Synthesis and radical polymerization of new vinylcyclopropane derivatives, 1,1-bis(ethoxycarbonyl)-2-isopropenylcyclopropane (4a) and 1,1-bis(ethoxycarbonyl)-2-isopropenyl-2-methylcyclopropane (4b), were carried out. 4a and 4b were prepared by treating 1,4-dibromo-2-butene derivatives and diethyl malonate with sodium ethoxide. The radical polymerization of 4 was carried out in the presence of an appropriate initiator (3 mol % vs monomer) at 60-120 degrees C. The polymers which were insoluble in methanol/water (volume ratio = 80:20), were obtained as transparent solids. The conversion of 4a in the polymerizations in both bulk and chlorobenzene was 95-99%. On the other hand, the conversion of 4b was 40-48%. From the detailed analyses of H-1 and C-13 NMR spectra of the polymers obtained, poly(4a) was determined to consist of 1,5-ring-opened units in cis and trans configurations in the main chain and a cyclobutane-containing unit. The content ratio of poly(4a) in the trans configuration to that of the cis configuration increased as the polymerization temperature was raised. Poly(4b) was determined to consist of a 1,5-ring-opened unit in the trans configuration and a cyclobutane-containing unit. MM and MO calculations were carried out to examine the difference in the selectivity of radical ring-opening polymerizations of 4a and 4b. The results of examination of the conformation of 4 and activation energy in the addition reaction of the trimethylmethyl radical to 4 agreed well with the selectivity in the configuration of the main chain of the polymers. The degrees of volume shrinkage on the polymerization of 4a and 4b were 10.4 and 10.2%, and the T-g’s of 4a and 4b were 24 and 3 degrees C, respectively.