Anion Exchange Fuel Cell Membranes Prepared from C–H Borylation

Article pubs.acs.org/Macromolecules

Anion Exchange Fuel Cell Membranes Prepared from C−H Borylation and Suzuki Coupling Reactions Angela D. Mohanty,‡ Yeong-Beom Lee,† Liang Zhu,§ Michael A. Hickner,§ and Chulsung Bae*,‡,† ‡

Department of Chemistry and Chemical Biology, New York State Center for Polymer Synthesis, Rensselaer Polytechnic Institute, 110 Eighth Street, Troy, New York 12180, United States † Department of Chemistry, University of Nevada Las Vegas, 4505 Maryland Parkway, Box 454003, Las Vegas, Nevada 89154-4003, United States § Department of Materials Science and Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802, United States S Supporting Information *

ABSTRACT: Of the polymer materials investigated as anion exchange membranes, trimethylbenzylammonium-containing polysulfones are the most extensively studied. They are commonly prepared by chloromethylation of the aromatic backbone or radical bromination of benzylic carbons, followed by a Menshutkin reaction between a tertiary amine and the benzyl halide. To overcome synthetic limitations involved with these two methods, we report the preparation of an anion exchange membrane by means of iridium-catalyzed C−H borylation followed by palladium-catalyzed Suzuki coupling. Owing to the use of mild reaction conditions and high efficiency of these metal-catalyzed reactions, we were able to minimize side reactions and easily control the degree of functionalization for a series of trimethylbenzylammonium-containing polysulfones. The resulting membranes exhibited lower water uptake while maintaining similar hydroxide conductivity and functional group stability as compared to the corresponding chloromethylation-prepared anion exchange membrane materials.

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INTRODUCTION The search for an alternative energy conversion process has become progressively intense because of the growing concern over our ever-increasing consumption of fossil fuels and its resulting effect on global climate impact. Among alternative energy technologies, fuel cells offer a promising solution to clean energy production because of their ability to convert energy stored in fuels directly and efficiently into electrical energy without emission of polluting chemicals.1,2 Among the various fuel cell types, alkaline fuel cells (AFCs), which utilize a concentrated KOH solution as the electrolyte, are among the oldest known. In spite of their successful employment in niche applications such as the Apollo Space Program, AFCs have to overcome several drawbacks in order to be widely adopted in power applications. These drawbacks include high basicity corrosion, electrolyte leakage, and low fuel cell performance due to carbonate salt buildup from the reaction of KOH with carbon dioxide.1,2 In contrast, in anion exchange membrane fuel cells (AEMFCs) hydroxide ions are transported through a solid-polymer electrolyte membrane; thus, the issues originating from AFC’s liquid electrolyte can be eliminated. Compared to proton exchange membrane fuel cells, in which solvated protons are transported through the acidic membrane and platinum-based catalysts are required, AEMFCs offer the advantage of using lower-cost, non-precious-metal electrocatalysts (Fe, Co, Ni, Ag, etc.) because of their enhanced © 2014 American Chemical Society

durability in basic media. In addition, AEMFCs exhibit faster cathode reaction rates (i.e., give higher energy efficiency) than proton exchange membrane fuel cells and offer greater fuel flexibility.1−6 In recent years, a variety of alkaline exchange membranes (AEMs) based on polysulfones,7−13 polyphenylenes,14 polystyrenes,15−17 polynorbornenes,18 polyethylenes,19−21 poly(arylene ether ketone)s,22,23 and poly(phenylene oxide)s24 have been studied. To these polymer systems, a variety of cationic moieties such as guanidinium,25−27 imidazolium,28,29 phosphonium,21 tertiary sulfonium,30 and trimethylbenzylammonium have been attached. Among them, trimethylbenzylammonium-containing polysulfones are the most extensively studied given that functionalized polysulfones are thermally and chemically stable, soluble in many organic solvents, and easy to modify and fabricate. To introduce quaternary ammonium (QA) groups onto polysulfone backbones, most synthetic methods have relied on chloromethylation,9,11,12,15,16,25,31 or radical bromination of an aromatic polymer,10,14,32 followed by the Menshutkin reaction of the benzyl halide with a tertiary amine. Although simple to conduct, chloromethylation of aromatic rings requires chloriReceived: January 16, 2014 Revised: February 27, 2014 Published: March 11, 2014 1973

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nated solvents, toxic reagents, in some cases long reaction times (typically from 1 to 7 days using the Wright method9), and extensive optimization to reach a desired degree of functionalization. Consequently, side reactions such as polymer gelation frequently occur over prolonged reaction times, and it is difficult to push the ion-exchange capacity (IEC) of the resulting polymers above 2.5 mequiv/g.33 Radical-initiated bromination of benzylic methyl groups of aromatic polymers has been reported to exhibit faster reaction rates and better control in functionalization but may cause polymer degradation when targeting high levels of bromination.34 After a halide is introduced to the benzylic position of aromatic polymers using either of the above methods, nucleophilic substitution with a tertiary amine generates QA groups tethered to the polymer chain. Because the benzylic position of the aromatic main-chain polymer backbone is sterically hindered (due to polymer chain conformation) and tertiary amine is not strong nucleophiles, the substitution reaction is limited to less-hindered amines bearing small alkyl moieties, such as trimethylamine. Complete conversion of benzyl halides to benzyl QA in the polymer system is not always observed, leaving some unreacted benzyl halides.14 Under alkaline conditions, hydroxide anion (a strong nucleophile) could react with the remaining benzyl halides and thus reduce the hydroxide anion concentration and cause unwanted polymer functionalization. Herein, we report successful preparation of an anion exchange polymer by means of iridium-catalyzed C−H borylation followed by palladium-catalyzed Suzuki coupling, a polymer functionalization method previously developed in our group for the preparation of robust ion-conducting membranes.35−38 The employment of these highly efficient metalcatalyzed reactions allowed the synthesis of an aromatic AEM material with short reaction times under mild conditions, thus minimizing side reactions along the polymer chains (e.g., degradation and gelation). Because both C−H borylation and Suzuki coupling reactions give reliable conversions, we could easily control the degree of functionalization as well as achieve complete conversion to the desired QA moiety. In addition, this synthetic method can provide the ability to incorporate bulky, more stable QA structures into AEMs by using appropriate amine substrates in the Suzuki coupling stage. In this article, we prepared Udel polysulfone (PSU)-based AEMs by incorporating trimethylbenzylammonium groups using two different synthetic methods: (i) commonly employed chloromethylation approach and (ii) our metal-catalyzed synthetic approach, and we compared their synthesis and membrane properties.

Scheme 1. Synthesis of TrimethylbenzylammoniumFunctionalized Polysulfone via C−H Borylation and Suzuki Coupling Reactions

Scheme 2. Synthesis of TrimethylbenzylammoniumFunctionalized Polysulfone via the Chloromethylation Route

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RESULTS AND DISCUSSION A. Synthesis of Anion-Conducting Polymers. We prepared a series of PSU-based AEMs using our transitionmetal-catalyzed polymer functionalization approach (Scheme 1). Because benzyltrimethylammonium is the most commonly investigated QA group for AEMs, we chose to utilize benzyltrimethylammonium-functionalized PSU as our model study. To directly compare our synthetic process and polymer membrane properties, we also prepared a series of similar PSUbased AEMs via the commonly employed chloromethylation synthetic approach (Scheme 2). A1. Synthesis of Anion-Conducting Polymers via C−H Borylation Route. The iridium-catalyzed aromatic C−H borylation of commercial PSU was conducted in THF to yield the corresponding borylated polymer B-x-PSU, where x indicates the degree of functionalization (DF) of pinacolbor1974

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Figure 1. Representative 1H NMR spectra for the borylated polymer series: (a) PSU in CDCl3, (b) B-70-PSU in CDCl3, (c) B-70-PSU-A in CDCl3, (d) B-70-PSU-NMe3I in DMSO-d6.

Reaction of the amine-functionalized polymer with methyl iodide afforded the trimethylbenzylammonium-functionalized PSU in iodide form (B-x-PSU-NMe3I). Complete methylation was observed after 12 h at 50 °C, as confirmed by 1H NMR spectroscopy. As seen in Figure 1, the methylene resonance became smaller, broadened, and shifted downfield to 4.45−4.6 ppm due to the electron-withdrawing effects from the QAcationic species. Likewise, a downfield shift was also observed for the resonance of the three N-methyl groups (at 2.9−3.1 ppm). The DF of trimethylbenzylammonium was estimated based on the relative intensity of the isopropylidene group in the polymer main chain (at 1.5−1.75 ppm) and the methylene group of the trimethylbenzylammonium moiety (at 4.45−4.6 ppm) and was confirmed to match within 5% of the DF of the starting benzylic amine. Films (approximately 10−30 μm thickness) were cast from DMSO in the iodide form to yield a transparent, dark-yellow-colored, flexible membrane samples. Ion exchange reaction to hydroxide form was conducted in an argon-filled glovebox (to eliminate CO2 contamination) by soaking in degassed 1 M NaOH at rt for 48 h, which yielded a transparent, light-yellow-colored film of B-x-PSU-NMe3OH. A2. Synthesis of Anion-Conducting Polymers via Chloromethylation Route. The Friedel−Crafts chloromethylation of PSU was conducted according to literature procedures12 using chloromethyl methyl ether (CMME) and zinc chloride as a Lewis acid catalyst (Scheme 2). This procedure was chosen over other routes9,25,31 because of its shorter reaction time. However, it should be noted that a large excess of highly toxic CMME (40 equiv) and extensive optimization were required to achieve the desired DF. The chloromethylated polymers, C-xPSU where x indicates the percent of DF of chloromethyl groups attached to the repeating unit, with 60, 90, 100, and 160% DF were prepared by adjusting the reaction time from 40 min to 3 h. However, the reliability of this method was poor given that certain reaction times did not always yield the same DF if repeated and gelation occurred in some cases. As seen in

onate ester attached to the repeating unit (Scheme 1). As shown in Figure 1, new resonances at 1.0−1.2 ppm in the 1H NMR of B-x-PSU appeared due to the addition of the pinacolboronate ester. The DF of boronate ester was estimated based on the relative intensity of the isopropylidene group in the polymer main chain (at 1.6−1.7 ppm) and the four methyl groups of the boronate ester (at 1.0−1.2 ppm). By using different molar ratios of bis(pinacolato)diboron (B2Pin2) to the polymer repeat unit, B-x-PSU with five different DFs (70, 110, 135, 150, and 210%) were prepared. Thus, we were able to conveniently and reproducibly control the DF by tuning the stoichiometric ratio of B2pin2 added. We recently reported that although the borylation in principle can occur at any of four C− H bonds of the polymer repeating unit, the boronate ester groups are preferentially incorporated into the aromatic rings of the sulfone unit since C−H borylation occurs faster at electrondeficient aromatic rings.38 To replace the boronate ester with an amine, palladiumcatalyzed Suzuki−Miyaura cross-coupling was conducted using an amine-containing aryl halide, (4-bromobenzyl)dimethylamine (A), as a substrate. Again this method reliably gave full conversion of the boronate ester to benzylic amine, yielding B-x-PSU-A. The 1H NMR spectrum of B-x-PSU-A in Figure 1 illustrates full conversion, as the resonances for the boronate ester (at 1.0−1.2 ppm) disappeared while new resonances at 2.2−2.3 and 3.4−3.5 ppm from the benzylic amine moiety were clearly visible. The DF of the benzylic amine was estimated based on the relative intensity of the isopropylidene group in the polymer main chain (at 1.6−1.7 ppm) and the methylene groups of the benzylic amine (at 3.4− 3.5 ppm) and was confirmed to match within 5% of the starting borylation degree. Similar to the results of our previous report,38 no degradation or gelation was observed in C−H borylation and Suzuki coupling given that no significant molecular weight changes were observed with GPC (Table S1 in Supporting Information). 1975

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Figure 2. Representative 1H NMR spectra for the chloromethylated polymer series: (a) PSU in CDCl3, (b) C-60-PSU in CDCl3, and (c) C-60-PSUNMe3Cl in DMSO-d6.

Figure 2, a sharp resonance at 4.52−4.55 ppm in the 1H NMR of C-x-PSU appeared due to the attachment of the chloromethyl moiety. The DF of chloromethylation was estimated based on the relative intensity of the isopropylidene group in the polymer main chain (at 1.6−1.7 ppm) and the methylene group of the chloromethyl moiety (at 4.52−4.55 ppm). Amination of the chloromethylated polymer in solution with trimethylamine under homogeneous conditions (rt, 48 h in DMSO) resulted in full conversion to C-x-PSU-NMe3Cl in chloride form. As shown in Figure 2, the methylene resonance became smaller and broadened while new resonances from the three N-methyl groups appeared at 2.95−3.1 ppm. The DF of QA was estimated based on the relative intensity of the isopropylidene group in the polymer main chain (at 1.5−1.75 ppm) and the methylene of the trimethylbenzylammonium group (at 4.45−4.6 ppm) and was confirmed to match within 5% of the DF of the starting chloromethyl group. Films (approximately 10−30 μm thickness) were directly cast from the DMSO reaction solution to yield a transparent, colorless, flexible membrane. Ion exchange reaction to hydroxide form was conducted in an argon-filled glovebox by soaking in degassed 1 M NaOH at rt for 48 h, yielding a transparent, offwhite-colored film of C-x-PSU-NMe3OH. B. Properties of Anion Exchange Membranes. IEC, water uptake, and hydroxide conductivity for each series of as synthesized AEMs are summarized in Table 1. Also included in Table 1 are experimental water uptake and conductivity results of the commercially available Tokyuama A901 membrane (Tokuyama Corp.)39 for comparison and validation of our experimental methods. B1. Water Uptake. The water uptakes of the AEM materials were measured at room temperature in hydroxide ion form. As shown in Table 1, the AEMs prepared by the traditional chloromethylation synthetic route had higher water uptakes than the corresponding AEMs prepared by our borylation route at most IEC levels. At low IEC (∼1.2 mequiv/g), B-70-PSUNMe3OH and C-60-PSU-NMe3OH had similar water uptake of

Table 1. Polysulfone-Based AEMs and Their IEC Values, Water Uptake, and Hydroxide Conductivity OH− σ (mS/cm)f sample

DFa (%)

IECb (mequiv/g)

WUc (%)

B-70-PSU-NMe3 B-110-PSU-NMe3 B-135-PSU-NMe3 B-150-PSU-NMe3 B-210-PSU-NMe3 C-60-PSU-NMe3 C-90-PSU-NMe3 C-100-PSU-NMe3 C-160-PSU-NMe3 Tokuyama A901

70 110 135 150 210 60 90 100 160 NA

1.27 1.79 2.02 2.19 2.64 1.23 1.75 1.90 2.71 1.70g

59 67 75 74 177 50 123 218 d 170

30 °C 60 °C 13 26 33 43 56 12 25 37 d 38e

22 33 49 69 96 18 40 66 d 80

a

Degree of functionalization calculated from NMR. bBased on NMR calculations otherwise stated. cWater uptake based on an average of 3 experiments, measured at rt in OH− form. dCould not measure since polymer is water-soluble. eReported OH− σ at 23 °C is 38 mS/cm for a 10 μm thick film.39 fAll OH− σ were measured in water under argon atmosphere and are an average of two measurements. gFrom Tokuyama Corporation material data sheet.

59 and 50%, respectively. However, as IEC levels are increased, water uptake increases to a greater extent for the chloromethylated route AEMs. For the B-x-PSU-NMe3OH series, water uptake increases from 67, 75, and 177% at IECs of 1.79, 2.02, and 2.64 mequiv/g, respectively, whereas water uptake dramatically increases for the C-x-PSU-NMe3OH series to 123% at IEC 1.75 and 218% at IEC 1.90, eventually becoming water-soluble at IEC 2.71 mequiv/g. We believe the differences in water uptake of the AEMs result from the difference in geometric proximity of the QA functional group to the PSU backbone. For C-x-PSU-NMe3 the QA moiety is closer to the polymer main chain, separated only by a methylene (−CH2−) group. On the other hand, for B-xPSU-NMe3 the QA group is located farther away from the PSU 1976

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Figure 3. Comparison of hydroxide (left, blue), bicarbonate (middle, green), and chloride (right, red) ion conductivities at 30 °C. B-x are AEMs prepared from the C−H borylation/Suzuki coupling route and C-x are AEMs prepared from the chloromethylation route. C-160 is not shown since it is water-soluble.

backbone (separated by a benzylic −C6H4−CH2− spacer) which may allow the polymer backbones to aggregate better than for the C-x-PSU-NMe3 samples. Since the mechanical strength of an AEM heavily depends on the hydrophobic effects of the polymer backbone and QA groups are solvated by water molecules, separating the QA groups away from the backbone would alter the water absorption properties and reduce swelling behavior of the membranes in water. B2. Ion Conductivity. Hydroxide conductivity, measured at 30 and 60 °C, is listed in Table 1. As expected, conductivity increased as IEC and temperature increased for all AEMs. It was interesting to note that conductivity at 30 °C was similar for each series of polymers at similar IEC levels regardless of the synthetic route. However, at higher temperatures, hydroxide conductivity increased more for the chloromethylated AEMs. For example, B-110-PSU-NMe3OH and C-90PSU-NMe3OH had similar conductivity values of 26 and 25 mS/cm at 30 °C, respectively. At 60 °C, the conductivity increased to 40 mS/cm for C-90-PSU-NMe3OH but reached only 33 mS/cm for B-110-PSU-NMe3OH. This trend is exemplified further when B-135-PSU-NMe3OH and C-100PSU-NMe3OH were compared (IEC ∼ 1.9−2.0). This is likely a result of the higher water uptake of C-x-PSU-NMe3OH, which further increases at the higher temperature. As described in the Experimental Section, hydroxide conductivity was measured in argon-bubbled deionized water under a blanket of gentle argon flow. The ion exchange reaction to hydroxide anion and subsequent sample preparations were also conducted in an argon-filled glovebox. This procedure was used to avoid contact with CO2 in air which may react with hydroxide anions in the membrane to form bicarbonate (HCO3−) anions. To confirm that our observed conductivity was predominantly from hydroxide conductivity, we measured hydroxide conductivity of the commercial Tokyuama A901 membrane. Our experimental conductivity measurement of 38 mS/cm matched well with the reported39 Tokuyama A901 hydroxide conductivity. To further confirm our results, we also

investigated the bicarbonate and chloride conductivities for each AEM. It has been reported that both bicarbonate and chloride conductivity are lower than that of hydroxide, with bicarbonate having the lowest dilute solution mobility and therefore the lowest membrane conductivity.10,20,40,41 To measure bicarbonate and chloride conductivities, hydroxide ion films were ion exchanged in 1 M NaHCO3 and 1 M NaCl, respectively, at rt for 48 h. As shown in Figure 3, all AEMs had much lower conductivity in bicarbonate and chloride form than in hydroxide form, with bicarbonate as the lowest in almost every case. B3. Cation Functional Group Stability. The long-term stability of the cation group under strongly basic conditions is a major issue in hydroxide-conducting polymer materials. The benzyltrimethylammonium cation is known to be relatively stable; however, more robust cation species are highly desired for practical applications in long-lived AEMFCs. To investigate whether different synthetic routes to introduce ammonium cation groups affect the material’s stability, two sets of films from each polymer series were treated with 1 M NaOH at 50 °C for 6 h. After base treatment, the films were washed with deionized water and vacuum-dried. The DFs of benzyltrimethylammonium before and after base treatment were compared by 1H NMR spectroscopy based on the relative intensity of the isopropylidene group in the polymer main chain (at 1.5−1.75 ppm) and the methylene group of the trimethylbenzylammonium moiety (at 4.45−4.6 ppm). The percentage of QA degradation was calculated by subtracting the starting DF by the remaining DF after base treatment (Table 2). Under these testing conditions, degradation products were not identified; however, loss in relative intensity of the methylene and trimethylbenzylammonium groups was apparent (see Figure S1 in Supporting Information). Both polymer systems appeared to have no differences in the amount of degradation that occurred. For example, B-70-PSUNMe3OH and C-60-PSU-NMe3OH showed a loss of 19.5 and 20.7%, respectively, of their QA functional groups. Likewise, B1977

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cyclooctadiene)iridium(I) dimer ([IrCl(COD)]2) were donated by Frontier Scientific Co. and Sinocompound Technology Co., respectively. [1,1′-Bis(diphenylphosphino)ferrocene]dichloropalladium(II) complex with dichloromethane (Pd(dppf)Cl2CH2Cl2), sodium hydroxide (NaOH), potassium carbonate (K2CO3), sodium bicarbonate (NaHCO3), magnesium sulfate (MgSO4), methyl iodide (CH3I), zinc chloride (ZnCl2), 1,1,2,2-tetrachloroethane (C2H2Cl4), trimethylamine 45% (w/w) aqueous solution (NMe3), diethyl ether, and 4-bromobenzyl bromide were purchased from Alfa Aesar. Anhydrous tetrahydrofuran (THF) was obtained from Acros Organics and stored in a nitrogen-filled glovebox. Dimethylamine 40% (w/w) aqueous solution (HNMe2) was obtained from Acros Organics. Chloromethyl methyl ether (CMME), dimethyl sulfoxide (DMSO), N,N-dimethylacetamide (DMA), and 4,4′-di-tert-butylbipyridine (dtbpy) were purchased from Sigma-Aldrich. Methanol, 2-propanol, and chloroform were reagent grade and used as received. Synthesis of (4-Bromobenzyl)dimethylamine (A). In a 50 mL round-bottom flask, 4-bromobenzyl bromide (5.00 g, 20.0 mmol) was dissolved in dioxane (10 mL). After cooling in an ice bath, an aqueous solution of HNMe2 (4.05 mL, 80.0 mmol) was added slowly. After stirring in the ice bath for 2 h, the reaction was quenched with 0.5 M NaHCO3 (3 mL) and water (10 mL) and extracted with diethyl ether (4 × 20 mL). The organic layer was dried over MgSO4, filtered, and concentrated on a rotary evaporator. Product A was isolated as a colorless liquid (3.18 g, 74% yield). 1H NMR (CDCl3): δ 7.44 (d, J = 8 Hz, 2H), 7.19 (d, J = 8 Hz, 2H), 3.37 (s, 2H), 2.23 (s, 6H). Ir-Catalyzed C−H Borylation of PSU. The following is a representative procedure for the synthesis of borylated PSU having 70% borylation degree (B-70-PSU). In a nitrogen-filled glovebox, PSU (3.00 g, 6.79 mmol of polymer repeating unit), B2Pin2 (86 mg, 3.39 mmol, 0.5 equiv), [IrCl(COD)]2 (34 mg, 1.5 mol % based on the amount of B2Pin2), dtbpy (27 mg, 3 mol % based on the amount of B2Pin2), THF (15 mL), and a magnetic stirring bar were placed into a 50 mL round-bottom flask. The flask was removed from the glovebox, fitted with a reflux condenser under a nitrogen atmosphere, and then stirred in an 80 °C oil bath for 15 h. After cooling to room temperature, the reaction solution was diluted with THF (20 mL) and filtered through a short plug of silica gel to remove the catalyst. The filtrate was concentrated using a rotary evaporator and poured into methanol to precipitate the polymer. The dissolution and precipitation process was repeated one more time. The borylated polymer was isolated as white fibers after drying under vacuum at 80 °C for 12 h (3.35 g). To reach a FD of 110, 135, 150, and 210%, the equivalents of B2Pin2 were increased to 0.7, 0.8, 0.9, and 1.0, respectively. The mol % of Bpin was estimated based on the relative intensity of resonances of −C(CH3)2− in the polymer main chain (at 1.6−1.7 ppm) and the four methyl groups of the Bpin (at 1.0−1.2 ppm). Pd-Catalyzed Suzuki Coupling of B-x-PSU. The following represents a typical procedure for the synthesis of dimethylbenzylammonium-containing PSU (B-x-PSU-A). In a nitrogen-filled glovebox, B-70-PSU (1.50 g, 2.00 mmol of Bpin), Pd(dppf)Cl2-CH2Cl2 (48 mg, 3 mol % based on the amount of boryl group of B-x-PSU), THF (15 mL), and a magnetic stirring bar were placed into a two-neck 50 mL round-bottom flask. The flask was removed from the glovebox and fitted with a reflux condenser under a nitrogen atmosphere. Degassed amine A (1.30 g, 6.00 mmol, 3 equiv based on the amount of boryl group of B-x-PSU) followed by K2CO3 (83 mg, 6.0 mmol, 3 equiv based on the amount of boryl group of B-x-PSU) dissolved in degassed distilled water (1 mL) were added to reaction mixture via syringe. After stirring at 75 °C for 12 h, the reaction was cooled and diluted with THF (15 mL) and then filtered through a short plug of silica gel to remove the catalyst. The filtrate was concentrated using a rotary evaporator and poured into methanol to precipitate the polymer. The dissolution and precipitation process was repeated one more time. The amine-functionalized polymer was isolated as off-white fibers after drying under vacuum at 80 °C for 12 h (1.22 g). Synthesis of B-x-PSU-NMe3OH. The following is a representative procedure for the methylation and ion exchange reaction of B-x-PSUA. In a 50 mL round-bottom flask equipped with a reflux condenser, B70-PSU-A (1.22 g, 1.61 mmol of amine) was dissolved in DMA (16

Table 2. Quaternary Ammonium Stability Comparison between the Different Polymer Systems

sample film tested

IEC (mequiv/g)

starting DF (%)a

B-70-PSU-NMe3 B-110-PSU-NMe3 C-60-PSU-NMe3 C-90-PSU-NMe3

1.27 1.79 1.23 1.75

70 110 60 90

DF (%) after NaOH treatmenta

% QA degraded after NaOH treatmentb

50.5 71.3 39.3 51.8

19.5 38.7 20.7 38.2

Estimated based on 1H NMR. bCalculated from (starting DF − DF after NaOH) = % QA degraded.

a

110-PSU-NMe3OH and C-90-PSU-NMe3OH lost 38.7 and 38.2% QA groups, respectively (see Table 2). Therefore, although there are differences in the way the QA functional groups are attached to the PSU backbone (i.e., −C6H4−CH2− vs −CH2− spacer), they appear to have no effect on the amount of degradation from attack by hydroxide anions.

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CONCLUSIONS A series of PSU-based AEMs were prepared by attaching trimethylbenzylammonium groups along the polymer chain via iridium-catalyzed C−H borylation followed by palladiumcatalyzed Suzuki coupling. This metal-catalyzed synthetic route displayed several advantages over the commonly employed chloromethylation approach for the preparation of AEM materials. Some of these advantages include safer and milder reaction conditions, high conversion efficiency, and increased reliability in the control of functionalization. It was discovered that the AEMs prepared from the chloromethylation route exhibited higher water uptake than the metal-catalyzed prepared materials. This difference in water uptake is likely a result of the type of trimethylammonium group attachment to the PSU main chain. In chloromethylation, the cation is separated from the polymer backbone by only a methylene unit. In our Suzuki-coupling approach, a longer aryl−benzyl cation pendant is employed, which likely allows the hydrophobic polymer backbones to reinforce the film to a greater extent. Because of their increased water uptake, the AEMs from the chloromethylated synthetic approach exhibited slightly increased hydroxide conductivity at higher temperatures. At lower temperature, however, no significant differences in conductivity were observed for both types of AEMs. In addition, no changes in QA stability were observed for the AEM materials. It should be noted that our metal-catalyzed synthetic method requires an additional step as well as more expensive metalbased reagents as compared to the chloromethylation route; however, one significant advantage of utilizing this metalcatalyzed approach is that it can provide the ability to incorporate a variety of more stable QA structures onto an aromatic-based polymer by using appropriate structures of amine substrate in the Suzuki coupling stage. This is of great interest for the improvement of AEM material long-term stability. This work is currently ongoing in our group and will be reported in future publications.

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EXPERIMENTAL SECTION

Materials. Udel polysulfone [PSU; Mw = 56 kg/mol, polydispersity index (PDI) = 2.27 measured from gel permeation chromatography (GPC) using THF as eluent] was purchased from Scientific Polymer Products, Inc. Bis(pinacolato)diboron (B2Pin2) and chloro(1,51978

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dried at 80 °C under vacuum for 15 h and weighed again. Water uptake (%) was calculated from

mL). Chilled CH3I (1.14 g, 8.00 mmol, 5 equiv based on the amount of amine group of B-x-PSU-A) was added all at once. After stirring at 50 °C for 12 h, the reaction was cooled and poured into 2-propanol to precipitate the polymer. The trimethylbenzylammonium-functionalized polymer in iodide form (B-70-PSU-NMe3I) was isolated as a dark-yellow solid after drying under vacuum at 80 °C for 12 h (1.40 g). To prepare a thin film of approximately 10−30 μm thickness, 0.15 g of polymer was dissolved in 3 mL of DMSO, filtered, and cast onto a glass plate about 8 × 8 cm2 in size. The film was dried in an oven at 60 °C for 12 h with a positive air flow, followed by 80 °C under vacuum for 4 h. The film was removed from the glass by immersion in water. In an argon-filled glovebox, the transparent, dark-yellow-colored, flexible film was ion exchanged from iodide to hydroxide form by soaking in degassed 1 M NaOH for 48 h at rt and washed with degassed deionized water until pH remained neutral, yielding a transparent, light-yellow-colored film of B-70-PSU-NMe3OH. Chloromethylation of PSU. The following is a representative procedure for the synthesis of chloromethylated PSU having 60% functionalization (C-60-PSU). In a two-neck 100 mL round-bottom flask, PSU (1.00 g, 2.26 mmol of polymer repeating unit) was dissolved in C2H2Cl4 (30 mL) under a nitrogen atmosphere. ZnCl2 (156 mg, 1.13 mmol) and CMME (7.30 g, 90.5 mmol) were mixed and added all at once. After stirring at 50 °C for 40 min, the reaction was cooled and poured into methanol to precipitate the polymer. The crude product was dissolved in chloroform and filtered through a short plug of silica gel. The filtrate was concentrated using a rotary evaporator and poured into methanol to precipitate the polymer. The chloromethylated polymer was isolated as white fibers after drying under vacuum at 80 °C for 12 h (0.95 g). To reach a FD of 90, 100, and 160%, the reaction times were increased to 45 min, 1 h, and 3 h, respectively. The mol % of −CH2Cl was estimated based on the relative intensity of resonances of −C(CH3)2− in the polymer main chain (at 1.6−1.7 ppm) and the methylene of the chloromethyl group (at 4.5−4.6 ppm). Synthesis of C-x-PSU-NMe3OH. The following is a representative procedure for homogeneous amination and ion exchange of C-x-PSU. In a 20 mL vial fitted with a Teflon-lined screw-cap, C-90-PSU (150 mg, 0.28 mmol of CH2Cl) was dissolved in DMSO (1.88 mL, 8% weight-to-volume ratio). Aqueous NMe3 solution (0.10 g, 1.68 mmol, 3 equiv based on the amount of CH2Cl groups) was added, and the reaction mixture was stirred with a magnetic stir bar at rt for 48 h. A film of approximately 10−30 μm thickness was prepared directly from the reaction mixture by first diluting with DMSO (1.8 mL) and then filtering onto a glass substrate about 8 × 8 cm2 in size. The film was dried in an oven at 60 °C for 12 h with a positive air flow, followed by 80 °C under vacuum for 4 h. The film was removed from the glass by immersion in water. In an argon-filled glovebox, the transparent, colorless, flexible film was ion exchanged from chloride to hydroxide form by soaking in degassed 1 M NaOH for 48 h at rt and washed with degassed deionized water until pH remained neutral, yielding a transparent, off-white-colored film of C-90-PSU-NMe3OH. Characterization. 1H NMR spectra were obtained with a Varian Unity 500 MHz spectrometer, and chemical shifts were referenced to the solvent residue peaks CDCl3 (at 7.26 ppm) and DMSO-d6 (at 2.50 ppm). Ion exchange capacities (IEC) were calculated from the relative intensity of the 1H NMR peaks from the methylene of the QA moiety (at 4.5−4.7 ppm) and the −C(CH3)2− in the polymer main chain (at 1.6−1.7 ppm). Molecular weight measurement was performed using a VISCOTEK GPC equipped with three Jordi-HPLC Series columns and tetra detector array, set at 30 °C with a flow rate of 0.7 mL/min having THF as the mobile phase. The instrument was calibrated using polystyrene standards. Because of polymer aggregation issues, molecular weight measurement of amine-containing B-x-PSU-A was performed using a FUTECS, NS-400 HPLC system, set at 150 °C with a flow rate of 1.0 mL/min having DMF with 4% LiBr as the mobile phase. Water Uptake (WU) Measurements. In an argon-filled glovebox, the fully hydrated anion exchange membranes (in hydroxide form) were taken out of water, blotted quickly with a KimWipe to remove surface liquid, and weighed immediately. The membranes were then

WU (%) = [(Wwet − Wdry )/Wdry ] × 100%

(1)

where Wwet and Wdry are the weight of the water-swollen and the corresponding dry membranes, respectively. Ionic Conductivity Measurements. The ion conductivities (σ, mS/cm) of each membrane (approximate size: 3 cm × 0.6 cm) were measured using a four-electrode method with BT-512 membrane conductivity test system (BekkTech LLC). Measurements were carried out under fully hydrated conditions at 30 and 60 °C, with the cell immersed in deionized water which was degassed and blanketed with a flow of argon gas. At a given temperature, the samples were equilibrated for at least 90 min before recording measurements. The ionic conductivity was calculated according to

σ = L /(RWT )

(2)

where L is the distance between the two inner platinum wires (0.425 cm), R is the resistance of the membrane in Ω, and W and T are the width and the thickness of the membrane in centimeters, respectively.

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ASSOCIATED CONTENT

S Supporting Information *

Molecular weight characterization and 1H NMR spectra of amine A and B-110-PSU-NMe3OH before and after NaOH treatment. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (C.B.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from NSF CAREER Award (C.B.) is greatly appreciated. The authors thank Shin Watanabe and Hiroyuki Yanagi of the Tokuyama Corporation for their donation of Tokuyama alkaline exchange membranes and Frontier Scientific Co. and Sinocompound Technology Co. for donation of B2pin2 and iridium complex, respectively. The authors also thank Dr. Yong Seok Kim of Korea Research Institute of Chemical Technology for obtaining GPC data of amine-functionalized PSU sample.

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REFERENCES

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dx.doi.org/10.1021/ma500125t | Macromolecules 2014, 47, 1973−1980