Conformational Statistics of Polyisobutene by a Monte Carlo Study

Abstract

The Monte Carlo method has been utilized to investigate the conformational distribution in the central section of a polyisobutene (PIB) decamer at various temperatures. The Monte Carlo results are in excellent agreement with the experimental data on the average dimensions of PIB chains, as well as with the molecular scattering functions of this polymer in solution and in bulk. The average values of the backbone bond angles centered at the CH2 and at the disubstituted C atom are 126° and 110°, respectively, at 400 K. The distribution of torsional angles is characterized by six distinct maxima centered at ±15°, ±130°, and ±105°. A six-state RIS model with states located in the latter positions constitutes a very good description of the conformational distribution in PIB, leading to calculated properties in good agreement with experiments. The Monte Carlo results are then at variance with the outcome of previous calculations based on constrained minimizations of the energy, which led to the inaccurate conclusion that torsional angles around ±105° are of negligible incidence in PIB. Therefore, this Monte Carlo based conformational analysis is shown to be particularly powerful in investigating chains with a number of highly interdependent conformational variables. © 1992, American Chemical Society. All rights reserved.