Change in Crystal Structure and Polymerization Reactivity for the Solid-State Polymerization of Muconic Esters

We report the polymerization mechanism and kinetics of muconic esters as the 1,3-diene monomers during solid-state polymerization via a crystal-to-crystal transformation. We have revealed a change in the structure of the muconate crystals accompanying the shrinking and expanding of the lattice lengths, on the basis of the X-ray single-crystal structure analysis of the monomers and polymers as well as a change in the transient structure monitored by powder X-ray diffraction during the continuous X-ray radiation. The polymerization rate is closely related to the molecular stacking distance in the monomer crystals because the polymer chain skeletons have the same conformational structure in the crystals. A lattice length in the direction along which a polymer chain is produced decreases during the transformation from monomer to polymer crystals when monomer molecules are arranged in a columnar assembly with a stacking distance greater than the fiber period of the resulting polymer, and vice versa. A strain in the crystals is formed by a mismatch between the monomer and polymer crystal lattices accumulated and then finally released during the polymerization.