Despite the multitude of available alternatives, the Grignard coupling-based synthesis of polycarbosilanes remains attractive, offering the benefit of easy structural design. Moreover, this method allows one to obtain a polymer precursor with the stoichiometric Si : C ratio required for SiC ceramic production and is also suited for the synthesis of polymers containing curable functional groups (e.g., allyl and vinyl). Herein, vinyl-substituted hydridopolycarbosilane was synthesized from three different starting materials (Cl₃SiCH₂Cl, (MeO)₃SiCH₂Cl, and (EtO)₃SiCH₂Cl) using Grignard coupling, and the effects of starting material on polymer growth behavior were investigated. The alkoxysilane starting materials had the advantage of being safe to handle, but had a limitation in polymer growth. The largest molecular weight was observed for Cl₃SiCH₂Cl, although side reactions occurred. This behavior was ascribed to the retention of unreacted –SiCH₂Cl groups contributing to polymer growth via coupling with neighboring polymer chains.