ORGANOSILICON POLYMERS - SYNTHESIS, ARCHITECTURE, REACTIVITY AND APPLICATIONS

Tailoring of polysilanes with given architectures and reactivities is a great challenge in the field of SiC pre-ceramic polymers, This paper reviews recent polysilane and related copolymer synthesis reactions, It is shown that the Wurtz-type polymerization of dichloro-, trichloro - or tetrachloro-silanes, so far the most extensively studied, enables access to a large variety of architectures ranging from one- to three -dimensional (3D) topologies, and based on secondary > SiR(2), tertiar y RSi(Si)(3) or quaternary Si(Si)(4) silicon units in the polymer back bone, These polysilanes usually present an intrinsic low reactivity, d etrimental for fiber processing, Examples are given to illustrate how this reactivity can be increased by secondary substitution reactions, which create reactive entities that can favor further crosslinking rea ctions, Secondly a novel route involving heterogeneously catalyzed dis proportionation of chloromethyldisilanes, developed in our laboratory, is reviewed which offers a direct access to polysilyne-type 3D archit ecture constituted by arrangements of fused rings, The Lewis-base cata lyzed disproportionation mechanism is discussed and seems to involve d onor-stabilized silylenes as key intermediates in the polymer formatio n process, The experimental results are supported by ab-initio quantum chemical calculations, Silylenes attack the Si sites of higher functi onality causing a high regioselectivity for the exclusive formation of branched oligosilanes. The oligomers undergo thermally induced branch ing and crosslinking reactions leading to poly(chloromethylsilane)s. O bviously, there are analogies to the oligomer and polymer formation of the transition-metal complex catalyzed dehydropolymerzation of methyl disilanes, Poly(chloromethylsilane)s exhibit a high reactivity due to the presence of Si-Cl bonds, Disproportionation of chloromethyldisilan es in presence of olefins such as styrene provides promising polymer p recursors for SiC fibers, Their rheological properties have been inves tigated for various styrene contents, The polymer fibers spun from mel t are cured under ammonia, and then pyrolyzed to silicon carbide fiber s, showing temperature resistance up to 1500 degrees C. (C) 1997 by Jo hn Wiley & Sons, Ltd.