Group 4 Salalen Complexes and Their Application for the Ring-Opening Polymerization of rac-Lactide

In this paper, we report the preparation and characterization of a series of unsymmetrical group 4 metal complexes based on ONNO salalen-type ligands. In these examples, the ligand is unsymmetrical with an amine and an imine nitrogen center. All complexes {except Hf(3)((OPr)-Pr-i)(2)} adopt a beta-cis configuration in the solid state, which is also in agreement with solution-state NMR spectroscopic measurements. The complexes have been tested for the HOP of rac-lactide (both in solution and under melt conditions). Narrow polydispersities have been seen (1.07-1.82). The Ti(IV) complexes produce atactic PLA, whereas the Zr(IV) complexes form PLA with a heterotactic bias. Hf(IV) complexes have been shown to produce isotactic PLA with a P-m of 0.75. The kinetics for all initiators have been investigated and are discussed.