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Poly(methylsilane) and poly(vinylsilane) were synthesized using a titanocene
catalyst, and their pyrolytic conversion to ceramics was followed using a
combination of thermal analysis and infrared spectroscopy. The two polymers
have distinctly different backbone structures, as determined by
29Si NMR; methylsilane polymerizes to a polysilane, while
vinylsilane polymers have a predominately polycarbosilane backbone, with
some polysilane structure as well. The pyrolysis path and char yield were
dependent primarily on backbone structure, with little influence of polymer
molecular weight. The majority of the weight loss on conversion occurs below
650 °C, although bond rearrangement continues to 1400 °C. Poly(vinylsilane)
produced a C-rich Si-C ceramic in which the carbon was dispersed on a
sufficiently fine level to show resistance to oxidation on heating in air to
Vinylsilane polymerizes to form predominantly a carbosilane polymer using dimethyltitanocene catalyst. This is in contrast to alkylsilanes, which afford polysilanes under the same conditions. The mechanism of polymerization of alkenylsilanes has been shown to be fundamentally different from that for the polymerization of alkylsilanes. The silyl substituent apparently activates a double bond to participate in a number of polymerization processes in this system, particularly hydrosilation. Isotopie labeling indicates the involvement of silametallocyclic intermediates, accompanied by extensive nuclear rearrangement. Polymers and copolymers derived from alkenylsilanes have relatively high char yields even for conditions which afford low molecular weight distributions. Formation of crystalline β-SiC is optimum for a copolymer of an alkylsilane and an alkenylsilane having a silane/carbosilane backbone ratio of 85/15 and a C/Si ratio of 1.3/1.
Oxidative coupling of primary alkylsilanes catalyzed by Group IVB metallocene complexes leads predominantly to linear polysilanes, as first reported by Harrod. We have investigated the polymerization of ethylsilane and vinylsilane using dimethyltitanocene in order to determine the suitability of such polymers as precursors to Si-C based ceramics for application as coatings or composite matrices. A photochemical procedure for initiation of polymerization is described. Ethylsilane forms polysilanes (which contain a -Si-Si-Si-Si- backbone) by a step growth mechanism. Vinylsilane shows some Si-Si formation; however, polymerization by several different routes, including formation of polycarbosilanes (which contain a -Si-C-SI-C- backbone) by hydrosilation reactions and crosslinking via metathesis, predominate. The carbosilane polymer has high char yield (60-75%) and appears advantageous for conversion into silicon carbide, as determined by X-ray diffraction.
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