Kinetic Aspects of the Alkaline Hydrolysis of Poly(acrylamide)

Abstract

The alkaline hydrolysis of poly(acrylamide) was followed by determining the residual amount of unreacted amide group. Contrary to the report of Moens and Smets, the plots of the reaction based on the simple second-order rate equation form smooth curves showing no distinction for steps involved, and a kind of conversion limit exists when hydrolysis is about 60% complete. Theoretical calculations based on the assumption that the reactivity of each amide group depends upon whether zero, one, or two of its nearest neighbors have reacted were compared with the experimental data. Under the ionic strength of about 0.5, the actual progress of the reaction is in satisfactory accord with the theoretical figure. KEY WORDS Poly(acrylamide) / Alkaline Hydrolysis / Electrostatic Repulsion Effect / Conversion Limit / Statistical Calculation / Alkaline hydrolysis of poly(vinylamides) 1-3 is one of the ideal subjects of study on the effects of neighboring groups in the reaction of chain molecules, as it can be followed over its entire course in a single solvent medium (aqueous medium). On this subject, however, no satisfactorily quantitative investigation has been reported. This may be attributed to the inaccuracy of experimental data. For instance, it has been reported by Moens and Smets 3 that the first-order plot of the hydrolysis of poly(acrylamide) in the presence of a large excess of sodium hy-droxide (0.5 N) is divided into two distinct lines, i.e., the reaction involves two different steps. On the other hand, in the case of poly(meth-acrylamide) the rate constant decreases progressively as the conversion increases. On the basis of these observations, Smets emphasized the role of polymer structure in the reaction of chain molecules. However, their results, especially on poly(acrylamide), are problematical. Moens and Smets adopted conductometric titration for determination of the degree of hydrolysis. Con-ductometric titration is, however, not suitable for determination of polymer electrolytes because of the obscurity of the critical points in the titration curve; and, particularly in the case where a large excess of alkali is present in the 370 reaction mixture, the obtained value for carbo-xyl content involves large error. By the way, Keller' and others' have investigated the kinetic problem of neighboring group effects for long-chain molecules. They derived general kinetic expressions for the entire course of the process assuming that the reactivity of each group depends upon whether zero, one, or two of its nearest neighbors have reacted. However, no fruitful comparison of theoretical calculations with experimental data has yet been achieved. We consider that the alkaline hydrolysis of poly(vinylamide)s will conform to the theoretical figure provided that the reaction condition is suitable. For evidence, some new precise experimental data on poly(acrylamide) are presented in this paper, and they are compared with theoretical calculations. The equations derived by Keller or others are, however, not always convenient for analysis of experimental data; in those equations the average fractions of the reacted group and un-reacted groups of the three species are expressed as functions of time. On that account, first we consider the problem from a different angle to facilitate the evaluation of conversion limit and rate constants for three-classified amide groups.