Abstract
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 mechanism of the catalysis by Et 3N was demonstrated 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 functioning 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.
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