X-ray diffraction and molecular modeling have been used to investigate the near planar solid-state phases of poly(di-n-hexylsilane) (pdHexSi) and poly(di-n-butylsilane) (pdBuSi). Structure refinement of diffraction data in conjunction with oligomer cluster energy calculations provides a more unified picture of both the main chain and side chain conformations. In both polymers the alkyl side chain construction is closely identified as an asymmetric cisoid-transoid conformational arrangement. Full refinement of the pdHexSi data identifies subtler features in the alkyl chain organization. All major trends and key structural features are paralleled in empirical MM3 based modeling of isolated di-n-butylsilane and di-n-butylsilane oligomers. Zero-temperature gas-phase calculations identify a well-defined topology of energetically accessible main chain and side chain conformations. Increasing the side chain length, from butyl to hexyl, changes both the distribution and make up of the low energy structures. A combination of intrachain and interchain packing energy constraints stabilize the planar form of poly(di-n-hexylsilane). Intrachain interactions are significantly weaker in poly(di-n-butylsilane).