Polymerization of lactide using achiral bis(pyrrolidene) schiff base aluminum complexes
A series of aluminum ethyls and isopropoxides based on a bis(pyrrolidene) Schiff base ligand framework has been prepared and characterized. NMR studies of the dissolved complexes indicate that they adopt a symmetric structure with a monomeric, five-coordinated aluminum center core. The aluminum ethyls used as catalysts in the presence of 2-propanol as initiator and the aluminum isopropoxides were applied for lactide polymerization in toluene to test their activities and stereoselectivities. All polymerizations are living, as evidenced by the narrow polydispersities and the good fit between calculated and found number-average molecular weights of the isolated polymers. All of these aluminum complexes polymerized (S,S)-lactide to highly isotactic PLA without epimerization of the monomer, furnished isotactic-biased polymer from rac-lactide, and gave atactic polymer from meso-lactide. The study of kinetics indicated that the activity of the bis(pyrrolidene) Schiff base aluminum initiator systems toward lactide polymerization decreases in the following order: (S,S)-lactide > rac-lactide > meso-lactide. The methyl substituents on the diimine bridge or on the pyrrole rings both exert significant influence on the course of the polymerizations, affecting both the stereoselectivity and the polymerization rate. Kinetics using [L2AlEt]/2-propanol (2a/2-propanol) and [L2AlOiPr] (2b) indicated that the polymerizations are both first-order with respect to rac-lactide monomer and catalyst. The higher polymerization rate constant (kp) values for [L2AlOiPr] (2b) compared with those of [L2AlEt]/2-propanol (2a/2-propanol) revealed that in this case the overall polymerization rate was influenced by the relatively slow in situ alcoholysis reaction of aluminum ethyls. Polymerization experiments with [L2AlOiPr] (2b) revealed that with this complex much faster (kp = 13.0 L·mol−1·min−1) lactide polymerizations can be achieved compared with other aluminum complexes.