Dissociation behavior of poly(maleic acid): Potentiometric titrations, viscometry, pulsed field gradient NMR, and model calculations

Potentiometric titrations, pulsed field gradient NMR, and viscometry of poly(maleic acid) have been carried out in different salt solutions of varying ionic strength. The shape of the titration curve depends strongly on the type of cation used, especially beyond alpha = 0.5. Above 5 times excess salt concentration, the potentiometric titration curves can be fit satisfactorily to an equation derived on the basis of a random Ising model, using three free parameters: the intrinsic dissociation constant K-O and two excess free energies epsilon(r) and epsilon(m) describing the nearest-neighbor interaction across a racemic and mesomeric bond, respectively. The stereochemistry of PMA is modeled using an additional parameter P-r, which is the probability of racemic propagation during synthesis. This parameter was determined by C-13 NMR. The results compare favorably with those obtained for other poly(carboxylic acid)s. Except for a gradual expansion of the polymer coil upon charging, no conformational changes are observed by PFG-NMR and viscometry. Rather, attractive intermolecular interactions appear to play a role at high degrees of dissociation.