Unexpected substituent’s effects on catalytic activity in the ring-opening polymerization of ε-CL and δ-VL catalyzed by β-pyridyl-enamino Al complexes

Abstract

This contribution illustrates the significant influence of the Al-catalyzed ε-caprolactone (ε-CL) and δ-valerolactone (δ-VL) ring-opening polymerizations. A series of β-pyridyl-enamino aluminum complexes [AlLMe2] 1–6 {L = [(2-C5H4N)-C(H)C(Ph)N(Ar)], Ar = C6H5 (1), 2,6-Me2C6H3 (2), 2,6-Et2C6H3 (3), 2,6-iPr2C6H3 (4), 4-FC6H4 (5), C6F5 (6)} were synthesized and characterized by NMR spectroscopy and single-crystal X-ray diffraction. The catalytic activities the polymerization of ε-caprolactone (ε-CL) and δ-valerolactone (δ-VL) correlated with the substituent’s character of the ortho and para groups on the Ar moieties of the β-pyridyl-enamino ligands. For the polymerizations of the both cyclic esters, complexes 1, 5 and 6 displayed excellent catalytic activities in the presence of the initiator benzyl alcohol, while complexes 2–4 exhibited much lower activities under the same conditions. Kinetic studies revealed that complex 5 polymerized ε-CL at a faster rate in comparison to 1 and 6, where highlighted a cumulative effect of both the electronic and steric effect on the ligands. By contrast, for the ROP of δ-VL, complex 1 was found to be more active than the catalysts 5 and 6. This unusual result was suggested the rate-determined step was the “insertion” step in ε-CL and δ-VL polymerization, the former’s insertion rate is faster than the latter’s.