Synthesis and characterization of half-salen complexes and their application in the polymerization of lactide and ε-caprolactone

A series of half-salen ligands and their aluminum Al(III) complexes that contained different substituents were designed and synthesized. All the ligands and their complexes were characterized by 1H, 13C NMR and elemental analysis. The complexes can be used as catalysts to produce polylactide and poly-ε-caprolactone. All polymerizations were living with narrow molar mass distributions. The Mn(obsd) values of the isolated polymers were in good agreement with Mn(calcd). The polymerization rate of the electrophilic substituted complex was higher than the non-electrophilic substituted analogues. The more bulky substituents with more steric hindrance of the complexes showed relatively low polymerizing activity. The aluminum complexes showed a moderate stereo-selectivity to the ring opening polymerization of rac-lactide to give isotactic enriched polylactide.