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-hexyl)silane, have been studied using conventional X-ray ray diffraction and direct measurements of the local structure (through a pair correlation function analysis) in combination with UV absorption spectroscopy. These data universally support Si backbone models which are dominated by the presence of mixed transoid (T), deviant (D), and, possibly, ortho (O) conformations with dihedral angles approximating ±170°, ±155°, and ±90°, 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 backbone, and this, locally, approximates a D+T+D−T− motif. Poly(methyl-n-hexyl)silane displays a number of low-temperature polymorphic structures, and one of these is found to exhibit Bragg-like scattering features. Temperature-dependent UV absorption measurements of poly(methyl-n-hexyl)silane resolve two distinguishable isosbestic points and are indicative of mesomorphic behavior.