Isoselective Ring-Opening Polymerization of <i>rac</i>-Lactide from Chiral Takemoto’s Organocatalysts: Elucidation of Stereocontrol

Abstract

Despite significant advances in organocatalysis, stereoselective polymerization reactions utilizing chiral organocatalysts have received very little attention, and much about the underlying mechanisms remains unknown. Here, we report that both commercially available (<i>R</i>,<i>R</i>)- and (<i>S</i>,<i>S</i>)-enantiomers of chiral thiourea-amine Takemoto's organocatalysts promote efficient control and high isoselectivity at room temperature of the ring-opening polymerization (ROP) of racemic lactide by kinetic resolution, yielding highly isotactic, semicrystalline and metal-free polylactide (PLA). Kinetic investigations and combined analyses of the resulting PLAs have allowed the stereocontrol mechanism, which eventually involves both enantiomorphic site control and chain-end control, to be determined. Moreover, epimerization of <i>rac</i>-LA to <i>meso</i>-LA is identified as being responsible for the introduction of some stereoerrors during the ROP process.