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

2,4-Dimethylglutaric acid dagger (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 x 10(8) (fully protonated form) and k = 1.6 x 10(9) dm(3) 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 x 10(8) dm(3) mol(-1) s(-1) at pH 3.2 and 1.3 x 10(7) dm(3) mol(-1) s(-1) at pH 10) by dimerisation and disproportionation (ratio 1:1.3 at: pH 3.6). In N2O-O-2-saturated solutions, 2,4-DMGA radicals react with oxygen (k = 1.2 x 10(9) dm(3) mol(-1) s(-1) at pH 3.6), The decay of the peroxyl radicals (2k = 7.1 x 10(7) dm(3) mol(-1) s(-1) at pH 3.6 and 2.6 x 10(6) dm(3) 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 alpha-position to the carboxyl group (C-2-O-2(-)) which abstracts intramolecularly the C-4-H. The rate constant for this intramolecular II-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.