Reactions of the tetraoxidosulfate(˙−) and hydroxyl radicals with poly(sodium α-methylstyrene sulfonate)

Abstract

Poly(α-methylstyrene sulfonic acid) (PAMS) represents a class of polymers that can form the protogenic constituent in electrolyte membranes for fuel cells. Oxidative stress is thought to play an important role in the degradation of the fuel cell membranes. Having previously established that damage may be mediated via abstraction of a benzylic hydrogen, we examined model compounds similar to those used before, but with a methyl group at the α-position. We studied the reaction of HO˙ and SO₄˙⁻, generated by pulse radiolysis, with model compounds in aqueous solution, and measured k = (2 ± 0.5) × 10¹⁰ M⁻¹ s⁻¹ and (2 – 3) × 10¹⁰ M⁻¹ s⁻¹ for the reaction of HO˙ with PAMS with average molecular weights of 2640 Da (PAMS-2640) and 6440 Da (PAMS-6440), respectively, at room temperature. At low pH, the decay of the hydroxycyclohexadienyl radical thus formed is accompanied by the formation of an absorption band in the visible region of the spectrum, which we tentatively assign to the radical cation of PAMS-2640 and -6440. The radical cation of PAMS-2640, formed by the reaction of SO₄˙⁻ with k = (6 ± 1) × 10⁸ M⁻¹ s⁻¹, has a local absorption maximum at 560 nm, with ε₅₆₀ ≥ 1400 M⁻¹ cm⁻¹. For the reaction of HO˙ and SO₄˙⁻ with the model compound benzenesulfonate, we measured k = (4–5) × 10⁹ M⁻¹ s⁻¹ and (1.0 ± 0.3) × 10⁸ M⁻¹ s⁻¹, respectively, while the reaction of SO₄˙⁻ with PAMS-6440 proceeds with (0.8–1) × 10⁹ M⁻¹ s⁻¹. The 4-sulfophenoxyl radical was generated via the reaction of N₃˙ with 4-hydroxybenzenesulfonate; ε₄₁₀ ≥ 2300 M⁻¹ cm⁻¹. Not unexpectedly, the radical cation of PAMS is longer-lived than that of polystyrene sulfonic acid. Furthermore, fragmentation may result in desulfonation.