Chapter 4
The Preparation of Well-Defined Water Soluble-Swellable (Co)Polymers by Atom Transfer Radical Polymerization Krzysztof Matyjaszewski, Scott G. Gaynor, Jian Qiu, Kathryn Beers, Simion Coca, Kelly Davis, Andreas Muhlebach, Jianhui Xia, and Xuan Zhang Center for Macromolecular Engineering, Department of Chemistry, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, PA 15213
The development of atom transfer radical polymerization (ATRP), a controlled/"living" radical polymerization system, has allowed for the preparation of a variety of well defined polymers (DP = Δ[Μ]/[Ι] ; M / M < 1.5) based on styrenes, (meth)acrylics, acrylonitrile, and other monomers. This paper is to review the extension of ATRP in preparing well-defined water soluble homopolymers and amphiphilic, water swellable copolymers. The monomers that have been polymerized using ATRP include: 2hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, 2(dimethylamino)ethyl methacrylate and 4-vinylpyridine. Well -defined poly(t-butyl acrylate) was also prepared and was transformed to poly(acrylic acid) by hydrolysis. Various amphiphilic block copolymers were prepared with these hydrophilic monomers and other, hydrophobic monomers. n
0
w
n
Introduction The recent explosion of interest in controlled/"living" radical polymerizations is the result of polymer chemists trying to develop new polymeric materials from existing monomers.(7) Controlled/"living" polymerizations offer the possibility of synthesizing polymers with precise control of the end groups, composition, functionality, and architecture of a polymer. Such variability, coupled with the ability of radical polymerization to polymerize a wide variety of monomers, is expected to offer the polymer chemist a virtually limitless array of polymeric materials for nearly
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© 2000 American Chemical Society
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every application. Towards this end, we have developed atom transfer radical polymerization (ATRP) as a method for preparing well-defined polymers. (2-8) ATRP is, at its essence, the reversible activation and deactivation of a polymer chain by reaction of a dormant chain end (R-X) with a transition metal catalyst (M") complexed with a ligand, Scheme 1. The dormant chain end is capped with a group, generally a halogen, that can be removed homolytically by reaction with the transition metal catalyst to form a radical and a transition metal halide (X-M ) (with rate constant k j . This radical can then initiate the polymerization of the monomer and propagate. As the radical chain propagates, the radical can do one of three things: react with monomer and continue to propagate (with rate constant kp), react with another radical to terminate by coupling or disproportionation (with rate constant kj, or react with the transition metal halide to cap the growing chain and reduce the transition metal catalyst to its lower oxidation state (with rate constant, k j . n+1
t
Scheme 1
Κ R—X
+
n
M /Ligand t
-
^
R* +
X~M
n + 1 t
/ Ligand
In order to obtain a controlled/"living" radical polymerization, termination of the polymer chain by bimolecular radical reactions must be suppressed. To do this, the concentration of the radicals in solution must be kept very low. This is accomplished by attenuation of the equilibrium between the active and dormant states of the growing polymer chain. If the equilibrium is shifted too far towards the active species, the system behaves as a simple redox initiated polymerization and no control is obtained. Contrarily, if the equilibrium is shifted too far towards the dormant species, no polymerization is observed. In this system, some termination (
