Dinuclear aluminum complexes bearing methylene-bridged phenoxy-imine ligands and their application in the ring-opening polymerization of rac-lactide

Abstract

The synthesis and characterization of dinuclear aluminum complexes supported by methylene-bridged phenoxy-imine ligands (1–12), and their catalysis in the ring-opening polymerization (ROP) of rac-lactide are reported. Reactions of trimethylaluminum with the proligands L¹H₂–L¹²H₂ in a molar ratio of 2 : 1 in toluene at 90 °C afforded the corresponding dinuclear aluminum complexes. The rac-LA polymerizations initiated by complexes 1–12 in the presence of BnOH exhibited living characters, yielding PLAs with predetermined molecular weights and narrow molecular weight distributions. In the presence of excess BnOH, all complexes promoted the living immortal ROP of rac-LA. Kinetic studies revealed the rate law, with a first-order dependence on both monomer and aluminum concentrations. The introduction of a tert-butyl ortho substituent on the phenoxy unit of the ligand resulted in a decrease of the catalytic activity of the dinuclear aluminum complex. The type of imino substituent was found to have an influence on both its catalytic activity and polymer microstructure. Polylactides with various microstructures ranging from heterotactically-biased (P_r = 0.60) to isotactically-biased (P_m = 0.71) enchainments were produced. Compared with the synthesized mononuclear aluminum complexes (13 and 14), the corresponding dinuclear aluminum congeners (6 and 12, respectively) were found to slightly accelerate the catalytic activity towards ROP of rac-LA by factors of ca. 20% in both cases. Density functional theory (DFT) calculations for the Al–Al distance of the dinuclear aluminum complex 6 suggested that the long Al–Al distance (10.0–12.8 Å) could be responsible for the comparable catalytic activities between the mononuclear and dinuclear aluminum complexes. The results were discussed and compared with other binuclear aluminum systems.