Conformational Characteristics of Polyisobutylene

Abstract

The intramolecular energy of segments in a poly(l,l-dimethylethylene) chain (PIB) has been estimated by calculations on structures CH3-[C(CH3)2CH2]X-H, with x = 2 (dimer), 3 (trimer), and 4 (tetramer), and on a hexad of structure CCH2-[C(CH3)2CH2]6-C. PIB chains are subject to large steric repulsions that computations on analogues of low molecular weight do not properly take into account since the steric strain is relieved at the chain ends. We introduce an iterative scheme that eliminates end effects and correctly estimates the effects of steric strain on the configuration of the long-chain PIB molecule. Strain energy estimations for 2,2,4,4-tetramethylpentane, as well as for a diad in a long PIB chain, agree well with experimental values. The two-bond repeat regular conformation of lowest computed energy is an 83 helix, with helix parameters in excellent agreement with the conformation in crystalline PIB according to X-ray crystallographic results. The detailed analysis of all diad, triad, and tetrad conformations and of more than 100 conformations for the hexad is complex; the steric stress in PIB chains is manifested in interactions that are transmitted beyond nearest neighbors. In good approximation, however, the significant conformations of PIB can be represented by a four-state rotational isomeric scheme, with only one adjustable statistical weight parameter. Rotational isomeric states are located at [formula-omitted]. Skeletal [formula-omitted]bond angles are 123° at the methylene groups and 109° at the quaternary carbon atoms. Conformations divide into a “+ class” and a “- class”. Bond conformations tend to be followed by those from the same class, changes from one to the other being rare. The characteristic ratio and its temperature coefficient, computed with the four-state scheme, agree well with experimental values. © 1983, American Chemical Society. All rights reserved.