In commerce, silicone elastomers are typically cured by platinum-catalyzed hydrosilylation or radical induced crosslinking (high temperature vulcanization (HTV)), both of which lead to crosslinks comprised of 2 or 3 carbon fragments. Alternatively, room temperature vulcanization (RTV) utilizes catalyzed nucleophilic substitution to give elastomers with Si-O-Si crosslinks. At end of life, the best current recycling protocols use aggressive acid/base conditions to regenerate cyclic monomers and mixtures of crosslinkers/fillers, etc. Over longer time periods all three types of elastomers undergo acid-catalyzed depolymerization initiated by HBr derived from bromine. However, we report that oxidative de-crosslinking with bromine of the Pt-cured elastomers occurs in minutes to hours to regenerate long chain silicone polymers, terminated with SiOH groups. It is therefore possible to devulcanize Pt-cured elastomers in the presence of RTV elastomers and reuse the high molar mass materials directly in new RTV elastomers, avoiding complete depolymerization, and thereby increasing the circularity of silicones. Oxidative bromination of hydrosilylation cured silicone elastomers occurs much more rapidly than acid-catalyzed depolymerization. The beta-effect facilitates SiC cleavage permitting decrosslinking to oils that can be reprocessed into RTV elastomers.
