Semicrystalline films and powders of poly(di-n-octylsilane) and poly(di-n-decylsilane) have been investigated using in situ quenching studies by X-ray scattering and UV-vis absorption in combination with detailed model calculations. These polymers displayed a surprisingly rich array of monotropic and enantiotropic structures during thermal cycling. Poly(di-n-decylsilane) was observed to have no less than five distinct ordered forms (crystalline or semicrystalline) which varied in their respective chain structure, packing, and optical properties. Of these only one appeared to be thermodynamically stable. One of the metastable polymorphic structures exhibited an unprecedented tripling of the basic unit cell structure. Poly(di-n-octylsilane) samples also exhibited up to five distinct structural forms at temperatures below that of a 25 degreesC transition to the thermotropic mesophase. Three of these were clearly ordered while the other two are mesophases with features suggestive of a less ordered state. Suggested structural models are used to document and identify the predominant side chain and main chain packing motifs in the various crystalline polymorphs.