Amphoteric poly(amido-amine) polymers containing the N,N'-ethylenediaminediacetic acid moiety: stability of copper(II) and calcium(II) chelates

The thermodynamics of protonation and complexing ability with CuII and CaII ions of synthetic polyampholytes containing ethylenediamine-N,N′-diacetic acid residues is reported. The free and two cross-linked polymers, of poly(amido-amino) structure, have been studied in aqueous media (0.1 M NaCl and 25°C) mainly by potentiometry and solution calorimetry. Potentiometric and calorimetric titration curves have been fitted only with two basicity constants and two enthalpy changes, both relative to the protonation equilibria of the two amino nitrogens in each repeating unit. The first protonation step always revealed greater thermodynamic values owing to electrostatic and inductive effects. The metal ion equilibrium constant revealed only a single complex species of MIIL stoichiometry (where L2- is the repeating unit of the polymer in the ionized form) with a considerably higher log β value for CuII. The CuL species was present over a wide range of acidity (pH 2-12), while CaL formed appreciably only above pH 5. The enthalpy change (-ΔH°) of the CuII complex species was also considerably greater than that of CaII, suggesting a strong chemical bond in the former case in which d orbitals are utilized. Both cross-linked polymers retained the same ability to form metal complexes in the same pH range.