Rapid and Stereoselective Ring-Opening Polymerization of rac-Lactide Enabled by a Chiral Cyclopropenimine-Thiourea...

Polylactide (PLA) is an important bioplastic derived from renewable resources, valued for its biocompatibility and biodegradability. The tacticity and microstructure of PLA significantly influence its thermomechanical properties, yet achieving both high reactivity and precise stereocontrol in the organocatalytic ring-opening polymerization of rac-lactide remains a major challenge. Herein, we show that chiral cyclopropenimine-thiourea organocatalysts enable a rapid and highly stereoselective polymerization of rac-lactide to produce isotactic stereoblock PLAs. This strategy supports fast propagation and living behavior, yielding PLAs with controlled molecular weights and narrow dispersities. Notably, our bifunctional organocatalyst exhibits a remarkably high turnover frequency of up to 1,710 h^-1 and delivers stereoregular PLA with a high stereoregularity parameter of 0.90 at 25°C, which increases to 0.99 at -78°C. The resulting PLAs undergo stereocomplexation, forming a long-range ordered lamellar structure with reduced interlamellar spacing, as confirmed by x-ray scattering analysis.