High Yield Polycarbosilane Precursors to Stoichiometric SiC. Synthesis, Pyrolysis and Application

Abstract

ABSTRACTThe synthesis and properties of two polycarbosilanes that have essentially a “SiH2CH2” composition is described. One of these polymers is a highly branched hydridopolycarbosilane (HPCS) derived from Grignard coupling of CI3SiCH2CI followed by LiAIH4 reduction. This synthesis is amenable to large scale production and we are exploring applications of HPCS as a source of SiC coatings and its allyl-derivative, AHPCS, as a matrix source for SiC- and C-fiber-reinforced composites. These polymers thermoset on heating at 200-400 °C (or at 100 °C with a catalyst) and give near stoichiometric SiC with low O content in ca. 80% yield on pyrolysis to 1000 °C. The second method involves ring-opening polymerization of 1,1,3,3-tetrachlorodisilacyclobutane and yields a high molecular weight, linear polymer that can be reduced to [SiH2CH2]n (PSE), the monosilicon analog of polyethylene. In contrast to high density polyethylene which melts at 135 °C, PSE is a liquid at room temperature which crystallizes at ca. 5 °C. On pyrolysis to 1000 °C, PSE gives stoichiometric, nanocrystalline, SiC in virtually quantitative yield. The polymer-to-ceramic conversion was examined for PSE by using TGA, mass spec, solid state NMR, and IR methods yielding information regarding the cross-linking and structural evolution processes. The results of these studies of the polymer-to-ceramic conversion process and our efforts to employ the AHPCS polymer as a source of SiC matrices are described.