A Greener and More Scalable Synthesis of Biogenic Polydisulfides from Lipoic Acid

Abstract

Ring-opening polymerization (ROP) of 1,2-dithiolanes form polydisulfides, an emergent class of dynamic covalent polymers. However, both monomer and polymer syntheses typically require anaerobic and moisture-free conditions, often employing hazardous reagents and solvents that limit scalability. Herein, efficient, scalable syntheses for poly(ethyl lipoate) and ethyl lipoate that incorporate Green Chemistry principles are disclosed. The synthesis of ethyl lipoate from lipoic acid on a 100-gram scale (>80% yield) is optimized lowering the E-factor (2.27) by an order of magnitude compared to conventional methods. Diphenyl phosphate, a nonhazardous commercial organic acid, is used to synthesize ultra-high-molecular-weight poly(ethyl lipoate) on a 50-gram scale from cationic ROP (CROP). The polymerizations proceed under ambient atmosphere in low-hazard and renewable solvents, and a mild depolymerization strategy to regenerate the monomer is developed. Due to their extreme molar mass, the materials possess unique mechanical and physical properties. Life cycle assessment (LCA) conducted on synthetic and recycling processes shows that the polydisulfide has competitive environmental impacts comparable to several commodity polymers, despite the latter having an efficiency advantage due to economies of scale. These discoveries establish an economical and scalable closed-loop polymer platform that can be broadly applied to various polydisulfides sourced from 1,2-dithiolanes such as lipoic acid.