Condensation of model linear siloxane oligomers possessing silanol and silyl chloride end groups. The mechanism of silanol silylation by a chlorosilane in the presence of neutral nucleophiles

Article abstract of DOI:10.1016/0022-328X(89)87202-X

The condensation of pentamethyldisiloxane-1-ol (1) with 1-chloro-1-isopropyltetramethyldisiloxane (2) in methylene chloride solution has been studied as model of the polyheterocondensation of functional oligosiloxanes.The process catalysed by triethylamine, hexamethylphosphoroamide (HMPA) or some nitrogen heterocycles showed a high selectivity towards heterocondensation.Triethylamine promotes the reaction by acting as the base bonding to the incipient proton on the silanol group entering into the condensation.This mechnism of the catalysis by Et3N was demostrated by kinetic studies involving use of gas-liquid chromatography in conjunction with study of the hydrogen bonding by IR spectroscopy.In contrast, the kinetic data and the reactivity pattern indicate that N-heterocycles and HMPA catalyse the reaction by fuctioning as nucleophiles to form a transient complex with the chloride substrate.A mixture containing a highly nucleophilic N-heterocycle, triethylamine has no effect on the rate of the reaction, acting only as a HCl acceptor.