RADICAL POLYMERIZATION BEHAVIOR OF 1,1-DISUBSTITUTED 2-VINYLCYCLOPROPANES

Radical polymerizations of 1,1-disubstituted 2-vinylcyclopropanes (1a-c) were carried out, and the structures of the obtained polymers, mechanism of the polymerization, and volume change on polymerization were studied. From the detailed analyses of the H-1 and C-13 NMR spectra of the obtained polymers, a unit which was supposed to have a cyclobutane ring was confirmed in addition to the 1,5-type ring-opened unit. On the basis of the structure of the polymer, the selectivity in the direction of cleavage of the cyclopropane ring of 1a was found not to be 100%. Molecular orbital calculations were carried out to discuss the mechanism of the polymerization. The results of the calculations suggested that the formation of the cyclobutane unit is thermodynamically possible, and the selectivity in the direction of cleavage of the cyclopropane ring of 1a is lower than that of 1b from the difference of two-center energies. The radical polymerization of 1a-c showed a volume shrinkage larger than those of usual ring-opening monomers. This result was suggested to be due to the occurrence of polymerization other than the 1,5-type ring-opening polymerization.