Evaluation of the kinetic parameters for styrene polymerization and their chain length dependence by kinetic simulation and pulsed laser photolysis

Pulsed laser photolysis (PLP) has been employed to determine propagation rate constants k_p for styrene polymerization in benzene over a wider temperature range (20?80°C) than previously converd. It is proposed that a small chain length dependence of k_p (overall) may, in part, be a consequence of a marked chain length dependence of k_p for the first few propagation steps i.e. k_p(1) > k_p(2) < k_p(3) ≥ k_p(≥4)]. The propagation rate constant for styrene polymerization is given by the expression: In k_p = 16,0₉ ? 289₅₀/(RT) (overall) or In k_p = 16,4₇ ? 300₈₄/(RT) (chain length ≥ 4). Kinetic simulation has been applied both as an aid in data analysis and to demonstrate the reliability of the PLP technique for evaluation of propagation rate constants (k_p) in radical polymerization. This has been achieved by examining the sensitivity of the molecular weight distribution of polymers formed in PLP experiments to the values of the kinetic parameters associated with polymerization and their chain length dependence. The termination rate constants (k_t = k_c + k_d) and the ratio of combination to disproportionation (k_c: k_d) markedly affect the molecular weight distribution of polymer formed in PLP experiments. The prospects for evaluating the values of k_t, its chain length dependence and k_c : k_d by direct analysis of the molecular weight distribution are discussed in the light of these results.