Determination of the Propagation Rate Constant in the Carbocationic Polymerization of 2,4,6-Trimethylstyrene.

Abstract

The living carbocationic polymerization of 2,4,6-trimethylstyrene (TMeSt) was studied using the 1-chloro-1-(2,4,6-trimethylphenyl)ethane/BCl3 initiating system in CH2Cl2 from −70 to −20 °C. The equilibrium constant of ionization (Ki) was calculated from the cation concentration determined by UV−visible spectroscopy. From the apparent rate constant of propagation (kpapp) and Ki, the absolute rate constant of propagation for ion pairs kp± = 1.4 × 104 L mol-1 s-1 was calculated at −70 °C. The rate constant of ion collapse, k-i was determined from the polydispersity vs conversion plots. From Ki and k-i, the rate constant of ionization, ki were also calculated. From the temperature dependence of Ki, and k-i the enthalpy and entropy of the ionization and deactivation were obtained, respectively. Competition experiments i.e., polymerization carried out in the presence of 2-chloropropene as a π-nucleophile, were also carried out at −70 °C to determine kp± separately. NMR spectroscopy, gel permeation chromatography, and elemental analysis suggested complete capping of the polymeric cation and the absence of side reactions. From the limiting number-average degree of polymerization and the known rate constant of capping kc = 62.0 L mol-1 s-1, kp± = 8.3 × 104 L mol-1 s-1 was determined. This value is similar to that determined from kpapp and Ki; thus, the utility of competition experiments is corroborated.