The reversible bond formation between cobalt(II) catalytic chain transfer agents and propagating radicals was studied using electron paramagnetic resonance and conventional kinetic measurements. It was found that this reversible cobalt−carbon bond formation has no significant effect on the catalytic chain transfer polymerization of methyl methacrylate but does affect the polymerization behavior of styrene. In both systems significant induction periods are observed which seem to disappear in the methyl methacrylate system but persist in the styrene system upon decreasing the initial concentration of the cobalt(II) complex. The overall rates of polymerizations are found to be readily described by “classical” free-radical polymerization kinetics, 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 entire conversion range, it was found that the molecular weight in styrene increases with conversion until a constant molecular weight is obtained which is given by the Mayo equation. The kinetic behavior and the molecular weight evolution could simply be modeled by a reaction scheme providing a constant radical concentration and the presence of a chain transfer agent.

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The free-radical bulk polymerization of styrene initiated by dimethyl 2,2′-azobisisobutyrate at 70°C has been studied employing the techniques of Fourier transform near-infrared spectroscopy, electron spin resonance spectroscopy and gel permeation chromatography. The initiator efficiency (f) as a function of monomer conversion was estimated via an equation relating the instantaneous and the cumulative number average degrees of polymerization. The result was subsequently employed to calculate the conversion dependence of the termination rate coefficient (k t) from zero to the limiting conversion for different initiator concentrations. Over the course of the reaction, kt decreases by 5 orders of magnitude (10 7 -10 2 M -1 s -1 ). At conversion levels approaching the limiting value, kt shows a marked dependence on initiator concentration; higher initiator concentration shifts the kt vs conversion curves toward higher conversion levels. The reaction diffusion constant was found to decrease with increasing conversion in the high-conversion regime.

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