Bulk polymerization of lactides initiated by aluminum isopropoxide. I. Mechanism and kinetics

Commercially available Al isopropoxide (Al(OiPr)₃) has proved to be an efficient initiator for the bulk polymerization of lactide. The ring-opening polymerization proceeds through a "coordination-insertion" mechanism and the selective rupture of the acyl-oxygen bond of the monomer. Polyester chains are selectively end-capped with an aluminum alkoxide and an isopropoxy group. Therefore, substitution of Al tris(4-penten-1-olate) for Al(OiPr)₃ leads to the formation of macromonomers. In the temperature range from 110 to 150°C, polymerization is "living", i.e. the molecular weight can be predicted by the initial monomer-to-Al molar ratio and the monomer conversion. The narrower polydispersity of poly(L,L-lactide) compared to the amorphous poly(D,L-lactide) (M̄w/M̄n 1.3 compared to 1.9, respectively), both prepared in the bulk, is thought to result from the restricted mobility of the crystallized isotactic polyester chains. When the temperature is increased up to 180°C, inter- and intramolecular transesterification reactions interfere with chain propagation.