SOLID-STATE STRUCTURE AND PHASE-TRANSITIONS OF POLY(DIMETHYLSILYLENE)

We have investigated the structure of poly(dimethylsilylene) (PDMS), which is of interest as the first member of the important and widely studied family of dialkyl-substituted Si-backbone polymers. We found that the chains adopt a trans conformation and propose a monoclinic packing with a = 1.218 nm, b = 0.800 nm, c (chain axis) = 0.388 nm, and gamma = 91-degrees. This trans conformation is in agreement with our findings from the next lowest homologues (diethyl and di-n-propyl) but does not lend support to energetic predictions of helical conformations or of a requirement of at least six carbon atoms in the side chain to cause planarity of the silicon backbone. Instead, it points to stabilization of the ground-state trans conformation by Si-sigma-bond delocalization as the conformational driving force. Solid-state NMR showed the trans chains in PDMS to be much more rigid than those in the diethyl polymer. PDMS undergoes two weak thermal transitions at 160 and 220-degrees-C. On the basis of our UV, X-ray, and electron diffraction results, we find the first of these to involve preservation of the trans conformation and adoption of orthorhombic packing on a metrically hexagonal lattice. The second thermal transition involves additionally some conformational and orientational disordering. Thus, in contrast to the widely studied poly(di-n-hexylsilylene), the dimethyl analogue exhibits no thermochromism or large-scale conformational disordering at its solid-state transformations, which are primarily intermolecular in nature.