Evaluation of the rate constant for primary radical termination in free radical polymerization

Styrene and methyl methacrylate were polymerized with azodiisobutyronitrile (AIBN) at 50, 60, and 70°C. The average degree of polymerization was kept constant while changing the initiator concentration, by using 1‐butanethiol as a chain transfer agent. In these polymerizations, a deviation from the simple kinetic rate law was noticed. This deviation was explained in terms of primary radical termination considering the effect of size dependence of the termination rate constant on the kinetics of free radical polymerization and taking into account the fraction of thermal polymerization. The temperature dependence of the characteristic constant, K tpr / K p K i , was estimated to be 5,408 · 10 −5 exp (12915 cal mol −1 / RT ) and 7,52 · 10 −3 exp (7791 cal mol −1 / RT ) for styrene/AIBN and methyl methacrylate/AIBN, respectively.