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

Abstract

A series of Al complexes containing phenoxyimine ligands of the types R 1 (R 2 )Al[O-2- t Bu-6-{(C 6 F 5 )N═CH}C 6 H 3 ] [R 1, R 2 = Me, Me ( 1a ); Et, Et ( 1b ); Me, Cl ( 1c )] and Me 2 Al[O-2- t Bu-6-(ArN═CH)C 6 H 3 ] [Ar = 2,6-F 2 C 6 H 3 ( 2a ), 2,4-F 2 C 6 H 3 ( 3a ), 3,4-F 2 C 6 H 3 ( 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 PhCH 2 OH 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 = C 6 H 5 ( 5a ), 2,6-Me 2 C 6 H 3 ( 6a )]−PhCH 2 OH 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 C 6 F 5 analogues ( 1a − c ). Therefore, the C 6 F 5 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.