Ring-opening polymerization of ε-caprolactone and cyclohexene oxide initiated by aluminum β-ketoamino complexes: steric and electronic effect of 3-position substituents of the ligands

Abstract

A series of aluminum complexes supported by β-ketoamino, ligand-bearing, 3-position substituentsLAlEt2 (L=CH3C(O)C(Cl)=C(CH3)NAr (L 1), L=CH3C(O)C(H)=C(CH3)NAr (L 2), L=CH3C(O)C(Ph)=C(CH3)NAr (L 3), and L=CH3C(O)C(Me)=C(CH3)NAr (L 4), Ar=2,6-Pr2C6H3) were synthesized in situ and employed in the ringopening polymerization (ROP) of e-caprolactone (e-CL) and cyclohexene oxide (CHO). The 3-position substituents on the β-ketoamino ligand backbone of the aluminum complexes influenced the catalyst activity remarkably for both ROP of e-CL and CHO. Aluminum β-ketoamino complexes displayed different catalytic behavior in ROP of e-CL and CHO. The order of the catalytic activity ofLAlEt2 wasL 1AlEt2>L 2AlEt2>L 3AlEt2>L 4AlEt2 for ROP of e-CL, being opposite to the electron-donating ability of the 3-position substituents on the β-ketoamino ligand, while the order of the catalytic activity for ROP of CHO wasL 1AlEt2>L 3AlEt2>L 4AlEt2>L 2AlEt2. The effects of reaction temperature and time on the ROP were also investigated for both e-CL and CHO.