Alkaline and acid hydrolysis of polyvinylamides

Abstract

AbstractThe alkaline and acid hydrolyses of polyvinylamides and their N‐substitution derivatives have been studied and compared to the alkaline and acid hydrolyses of simple homologous substances. The rates of reaction have been followed conductometrically, and the rate constants evaluated at different temperatures in order to determine the energies of activation and the log PZ of the Arrhenius equation. The influence of the polymeric structure on the course of the reactions has been shown. In alkaline medium the increasing negative charges on the polymeric chain exert an electrostatic repulsion effect toward the hydroxyl ion, and, as a consequence, cause a progressive decrease of the rate of reaction. In the case of polyacrylamide (PAA), hydrolysis enhances an uncoiling of the polymeric chain when the degree of conversion reaches about 40%. Therefore the reaction is characterized by two different steps, a first, rapid step analogous to the reaction of aliphatic monoamides (Ea = 13.7 kcal.), and a second, slow step related to the increasing electrostatic effect. This interpretation has been confirmed by hydrolysis measurements on copolymers of acrylamide‐vinylpyrrolidone (42/58) and acrylamide‐acrylic acid (64/36). In the case of polymethacrylamide (PMA), N‐methyl‐PAA, and N,N′‐diethyl‐PAA steric hindrance along the chains confers a stretched configuration on the macromolecules, and reinforces the electrostatic effects; therefore, the rate constants, can be determined only at low degree of conversion. Moreover, the maximum degree of hydrolysis becomes limited and is 72, 55, and 35 for PMA (Ea = 16 kcal.), N‐methyl‐PAA (Ea = 18.3), and N,N′‐diethyl‐PAA (Ea = 22 kcal.), respectively. N,N′‐Diethylpolymethacryamide is completely alkali‐resistant. In acidic medium the hydrolysis of PAA and PMA is characterized by a secondary intramolecular cyclization due to imide formation between amide and acid functional groups. This secondary reaction, which becomes appreciable at 85°C., causes an increase in the overall energy of activation of the hydrolysis, which is consequently 3 to 4 kcal. higher than those of aliphatic low molecular weight compounds. On the other hand, this imidization is absent in the case of the N‐substituted polymers, of which the energies of activation are equal to those of the aliphatic homologous substances (N‐methyl‐PAA = 21.7; N,N′‐diethyl‐PAA = 24.6 kcal.).