STRUCTURE ELUCIDATION OF CRYSTALLINE POLY(DIPHENYLSILOXANE)

This study shows that the structure of the poly(diphenylsiloxane) (PDP hS) backbone is quasi-planar rather than helical. The nearest-neighbor chains in the crystal are packed in a hexagonal configuration. Using molecular mechanics calculations and analysis of X-ray powder diffract ion data, it has been determined that the crystalline structure of PDP hS has Pbn2(1) symmetry and the backbone consists of siloxane bonds in a quasi cis-trans sequential conformation. The deviation of each bond from the planar configuration is about 15 degrees. The backbones of t he two chains in the unit cell are antisymmetrical. The Si-O-Si bond a ngle is 145 degrees and the torsional angles of the vicinal phenyl gro ups relative to each Ether are 125 and 72 degrees. The orientation of the phenyl groups in the crystal consists of parallel and perpendicula r nearest neighbors. The calculated X-ray results are able to explain the main features of the experimental pattern in terms of position and intensity of the peaks. Molecular mechanics calculations suggest that the density of ideal crystals of poly(diphenylsiloxane) at low temper atures should be in the range 1.26-1.3 g/cm(3), whereas the reported e xperimental value is 1.22 g/cm(3). A new set of molecular mechanics pa rameters for the UFF method to more accurately describe polysiloxanes in the condensed state is given.