Living Carbocationic Polymerization ofp-Methoxystyrene Usingp-Methoxystyrene Hydrochloride/SnBr₄Initiating System:  Determination of the Absolute Rate Constant of Propagation for Ion Pairs.

Abstract

The carbocationic polymerization of p-methoxystyrene (p-MeOSt) was studied in CH2Cl2 at −30 °C using 1-chloro-1-(p-methoxyphenyl)ethane (p-MeOStCl) as initiator in conjunction with different Lewis acids, BF3·O(C2H5)2, BCl3, ZnCl2, TiCl4, SnCl4, and SnBr4. The best results, moderate rates, theoretical Mns, and low polydispersities, were obtained in conjunction of SnBr4. The living nature of the polymerization was verified by linear first-order ln([M]0/[M]) vs time and linear Mn vs conversion plots in the temperature range of −60 to −20 °C. The number-average molecular weight of the polymers increased in direct proportion to monomer conversion up to Mn = 120 000 and agreed with the calculated molecular weight, assuming that one polymer chain forms per molecule of p-MeOStCl. Kinetic studies suggest that the polymerization is first order in [SnBr4], and the rate, Mn, and polydispersity index are not affected by excess proton trap, 2,6-di-tert-butylpyridine. The living nature of the polymerization was further demonstrated by chain extension experiment. The stability of the propagating chain end of poly(p-MeOSt) and monomeric chain end of p-MeOSt+ was studied, and a slow decomposition of the active chain end was observed under monomer starved conditions. Employing the model compound p-MeOStCl in conjunction with SnBr4 in CH2Cl2 UV−vis spectroscopy was used to determine the equilibrium constant of ionization (Ki) at −30 and −60 °C. From Ki values and the apparent rate constant of propagation (kapp), the absolute rate constant of propagation for ion pairs, kp± = 1.07 × 105 L mol-1 s-1 for −30 °C and 3.83 × 104 L mol-1 s-1 for −60 °C, was calculated. To determine kp± separately, competition experiments were carried out in the presence of a nucleophile, phenylsilane. NMR spectroscopy, gel permeation chromatography, and MALDI−TOF MS analysis suggested complete capping of the polymeric cation and the absence of side reactions. From the limiting conversion and limiting number-average degree of polymerization kp± was calculated using the known rate constant of capping. The kp± values obtained from the competition experiments agreed well with those determined from the UV−vis spectroscopy.