DISSOCIATION BEHAVIOR OF POLY(FUMARIC ACID) AND POLY(MALEIC ACID) .2. MODEL CALCULATION

The model calculation for the dissociation behavior of poly(fumaric acid) and poly(maleic acid) reported in the previous paper was carried out using the Ising model. The interaction between a proton and ionized groups was divided into short-range and long-range electrostatic interactions, the former being evaluated from the ratio of the activity of an ionized group to that of an unionized one and the latter from the contribution of the interaction between pairs of the ionized groups not counted in the former. The factors examined here which may affect the Gibbs free energy change in the dissociation process are the range of the short-range interaction, the configuration of ionized groups, the dielectric constant around ionized groups, and the possibility of hydrogen bonding between neighboring carboxyl groups. Agreement of the model calculation with experimental data was not as quantitative as that for the alternating copolymer of maleic acid reported previously, probably because of overestimation of the long-range interaction assuming the Debye–Hückel potential. The difference in the potentiometric titration behavior between poly(fumaric acid) and poly(maleic acid) could be explained qualitatively by their configurational difference. The strong short-range electrostatic interaction proves to play a dominant role in the dissociation behavior of these polyacids with high charge densities. © 1990, American Chemical Society. All rights reserved.