SYNTHESIS AND PROPERTIES OF (D)-LACTIDE AND (L)-LACTIDE STEREOCOPOLYMERS USING THE SYSTEM ACHIRAL SCHIFFS BASE ALUMINUM METHOXIDE AS INITIATOR

The polymerization of lactides with various D/L enantiomeric compositi ons using achiral Schiff's base/aluminium methoxide as initiator (SALE NAlOCH(3)) in dichloromethane solution at 70 degrees C is reported. Th e conversion was kept below 70% in order to limit transesterification reactions. The polymers obtained after precipitation show a narrow mol ecular weight distribution (ratio of weight- to number-average molecul ar weights (M) over bar(w)/(M) over bar(n) = 1,1-1,2) and an optical r otation higher than that expected from the optical purity of the start ing monomers. The examination of the thermal properties reveals that w hatever their enantiomeric composition all the prepared polymers are c rystalline. This unusual behaviour is explained by an end-chain propag ation mechanism producing stereocopolymers with long enantiomeric sequ ences, i.e., increased isotacticity, as substantiated by an examinatio n of the microstructure of the polymers by means of C-13 nuclear magne tic resonance. A stereocomplex formation was observed for stereocopoly mers with optical purities below 40%.