Polymerization of two phenyl-substituted 1,3-disilacyclobutanes with or without catalyst was studied for a better understanding of the synthesis of poly(silylenemethylene)s. Copper compounds were found to be effective catalysts for ring-opening polymerization of 1,3-dimethyl-1,3-diphenyl-1,3-disilacyclobutane (DDDC) as well as 1,1,3,3-tetraphenyl-1,3-disilacyclobutane regardless of the valence of copper and the counter anion. The catalytic activities of the copper compounds toward polymerization of DDDC, however, were found to be quite low compared with platinum or rhodium catalysts. Copolymerization of these two cyclic dimers without catalyst afforded copolymers with [MePhSiCH2]/[Ph2SiCH2] ratios very close to the loaded values. It is suggested from the dimer reactivity ratio that DDDC copolymerize with TPDC to yield almost random copolymers, while the dimer sequential length distributions in the copolymers are slightly different from an ideal distribution of two dimeric units.