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 ± ≈ 2 × 10 9 L mol -1 s -1, 6 orders of magnitude higher than previously accepted, for the absolute rate constant of propagation of the TiCl 4 -induced polymerization of St in methylcyclohexane/methyl chloride 60/40 (v/v) at −80 °C. The k p ± 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 k p ± as similar values have been obtained with TiCl 4 or SnCl 4 . 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.