Living ring-opening polymerization of ε-caprolactone catalyzed by β-quinolyl-enamino aluminium complexes: ligand electronic effects

Abstract

A series of β-quinolyl-enamino aluminium complexes [AlLMe₂] 1–7 {L = [(2-C₉H₆N)-CHC(4-MeC₆H₄)-N(Ar)–], Ar = C₆H₅ (1), 4-FC₆H₄ (2), 3,4,5-F₃C₆H₂ (3), C₆F₅ (4), 5-^tBuC₆H₄ (5), 2,6-Me₂C₆H₃ (6), and 2,6-ⁱPr₂C₆H₃ (7)} were synthesized by the reaction of AlMe₃ with the corresponding β-quinolyl-enamine ligands. All the complexes were characterized by ¹H and ¹³C NMR spectroscopy and elemental analysis. The molecular structures of complexes 1–3 and 7 were confirmed by single-crystal X-ray diffraction, which revealed that all the Al atoms adopt a distorted tetrahedral geometry. These Al complexes were tested as the initiators for the ring-opening polymerization (ROP) of ε-caprolactone (ε-CL). The polymerization results showed that 1–5 in the presence of BnOH proceeded efficiently and their catalytic behaviors toward the ROP of ε-CL were significantly affected by the substituents of the aryl ring (Ar) on the end of the enamino framework, where the placement of electron-withdrawing substituents caused higher catalytic activity. However 6 and 7 displayed poor activity under the same conditions, suggesting that ortho-substituents on the Ar moieties hamper the coordination–insertion of the ε-CL monomer. The ROP of ε-CL by efficient binary catalytic systems proceeded in a living manner evidenced by the narrow PDIs and the linear nature of M_nversus conversion plots and monomer/catalyst ratios.