Radiolysis of the poly(acrylic acid) model 2,4-dimethylglutaric acid: a pulse radiolysis and product study

Abstract

2,4-Dimethylglutaric acid (2,4-DMGA), a low molecular weight model compound for poly(acrylic acid), reacts in aqueous solution with radiolytically generated hydroxyl radicals with k= 9.5 × 108(fully protonated form) and k= 1.6 × 109 dm3 mol–1 s–1(doubly charged anion), as determined by pulse radiolysis. In deoxygenated solutions, the resulting 2,4-DMGA radicals decay bimolecularly (2k= 4.0 × 108 dm3 mol–1 s–1 at pH 3.2 and 1.3 × 107 dm3 mol–1 s–1 at pH 10) by dimerisation and disproportionation (ratio 1:1.3 at pH 3.6). In N2O–O2-saturated solutions, 2,4-DMGA radicals react with oxygen (k= 1.2 × 109 dm3 mol–1 s–1 at pH 3.6). The decay of the peroxyl radicals (2k= 7.1 × 107 dm3 mol–1 s–1 at pH 3.6 and 2.6 × 106 dm3 mol–1 s–1 at pH 10) leads to the formation of hydroxy- and carbonyl-containing as well as to C–C fragmentation and decarboxylation products, the major ones being (G values in units of 10–7 mol J–1 in parentheses): 2-methyl-4-oxovaleric acid (1.4), 2,4-dimethyl-3-oxoglutaric acid (0.75), acetylacetone (0.5), 2-formyl-4-methylglutaric acid (0.45), 2-hydroxymethyl-4-methylglutaric acid (0.2), 2- and 3-hydroxy-2,4-dimethylglutaric acid (0.2), 2-methylglutaraldehydic acid (0.2), 2-methylmalonaldehydic acid (0.2) and acetaldehyde (0.25). The formation of acetylacetone is of special interest, since a similar product is also formed in poly(acrylic acid) in relatively high yields. It is suggested to be induced by the peroxyl radical in the α-position to the carboxyl group (C2–O2–) which abstracts intramolecularly the C4–H. The rate constant for this intramolecular H-abstraction has been estimated at 0.5 s–1. The hydroperoxide product formed in subsequent steps decomposes into acetylacetone. The other products listed are explained on the basis of known peroxyl radical reactions.