Evidence for Ester-Exchange Reactions and Cyclic Oligomer Formation in the Ring-Opening Polymerization of Lactide with Aluminum Complex Initiators

Four polylactide samples, obtained by ring-opening polymerization with an aluminum alkoxide initiator derived from a Schiff's base, were characterized by MALDI-TOF mass spectrometry. The MALDI mass spectra of these polylactides show well-resolved signals that can be reliably assigned to polylactide oligomers. Remarkably, both even-membered and odd-membered oligomers are present in these MALDI spectra. The presence of odd-membered oligomers cannot be explained on the basis of the lactide ring-opening polymerization, and one must admit that ester-exchange reactions do occur parallel to the polymerization process, causing a random cleavage of the polylactide chain. Furthermore, evidence for the presence of cyclic lactides was found in the MALDI-TOF spectrum of a low molecular weight polylactide fraction, indicating that ester exchange occurs also in polylactides by intramolecular end-biting reactions (ring−chain equilibration), with formation of cyclic oligomers.