Evolution of aluminum aminophenolate complexes in the ring-opening polymerization of ε-caprolactone: electronic and amino-chelating effects

A series of aluminum complexes bearing phenolate (O-Al and O2-Al), biphenolate (OO-Al type), aminophenolate (ON-Al), aminobiphenolate (ONO-Al), bis(phenolato)bis(amine) (NNOO-Al), and Salan (ONNO-Al) type ligands were synthesized. ε-Caprolactone (CL) polymerization using these aluminum complexes as catalysts was investigated. The overall polymerization rates of Al catalysts with different ligands were found to be in the following order (<i>k</i>_obs values): ONBr-Al (0.124 min^-1) ≥ OBr2-Al (0.121 min^-1) > ONNOBr-Al (0.054 min^-1) > NNOBr-Al (0.044 min^-1) ≥ ONOBr-Al (0.043 min^-1) > OBr-Al (0.033 min^-1) > NNOOBr-Al (0.015 min^-1) ≥ BuONNOBu-Al (0.001 min^-1) = OOBr-Al (0.001 min^-1). In addition, Al complexes with electron-donating substituents on ligands exhibited higher catalytic activity than those with bromo substituents. Density functional theory (DFT) calculations revealed that a dinuclear Al complex with two bridging methoxides had to rearrange to a phenolate bridged dinuclear Al complex with terminal methoxides. This is due to the low initiating ability of two bridging benzyl alkoxides. Combining the polymerization data and DFT results, it was concluded that the electron-donating substituents on the phenolate ring and chelating amino group enhance the electron density of the Al center. This may prevent the formation of a less active dinuclear Al complex with two bridging alkoxides (initiators) or facilitate its structural rearrangement. OOMe-Al has been established as a powerful candidate with a high polymerization rate and it exhibits well-controlled polymerization for synthesizing the <i>m</i>PEG-<i>b</i>-PCL copolymer.