Mesoionic 5-methyl-4,6-dioxo-3-phenyl-1-(4-vinylphenyl)-1,3-diazine (5) was obtained by condensation of 3-phenyl-1-(4-vinylphenyl)formamidine (3) with methylmalonic acid (4). This vinyl-substituted mesoionic heterocyclic compound was polymerized by usual radical initiation and, additionally, by thermally controlled polycycloaddition in solution, yielding two different types of polymers, 6, 7. According to the X-ray diffraction analysis, the structure of a low molecular weight model proves the regioselectivity of the polycycloaddition.