Kinetics and molecular weight control of the polymerization of acrylamide via RAFT

The reversible addition-fragmentation chain transfer (RAFT) polymerization of acrylamide (AM) was studied in order to establish reaction conditions which would provide optimal rates of monomer conversion and to determine reasons for deviation of theoretical and experimental molecular weights, the former predicted from current models. To this end, chain transfer agents (CTAs) and initiators were selected and experiments performed in water and in dimethyl sulfoxide (DMSO) at specified CTA/initiator ratios and temperatures. Higher apparent rates of polymerization were achieved utilizing CTAs with higher intermediate fragmentation rates, larger initiator concentrations, and higher temperatures. For RAFT polymerization of acrylamide under these experimental conditions, a continuing supply of radicals was required in order to achieve reasonable conversions. The deviations of experimentally measured molecular weights from those theoretically predicted are a function of the CTA utilized and parallel the extent of rate retardation. The deviations are, at least in part, consistent with significant early radical coupling of stable intermediate species during the preequilibrium period (or the recently proposed CTA "initialization" period). These effects are apparent in both aqueous buffer and DMSO. The retardation effects and eventual loss of linearity of the first-order kinetic plots at extended times are also consistent with termination processes although these experiments alone do not rule out alternative mechanisms of reversible termination or slow fragmentation of intermediate species. For RAFT polymerizations in DMSO mediated by the trithiocarbonate CTA, reaction rates are significantly faster, and near quantitative conversions can be reached with proper initiator choice.