Controlled/"living" atom transfer radical polymerization of methyl methacrylate using various initiation systems

The homogeneous controlled/"living" free radical polymerization of methyl methacrylate (MMA) by atom transfer radical polymerization (ATRP) using a (CuX)-X-I/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/(CuCl)-Cl-I 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 (CuCl)-Cl-II, 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 Cu-II formation, and led to an increase in the rate of polymerization. p-Toluenesulfonyl chloride/(CuBr)-Br-I gave better control of molecular weight and lower polydispersities than p-TsCl/(CuCl)-Cl-I, possibly due to the faster deactivation step in ATRP. Ethyl 2-bromoisobutyrate/(CuBr)-Br-I gave the fastest rate of polymerization among all the initiation systems but showed some deviation in M, 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.