DISSOCIATION BEHAVIOR OF POLY(MALEIC ACID) - POTENTIOMETRIC TITRATIONS, VISCOMETRY, PULSED-FIELD GRADIENT NMR, AND MODEL-CALCULATIONS

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