Notable Effect of Fluoro Substituents in the Imino Group in Ring-Opening Polymerization of ε-Caprolactone by Al Complexes Containing Phenoxyimine Ligands

A series of Al complexes containing phenoxyimine ligands of the types R¹(R²)Al[O-2-^tBu-6-{(C₆F₅)NCH}C₆H₃] [R¹, R² = Me, Me (1a); Et, Et (1b); Me, Cl (1c)] and Me₂Al[O-2-^tBu-6-(ArNCH)C₆H₃] [Ar = 2,6-F₂C₆H₃ (2a), 2,4-F₂C₆H₃ (3a), 3,4-F₂C₆H₃ (4a)] have been prepared and identified on the basis of NMR spectra and elemental analyses. Their structures were determined by X-ray crystallography, and these complexes fold a distorted tetrahedral geometry around Al. The ring-opening polymerizations (ROPs) of ε-caprolactone (CL) using 1a−c in the presence of PhCH₂OH proceeded efficiently in a living manner, and the propagation rates were somewhat influenced by the anionic donor ligand employed (Me, Et, or Cl). The catalytic activity by 1a−6a [Ar = C₆H₅ (5a), 2,6-Me₂C₆H₃ (6a)]−PhCH₂OH catalyst systems was strongly affected by the aromatic substituent (Ar) in the imino group, and placement of fluorine substituents especially in the ortho-position strongly affected the catalytic activity. The ROPs by 2a−6a were accompanied by a certain degree of side reactions (transesterification), whereas the living polymerization systems can be accomplished using the C₆F₅ analogues (1a−c). Therefore, the C₆F₅ substituent in the imino group plays an essential key role in the ROP of CL in terms of both the catalytic activity and maintaining a living manner.