Halide exchange during atom transfer radical polymerization (ATRP) using mixed halide initiation systems, R-X/Cu-Y (X, Y ) Cl or Br), was investigated. Model studies of mixed halide initiation systems (i.e., R-X/Cu-Y, X * Y) demonstrated that exchange occurs rapidly at 90 °C, and there is a clear preference for alkyl chlorides to be formed in over alkyl bromides. This was attributed mainly to the carbon-chlorine bond being stronger than the corresponding carbon-bromine bond. This implies that, in ATRP with a mixed halide initiator/catalyst system, the bulk of the polymer chain ends are terminated by chlorine if [CuCl] 0 g [RBr]0. Examples of using this information to improve the control in ATRP of methyl methacrylate (MMA) are presented. It was shown that, when benzyl halides were used as the initiator in the ATRP of MMA, the rate of initiation was increased relative to the rate of propagation, thus providing better control by using the benzyl bromide/copper chloride mixed halide system. Better molecular weight control at high conversions of monomer to polymer was observed when using ethyl 2-bromoisobutyrate and copper chloride as initiator/catalyst in comparison to using ethyl 2-bromoisobutyrate/copper bromide, indicating that side reactions are less significant in the former.

The synthesis of di- and triblock copolymers, involving methyl methacrylate (MMA), butyl acrylate, and methyl acrylate, using copper-based atom transfer radical polymerization (ATRP) is reported. It was found that poly(MMA) macroinitiator is able to initiate the ATRP of acrylic monomers. However, for polyacrylates to effectively initiate the ATRP of MMA, the end group should be a bromine atom and the catalyst CuCl; that is, halogen exchange should take place. ABA-type triblock copolymers, where B = poly(butyl acrylate) and A = poly(methyl methacrylate), were synthesized by growing the center block first using a difunctional initiator and then adding MMA to the ends.

The homogeneous controlled/“living” free radical polymerization of methyl methacrylate (MMA) by atom transfer radical polymerization (ATRP) using a CuIX/4,4‘-di(5-nonyl)-2,2‘-bipyridine catalytic system (X = Cl, Br) with various initiators R−X was investigated. The rates of polymerization initiated by most of the systems exhibited first-order kinetics with respect to the monomer. A linear increase of number average molecular weight (M n) versus monomer conversion was observed for most of these initiation systems. The benzhydryl chloride/CuICl system yielded the lowest rate of polymerization, which could be increased by slow addition of the initiator. The reduced rate of polymerization was due to an increase in the concentration of CuIICl, which results from the coupling of benzhydryl radicals during initiation. The slow addition of benzhydryl chloride prevented the formation of a large amount of benzhydryl radicals in the initiation step, thereby reducing radical−radical termination and CuII formation, and led to an increase in the rate of polymerization. p-Toluenesulfonyl chloride/CuIBr gave better control of molecular weight and lower polydispersities than p-TsCl/CuICl, possibly due to the faster deactivation step in ATRP. Ethyl 2-bromoisobutyrate/CuIBr gave the fastest rate of polymerization among all the initiation systems but showed some deviation in M n at high conversions. The initiation efficiencies of diethyl 2-bromomalonate and diethyl 2-bromo-2-methylmalonate in the ATRP of MMA were examined. The latter can initiate polymerization efficiently, while the former gave no polymerization. This can be explained by the difference in the electronic nature of the two malonyl radicals generated during initiation. Such experimental observations, coupled with data from the literature, lead to some general “rules” by which successful initiation of ATRP can be achieved.

Atom transfer radical polymerization has been used to successfully synthesize polyacrylonitrile (PAN) with predictable molecular weights and narrow polydispersities. This was achieved by using CuX/2,2‘-bipyridine (X = Br or Cl) as the catalyst, 2-halopropionitriles as initiators, and ethylene carbonate as a solvent. Monomer consumption showed significant curvature in the first-order kinetic plot, indicating termination is present. 1H NMR spectroscopy showed that the halide end group is irreversibly removed during the polymerization. Possible reasons for this reaction are given, with the most probable being the reduction of the radical by CuIX to form an anion that subsequently deactivates very quickly. Such side reactions limit the molecular weight achievable to M n < 30 000 while still keeping low polydispersity, M w/M n < 1.5.

Controlled/“living” atom transfer radical polymerization (ATRP), which is subject to the persistent radical effect, has been examined through computer simulations. It was found that the peculiar time-dependent rates predicted by the persistent radical effect may not be observed experimentally because termination is dependent on chain length and viscosity. In the early stages of the polymerizations the deactivating species is produced very fast through irreversible radical−radical termination; however, as the polymerization progresses, less deactivator is produced because termination reactions are slowed, thus resulting in a steady rate of polymerization. Thermal initiation of styrene was found to be insignificant in ATRP under the conditions examined, in contrast to systems characterized by a low equilibrium constant, such as TEMPO-mediated polymerization, where the rate of styrene polymerization is dominated by thermal initiation. Inclusion of termination rate coefficients that decrease during polymerization in the simulation model led to adequate reproduction of experimental results.