Inter- and intramolecular ester exchange reactions in the ring-opening polymerization of (D,L)-lactide using lanthanide alkoxide initiators

Abstract

Polylactide samples obtained by the ring-opening polymerization of (D,L)-lactide by using different lanthanide alkoxide initiators were examined by SEC, 13 C NMR and MALDI-TOF MS techniques. The evidence of substantial transesteriffcation was observed in the case of a polymerization initiated by lanthanum alkoxide and bimetallic aluminium-yttrium alkoxide initiators. In the case of Y and Sm alkoxide initiators, the MWD remains narrow up to high conversions for reasonable polymerization times, but the prolongation of the post-polymerization time increases the amount of ester-eschange reactions, The extent of the transesterification reactions was evaluated on the basis of the intensity of the stereosequence resonances for the methine carbon signal in the 13 C NMR spectra. MALDI-TOF MS analysis revealed the presence of add-numbered oligomers in all spectra. Thus, transesterification reactions occus from the beginning of the polymerization. Linear and cyclic oligomers were detected in some cases indicating the simultaneous occurrence of inter- and intramolecular exchange reactions. The microstructure analysis of the polymers by 13 C NMR spectroscopy indicates a preference for a syndiotactic addition during the polymerization process.