Heterofunctional condensation of alkoxysilanes and silanols—I. Synthesis of definite polysiloxane networks

Heterofunctional condensation of alkoxysilanes with silanols may be useful for the synthesis of polysiloxane networks, leading to more definite structures than in the case of hydrolysis of chlorosilanes, provided that secondary reactions are avoided. Using methyltrimethoxysilane or γ-aminopropyltrimethoxysilane (APTMS) and a monosilanol (phenyldimethylsilanol) as a model, the various possible side-reactions (alkoxylation of the silanol, hydrolysis of the alkoxysilane, homocondensation of the silanol) were evaluated. Heterocondensation does not occur in the absence of a catalyst. Amines are good catalysts giving less side-reactions than acids. With APTMS, the optimal reaction temperature is about 100° and the evolved methanol should be continuously removed. Under these conditions, polysiloxane networks have been prepared from α,ω-hydroxypolysiloxanes of various molecular weights and APTMS. Resins of good mechanical properties have been obtained with a content of trifunctional units much lower than in the case of cohydrolysis, indicating a better distribution of the branching points (probably less clustering of the trifunctional units and less cyclization).