The structure of methacrylate radicals and other related enolate radicals with various molecular sizes from small molecules to polymers was investigated by ESR spectroscopy. The bromides as precursors for the radicals were prepared by the reaction of the corresponding alkenes with bromotrichloromethane. The radicals were formed by photoirradiation of a toluene solution of the precursor in the presence of hexabutylditin at 213-318 K. The polymer radicals were produced in the ESR tube by the radical polymerization of the corresponding methacrylate with an azo initiator in toluene at 373 K or in p-xylene at 393 K. The ESR spectra of the adduct radicals 1 was revealed to consist of binary components characterized by the following coupling constants and g-values : alpha(1)(3H) = 23.4-23.6 G, alpha(2)(1H) = 21.6-22.3 G, and g = 2.0043-2.0044 for conformation A and alpha(1)(3H) = 23.4-23.5 G and g = 2.0043-2.0044 for conformation B. The alpha-nitrile radical 9, the dimer or trimer radicals 5-7, and the polymer radicals 8 also gave similar dual-conformation Spectra. On the basis of the coupling constants determined for these spectra of the Various enolate radicals, the conformation model of the radicals was discussed. It has been revealed that the pyramidalization of the radical center and the A-strain-controlled conformation due to the steric strain between the large substituents account for the coupling constants for the ESR spectra of the radicals. The propagation mechanism was also discussed in relation to the tacticity of the polymer.