Poly(di-n-propylsilylene) and poly(diethylsilylene-co-di-n-propylsilylene) have been investigated by Raman, IR, and UV spectroscopy, TGA, DSC, wide angle X-ray diffraction, and solid state C-13 and Si-29 NMR. No IR-Raman coincidences were found in the Si-Si or Si-C stretching regions for (n-Pr2Si)n, demonstrating that the main chain has a planar all-trans configuration, T(infinity), consistent with the UV absorption at 355 nm. From the Si-C stretching bands, all of the n-propyl groups appear to have the same conformation. (n-Pr2Si)n(Et2Si)m also has the T(infinity) configuration at room temperature, and a tetragonal crystal lattice. Both polymers undergo a first-order transition to a mesomorphic phase, above 222-degrees-C for (n-Pr2Si)n and above 113-degrees-C for the copolymer.