Covalent Borane-Thiourea Organocatalyst for Stereoselective Ring-Opening Polymerization

While the chemistry of ring-opening polymerization has advanced significantly to alleviate the escalating plastic waste crisis, highly selective and efficient catalysts to synthesize chemically recyclable, high-performance polythioesters remain needed. Typical ring-opening catalysts, however, face an intrinsic stereochemical challenge: unavoidable monomer racemization due to elevated α-hydrogen acidity, which severely hampers the synthesis of highly tactic crystalline materials and diminishes the performance of the resulting plastics. Here, we report the design and synthesis of a covalent borane-thiourea organocatalyst for the stereoselective ring-opening polymerization of enantiopure dithiolactones. Through incorporation of a strongly Lewis acidic 9-borafluorene moiety, this metal-free system effectively reduces the basicity of propagating thiolate chain ends while enabling thiourea-mediated monomer activation, thereby ensuring rapid polymerization with minimal racemization. Consequently, this approach affords polythioesters with near-perfect isotacticity (P_m = 0.97) and high molecular weights (M_n up to 58.1 kDa). Notably, the resulting stereoregular polymers are tough, semicrystalline materials with properties comparable to commercial polyolefins like low-density polyethylene, while exhibiting complete chemical recyclability to realize a sustainable cradle-to-cradle closed loop. Overall, this covalent borane-thiourea organocatalyst solves the intrinsic stereochemical challenges in typical ring-opening polymerizations, providing a powerful strategy to access chemically recyclable and tough thermoplastics from dithiolactones as promising next-generation sustainable polymers.