Hydroxyl-radical-induced reactions of poly (acrylic acid); a pulse radiolysis, EPR and product study. Part II. Oxygenated aqueous solutions

Abstract

Hydroxyl radicals were generated radiolytically in N₂O–O₂(4:1)-saturated dilute aqueous solutions of poly(acrylic acid), PAA. They react with PAA by abstracting H atoms from PAA in both the α- and βpositions to the carboxy group. The resulting PAA radicals are converted by O₂ into the corresponding peroxyl radicals (k= 3.1 × 10⁸ dm³₃ mol^–1 s^–1 at pH 3.5 and k= 1 × 10⁸ dm³ mol^–1 s^–1 at pH 10, as measured by pulse radiolysis). The lowering of the rate constant at high pH has been attributed to a diffusion barrier for O₂ exerted by the ion cloud around the charged macroradical. Although the lifetimes of the PAA peroxyl radicals (measured by EPR spectroscopy) increase with increasing pH, they still decay much faster than the very long-lived PAA radicals formed in the absence of O₂. Concomitant with an increase in the lifetime of the peroxyl radicals, O₂ uptake (in units of mol per mol PAA radicals initially formed) increases from 1.7 at pH 2.5 to 17 at pH 10 as does decarboxylation (from 1.0 at pH 3.5 to 13 at pH 10), scission (from 0.05 at pH 2 to 2.4 at pH 9) and the formation of acetylacetone-like products (from 0.35 at pH 3.5 to 1.05 at pH 10). The high yield of O₂ uptake at pH 10 is mainly the result of repetitive abstraction of α-hydrogens by PAA α-peroxyl radicals. The acetylacetone-like products are formed by intramolecular 1,5-H-transfer reactions followed by a decomposition of the ensuing hydroperoxides. The chain breaks are formed in the course of the bimolecular termination reactions as has been shown by pulse radiolysis with low-angle laser light-scattering detection.