A novel bicyclic bis(gamma-butyrolactone), 2,8-dioxa-1-isopropenylbicyclo[3.3.0]octane-3,7-dione (2), was successfully synthesized by the reaction of 1,2,3-propanetricarboxylic acid and methacrylic anhydride at 140 degrees C in the presence of N,N-dimethyl-4-aminopyridine and CuCl2. Anionic ring-opening copolymerization of 2 with glycidyl phenyl ether (GPE) using PPh3 as an initiator proceeded in a 1:1 alternating manner to give the corresponding polyester 3 bearing alpha,beta-unsaturated ketone groups in the side chains. Prolongation of polymerization time resulted in the production of an insoluble networked copolymer due to reactions of the alpha,beta-unsaturated ketone groups in the side chains. In order to demonstrate the potential of the copolymer 3 as a reactive polymer, Michael addition of thiols to the alpha,beta-unsaturated ketone groups in the side chains was performed.