B(C6F5)3 catalyzed dehydrocarbon polycondensation of PhSiH3 with (MeO)4Si as model polyfunctional comonomers in new route to hydrophobic silicone TQ resins

Highly branched methoxy substituted phenylsilsesquioxane-silicate copolymers were obtained by dehydrocarbon polycondensation of phenylsilane (PhSiH₃) with tetramethoxysilane (TMOS) in toluene solution catalyzed by tris(pentafluorophenyl)borane [B(C₆F₅)₃]. This model reaction represents a new non-hydrolytic route to TQ silicone resins, which do not contain reactive and hydrophilic silanol groups and therefore, should be more stable and have better miscibility with some organic polymers. The soluble siloxane copolymers, which do not contain volatile oligomers, were obtained using the PhSiH₃:TMOS molar ratios below 0.9. The dehydrocarbon condensation is accompanied by metathetic exchange of hydrosilyl and methoxysilyl functions, which is responsible for the formation of links between units originating from the same comonomer. Competition between the dehydrocarbon condensation and metathesis is consistent with a mechanism involving the transient formation of a tertiary oxonium ion intermediate. ²⁹Si NMR analysis of the copolymer product proves that extensive cyclization occurs during the polycondensation, most likely leading to a significant proportion of ladder and cage like structures.