Damage to fuel cell membranes. Reaction of HO center dot with an oligomer of poly(sodium styrene sulfonate) and subsequent reaction with O-2

An understanding of the reactivity of oligomeric compounds that model fuel cell membrane materials under oxidative-stress conditions that mimic the fuel cell operating environment can identify material weaknesses and yield valuable insights into how a polymer might be modified to improve oxidative stability. The reaction of HO center dot radicals with a polymer electrolyte fuel cell membrane represents an initiation step for irreversible membrane oxidation. By means of pulse radiolysis, we measured k = (9.5 +/- 0.6) x 10(9) M-1 s(-1) for the reaction of HO center dot with poly(sodium styrene sulfonate), PSSS, with an average molecular weight of 1100 Da (PSSS-1100) in aqueous solution at room temperature. In the initial reaction of HO center dot with the oligomer (90 +/- 10)% react by addition to form hydroxycyclohexadienyl radicals, while the remaining abstract a hydrogen to yield benzyl radicals. The hydroxycyclohexadienyl radicals react reversibly with dioxygen to form the corresponding peroxyl radicals; the second-order rate constant for the forward reaction is k(f) = (3.0 +/- 0.5) x 10(7) M-1 s(-1), and for the back reaction, we derive an upper limit for the rate constant k(r) of (4.5 +/- 0.9) x 10(3) s(-1). These data place a lower bound on the equilibrium constant K of (7 +/- 2) x 10(3) M-1 at 295 K, which allows us to calculate a lower limit of the Gibbs energy for the reaction, (-21.7 +/- 0.8) kJ mol(-1). At pH 1, the hydroxycyclohexadienyl radicals decay with an overall first-order rate constant k of (6 +/- 1) x 10(3) s(-1) to yield benzyl radicals. The second-order rate constant for reaction of dioxygen with benzyl radicals of PSSS-1100 is k = (2-5)x 10(8) M-1 s-(1). We discuss hydrogen abstraction from PSSS-1100 in terms of the bond dissociation energy, and relate these to relevant electrode potentials. We propose a reaction mechanism for the decay of hydroxycyclohexadienyl radicals and subsequent reaction steps.