Synthesis and radical polymerization of spiro orthocarbonates (SOCs) bearing exomethylene groups at the alpha to the ether oxygen (1a-1e) were studied. SOCs 1a-1e were prepared by the successive reactions of dichlorodiphenoxymethane with two different diols followed by dehydrochlorination. Radical polymerization of the SOCs was carried out in the presence of an appropriate initiator (3 mol % versus monomer) at 130 and 180 degrees C. The obtained polymer insoluble in n-hexane contained both vinyl polymerization unit 13 and double ring-opening polymerization unit 14 (ketone-carbonates). The degree of ring-opening followed the order : 1a < 1b < 1d, 1e. n-Hexane-insoluble polymer was not obtained in the polymerization of 1c. Both steric hindrance of methyl group and ring size affected the degree of ring-opening. The introduction of methyl group into SOC increased the degree of ring-opening (1a < 1b), whereas the degree of ring-opening of either 1d, consisting of six- and seven-membered rings, and 1e, consisting of two seven-membered rings, was higher than those of 1a and 1b, consisting of five and seven-membered rings. From the molecular orbital calculation (PM3, UHF method), it was concluded that if the first single ring-opening occurs, then the successive second ring-opening takes place more smoothly. The first ring-opening requires more energy than the vinyl polymerization does.