Zinc Complexes Bearing Dinucleating Bis(imino- pyridine)binaphthol Ligands: Highly Active and Robust Catalysts for the Lactide Polymerization

We report the synthesis of new hexadentate ligands bearing imine and pyridine nitrogen neutral donors with a binaphthol backbone. Such ligands have been tailored to have two coordinative pockets thus promoting the formation of bimetallic complexes. By direct reaction of the enantiomerically pure and the racemic ligand with two equivalents of zinc amide precursor, the bimetallic complexes S-1 and rac-1 were respectively obtained. Both complexes were highly active in the polymerization of lactides under several reaction conditions, preserving exceptional activities even when working in technical grade melted lactide, thus showing the robustness of the catalysts. Interestingly, complex S-1 allows the production of either linear or purely macrocyclic PLAs by back-biting reactions, modifying the reaction conditions. Mechanistic studies have been carried out in NMR-scale solution to get information on the active species of the polymerization process.

Automated summary

We synthesized new hexadentate ligands with imine and pyridine nitrogen neutral donors, which have two coordinative pockets to promote the formation of bimetallic complexes. The bimetallic complexes S-1 and rac-1 were obtained by reacting the enantiomerically pure and racemic ligands with two equivalents of zinc amide precursor. Both complexes showed high activity in the polymerization of lactides under various reaction conditions and remained active even in technical grade melted lactide, demonstrating the robustness of the catalysts. Complex S-1 allows the production of linear or purely macrocyclic PLAs by back-biting reactions, depending on the reaction conditions. Mechanistic studies in NMR-scale solution were conducted to understand the active species in the polymerization process.