Structural and chemical changes in the topochemical 1,4-polymerization of 3,4-bis(methylene)hexanedioic acid

Since the discovery of the topochemical 1,4-polymerization of 3,4-bis(methylene)hexanedioic acid (BMHA), the first reported polymerization of an internally substituted butadiene, an objective of our research group has been to characterize the chemical and structural changes undergone by the crystalline monomer during polymerization. Here, we report a method for the kinetic study of topochemical reactions. BMHA exhibits first order kinetics with a rate constant, k, of 0.215 min-1. Compared to the polymerization of (Z,Z)-2,4-hexadienedioate (EMU), discovered by Matsumoto et al., which also has first order kinetics with a rate constant, k, of 0.394 min-1, BMHA reacts slightly slower than EMU. In addition to kinetic data, structural changes in the polymerization of BMHA were assessed by X-ray powder diffraction. The reaction of BMHA is heterogeneous with polymerization beginning at defect sites in the crystal which serve to nucleate polymer growth in distinct domains that propagate through the monomer lattice. Further, differential thermal analysis demonstrates the polymerization of BMHA by heating (Tp = 165 °C). By comparing the heat of polymerization of BMHA to the heats of polymerization of liquid butadiene analogues, the energy of the lattice strain overcome during the crystal structure transformation can be approximated at 43.3 kJ/mol. The information reported in this work serves to characterize the topochemical polymerization of 3,4-bis(methylene)hexanedioic acid.