Bis(8-quinolinolato)aluminum ethyl complexes: Iso-Selective Initiators for rac-Lactide Polymerization

The synthesis and characterization of a series of bis(8-quinolinato)aluminum ethyl complexes, substituted at the 2-, 5-, and 7-positions on the 8-quinolinol ligand, are presented. These complexes are viable initiators for the ring-opening polymerization of rac-lactide in the presence of 1 equiv of isopropyl alcohol. The polymerization control is good, it shows a linear evolution of molecular weight as the polymerization progresses, the polylactide molecular weights are in close agreement with those determined on the basis of the reaction stoichiometry, and the polydispersity indices are narrow. The polymerization kinetics have been monitored, and the influence of the site of ligand substitution has been related to the rates. Some of the initiators show stereocontrol, producing PLA with a good probability of isotactic enchainment (P-i = 0.76).