Poly(silylenemethylene)s of the types [SiMeRCH2](n) and [SiHRCH2](n) were prepared by the ring-opening polymerization (ROP) of 1,3-disilacyclobutanes (DSCBs) containing n-alkyl substituents, such as C2H5, n-C3H7, n-C4H9, n-C5H11, and n-C6H13, or a phenyl group on the Si. These new polymers include a monosilicon analog of poly(styrene), [SiHPhCH2](n). Improved synthesis routes to the DSCB monomers were developed which proceed through Grignard ring closure reactions on alkoxy-substituted chlorocarbosilanes. All of these asymmetrically substituted polymers were obtained in high molecular weight form, except for [SiHPhCH2](n). The configurations of all of the polymers were found to be atactic. The aryl-substituted polymers have higher glass transition temperatures (T(g)s) and thermal stability than those of the alkyl-substituted poly(silylenemethylene)s. Unlike the polyolefins of the type [C(H)(R)CH2](n), where T-g drops continuously from R = Me to n-Hex, the T(g)s of the n-CnH2n+1 (n = 2-6)-substituted [SiMeRCH2](n) PSM’s appear to reach a maximum (at -61 degrees C) for the R = n-Pr-substituted polymer. Moreover, where it was possible to make direct comparisons among similarly substituted atactic polymers, all of the poly(silylenemethylene)s were found to have lower T(g)s than their all-carbon analogs.