Synthesis and radical polymerization of a few hybrid monomers between vinylcyclopropane and cyclic ketene acetal (vinylcyclopropanone cyclic acetals, 1-vinyl-4,7-dioxaspiro[2,4]heptane (1a), 1-vinyl-4,9-dioxaspiro[2.6] nonane (1b), 1-vinyl-5-phenyl-4,7-dioxaspiro[2.4]heptane (1c), and 1-vinyl-6,7-benzo-4,9-dioxaspiro[2.6]nonane (1d)) were carried out. 1a-d were prepared by the reaction of 1,1-dichloro-2-vinylcyclopropane with corresponding diols in the presence of a base. Radical polymerization of 1 was carried out in the presence of an appropriate initiator (3 mol % vs monomer) at 60-120-degrees-C. Polymers which were soluble in n-hexane, methanol, chloroform, ether, and tetrahydrofuran (THF) were obtained as pale yellow transparent viscous oils, except for the polymerizations of 1a and 1b with DTBP at 120 -degrees-C, in which they were obtained as gelled polymers insoluble in common organic solvents. Structures of the polymers obtained were determined by H-1 NMR and IR spectra referring to those of model compounds of double ring-opened units. Poly(1a) was determined to consist mainly of a single ring-opened unit. On the other hand, poly(1b) was determined to consist mainly of a double ring-opened unit. 1c, which has the structure of phenyl-substituted la, afforded a double ring-opened unit in addition to a single ring-opened unit. 1d, which has a structure of benzo-substituted lb, afforded mainly a double ring-opened unit. Calculation of two-center energies with the semiempirical molecular orbital method (PM3) was carried out to examine the mechanism of the radical ring-opening polymerization of 1. Results of the examination of two-center energies agreed well with the selectivity in the bond cleavage of the cyclopropane ring of 1. Moreover, it was confirmed that the higher the polymerization temperature becomes, the more thermodynamically superior path proceeds, from the comparison of the formation energies of the various radical intermediates. The volume shrinkages of 1c and 1d were 8.86 and 3.15% in the polymerization with DTBP at 120-degrees-C, respectively, which were regarded as the smallest ones for a radically polymerizable monomer.