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 R-1(R-2)Al[O-2'-Bu-6-{(C6F5)N=CH}C6H3] [R-1, R-2 = Me, Me (1a); Et, Et (1b); Me, Cl (1c)] and Me2Al[O-2-'Bu-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 E-caprolactone (CL) using la-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.