Kinetic investigations of the boron trichloride co-initiated carbocationic polymerization of isobutylene by real-time ATR-FTIR spectroscopy
The primary objective of this research was the development and application of real time in-situ ATR-FTIR spectroscopic analysis to accurately generate kinetic data on the carbocationic polymerization of isobutylene. Although the kinetic data generated by ATR-FTIR agreed fairly well with the kinetic data obtained gravimetrically, factors such as thermal fluctuations, spectral overlap, reagent dilution, and solution homogeneity greatly affected data collection. We were particularly interested in applying this new method to the study of a system facilitating the synthesis of well-defined low molecular weight PIB. Specifically, we investigated the effect of several factors on the polymerization of IB via tert -chloride/BCl3 initiating systems including the concentration of monomer and Lewis acid, the reaction temperature, the type of initiator employed and the use of external additives such as Lewis bases (ED) and common ion salt. Systematic investigations of the influence of the effective BCl 3 concentration ([BCl3 ]eff = [BCl3 ] o = [ED]o ) and IB concentration on the kinetics of IB polymerization were conducted using the BCl3 /t -Bu-m -DCC/DMP initiating system in MeCl at -80°C. The effect of temperature on the system was investigated over the range from -80 to -30°C. The kapp s obtained from first-order kinetic plots doubled as the reaction temperature was increased incrementally from -80 to -30°C. An Arrhenius plot of the temperature data was constructed and the apparent activation energy, Ea , was calculated to be 1.7 kcal/mol. The effect of initiator and Lewis base on the these systems was investigated by conducting a series of polymerizations in pure MeCl at -40°C, utilizing either an aliphatic initiator (TMPCl) or an aromatic initiator ( t -Bu-m -DCC) in combination with either DMP or DTBP to serve as the Lewis base at concentrations ranging from 0 to 20 × 10-3 M. The first-order kinetic plots were linear over the entire range of [DMP]o and [DTBP]o examined, and passed through the origin for TMPCl initiated polymerizations, while those employing t -Bu-m -DCC exhibited a small, but finite, y-intercept. The different behavior of the two systems was attributed to the fact that the aromatic initiator, t -Bu-m -DCC, is ionized much more rapidly than the aliphatic initiator, TMPCl. The dependence on [DMP]o was found to be -0.28 and -0.24 for TMPCl and t -Bu-m -DCC respectively. In contrast to polymerizations in the presence of DMP, there was only a very slight change in the value obtained for kapp as [DTBP] o was varied. In order to investigate the effect of common ion salt, a series of polymerizations were carried out in which nBu4 NCl was added to the system instead of a Lewis base such that [nBu4 NCl] o was varied over the range of 0.1 to 20 × 10-3 M. It was found that kapp decreased as [nBu 4 NCl]o was increased incrementally from 1.0 × 10 -4 to 2.0 × 10-3 M. which was attributed to suppression of free ion formation by the common ion effect.