A single-site ethyl aluminum complex, [2,2-diethyl-1,3-propylenebis(3,5-di-tert-butyl-salicylideneimi- nato)] ethyl aluminum (2), with a geminal diethyl substitutent on the diamino bridge was synthesized by the reaction of AlEt3 with 1 equiv of N,N'-(2,2-diethyl-1,3-propylene)bis(3,5-di-tert-butylsalicylideneimine). X-ray diffraction showed that complex 2 contained a five-coordinate aluminum atom with a distorted trigonal bipyramidal geometry in the solid state. H-1-NMR and C-13-NMR spectra indicated that the two conformational enantiomers of 2 tautomerized quickly on the NMR timescale in solution. In the presence of isopropyl alcohol, the ring-opening polymerization (ROP) of rac-lactide with complex 2 produced a crystalline stereoblock polylactide (PLA). The stereoblocks contained an average of 12 units (L = 12) of enantiomerically pure lactic acid. There was a linear relationship between the monomer conversion and number-average molecular weights of the polymer. An induction period was observed for the polymerization. The induction period increased with decreasing concentration of catalyst 2 and isopropyl alcohol. In the presence of poly(ethylene glycol) (PEG), a PLA/PEG/PLA stereocomplex was prepared directly by the ROP of rac-lactide with complex 2, which was confirmed by NMR, gel permeation chromatography, wide-angle X-ray diffraction, and differential scanning calorimetry. (c) 2005 Wiley Periodicals, Inc.