Optimization of six-membered ring aluminum complexes in ε-caprolactone polymerization

A series of 6-membered ring aluminum (Al) complexes bearing Schiff base, benzotriazole phenolate, aminophenolate, carboxamide phenolate, and carboxylate phenolate were synthesized, and their applications in ε-caprolactone (CL) polymerizations were studied. The decreasing catalytic rates for CL polymerization were ranked as follows: Al complexes bearing aminophenolate > benzotriazole phenolate > carboxylate phenolate > Schiff base > carboxamide phenolate. The Al complex bearing aminophenolate (L ^Bu-prop ) revealed an appreciably higher (10–100 fold) polymerization rate and a shorter induction period than the other 6-membered ring Al complexes in the CL polymerization. Further investigation of the steric effect in the phenolate ring of aminophenolate ligands demonstrated that Al complexes with the tert-butyl groups on 2,4 positions of the phenolate ring had the highest catalytic activity, followed by that of the cumyl groups, and it was also discovered that the hydrogen substituents were responsible for the largest decrease in the catalytic activity of Al complexes. The bulky substituents on the amino group increased the catalytic activity of associated Al complexes as well as the aminophenolate ligands with the third chelating atoms on the amino group. However, the aminophenolate ligands with the fourth chelating atoms on the amino group slightly decreased the catalytic activity.