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

A series of novel five-coordinate aluminium mono alkoxide complexes supported by (R1,R2)BPBA (bis-3-R-1-5-R-2-phenoxymethyl-bisamine) 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.