Covalent Borane‐Thiourea Organocatalyst for Stereoselective Ring‐Opening Polymerization

ABSTRACT 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.