Light scattering and vicosity measurements were made on fractionated samples of poly-{n-hexyl-{(S)-2-methylbutyl] silylene)} (PH2MBS, 1) and poly{n-hexyl-[(S)-3-methylpentyl]silylene} (PH3MPS, 2) in isooctane to determine their stiffness, which are expressed in terms of the persistence length q of Kraty and Pored. The q of polymer 1 is as large as 85 nm and depends only slightly on temperature, indicating that its conformation is almost rodlike, whereas the a of polymer 2 is 6.2 nm, its global conformation being close to a random coil. It is concluded that the global conformation of polysilylene is determined by its side chain; polysilylene with a beta -branch is much stiffer compared to those without it.