Temperature Effects on the Living Cationic Polymerization of Isobutylene: Determination of Spontaneous Chain-Transfer Constants in the Presence of Terminative Chain Transfer

Abstract

The living cationic polymerization of isobutylene (IB) was studied using the 2-chloro-2,4,4-trimethylpentane (TMPCl)/TiCl4/2,6-di-tert-butylpyridine (DTBP) system in hexane (Hex)/methyl chloride (MeCl) (60/40 and 40/60, v/v) solvent mixtures at various temperatures ranging from −25 to −80 °C. From the Arrhenius plots of the apparent rate constants for propagation, negative apparent activation energies were observed and calculated to be −8.5 and −6.9 kcal/mol using Hex/MeCl, 60/40, v/v and 40/60, v/v solvent mixtures, respectively. At temperatures ≤ −60 °C, linear first-order plots and linear Mn versus conversion plots were obtained, suggesting the absence of termination and chain transfer. Irreversible termination was conspicuously observed from the curved ln([M]0/[M]) versus time plots when the polymerizations were carried out at temperatures ≥ −40 °C in both solvent mixtures. Mn versus conversion plots, however, exhibited linear growth of Mn, on the theoretical line, with increased conversion indicating the absence of chain transfer to monomer. Structural analysis of products obtained at −40 °C using 1H NMR spectroscopy revealed the presence of olefinic end groups, increasing in content with increased conversion. On the basis of these results, it is concluded that termination at higher temperature involves terminative chain transfer, that is, β-proton elimination from the living chain ends with the eliminated proton being instantaneously entrapped by a proton trap, DTBP. By a kinetic treatment of the terminative chain transfer, the spontaneous chain-transfer constants (ktr/kp's), zero order in monomer, were determined for the first time. In Hex/MeCl (40/60, v/v), ktr/kp was calculated to be 1.1 × 10-2 and 7.2 × 10-4 mol/L at −25 and −40 °C, respectively. In Hex/MeCl (60/40, v/v), ktr/kp was calculated to be 3.3 × 10-3 mol/L at −40 °C.