Microstructure, neighboring group inhibition, and electrostatic effects in the base-catalyzed degradation of polyacrylamide

Abstract

The degradation of polyacrylamide in basic solution has an extremely fast component characterized by the buildup and disappearance of an imide absorption band at 235 nm. Comparison with the behavior of low molecular weight model compounds shows that such imides are formed from monomer residues added head-to-head. Results suggest that 4.5% of amide groups belong to such pairs of residues and that about two-thirds of these have the racemic configuration. Initial amide hydrolysis rates of polyacrylamide and acrylamide-acrylic acid copolymers in 0.2 N NaOH at 53 °C showed that the reactivities of amide groups with one and two nearest-neighbor carboxyls are reduced by factors of 0.108 and 0.013, respectively, relative to amides flanked by two amide groups. Using these data to predict the course of polyacrylamide hydrolysis, we find that Keller's theory of polymer reactions with neighboring group effects is able to match experimental results in the early stages of the process but leads to low estimates for the conversion at high reaction times.The assumption that the electrical free energy of activation for the hydroxyl ion attack on chains with acrylamide and acrylic acid residues is equal to the electrical free energy of ionization for partially ionized poly(acrylic acid) with the same charge density leads to satisfactory predictions for the reactivity of random copolymers of acrylamide and acrylic acid but underestimates grossly the decrease in the rate constant with an increasing hydrolysis of polyacrylamide. The inadequacy of Keller's model to account fully for the course of polyacrylamide hydrolysis is due to long-range electrostatic effects and possibly to the effect of stereoisomerism.

Alternative extraction techniques for analysis of acrylamide in food: Influence of pH and digestive enzymes

“…In our experiments (not shown), polyacrylamide had to be extracted 1-2 days with only water, in order to have an effective extraction of the residual acrylamide in polyacrylamide. When increasing pH to 10-12 in a solution of polyacrylamide, it starts to deamidate (Muller, 1981;Morawetz, Sawant, & Suen, 1981;Sawant & Morawetz, 1984) and swell (Mallo, Candau, & Cohen, 1985). This swelling can open up the polyacrylamide structure, so that residual acrylamide can be released into the solution.…”

Alkaline hydrolysis of polyacrylonitrile, 2. On the product swelling

“…[9,10] It was proved [9] that intrachain imidization up to several percents may take place in dilute solution but the imide groups formed are readily hydrolyzed. In a semidilute solution one may expect crosslinking due to the formation of imides from two amide groups belonging to different chains.…”

“…Imide groups formed are easily accessible to hydroxyl ions and their reactivity in a semidilute solution should not be very different from the high value reported for the dilute solution. [9] Consider now the PAN hydrolysis after decoloration (corresponding to more than 95% of CN groups conversion). During this period, the number of b-diketone-like crosslinks could hardly increase because of the low content of CN groups, so swelling capacity of HPAN would not decrease.…”

Bioelectricity-assisted partial degradation of linear polyacrylamide in a bioelectrochemical system

“…It is commonly agreed the monomer units in the PAM are predominantly connected in the regular "head-to-tail" mode. Besides, the carbon chain of PAM also contains a proportion of abnormal "head-to-head" and "tailto-tail" linkages which are formed by the recombination of macroradicals under conditions of PAM preparation (Arcus 1949;Sawant and Morawetz 1984). The head-to-head structure has been proved to be susceptible to the attack of radicals and easily cleaved in the radical degradation of PAM (Kurenkov et al 2002).…”