Complexes of stannic chloride and alkyl phenols and the influence of these complexes and of free phenol on the cationic polymerization of isobutene

Abstract

The polymerization of isobutene in ethyl chloride solution at −78.5 °C has been studied using stannic chloride as catalyst and alkyl substituted phenols as cocatalysts. In preliminary experiments, the nature and extent of the complexing between phenols and stannic chloride was examined and it was shown that σ-complexing occurs between the oxygen of the phenol and the stannic chloride. Equilibrium constants were determined over the temperature range 30 to −50 °C and were used, together with hydrogen bonding data, to estimate the concentrations of complex and free phenol in the polymerization mixtures at −78.5 °C. The active initiator in the polymerization is probably the σ-complex but the free phenol acts as a chain terminating agent. This explains the ability of the cocatalyst to decrease the rate of polymerization under some experimental conditions. For isobutene concentrations up to 3 M and at constant dielectric constant the rate of polymerization and the molecular weight are proportional to the monomer concentration. A mechanism is proposed to account for the main features of the experimental results.