Kinetic simulation of living carbocationic polymerizations. II. Simulation of living isobutylene polymerization using a mechanistic model

Abstract

This paper discusses the kinetic simulation of TiCl 4––coinitiated living carbocationic isobutylene (IB) polymerizations governed by dormant-active equilibria, using a mechanistic model. Two kinetic models were constructed from the same underlying mechanism: one using a commercial simulation software package (Predici ®), and the other using the method of moments. Parameter estimation from experimental batch reactor data with Predici yielded a rate constant of propagation k p = 4.64 × 10 8 ± 2.75 × 10 8 L/mol s, with no constraints imposed. This agrees with k p data measured with diffusion clock and competition methods, but disagrees with kinetically obtained k p values. Estimation of rate constants with Predici ® and the GREG parameter estimation software packages revealed that it was difficult to estimate the complete set of kinetic parameters, due to correlated effects of the parameters on model predictions. Estimability analysis confirmed that some of the strongly correlating parameters could not be estimated simultaneously using the available experimental data. Using k p = 6 × 10 8 ± 2.75 × 10 8 L/mol s measured by Mayr, and using starting estimates of other rate constants defined by experimentally observed correlations, yielded the set of rate constants required for the simulations. Both kinetic models yielded good agreement with experimental data, with the exception of M w values that slightly diverged from the theoretically predicted ‘ M w− M n = constant’ relationship. This may indicate the occurrence of a minor side reaction. However, the k p/ k −1 = 17.5 L/mol average run length calculated from measured and simulated MWD data agrees well with earlier literature values.