Polysiloxane depolymerization with dimethyl carbonate using alkali metal halide catalysts

Polydimethylsiloxane terminated by a methyl group was almost completely depolymerized in the mixture of dimethyl carbonate (DMC) and methanol using alkali metal halide catalysts to form dimethoxydimethylsilane (DMDMS) and methoxytrimethylsilane (MTMS) monomers. The various alkali metal halides tested as catalysts gave the same monomer yield. The depolymerization rate depended on the kind of alcohol but not on dialkyl carbonate. The results indicate that the siloxane bond is cleaved by methanol attack to form the corresponding monomers together with water and that dimethyl carbonate reacts with water formed as a byproduct to give methanol, whose amount is the same as that of the methanol consumed for cleavage of siloxane bonds. Therefore, dimethyl carbonate, not methanol, acts as the depolymerization reagent in the apparent reaction. Depolymerization of poly(methylphenylsiloxane-ran-dimethylsiloxane) and poly(methylvinylsiloxane-ran-dimethylsiloxane) with dimethyl carbonate in methanol using potassium fluoride catalyst also proceeded to obtain dimethoxymethylphenylsilane and dimethoxymethylvinylsilane, respectively, together with dimethoxydimethylsilane and methoxytrimethylsilane from polydimethylsiloxane parts. Silicone rubber containing silica as an additive was also depolymerized to form dimethoxydimethylsilane together with a small amount of tetramethoxysilane formed from silica additive. Therefore, the depolymerization with DMC using alkali metal salt catalyst can be counted on for recycling silicone waste.