Reversible cobalt-carbon bond formation in catalytic chain transfer polymerization

The reversible bond formation between cobalt(II) catalytic chain transfer a gents and propagating radicals was studied using electron paramagnetic reso nance and conventional kinetic measurements. It was found that this reversi ble cobalt-carbon bond formation has no significant effect on the catalytic chain transfer polymerization of methyl methacrylate but does affect the p olymerization behavior of styrene. In both systems significant induction pe riods are observed which seem to disappear in the methyl methacrylate syste m but persist in the styrene system upon decreasing the initial concentrati on of the cobalt(II) complex. The overall rates of polymerizations are foun d to be readily described by "classical" free-radical polymerization kineti cs, including a chain-length-dependent average termination rate coefficient . Furthermore, in contrast to the situation observed in methyl methacrylate polymerization where constant molecular weights are produced over the enti re conversion range, it was found that the molecular weight in styrene incr eases with conversion until a constant molecular weight is obtained which i s given by the Mayo equation. The kinetic behavior and the molecular weight evolution could simply be modeled by a reaction scheme providing a constan t radical concentration and the presence of a chain transfer agent.