On the Kinetics of Polyacrylamide Alkaline Hydrolysis

Abstract

Alkaline hydrolysis of polyacrylamide (PAM) proceeds with a pronounced retardation and, under mild conditions, almost ceases near 40-50% conversion. The retardation is usually interpreted in terms of the nearest neighbor effect due to the electrostatic repulsion between -COOgroups formed and hydroxyl ions catalyzing the reaction1-4 (also a certain change of OHion activity has been considered5). Various approximate equations4-6 were used to describe the kinetics.3,4 Sawant and Morawetz2 were the first who employed correct Keller’s equations7 to describe the reaction kinetics in the framework of neighbor effect model. The equations contain three individual rate constants k0, k1, and k2 characterizing the reactivity of amide groups with 0, 1, and 2 neighboring carboxylate groups, respectively. Using polymer models (statistical copolymers acrylamide-acrylic acid of various composition), the authors2 estimated the rate constant values (k0:k1: k2 ) 1:0.11:0.013) and found that the calculated kinetic curve did not fit the experiment. Moreover, they showed that even for the maximum retarding effect k0:k1:k2 ) 1:0:0 calculations predicted a greater reaction rate than observed in their experiments. Then Sawant and Morawetz2 tried another approach. They supposed that more remote units, not only the nearest ones, also affect the reactivity of any amide group. Only one rate constant dependent on the total charge of a macromolecular coil was introduced. Following the ideas of Katchalsky et al.,8,9 they assumed that (a) the concentration of OHions near the charged coil is exponentially small in comparison with their bulk concentration and (b) the electrostatic work Wel required to bring a hydroxyl ion to any amide group of a charged reacting PAM coil (in alkaline medium) against the polyion repulsion is the same as that to remove a hydrogen ion from poly(acrylic acid) (PAA) with the same charge density. Using Mandel data on the PAA titration,10 they obtained the equation k/k0 ) exp[-2.3(0.9R + 0.6R2)] describing the rate constant dependency on a conversion degree R, where k0 is the initial rate constant of hydrolysis of uncharged PAM. The equation described well a dependency of the initial rate constants of hydrolysis of random copolymers acrylamide-acrylic acid on copolymer composition; however, “it completely failed to predict the rapid decrease of the rate of polyacrylamide hydrolysis”.2 It should be mentioned that the authors of ref 2 were aware of the fact that neither of the two alternative approaches would work. In their concluding remarks they wrote: “it is reasonable to assume that the three parameters k0, k1, k2 are also functions of the overall polymer charge”. Putting it in other words, they pointed out that the separate treatment of retarding action of the nearest and more remote units on the reaction rate could not fit experimental data. Our idea is to propose a model combining the nearest neighbor effect with the effect of a charged coil. This model demonstrates its efficiency to describe quantitatively the kinetics of PAM hydrolysis. According to ref 11 the data concerning both the reaction kinetics and units distribution in the chain formed are necessary to elucidate peculiarities of a macromolecular reaction. The only publication containing the proper information is the paper of Truong et al.3 So we used their results to test both the model of the pure neighbor effect and our model.