Oligomer samples obtained by the anionic copolymerization of a bis(gamma-lactone), 2,8-dioxa-1-methylbicyclo[3.3.0]octane-3,7-dione (1), and glycidyl phenyl ether with potassium tert-butoxide have been analyzed by matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry. The MALDI-TOF mass spectra of these cooligomers show well-resolved signals that can be reliably assigned to linear, alternating cooligomers that have carboxylate chain ends or alkoxide chain ends and cyclic ones. The formation of these three series of cooligomers suggests that the polymerization process involves concomitant intermolecular transesterification and intramolecular back-biting. The intramolecular back-biting reaction causes the formation of cyclic cooligomers, whereas the intermolecular transesterification causes the reduction of the molecular weight and the transformation of the alkoxide active chain end into a carboxylate chain end. The MALDI-TOF mass spectrometry study has shown that an excess of monomer 1 enhances the selectivity of propagation by increasing the probability of the attack of the alkoxide chain end to 1. (c) 2005 Wiley Periodicals, Inc.