Living omega -aluminum alkoxide poly-epsilon -caprolactone and poly-D,L-lactide chains were synthesized by the ring-opening polymerization of epsilon -caprolactone (E-CL) and D,L-lactide (D,L-LA), respectively, and were used as macroinitiators for glycolide (GA) polymerization in tetrahydrofuran at 40 degreesC. The P(CL-b-GA) and P(LA-b-GA) diblock copolymers that formed were fractionated by the use of a selective solvent for each block and were characterized by H-1 NMR spectroscopy and differential scanning calorimetry analysis. The livingness of the operative coordination-insertion mechanism is responsible for the control of the copolyester composition, the length of the blocks, and, ultimately, the thermal behavior. Because of the inherent insolubility of the polyglycolide blocks, microphase separation occurs during the course of the sequential polymerization, resulting in a stable, colloidal, nonaqueous copolymer dispersion, as confirmed by photon correlation spectroscopy. (C) 2000 John Wiley & Sons, Inc.