Living ring-opening homo- and copolymerisation of ε-caprolactone and L-lactide by cyclic β-ketiminato aluminium complexes.

A series of novel aluminium complexes containing cyclic β-ketiminato ligands of type Me2Al{O-[(ArN=CHC4H4(C6H4))]} (3a, Ar = 2,6-(i)Pr2C6H3; 3b, Ar = C6H5; 3c, Ar = C6F5) have been prepared in high yields. These complexes were identified by (1)H, (13)C NMR spectroscopy and elemental analysis. X-ray structural analyses for 3a-c revealed that these complexes have a distorted tetrahedral geometry around Al, and both bond distances and bond angles were considerably influenced by the ligand structure. These complexes were tested as catalyst precursors for ring-opening polymerisation of ε-caprolactone (ε-CL) and L-lactide (L-LA) in the presence of 2-propanol as an initiator. Complex 3a could polymerize ε-CL in a controlled manner with high efficiency. Based on the living characteristics, the preparation of well-defined block copolymers PCL-b-PLLA via sequential addition of monomers was performed by 3a. Note that complex 3c exhibited rather high catalytic activity for the ROP of L-LA with narrow molecular weight distribution. The monomer conversion reached completion only in 4 h when the L-LA/Al molar ratio was 100 at 80 °C. PLLA-b-PCL copolymers were thus easily produced by 3c.