Effect of temperature, solvent polarity, and nature of lewis acid on the rate constants in the carbocationic polymerization of isobutylene.

Abstract

The absolute rate constant of propagation for ion pairs (kp±) was determined by the diffusion clock method in the living carbocationic polymerization of isobutylene at different solvent polarity and temperature in conjunction with TiCl4, Me2AlCl, and BCl3 as Lewis acids. The kp± ((3.6−5.7) × 108 L mol-1 s-1 in hexanes/MeCl 60/40, v/v) was independent of temperature and nature of Lewis acid and increased moderately with increasing solvent polarity to a nearly diffusion-limited value (∼1.7 × 109 L mol-1 s-1) in pure MeCl. A similar kp± (∼(5−6) × 108 L mol-1 s-1) value was obtained in the nonliving polymerization of isobutylene in conjunction with EtAlCl2 in hexanes/MeCl 60/40 (v/v) at −80 °C, indicating that living and nonliving polymerizations proceed on identical propagating centers. The apparent equilibrium constant of ionization (activation) Kiapp (= KiKD0, where Ki is the absolute equilibrium constant of ionization and KD0 is the equilibrium constant of TiCl4 dimerization) was calculated from the apparent and absolute rate constant of propagation. The rate constant of deactivation, k-i was determined from the conversion vs polydispersity plots. From Kiapp and k-i, the apparent values of ki (kiapp = kiKD0, where ki is the absolute rate constant of ionization) were also calculated. On the basis of the results, the observed large differences in the overall polymerization rates with different solvent polarity and temperature can be attributed solely to the changes in the active center concentration, which decreases with decreasing solvent polarity and increasing temperature. From the temperature dependence of Kiapp, the apparent standard enthalpy (= ΔHi° + ΔHD0°) and entropy (= ΔSi° + ΔSD0°) of ionization, and from the temperature dependence of kiapp and k-i, the apparent activation enthalpy and entropy of the activation/deactivation process were calculated.