We present the results of ab initio 3-21G* geometry optimizations and valence effective Hamiltonian (VEH) band structure calculations aimed at determining the evolution of the geometric and electronic (ionization potential, electron affinities, and band gaps) properties of all-trans poly(dimethylsilane), poly(diethylsilane), poly(di-n-propylsilane), and poly(di-n-butylsilane) when increasing the size of the-alkyl group. In the latter polymer, we have also studied the 7/3 conformation, in order to analyze the effect of the backbone conformation on the geometric and electronic structure. The VEH ionization potentials of all-trans poly(di-n-alkylsilanes) are almost equal, and as experimental photoemission data show, only slight differences are appreciated. The band gap decreases in going from poly(di-n-butylsilane) to poly(di-n-propylsilane) and to poly(diethylsilane), and increases when passing to poly(dimethylsilane), which coincides with experimental evidences on poly(di-n-alkylsilanes). The change from all-trans to 7/3 conformation of poly(di-n-butylsilane) implies an increase of both, ionization potential and band gap, in perfect agreement with experimental photoemission and absorption data. The applicability of VEH to deal with poly(di-n-alkylsilanes) is discussed.