A new vinyl monomer containing the lactone moiety 3-(4-vinylphenoxy)phthalide (1) was synthesized and copolymerized with acrylonitrile (2). The sequence distributions of the copolymers were investigated by 13C NMR spectroscopy. A penultimate model was used to interpret the propagation process in the copolymerization. The reactivity ratios in the solution polymerization at 45 °C were evaluated: r^= 0.23, ri = 0.41, r2 = 0.08, and r2' = 0.11. Radical reactivity indices and frontier electron densities for 1 and 2 were calculated. The membrane prepared from this copolymer showed selective transport properties for alkali metal ions. The selectivity in the transport of alkali metal ions by electrodialysis depended on the fraction of the opened phthalide moiety, i.e., lactone moiety. The present membrane, of which the fraction of opened lactone is small, showed an appreciable ability to separate Li+ from Na+ and K+.