Stereogradient Poly(Lactic Acid) from meso‐Lactide/L‐Lactide Mixtures

The production of L‐lactide from L‐lactic acid involves a substantial formation of meso‐lactide as an impurity, and, upon polymerization with the industrial catalyst tin octanoate, results in poly(L‐lactic acid) of reduced crystallinity due to stereoerrors randomly distributed along the polymer chains. We describe a new approach wherein, instead of avoiding stereoerrors by removing the meso‐lactide prior to polymerization, the stereoerrors in the polymer are tolerated, by crowding them in a stereogradient copolymer. A zirconium complex of an amine tris(phenolate) ligand is found to exhibit very high syndioselectivity in the ring opening polymerization catalysis of meso‐lactide at room temperature, and gives rise to stereogradient copolymers in the polymerization of mixtures of meso‐lactide/L‐lactide in the melt at 180 °C. Relative to the stereo‐random copolymers obtained with tin octanoate, the stereogradient copolymers exhibit enhanced crystallinities manifested in lower solubilities and higher melting temperatures and enthalpies.