A chiral Schiff's base/aluminium alkoxide initiator bearing a ligand derived from R-(+)-111'-binaphthyl-2,2'-diamine was synthesized and used for the stereoelective polymerization of rac-(o,L)-lactide. Rather high stereoelectivity is observed: a polymer with 88% enantiomeric enrichment in D units is obtained at 19% conversion. At high conversions a stereocomplex between D-and L-enriched stereocopolymers is formed. The polymerization --reaction shows living type features, and narrow molecular weight distributions (MJM" = 1,05-1,30) are obtained up to very high conversions. This indicates that transesterification reactions do not occur significantly with this sterically hindered initiator.

Six polylactides, polymerized with Salen−Al−OCH3 initiator and having optical purities between 43% and 100%, were analyzed by differential scanning calorimetry, X-ray diffraction, and optical microscopy, following various crystallization conditions. It was found that each of those polylactides can crystallize, even those with low optical purities; their crystallization rate is, however, slower than those for high optical purity polyesters. Moreover, the low optical purity polymers tend to form stereocomplexes between the l and d sequences of the same polylactide, which behavior is ascribed to their multiblock microstructure. A correlation was found between the measured melting temperature of optically active polylactides and their average sequence length.