Determination of Rate Constants of Free-Ion and Paired-Ion Propagations in the Cationic Polymerization of Styrene by Trifluoromethanesulfonic Acid

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 K_{p, 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 k_{p, app} was separated into the rate constants of the free-ion and paired-ion propagations (k_p⁺ and k_p^±) on the basis of the dependence of kpfipp on [P⁺] and of the effect of a common ion salt. The values of k_p⁺ and k_p^± 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 k_p⁺ / k_p^α was 6-24, which is much smaller than those observed for the anionic polymerization.