We have investigated a change in the crystal structures of di(4-alkoxybenzyl) (E,E)- and (Z,Z)-muconates during the solid-state polymerization by X-ray crystal structure analyses and have discussed a reaction mechanism for the polymerization. The solid-state polymerization is divided into a homogeneous polymerization which proceeds in the solid solution of the remaining monomer and the resulting polymer and a heterogeneous polymerization in which a crystal phase separation occurs between the monomer and polymer crystal domains during the reaction. According to the former model, the monomer crystal phase continuously changes into the polymer crystal phase without any phase separation when the slightest movement of atoms induces less strain in the crystals. The molecular dynamics of the muconates in the solid state were monitored by in situ single and powder X-ray diffraction experiments. In fact, we succeeded in the direct observation of the expanding crystal lattices and cell volume at the initial stage of polymerization, followed by subsequent shrinking in the process of the homogeneous reaction. On the other hand, a phase separation was observed during the reaction according to the heterogeneous polymerization model, in which a considerable movement of atoms is required. The monomer stacking structure determines the reaction path during the solid-state polymerization.