Thermoplastic, redox recyclable silicone–lipoamide elastomers

Lipoamide formation, from lipoic acid and aminopropylsilicones, is accompanied by ring-opening polymerization to generate thermoplastic, silicone lipoamide copolymers. The materials are readily degraded by reduction of the disulfide linkages. Silicone elastomers, renowned for their exceptional properties, have historically been thermosets. Their high stability during use compromises the ability to repurpose them at end of life. In this study, we present an approach to repurposable and readily degradable silicone elastomers based on the incorporation of α-lipoic acid, a naturally occurring dithiolane. Unlike traditional methods for ROP of LPA by use of heat, the amine used to form silicone amides also catalyzes ROP to give elastomers whose physical properties, as expected, are closely tied to crosslink density; both telechelic and pendent aminopropylsilicones serve as starting materials. The materials produced are thermoplastic elastomers whose properties remain essentially unchanged after several heating cycles. Reduction of the disulfide linkages in the elastomer using dithiothreitol (DTT) converts the elastomer to oils that contain DTT residues. These oils crosslink under air, but not nitrogen, to give new elastomers with slightly higher mechanical properties that are ascribed to the presence of vicinal diols. This process leads to elastomers that are more sustainable than traditional silicone thermosets because of the many attributes that facilitate reuse, repurposing and recycling.