A comparative study of titanium complexes bearing 2-(arylideneamino)phenolates and 2-((arylimino)methyl)phenolates as catalysts for ring-opening polymerization of ε-caprolactone and l-lactide

Abstract

Titanium complexes bearing 2-(arylideneamino)phenolates and 2-((arylimino)methyl)phenolates were synthesized, and their catalytic activities in the polymerization of ε-caprolactone and L-lactide were studied. Among five-membered ring Ti complexes bearing 2-(arylideneamino)phenolates, F^Cl-Ti exhibited the highest level of catalytic activity ([CL] : [F^Cl-Ti] = 100 : 1, where [CL] = 2 M, and conv. = 86% at 60 °C after 9 h). For six-membered ring Ti complexes bearing 2-((arylimino)methyl)phenolates, S^Cl-Ti exhibited the highest level of catalytic activity ([CL] : [S^Cl-Ti] = 100 : 1, where [CL] = 2 M, and conv. = 88% at 60 °C after 118 h). The five-membered ring Ti complexes bearing 2-(arylideneamino)phenolates exhibited a higher level of catalytic activity (6.1–12.8 fold for the polymerization of ε-caprolactone and 6.2–23.0 fold for the polymerization of L-lactide) than the six-membered ring Ti complexes bearing 2-((arylimino)methyl)phenolates. The density functional theory (DFT) results revealed that the free energy of the first transition state F^H-Ti-TS1 is 36.49 kcal mol⁻¹ which is lower than that of S^H-Ti-TS1 (46.58 kcal mol⁻¹), which was ascribed to the fact that the Ti–N^im bond (2.742 Å) of F^H-Ti-TS1 is longer than that of S^H-Ti-TS1 (2.229 Å) and reduces the repulsion between ligands.