Synthesis, characterization, and pyrolytic conversion of a novel liquid polycarbosilane

Partially ethynyl-substituted ethynylhydridopolycarbosilane (EHPCS) is prepared by Grignard coupling of chloromethyltrichlorosilane, chloromethylmethyldichlorosilane, and ethynylmagnesium bromide, followed by reduction with lithium aluminum hydride. The as-synthesized EHPCS, with a hyperbranched structure and an approximate formula of [SiH _1.35(CH₃)_0.37(C≡CH)_0.28CH ₂]n, has been characterized by GPC, FT-IR, and ¹H, ¹³C, and ²⁹Si NMR. The EHPCS can be cured readily in an inert atmosphere at 140°C for 6 h. Pyrolysis behavior and structure evolution of the cured EHPCS were studied by means of thermal gravimetric analysis-differential scanning calormetry, FT-IR, X-ray diffraction, and elemental analysis. It is suggested that hydrosilylation, ethynyl group polymerization, dehydrocoupling, and demethanation are involved in the cross-linking of EHPCS. The polymer-ceramic conversion is complete at 900°C, and it gives a stoichiometric and amorphous silicon carbide (SiC) with a yield of 78%. Further heating at 1200°C induces partial crystallization, forming locally ordered SiC₄. The characteristic peaks of β-SiC are observable above 1600°C.