Determination of rate constants in the carbocationic polymerization of styrene: Effect of temperature, solvent polarity, and Lewis acid

The electrophilicity parameter (E = 9.6) of the 1-phenylethyl cation, 1(+), has been determined and combined with the nucleophilicity parameter (N = 0.78, s = 0.95) of styrene (St) to predict diffusion-limited propagation in the cationic polymerization of St by the linear free energy relationship log k = s(N + E). This prediction has been experimentally verified using two different diffusion clock methods, which provided a value of k(p)(+/-) approximate to 2 x 10(9) L mol(-1) s(-1), 6 orders of magnitude higher than previously accepted, for the absolute rate constant of propagation of the TiCl4-induced polymerization of St in methylcyclohexane/methyl chloride 60/40 (v/v) at -80 degreesC. The k(p)(+/-) value remained unchanged in the temperature range -50 to -80 degreesC, indicating that propagation does not have an enthalpic barrier; however, it increased moderately with increasing solvent polarity. The nature of the Lewis acid has little effect on k(p)(+/-) as similar values have been obtained with TiCl4 or SnCl4. The apparent rate constant of ionization, k(i)(app), the rate constant of deactivation, k(-i), and the apparent equilibrium constant of ionization, K-i(app), have also been determined as a function of temperature. The k(i)(app) increases slightly and k(-i) increases moderately with increasing temperature; therefore, K-i(app) and the overall polymerization rate decrease moderately with increasing temperature.