BIFUNCTIONAL CATALYSIS IN THE CONDENSATION OF SILANOLS AND ALKOXYSILANES

The condensation reaction of a model silanol, nonamethyltetrasiloxane-1-ol, with methyltrimethoxysilane to form 1,1-dimethoxydecamethylpentasiloxane was studied. Various catalysts were used to study the rate of the condensation. A reactivity order of CCl3COOH > Et2NOH > CH3COOH > MeEtC = NOH >> Et3N was observed for the catalysis. Formation of a transition state from slow protonation of the alkoxysilane by catalyst followed by its condensation with the silanol is deemed insufficient to explain the catalysis. Reaction of catalyst with the alkoxysilane to form an intermediate followed by silanol condensation with this intermediate is also considered and cannot be ruled out. Rate considerations, however, favors a cyclic transition state where bifunctional catalysis is invoked. With the slow triethylamine catalysis, still another mechanism is needed. A transition state where the amine is acting as a base in assisting the dissociation of the proton on the silanol is suggested.