Alpha-Lipoic Acid-Based Sustainable Silicone Elastomers

Abstract

The fate of crosslinked plastic at the end of life has urged today’s scientists to present sustainable alternatives. These plastics are typically composed of organic polymeric chains, classified as either thermoplastics or thermosets, with the majority falling under the classification of thermosets. Although thermosets have great mechanical properties, they cannot be recycled or repurposed following their use. This highlights the urgent need for sustainable alternatives to thermosets that offer similar mechanical properties yet can be recycled, repurposed, and degraded in a controlled manner. Silicones are polymers consisting of Si-O repeating units (polysiloxanes) as the backbone of the polymeric chain, widely used in various industries with unique properties compared to organic polymers. By combining polysiloxanes with other substances like small molecules or different polymers, functional silicones can be tailored for specific applications. These materials are created through established and specialized reactions unique to silicone polymers, with limited advancements in new crosslinking techniques. Current crosslinking methods for silicones have drawbacks, such as requiring harsh reaction conditions (like high temperature or toxic metal catalysts) or having limited compatibility with many organic functional groups. The development of innovative crosslinking technologies is essential for advancing silicones as materials, particularly in areas like functional coatings, polymers, and elastomers. Crosslinking in silicones occurs through covalent and non-covalent interactions, allowing the retention of many desirable silicone properties while also adding the ability to be recycled, reprocessed, and/or degraded. In this study, the crosslinking occurs through a naturally occurring anti-oxidant, -lipoic acid (LPA), which presents a multi-functional compound with a 5-membered cyclic disulfide ring with a carboxy group (COOH) bearing at the sidechain. By targeting the COOH with aminopropylsilicone (APS) a series of silicone elastomers were prepared through amide formation. These elastomers displayed thermoplasticity, enabling them to be reprocessed through heating, and allowed for controlled degradation at the end of life using a disulfide-reducing agent. This allows one to dilute the silicones with LPA while maintaining silicone-like properties in the materials, thereby enabling repurposing and redox recycling. Knowing LPA can work in solution, the dynamic disulfide linkages also opened the gateway to perform disulfide exchange chemistry on amine-rich surfaces. By anchoring LPA on amine-rich surfaces, ionically and covalently, one can extend the coating on the surface through disulfide/thiol-exchange chemistry. Displaying distinct properties for each kind of bonding, allows the recycling and reusing of the amine-rich surfaces incorporate sustainability in already developed methods of producing silicone elastomers.