Ring-opening polymerization of rac-lactide using anilinotropone-based aluminum complexes-sidearm effect on the catalysis

• A series of anilinotropone-based aluminum complexes were synthesized.
• They are highly active in the ring-opening polymerization of rac-lactide.
• Small aluminum chelate ring may contribute to the high activity.
• Side-arm strongly affects the catalytic properties of the catalysts.

A series of anilinotropone-based aluminum complexes of general formula (Ar-NC7H5O)AlMe2 (1, Ar = Ph; 2, Ar = 2,6-iPrC6H3; 3, Ar = (2-O-Ph)C6H4; 4, Ar = (2-O-2,6-iPr2-Ph)C6H4; 5, Ar = (2-S-Ph)C6H4) were synthesized at high yields. These complexes were characterized by 1H, 13C NMR spectroscopy, elemental analysis and mass spectroscopy. X-ray analysis of complexes 1, 3 and 5 showed that they exist as dimeric species in solid state and the Al center adopts a distorted trigonal-bipyramidal geometry. Complexes 1–5 are highly active in the ring-opening polymerization (ROP) of rac-lactide (rac-LA) in the presence of benzyl alcohol as an initiator. Sidearm on the ligand has a dramatic effect on the catalysis, and complexes 3–5 bearing heteroatom containing sidearm are much more active than complexes 1 and 2. Especially, near complete consumption of the monomer was observed for complex 3 at rac-LA/Al ratio of 100 and 80 °C, representing one of the most active aluminum catalysts for ROP of rac-LA. The smaller aluminum chelate ring size and the resulting bigger ring tension may contribute to the high activity. In addition to the catalytic activity, the isotacticity of the resulting polylactide is also affected by the sidearm.

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