POLY(CARBOSILANE) PRECURSORS OF SILICON-CARBIDE - THE EFFECT OF CROSS-LINKING ON CERAMIC RESIDUE

Poly(carbosilanes) of general structure X[-Si(H)(Y)C2H4-]n-SiH2Z (I, X = Y = H, Z = vinyl; II, X = Y = Z = H; III, X = Z = vinyl, Y = H; IV, X = Z = vinyl, Y = vinyl and H) have been prepared by means of hydrosilylation polymerization, followed by chemical transformation of the resulting polymer including a final LiAlH4 reduction step. Thus, I was prepared from dichlorovinylsilane (V), II from V together with trichlorosilane as a chain-terminating agent, and III from V together with dichlorodivinylsilane as a chain-terminating agent. IV was made by polymerization of anisylvinylsilane followed by anisyl group cleavage with HCl/AlCl3 and then reaction with vinyl Grignard reagent. Ceramic yield from I-IV was found to increase with polymer vinyl group content, ranging from a low yield for I (12%) to a high yield for IV (60%). This is explained by cross-linking through hydrosilylation occurring during pyrolysis, as shown by the exotherm at ca. 200-degrees-C, and also by changes in the IR spectrum.