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 Schiffs base, were char acterized by MALDI-TOF mass spectrometry. The MALDI mass spectra of th ese polylactides show well-resolved signals that can be reliably assig ned to polylactide oligomers. Remarkably, both even-membered and odd-m embered 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 r eactions do occur parallel to the polymerization process, causing a ra ndom cleavage of the polylactide chain. Furthermore, evidence for the presence of cyclic lactides was found in the MALDI-TOF spectrum of a l ow molecular weight polylactide fraction, indicating that ester exchan ge occurs also in polylactides by intramolecular end-biting reactions (ring-chain equilibration), with formation of cyclic oligomers.