Rapid and Enantioselective Synthesis of Stereodiblock Polylactides Enabled by Chiral Cyclopropenimine-Thiourea Catalysts
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Polylactide (PLA) is one of the most important commercial bioplastics. The microstructure of PLA is crucial for its thermomechanical properties. However, achieving catalytic asymmetric synthesis of stereodiblock PLAs via ring-opening polymerization (ROP) of rac-LA in sigle step is challenging. We present a rapid and stereoselective method for synthesizing stereodiblock PLAs through the introduction of chiral cyclopropenimine-thiourea organocatalysts. This enables highly stereocontrolled ROP of rac-LA with rapid propagation and living polymerization, resulting in PLAs with controlled molecular weight and narrow dispersity. The process exhibits both high reactivity and isotacticity (stereoregularity parameter of 0.99) at –78 °C. Kinetic studies reveal that enantiomorphic site control and chain-end control mechanisms are involved. The resulting stereodiblock PLAs undergo stereocomplexation, forming long-range lamellar structures with reduced inter-lamellar spacing, as confirmed by X-ray scattering analysis.