The reaction of methacryloyl chloride with 5-aminotetrazole gave the polymerizable methacrylamide derivative 5-(methacrylamido)tetrazole (4) in one step. The monomer had an acidic tetrazole group with a pK(a) value of 4.50 +/- 0.01 in water methanol (2:1). Radical polymerization proceeded smoothly in dimethyl formamide or, after the conversion of monomer 4 into sodium salt 4-Na, even in water. A superabsorbent polymer gel was obtained by the copolymerization of 4-Na and 0.08 mol % N,N'-methylenebisacrylamide. Its water absorbency was about 200 g of water/g of polymer, although the extractable Sol content of the gel turned out to be high. The consumption of 4-Na and acrylamide (as a model compound for the crosslinker) during a radical polymerization at 57 degreesC in D2O was followed by H-1 NMR spectroscopy. Fitting the changes in the monomer concentration to the integrated form of the copolymerization equation gave the reactivity ratios r(4-Na) = 1.10 +/- 0.05 and r(acrylamide) = 0.45 +/- 0.02, which did not differ much from those of an ideal copolymerization.