Stability of Hydrocarbon Fuel Cell Membranes: Reaction of Hydroxyl Radicals with Sulfonated Phenylated Polyphenylenes

Article abstract of DOI:10.1021/acs.chemmater.8b05302

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.

Full text of DOI:10.1021/acs.chemmater.8b05302

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Article
On the Stability of Hydrocarbon Fuel Cell Membranes: Reaction
of Hydroxyl Radicals with Sulfonated Phenylated Polyphenylenes
Thomas Holmes, Thomas J. G. Skalski, Michael Adamski, and Steven Holdcroft
Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b05302 • Publication Date (Web): 04 Feb 2019
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Chemistry of Materials
On the Stability of Hydrocarbon Fuel Cell Membranes: Reaction of Hydroxyl
Radicals with Sulfonated Phenylated Polyphenylenes
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Thomas Holmes, Thomas J. G. Skalski, Michael Adamski, & Steven Holdcroft*
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Department of Chemistry, Simon Fraser University, 8888 University Drive, Burnaby BC, V5A 1S6,
Canada
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E-mail: holdcrof@sfu.ca
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Abstract
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The perceived poor durability of hydrocarbon polymer electrolyte membranes remains a
significant hurdle for the integration of non-fluorous, 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.
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Introduction
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Since their introduction in the 1960’s, perfluorosulfonic acid (PFSA) ionomers such as Nafion®
have remained the benchmark proton exchange membrane (PEM) for solid polymer electrolyte fuel
cells and electrolyzers. Although PFSA ionomer membranes display high chemical stability, good
mechanical toughness, and high proton conductivity, their high cost of manufacturing, high gas
permeability, combined with increasing environmental concerns related to their disposal, have led
researchers to seek alternative PEMs for fuel cell applications.1–4 A significant amount of research has
thus been directed to investigating hydrocarbon membranes based on functionalized polystyrene,2,5–7
poly(arylene ether ketone),8–10 poly(imide),11–13 and poly(phenylene) polymer backbones.14–21
Recently, polyphenylene membranes have attracted renewed attention because of their high
oxidative stability, mechanical properties and proton conductivity; however their synthesis is perceived
as difficult due to the challenges associated with preparing high molecular weight polymers using
traditional transition metal-catalyzed polycondensation reactions,18,19,21 and their low solubility in polar
solvents.22 Phenylated polyphenylenes (PPP) such as those prepared by Stille,23–26 Kallitsis,27 and
Müllen28 have attracted attention due to their simpler synthesis, aided by the polymer’s solubility in
common organic solvents. These polymers may be sulfonated to form sulfonated phenylated
polyphenylene (sPPP), as demonstrated by the Sandia National Laboratory research group.14,15,29
Recently, we reported the syntheses of sPPP-based polymers through a strategy utilizing pre-
functionalized monomers, yielding structurally well-defined sPPP architectures which possessed high
degrees of functionalization, as well as high molecular weights.16,17,30
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In situ, accelerated stress tests performed on sPPP membranes operated in fuel cells under open
circuit potential, at 30% RH and 90°C have demonstrated that these membranes possess excellent in situ
durability relative to Nafion® N211 references.16 This is in part due to the low rates of gas crossover
through sPPP membranes, which consequently lowers the rates of free radical formation that are
derived therefrom. Similar studies on polyphenylene PEMs using in situ accelerated degradation tests
have reported similar results.21 In situ degradation is often attributed to a combination of mechanical and
chemical processes that include membrane swelling and shrinking caused by temperature and relative
humidity cycling, as well as the presence of generated radicals.1,31 However, the fundamental aspects of
hydrocarbon PEM degradation in the presence of free radicals alone remains relatively unknown. While
numerous extensive performance and degradation studies have been reported for PFSA-based
membranes,1,32–36 similarly comprehensive studies on polyaromatic hydrocarbon membranes, in general,
are relatively sparse,1,30,37–41 and reports on their degradation even more so.2,5,39,42–45
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In previous reports on sPPP,17,30 we showed that sPPP membranes possessing high ion
exchange capacities (IEC) exhibited no signs of degradation by mass loss or 1H NMR after exposure to
Fenton’s reagent for 1 hour (3 ppm FeSO4, 3% H2O2, 80°C), however after 4-6 hours all samples
displayed a substantial amount of degradation. In contrast, a phenylated, sulfonated polyarylene ether
exhibited a mass loss of 20% after 1 hour under similar conditions.46 In order to gain further insight into
the stability of sPPP to oxidative free radicals, the polymer was exposed to hydroxyl radicals for
prolonged periods to observe changes in the chemical structure. Due to the complexity of characterizing
sPPP polymers at the molecular level, an oligophenylene model compound sPP (Figure 1) was
designed to mimic the structural motifs of the polymer. Although Fenton’s reagent is commonly
employed as an accelerated oxidative stability test for PEMs, the radicals generated therein are in
relatively high concentration (k = 63 M-1s-1 for Fe2+),37,47 which promotes secondary degradation
pathways and creates difficulty in detecting potentially vital intermediates.48,49 To counter this, a milder
route to free radical formation was employed which involved the thermal decomposition of H2O2 (k =
1.2  10-7 s-1).37,50,51 This method was previously employed to elucidate the oxidative degradation route
of a sulfonated poly(arylene ether ketone) (sPEAK) model compound, which shed meaningful light on
the degradation pathways and chemical sensitivities of analogous polymers.39,43
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Figure 1: Chemical Structure of sulfonated phenylated polyphenylene (sPPP) and the
corresponding model compound (sPP).
Experimental
Reagents
The following reagents were purchased and used as received: Triethylamine (99%, Combi-Blocks), 1,4-
diiodobenzene (98%, Combi-Blocks Inc.), n-butanol (Fisher Scientific), hydrogen peroxide (30%,
Fisher Scientific), potassium hydroxide (Caledon Laboratories), nitrobenzene (Sigma Aldrich),
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dimethylformamide (HPLC grade, Sigma Aldrich), acetonitrile (HPLC grade, Fisher Scientific)
diphenylphosphineferrocene palladium dichloride (Strem Chemicals), trimethylsilylethyne (Tokyo
Chemical Industry Co.), trimethylsilyl chlorosulfonate (99%, Sigma Aldrich), 1,3-(diphenyl)propan-2-
one (98%, Tokyo Chemcial Industry Co.), bisbenzyl (98%, Tokyo Chemcial Industry Co.), copper
iodide (99.9%, Santa Cruz Biotechnology), dichloroethane (Caledon Laboratories Ltd.),
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phenylacetylene (98%, Combi-Blocks Inc.), and ammonium acetate (ACP Chemicals). Tetrasulfonated
bistetracyclone triethylammonium, and disulfoanted tetracyclone were prepared according to previously
reported literature procedures.16,17,30 All experiments were performed using deionized water which was
further purified using Millipore Milli-Q water purification system (≥18M·cm).
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Synthesis
Sulfonated phenylated polyphenylene (sPPP).17 A 60 mL Schlenk tube equipped with a stir bar was
charged with 2.50g (1.77 mmol) of a tetrasulfonated bistetracyclone triethylammonium salt, 24 mL of
nitrobenzene, and 0.23g (1.79 mmol) of 1,4-diethynyl benzene. The mixture was stirred for 15 minutes
at room temperature, then heated to 185°C. After 48 hours, the reaction was allowed to cool to room
temperature and precipitated into ethyl acetate (500 mL) and refluxed for 4 hours, then filtered and
washed twice with boiling ethyl acetate, and once with boiling acetone. The polymer was dried
overnight in a vacuum oven at 80°C. Next, the polymer was added to a 250 mL round bottom flask
containing 70 mL of methanol and stirred for 4 hours, then 25 mL of 2M KOH in methanol was added
dropwise and the polymer sPPP-K+ precipitated and was stirred for 2 hours then was collected via
suction filtration, washed with cold methanol and diethyl ether, and dried in a vacuum oven. Lastly,
sPPP-K+ was placed in a 250 mL round bottom flask containing 75 mL of H2O, then 25 ml of 2M
H2SO4 was added dropwise and sPPP precipitated. The mixture was stirred for 2 hours, then collected
via suction filtration, washed with cold H2O and diethyl ether, then dried in a vacuum oven at 80°C to
yield sPPP as a dark brown solid (1.65g, 81% yield). GPC analysis: Mw = 154,400 Da and Mn =
109,900 Da (Đ = 1.31).
Sulfonated Phenylated Phenylene (sPP).17 A 100 ml round bottom flask equipped with a stir bar was
charged with 8.00g (10.7 mmol) of disulfonated tetracyclone triethylammonium salt, 1.12g (10.9 mmol)
phenylacetylene, and 40 mL of nitrobenzene. The mixture was stirred for 15 minutes at room
temperature, then heated to 185°C. After 8 hours, the mixture was allowed to cool, then precipitated into
500 mL of ethyl acetate and refluxed for 2 hours, then collected via suction filtration and washed twice
with boiling ethyl acetate and once with boiling acetone. The powder was dried in a vacuum oven at
80°C for 2 hours, then dissolved into 200 mL of methanol in a round bottom flask equipped with a stir
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bar and 50 mL of 2M KOH in Methanol was added dropwise and a sPP-K+ precipitated. The mixture
was stirred for 2 hours, then collected via suction filtration and washed with cold methanol and diethyl
ether, then dried in a vacuum oven at 80°C for 2 hours. Lastly, the powder was dissolved in 200 mL of
H2O in a 500 mL round bottom flask equipped with a stir bar and 50 mL of 2M H2SO4 was added
dropwise and sPP precipitated. The mixture was stirred for 2 hours then collected via suction filtration
and washed with cold water and diethyl ether, followed by drying in a vacuum oven at 80°C to yield
sPP (5.61g, 85%).1H NMR (601 MHz, Methanol-d4) δ 7.64 (d, J = 8.3 Hz, 4H), 7.50 (d, J = 8.1 Hz,
4H), 7.47 (s, 2H), 7.25 (d, J = 8.3 Hz, 4H), 6.97 (s, 4H), 6.96 – 6.81 (m, 24H). 13C NMR (151 MHz,
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Methanol-d4) δ 145.20, 143.89, 143.76, 143.33, 143.31, 141.74, 141.56, 141.17, 140.89, 140.68, 140.09,
132.65, 132.54, 132.49, 131.85, 130.91, 130.49, 128.10, 127.85, 126.94, 126.73, 126.30, 125.77. HRMS
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[M-H]- Calculated for C66H45O12S4 1157.1796, found 1157.1676, [M-2H]2- 578.0863, [M-3H]3-
385.0556, [M-4H]4- 288.5400.
Equipment
13C NMR spectra were recorded on a Bruker AVANCE II 600 MHz NMR spectrometer
equipped with a 5 mm QNP cryoprobe. 1H, 2D (COSY), and 1D NOE were recorded on Bruker
AVANCE II 600 MHz NMR spectrometer equipped with a 5 mm TCI cryoprobe. Mass spectra were
recorded either on a Bruker Maxis Ultra-High Resolution tandem TOF (UHR-Qq-TOF) mass
spectrometer or with an Agilent 6210 TOF LC/MS. Size exclusion chromatography analyses were
obtained using a Malvern Instruments (Houston, Texas, USA) system with Viscotek D6000M column
equipped with a refractive index, UV, and light scattering detectors. The mobile phase consisted of
HPLC grade DMF (0.01M LiBr) as eluent at 298K with a flow rate of 1 ml/min, calibrated using a
poly(methyl methacrylate) standard (Mw = 51092 Da, Mn = 49231 Da).
HPLC analysis was obtained using an Agilent 1100 HPLC equipped with a PDA detector at 210
and 254 nm, equipped with an autosampler and fraction collector. A reverse phase Kinetex 5u C18
150x10 mm column was used. The mobile phase was composed of 5mM ammonium acetate in 90%
H2O and 10% acetonitrile (solvent A), and 5mM ammonium acetate in 95% acetonitrile and 5% H2O.
The mobile flow rate was set to 2 ml/min; linear solvent gradient 0-1 minute 5% (solvent B), 1-40 min
5-30% (solvent B), 40-45 min 30% (solvent B).
Degradation Procedure
Degradation experiments were performed similar to previously reported literature
procedures.39,40 Reactions were conducted in sealed glass pressure reactors, heated to 80 or 130°C and
allowed to react for 100 or 24 hours, respectively. Reactions were performed with molar ratios of 1-10
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(H2O2/sPP), and at concentrations of 0.07 – 0.70 vol.% H2O2. Typical experiments were performed with
volumes of 10 cm3. After each reaction, samples were withdrawn for HPLC and LCMS analysis, or
freeze-dried for NMR experiments.
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Results and Discussion
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Degradation of sPPP Polymer - Analyses
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Preliminary degradation experiments on sPPP with 0.35% H2O2 at 80°C for 100 hours indicated
sPPP Mw and Mn decreased from 156,400 to 82,800 Da, and 109,900 Da to 77,600 Da, respectively.
Despite this decrease in molecular weight, NMR spectra recorded a minimal change, with only two
small doublets appearing at 8.08 and 7.91 ppm in the NMR spectrum (see following section). Model
compound sPP was therefore examined as a means to shed light on the radical degradation pathway of
sPPP polymers.
Degradation of sPP model compound - Analyses and proposed mechanism
Previous studies have shown that the oxidation of aromatic compounds by hydroxyl radicals
originates by either abstraction of H (see Scheme 1) to form a phenyl radical (A) and water, or through
an addition reaction to form a hydroxy cyclohexadienyl radical (B). The latter of the two mechanisms
has been shown to be the dominant pathway. Once formed, B has a high probability of reacting with O2,
to form phenol (C) and HOO; alternatively, it may undergo an intramolecular reaction forming a
bicyclic compound (E) which leads to ring-opening bond scission.42,52–57
Scheme 1: Oxidative degradation mechanism of benzene.54,55,57
In order to observe the formation of intermediates, degradation experiments were initially
conducted on sPP using low concentrations of H2O2 (0.35%) at 130 and 80°C for 24 and 100 hours,
respectively, to ensure complete decomposition of H2O2.39 After subjecting sPP to 4 equivalents of
H2O2 at 0.35% and 130°C for 24 hours, it was found that < 10% of sPP had degraded, as characterized
by HPLC analysis (see Figure S28 for details). For comparison, it is reported in the literature that a
model compound of sulfonated poly(arylene ether ketone) degraded by 53% under the same
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conditions.39 Due to the low degradation rate observed, sPP was subject to harsher conditions (10
equivalents of 0.70% H2O2) for a total of 120 hours. Every 24 hours, an aliquot was withdrawn for
analysis, and additional H2O2 was added to continue the degradation experiment. A chromatographic
overlay of the HPLC response obtained is provided in Figure 2. Each 24-hour period consumed ≤ 30%
of sPP. After 120 hours, approx. 83% of sPP had been consumed.
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Figure 2: HPLC chromatographic overlay of sPP degraded for 120 hours, initiated by reaction
with 10 equivalents of 0.7% H2O2 at 130°C. H2O2 was replenished every 24 hours. Arrows and
numbers highlight compounds that were collected and further characterized. Refer to Figure 5 for
respective chemical structures. a Denotes products with more than one isomer.
Preparative HPLC was used to separate each significant degradation product. Each compound
was subsequently characterized using NMR and MS techniques. A description of the analysis of
compound a3 which eluted at 26.5 minutes is provided to highlight the methodology (see Figure 3). As
illustrated in Figure 3b, the [M-H]- peak at 1107.13 (m/z) indicates that the molecular weight of a3 is
1108.14 Da. The peaks observed at 553.06, 368.37, and 276.03 (m/z) are indicative of [M-2H]2-, [M-
3H]3-, and [M-4H]4- peaks, respectively; suggesting the compound still contained four acidic groups.
HRMS was then employed to obtain a molecular weight of 1108.135  0.001 Da, which was used to
calculate a chemical formula of C61H40O13S4. In the 1H NMR spectrum, 9 new resonances were
observed (labeled 1-9) when compared with the 1H NMR spectrum of sPP (Figure S4). A series of four
doublets (1-4) were observed, and the corresponding 2D COSY spectrum (Figure S21) was used to
outline correlations between doublets 1-2 and 3-4, which were assigned to the  and  protons on the
sulfonated phenyl ring. Next, the 2D COSY spectrum was used to identify a spin system comprised of 4
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aromatic protons (5-8), attributed to an ortho-substituted aromatic ring. Lastly, a resonance at 191.7
ppm observed in the 13C NMR spectrum (Figure S23) suggests that a3 contains a ketone. With this
information, it is proposed that a3 contains a fluorenone substructure as shown in Figure 3d. An
unusually low chemical shift at 6.2 ppm was observed for H5 of a3, which is caused by anisotropic
effects of the adjacent phenyl ring and proven through the detection of H1 and H2 peaks at 7.94 and 7.32
ppm using selective 1D NOE, as outlined in Figure 3c. A similar compound (b3) was also detected with
a molecular weight of 1028.18 Da, which is also purported to possess a fluorenone substructure, but
with a ketone replacing a sulfonated phenyl ring. Using a similar analytical strategy, all significant
degradation products highlighted in Figure 2 (with the exception of compounds a1/b1 and a4/b4) were
characterized. The total characterization for each compound is provided in the Supporting Information.
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Figure 3: Characterization of degradation product a3. (a) 1H NMR spectrum; (b) MS spectrum;
(c) Selective 1H 1D NOE irradiation; (d) Chemical structure of b3.
Addition products from the reaction of HO with sPP to form phenolic compounds a1 and b1
were detected by MS, but only in trace amounts. This is not unexpected, as phenol-containing
compounds are known to have relatively low oxidative stability.39,50,58,59 The oxidation of phenyl rings
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into carboxylic acids was consistently observed throughout this study, occurring at both sulfonated (b2),
and non-sulfonated (a2) phenyl rings, as well as via the oxidation of multiple phenyl rings into
carboxylic acids (5 and 6) (See Scheme 2 for full chemical structures). While the degradation product
resulting from the oxidation of two non-sulfonated phenyl rings (5) was collected and characterized, no
degradation product for the oxidation of two sulfonated phenyl rings was detected in this study.
Similarly, the oxidation product of three or more phenyl rings was not detected. Benzoic acid (BA) and
sulfobenzoic acid (SBA) were both consistently detected throughout the study, as well as compounds 7
and 8. A trace of ring-fused product (9) was also detected. Theoretical studies on polyaromatic
hydrocarbon model compounds have indicated that the attack on the C1 atom of sulfonated aromatic
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compounds may lead to the breaking of C-S bonds, forming phenol and SO3 , however no evidence for
this degradation product was detected.52
HPLC was used to analyze the evolution of each degradation product over the course of 120
hours (Figure 4). The plots provided the rate of increase and decrease of a particular compound. It is
important to note that the relative amounts of each product should not be directly compared to one other
due to discrepancies in their molar extinction coefficients.39 Instead, information was collected by
analyzing the evolution of each individual degradation product over time. Each 24-hour degradation
period degrades approximately 30% of sPP, and after a total of 120 hours, 83% of the original sPP had
been degraded. Many of the degradation products, such as carboxylic acids a2 and b2, fluorenones a3
and b3, and dicarboxylic acids 5 and 6 (Figure 4) display a slow increase in their concentration over the
first 48-72 hours, then decrease in concentration. These results are indicative of intermediate species
involved in the degradation mechanism. Degradation product 7 shows a similar trend, reaching a
maximum concentration between 72-96 hours. This would suggest 7 is a late-stage intermediate
product. Only benzoic acid (BA), sulfobenzoic acid (SBA), and 8 continuously increase in their
concentration throughout the entire oxidation process, indicating that these products are unlikely to
undergo further oxidation once formed. Lastly, ring-fused compound 9 also displays a maximum
concentration after 48 hours, followed by a continuous decrease in concentration, indicating it has a
lower oxidative stability than sPP.
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Figure 4: HPLC analysis data showing the evolution of sPP degradation products over the course
of 120 hours in the presence of 10 equivalents of H2O2 at 0.70% and 130 °C. H2O2 was added every
24 hours. See Scheme 2 for respective chemical structures.
The major degradation routes of sPP appear to originate at both the sulfonated phenyl (Scheme
2, route a) and non-sulfonated phenyl rings (Scheme 2, route b). Even when exposed to H2O2 at much
lower concentrations and temperatures (e.g., 4 equivalents, 0.30%, and 80°C), the degradation route of
sPP did not change (see Figure S31). In all cases, it began with the oxidation of a pendant phenyl ring,
initiated by radical addition of HO to form a hydroxycyclohexadienyl radical, which then leads to the
formation of hydroxylated sPP (a1 and b1) via elimination of H (as HOO through reaction with O2).
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Scheme 2: HO radical-induced oxidative degradation of sPP. a Products detected only by Mass
Spectrometry.
Having established that carboxylic acids a2 and b2, fluorenones a3 and b3, and dicarboxylic
acids 5 and 6 (Scheme 2) are intermediate species in the degradation process, and that they arise from
oxidation of pendant phenyl rings, it is postulated that carboxylic acids a2 and b2 may be formed
through the oxidation of phenols a1 and b1, or via ring-opening of a hydroxycyclohexadienyl radical
intermediate (see B, Scheme 1). Once formed, a2 and b2 undergo an acid-catalyzed intramolecular
Friedel-Crafts acylation reaction forming fluorenones a3 and b3. Similar acylation reactions have
previously been observed for aromatic polymers.60–62 Upon reaction with H2O2, fluorenones a3 and b3
undergo a Baeyer-Villiger oxidation to form lactones a4 and b4, which were only detectable via mass
spectrometry. In the presence of H2O2 and under acidic conditions, fluorenone has been shown to
undergo a Baeyer-Villiger reaction forming the corresponding lactone.63 It is proposed that upon further
oxidation, lactones a4 and b4 result in the formation of dicarboxylic acids 5 and 6. As no degradation
compound possessing three carboxylic acids was detected, it is postulated that further oxidation of
dicarboxylic acids 5 and 6 may result in the oxidation of a core phenylene ring, forming compound 7,
benzoic acid, and sulfobenzoic acid in the process. It is important to note that the degradation of a core
phenylene ring in compounds 5 and 6 would be the equivalent of a chain-scission process in the
analogous polymer, sPPP. Lastly, further oxidation of 7 results in the formation of compound 8.
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Although oxidation via hydroxyl radical attack is demonstrated to occur at both sulfonated and
non-sulfonated phenyl rings, the latter is assumed to be the predominant route due to the much higher
concentration of sulfobenzoic acid found compared to benzoic acid. This result is not unexpected, as
hydroxyl radicals are known to undergo a higher rate of reaction with benzene (k = 7.8 x 109 M-1s-1)
than with benzenesulfonic acid (k = 1.6 x 109).64 It is worth noting that the oxidation of a non-sulfonated
phenyl ring may lead to ring-opening, consuming said phenyl ring in the process. Further oxidation may
ultimately result in the ring-opening of a core phenylene ring, which would result in the production of
sulfobenzoic acid, as outlined in Scheme 2. That is, oxidation of a non-sulfonated phenyl ring may lead
to the production of sulfobenzoic acid as a degradation product, while oxidation of a sulfonated phenyl
ring may conversely lead to production of benzoic acid as a degradation product. As such, the addition
of electron withdrawing groups to phenyl rings appears to enhance their chemical stability by lowering
their rate of reaction with hydroxyl radicals.
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Scheme 3: Proposed oxidative ring-fusion mechanism of sPP initiated through H abstraction.
Note, multiple isomers are detected.
A trace amount of ring-fused product (9, Scheme 3) was detected; however, the amounts were
minimal, and likely formed via H-abstraction from sPP by HO, forming a phenyl radical that leads to
the fusion of phenyl rings according to the mechanism shown in Scheme 3. This observation is in
agreement with a theoretical study that suggests H-abstraction from phenyl rings by hydroxyl radicals is
expected to play only a minor role in the degradation of aromatic compounds.53–55
Degradation of sPPP polymer – Mechanism
Having rationalized a degradation mechanism for sPP, the analogous sPPP polymer was
subjected to degradation experiments using H2O2 concentrations of 0.07%, 0.15%, and 0.35%, and
heated to 80°C for 100 hours to ensure complete decomposition of H2O2 and to ensure radicals were
generated at a relatively low rate. The results from this experiment are outlined in Figures 5 and 6.
After degrading sPPP with 0.35% H2O2, two doublets were observed in the 1H NMR spectrum (Figure
5b) at 7.9 and 8.1 ppm, matching the NMR pattern for sulfobenzoic acid. No other significant changes
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in the 1H NMR spectra were observed. ATR-IR analysis (Figure 5c) revealed a broad carbonyl stretch
near 1690 cm-1, however this peak is difficult to interpret as it may be due to a) the formation of
carboxylic acid groups on the polymer formed by the oxidation of pendant phenyl rings; b) the
formation of fluorenone-like structures; c) due to residual sulfobenzoic acid remaining in the polymer;
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or d) due to a stronger H2O/H3O+ deformation mode. After degradation with H2O2, observed changes in
the GPC chromatograms (Figure 5a and Table 1) show minimal changes in peak shape, with small
shifts of the entire peak. Upon degrading sPPP with 0.07% H2O2, GPC analysis revealed a decrease in
the Mw from 156,400 Da to 98,500 Da, and the Mn from 109,900 to 95,700. When the amount of H2O2
was increased by a factor of 5 to 0.35% H2O2, the Mw and Mn only decreased to 82,800 Da and 77,600
Da, respectively.
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Figure 5: Hydroxyl radical-induced changes in sPPP after reaction with H2O2 at 80°C for 100
hours. (a) GPC (UV) curves; (b) 1H NMR spectrum; (c) ATR-IR spectrum.
Table 1: GPC molecular weight data of sPPP after reaction with H2O2 at 80°C for 100 hours.
H2O2
(%)
0
Mw (Da) Mn (Da)
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Although the characterization of degraded sPPP still remains a significant issue, similarities
between degraded sPPP and sPP can be seen. The high concentration of sulfobenzoic acid formed in
both cases would suggest that degradation originates with the oxidation of non-sulfonated phenyl rings,
forming phenolic rings. Once formed, these phenolic groups may be further oxidized, resulting in the
formation of carboxylic acids, which upon reaction with adjacent phenyl rings may form fluorenone
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substructures. Further oxidation may then convert the fluorenes to their corresponding lactones, which
upon further oxidation may culminate in chain-scission along the polymer backbone, forming
sulfobenzoic acid in the process (Scheme 4). This hypothesis explains why the NMR spectra of the
degraded polymer shows only minor changes with the appearance of sulfobenzoic acid, while the GPC
results show a decrease in the molecular weight of the polymer.
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Scheme 4: Proposed hydroxyl radical-induced oxidative degradation route of sPPP polymers.
Conclusion
In summary, we elucidate the route of free radical degradation of sulfonated phenylated
polyphenylenes (sPPP) using a structurally-analogous oligophenylene model compound sPP which
mimics the structural motifs of sPPP. Degradation is demonstrated to occur primarily on non-sulfonated
phenyl rings into fluorenone substructures, and upon further oxidation, may result in ring-opening of a
core phenylene ring and lead to the formation of sulfobenzoic acid. In the analogous polymer, the same
or similar oxidation of a fluorenone substructure would lead to chain-scission of the polymer backbone.
Indeed, upon exposure of sPPP to hydroxyl radicals, the molecular weight is found to decrease
substantially while the NMR spectra are relatively unchanged, save for the appearance of sulfobenzoic
acid. A potential strategy to improve the oxidative stability of sPPP membranes lies in inhibiting the
formation of the reactive fluorenone intermediates, which appear to promote chain-scission. The results
obtained in this paper suggest the addition of electron withdrawing groups to phenyl rings may serve to
enhance the chemical stability of hydrocarbon polymer electrolyte membranes. Although hydrocarbon
PEMs might never obtain the levels of ex situ chemical stability of their perfluorinated analogues, their
generally observed lower gas crossover rates have led to several studies showing significantly longer in
situ membrane lifetimes for hydrocarbon PEMs under accelerated OCV degradation conditions. Hence,
the future in situ use of hydrocarbon PEMs remains promising.
Associated Content
The Supporting Information is available free of charge on the ACS Publications website at DOI:
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All characterization data for sPPP, degraded sPPP, sPP, a2, b2, a3, b3, 4, 5, 6, 7, 8, and 9,
HPLC traces for sPP and degraded sPP, FTIR-ATR for sPPP and degraded sPPP.
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Acknowledgements
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This work was supported by NSERC (Discovery Grant RGPIN-2018-03698). The authors thank Eric
Ye for his assistance with NMR spectra, Prof. Robert Young for the use of HPLC and LCMS, and Dr.
Shaoyi Xu for useful discussions.
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