A study of the interaction of Ni(II) and Cu(II) with poly(acrylic acid)

Potentiometric titrations have been carried out on a sample of poly(acrylic acid) of very high molecular weight (Mw = 1.92 × 106). At polymer concentrations greater than about 1 × 10−4 equiv./liter, this material behaves as a weaker acid, under given conditions, than when it is heat degraded, and also compared with a lower molecular weight sample of poly(acrylic acid) (Mw = 3.52 × 105). Only at very high dilutions are there indications that the three materials have the same pKa values. The titration curves reveal a conformational transition in poly (acrylic acid) in the range of α's of 0.2–0.5. Formation curves for the complexes between the high molecular weight poly‐(acrylic acid) and Cu2+ and Ni2+ have been calculated from the titration curves. At a polymer concentration of 2.4 × 10−3 equiv./liter, these metals form complexes in which two carboxylate groups are involved, whereas at a polymer concentration of 1.5 × 10−4 equiv./liter, the formation curves suggest the existence of complexes involving more than two carboxylate groups. This is thought to be due to enhanced intermolecular interaction which results in a reversible physically crosslinked system.