Sulfur-Substitution as a Design Principle for Closed-Loop Plastics: Dithiolactones, Monothiodilactones, and Beyond

Plastics capable of true closed-loop recycling require molecular design strategies that reconcile polymerizability, recyclability, and high material performance within a single system. Yet many chemically recyclable polymers remain limited by narrow monomer scope, inefficient depolymerization, or the persistent trade-off between recyclability and useful material properties. While most established systems have arisen from oxygen-based cyclic monomers, sulfur substitution offers a conceptually distinct route by reshaping ring-closing tendencies, polymerization behavior, and end-of-life depolymerization pathways. In this Account, we summarize how sulfur substitution evolved in our work from an initial polymerization insight into a broader design principle for closed-loop plastics. This story begins with S-carboxyanhydrides (SCAs), whose ultrafast and selective ring-opening polymerization first demonstrated the synthetic potential of sulfur-containing cyclic monomers for well-defined polythioesters. During these studies, a thermodynamic comparison between thiolactic acid-derived SCAs and sulfur-substituted lactide analogues unexpectedly revealed that thiolactide exhibits near-equilibrium polymerization thermodynamics. This finding identified dithiolactones as promising closed-loop monomer candidates and led to a systematic exploration of sulfur-substituted cyclic esters. We then show how this design logic was established with dithiolactones and further developed through stereocontrolled polymerization. Dithiolactones demonstrated that O-to-S substitution can rebalance conflicting cyclizability, polymerizability, and recyclability. Building on this foundation, ligand-tailored salen-Al catalysis, covalent borane-thiourea organocatalysis, and quaternary ammonium/thiophenolate ion-pair catalysis consistent with nonclassical C-H···X interactions expanded dithiolactones into a stereocontrolled platform. These complementary strategies enabled high-molecular-weight, stereoregular, and chemically recyclable polythioesters from both enantiopure and racemic dithiolactones. Monothiodilactones, in turn, refined this design logic by showing how partial sulfur substitution can improve regio- and chemoselectivity while suppressing deleterious transthioesterification. Beyond these monomer platforms, interfacial chain-growth polymerization ultimately translated sulfur substitution into high-molecular-weight, polypropylene-like circular polythioglycolide (PTGA), thereby demonstrating the material potential of this design principle. Overall, this Account presents sulfur substitution not only as a structural modification but also as a design principle connecting monomer thermodynamics, stereochemical control, materials performance, and closed-loop recovery. More broadly, this Account highlights sulfur substitution as a versatile molecular design strategy for integrating polymerization control, material performance, and closed-loop recyclability in next-generation circular plastics.