Preparation and titration of amphoteric polyelectrolytes

A series of amphoteric polyelectrolytes have been made by copolymerizing N ‐dialkylaminoethyl acrylates and methacrylates with acrylic, methacrylic, or itaconic acids in unbuffered aqueous solutions or in nonaqueous solutions at 70°C. using potassium persulfate or azobisisobutyronitrile as the catalyst. The purified copolymers were titrated electrometrically on a semimicro scale. The composition of the copolymers as obtained by titration agreed with calculations based on the analysis of nitrogen content. Except for itaconic acid copolymers, the titration curves exhibited separate and nonoverlapping ionization regions for the acid and basic components and allowed the assessment of the p K values of the acid and amine groups and also the p I values of the copolymers. The p K and p I values varied with the composition of the copolymers in the expected manner. As the frequency of a given group along the main chain increased, its apparent strength decreased. The p I values also shifted to lower pH values as the number fraction of acid groups increased. All these effects could be explained on the basis of the theory of the electrical free energy of ionization. The p I values computed graphically agreed within experimental error with those calculated from the equation: where α is the degree of ionization of the group present in excess in the polyelectrolyte. The implication is that the electrical free energy of ionization does not appear to vanish at the iso‐electric point.