Communication: gamma-Acryloyloxyethyl-gamma-butyrolactone is formed as a byproduct when the polymerization of gamma-acryloyloxy-epsilon-caprolactone is initiated with aluminium isopropoxide in toluene. The extent of this side reaction decreases with decreasing temperature and is dependent on whether the reaction is stopped as soon as monomer conversion is complete or not. A two-step backbiting mechanism is proposed for this intramolecular transesterification reaction.