Synthesis, Characterization, and Pyrolytic Conversion of a Novel Liquid Polycarbosilane

Partially ethynyl-substituted ethynylhydridopolycarbosilane (EHPCS) is prepared by Grignard coupling of chloromethyl-trichlorosilane, chloromethylmethyldichlorosilane, and ethynyl-magnesium bromide, followed by reduction with lithium aluminum hydride. The as-synthesized EHPCS, with a hyperbranched structure and an approximate formula of [SiH1.35(CH3)(0.37) (C CH)(0.28)CH2]n, has been characterized by GPC, FT-IR, and H-1, C-13, and Si-29 NMR. The EHPCS can be cured readily in an inert atmosphere at 140 degrees 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 degrees C, and it gives a stoichiometric and amorphous silicon carbide (SiC) with a yield of 78%. Further beating at 1200 degrees C induces partial crystallization, forming locally ordered SiC4. The characteristic peaks of beta-SiC are observable above 1600 degrees C.