Crystal structure of polyisobutylene

Abstract

Abstract The crystal structure of polyisobutylene was determined by x‐ray analysis. The orthorhombic cell, with a = 6.88 Å, b = 11.91 Å, c (fiber axis) = 18.60 Å (space group: P 2 1 2 1 2 1 − D ), contains two molecular chains each consisting of eight monomeric units in the fiber identity period. The chain conformation is essentially an (8/3) helix, but deviates appreciably from the exact (8/3) helix symmetry. The symmetry of the molecular chain is only a twofold screw axis in exact sense, and a crystallographic asymmetric unit consists of four monomeric units. The torsional angles are equation image where M denotes the methyl group. The averaged skeletal CCH 2 C and CCM 2 C bond angles are 128° and 110°, respectively. The large CCH 2 C bond angles may be due to steric respulsion between the adjacent methyl groups, giving intramolecular distances larger than 3.09 Å.