Electronic influence of ligand substituents on the rate of polymerization of (epsilon)-caprolactone by single-site aluminium alkoxide catalysts

A series of novel five-coordinate aluminium mono alkoxide complexes supported by R1,R2BPBA (bis-3-R1-5-R2-phenoxymethylbisamine) ligands were synthesized to probe the effect of electronic variation in the supporting ligand on the rate of (epsilon)caprolactone polymerization. Substitution on the aromatic position para to the phenoxide donor oxygen by tert-butyl, methoxy and bromo substituents furnished aluminium complexes that catalyzed the polymerization of (epsilon)-caprolactone at different rates. We propose that a subtle interplay between complex Lewis acidity and alkoxide nucleophilicity determines the overall rate of polymerization in these systems.