Disproportionation of oligodimethylsiloxanols in the presence of a protic acid in dioxane

Article abstract of DOI:10.1016/0022-328X(93)80039-E

The kinetics of the acid-catalyzed disproportionation of α,ω-dihydroxyoligodimethylsiloxanes HOnH, n = 2, 5, and their analogues having one hydroxyl function replaced by methyl MenH, n = 2, 5, were studied in dioxane in the presence of water.The formation of the primary disproportionation products was monitored by gas-liquid chromatography.The reaction in dioxane solution can compete with the condensation process only in the presence of water.It is of first order with respect to the substrate, catalyst, and water.The results are interpreted in terms of a stepwise mechanism involving rate determining cleavage of the terminal siloxane unit by water, followed by condensation of transiently-formed dimethylsilanediol with the substrate.The unzipping mechansim involving the dimethylsilanediol intermediate is suggested to operate in hydrolytic cleavage of polydimethylsiloxane exposed to water or water vapour.