Highly Isoselective Polymerization of rac-Lactide by Zinc Complexes of Sequential Tetradentate {ONNN} Ligands

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}ZnEt 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.