We describe here the novel phase transition of organic polymer crystals that are directly fabricated by the topochemical polymerization of the 4-bromo- and 4-chlorobenzyl esters of (ZZ)-muconic acid as the 1,3-diene monomers in the crystalline state. The obtained polymers are thermally stable, and their crystal structures reversibly change in two steps according to the temperature. The phase transition of the organic polymer crystals has been revealed on the basis of the results of thermal analyses including thermogravimietric analysis (TG), differential thermal analysis (DTA), and differential scanning calorimetry (DSC), as well as powder X-ray diffraction (XRD), single-crystal structure analysis, and IR spectroscopy under temperature control. We have evaluated a change in cell lengths from d-spacing values observed by XRD at various temperatures and concluded that the conformational change in the ester moiety of the polymer side chain induces a noncontinuous change in the crystal structures as the phase transition, which can be detected by X-ray diffraction measurement and thermal analyses.