German Edition: DOI: 10.1002/ange.201703916 Proton-Exchange Membranes International Edition: DOI: 10.1002/anie.201703916 Highly Stable, Low Gas Crossover, Proton-Conducting Phenylated Polyphenylenes Michael Adamski, Thomas J. G. Skalski, Benjamin Britton, Timothy J. Peckham, Lukas Metzler, and Steven Holdcroft* Abstract: Two classes of novel sulfonated phenylated poly- phenylene ionomers are investigated as polyaromatic-based proton exchange membranes. Both types of ionomer possess high ion exchange capacities yet are insoluble in water at elevated temperatures. They exhibit high proton conductivity under both fully hydrated conditions and reduced relative humidity, and are markedly resilient to free radical attack. Fuel cells constructed with membrane-electrode assemblies contain- ing each ionomer membrane yield high in situ proton conductivity and peak power densities that are greater than obtained using Nafion reference membranes. In situ chemical stability accelerated stress tests reveal that this class of the polyaromatic membranes allow significantly lower gas cross- over and lower rates of degradation than Nafion benchmark systems. These results point to a promising future for molecularly designed sulfonated phenylated polyphenylenes as proton-conducting media in electrochemical technologies. Hydrocarbon-based proton exchange membranes (PEMs) and ionomers, intended for electrochemical applications (fuel cells, electrolyzers, and water treatment) [1, 2] are actively sought after as alternatives to traditional perfluorosulfonic acid (PFSA) ionomers [2–4] due to their ease of synthesis, low cost, low gas crossover, high T g , and fewer environmental concerns. [5, 6] Many different ion-containing polymers have been investigated with significant focus on those incorporat- ing aromatic groups as part of the polymer main chain, such as sulfonated derivatives of poly(arylene ether)s, [7] poly(arylene ether ketone)s, [8–10] poly(arylene sulfone)s, [9] poly(imide)s, [11] poly(benzimidazole)s, [2, 12] and poly(para-phenylene)s. [8, 13, 14] However, it is the general consensus that hydrocarbon- based ionomers to date are inhibited by a greater sensitivity to oxidative degradation either ex situ (e.g., Fentons Reagent test) and/or in situ (e.g., in PEM fuel cells). [2, 8] Recent attention has therefore focused on the rational design of hydrocarbon ionomers with enhanced chemical stabil- ity. [6, 15, 16] Sulfonated phenylated polyphenylenes (sPPPs) have been of particular interest as PEMs due to the inherent chemical and mechanical stability of a fully aromatic back- bone. [17, 18] Work in this area, however, had been limited by the challenge of synthesizing well-defined polymer backbones composed of sterically encumbered, rigid, aryl–aryl link- ages, [18, 19] their limited solubility in polar solvents, [8, 18] and ill- defined molecular structures as a result of the post-sulfona- tion technique commonly employed. These challenges lead to a random distribution of ionic groups on the multitude of available phenyl rings, [20–22] as well as the uncertainty of the ratio of meta :para linkages between phenyl rings along the polymer backbone. [18, 23, 24] Recently, we reported the synthesis of a well-defined, branched, sulfonated polyphenylene homopolymer (sPPP- H + ) using pre-sulfonated monomers. [24] Membranes cast from this polymer exhibited high proton conductivity and ex situ stability to oxidative degradation (as determined by 1 H NMR). When employed as a membrane and/or ionomer in the catalyst layer of a fuel cell, sPPP-H + supported a power density comparable or exceeding that of Nafion, the arche- typal PFSA ionomer. However, while sPPP-H + membranes remain intact in H 2 O at RT, they swelled excessively at higher temperatures, thus limiting research to in situ durability. In this paper, we explore the syntheses of novel sulfonated phenylated polyphenylenes using Diels–Alder (D–A) poly- merization reactions with emphasis on molecular design to enhance the positive attributes of sPPP-H + . This is accom- plished by incorporation of spacer units, biphenyl and naphthyl, in the polymer backbone. Optimization of condi- tions for synthesis of the polymers is aided by synthetic studies of oligophenylene model compounds which bear structural similarities to the analogous polymers, but are simpler to characterize. [18, 23, 25] Biphenyl and naphthyl-linked small molecules SM-B and SM-N were obtained through [4+2] D–A cycloaddition between 3c and linkers 2b or 2c, respectively (Scheme 1 a). Reaction conditions identical to the intended polymerization conditions were employed in order to confirm the stability of the desired spacer units at the temperatures necessary to facilitate the D–A reaction. [17, 24] Use of pre-sulfonated monomers allows for the synthesis of polymers containing four sulfonic acid groups per repeat- ing unit, with precise control over their positioning. [24] Syntheses were accomplished through [4+2] D–A cycloaddi- tion between monomer 1c and linkers 2b or 2c to yield sPPB- HNEt 3 + and sPPN-HNEt 3 + , respectively (Scheme 1 b). A detailed synthesis of each compound is outlined in the Supporting Information (SI). Gel permeation chromatogra- phy (GPC) analyses indicated a M w of 175 000 Da (M w /M n = 1.56) for sPPB-HNEt 3 + , and 329 000 Da (M w /M n = 2.33) for sPPN-HNEt 3 + . Successful polymerizations were confirmed by 1 H NMR spectroscopic analysis, using the triethylammo- [*] M. Adamski, T. J.G. Skalski, B. Britton, Dr. T. J. Peckham, L. Metzler, Prof. S. Holdcroft Department of Chemistry, Simon Fraser University 8888 University Drive, Burnaby BC, V5A 1S6 (Canada) E-mail: holdcrof@sfu.ca Supporting information and the ORCID identification number(s) for the author(s) of this article can be found under: https://doi.org/10.1002/anie.201703916. A ngewandte Chemie Communications 9058  2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. Int. Ed. 2017, 56, 9058 –9061