Synthesis and properties of (D)- and (L)-lactide stereocopolymers using the system achiral Schiff's base aluminium methoxide as initiator

The polymerization of lactides with various D/L enantiomeric compositions using achiral Schiff's base/aluminium methoxide as initiator (SALENAlOCH(3)) in dichloromethane solution at 70 degrees C is reported. The conversion was kept below 70% in order to limit transesterification reactions. The polymers obtained after precipitation show a narrow molecular weight distribution (ratio of weight- to number-average molecular weights (M) over bar(w)/(M) over bar(n) = 1,1-1,2) and an optical rotation higher than that expected from the optical purity of the starting monomers. The examination of the thermal properties reveals that whatever their enantiomeric composition all the prepared polymers are crystalline. This unusual behaviour is explained by an end-chain propagation mechanism producing stereocopolymers with long enantiomeric sequences, i.e., increased isotacticity, as substantiated by an examination of the microstructure of the polymers by means of C-13 nuclear magnetic resonance. A stereocomplex formation was observed for stereocopolymers with optical purities below 40%.