Radical copolymerization of 1,1-bis(ethoxy-carbonly)-2-vinylcyclopropane (ECVCP) with allyl carbonates that contain isopropyl groups yields highly branched polyvinylcyclopropanes. The polymerizations were carried out in the presence of 2,2-azoisobutyronitrile at 150 degrees C in chlorobenzone. Structural analysis of the polymers suggested that radical ring-opening polymerization proceeded through 1,5-ring-opening followed by transfer to the allylic carbonate comonomers. Intra-molecular cyclization, which yields poly-cyclobutane units, was also observed during the polymerization.