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

A series of β-quinolyl-enamino aluminium complexes [AlLMe2 ] 1-7 {L = [(2-C9 H6 N)-CH[double bond, length as m-dash]C(4-MeC6 H4 )-N(Ar)-], Ar = C6 H5 (1), 4-FC6 H4 (2), 3,4,5-F3 C6 H2 (3), C6 F5 (4), 5-t BuC6 H4 (5), 2,6-Me2 C6 H3 (6), and 2,6-i Pr2 C6 H3 (7)} were synthesized by the reaction of AlMe3 with the corresponding β-quinolyl-enamine ligands. All the complexes were characterized by 1 H and 13 C 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 Mn versus conversion plots and monomer/catalyst ratios.