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

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)[dbnd]C(Ph)[sbnd]N(Ar)[sbnd]], 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.