Titration of poly(carboxylic acid)s in methanol solution. Polymer chain extension, ionization equilibria, and conformational mobility

The titration of poly(acrylic acid) (PAA) and poly(methacrylic acid) (PMA) with LiOCH ₃ and NaOCH ₃ in methanol was studied by solution viscosity, potentiometry, and NMR spectroscopy. When titrating with LiOCH ₃, the reduced viscosity (rv) first increased with an increasing degree of neutralization (α) but then decreased due to counterion condensation and the mutual attraction of the ion-pair dipoles. The earlier rv maximum and the steeper rv decline in the case of PMA were attributed to a lower local dielectric constant. The rise of the pK with α continued for PAA even after the onset of counterion condensation as judged by the rv reversal, although Manning's theory predicts that the polymer charge density should remain constant. The NMR spectra of PMA and PAA showed a significantly greater line broadening with increasing neutralization in methanol than in D ₂O, indicating a sharp decrease of conformational mobility due to the energetic interactions between the polymer-bound ionic charges and ion-pair dipoles in the medium of low dielectric constant. When PAA or PMA was titrated with NaOCH ₃, where the two polymers showed a similar behavior, the collapse of the polymer chain (as evidenced from both rv and NMR intensity) occurred at a much lower a than with lithium methoxide titration, and led to a rv much lower than that of the two unneutralized polymers. This was attributed to a much stronger tendency of Na ion-pairs than Li ion-pairs to dimerize.