from WILEY-VCH
Verlag GmbH&Co.
Brush polymers with high grafting density (numberaverage ratio of side chains to backbone monomer units
>90%) have been reported, where well-defined side chains
synthesized by living anionic polymerization were grafted
onto the polymer backbone via reaction with functional side groups, such as alkyl halides [337,338]. Highly
efficient click reactions, particularly copper(I)-catalyzed
azide–alkyne cycloaddition reactions, were also used for
the synthesis of molecular brushes and/or grafted copolymers by combining the click reactions with radical polymerization techniques [309,314,322,323,339–343]. Fréchet
and Hawker successfully coupled azide-functionalized
dendrons to linear polymeric backbones containing an
acetylene side group on every monomer unit [314,339].
When poly(vinyl acetylene) was reacted with benzyl ether
dendritic azides, the coupling reaction was quantitative for
the preparation of first and second generations of dendritic
azides.
Recently, poly(2-hydroxyethyl methacrylate) (polyHEMA, DP = 210, Mw /Mn = 1.22) synthesized by ATRP was
modified with 4-pentynoic acid in order to incorporate
an alkyne functionality onto essentially every monomer
unit (polyHEMA-alkyne) along the backbone. Subsequent
reaction with azide-terminated polymers enabled efficient
synthesis of well-defined brushes [322]. The efficiency of
“grafting-onto” reactions strongly depended on the size of
side chains. For short “thin” PEO side chains, the efficiency
of grafting reached 88%. But the efficiency was much lower
for longer PEO chains and for “thicker” polySt or polyBA side
chains. Alternatively, grafted copolymers were also synthesized by grafting acetylene-terminated side chains onto
multifunctional backbones containing azide side groups
[323].
As another efficient click reaction, the DA coupling reaction, [4 + 2] system, between a diene and a dienophile
was also applied for the synthesis of grafted copoly-
H. Gao, K. Matyjaszewski / Progress in Polymer Science 34 (2009) 317–350
mers. For example, two types of well-defined polySt-g-PEO
and polySt-g-polyMMA grafted copolymers were successfully prepared via grafting maleimide-terminated side
chains onto multifunctional polymeric backbones containing anthryl side groups [344]. Hetero-grafted copolymers
containing two types of side chains, polyMMA and PEO,
were also synthesized by combining copper(I)-catalyzed
azide–alkyne coupling reaction and DA coupling reactions
[345].
5.3.3. Synthesis of model networks by using MCA in CRPs
A degradable model network was prepared by coupling telechelic linear polymers, synthesized via ATRP,
with MCAs using the Cu(I)-catalyzed azide–alkyne cycloaddition [321]. A difunctional initiator with an ozonizable
group at the center of the molecule was used to initiate an ATRP of tBA. Azide functionality was introduced
into the telechelic polymer by reaction of the dibromineterminated Br-polytBA-Br with NaN3 in DMF. Regular
well-defined networks were obtained by coupling the
diazide-functionalized N3 -polytBA-N3 with tri- or tetraacetylene MCAs using “click chemistry” (Scheme 21). The
resulting networks were ozonized in order to obtain soluble polymer products, which were then analyzed by GPC.
The cleaved product of the network from tri-acetylene
MCA exhibited a Mn value equal to 1.5 times that of the
linear N3 -polytBA-N3 polymer precursor, indicating the
cleaved product was a three-arm star polymer. The product
obtained from the cleavage of tetra-functional network displayed a Mn equal to 2 times that of N3 -polytBA-N3 , i.e., a
four-arm star. However, the GPC analysis revealed that both
systems also contained polymers with molecular weight
equal to one-half that of the parent polymer, suggesting
the presence of unreacted azide or alkyne groups.
Thereafter, different types of polymeric networks were
synthesized via click coupling reactions between multifunctional prepolymers containing azide and alkyne groups
[317,346,347] or diene and dienophile groups [348]. The
use of well-defined telechelic polymers produced model
networks [80] with controlled molecular weight between
cross-links [321,346].
5.4. Synthesis of brush/grafted copolymers using the
“grafting-through” method
The “grafting-through” method involves the homopolymerization and/or copolymerization of MMs through their
terminal vinyl groups, which generates “T” shaped branching points along the polymer backbone. It is worth noting
that a linear MM, to some extent, can be treated as a polymer chain containing a pendant vinyl group at the chain
end. If the pendant vinyl group was located randomly along
the backbone of the MM, its polymerization will generate an “X” shaped branching point along the backbone of
the grafted polymer. An attractive feature of the “graftingthrough” method allows every repeat unit of the backbone
containing one side chain. However, due to the low concentration of polymerizable chain-end vinyl groups and
the high steric hindrance around the chain-end propagating radical, the DP of the backbone is low when high
molecular weight MM with a “bulky” structure was used
341
[349]. The incomplete conversion of linear MM requires
an extra purification step (such as fractionation and dialysis) to remove the unincorporated MM. Conventional RP
[350–352], anionic polymerization [353,354], ring-opening
metathesis polymerization [355–358], and coordination
polymerization [359,360] techniques have been applied to
the synthesis of various types of cylindrical brushes using
the “grafting-through” route. The recently developed CRP
methods, allowing a better control of the backbone structure than conventional RP, was successfully employed for
homopolymerization of less stericly hindered PEO MM and
produced a cylindrical brush with backbone DP = 425 at 90%
MM conversion [361]. However, the ATRP of a bulky MM,
such as polyBA MM with an acrylate chain-end group, was
very slow and resulted in limited DP of the backbone [362].
RcP of two MMs allows the synthesis of hetero-grafted
brushes with side chains of different chemical structures.
The relative reactivity of the MMs, affected by the identity of the chain-end vinyl groups, the nature of side
chains and the presence of additives [363–367], determines
the chain-sequence distribution of the two side chains
along the brush backbone. For instance, conventional RcP
of vinylbenzyl-terminated polySt MM and methacryloylterminated PEO MM induced phase separation during the
copolymerization, due to the dramatic polarity difference
of these two MMs. When an additive, such as tin tetrachloride, was added, the resulting hetero-grafted brushes
showed an alternating distribution of the two side chains
along the backbone and interesting aggregation behavior in solution [368]. ATRP was used to copolymerize a
poly(dimethylsiloxane) (PDMS) MM and a PEO MM [369].
After annealing at high temperature the hetero-grafted
copolymer demonstrated a super-soft elastomeric behavior
which indicated the presence of a phase separated amorphous PDMS fraction and crystallized PEO segments [370].
6. Summary and outlook
The explosive development of various CRP techniques
provides an unparallel opportunity for the synthesis of
various types of branched polymers with controlled architecture and site-specific functionality. The employment of
a divinyl cross-linker in the CRP process enables the synthesis of new polymer materials with tailored structures
targeting specialty applications. A variety of functionalities
can be incorporated into the polymers in a predetermined
manner through rational selection of functional initiators,
monomers and/or divinyl cross-linkers. The structure of
polymers varies from star polymers containing a crosslinked core and multiple radiating arms with preselected
functionality to highly branched polymers and/or insoluble
gels by simply changing the sequence of polymerization of
the monomer and cross-linker. Polymerization of monomer
prior to addition of the cross-linker, or polymerization of
monomer after the polymerization of cross-linker, results
in formation of star-like polymers with a cross-linked core,
but with different site-specific functionality. Concurrent
copolymerization of monomer and cross-linker generates
“randomly” branched polymers or gels.
Star polymers are of particular interest in biomedical
and/or pharmaceutical fields and the synthesis of biocom-
342
H. Gao, K. Matyjaszewski / Progress in Polymer Science 34 (2009) 317–350
Scheme 21. Synthesis of degradable model network by ATRP and click chemistry [321]. Reproduced with permission from American Chemical Society.
patible and degradable star nanomaterials with minimal
environmental impact is of increasing importance. ATRP
is by far the most efficient and hence widely applied
CRP technique for the synthesis of star polymers with
core–shell structures and amphiphilic compositions. The
recent development of new ATRP techniques, such as
activators regenerated by electron transfer (ARGET) [39],
and initiators for continuous activator regeneration (ICAR)
[41], dramatically decreased the use of copper catalyst
to ppm level and allowed the polymerization to be performed in the presence of limited amounts of air [371,372].
Moreover, conducting the polymerization in a confined
space, e.g., heterogeneous media, can potentially suppress
the intermolecular termination reactions between multifunctional MIs, while maintaining economically viable
polymerization rates and attaining high monomer conversion [373,374]. The application of these new polymerization
techniques and processes, when combined with the various
developed synthetic methods, will attract much more commercial interest for synthetic star polymers in a number of
diverse applications.
The CRcP of a monomer and a divinyl cross-linker shows
a retarded gelation process in the cross-linking reaction
because of the features of CRP, such as fast initiation and
quick deactivation reactions. Consequently, the network
produced by a CRP has a more homogeneous structure
and preserves the chain-end functionality when compared
to that from conventional RP. The structural difference in
the networks should influence their properties, such as
mechanical strength, swelling ratios and deswelling rates.
A multi-vinyl cross-linker can also be used in CRP crosslinking reactions. It is expected that the study of gels with
controlled structure and functionality will increase in the
future as the materials with improved properties will be
important for membranes, controlled drug release and special separation techniques.
The synthetic advances resulting in predetermined
manipulation of the polymer structure will stimulate the
development of novel characterization techniques to quantitatively determine the topology, composition, level of
functionality, as well as the existence of imperfections in
these complex polymers. A comprehensive understanding
of the structure–property correlation on these materials
will be required in order to rationally design the macromolecular structures based on the final application.
Acknowledgements
The financial support from NSF (DMR-05-49353) and
the CRP Consortium at Carnegie Mellon University is
greatly appreciated. H. Gao acknowledges the support from
McWilliams Fellowship at Carnegie Mellon University.
References
[1] Matyjaszewski K, Davis TP, editors. Handbook of radical polymerization. Hoboken: Wiley; 2002.
[2] Braunecker WA, Matyjaszewski K. Controlled/living radical polymerization: features, developments, and perspectives. Prog Polym
Sci 2007;32:93–146.
H. Gao, K. Matyjaszewski / Progress in Polymer Science 34 (2009) 317–350
[3] Patten TE, Xia JH, Abernathy T, Matyjaszewski K. Polymers with very
low polydispersities from atom transfer radical polymerization. Science 1996;272:866–8.
[4] Coessens V, Pintauer T, Matyjaszewski K. Functional polymers by atom transfer radical polymerization. Prog Polym Sci
2001;26:337–77.
[5] Davis KA, Matyjaszewski K. Statistical, gradient, block and graft
copolymers by controlled/living radical polymerizations. Adv
Polym Sci 2002;159:2–166.
[6] Hawker CJ, Wooley KL. The convergence of synthetic organic and
polymer chemistries. Science 2005;309:1200–5.
[7] Hadjichristidis N, Iatrou H, Pitsikalis M, Mays J. Macromolecular
architectures by living and controlled/living polymerizations. Prog
Polym Sci 2006;31:1068–132.
[8] Matyjaszewski K, Gnanou Y, Leibler L, editors. Macromolecular engineering: from precise macromolecular synthesis to macroscopic
materials properties and applications. Weinheim: Wiley-VCH;
2007.
[9] Zhang M, Mueller AHE. Cylindrical polymer brushes. J Polym Sci,
Part A: Polym Chem 2005;43:3461–81.
[10] Sheiko SS, Sumerlin BS, Matyjaszewski K. Cylindrical molecular
brushes: synthesis, characterization, and properties. Prog Polym Sci
2008;33:759–85.
[11] Fréchet JMJ, Henmi M, Gitsov I, Aoshima S, Leduc MR, Grubbs RB.
Self-condensing vinyl polymerization: an approach to dendritic
materials. Science 1995;269:1080–3.
[12] Matyjaszewski K, Spanswick J. Controlled/living radical polymerization. Mater Today 2005;8:26–33.
[13] Odian G. Principles of polymerization. Hoboken, NJ: Wiley; 2004.
[14] Szwarc M. Living polymers. Nature 1956;178:1168–9.
[15] Penczek S, Cypryk M, Duda A, Kubisa P, Slomkowski S. Living ringopening polymerizations of heterocyclic monomers. Prog Polym Sci
2007;32:247–82.
[16] Uhrig D, Mays JW. Experimental techniques in high-vacuum
anionic polymerization. J Polym Sci, Part A: Polym Chem
2005;43:6179–222.
[17] Georges MK, Veregin RPN, Kazmaier PM, Hamer GK. Narrow
molecular weight resins by a free-radical polymerization process.
Macromolecules 1993;26:2987–8.
[18] Hawker CJ, Bosman AW, Harth E. New polymer synthesis by
nitroxide mediated living radical polymerizations. Chem Rev
2001;101:3661–88.
[19] Wayland BB, Poszmik G, Mukerjee SL, Fryd M. Living radical polymerization of acrylates by organocobalt porphyrin complexes. J Am
Chem Soc 1994;116:7943–4.
[20] Le Grognec E, Claverie J, Poli R. Radical polymerization of styrene
controlled by half-sandwich Mo(III)/Mo(IV) couples: all basic mechanisms are possible. J Am Chem Soc 2001;123:9513–24.
[21] Wang J-S, Matyjaszewski K. Controlled/“Living” radical polymerization. Atom transfer radical polymerization in the presence
of transition-metal complexes. J Am Chem Soc 1995;117:5614–
5.
[22] Matyjaszewski K, Xia JH. Atom transfer radical polymerization.
Chem Rev 2001;101:2921–90.
[23] Kamigaito M, Ando T, Sawamoto M. Metal-catalyzed living radical
polymerization. Chem Rev 2001;101:3689–745.
[24] Tsarevsky NV, Matyjaszewski K. “Green” atom transfer radical
polymerization: from process design to preparation of welldefined environmentally friendly polymeric materials. Chem Rev
2007;107:2270–99.
[25] Kwak Y, Matyjaszewski K. Effect of initiator and ligand structures on
ATRP of styrene and methyl methacrylate initiated by alkyl dithiocarbamate. Macromolecules 2008;41:6627–35.
[26] Matyjaszewski K, Gaynor S, Wang J-S. Controlled radical polymerizations: the use of alkyl iodides in degenerative transfer.
Macromolecules 1995;28:2093–5.
[27] Gaynor SG, Wang J-S, Matyjaszewski K. Controlled radical polymerization by degenerative transfer: effect of the structure of the
transfer agent. Macromolecules 1995;28:8051–6.
[28] Lacroix-Desmazes P, Severac R, Boutevin B. Reverse iodine transfer
polymerization of methyl acrylate and n-butyl acrylate. Macromolecules 2005;38:6299–309.
[29] Boyer C, Lacroix-Desmazes P, Robin J-J, Boutevin B. Reverse iodine
transfer polymerization (RITP) of methyl methacrylate. Macromolecules 2006;39:4044–53.
[30] Chiefari J, Chong YK, Ercole F, Krstina J, Jeffery J, Le TPT,
et al. Living free-radical polymerization by reversible additionfragmentation chain transfer: the RAFT process. Macromolecules
1998;31:5559–62.
343
[31] Moad G, Rizzardo E, Thang SH. Living radical polymerization by the
RAFT process. Aust J Chem 2005;58:379–410.
[32] Moad G, Rizzardo E, Thang SH. Toward living radical polymerization.
Acc Chem Res 2008;41:1133–42.
[33] Fischer H. The persistent radical effect: a principle for selective
radical reactions and living radical polymerizations. Chem Rev
2001;101:3581–610.
[34] Goto A, Fukuda T. Kinetics of living radical polymerization. Prog
Polym Sci 2004;29:329–85.
[35] Tang W, Tsarevsky NV, Matyjaszewski K. Determination of equilibrium constants for atom transfer radical polymerization. J Am Chem
Soc 2006;128:1598–604.
[36] Gromada J, Matyjaszewski K. Simultaneous reverse and normal initiation in atom transfer radical polymerization. Macromolecules
2001;34:7664–71.
[37] Min K, Gao H, Matyjaszewski K. Preparation of homopolymers and
block copolymers in miniemulsion by ATRP using activators generated by electron transfer (AGET). J Am Chem Soc 2005;127:3825–
30.
[38] Jakubowski W, Matyjaszewski K. Activator generated by electron
transfer for atom transfer radical polymerization. Macromolecules
2005;38:4139–46.
[39] Jakubowski W, Min K, Matyjaszewski K. Activators regenerated
by electron transfer for atom transfer radical polymerization of
styrene. Macromolecules 2006;39:39–45.
[40] Jakubowski W, Matyjaszewski K. Activators regenerated by
electron transfer for atom-transfer radical polymerization of
(meth)acrylates and related block copolymers. Angew Chem, Int Ed
2006;45:4482–6.
[41] Matyjaszewski K, Jakubowski W, Min K, Tang W, Huang J, Braunecker WA, et al. Diminishing catalyst concentration in atom
transfer radical polymerization with reducing agents. Proc Natl
Acad Sci USA 2006;103:15309–14.
[42] Burchard W. Solution properties of branched macromolecules. Adv
Polym Sci 1999;143:113–94.
[43] Vlassopoulos D, Fytas G, Pakula T, Roovers J. Multiarm star polymers
dynamics. J Phys: Condens Matter 2001;13:R855–76.
[44] Helms B, Guillaudeu SJ, Xie Y, McMurdo M, Hawker CJ, Fréchet JMJ.
One-pot reaction cascades using star polymers with core-confined
catalysts. Angew Chem, Int Ed 2005;44:6384–7.
[45] Wang F, Bronich TK, Kabanov AV, Rauh RD, Roovers J, Synthesis.
Evaluation of a star amphiphilic block copolymer from poly(␧caprolactone) and poly(ethylene glycol) as a potential drug delivery
carrier. Bioconjugate Chem 2005;16:397–405.
[46] Kreutzer G, Ternat C, Nguyen TQ, Plummer CJG, Mnson JAE, Castelletto V, et al. Unimolecular containers based on
amphiphilic multiarm star block copolymers. Macromolecules
2006;39:4507–16.
[47] Fukukawa K-i, Rossin R, Hagooly A, Pressly ED, Hunt JN, Messmore
BW, et al. Synthesis and characterization of core–shell star copolymers for in vivo PET imaging applications. Biomacromolecules
2008;9:1329–39.
[48] Hadjichristidis N. Synthesis of miktoarm star (␮-star) polymers. J
Polym Sci, Part A: Polym Chem 1999;37:857–71.
[49] Zilliox JG, Rempp P, Parrod J. Preparation of star-shaped macromolecules by anionic copolymerization. J Polym Sci, Polym Symp
1968;22:145–56.
[50] Bi L-K, Fetters LJ. Synthesis and properties of block copolymers.
3. Polystyrene–polydiene star block copolymers. Macromolecules
1976;9:732–42.
[51] Taton D. In: Matyjaszewski K, Gnanou Y, Leibler L, editors. Macromolecular engineering: from precise macromolecular synthesis
to macroscopic materials properties and applications. Weinheim:
Wiley-VCH; 2007. p. 1007–56.
[52] Xia JH, Zhang X, Matyjaszewski K. Synthesis of star-shaped
polystyrene by atom transfer radical polymerization using an “arm
first” approach. Macromolecules 1999;32:4482–4.
[53] Zhang X, Xia JH, Matyjaszewski K. End-functional poly(tert-butyl
acrylate) star polymers by controlled radical polymerization.
Macromolecules 2000;33:2340–5.
[54] Furukawa T, Ishizu K. Synthesis and viscoelastic behavior of multiarm star polyelectrolytes. Macromolecules 2005;38:2911–7.
[55] Gao H, Matyjaszewski K. Structural control in ATRP synthesis
of star polymers using the arm-first method. Macromolecules
2006;39:3154–60.
[56] Baek KY, Kamigaito M, Sawamoto M. Star-shaped polymers
by metal-catalyzed living radical polymerization. 1. Design of
Ru(II)-based systems and divinyl linking agents. Macromolecules
2001;34:215–21.
344
H. Gao, K. Matyjaszewski / Progress in Polymer Science 34 (2009) 317–350
[57] Baek K-Y, Kamigaito M, Sawamoto M. Synthesis of star-shaped
copolymers with methyl methacrylate and n-butyl methacrylate
by metal-catalyzed living radical polymerization: block and random copolymer arms and microgel cores. J Polym Sci, Part A: Polym
Chem 2002;40:633–41.
[58] Baek K-Y, Kamigaito M, Sawamoto M. Star-shaped polymers by
Ru(II)-catalyzed living radical polymerization. II. Effective reaction conditions and characterization by multi-angle laser light
scattering/size exclusion chromatography and small-angle X-ray
scattering. J Polym Sci, Part A: Polym Chem 2002;40:2245–55.
[59] Baek KY, Kamigaito M, Sawamoto M. Core-functionalized star polymers by transition metal-catalyzed living radical polymerization. 1.
Synthesis and characterization of star polymers with PMMA arms
and amide cores. Macromolecules 2001;34:7629–35.
[60] Terashima T, Kamigaito M, Baek K-Y, Ando T, Sawamoto M. Polymer catalysts from polymerization catalysts: direct encapsulation of
metal catalyst into star polymer core during metal-catalyzed living
radical polymerization. J Am Chem Soc 2003;125:5288–9.
[61] Baek K-Y, Kamigaito M, Sawamoto M. Star poly(methyl methacrylate) with end-functionalized arm chains by ruthenium-catalyzed
living radical polymerization. J Polym Sci, Part A: Polym Chem
2002;40:1972–82.
[62] Matyjaszewski K. The synthesis of functional star copolymers as an
illustration of the importance of controlling polymer structures in
the design of new materials. Polym Int 2003;52:1559–65.
[63] Taton D, Gnanou Y, Matmour R, Angot S, Hou S, Francis R, et al.
Controlled polymerizations as tools for the design of star-like and
dendrimer-like polymers. Polym Int 2006;55:1138–45.
[64] Wiltshire JT, Qiao GG. Recent advances in star polymer design:
degradability and the potential for drug delivery. Aust J Chem
2007;60:699–705.
[65] Blencowe A, Kit GT, Best SP, Qiao GG. Synthesis of Buckminsterfullerene C60 functionalised core cross-linked star polymers.
Polymer 2008;49:825–30.
[66] Adkins CT, Harth E. Synthesis of star polymer architectures with siteisolated chromophore cores. Macromolecules 2008;41:3472–80.
[67] Spiniello M, Blencowe A, Qiao GG. Synthesis and characterization of
fluorescently labeled core cross-linked star polymers. J Polym Sci,
Part A: Polym Chem 2008;46:2422–32.
[68] Gao H, Tsarevsky NV, Matyjaszewski K. Synthesis of degradable miktoarm star copolymers via atom transfer radical polymerization.
Macromolecules 2005;38:5995–6004.
[69] Themistou E, Patrickios CS. Synthesis and characterization of
polymer networks and star polymers containing a novel, hydrolyzable acetal-based dimethacrylate cross-linker. Macromolecules
2006;39:73–80.
[70] Kafouris D, Themistou E, Patrickios CS. Synthesis and characterization of star polymers and cross-linked star polymer model networks
with cores based on an asymmetric, hydrolyzable dimethacrylate
cross-linker. Chem Mater 2006;18:85–93.
[71] Themistou E, Patrickios CS. Synthesis and characterization of star
polymers and cross-linked star polymer model networks containing
a novel, silicon-based, hydrolyzable cross-linker. Macromolecules
2004;37:6734–43.
[72] Connal LA, Vestberg R, Hawker CJ, Qiao GG. Synthesis of dendron
functionalized core cross-linked star polymers. Macromolecules
2007;40:7855–63.
[73] Bouilhac C, Cramail H, Cloutet E, Deffieux A, Taton D.
Benzophenone-functionalized, starlike polystyrenes as organic
supports for a tridentate bis(imino)pyridinyliron/trimethylaluminum catalytic system for ethylene polymerization. J Polym Sci,
Part A: Polym Chem 2006;44:6997–7007.
[74] Bouilhac C, Cloutet E, Cramail H, Deffieux A, Taton D. Functionalized star-like polystyrenes as organic supports of a
tridentate bis(imino)pyridinyliron/aluminic derivative catalytic
system for ethylene polymerization. Macromol Rapid Commun
2005;26:1619–25.
[75] Kronenthal RL. Biodegradable polymers in medicine and surgery.
Polym Sci Technol 1975;8:119–37.
[76] Du J, Chen Y. PCL star polymer, PCL-PS heteroarm star polymer
by ATRP, and core-carboxylated PS star polymer thereof. Macromolecules 2004;37:3588–94.
[77] Wiltshire JT, Qiao GG. Selectively degradable core cross-linked star
polymers. Macromolecules 2006;39:9018–27.
[78] Jakubowski W, Lutz J-F, Slomkowski S, Matyjaszewski K. Block
and random copolymers as surfactants for dispersion polymerization. I. Synthesis via atom transfer radical polymerization and
ring-opening polymerization. J Polym Sci, Part A: Polym Chem
2005;43:1498–510.
[79] He T, Adams DJ, Butler MF, Yeoh CT, Cooper AI, Rannard SP. Direct
synthesis of anisotropic polymer nanoparticles. Angew Chem, Int
Ed 2007;46:9243–7.
[80] Hild G. Model networks based on ‘endlinking’ processes: synthesis,
structure and properties. Prog Polym Sci 1998;23:1019–149.
[81] Patrickios CS, Georgiou TK. Covalent amphiphilic polymer networks. Curr Opin Colloid Interface Sci 2003;8:76–85.
[82] Lee H-J, Lee K, Choi N. Controlled anionic synthesis of star-shaped
polystyrene by the incremental addition of divinylbenzene. J Polym
Sci, Part A: Polym Chem 2005;43:870–8.
[83] Gao H, Ohno S, Matyjaszewski K. Low polydispersity star polymers via cross-linking macromonomers by ATRP. J Am Chem Soc
2006;128:15111–3.
[84] Gao H, Matyjaszewski K. Low-polydispersity star polymers with
core functionality by cross-linking macromonomers using functional ATRP initiators. Macromolecules 2007;40:399–401.
[85] Ishizu K, Sunahara K. Synthesis of star polymers by organized polymerization of macromonomers. Polymer 1995;36:4155–7.
[86] Furukawa T, Ishizu K. Synthesis and characterization of
poly(ethylene oxide) star polymers possessing a tertiary amino
group at each arm end by organized polymerization using
macromonomers. J Colloid Interface Sci 2002;253:465–9.
[87] Schon F, Hartenstein M, Muller AHE. New strategy for the synthesis
of halogen-free acrylate macromonomers by atom transfer radical
polymerization. Macromolecules 2001;34:5394–7.
[88] Muehlebach A, Rime F. Synthesis of well-defined macromonomers
and comb copolymers from polymers made by atom transfer radical
polymerization. J Polym Sci, Part A: Polym Chem 2003;41:3425–
39.
[89] Vogt AP, Sumerlin BS. An efficient route to macromonomers via
ATRP and click chemistry. Macromolecules 2006;39:5286–92.
[90] Abrol S, Kambouris PA, Looney MG, Solomon DH. Studies on microgels. Part 3. Synthesis using living free-radical polymerization.
Macromol Rapid Commun 1997;18:755–60.
[91] Pasquale AJ, Long TE. Synthesis of star-shaped polystyrenes via
nitroxide-mediated stable free-radical polymerization. J Polym Sci,
Part A: Polym Chem 2001;39:216–23.
[92] Tsoukatos T, Pispas S, Hadjichristidis N. Star-branched polystyrenes
by nitroxide living free-radical polymerization. J Polym Sci, Part A:
Polym Chem 2001;39:320–5.
[93] Narumi A, Satoh T, Kaga H, Kakuchi T. Glycoconjugated polymer. 3.
Synthesis and amphiphilic property of core-glycoconjugated starshaped polystyrene. Macromolecules 2002;35:699–705.
[94] Narumi A, Yamane S, Miura Y, Kaga H, Satoh T, Kakuchi T. Starshaped polystyrenes with glycoconjugated periphery and interior:
synthesis and entrapment of hydrophilic molecule. J Polym Sci, Part
A: Polym Chem 2005;43:4373–81.
[95] Bosman AW, Heumann A, Klaerner G, Benoit D, Fréchet JMJ, Hawker
CJ. High-throughput synthesis of nanoscale materials: structural
optimization of functionalized one-step star polymers. J Am Chem
Soc 2001;123:6461–2.
[96] Bosman AW, Vestberg R, Heumann A, Fréchet JMJ, Hawker CJ.
A modular approach toward functionalized three-dimensional
macromolecules: from synthetic concepts to practical applications.
J Am Chem Soc 2003;125:715–28.
[97] Moad G, Mayadunne RTA, Rizzardo E, Skidmore M, Thang SH.
Synthesis of novel architectures by radical polymerization with
reversible addition fragmentation chain transfer (RAFT polymerization). Macromol Symp 2003;192:1–12.
[98] Lord HT, Quinn JF, Angus SD, Whittaker MR, Stenzel MH, Davis TP.
Microgel stars via Reversible Addition Fragmentation Chain Transfer
(RAFT) polymerisation—a facile route to macroporous membranes,
honeycomb patterned thin films and inverse opal substrates. J Mater
Chem 2003;13:2819–24.
[99] Zheng G, Pan C. Preparation of star polymers based on polystyrene
or poly(styrene-b-N-isopropyl acrylamide) and divinylbenzene via
reversible addition-fragmentation chain transfer polymerization.
Polymer 2005;46:2802–10.
[100] Zheng G, Pan C. Reversible addition-fragmentation transfer polymerization in nanosized micelles formed in situ. Macromolecules
2006;39:95–102.
[101] Zheng Q, Zheng G-h, Pan C-y. Preparation of nano-sized
poly(ethylene oxide) star microgels via reversible additionfragmentation transfer polymerization in selective solvents. Polym
Int 2006;55:1114–23.
[102] Otsu T, Yoshida M, Tazaki T. A model for living radical polymerization. Makromol Chem, Rapid Commun 1982;3:133–40.
[103] Otsu T. Iniferter concept and living radical polymerization. J Polym
Sci, Part A: Polym Chem 2000;38:2121–36.
H. Gao, K. Matyjaszewski / Progress in Polymer Science 34 (2009) 317–350
[104] Iovu MC, Matyjaszewski K. Controlled/living radical polymerization of vinyl acetate by degenerative transfer with alkyl iodides.
Macromolecules 2003;36:9346–54.
[105] Goto A, Kwak Y, Fukuda T, Yamago S, Iida K, Nakajima M, et al.
Mechanism-based invention of high-speed living radical polymerization using organotellurium compounds and azo-initiators. J Am
Chem Soc 2003;125:8720–1.
[106] Yamago S, Ray B, Iida K, Yoshida J, Tada T, Yoshizawa K, et al. Highly
versatile organostibine mediators for living radical polymerization.
J Am Chem Soc 2004;126:13908–9.
[107] Wayland BB, Peng C-H, Fu X, Lu Z, Fryd M. Degenerative transfer and reversible termination mechanisms for living radical
polymerizations mediated by cobalt porphyrins. Macromolecules
2006;39:8219–22.
[108] Braunecker WA, Itami Y, Matyjaszewski K. Osmium-mediated radical polymerization. Macromolecules 2005;38:9402–4.
[109] Ishizu K, Katsuhara H, Itoya K. Synthesis of amphiphilic star block
copolymers via diethyldithiocarbamate-mediated living radical
polymerization. J Polym Sci, Part A: Polym Chem 2006;44:3321–7.
[110] Yamauchi K, Takahashi K, Hasegawa H, Iatrou H, Hadjichristidis N,
Kaneko T, et al. Microdomain morphology in an ABC 3-miktoarm
star terpolymer: a study by energy-filtering TEM and 3D electron
tomography. Macromolecules 2003;36:6962–6.
[111] Mavroudis A, Avgeropoulos A, Hadjichristidis N, Thomas EL, Lohse
DJ. Synthesis and morphological behavior of model linear and miktoarm star copolymers of 2-methyl-1,3-pentadiene and styrene.
Chem Mater 2003;15:1976–83.
[112] Mavroudis A, Avgeropoulos A, Hadjichristidis N, Thomas EL, Lohse
DJ. Synthesis and morphological behavior of model 6-miktoarm
star copolymers, PS(P2MP)5 , of styrene (S) and 2-methyl-1,3pentadiene (P2MP). Chem Mater 2006;18:2164–8.
[113] Li ZB, Kesselman E, Talmon Y, Hillmyer MA, Lodge TP. Multicompartment micelles from ABC miktoarm stars in water. Science
2004;306:98–101.
[114] Li ZB, Hillmyer MA, Lodge TP. Morphologies of multicompartment
micelles formed by ABC miktoarm star terpolymers. Langmuir
2006;22:9409–17.
[115] Peleshanko S, Jeong J, Gunawidjaja R, Tsukruk VV. Amphiphilic heteroarm PEO-b-PSm star polymers at the air–water interface: aggregation and surface morphology. Macromolecules 2004;37:6511–
22.
[116] Sumerlin BS, Neugebauer D, Matyjaszewski K. Initiation efficiency
in the synthesis of molecular brushes by grafting from via atom
transfer radical polymerization. Macromolecules 2005;38:702–8.
[117] Gao H, Matyjaszewski K. Synthesis of miktoarm star polymers via
ATRP using the “In-Out” method: determination of initiation efficiency of star macroinitiators. Macromolecules 2006;39:7216–23.
[118] Du J, Chen Y. Preparation of poly(ethylene oxide) star polymers and
poly(ethylene oxide)–polystyrene heteroarm star polymers by atom
transfer radical polymerization. J Polym Sci, Part A: Polym Chem
2004;42:2263–71.
[119] Gao H, Matyjaszewski K. Arm-first method as a simple and general
method for synthesis of miktoarm star copolymers. J Am Chem Soc
2007;129:11828–34.
[120] Pasch H, Trathnigg B. HPLC of polymers. Berlin: Springer; 1997.
[121] Gao H, Min K, Matyjaszewski K. Characterization of linear and 3-arm
star block copolymers by liquid chromatography at critical conditions. Macromol Chem Phys 2006;207:1709–17.
[122] Gao H, Matyjaszewski K. Synthesis of low-polydispersity miktoarm
star copolymers via a simple “Arm-First” method: macromonomers
as arm precursors. Macromolecules 2008;41:4250–7.
[123] Eschwey H, Hallensleben M, Burchard W. Preparation and some
properties of star-shaped polymers with more than a hundred side
chains. Makromol Chem 1973;173:235–9.
[124] Gnanou Y, Lutz P, Rempp P. Synthesis of star-shaped poly(ethylene
oxide). Makromol Chem 1988;189:2885–92.
[125] Gao H, Matyjaszewski K. Synthesis of star polymers by a new
“Core-First” method: sequential polymerization of cross-linker and
monomer. Macromolecules 2008;41:1118–25.
[126] Deng G, Chen Y. A novel way to synthesize star polymers in one
pot by ATRP of N-[2-(2-bromoisobutyryloxy)ethyl]maleimide and
styrene. Macromolecules 2004;37:18–26.
[127] Greenley RZ. In: Brandup J, Immergut EH, Grulke EA, editors. Polymer handbook. New York: Wiley; 1999.
[128] Deng G, Cao M, Huang J, He L, Chen Y. One-pot synthesis of star
polymer by ATRP of bismaleimide and an excess of styrene with a
conventional initiator. Polymer 2005;46:5698–701.
[129] Ishizu K, Ochi K. Architecture of star-block copolymers consisting
of triblock arms via a N,N-diethyldithiocarbamate-mediated living
[130]
[131]
[132]
[133]
[134]
[135]
[136]
[137]
[138]
[139]
[140]
[141]
[142]
[143]
[144]
[145]
[146]
[147]
[148]
[149]
[150]
[151]
[152]
[153]
[154]
[155]
345
radical photo-polymerization and application for nanocomposites
by using as fillers. Macromolecules 2006;39:3238–44.
Ishizu K, Uchida S. Synthesis and microphase-separated structures
of star-block copolymers. Prog Polym Sci 2000;24:1439–80.
Ouchi M, Terashima T, Sawamoto M. Precision control of radical
polymerization via transition metal catalysis: from dormant species
to designed catalysts for precision functional polymers. Acc Chem
Res 2008;41:1120–32.
Terashima T, Ouchi M, Ando T, Kamigaito M, Sawamoto M. Metalcomplex-bearing star polymers by metal-catalyzed living radical
polymerization: synthesis and characterization of poly(methyl
methacrylate) star polymers with Ru(II)-embedded microgel cores.
J Polym Sci, Part A: Polym Chem 2006;44:4966–80.
Terashima T, Ouchi M, Ando T, Kamigaito M, Sawamoto M.
Amphiphilic, thermosensitive ruthenium(II)-bearing star polymer
catalysts: one-pot synthesis of PEG armed star polymers with
ruthenium(II)-enclosed microgel cores via metal-catalyzed living
radical polymerization. Macromolecules 2007;40:3581–8.
Chi Y, Scroggins ST, Fréchet JMJ. One-pot multi-component asymmetric cascade reactions catalyzed by soluble star polymers with
highly branched non-interpenetrating catalytic cores. J Am Chem
Soc 2008;130:6322–3.
Youk JH, Park M-K, Locklin J, Advincula R, Yang J, Mays J. Preparation
of aggregation stable gold nanoparticles using star-block copolymers. Langmuir 2002;18:2455–8.
Ishizu K, Furukawa T, Yamada H. Silver nanoparticles dispersed
within amphiphilic star-block copolymers as templates for plasmon
band materials. Eur Polym J 2005;41:2853–60.
Murray CB, Kagan CR, Bawendi MG. Synthesis and characterization of monodisperse nanocrystals and close-packed nanocrystal
assemblies. Annu Rev Mater Sci 2000;30:545–610.
Decher G. Fuzzy nanoassemblies: toward layered polymeric multicomposites. Science 1997;277:1232–7.
Caruso F. Nanoengineering of particle surfaces. Adv Mater
2001;13:11–22.
Hammond PT. Form and function in multilayer assembly: new
applications at the nanoscale. Adv Mater 2004;16:1271–93.
Yang S, Zhang Y, Wang L, Hong S, Xu J, Chen Y, et al. Composite thin film by hydrogen-bonding assembly of polymer brush and
poly(vinylpyrrolidone). Langmuir 2006;22:338–43.
Connal LA, Li Q, Quinn JF, Tjipto E, Caruso F, Qiao GG. pH-responsive
poly(acrylic acid) core cross-linked star polymers: morphology
transitions in solution and multilayer thin films. Macromolecules
2008;41:2620–6.
Kim B-S, Gao H, Argun AA, Matyjaszewski K, Hammond PT. All star
polymer multilayers as pH-responsive nanofilms. Macromolecules
2008. ASAP.
Flory PJ. Principles of polymer chemistry. Ithaca, NY: Cornell University Press; 1953. p. 347–398.
Stockmayer WH. Theory of molecular size distribution and gel formation in branched polymers. II. General cross linking. J Chem Phys
1944;12:125–31.
Flory PJ. Molecular size distribution in three dimensional polymers. III. Tetrafunctional branching units. J Am Chem Soc
1941;63:3096–100.
Walling CJ. Gel formation in addition polymerization. J Am Chem
Soc 1945;67:441–7.
Gordon M. Network theory of the gel point and the “incestuous”
polymerization of diallyl phthalate. J Chem Phys 1954;22:610–3.
Matsumoto A. Polymerization of multiallyl monomers. Prog Polym
Sci 2001;26:189–257.
Dusek K, Duskova-Smrckova M. Network structure formation
during crosslinking of organic coating systems. Prog Polym Sci
2000;25:1215–60.
Zhu S, Hamielec AE. Influence of cross-link density distribution on
network formation in free-radical copolymerization of vinyl/divinyl
monomers. Macromolecules 1992;25:5457–64.
Matsumoto A. Free-radical crosslinking polymerization and
copolymerization of multivinyl compounds. Adv Polym Sci
1995;123:41–80.
Bastide J, Leibler L. Large-scale heterogeneities in randomly crosslinked networks. Macromolecules 1988;21:2647–9.
Kannurpatti AR, Anseth JW, Bowman CN. A study of the evolution
of mechanical properties and structural heterogeneity of polymer networks formed by photopolymerizations of multifunctional
(meth)acrylates. Polymer 1998;39:2507–13.
Funke W, Okay O, Joos-Muller B. Microgels-intramolecularly
crosslinked macromolecules with a globular structure. Adv Polym
Sci 1998;136:138–234.
346
H. Gao, K. Matyjaszewski / Progress in Polymer Science 34 (2009) 317–350
[156] Gordon M, Roe R-J. Diffusion and gelation in polyadditions. IV. Statistical theory of ring formation and the absolute gel point. J Polym
Sci 1956;21:75–90.
[157] Ide N, Fukuda T. Nitroxide-controlled free-radical copolymerization of vinyl and divinyl monomers. Evaluation of pendant-vinyl
reactivity. Macromolecules 1997;30:4268–71.
[158] Ide N, Fukuda T. Nitroxide-controlled free-radical copolymerization of vinyl and divinyl monomers. 2. Gelation. Macromolecules
1999;32:95–9.
[159] Yu Q, Zeng F, Zhu S. Atom transfer radical polymerization of poly(ethylene glycol) dimethacrylate. Macromolecules
2001;34:1612–8.
[160] Isaure F, Cormack PAG, Graham S, Sherrington DC, Armes SP,
Buetuen V. Synthesis of branched poly(methyl methacrylate)s
via controlled/living polymerisations exploiting ethylene glycol
dimethacrylate as branching agent. Chem Commun 2004:1138–9.
[161] Tsarevsky NV, Matyjaszewski K. Combining atom transfer radical
polymerization and disulfide/thiol redox chemistry: a route to welldefined (Bio)degradable polymeric materials. Macromolecules
2005;38:3087–92.
[162] Li Y, Armes SP. Synthesis and chemical degradation of branched
vinyl polymers prepared via ATRP: use of a cleavable disulfide-based
branching agent. Macromolecules 2005;38:8155–62.
[163] Liu B, Kazlauciunas A, Guthrie JT, Perrier S. One-pot hyperbranched
polymer synthesis mediated by reversible addition fragmentation chain transfer (RAFT) polymerization. Macromolecules
2005;38:2131–6.
[164] Liu B, Kazlauciunas A, Guthrie JT, Perrier S. Influence of reaction parameters on the synthesis of hyperbranched polymers via
reversible addition fragmentation chain transfer (RAFT) polymerization. Polymer 2005;46:6293–9.
[165] Wang AR, Zhu S. Control of the polymer molecular weight in
atom transfer radical polymerization with branching/crosslinking.
J Polym Sci, Part A: Polym Chem 2005;43:5710–4.
[166] Wang AR, Zhu S. Branching and gelation in atom transfer radical polymerization of methyl methacrylate and ethylene glycol
dimethacrylate. Polym Eng Sci 2005;45:720–7.
[167] Taton D, Baussard J-F, Dupayage L, Poly J, Gnanou Y, Ponsinet V, et
al. Water soluble polymeric nanogels by xanthate-mediated radical
crosslinking copolymerization. Chem Commun 2006:1953–5.
[168] Yu Q, Zhang J, Cheng M, Zhu S. Kinetic behavior of atom transfer
radical polymerization of dimethacrylates. Macromol Chem Phys
2006;207:287–94.
[169] Bannister I, Billingham NC, Armes SP, Rannard SP, Findlay P. Development of branching in living radical copolymerization of vinyl and
divinyl monomers. Macromolecules 2006;39:7483–92.
[170] Bouhier M-H, Cormack P, Graham S, Sherrington DC. Synthesis of
densely branched poly(methyl methacrylate)s via ATR copolymerization of methyl methacrylate and ethylene glycol dimethacrylate.
J Polym Sci, Part A: Polym Chem 2007;45:2375–86.
[171] Gao H, Min K, Matyjaszewski K. Determination of gel point during
atom transfer radical copolymerization with cross-linker. Macromolecules 2007;40:7763–70.
[172] Saka Y, Zetterlund PB, Okubo M. Gel formation and primary chain
lengths in nitroxide-mediated radical copolymerization of styrene
and divinylbenzene in miniemulsion. Polymer 2007;48:1229–
36.
[173] Vo C-D, Rosselgong J, Armes SP, Billingham NC. RAFT synthesis of
branched acrylic copolymers. Macromolecules 2007;40:7119–25.
[174] Yu Q, Zhou M, Ding Y, Jiang B, Zhu S. Development of networks in
atom transfer radical polymerization of dimethacrylates. Polymer
2007;48:7058–64.
[175] Gao H, Li W, Matyjaszewski K. Synthesis of polyacrylate networks
by ATRP: parameters influencing experimental gel points. Macromolecules 2008;41:2335–40.
[176] Yu Q, Zhu Y, Ding Y, Zhu S. Reaction behavior and network development in RAFT radical polymerization of dimethacrylates. Macromol
Chem Phys 2008;209:551–6.
[177] Li Y, Ryan AJ, Armes SP. Synthesis of well-defined branched
copolymers by quaternization of near-monodisperse homopolymers. Macromolecules 2008;41:5577–81.
[178] Gao H, Miasnikova A, Matyjaszewski K. Effect of cross-linker reactivity on experimental gel points during ATRcP of monomer and
cross-linker. Macromolecules 2008;41:7843–9.
[179] Liu QF, Zhang P, Qing AX, Lan YX, Lu MG. Poly(N-isopropylacrylamide) hydrogels with improved shrinking kinetics by RAFT polymerization. Polymer 2006;47:2330–6.
[180] Lovell PA, El-Aasser, Mohamed S, editors. Emulsion polymerization
and emulsion polymers. New York, NY: John Wiley & Sons, Inc.; 1997.
[181] Kawaguchi H. Functional polymer microspheres. Prog Polym Sci
2000;25:1171–210.
[182] Qiu J, Charleux B, Matyjaszewski K. Controlled/living radical polymerization in aqueous media: homogeneous and heterogeneous
systems. Prog Polym Sci 2001;26:2083–134.
[183] Cunningham MF. Living/controlled radical polymerizations in dispersed phase systems. Prog Polym Sci 2002;27:1039–67.
[184] Save M, Guillaneuf Y, Gilbert RG. Controlled radical polymerization
in aqueous dispersed media. Aust J Chem 2006;59:693–711.
[185] McLeary JB, Klumperman B. RAFT mediated polymerisation in heterogeneous media. Soft Matter 2006;2:45–53.
[186] Cunningham MF. Controlled/living radical polymerization in aqueous dispersed systems. Prog Polym Sci 2008;33:365–98.
[187] Oh JK, Drumright R, Siegwart DJ, Matyjaszewski K. The development
of microgels/nanogels for drug delivery applications. Prog Polym Sci
2008;33:448–77.
[188] Zetterlund PB, Kagawa Y, Okubo M. Controlled/living radical
polymerization in dispersed systems. Chem Rev 2008;108:3747–
94.
[189] O’Brien N, McKee A, Sherrington DC, Slark AT, Titterton A. Facile, versatile and cost effective route to branched vinyl polymers. Polymer
2000;41:6027–31.
[190] Isaure F, Cormack PAG, Sherrington DC. Facile synthesis of branched
poly(methyl methacrylate)s. J Mater Chem 2003;13:2701–10.
[191] Weaver JVM, Williams RT, Royles BJL, Findlay PH, Cooper AI, Rannard
SP. pH-responsive branched polymer nanoparticles. Soft Matter
2008;4:985–92.
[192] Durie S, Jerabek K, Mason C, Sherrington DC. One-pot suspension
polymerization of branched poly(styrene-divinylbenzene). Macromolecules 2002;35:9665–72.
[193] Isaure F, Cormack PAG, Sherrington DC. Synthesis of branched
poly(methyl methacrylate)s: effect of the branching comonomer
structure. Macromolecules 2004;37:2096–105.
[194] Sato T, Ihara H, Hirano T, Seno M. Formation of soluble hyperbranched polymer through the initiator-fragment incorporation
radical copolymerization of ethylene glycol dimethacrylate with
N-methylmethacrylamide. Polymer 2004;45:7491–8.
[195] Sato T, Arima Y, Seno M, Hirano T. Initiator-fragment incorporation radical polymerization of divinyl adipate with dimethyl
2,2 -azobis(isobutyrate): kinetics and formation of soluble hyperbranched polymer. Macromolecules 2005;38:1627–32.
[196] Mueller AHE, Yan D, Wulkow M. Molecular parameters of
hyperbranched polymers made by self-condensing vinyl polymerization. 1. Molecular weight distribution. Macromolecules
1997;30:7015–23.
[197] Yan D, Mueller AHE, Matyjaszewski K. Molecular parameters of
hyperbranched polymers made by self-condensing vinyl polymerization. 2. Degree of branching. Macromolecules 1997;30:7024–
33.
[198] Baskaran D. Synthesis of hyperbranched polymers by anionic
self-condensing vinyl polymerization. Macromol Chem Phys
2001;202:1569–75.
[199] Paulo C, Puskas JE. Synthesis of hyperbranched polyisobutylenes by
inimer-type living polymerization. 1. Investigation of the effect of
reaction conditions. Macromolecules 2001;34:734–9.
[200] Gaynor SG, Edelman S, Matyjaszewski K. Synthesis of branched and
hyperbranched polystyrenes. Macromolecules 1996;29:1079–81.
[201] Matyjaszewski K, Gaynor SG, Kulfan A, Podwika M. Preparation of
hyperbranched polyacrylates by atom transfer radical polymerization. 1. Acrylic AB* monomers in “living” radical polymerizations.
Macromolecules 1997;30:5192–4.
[202] Matyjaszewski K, Gaynor SG, Muller AHE. Preparation of hyperbranched polyacrylates by atom transfer radical polymerization. 2.
Kinetics and mechanism of chain growth for the self-condensing
vinyl polymerization of 2-((2-bromopropionyl)oxy)ethyl acrylate.
Macromolecules 1997;30:7034–41.
[203] Matyjaszewski K, Pyun J, Gaynor SG. Preparation of hyperbranched
polyacrylates by atom transfer radical polymerisation. 4. The use of
zero-valent copper. Macromol Rapid Commun 1998;19:665–70.
[204] Hawker CJ, Fréchet JMJ, Grubbs RB, Dao J. Preparation of hyperbranched and star polymers by a “Living”, self-condensing free
radical polymerization. J Am Chem Soc 1995;117:10763–4.
[205] Carter S, Rimmer S, Sturdy A, Webb M. Highly branched stimuli
responsive poly[(N-isopropyl acrylamide)-co-(1,2-propanediol-3methacrylate)]s with protein binding functionality. Macromol
Biosci 2005;5:373–8.
[206] Sunder A, Hanselmann R, Frey H, Muelhaupt R. Controlled synthesis of hyperbranched polyglycerols by ring-opening multibranching
polymerization. Macromolecules 1999;32:4240–6.
H. Gao, K. Matyjaszewski / Progress in Polymer Science 34 (2009) 317–350
[207] Sunder A, Bauer T, Muelhaupt R, Frey H. Synthesis and thermal
behavior of esterified aliphatic hyperbranched polyether polyols.
Macromolecules 2000;33:1330–7.
[208] Sunder A, Mulhaupt R, Haag R, Frey H. Hyperbranched polyether
polyols. A modular approach to complex polymer architectures. Adv
Mater 2000;12:235–9.
[209] Litvinenko GI, Simon PFW, Mueller AHE. Molecular parameters of hyperbranched copolymers obtained by self-condensing
vinyl copolymerization. 1. Equal rate constants. Macromolecules
1999;32:2410–9.
[210] Mori H, Seng DC, Lechner H, Zhang M, Mueller AHE. Synthesis
and characterization of branched polyelectrolytes. 1. Preparation of
hyperbranched poly(acrylic acid) via self-condensing atom transfer
radical copolymerization. Macromolecules 2002;35:9270–81.
[211] Mori H, Walther A, Andre X, Lanzendoerfer MG, Mueller
AHE. Synthesis of highly branched cationic polyelectrolytes via
self-condensing atom transfer radical copolymerization with
2-(diethylamino)ethyl methacrylate. Macromolecules 2004;37:
2054–66.
[212] Cheng G, Simon PFW, Hartenstein M, Muller AHE. Synthesis of
hyperbranched poly(tert-butyl acrylate) by self-condensing atom
transfer radical polymerization of a macroinimer. Macromol Rapid
Commun 2000;21:846–52.
[213] Peeters JW, Palmans ARA, Meijer EW, Koning CE, Heise A. Chemoenzymatic synthesis of branched polymers. Macromol Rapid Commun
2005;26:684–9.
[214] Simon PFW, Muller AHE. Molecular parameters of hyperbranched
polymers made by self-condensing vinyl polymerization of
macroinimers. Macromol Theory Sim 2000;9:621–7.
[215] Hawker CJ. Architectural control in “living” free radical polymerizations: preparation of star and graft polymers. Angew Chem, Int Ed
1995;34:1456–9.
[216] Wang J-S, Greszta D, Matyjaszewski K. Atom transfer radical
polymerization (ATRP): a new approach towards well-defined
(co)polymers. Polym Mater Sci Eng 1995;73:416–7.
[217] Matyjaszewski K, Miller PJ, Fossum E, Nakagawa Y. Synthesis of
block, graft and star polymers from inorganic macroinitiators. Appl
Organomet Chem 1998;12:667–73.
[218] Ueda J, Matsuyama M, Kamigaito M, Sawamoto M. Multifunctional initiators for the ruthenium-mediated living radical
polymerization of methyl methacrylate: di- and trifunctional
dichloroacetates for synthesis of multiarmed polymers. Macromolecules 1998;31:557–62.
[219] Angot S, Murthy KS, Taton D, Gnanou Y. Atom transfer radical
polymerization of styrene using a novel octafunctional initiator: synthesis of well-defined polystyrene stars. Macromolecules
1998;31:7218–25.
[220] Ueda J, Kamigaito M, Sawamoto M. Calixarene-core multifunctional
initiators for the ruthenium-mediated living radical polymerization
of methacrylates. Macromolecules 1998;31:6762–8.
[221] Hedrick JL, Trollss M, Hawker CJ, Atthoff B, Claesson H, Heise A, et
al. Dendrimer-like star block and amphiphilic copolymers by combination of ring opening and atom transfer radical polymerization.
Macromolecules 1998;31:8691–705.
[222] Matyjaszewski K, Miller PJ, Pyun J, Kickelbick G, Diamanti S. Synthesis and characterization of star polymers with varying arm number,
length, and composition from organic and hybrid inorganic/organic
multifunctional initiators. Macromolecules 1999;32:6526–35.
[223] Heise A, Hedrick JL, Frank CW, Miller RD. Starlike block copolymers
with amphiphilic arms as models for unimolecular micelles. J Am
Chem Soc 1999;121:8647–8.
[224] Coessens V, Pyun J, Miller PJ, Gaynor SG, Matyjaszewski K. Functionalization of polymers prepared by ATRP using radical addition
reactions. Macromol Rapid Commun 2000;21:103–9.
[225] Ohno K, Wong B, Haddleton DM. Synthesis of well-defined
cyclodextrin-core star polymers. J Polym Sci, Part A: Polym Chem
2001;39:2206–14.
[226] Stenzel-Rosenbaum M, Davis TP, Chen V, Fane AG. Star-polymer synthesis via radical reversible addition-fragmentation chain-transfer
polymerization. J Polym Sci, Part A: Polym Chem 2001;39:2777–83.
[227] Moschogianni P, Pispas S, Hadjichristidis N. Multifunctional ATRP
initiators: synthesis of four-arm star homopolymers of methyl
methacrylate and craft copolymers of polystyrene and poly(t-butyl
methacrylate). J Polym Sci, Part A: Polym Chem 2001;39:650–5.
[228] Mayadunne RTA, Jeffery J, Moad G, Rizzardo E. Living free radical polymerization with reversible addition-fragmentation chain
transfer (RAFT polymerization): approaches to star polymers.
Macromolecules 2003;36:1505–13.
347
[229] Zheng Q, Pan C-y. Synthesis and characterization of dendrimer-star
polymer using dithiobenzoate-terminated poly(propylene imine)
dendrimer via reversible addition-fragmentation transfer polymerization. Macromolecules 2005;38:6841–8.
[230] Hovestad NJ, van Koten G, Bon SAF, Haddleton DM. Copper(I)
bromide/N-(n-octyl)-2-pyridylmethanimine-mediated
livingradical polymerization of methyl methacrylate using carbosilane
dendritic initiators. Macromolecules 2000;33:4048–52.
[231] Heise A, Diamanti S, Hedrick JL, Frank CW, Miller RD. Investigation of
the initiation behavior of a dendritic 12-arm initiator in atom transfer radical polymerization. Macromolecules 2001;34:3798–801.
[232] Zhao Y, Shuai X, Chen C, Xi F. Synthesis of star block copolymers
from dendrimer initiators by combining ring-opening polymerization and atom transfer radical polymerization. Macromolecules
2004;37:8854–62.
[233] Zhao Y, Chen Y, Chen C, Xi F. Synthesis of well-defined star polymers and star block copolymers from dendrimer initiators by atom
transfer radical polymerization. Polymer 2005;46:5808–19.
[234] Zhang X, Chen Y, Gong A, Chen C, Xi F. Dendrigraft polystyrene initiated by poly(p-chloromethyl styrene): synthesis and properties.
Polym Int 1999;48:896–900.
[235] Maier S, Sunder A, Frey H, Mulhaupt R. Synthesis of poly(glycerol)block-poly(methyl acrylate) multi-arm star polymers. Macromol
Rapid Commun 2000;21:226–30.
[236] Shen Z, Chen Y, Barriau E, Frey H. Multi-arm star polyglycerol-blockpoly(tert-butyl acrylate) and the respective multi-arm poly(acrylic
acid) stars. Macromol Chem Phys 2006;207:57–64.
[237] Hong H, Mai Y, Zhou Y, Yan D, Cui J. Self-assembly of large
multimolecular micelles from hyperbranched star copolymers.
Macromol Rapid Commun 2007;28:591–6.
[238] Von Werne T, Patten TE. Preparation of structurally well-defined
polymer-nanoparticle hybrids with controlled/living radical polymerizations. J Am Chem Soc 1999;121:7409–10.
[239] Pyun J, Matyjaszewski K, Kowalewski T, Savin D, Patterson G, Kickelbick G, et al. Synthesis of well-defined block copolymers tethered to
polysilsesquioxane nanoparticles and their nanoscale morphology
on surfaces. J Am Chem Soc 2001;123:9445–6.
[240] Pyun J, Kowalewski T, Matyjaszewski K. Synthesis of polymer
brushes using atom transfer radical polymerization. Macromol
Rapid Commun 2003;24:1043–59.
[241] Heise A, Trollsas M, Magbitang T, Hedrick JL, Frank CW, Miller
RD. Star polymers with alternating, arms from miktofunctional
mu-initiators using consecutive atom transfer radical polymerization and ring-opening polymerization. Macromolecules
2001;34:2798–804.
[242] Feng X-S, Pan C-Y. Block and star block copolymers by mechanism
transformation. 7. Synthesis of polytetrahydrofuran/poly(1,3dioxepane)/polystyrene ABC miktoarm star copolymers by combination of CROP and ATRP. Macromolecules 2002;35:2084–9.
[243] Magbitang T, Lee VY, Connor EF, Sundberg LK, Kim HC, Volksen
W, et al. Templating organosilicate vitrification using unimolecular self-organizing polymers prepared from tandem ring opening
and atom transfer radical polymerizations. Macromol Symp
2004;215:295–305.
[244] Dichtel WR, Baek K-Y, Fréchet JMJ, Rietveld IB, Vinogradov SA.
Amphiphilic diblock star polymer catalysts via atom transfer radical
polymerization. J Polym Sci, Part A: Polym Chem 2006;44:4939–51.
[245] Zhang L, Niu H, Chen Y, Liu H, Gao M. Preparation of platinum
nanoparticles using star-block copolymer with a carboxylic core.
J Colloid Interface Sci 2006;298:177–82.
[246] Fustin C-A, Colard C, Filali M, Guillet P, Duwez A-S, Meier MAR, et
al. Tuning the hydrophilicity of gold nanoparticles templated in star
block copolymers. Langmuir 2006;22:6690–5.
[247] Shen Z, Duan H, Frey H. Water-soluble fluorescent Ag nanoclusters obtained from multiarm star poly(acrylic acid) as “molecular
hydrogel” templates. Adv Mater 2007;19:349–52.
[248] Heise A, Nguyen C, Malek R, Hedrick JL, Frank CW, Miller RD. Starlike
polymeric architectures by atom transfer radical polymerization:
templates for the production of low dielectric constant thin films.
Macromolecules 2000;33:2346–54.
[249] Yuan W, Yuan J, Zheng S, Hong X. Synthesis, characterization, and
controllable drug release of dendritic star-block copolymer by ringopening polymerization and atom transfer radical polymerization.
Polymer 2007;48:2585–94.
[250] Stenzel MH, Davis TP. Star polymer synthesis using trithiocarbonate
functional b-cyclodextrin cores (reversible addition-fragmentation
chain-transfer polymerization). J Polym Sci, Part A: Polym Chem
2002;40:4498–512.
348
H. Gao, K. Matyjaszewski / Progress in Polymer Science 34 (2009) 317–350
[251] Jesberger M, Barner L, Stenzel MH, Malmstroem E, Davis TP,
Barner-kowollik C. Hyperbranched polymers as scaffolds for
multifunctional reversible addition-fragmentation chain-transfer
agents: a route to polystyrene-core-polyesters and polystyreneblock-poly(butyl acrylate)-core-polyesters. J Polym Sci, Part A:
Polym Chem 2003;41:3847–61.
[252] Darcos V, Dureault A, Taton D, Gnanou Y, Marchand P, Caminade AM, et al. Synthesis of hybrid dendrimer-star polymers by the RAFT
process. Chem Commun 2004:2110–1.
[253] Whittaker MR, Monteiro MJ. Synthesis and aggregation behavior
of four-arm star amphiphilic block copolymers in water. Langmuir
2006;22:9746–52.
[254] Boschmann D, Vana P. Z-RAFT star polymerizations of acrylates:
star coupling via intermolecular chain transfer to polymer. Macromolecules 2007;40:2683–93.
[255] Froehlich MG, Vana P, Zifferer G. Shielding effects in
polymer–polymer reactions. 1. Z-RAFT star polymerization of
four-arm stars. Macromol Theory Sim 2007;16:610–8.
[256] Celik C, Hizal G, Tunca U. Synthesis of miktoarm star and miktoarm
star block copolymers via a combination of atom transfer radical
polymerization and stable free-radical polymerization. J Polym Sci,
Part A: Polym Chem 2003;41:2542–8.
[257] Shi P-J, Li Y-G, Pan C-Y. Block and star block copolymers by mechanism transformation X. Synthesis of poly(ethylene oxide) methyl
ether/polystyrene/poly(l-lactide) ABC miktoarm star copolymers
by combination of RAFT and ROP. Eur Polym J 2004;40:1283–
90.
[258] He T, Li DJ, Sheng X, Zhao B. Synthesis of ABC 3-miktoarm star
terpolymers from a trifunctional initiator by combining ringopening polymerization, atom transfer radical polymerization,
and nitroxide-mediated radical polymerization. Macromolecules
2004;37:3128–35.
[259] Tunca U, Ozyurek Z, Erdogan T, Hizal G. Novel miktofunctional initiator for the preparation of an ABC-type miktoarm star polymer
via a combination of controlled polymerization techniques. J Polym
Sci, Part A: Polym Chem 2004;42:4228–36.
[260] Priftis D, Pitsikalis M, Hadjichristidis N. Miktoarm star copolymers
of poly(␧-caprolactone) from a novel heterofunctional initiator. J
Polym Sci, Part A: Polym Chem 2007;45:5164–81.
[261] Guo Y-M, Xu J, Pan C-Y. Block and star block copolymers by mechanism transformation. IV. Synthesis of S-(PSt)2 (PDOP)2 miktoarm
star copolymers by combination of ATRP and CROP. J Polym Sci, Part
A: Polym Chem 2001;39:437–45.
[262] Francis R, Lepoittevin B, Taton D, Gnanou Y. Toward an easy access
to asymmetric stars and miktoarm stars by atom transfer radical
polymerization. Macromolecules 2002;35:9001–8.
[263] Cai Y, Armes SP. Synthesis of well-defined y-shaped zwitterionic
block copolymers via atom-transfer radical polymerization. Macromolecules 2005;38:271–9.
[264] Liu C, Zhang Y, Huang J. Well-defined star polymers with
mixed-arms by sequential polymerization of atom transfer radical polymerization and reverse addition-fragmentation chain
transfer on a hyperbranched polyglycerol core. Macromolecules
2008;41:325–31.
[265] Babin J, Taton D, Brinkmann M, Lecommandoux S. Synthesis and
self-assembly in bulk of linear and mikto-arm star block copolymers
based on polystyrene and poly(glutamic acid). Macromolecules
2008;41:1384–92.
[266] Lee H, Jakubowski W, Matyjaszewski K, Yu S, Sheiko SS. Cylindrical
core–shell brushes prepared by a combination of ROP and ATRP.
Macromolecules 2006;39:4983–9.
[267] Yuan W, Yuan J, Zhang F, Xie X, Pan C. Synthesis, characterization, crystalline morphologies, and hydrophilicity of brush
copolymers with double crystallizable side chains. Macromolecules
2007;40:9094–102.
[268] Lee H-i, Matyjaszewski K, Yu-Su S, Sheiko SS. Hetero-grafted
block brushes with PCL and PBA side chains. Macromolecules
2008;41:6073–80.
[269] Beers KL, Gaynor SG, Matyjaszewski K, Sheiko SS, Moller M. The
synthesis of densely grafted copolymers by atom transfer radical
polymerization. Macromolecules 1998;31:9413–5.
[270] Boerner HG, Beers K, Matyjaszewski K, Sheiko SS, Moeller
M. Synthesis of molecular brushes with block copolymer side
chains using atom transfer radical polymerization. Macromolecules
2001;34:4375–83.
[271] Cheng G, Boker A, Zhang M, Krausch G, Muller AHE. Amphiphilic
cylindrical core–shell brushes via a “Grafting From” process using
ATRP. Macromolecules 2001;34:6883–8.
[272] Neugebauer D, Sumerlin BS, Matyjaszewski K, Goodhart B, Sheiko
SS. How dense are cylindrical brushes grafted from a multifunctional macroinitiator? Polymer 2004;45:8173–9.
[273] Pietrasik J, Sumerlin BS, Lee RY, Matyjaszewski K. Solution behavior of temperature-responsive molecular brushes prepared by ATRP.
Macromol Chem Phys 2007;208:30–6.
[274] Lee H-i, Pietrasik J, Matyjaszewski K. Phototunable temperatureresponsive molecular brushes prepared by ATRP. Macromolecules
2006;39:3914–20.
[275] Tang C, Dufour B, Kowalewski T, Matyjaszewski K. Synthesis and
morphology of molecular brushes with polyacrylonitrile block
copolymer side chains and their conversion into nanostructured
carbons. Macromolecules 2007;40:6199–205.
[276] Zhang M, Breiner T, Mori H, Muller AHE. Amphiphilic cylindrical
brushes with poly(acrylic acid) core and poly(n-butyl acrylate) shell
and narrow length distribution. Polymer 2003;44:1449–58.
[277] Qin S, Matyjaszewski K, Xu H, Sheiko SS. Synthesis and visualization of densely grafted molecular brushes with crystallizable
poly(octadecyl methacrylate) block segments. Macromolecules
2003;36:605–12.
[278] Ishizu K, Tsubaki K, Uchida S. Encapsulation of polypyrrole by
internal domain modification of double-cylinder-type copolymer
brushes. Macromolecules 2002;35:10193–7.
[279] Djalali R, Li S-Y, Schmidt M. Amphipolar core–shell cylindrical brushes as templates for the formation of gold clusters and
nanowires. Macromolecules 2002;35:4282–8.
[280] Zhang M, Estournes C, Bietsch W, Mueller AHE. Superparamagnetic
hybrid nanocylinders. Adv Funct Mater 2004;14:871–82.
[281] Boerner HG, Duran D, Matyjaszewski K, da Silva M, Sheiko SS. Synthesis of molecular brushes with gradient in grafting density by
atom transfer polymerization. Macromolecules 2002;35:3387–94.
[282] Lee H-I, Matyjaszewski K, Yu S, Sheiko SS. Molecular brushes with
spontaneous gradient by atom transfer radical polymerization.
Macromolecules 2005;38:8264–71.
[283] Matyjaszewski K, Qin S, Boyce JR, Shirvanyants D, Sheiko SS. Effect of
initiation conditions on the uniformity of three-arm star molecular
brushes. Macromolecules 2003;36:1843–9.
[284] Gu L, Shen Z, Zhang S, Lu G, Zhang X, Huang X. Novel amphiphilic
centipede-like copolymer bearing polyacrylate backbone and
poly(ethylene glycol) and polystyrene side chains. Macromolecules
2007;40:4486–93.
[285] Zhang A, Barner J, Goessl I, Rabe JP, Schuleter AD. A covalentchemistry approach to giant macromolecules and their wetting
behavior on solid substrates. Angew Chem, Int Ed 2004;43:5185–8.
[286] Liu YF, Abetz V, Muller AHE. Janus cylinders. Macromolecules
2003;36:7894–8.
[287] Bo Z, Rabe JP, Schluter AD. A poly(para-phenylene) with hydrophobic and hydrophilic dendrons: prototype of an amphiphilic cylinder
with the potential to segregate lengthwise. Angew Chem, Int Ed
1999;38:2370–2.
[288] Roovers J, Zhou LL, Toporowski PM, Vanderzwan M, Iatrou H, Hadjichristidis N. Regular star polymers with 64 and 128 arms—models
for polymeric micelles. Macromolecules 1993;26:4324–31.
[289] Bernard J, Favier A, Zhang L, Nilasaroya A, Davis TP, Barner-Kowollik
C, et al. Poly(vinyl ester) star polymers via xanthate-mediated living
radical polymerization: from poly(vinyl alcohol) to glycopolymer
stars. Macromolecules 2005;38:5475–84.
[290] Ruokolainen J, Torkkeli M, Serimaa R, Komanschek E, ten
Brinke G, Ikkala O. Order–disorder transition in comblike block
copolymers obtained by hydrogen bonding between homopolymers and end-functionalized oligomers: poly(4-vinylpyridine)pentadecylphenol. Macromolecules 1997;30:2002–7.
[291] Ruokolainen J, Tanner J, ten Brinke G, Ikkala O, Torkkeli M, Serimaa
R. Poly(4-vinyl pyridine)/zinc dodecyl benzene sulfonate mesomorphic state due to coordination complexation. Macromolecules
1995;28:7779–84.
[292] Antonietti M, Conrad J, Thuenemann A. Polyelectrolyte–surfactant
complexes: a new type of solid, mesomorphous material. Macromolecules 1994;27:6007–11.
[293] Ikkala O, Ruokolainen J, ten Brinke G, Torkkeli M, Serimaa R.
Mesomorphic state of poly(vinylpyridine)-dodecylbenzenesulfonic
acid complexes in bulk and in xylene solution. Macromolecules
1995;28:7088–94.
[294] Kolb HC, Finn MG, Sharpless KB. Click chemistry: diverse chemical function from a few good reactions. Angew Chem, Int Ed
2001;40:2004–21.
[295] Huisgen R. 1,3-Dipolar cycloadditions. Past and future. Angew
Chem, Int Ed 1963;2:565–98.
H. Gao, K. Matyjaszewski / Progress in Polymer Science 34 (2009) 317–350
[296] Rostovtsev VV, Green LG, Fokin VV, Sharpless KB. A stepwise
Huisgen cycloaddition process: copper(I)-catalyzed regioselective
“ligation” of azides and terminal alkynes. Angew Chem, Int Ed
2002;41:2596–9.
[297] Tornoe CW, Christensen C, Meldal M. Peptidotriazoles on solid
phase: [1,2,3]-triazoles by regiospecific copper(i)-catalyzed 1,3dipolar cycloadditions of terminal alkynes to azides. J Org Chem
2002;67:3057–64.
[298] Binder WH, Sachsenhofer R. ‘Click’ chemistry in polymer and materials science. Macromol Rapid Commun 2007;28:15–54.
[299] Lutz J-F. 1,3-Dipolar cycloadditions of azides and alkynes: a universal ligation tool in polymer and materials science. Angew Chem, Int
Ed 2007;46:1018–25.
[300] Golas PL, Matyjaszewski K. Click chemistry and ATRP: a beneficial
union for the preparation of functional materials. QSAR Comb Sci
2007;26:1116–34.
[301] Fournier D, Hoogenboom R, Schubert US. Clicking polymers: a
straightforward approach to novel macromolecular architectures.
Chem Soc Rev 2007;36:1369–80.
[302] Evans RA. The rise of azide–alkyne 1, 3-dipolar ‘Click’ cycloaddition
and its application to polymer science and surface modification.
Aust J Chem 2007;60:384–95.
[303] Binder WH, Kluger C. Combining ring-opening metathesis polymerization (ROMP) with sharpless-type “click” reactions: an
easy method for the preparation of side chain functionalized
poly(oxynorbornenes). Macromolecules 2004;37:9321–30.
[304] Lutz J-F, Boerner HG, Weichenhan K. Combining atom transfer radical polymerization and click chemistry: a versatile method for the
preparation of end-functional polymers. Macromol Rapid Commun
2005;26:514–8.
[305] Mantovani G, Ladmiral V, Tao L, Haddleton DM. One-pot tandem living radical polymerization-Huisgens cycloaddition process
(“click”) catalyzed by N-alkyl-2-pyridylmethanimine/Cu(I)Br complexes. Chem Commun 2005:2089–91.
[306] Sumerlin BS, Tsarevsky NV, Louche G, Lee RY, Matyjaszewski K.
Highly efficient “Click” functionalization of poly(3-azidopropyl
methacrylate) prepared by ATRP. Macromolecules 2005;38:7540–
5.
[307] Gao H, Louche G, Sumerlin BS, Jahed N, Golas P, Matyjaszewski
K. Gradient polymer elution chromatographic analysis of a,wdihydroxypolystyrene synthesized via ATRP and click chemistry.
Macromolecules 2005;38:8979–82.
[308] Englert BC, Bakbak S, Bunz UHF. Click chemistry as a powerful tool
for the construction of functional poly(p-phenyleneethynylene)s:
comparison of pre- and postfunctionalization schemes. Macromolecules 2005;38:5868–77.
[309] Parrish B, Breitenkamp RB, Emrick T. PEG- and peptidegrafted aliphatic polyesters by click chemistry. J Am Chem Soc
2005;127:7404–10.
[310] Ladmiral V, Mantovani G, Clarkson GJ, Cauet S, Irwin JL, Haddleton DM. Synthesis of neoglycopolymers by a combination of
“Click Chemistry” and living radical polymerization. J Am Chem Soc
2006;128:4823–30.
[311] Li M, De P, Gondi SR, Sumerlin BS. Responsive polymer–protein
bioconjugates prepared by RAFT polymerization and coppercatalyzed azide–alkyne click chemistry. Macromol Rapid Commun
2008;29:1172–6.
[312] Geng J, Mantovani G, Tao L, Nicolas J, Chen G, Wallis R, et al.
Site-directed conjugation of “Clicked” glycopolymers to form glycoprotein mimics: binding to mammalian lectin and induction of
immunological function. J Am Chem Soc 2007;129:15156–63.
[313] Wu P, Feldman AK, Nugent AK, Hawker CJ, Scheel A, Voit B, et al. Efficiency and fidelity in a click-chemistry route to triazole dendrimers
by the copper(I)-catalyzed ligation of azides and alkynes. Angew
Chem, Int Ed 2004;43:3928–32.
[314] Helms B, Mynar JL, Hawker CJ, Fréchet JMJ. Dendronized linear polymers via “Click Chemistry”. J Am Chem Soc 2004;126:15020–1.
[315] Tsarevsky NV, Sumerlin BS, Matyjaszewski K. Step-growth “Click”
coupling of telechelic polymers prepared by atom transfer radical
polymerization. Macromolecules 2005;38:3558–61.
[316] Joralemon MJ, O’Reilly RK, Matson JB, Nugent AK, Hawker CJ, Wooley KL. Dendrimers clicked together divergently. Macromolecules
2005;38:5436–43.
[317] Ossipov DA, Hilborn J. Poly(vinyl alcohol)-based hydrogels formed
by “Click Chemistry”. Macromolecules 2006;39:1709–18.
[318] Laurent BA, Grayson SM. An efficient route to well-defined
macrocyclic polymers via “Click” cyclization. J Am Chem Soc
2006;128:4238–9.
349
[319] Fernandez-Megia E, Correa J, Rodriguez-Meizoso I, Riguera R. A
click approach to unprotected glycodendrimers. Macromolecules
2006;39:2113–20.
[320] Gao H, Matyjaszewski K. Synthesis of star polymers by a combination of ATRP and the “Click” coupling method. Macromolecules
2006;39:4960–5.
[321] Johnson JA, Lewis DR, Diaz DD, Finn MG, Koberstein JT, Turro NJ. Synthesis of degradable model networks via ATRP and click chemistry.
J Am Chem Soc 2006;128:6564–5.
[322] Gao H, Matyjaszewski K. Synthesis of molecular brushes by “Grafting onto” method: combination of ATRP and click reactions. J Am
Chem Soc 2007;129:6633–9.
[323] Tsarevsky NV, Bencherif SA, Matyjaszewski K. Graft copolymers by a
combination of ATRP and two different consecutive click reactions.
Macromolecules 2007;40:4439–45.
[324] Hoogenboom R, Moore BC, Schubert US. Synthesis of star-shaped
poly(␧-caprolactone) via ‘click’ chemistry and ‘supramolecular
click’ chemistry. Chem Commun 2006:4010–2.
[325] Altintas O, Yankul B, Hizal G, Tunca U. A3-type star polymers via
click chemistry. J Polym Sci, Part A: Polym Chem 2006;44:6458–65.
[326] Gao H, Min K, Matyjaszewski K. Synthesis of 3-arm star block
copolymers by combination of “core-first” and “coupling-onto”
methods using ATRP and click reactions. Macromol Chem Phys
2007;208:1370–8.
[327] Whittaker MR, Urbani CN, Monteiro MJ. Synthesis of 3-miktoarm
stars and 1st generation mikto dendritic copolymers by “Living”
radical polymerization and “Click” chemistry. J Am Chem Soc
2006;128:11360–1.
[328] Altintas O, Hizal G, Tunca U. ABC-type hetero-arm star terpolymers through “Click” chemistry. J Polym Sci, Part A: Polym Chem
2006;44:5699–707.
[329] Zhu J, Zhu X, Kang ET, Neoh KG. Design and synthesis of star polymers with hetero-arms by the combination of controlled radical
polymerizations and click chemistry. Polymer 2007;48:6992–9.
[330] Deng GH, Ma DY, Xu ZZ. Synthesis of ABC-type miktoarm star
polymers by “click” chemistry, ATRP and ROP. Eur Polym J
2007;43:1179–87.
[331] Gungor E, Cote G, Erdogan T, Durmaz H, Demirel AL, Hizal G, et al.
Heteroarm H-shaped terpolymers through click reaction. J Polym
Sci, Part A: Polym Chem 2007;45:1055–65.
[332] Wang G, Luo X, Liu C, Huang J. Synthesis of ABCD 4-miktoarm starshaped quarterpolymers by combination of the “click” chemistry
with multiple polymerization mechanism. J Polym Sci, Part A: Polym
Chem 2008;46:2154–66.
[333] Gungor E, Durmaz H, Hizal G, Tunca U. H-shaped (ABCDE type)
quintopolymer via click reaction [3 + 2] strategy. J Polym Sci, Part
A: Polym Chem 2008;46:4459–68.
[334] Dag A, Durmaz H, Hizal G, Tunca U. Preparation of 3-arm star polymers (A3) via Diels–Alder click reaction. J Polym Sci, Part A: Polym
Chem 2008;46:302–13.
[335] Sinnwell S, Inglis AJ, Stenzel MH, Barner-Kowollik C. Access
to three-arm star block copolymers by a consecutive combination of the copper(I)-catalyzed azide–alkyne cycloaddition and
the RAFT hetero Diels–Alder concept. Macromol Rapid Commun
2008;29:1090–6.
[336] Inglis AJ, Sinnwell S, Davis TP, Barner-Kowollik C, Stenzel MH.
Reversible addition fragmentation chain transfer (RAFT) and
hetero-Diels–Alder chemistry as a convenient conjugation tool
for access to complex macromolecular designs. Macromolecules
2008;41:4120–6.
[337] Schappacher M, Deffieux A. New comblike copolymers of controlled
structure and dimensions obtained by grafting polystyryllithium
onto poly(chloroethyl vinyl ether) chains. Macromol Chem Phys
1997;198:3953–61.
[338] Gauthier M, Moeller M. Uniform highly branched polymers by
anionic grafting: arborescent graft polymers. Macromolecules
1991;24:4548–53.
[339] Mynar JL, Choi T-L, Yoshida M, Kim V, Hawker CJ, Fréchet
JMJ. Doubly-dendronized linear polymers. Chem Commun
2005:5169–71.
[340] Riva R, Schmeits S, Jerome C, Jerome R, Lecomte P. Combination of
ring-opening polymerization and “Click Chemistry”: toward functionalization and grafting of poly(␧-caprolactone). Macromolecules
2007;40:796–803.
[341] Van Camp W, Germonpre V, Mespouille L, Dubois P, Goethals EJ, Du
Prez FE. New poly(acrylic acid) containing segmented copolymer
structures by combination of “click” chemistry and atom transfer
radical polymerization. React Funct Polym 2007;67:1168–80.
350
H. Gao, K. Matyjaszewski / Progress in Polymer Science 34 (2009) 317–350
[342] Quemener D, Le Hellaye M, Bissett C, Davis TP, Barner-Kowollik C,
Stenzel MH. Graft block copolymers of propargyl methacrylate and
vinyl acetate via a combination of RAFT/MADIX and click chemistry:
reaction analysis. J Polym Sci, Part A: Polym Chem 2008;46:155–73.
[343] Jiang X, Vogel EB, Smith III MR, Baker GL. “Clickable” polyglycolides: tunable synthons for thermoresponsive, degradable polymers.
Macromolecules 2008;41:1937–44.
[344] Gacal B, Durmaz H, Tasdelen MA, Hizal G, Tunca U, Yagci Y, et
al. Anthracene-maleimide-based Diels–Alder “click chemistry” as a
novel route to graft copolymers. Macromolecules 2006;39:5330–6.
[345] Dag A, Durmaz H, Demir E, Hizal G, Tunca U. Heterograft copolymers
via double click reactions using one-pot technique. J Polym Sci, Part
A: Polym Chem 2008;46:6969–77.
[346] Malkoch M, Vestberg R, Gupta N, Mespouille L, Dubois P, Mason
AF, et al. Synthesis of well-defined hydrogel networks using Click
chemistry. Chem Commun 2006:2774–6.
[347] Xia Y, Verduzco R, Grubbs RH, Kornfield JA. Well-defined liquid crystal gels from telechelic polymers. J Am Chem Soc
2008;130:1735–40.
[348] Kavitha AA, Singha NK. A tailor-made polymethacrylate bearing a
reactive diene in reversible Diels–Alder reaction. J Polym Sci, Part
A: Polym Chem 2007;45:4441–9.
[349] Ito K, Kawaguchi S. Poly(macromonomers): homo- and copolymerization. Adv Polym Sci 1999;142:129–78.
[350] Tsukahara Y, Mizuno K, Segawa A, Yamashita Y. Study on the radical polymerization behavior of macromonomers. Macromolecules
1989;22:1546–52.
[351] Tsukahara Y, Tsutsumi K, Yamashita Y, Shimada S. Radical polymerization behavior of macromonomers. 2. Comparison of styrene
macromonomers having a methacryloyl end group and a vinylbenzyl end group. Macromolecules 1990;23:5201–8.
[352] Nomura E, Ito K, Kajiwara A, Kamachi M. Radical polymerization
kinetics of poly(ethylene oxide) macromonomers. Macromolecules
1997;30:2811–7.
[353] Ederle Y, Isel F, Grutke S, Lutz PJ. Anionic polymerization and
copolymerization of macromonomers. Kinetics, structure control.
Macromol Symp 1998;132:197–206.
[354] Pantazis D, Chalari I, Hadjichristidis N. Anionic polymerization of
styrenic macromonomers. Macromolecules 2003;36:3783–5.
[355] Heroguez V, Breunig S, Gnanou Y, Fontanille M. Synthesis of a-norbornenylpoly(ethylene oxide) macromonomers and
their ring-opening metathesis polymerization. Macromolecules
1996;29:4459–64.
[356] Mecerreyes D, Dahan D, Lecomte P, Dubois P, Demonceau A,
Noels AF, et al. Ring-opening metathesis polymerization of new
a-norbornenyl poly(␧-caprolactone) macromonomers. J Polym Sci,
Part A: Polym Chem 1999;37:2447–55.
[357] Allcock HR, de Denus CR, Prange R, Laredo WR. Synthesis of norbornenyl telechelic polyphosphazenes and ring-opening metathesis
polymerization reactions. Macromolecules 2001;34:2757–65.
[358] Nomura K, Takahashi S, Imanishi Y. Synthesis of poly
(macromonomer)s by repeating ring-opening metathesis
polymerization (ROMP) with Mo(CHCMe2 Ph)(NAr)(OR)2 initiators.
Macromolecules 2001;34:4712–23.
[359] Shiono T, Moriki Y, Soga K. Copolymerization of poly(propylene)
macromonomer and ethylene with metallocene catalysts. Macromol Symp 1995;97:161–70.
[360] Endo K, Senoo K. Syndiospecific copolymerization of styrene
with styrene-terminated isoprene macromonomer by CpTiCl3 methylaluminoxane catalyst. Macromol Rapid Commun 1998;
19:563–6.
[361] Neugebauer D, Zhang Y, Pakula T, Sheiko SS, Matyjaszewski K.
Densely-grafted and double-grafted PEO brushes via ATRP. A route
to soft elastomers. Macromolecules 2003;36:6746–55.
[362] Ohno S, Matyjaszewski K. Controlling grafting density and side
chain length in poly(n-butyl acrylate) by ATRP copolymerization of macromonomers. J Polym Sci, Part A: Polym Chem
2006;44:5454–67.
[363] Ishizu K, Shen XX. Radical copolymerization reactivity of maleateterminated poly(ethylene glycol) with vinylbenzyl-terminated
polystyrene macromonomers. Polymer 1999;40:3251–4.
[364] Roos SG, Muller AHE, Matyjaszewski K. Copolymerization of n-butyl
acrylate with methyl methacrylate and PMMA macromonomers:
comparison of reactivity ratios in conventional and atom transfer
radical copolymerization. Macromolecules 1999;32:8331–5.
[365] Ishizu K, Shen XX, Tsubaki KI. Radical copolymerization reactivity of methacryloyl-terminated poly(ethylene glycol methyl ether)
with vinylbenzyl-terminated polystyrene macromonomers. Polymer 2000;41:2053–7.
[366] Shinoda H, Matyjaszewski K. Structural control of poly(methyl
methacrylate)-g-poly(lactic acid) graft copolymers by atom transfer radical polymerization (ATRP). Macromolecules 2001;34:6243–
8.
[367] Shinoda H, Miller PJ, Matyjaszewski K. Improving the structural control of graft copolymers by combining ATRP with the
macromonomer method. Macromolecules 2001;34:3186–94.
[368] Tsubaki K, Kobayashi H, Sato J, Ishizu K. Dilute solution properties and aggregation behavior of alternate hetero-arm copolymer
brushes. J Colloid Interface Sci 2001;241:275–9.
[369] Neugebauer D, Zhang Y, Pakula T, Matyjaszewski K. PDMSPEO densely grafted copolymers. Macromolecules 2005;38:8687–
93.
[370] Zhang Y, Costantini N, Mierzwa M, Pakula T, Neugebauer D, Matyjaszewski K. Super soft elastomers as ionic conductors. Polymer
2004;45:6333–9.
[371] Min K, Jakubowski W, Matyjaszewski K. AGET ATRP in the presence of air in miniemulsion and in bulk. Macromol Rapid Commun
2006;27:594–8.
[372] Matyjaszewski K, Dong H, Jakubowski W, Pietrasik J, Kusumo A.
Grafting from surfaces for “Everyone”: ARGET ATRP in the presence
of air. Langmuir 2007;23:4528–31.
[373] Min K, Yu S, Lee H-i, Mueller L, Sheiko SS, Matyjaszewski K. High
yield synthesis of molecular brushes via ATRP in miniemulsion.
Macromolecules 2007;40:6557–63.
[374] Bombalski L, Min K, Dong H, Tang C, Matyjaszewski K. Preparation
of well-defined hybrid materials by ATRP in miniemulsion. Macromolecules 2007;40:7429–32.