Homoleptic titanium and zirconium complexes exhibiting unusual Oiminol–metal coordination: application in stereoselective ring-opening polymerization of lactide

The synthesis and characterization of novel homoleptic Ti and Zr complexes with tridentate ONO-type Schiff base ligands and their catalytic activities towards the ring-opening polymerization (ROP) of lactide are reported. The solid state structure confirms the trigonal prismatic geometry in group 4 complexes with Schiff base ligands around the metal centers. The highly conjugated metal complexes also exhibit unusual Oiminol–metal coordination. All the complexes were proved to be effective catalysts for the ROP of lactide (LA) under both solvent and melt conditions. The Zr complexes afforded isotactic enriched PLA (up to 74%) from rac-LA, whereas an atactic polymer was formed from Ti complexes. The ligand initiated the polymerization as revealed by NMR and MALDI-TOF analyses. The polymerization proceeded through the coordination–insertion mechanism. The polymerization followed first-order kinetics with respect to monomer concentration. The reactivity order towards the ROP of LA was also verified by DFT studies.