Syntheses and radical ring-opening polymerizations of vinylcyclopropanone derivertives having cyclic six-membered acetal, exomethylene, and phenyl groups; 1-vinyl-6-methylene-4,8-dioxaspiro[2.5]octane (2b), 1-vinyl-5,7-dimethyl-6-methylene-4,8-dioxaspiro[2.5]octane (2c), 1-vinyl-5-phenyl-4,8-dioxaspiro[2.5] octane (2d), and 1-vinyl-5,7-diphenyl-4,8-dioxaspiro[2.5]octane (2e), were carried out. The monomers were prepared by reactions of 1,1-dichloro-2-vinylcyclopropane and the corresponding diols in DMF in the presence of sodium hydride. Radical polymerizations of 2b-2e were carried out at 60, 80, and 120 degrees C in the presence of an appropriate initiator (3 mol % vs. monomer) in degassed sealed ampoules for 20 h. Although colorless transparent polymers ((M) over bar(n) 2300-13,500) were isolated by preparative HPLC for the most monomers, a crosslinked polymer was obtained in the case of 2b. The structures of the polymers were determined to consist of single and double ring-opening units. The content of the double ring-opened unit was 25-75% by comparison of LR spectra to a model compound. It is suggested that the double ring-opened propagating chain end is stabilized by the substituents on the cyclic acetal rings. The two-center energies of the cyclopropane ring and activation energy of ring-opening calculated by molecular orbital methods may explain the selectivity in the cleavage of the cyclopropane ring, and the degree of double ring-opening.