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