Zirconium vs Aluminum Salalen Initiators for the Production of Biopolymers

Herein we report the synthesis and full characterization (NMR and solid-state structures) for a series of Zr­(IV), Hf­(IV), and Al­(III) salalen complexes, together with salen bimetallic counterparts. With the salalen ligand, 1H2, monometallic complexes were observed in solution and solid state. Complex Zr­(1)­(OiPr)2 was able to produce isotactic polylactide (PLA) from rac-lactide (Pm up to 0.85) in solution at 50 °C; in the melt (130 °C) this reduced to ca. 0.75. Al­(1)­Me was significantly less active and produced PLA with only a very modest isotactic enchainment (Pm ≈ 0.6). Zr­(1)­(OiPr)2 was also able to produce copolymers with lactide and ε-caprolactone, producing copolymers of a “blocky” nature.