The propagation step of the cationic polymerization of styrene was studied in detail with the CF3SO3H initiator in dichloroethane at -1 to 30 °C. The apparent rate constant of propagation Kp, app was estimated from the time course of the carbocation (P+) formation and monomer consumption by the combination of stopped-flow spectroscopy and the rapid quenching technique. Then kp, app was separated into the rate constants of the free-ion and paired-ion propagations (kp+ and kp±) on the basis of the dependence of kpfipp on [P+] and of the effect of a common ion salt. The values of kp+ and kp± were (2-30) × 105 M-1 s-1 and (0.4-1.2) X 105 M-1 s-1, respectively, in the temperature range studied. The ratio kp+ / kpα was 6-24, which is much smaller than those observed for the anionic polymerization.
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