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

• Published in: Macromolecular chemistry and physics 1997-04, Vol.198 (4), p.1227-1238
• Main Authors: Wisniewski, Muriel, Borgne, Alain Le, Spassky, Nicolas
• Format: Article
• Language: English
• Subjects: Applied sciences; Exact sciences and technology; Organic polymers; Physicochemistry of polymers; Polymerization; Preparation, kinetics, thermodynamics, mechanism and catalysts
• ISSN: 1022-1352; 1521-3935
• DOI: 10.1002/macp.1997.021980424

The polymerization of lactides with various D/L enantiomeric compositions using achiral Schiff's base/aluminium methoxide as initiator (SALENAlOCH3) in dichloromethane solution at 70°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 Mw/Mn = 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 behavior 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 13C nuclear magnetic resonance. A stereocomplex formation was observed for stereocopolymers with optical purities below 40%.