Structure and chain conformation in poly(methyl-n-alkyl)silanes

Three asymmetrically substituted methyl-alkyl-substituted polysilanes, poly (methyl-n-propyl)silane, poly(methyl-n-butyl)silane, and poly(methyl-n-hexy l)silane, have been studied using conventional X-ray ray diffraction and di rect measurements of the local structure (through a pair correlation functi on analysis) in combination with UV absorption spectroscopy. These data uni versally support Si backbone models which are dominated by the presence of mixed transoid (T), deviant (D), and, possibly, ortho (O) conformations wit h dihedral angles approximating +/- 170 degrees, +/- 155 degrees, and +/- 9 0 degrees, respectively. There is no evidence for a significant fraction of all-anti (A, i.e., trans-planar), all-transoid, or anti-gauche conformers. Crystalline poly(methyl-n-hexyl)silane incorporates a D/T-based Si backbon e, and this, locally, approximates a D+T+D-T-motif. Poly(methyl-n-hexyl)sil ane displays a number of low-temperature polymorphic structures, and one of these is found to exhibit Bragg-like scattering features. Temperature-depe ndent UV absorption measurements of poly(methyl-n-hexyl)silane resolve two distinguishable isosbestic points and are indicative of mesomorphic behavio r.