Polymer NMR spectroscopy. IX. Polyacrylamide and polymethacrylamide in aqueous solution

Abstract The NMR spectrum of polyacrylamide in aqueous solution exhibits a doublet resonance in the amide proton region, in contrast to the broad singlet shown by most smallmolecule amides. This is attributed to the relatively slow reorientation rate of the polymer segments, permitting effective coupling of the electric quadrupole moment of the N14 nucleus to the motions of the molecular framework. This, in turn, narrows the amide resonance to such a degree that the two protons can be discriminated. By observation of the collapse of this doublet, the rate of rotation of the amide groups can be measured; at pH 4.5 this rotation requires an activation energy of 10.5 ± 1 kcal. and is more rapid than proton exchange between the amide group and water. Rotation is not catalyzed by acid or base, where as exchange is catalyzed by both and becomes more rapid than rotation at high and low pH. The two amide protons exchange at equal rates; if intramolecular hydrogen bonding of amide groups is present, it has no apparent retarding effect on proton exchange. These results are discussed in terms of possible protonated intermediates.