Radical polymerization of dialkyl muconates (dialkyl 2,4-hexadienoates) was investigated in an isotropic (amorphous) state, that is, solution or bulk polymerization, and in a crystalline state. In the bulk polymerization at 120 degrees C in the presence of a radical initiator, high molecular weight polymers of more than 10(5) were obtained in high yields, compared with those in solution polymerization. Three kinds of isomers, cis,cis-,cis,trans-, and trans,trans-muconates, showed similar polymerization reactivities. The microstructure in the polymers was determined by H-1 and C-13 NMR spectroscopies to be 84-91%, 6-13%, and 2-4% for trans-1,4-, cis-1,4-, and 1,2-structures, respectively, which depended on the polymerization temperature. In contrast with the bulk (in the melt) and solution polymerizations, we discovered that diethyl cis,cis-muconate underwent highly stereospecific polymerization in a crystalline state under UV irradiation via a topochemical polymerization mechanism. The resulting polymer was of ultrahigh molecular weight ([eta]) = 10(3) cm(3)/g) with high stereoregularity and crystallinity. This polymer has been clarified to be a tritactic polymer, trans-1,4-meso-diisotactic or trans-1,4-meso-disyndiotactic polymer.