Macromolecular engineering of polylactones and polylactides. 4. Mechanism and kinetics of lactide homopolymerization by aluminum isopropoxide

Aluminum isopropoxide is an effective initiator for the polymerization of lactides in toluene at 70-degrees-C. The ring-opening polymerization proceeds through a "coordination-insertion" mechanism and selective rupture of the acyl oxygen bond of the monomer. A kinetic study has shown that the polymerization is first order in both the monomer and initiator. The polymerization is typically "living" until a molecular weight of ca. 90 000 is reached. At higher temperatures (100-degrees-C), inter- and intramolecular transesterification reactions are responsible for a limitation in the molecular weight of the growing chains. In toluene, at 70-degrees-C, each alkoxide of the aluminum isopropoxide participates in the polymerization. Any molecular weight below 90 000 can accordingly be predicted on the basis of the monomer/Al molar ratio, and the polydispersity is rather narrow (1.1-1.4).