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The perceived poor durability of hydrocarbon polymer electrolyte membranes remains a significant hurdle for the integration of nonfluorous, solid polymer electrolytes into electrochemical systems such as fuel cells. In order to elucidate the mechanism of free radical degradation in a promising class of hydrocarbon polymer electrolyte membranes based on sulfonated phenylated polyphenylenes (sPPP), we synthesized and studied the degradation of a structurally analogous oligophenylene model compound in the presence of hydroxyl radicals using NMR spectroscopy and mass spectrometry. Degradation is demonstrated to be initiated by the oxidation of pendant phenyl rings to carboxylic acids, which form fluorenone substructures via intramolecular reaction with a juxtaposed phenyl ring. Upon further oxidation, these substructures can lead to ring-opening of a core phenyl ring which, if occurring in sPPP, leads to chain-scission of the polymer backbone. In keeping with this hypothesis, molecular weights of sPPP are found to decrease when subject to hydroxyl radicals. Although degraded polymer NMR spectra remain unchanged, resonances consistent with the elimination of sulfobenzoic acid emerge.
Holmes et al. (Mon,) studied this question.