To provide a process for producing substituted polysilylenemethylenes excellent in thermal resistance and moldability without using any metal catalyst at a relatively low temp.

A 1,3-disilacyclobutane of the formula: (RR'SiCH2)2 (wherein R and R' may be the same or different and represent each hydrogen, a 1-8C alkyl, a halogenated alkyl, an alkenyl or an aryl, provided they both cannot be hydrogen at the same time) is heated in the presence of a lipophilic org. peroxide and a lipophilic organic azo compd. as a radical-forming agent to open the ring and thereby to form a substd. polysilylenemethylene having repeating units of the formula: -(RR'SiCH2)2-.