The solid-state electronic properties and structures of poly(n-pentyl-n-hexylsilane) (C5-C6), poly(n-pentyl-n-octylsilane) (C5-C8), and poly(n-pentyl-n-dodecylsilane) (C5-C12) have been investigated using UV absorption, differential scanning calorimetry, and X-ray diffraction. At low temperatures, C5-C6 is semicrystalline with an all-trans backbone conformation. Upon heating, it transforms to the hexagonal, columnar, condis crystalline phase observed in many polysilanes. C5-C8 exhibits the hexagonal, columnar mesophase both above and below its first-order transition temperature of -32 degrees C. This type of behavior is found to be characteristic of poly(n-alkyl-n-alkyl’silanes) with a side-chain asymmetry of two or three carbon atoms, The existence of first-order transitions and the observation of UV thermochromic red shifts upon cooling for these polymers which do not crystallize are most likely the result of polarization interactions between the a-bonded silicon backbone and the alkyl side chains, which is consistent with the predictions of the model of Schweizer for conjugated polymers.