CRYSTAL-STRUCTURES OF POLY(DI-N-BUTYL SILANE) AND POLY(DI-N-PENTYL SILANE)

The crystal structures of poly(di-n-butyl silane) and poly(di-n-pentyl silane) have been determined by comparing the experimental x-ray diffraction pattern with the theoretical pattern. The side-chain conformation is mostly determined by the intramolecular steric hindrance as well as the van der Waals interaction between the side chains. The Si backbone conformation of both crystals is a 7/3 helix, which is mainly determined by the intramolecular van der Waals interaction between the nearest-neighbour side chains. On the other hand, the Si backbone conformation of both crystals after a pressure treatment of 1500 MPa is all trans, which is caused by the van der Waals interaction between the side chains attached to the second-nearest-neighbour Si atoms. In all cases, one of the two C-Si-C-C groups in Si(C4H9)(2) or Si(C5H11)(2) is an essentially trans conformation, whereas the other C-C-Si-C group has a cis-like conformation, which is different from that generally accepted.