CONTROLLED LIVING ATOM-TRANSFER RADICAL POLYMERIZATION OF METHYL-METHACRYLATE USING VARIOUS INITIATION SYSTEMS/

The homogeneous controlled/''living'' free radical polymerization of m ethyl 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 o f polymerization initiated by most of the systems exhibited first-orde r kinetics with respect to the monomer. A linear increase of number av erage molecular weight (M-n) versus monomer conversion was observed fo r most of these initiation systems. The benzhydryl chloride/(CuCl)-Cl- I system yielded the lowest rate of polymerization, which could be inc reased by slow addition of the initiator. The reduced rate of polymeri zation was due to an increase in the concentration of (CuCl)-Cl-II, wh ich 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 a n increase in the rate of polymerization. p-Toluenesulfonyl chloride/( CuBr)-Br-I gave better control of molecular weight and lower polydispe rsities than p-TsCl/(CuCl)-Cl-I, possibly due to the faster deactivati on step in ATRP. Ethyl 2-bromoisobutyrate/(CuBr)-Br-I gave the fastest rate of polymerization among all the initiation systems but showed so me deviation in M, at high conversions. The initiation efficiencies of diethyl 2-bromomalonate and diethyl 2-bromo-2-methylmalonate in the A TRP of MMA were examined. The latter can initiate polymerization effic iently, while the former gave no polymerization. This can be explained by the difference in the electronic nature of the two malonyl radical s generated during initiation. Such experimental observations, coupled with data from the literature, lead to some general ''rules'' by whic h successful initiation of ATRP can be achieved.