Determination of rate constants in the carbocationic polymerization of styrene:  effect of temperature, solvent polarity, and lewis acid.

Abstract

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 kp± ≈ 2 × 109 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 °C. The kp± value remained unchanged in the temperature range −50 to −80 °C, 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 kp± as similar values have been obtained with TiCl4 or SnCl4. The apparent rate constant of ionization, , the rate constant of deactivation, k-i, and the apparent equilibrium constant of ionization, , have also been determined as a function of temperature. The increases slightly and k-i increases moderately with increasing temperature; therefore, and the overall polymerization rate decrease moderately with increasing temperature.