Ring-opening polymerization of various cyclic esters by Al complex catalysts containing a series of phenoxy-imine ligands: Effect of the imino substituents for the catalytic activity

Abstract

Ring-opening polymerizations (ROPs) of various cyclic esters [ɛ-caprolactone (CL), δ-valerolactone (VL), rac-lactide (LC)] using a series of Al complexes containing phenoxy-imine ligands of type, Me 2Al[O-2- t Bu-6-(RN CH)C 6H 3], [R = t Bu ( 1a), cyclohexyl ( 1b), adamantyl ( 1c), C 6H 5 ( 1d), 2,6-Me 2C 6H 3 ( 1e), 2,6- i Pr 2C 6H 3 ( 1f), 2,4,6-Me 3C 6H 2 ( 1g), 2,4,6- t Bu 3C 6H 2 ( 1h), C 6F 5 ( 1i)], have been explored in the presence of n BuOH. Synthesis and identification of 1g and 1h including structural analysis of 1g by X-ray crystallography have also been explored. Both the catalytic activity and the catalyst efficiency in the ROPs were found to be highly affected by the imino substituents (R) employed. The 2,4,6- t Bu 3C 6H 2 analogue ( 1h) showed the notable catalytic activity in spite of its low catalyst efficiency in the ROP of CL, and the C 6F 5 analogue ( 1i) was most effective in terms of both the activity and the efficiency. The C 6F 5 analogue ( 1i) also showed the highest catalytic activity for the ROP of VL, and the polymerization proceeded in a living manner. The C 6F 5 analogue also showed relatively high catalytic activity in the ROP of LC, but the resultant polymer had no stereo-regularity. Although the Al complexes containing alkyl substituents ( 1a– c) exhibited low or negligible catalytic activities in the ROPs of CL and VL, the cyclohexyl analogue ( 1b) showed moderate catalytic activity for the ROP of LC. Attempts for the ROP of β-butyrolactone (β-BL) and γ-butyrolactone (γ-BL) using 1e,h, i- n BuOH catalyst systems afforded negligible amount of polymers under the similar conditions.