A greener and more scalable synthesis of biogenic polydisulfides from lipoic acid

Ring-opening polymerization (ROP) of 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 limits scalability and transformation into practical materials. Here, we disclose efficient, scalable syntheses for poly(ethyl lipoate) and ethyl lipoate; these syntheses incorporate Green Chemistry principles. We optimized the synthesis of ethyl lipoate on a 100-gram scale (>80% yield) with an improved Fischer esterification: E-factor = 2.27, an order of magnitude above conventional methods. We used diphenyl phosphate, a non-hazardous commercial organic acid, to yield ultra-high-molecular-weight poly(ethyl lipoate) on a 50-gram scale from cationic ring-opening polymerization (CROP). The polymerizations proceeded under ambient atmosphere in benign solvents that were used as received. We then developed a mild depolymerization strategy to regenerate monomer. Due to their extreme molar masses, these polymers possess unique mechanical and physical properties, which we briefly surveyed by assessing thin films. Life cycle analysis (LCA) conducted on both synthetic processes and polymer recycling show that this dynamic polydisulfide has competitive sustainability attributes. In fact, both processes have similar environmental impacts compared to those of several commodity polymers, despite the latter having a significant 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 lipoic acid.