Stereoselective Ring-Opening Polymerization of rac-Lactide with a Single-Site, Racemic Aluminum Alkoxide Catalyst: Synthesis of Stereoblock Poly(lactic acid)

The polymerization of racemic lactide with a racemic aluminum alkoxide catalyst is reported. Microstructural analysis of the polymer produced with H NMR spectroscopy revealed that an isotactic stereoblock poly(lactic acid) formed, where each enantiomerically pure block contained an average of 11 lactide monomer units. The melting point of this polymer, 179 °C, was higher than that of the enantiomerically pure polymer, consistent with the cocrystallization of the enantiomeric blocks of the polymer. The mechanism of the polymer formation is currently unknown, although a polymer exchange pathway, where living chain ends switch between metal centers to produce diastereomeric active species, is proposed. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 4686–4692, 2000