Exploration of solvent, volume, and catalyst effects on fluoride-catalyzed end-of-life depolymerization of silicones to cyclic monomers

Abstract

Recycling end-of-life silicones is of interest due to the high energy cost associated with producing new materials from virgin raw materials. This work explores the chemical recycling of various commercially available silicone rubbers and fluids using catalytic amounts of tetrabutylammonium fluoride (TBAF) in different solvents and solvent volumes. This method allows for the room-temperature depolymerization of each silicone tested to cyclic siloxane oligomers, with the major product being D₄ siloxane. Suitable sustainable solvents such as ethyl acetate and cyclopentylmethyl ether were identified for the TBAF-catalyzed depolymerization, and the relationship between the solvent volume used in the reaction and the product distribution was determined and optimized, showing a 1 : 10 polymer mass to solvent volume ratio as ideal. The depolymerization was carried out on a large scale using a consumer product and silicone elastomer, and it was found that the cyclic siloxanes could be isolated via fractional distillation. The fluoride-induced rearrangement of D₆ siloxane was used to study the reaction kinetics by ¹H NMR array experiments. Alternative fluoride catalyst systems were explored which utilized a combination of a phase transfer catalyst, such as polyethylene glycol, and an alkali fluoride salt. These systems were found to convert silicone rubber to cyclic oligomers with product distributions similar to those observed for the TBAF catalyzed process.