Determination of the Absolute Rate Constant of Propagation for Ion Pairs in the Cationic Polymerization of p-Methylstyrene

Abstract

The cationic polymerization of p-methylstyrene (p-MeSt) was studied in dichloromethane, chloroform, and methylcyclohexane/methyl chloride 60/40 (v/v) at different temperatures with various initiators in conjunction with different Lewis acids, TiCl4, BCl3, SnBr4, and SnCl4. Well-controlled living cationic polymerization was obtained in dichloromethane, in conjunction with SnCl4 as Lewis acid, and the living nature of the polymerization was verified by linear first-order ln([M]0/[M]) vs time and linear number-average molecular weight (Mn) vs conversion plots in the temperature range of −15 to −70 °C. The number-average molecular weight of the polymers increased in direct proportion to monomer conversion up to Mn ≈ 90 000 and agreed reasonably well with the calculated molecular weight, assuming that one polymer chain forms per molecule of initiator. The kinetics of p-MeSt polymerization suggests that the polymerization is first order with respect to SnCl4 concentration. Employing the model compound 1-chloro-1-(4-methylphenyl)ethane in conjunction with SnCl4 in CH2Cl2, UV−vis spectroscopy was used to follow the capping reaction with 2-phenylfuran to determine the equilibrium constant of ionization (Ki) at −30 °C. From the Ki value and the apparent rate constant of propagation (kapp) the absolute rate constant of propagation for ion pairs, kp± = 6.8 × 108 L mol-1 s-1 at −30 °C, was calculated as a lower limit. The absolute rate constant of propagation for ion pairs, kp±, was also determined at different temperatures from competition experiments, where polymerizations were carried out in the presence of 2-phenylfuran as capping agent. Gel permeation chromatography and NMR spectroscopy suggested complete capping of the polymeric cation and the absence of side reactions. From the limiting conversion and limiting number-average degrees of polymerization kp± = 1 × 109 L mol-1 s-1 was calculated using the known rate constant of capping. The kp± value remained unaffected in the temperature range of −15 to −70 °C, indicating that propagation does not have an enthalpic barrier.