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

Polylactide samples obtained by the ring-opening polymerization of (D,L)-la ctide by using different lanthanide alkoxide initiators were examined by SE C, C-13 NMR and MALDI-TOF MS techniques. The evidence of substantial transe sterification was observed in the case of a polymerization initiated by lan thanum alkoxide and bimetallic aluminium-yttrium alkoxide initiators. In th e case of Y and Sm alkoxide initiators, the MWD remains narrow up to high c onversions for reasonable polymerization times, but the prolongation of the post-polymerization time increases the amount of ester-exchange reactions. The extent of the transesterification reactions was evaluated on the basis of the intensity of the stereosequence resonances for the methine carbon s ignal in the C-13 NMR spectra. MALDI-TOF MS analysis revealed the presence of odd-membered oligomers in all spectra. Thus, transesterification reactio ns occur from the beginning of the polymerization. Linear and cyclic oligom ers were detected in some cases indicating the simultaneous occurrence of i nter- and intramolecular exchange reactions. The microstructure analysis of the polymers by C-13 NMR spectroscopy indicates a preference for a syndiot actic addition during the polymerization process.