Moshe Kol

Zinc is a nontoxic and readily available metal whose complexes have been extensively studied as catalysts for the ring‐opening polymerization of lactide to poly(lactic acid). When employed in polymerization of racemic lactide, zinc‐based catalysts are typically found to be nonstereoselective and the desired isoselective catalysts are particularly scarce. Herein, a new family of sequential pyridine‐imine/amine‐phenolate tetradentate‐monoanionic {ONNN}‐type ligands which leads to well‐defined {ONNN}ZnEt complexes is studied. The relative location of the internal imine and amine donors affects their wrapping tendencies around the zinc. Upon activation with benzyl alcohol, active catalysts for the ring‐opening polymerization of racemic lactide are formed. The activities and stereoselectivities of the catalysts are strongly affected by the ligand substitution pattern and in particular by the relative location of the imine and amine donors. Thus, pyridine‐amine‐imine‐phenolate zinc complexes are isoselective and polymerize racemic lactide to give isotactic multiblock poly(lactic acid), whereas the isomeric pyridine‐imine‐amine‐phenolate zinc complex is essentially nonstereoselective. A rare combination of very high activity and very high isoselectivity reaching a P_m = 0.90 at room temperature is achieved with selected members of this catalyst family.

Aspects of the proposed pathway combining chain-end and enantiomorphic site control for the stereospecific polymerization of lactide (LA) were investigated through studies of aluminum complexes supported by enantiopure and racemic bipyrrolidine-based salan ligands, Lig¹AlOBn and Lig²AlOBn. Spectroscopic analysis of stoichiometric initiation reactions and the definition of the stereochemistry of the selective formation of the "match" single-insertion products by X-ray crystallography led to key conclusions about the observed stereocontrol. Notably, it was determined to rely heavily on the preference for the trio of stereocenters around the metal to have a "match" formation (RR-ligand + S-polymer), which works synergistically with the enantiomorphic site preference of the catalyst to ring-open next to a stereocenter of a monomer of the same chirality as that of the ligand, resulting in highly heterotactic or syndiotactic PLA from rac- or meso-LA, respectively.

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.

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.