A novel ladder polymer, i.e., a "fused 15-crown-4 polymer", has been synthesized by a two-step polymerization of 3-ethyl-3-oxiranylmethoxymethyloxetane (1). The anionic polymerization of 1 produced the polyoxirane (2) with multiple pendant oxetanyl groups. The second cationic cyclopolymerization of 2 homogeneously proceeded to form gel-free polymers (3), i.e., "fused 15-crown-4 polymers", having M(n)s of ca. 13 000 in 60-90% yield. The cation-binding ability of 3 has also been evaluated by a liquid-liquid extraction method using picrates of alkali, alkaline-earth, and transition metals. The metal cation-binding property of 3 strongly depended on the metal cation radius. For 3 having an M. of 13 200 and M-w/M-n of 2.42, the extraction yields of Li+, CO2+, Ni2+, and Cu2+ (with radii of 0.68-0.72 Angstrom) were relatively high. Ion selectivity values were similar to those of 14-crown-4.