Systematic Alkaline Stability Study of Polymer Backbones for Anion

Apr 26, 2016 - National Institute of Advanced Industrial Science & Technology, Tsukuba, ... Woo-Hyung Lee , Eun Joo Park , Junyoung Han , Dong Won Shi...
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Systematic Alkaline Stability Study of Polymer Backbones for Anion Exchange Membrane Applications Angela D. Mohanty,† Steven E. Tignor,† Jessica A. Krause,† Yoong-Kee Choe,*,‡ and Chulsung Bae*,† †

Department of Chemistry and Chemical Biology, Rensselaer Polytechnic Institute, Troy, New York 12180, United States National Institute of Advanced Industrial Science & Technology, Tsukuba, 305-8568, Japan

‡

S Supporting Information *

ABSTRACT: Anion exchange membranes are an important component in alkaline electrochemical energy conversion and storage devices, and their alkaline stability plays a crucial role for the long-term use of these devices. Herein, a systematic study was conducted for the analysis of polymer backbone chemical stability in alkaline media. Nine representative polymer structures including poly(arylene ether)s, poly(biphenyl alkylene)s, and polystyrene block copolymers were investigated for their alkaline stability. Polymers with aryl ether bonds in their repeating unit showed poor chemical stability when treated with KOH and NaOCH3 solutions, whereas polymers without aryl ether bonds [e.g., poly(biphenyl alkylene)s and polystyrene block copolymers] remained stable. Additional NMR studies and density functional theory (DFT) calculations of small molecule model compounds that mimic the chemical structures of poly(arylene ether)s confirmed that electron-withdrawing groups near to the aryl ether bonds in the repeating unit accelerate chemical degradation. Results from this study suggest that the use of allcarbon-based polymer repeating units (i.e., polymers not bearing aryl ether bonds) can enhance long-term alkaline stability of anion exchange membranes in electrochemical energy devices.

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fones, 21−27 polyphenylenes,28,29 poly(arylene ether ketones),30,31 poly(phenylene oxides),32,33 polystyrenes,34−38 polynorbornenes,39 polybenzimidazoliums,40,41 polypropylenes,42 and polyethylenes43,44 have been employed. While investigations regarding cation functional group stability are still under debate and need further study, polymer backbone degradation is another important issue for durable AEMs. So far, only a few studies about alkaline degradation of AEMs based on QA-functionalized polysulfone and poly(phenylene oxide) polymer backbones have been reported,45−49 and systematic analysis of polymer backbone degradation in alkaline media is still lacking. The degradation studies in the literature were typically conducted by immersing membranes in alkaine conditions and measuring the changes in mechanical property, ion conductivity, and 2-D NMR spectrum. It has been proposed that aryl ether-containing polymers undergo cleavage of the C−O bonds via nucleophilic aromatic substitution (SNAr) of hydroxide anion when benzyltrimethylammonium

INTRODUCTION Anion exchange membranes (AEMs) have been intensively researched over the past decade for potential applications in electrochemical energy conversion/storage devices such as fuel cells, electrodialysis, redox flow batteries, and electrolyzers. Utilizing AEMs for these devices potentially reduces the costs because operations in alkaline conditions permit the use of nonprecious-metal electrocatalysts.1−3 So far, poor stability of AEMs particularly under high temperatures (>80 °C) and high pH operating conditions has been one of the major obstacles for successful adoption of these technologies. Thus, significant efforts in recent years have focused on improving chemical stability of both polymer backbone and quaternary ammonium (QA) functional group. To improve alkaline stability of cation functional groups, several systematic small molecule studies have been reported.4−10 Additionally, cations other than the common QAs have been incorporated into AEMs; these examples include guanidinium,11−13 imidazolium,14,15 phosphonium,16,17 tertiary sulfonium, 18 and metal-based cations such as ruthenium19 and cobaltocenium.20 For polymer backbones of AEMs, a variety of polymer structures such as polysul© XXXX American Chemical Society

Received: November 25, 2015 Revised: April 14, 2016

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Macromolecules groups were at the ortho position of C−O bonds; the cation groups on the neighboring site pull electron from the aromatic rings of polysulfone backbone and facilitate cleavage of C−O bonds under alkaline condition.47,48 The poor backbone stability of polysulfone AEMs has been ascribed to the presence of QA at the benzylic position which acts as a strong electronwithdrawing group. When those cation groups are absent (e.g., chloromethylated polysulfone), the backbone of polysulfone is known to be chemically stable in an aqueous alkaline environment. Except for a few studies described above, little studies have been conducted about degradation of polymer backbones in AEMs. Since most polymers for AEMs are synthesized by condensation polymerization via nucleophilic aromatic substitution (e.g., polysulfone), they inherently contain aryl ether bonds in the backbone. Thus, it is important to understand whether the degradation of C−O bonds is unique for polymer backbones that have a neighboring QA or if it can occur without QA groups. Incorporation of QA will also affect the hydrophilicity of polymer chains, which can have a huge impact on alkaline stability in comparison to hydrophobic nonionic polymer analogues. So far, whether the facilitated C−O bond backbone degradation of polysulfone AEMs is a result of electron-withdrawing electronic effect or hydrophilicity change of cation group is still an open question and has never been investigated. Therefore, we believe that stability studies of nonionic aryl ether polymers could provide more insightful chemical stability of polymer backbone. Herein, a systematic study that investigates the alkaline stability of various aromatic-based precursor polymer backbones is reported to expand the scope of polymer backbone stability analysis in alkaline media. We investigated degradation of polymer backbones using gel permeation chromatography (GPC) to identify which polymer repeating unit structures undergo degradation when they are subjected to a base (i.e., aqueous NaOH and NaOCH3 in CH3OH). Although NMR spectroscopy is an excellent technique for determining changes in molecular structures, it is often difficult to quantitatively identify degradation of complex polymeric structures because characterization of end groups of degraded polymer chains is challenging. By analyzing GPC data of polymers before and after alkaline tests, we will able to more clearly identify degradation of polymer chains by comparing polymer’s molecular weights. In addition to the polymer backbone stability analysis, we performed small molecule model compound stability studies to compare the effect of substituents on aryl ether bond stability and elucidate detailed routes of degradation. Finally, computational study was conducted to gain better theoretical understanding of aryl ether bond degradation rates of different model compounds. Results from this study will enhance our understanding of polymer backbone degradation of AEMs and help to discover what polymer repeating unit structures would be better for long-term alkaline stability in various AEM electrochemical energy devices.

Figure 1. (a) Structures of alkyl-tethered QA polysulfones prepared for attempted AEM applications. (b) Images of brittle AEM film pieces of PSU-CH2O-QA (left) and PSU-O-QA (right) in Br− form.

scheme). We postulated that if a QA group is separated further away from the aromatic rings of PSU by insertion of a long alkyl chain spacer, the electron-withdrawing effect of the cation would not cause the cleavage of the C−O bonds in the polymer backbone. However, we observed that films of these alkyltethered QA polymers were extremely brittle, and we could not investigate their hydroxide ion conductivities (Figure 1b). After further investigations, we found that the brittleness of the membrane was a result of significant degradation of the PSU backbone that occurred during alkaline treatments (i.e., aqueous NaOH or KOH solution) while doing chemical modifications (Scheme 1, Table S1, and Figure S1). The pristine polymer PSU has a Mn = 34.5 kg/mol and a polydispersity index (PDI) of 1.76 by GPC measurement with THF eluent. Although the chloromethylation and the iridium-catalyzed C−H borylation of PSU affected negligible changes in Mn (Mn = 31.8 kg/mol for PSU-CH2Cl and Mn = 30.4 kg/mol for PSU-Bpin), when these polymers were treated with alkaline hydroxide solution for further chemical modifications, their molecular weight values dropped sharply (Mn = 14.4 kg/mol for PSU-CH2OH and Mn = 20.7 kg/mol for PSU-O-Br). Note that the molecular weight information on PSU-OH could not be determined due to its poor solubility in THF. The reduction in polymer molecular weights significantly impacted the toughness of the subsequent polymers PSUCH2O-QA and PSU-O-QA, thus preventing their potential use as AEM materials. After we discovered these unexpected results, we set out to expand the scope of polymer backbone stability analysis in the hopes of identifying polymer structures that afford enhanced chemical stability in alkaline media. As shown in Figure 2, polymers investigated in this study include PSU, chloromethylated PSU (PSU-CH2Cl), fluoromethylated PSU (PSUCH2F), poly(phenylene oxide) (PPO), bromomethylated PPO (PPO-CH2Br), trifluoromethyl-containing poly(biphenylalkylene) (PB), bromoalkyl-tethered PB (PBA), polystyreneb-poly(ethylene-co-butylene)-b-polystyrene (SEBS), and

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RESULTS AND DISCUSSION Degradation Study of Polymers by GPC. Our initial investigations to improve alkaline stability of AEMs started with incorporation of alkyl-tethered QAs to Udel polysulfone (PSU) backbone to insulate the electron-withdrawing effect of the cation from the aromatic C−O bonds (PSU-CH2O-QA and PSU-O-QA of Figure 1a; see Scheme 1 for detailed synthetic B

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Macromolecules Scheme 1. Synthesis of Alkyl-Tethered Quaternary Ammonium Polysulfones

heated at 60 °C for 2 h then at room temperature for 12 h, (ii) polymer was dissolved in a mixture of NaOCH3, methanol, and THF and heated at 60 °C for 14 h, and (iii) polymer in film form was immersed in a 2 M NaOCH3/methanol solution and heated at 60 °C for 7 days. Condition i was performed to mimic the hydroxylation conditions of PSU-CH 2 Cl used for preparation of PSU-CH2OH as discussed in Scheme 1. The KOH treatment of condition i can induce substitution of alkyl halide on side chains to OH group, which results in changes in solubility and retention time of the resulting polymers in GPC analysis. To mitigate such solubility problem, NaOCH3 was used for conditions ii and iii as the substitution of alkyl halides by OCH3 group would not affect polymer’s solubility much in THF. Although the base strengths of OH− and OCH3− depend on solvents in the medium, on the basis of minor difference in pKa values of H2O and CH3OH in water (15.7 and 15.5, respectively),53 we expect that their reactivities toward polymer backbone will not be significantly different. Although both experiments in conditions i and ii were conducted under homogeneous conditions, the condition ii is more severe

chloromethylated SEBS (SEBS-CH2Cl). While PSU and PPO have aryl ether bonds in their repeating units, PB and SEBS have all carbon-based repeating units. PSU, PPO, and SEBS were commercially available polymers while PB and PBA were prepared via acid-catalyzed condensation polymerization according to reported procedures (Scheme S5).50,51 Chemical modifications of the polymers such as fluoro-, chloro-, and bromo-methylations are described in the Supporting Information. The degrees of functionalization (DF; mol % of functional group per polymer repeat unit) of the modified polymers were as follows: PSU-CH2Cl with 81 mol % −CH2Cl, PSU-CH2F with 130 mol % −CH2F, PPO-CH2Br with 39 mol % −CH2Br, PBA with 100 mol % −(CH2)6Br, and SEBS-CH2Cl with 7 mol % −CH2Cl. Because of extensive side reactions we observed in radical bromination of PPO and chloromethylation of SEBS,34,38,52 a low DF was targeted for preparation of PPOCH2Br and SEBS-CH2Cl. All polymers of Figure 2 were subjected to three different alkaline conditions: (i) polymer was dissolved in an aqueous KOH/THF solution (1:4 volume ratio of H2O/THF) and C

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condition iii, Table 1). This suggests that OCH3− anion cannot readily attack the polymer backbones, possibly because the anion cannot diffuse effectively through tightly packed polymer chains in membrane form. Although no decrease in molecular weights was observed for all polymers under the condition iii, direct comparison of this result and typical degradation of AEMs under alkaline conditions needs caution because QA polymers in AEMs are more hydrophilic and swell more than the nonionic polymers tested in Table 1. When the polymer materials are fully dissolved in solution (e.g., in THF for alkaline condition ii), OCH3− anions would have a greater accessibility to come into contact with polymer chains in solution, which would simulate better the test condition of swollen state of AEMs in fuel cells. As shown in Table 1, after treatment with alkaline condition ii, some polymers indeed underwent chain scission in spite of a shorter period of exposure to NaOCH3/methanol/THF solution (14 h for condition ii vs 7 days for condition iii, Table 1). PSU is one of the most commonly used polymer backbones in AEM applications because of its good thermal and mechanical stability, good film forming properties, and ease of chemical modifications. PSU membrane has been suggested to be stable under high pH conditions as well.21 However, when PSU is dissolved in a solvent and treated under alkaline conditions in our study, it showed a sign of backbone degradation. Actually, PSU exhibited the worst chemical stability in alkaline media among all pristine polymers investigated. For example, PSU degraded in the presence of NaOCH3 of condition ii whereas PPO did not. As shown in condition ii of Figure 3 (also refer to Table S2 in Supporting Figure 2. Chemical structures of polymers investigated for backbones stability under alkaline conditions.

condition than the condition i as it employs a higher pH condition, a higher temperature, and a longer test period (i.e., 10 equiv of NaOCH3 at 60 °C for 14 h). After treating the polymers with the aforementioned alkaline conditions, they were analyzed with GPC and 1H NMR spectroscopy. Table 1 summarizes degradation results of the alkaline-treated polymers, where a plus sign (+) signifies degradation was observed and a minus sign (−) indicates no degradation. It was apparent that all polymer materials remained stable when tested in their film form (alkaline Table 1. Summary of Alkaline Stability Tests for Polymer Backbonesa condition:b

i

ii

iii

PSU PSU-CH2Cl PSU-CH2F PPO PPO-CH2Br PB PBA SEBS SEBS-CH2Cl

− + − − NAc − − − −

+ + + − SRd − − − −

− − − − − − − − −

Figure 3. Changes in number-average molecular weight (Mn) of PSU, PSU-CH2Cl, and PSU-CH2F after alkaline treatments (conditions i, ii, and iii). The far left bars are Mn values of pristine polymer before alkaline treatment.

Information), a sharp decrease in molecular weight was observed for PSU after treatment with 10 equiv. NaOCH3 at 60 °C for 14 h, while no molecular weight changes were observed for PPO under the same alkaline treatment (Figure 4). Recently, Amel et al. reported that QA-functionalized PSU degraded faster in alkaline condition than QA-functionalized PPO due to the presence of electron-withdrawing sulfone linkage in PSU.49 Our results from Figures 3 and 4 suggest that PSU degrades faster than PPO even without the presence of QA groups due to the electron-withdrawing ef fect of sulfone linkage.

+ indicates degradation occurred, and − indicates no degradation. Alkaline conditions: (i) 3 equiv of KOH, 18-crown-6, THF/H2O (4:1, v/v), 60 °C for 2 h and then rt for 12 h; (ii) 10 equiv of NaOCH3, THF/methanol, 60 °C for 14 h; (iii) polymer film (20−40 μm thick) immersed in 2 M NaOCH3/methanol, 60 °C for 7 days. c Not able to run GPC due to poor solubility. dSide reactions occurred. a b

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underwent chain scission only under alkaline condition ii, PSUCH2Cl degraded under both alkaline treatments i and ii. One possible explanation for the reversed stability order is that introducing −CH2F side groups on PSU makes the polymer chains more hydrophobic, which diminishes the approach of the hydrophilic hydroxide anion. As shown in the 1H and 19F NMR spectra of PSU-CH2F (Figure S9), the −CH2F group on side chains remained intact after the alkaline treatment with KOH and NaOCH3. However, when treated with more excess of nucleophile at higher temperature in condition ii, the C−O bonds of PSU-CH 2 F also underwent degradation by nucleophilic attack of OCH3− via the SNAr reaction mechanism. To understand better the difference in electron-withdrawing effects of −CH2Cl and −CH2F substituents, we calculated the NBO charges of 2-Cl and 2-F, model compounds that mimic the chemical structures of PSU-CH2Cl and PSU-CH2F, respectively. As shown in Figure 5 and Figure S79, in spite of

Figure 4. Changes in number-average molecular weight (Mn) of PPO and PPO-CH2Br after alkaline treatments (conditions i, ii, and iii). The far left bars are Mn values of pristine polymer before alkaline treatment. Note the absence of Mn data for PPO-CH2Br after treatment with condition i; this is a result of poor solubility of the alkaline-treated polymer. Figure 5. Molecular structures of chloromethylated and fluoromethylated sulfone model compounds 2-Cl and 2-F, respectively, and their selected NBO charge.

Furthermore, when PSU is functionalized with electronwithdrawing substituents, such as −CH2Cl, its alkaline degradation was facilitated. As Table 1 highlights, PSUCH2Cl underwent chain scission as a result of treatment with alkaline conditions i and ii (see Figure 3 for changes in Mn). Since pristine PSU did not degrade in the presence of 3 equiv of KOH (condition i), we suggest that having an additional electron-withdrawing substituent, such as −CH2Cl, in close proximity to the aryl ether bonds facilitates nucleophilic attack of hydroxide anion and causes faster degradation even under milder condition i. Arges et al. reported that they observed pristine PSU and chloromethylated PSU films to remain stable, but QA-functionalized PSU in membrane form underwent ether bond hydrolysis after alkaline treatments with 1 M KOH at 60 C.47 Our results of Figure 3 indicate that the aryl ether bonds of those two nonionic polymers are inherently unstable and readily cleaved if a base accesses to polymer chains. Subjecting PSU-CH2Cl under basic condition also replaced the −CH2Cl, which contains a potential leaving group, to −CH2OH and −CH2OCH3 under conditions i and ii, respectively (see Figure S7 for 1H NMR spectra). To study the electron-withdrawing effects of a benzylsubstituted side group further, −CH2F was introduced to the PSU backbone. Unlike benzyl chloride and benzyl QA substituents, where the potential leaving groups readily undergo substitution reaction with OH− or OCH3− nucleophiles, benzyl fluorides (in general alkyl fluorides) do not undergo SN2 reactions with a nucleophile because of the strong covalent C− F bond. Thus, incorporating −CH2F into PSU would allow investigation of the electron-withdrawing group effect on the cleavage of polymer backbone C−O bonds while decoupling the SN2 reactions of the functional group on the side chains. Because F is more electronegative than Cl, if we simply consider the electron-withdrawing effect of the side chains on degradation of aryl ether bonds, we expect PSU-CH2F to degrade faster than PSU-CH2Cl. However, the alkaline test results of Figure 3 showed opposite outcome; while PSU-CH2F

clear difference in electronegativity between Cl and F and their electron-withdrawing effect on the adjacent benzylic carbon, the difference in atomic charge on neighboring carbon diminishes as they are further away from the halogen atoms. The etherconnected ipso carbon atoms of 2-Cl and 2-F have almost the same atomic charge, which suggests that the difference in C−O bond stability of PSU-CH2Cl and PSU-CH2F is primarily due to more hydrophobic nature of the latter, and the inductive electron-withdrawing effect plays a minor role. Although PPO did not show any sign of degradations under all three conditions in Table 1, it is unclear if the addition of electron-withdrawing substituents to PPO caused degradation; the degradation results of PPO-CH2Br were inconclusive. It is of interest to note that NBS-mediated radical bromination at the benzylic side chains of PPO (Scheme S2) resulted in side reactions; a decrease in Mn and an increase in PDI (Mn = 22.2 kg/mol and PDI = 2.01 for PPO vs Mn = 14.7 kg/mol and PDI = 3.87 for PPO-CH2Br, respectively; Table S6 and Figure S12) were observed after the bromination. After treating PPOCH2Br with 3 equiv of KOH (condition i), the polymer became insoluble in organic solvents, probably due to the formation of hydrogen bonded networks from the −CH2OH side chains as a result of OH− attack to −CH2Br. Therefore, its chemical structure and molecular weight property could not be analyzed by NMR and GPC. After treating PPO-CH2Br with 10 equiv of NaOCH3 (condition ii, Figure 4), the polymer remained soluble but its molecular weight was increased with bimodal broadening of PDI (Figure S12). We suspect that some side reactions that increase the polymer’s molecular weights may have occurred, but definitive conclusions cannot be made regarding degradation of PPO-CH2Br under the aforementioned test conditions. To gain better understanding, we decided to investigate alkaline stability of small molecule model E

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Macromolecules compounds that mimic PPO structure, which will be discussed in the next section. It has been suggested that polymer backbone stability can be improved in AEM applications by utilizing polymers that do not contain aryl ether bonds, such as Hibbs’ poly(phenylene) and our polyfluorene backbones.29,45,53 As expected, the four polymers without aryl ether bonds in Figure 2 (i.e., PB, PBA, SEBS, and SEBS-CH2Cl) remained stable under all alkaline treatments. PB and PBA, which were recently developed by our group using an acid-catalyzed polycondensation method, are new examples of polymers that are free of aryl ether bonds.51 Although they contain a trifluoromethyl group, a strong electron-withdrawing substituent, it is not attached to the aromatic rings of polymer backbone, and the backbone does not contain aryl ether bonds; thus, the trifluoromethyl group did not induce degradation under any of the alkaline test conditions (Figure 6).

Figure 7. Changes in number-average molecular weight (Mn) of SEBS and SEBS-CH2Cl after alkaline treatments (conditions i, ii, and iii). The far left bars are Mn values of pristine polymer before alkaline treatment.

stability to facilitate analysis of chemical degradation of aryl ether-containing polymers in alkaline environment. Figure 8 shows seven model compounds that represent components of the PSU repeat unit structure. Synthesis of these model compounds is described in the Supporting Information. Compound 1 was prepared to mimic the sulfone-linked aromatic rings of unfunctionalized PSU. The most commonly employed route to functionalize PSU with a QA functional group is to conduct chloromethylation (as described above for synthesis of PSU-CH2Cl) followed by SN2 attack of trimethylamine. This results in the incorporation of benzyltrimethylammonium to the electron-rich aromatic rings of PSU at the ortho position to the aryl ether bond. To mimic this structure of benzyltrimethylammonium functionalized PSU, compounds 2 and 8 were prepared. Compounds 3 and 9 were additionally prepared in order to compare the effects of substituents at ortho- vs meta-positions on aryl ether bond stability. Compound 10 was prepared to study the electron-withdrawing effect of sulfone linkage in compound 1 by replacing to an electrondonating tert-butyl group. As this model compound does not contain an electron-withdrawing sulfone linkage, it can also be used to mimic the backbone structure of unfunctionalized PPO. Lastly, compound 14 that has a benzyltrimethylammonium at ortho to the aryl ether bond was prepared, as a counterpart of sulfone-linked compound 8. The direct comparison of compounds 8 and 14 will help to identify which electronwithdrawing substituent (i.e., the sulfone linkage or benzyltrimethylammonium functional group) has a greater influence on aryl ether bond cleavage in an alkaline environment. All model compounds of Figure 8 were treated with 4 equiv of NaOCH3 in a DMSO-d6/CD3OD solution at room temperature for 20 h and were investigated for aryl ether bond cleavage and QA functional group degradation by 1H NMR spectroscopy. 18-Crown-6 was used as an internal standard since it was shown to work well for previous small molecule model compound NMR studies. 7 Instead of hydroxide, methoxide was chosen as the base for these small molecule alkaline stability experiments because we could better

Figure 6. Changes in number-average molecular weight (Mn) of PB and PBA after alkaline treatments (conditions i, ii, and iii). The far left bars are Mn values of pristine polymer before alkaline treatment.

No changes in molecular weight and 1H NMR spectrum were observed for pristine SEBS under all alkaline test conditions (Figure 7, Figures S18 and S19). For SEBSCH2Cl, although a low DF was targeted to prevent gelation in the chloromethylation of SEBS, a small amount of unknown side reaction was unavoidable. As shown in Figure S21, a new broad peak at 3.30 ppm, which could be resulted from side reactions of −CH2Cl groups, appeared in the 1H NMR spectrum of SEBS-CH2Cl, and the intensity of this resonance increased when treated with alkaline test conditions i−iii. The greater extent of side reaction (presumably cross-linking reaction) on the polymer chains from the alkaline tests is also reflected in a minor increase in PDI of SEBS-CH2Cl samples (PDI increased from 1.05 to 1.21−1.48; Table S10). However, the side reaction did not significantly affect the Mn values of the alkaline-treated SEBS-CH2Cl polymers (Figure 7), and more importantly, no sign of degradation on the polymer chain was observed. Small Molecule Model Compound Degradation Study. Because it is difficult to elucidate mechanisms and exact chemical degradation routes of polymers, we designed a set of small molecule model compounds and investigated their F

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Figure 8. Small molecule model compounds synthesized to mimic the PSU repeat unit.

Figure 9. (a−c) Potential routes of methoxide attack and their possible byproducts for the various structures of model compounds investigated for the small molecule stability study. (d) Summarized results from byproduct analysis.

identify which position of C−O bonds the anion attacked based on the analysis of byproducts. For example, as shown in Figure 9a, OCH3− attack via route 1 would result in the formation of 1-methoxy-4-(phenylsulfonyl)benzene and phenol as byproducts. Whereas OCH3− attack via route 2 in Figure 9a would result in the formation of 4-(phenylsulfonyl)phenol and methoxy-substituted benzene. Figure 9 illustrates potential degradation routes of the small model compounds listed in Figure 8 and their identified byproducts by GC-MS and NMR spectroscopy. As summarized in Figure 9d, all sulfonecontaining model compounds (1, 2, 3, 8, 9) were found to have C−O bond cleavage near to sulfone linkage. The mechanism for sulfone-free model compounds (10 and 14) was inconclusive as the C−O bonds of these compounds did

not undergo degradation under the room temperature condition (see Figure 10a). Figure 10 shows the comparison of chemical degradation for model compounds 1, 2, 3, 8, 9, 10, and 14 after treatment with 4 equiv of NaOCH3 in DMSO-d6/CD3OD at room temperature for 20 h. Although these alkaline stability test results cannot be directly compared to those of polymer backbone (because different solvents and temperatures were employed), the data from the model compounds can be analyzed quantitatively and can give better insights into how the aryl ether bond is cleaved. The percentage of remaining aryl ether bonds in Figure 10a was calculated based on the change in integral values of the aromatic peaks relative to 18-crown-6 internal standard (see Figures S22−S75 for detailed 1H NMR spectra). The percentage of remaining cation functional groups G

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Figure 10. Percentage of remaining (a) aryl ether bonds and (b) cation functional groups for model compounds 1, 2, 3, 8, 9, 10, and 14 after treatment with 4 equiv of NaOCH3 in DMSO-d6/CD3OD (20:1, v/v) at room temperature for 20 h.

compound was lower (85.8 kJ/mol) than the meta-substituted one (111.3 kJ/mol) when hydroxide anion nucleophile attacks via SNAr.46 Interestingly, the concurrent degradation of the benzyltrimethylammonium cation functional group for compounds 8 and 9 proceeded almost at the same rate as aryl ether bond cleavage (Figure 10b). Both compounds 8 and 14 contain a benzyltrimethylammonium group at ortho position to the aryl ether bonds. However, the incorporation of benzyltrimethylammonium group into compound 14 did not cause any cleavage of the aryl ether bond at room temperature (Figure 10a). After treatment with 4 equiv of NaOCH3 for 20 h, compound 14 still had 100% of remaining aryl ether bond, whereas compound 8 degraded completely. This result strongly suggests that the electron-withdrawing effects are stronger for the sulfone linkage in comparison to the benzyltrimethylammonium functional group. Thus, we believe that the sulfone moiety is the primary cause for aryl ether bond cleavage in alkaline media. Although benzyltrimethylammonium is an electron-withdrawing group, a methylene spacer exists between the positively charged nitrogen cation and aromatic ring, thus exerting less influence on the instability of aryl ether bond than directly connected sulfone linkage. Byproduct analysis of the sulfone-containing model compounds (1, 2, 3, 8, and 9) revealed that OCH3− attacked exclusively at the C−O bond adjacent to the sulfone-linked aromatic ring (routes 1 and 3 of Figures 9a and 9b). The data from GC-MS, 1H, 13C, and 2D-NMR spectra showed that 1methoxy-4-(phenylsulfonyl)benzene and phenol derivatives were the only byproducts formed as a result of OCH3− attack at aryl ether bond for compounds 1, 2, 3, 8, and 9 (see Figures S25−S53). These results further confirm that the sulfone linkage is the primary factor in promoting OCH3− attack at the aryl ether bonds via SNAr. PPO Model Compound Degradation Study. Since small molecule model compounds 10 and 14, which resemble the aryl ether-containing PPO structure, did not degrade at room temperature in Figure 10, we investigated their stability further at higher temperature 100 °C for a longer period of time. As shown in Figure 11, the C−O bond of unfunctionalized compound 10 remained almost intact (with 90% remaining aryl ether bonds after 792 h). However, the C−O bond of compound 14 fully degraded in less than 6 h. The results of Figure 11 support that aryl ether bond cleavage is accelerated

for compounds 8, 9, and 14 in Figure 10b was calculated based on the change in integral values of the CH3 peaks of trimethylammonium relative to 18-crown-6. For model compounds 1 and 10, which do not contain any substituent, it was found that 1 degraded rapidly even at room temperature (18% aryl ether bonds remained after 20 h; see Figure 10a), whereas 100% of aryl ether bonds remained for compound 10. This evidence clearly indicates that the electronwithdrawing sulfone linkage of compound 1 enhances aryl ether bond cleavage in alkaline media. This result is also consistent with our previous observations that the sulfone-containing pristine PSU degraded in alkaline media, but the pristine PPO did not (see Table 1). When we compared the stability of aryl ether bonds of the methyl-substituted model compounds 2 and 3 with that of unfunctionalized 1, we found that both compounds 2 and 3 were more stable than compound 1 (about 60% remaining aryl ether bonds for both 2 and 3, Figure 10a). This result suggests that the incorporation of a methyl group inhibits the nucleophilic attack of methoxide anion at the neighboring aryl ether bonds. Because both ortho-substituted compound 2 and meta-substituted compound 3 have a similar degradation rate in Figure 10a, we suggest that the enhanced aryl ether bond stability compared to unsubstituted compound 1 is due to the inductive effect of electron-donating methyl group rather than increased steric hindrance from the methyl substitutions. Introduction of an electron-withdrawing group, such as benzyltrimethylammonium in compounds 8 and 9, increased the rate of aryl ether bond cleavage in comparison to unsubstituted compound 1. This observation reflects our polymer degradation studies in which PSU-CH2Cl backbone degraded faster in alkaline media than pristine PSU (see Table 1). We additionally observed that benzyltrimethylammonium at the ortho position of aryl ether bond enhanced the degradation rate the C−O bond compared to that with meta position. As shown in Figure 10a, compound 8 with ortho-substituted benzyltrimethylammonium degraded completely (0% remaining aryl ether bonds) whereas compound 9 with metasubstituted benzyltrimethylammonium had 7% remaining aryl ether bonds after 20 h. The accelerated cleavage of aryl ether bonds for ortho-substituted benzyltrimethylammonium compared to the corresponding meta-substituted one was suggested by a density functional theory calculation study: the barrier height of the C−O degradation for ortho-substituted model H

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Figure 11. Percentage of aryl ether bonds remaining for model compounds 10 and 14 after treatment with 4 equiv of NaOCH3 in DMSO-d6/CD3OD (20:1, v/v) at 100 °C for 792 h. Note that compound 14 completely degraded within 6 h.

when an electron-withdrawing functional group, such as benzyltrimethylammonium, is in close proximity even though is separated from aromatic ring by a methylene spacer. The byproduct analysis of compound 14 revealed demethylated 14 and a higher-molecular-weight byproduct that could not be identified (Figures S67 and S68). This suggests that upon aryl ether bond cleavage, the resulting initial byproducts reacted further generating complex byproducts. We suspect that this formation of higher-molecular-weight byproduct may have caused the broadening of PDI when PPO-CH2Br was treated under condition ii (Table S6 and Figure S12). To investigate aryl ether bond stability of PPO derivatives further, compound 18 in which the trimethylammonium cation is separated from the aryl ether bond by a six-carbon alkyl spacer was prepared (Figure 12). Compounds 14 and 18 were

Figure 13. Percentage of (a) aryl ether bonds and (b) QA functional groups remaining for model compounds 14 and 18 after treatment with 4 equiv of NaOCH3 in DMSO-d6/CD3OD (20:1, v/v) at 80 °C for 48 h.

in close proximity. However, when the benzyltrimethylammonium group is positioned further away as with compound 18, the substituent has little effect on the stability of aryl ether bond and the C−O bond could remain unaffected. It was interesting to note, however, that the QA functional groups of compounds 14 and 18 degraded at a similar rate (Figure 13b). From byproduct analysis, it was found that the QA functional group from compound 14 underwent SN2 degradation by attack at the N-methyl groups in addition to the formation of unknown higher-molecular-weight byproduct (Figures S64−S67), whereas compound 18 underwent both SN2 substitution and E2 β-Hofmann elimination (Figures S72− S75). In our previous report, we showed that benzyl-QA groups degrade at a faster rate than alkyl-tethered QA groups in aqueous solutions and alkyl-tethered QAs do not undergo a significant amount of degradation even under boiling alkaline aqueous conditions.7 The difference in alkaline stability of alkyltethered QAs from the previous study is due to the use of different solvents in degradation study (i.e., in D2O vs DMSOd6). While OH− in protic solvent like D2O is highly solvated and results in diminished strength of basicity, CH3O− in the current study cannot be solvated effectively in polar aprotic solvent DMSO (with a small amount of methanol; 20:1 for v/v ratio of DMSO-d6 and methanol-d4). This difference in basicity is also reflected in the dependence of pKa values of H2O (or CH3OH) on different solvents: H2O has pKa of 15.7 and 31.2 in H2O and DMSO, respectively, and CH3OH has pKa value of 15.5 and 27.9 in H2O and DMSO, respectively. Anion (OH− and CH3O−) in H2O is significantly less basic than that of DMSO; thus, CH3O− in DMSO could have enhanced reactivity

Figure 12. Chemical structures of small molecule model compound 18 and model polymers PPO-CH2QA, PPO-(CH2)6Br, and PPO-(CH2)6QA.

treated with 4 equiv of NaOCH3 under identical conditions at 80 °C. The degradation rate of compound 14 was slower at this temperature compared with the 100 °C study, allowing us to monitor the progressive degradation of the C−O bond: 20% of the aryl ether bond still remained after 48 h at 80 °C (Figure 13a). In contrast, the aryl ether bond of compound 18 remained almost intact (96% remaining after 48 h, Figure 13a). Similar to the results of Figure 11, the C−O bond of compound 14 degraded at a faster rate because of the presence of an electron-withdrawing group (i.e., benzyltrimethylammonium) I

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alkaline stability test results of small molecule model compounds at 80 °C, it was found that the aryl ether bond degrades more rapidly when it is in close proximity to an electron-withdrawing group (e.g., compound 14 degraded rapidly while compounds 10 and 18 remained stable). Additionally, the byproduct analysis of compound 14 revealed unknown higher molecular weight compounds, likely a result from side reactions after degradation. From the investigation of polymer systems, it was discovered that both PPO-CH2QA and PPO-(CH2)6QA had decreases in molecular weight after condition iv in DMSO at 80 °C. Pristine PPO remained stable while the data from PPO-CH2Br were inconclusive after treatment with condition ii in THF. We suspect that some degradation followed by side reactions may have occurred for PPO-CH2Br, in a similar way as with small molecule model compound 14. After treatment of bromohexyl-tethered PPO(CH2)6Br of Figure 12 with condition ii in THF, we also observed a sharp decrease in Mn from 28.8 to 16.8 kg/mol (Figure 14). The overall combined data suggest that PPOrelated polymers undergo degradation in alkaline media, but it is still unclear about the extent of degradation that may occur under alkaline-mediated electrochemical device operating conditions. Computational Degradation Study of Small Molecule Model Compounds. The experimental degradation data above demonstrate differences in relative stability of aryl ether bond of polymer backbones. To gain mechanistic insight into their stability difference, we investigated the effect of substituents on the stability of aryl ether bonds using density functional theory (DFT) simulations. Among model compounds in Figure 8, we selected model compounds 1, 2, 8, and 14 and computed their energy barriers required for the cleavage of C−O bond. As shown in Figure 15 as well as Figure S80 and Scheme S19, the degradation of aryl ether bond occurs via nucleophilic

toward electrophilic sites, accelerating simultaneous SN2 and E2 degradations of the QA group in compound 18. Representative polymeric model compounds of PPO were prepared and are shown in Figure 12. PPO-CH2QA and PPO(CH2)6QA are the polymer analogues of compounds 14 and 18, respectively. PPO-CH2QA and PPO-(CH2)6QA were treated with 4 equiv of NaOCH3 in DMSO-d6/CD3OD (20:1, v/v) at 80 °C for 72 h (termed condition iv). However, both polymers precipitated from the solution during this 72 h period, making NMR an unsuitable characterization technique for monitoring degradation. Thus, the polymer solutions were poured into an acidic aqueous solution after 72 h period instead, and the soluble fraction of the recovered polymers was analyzed with GPC. As shown in Figure 14 as well as Figure

Figure 14. Changes in Mn of PPO-(CH2)6Br, PPO-(CH2)6QA, and PPO-CH2QA after alkaline treatments (conditions ii and iv). The far left bars are Mn values of polymer before alkaline treatment. Note: the Mn values for PPO-(CH2)6QA and PPO-CH2QA after alkaline treatment are for only the soluble fractions collected.

S78 and Table S12, both PPO-CH2QA and PPO-(CH2)6QA showed a significant reduction in Mn after alkaline treatment of condition iv in comparison to those of precursor polymers, PPO-CH2Br and PPO-(CH2)6Br. This molecular weight data strongly suggest that the cleavage of aryl ether bonds has occurred for both PPO-CH2QA and PPO-(CH2)6QA. Regardless of the presence or absence of a long alkylene spacer between QA and PPO aromatic ring, it appears that both backbone degradation (via C−O bond cleavage) and insoluble gel formation (via cross-linking of polymer chains) occur simultaneously under these alkaline conditions. At this moment, it is not clear why the aryl ether bond degradation result of PPO-(CH2)6QA is inconsistent with the aryl ether stability data of small molecule compound 18 as discussed in Figure 13a. It should be emphasized, however, that the molecular weight information for PPO-CH2QA and PPO(CH2)6QA after alkaline treatment was obtained only from the soluble fractions of the recovered materials. A major portion of the recovered polymers was insoluble in common organic solvents after treatment of condition iv. Therefore, the molecular weight information on PPO-CH2QA and PPO(CH2)6QA shown in Figure 14 may not accurately represent information from the entire polymer batch. Overall, the interpretation of the chemical stability of PPOrelated polymer materials still remains a challenge due to insolubility of the polymers after alkaline treatment. From the

Figure 15. Free energy profiles of cleavage of aryl ether bonds in model compounds 2 and 8.

aromatic substation reaction in which the formation of the Meisenheimer complex intermediate by nucleophilic attack at the aromatic ring is the rate-determining step. Thus, the barrier height for this transition state should correlate with the experimental degradation rate: the higher the barrier is, the slower the degradation rate is. The barrier height of aryl ether bond cleavage was 7.8, 10.6, and 5.4 kcal/mol for each compound 1, 2, and 8, respectively (Figure 15 and Figure S80). J

DOI: 10.1021/acs.macromol.5b02550 Macromolecules XXXX, XXX, XXX−XXX

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The trend of simulated C−O bond stability follows the same order of the model compounds’ experimental data shown in Figure 10a (stability: 2 > 1 > 8). Furthermore, model compound 14 was found to have 18.4 kcal/mol for barrier height of aryl ether bond cleavage. This value is significantly higher than that of sulfone-containing derivative 8 (5.4 kcal/ mol), which clearly supports our experimental conclusion that the absence of sulfone-linkage dramatically enhanced the stability of aryl ether bond (Figure 10a and Figure S80).

AUTHOR INFORMATION

Corresponding Authors

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from Rensselaer Polytechnic Institute (startup for C.B., Slezak Felowship for A.D.M., and School of Science Summer Undergraduate Research Fellowship for S.E.T.) and NSF (DMR 1506245 and CHE 1534289 for C.B.) is greatly appreciated. Y.-K.C. also acknowledges financial support from the Ministry of Economy, Trade and Industry (METI) of Japan.

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CONCLUSION A systematic alkaline stability study was conducted by analyzing molecular weight properties of aromatic polymers with diverse backbone structures. Among unsubstituted aromatic polymers studied, PSU showed the poorest chemical stability when treated with KOH and NaOCH3 solutions. Installing electronwithdrawing substituents to PSU accelerated degradation further in alkaline media. Aromatic polymers without aryl ether bonds (e.g., PB, PBA, SEBS, and SEBS-CH2Cl) did not show any sign of backbone degradation. Although PBA contains two electron-withdrawing substituents (CF3 and Br), it remained stable under all alkaline conditions investigated because of absence of potentially labile aryl ether bond. Small molecule model compound studies were consistent with polymer studies, such that aryl ether bonds readily degraded in alkaline media when a strong electron-withdrawing functional group such as sulfone linkage is in close proximity. Unsubstituted PPO and its small molecule PPO-analogue remained chemically stable under alkaline treatments. However, possible degradations and side reactions were observed for substituted PPO materials, such as with PPO-CH2Br, PPO(CH2)6Br, PPO-CH2QA, and PPO-(CH2)6QA. Unfortunately, because of poor solubility issues of the alkaline treated polymers and difficulties with byproduct analysis, we cannot determine how the aryl ether bonds of PPO materials were cleaved in alkaline media. The computational simulation study of model compounds confirmed the effect of neighboring substituents on the stability of aryl ether bonds observed by experiments. The presence of strong electron-withdrawing group, such as sulfone linkage and benzyltrimethylammonium group, reduced the activation energy barrier for cleavage of aryl ether bond. Between sulfone linkage and benzyltrimethylammonium group, the former influences greater because it is directly attached aromatic ring while the latter is separated by a methylene spacer. The results from this study suggest that all carbon-based polymer backbone structures (i.e., polymers not bearing aryl ether bonds) are the most promising repeating unit structures for long-term alkaline stability in various AEM electrochemical energy devices.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b02550. Synthesis of polymer materials and small molecules; general procedures for alkaline stability tests; GPC and NMR data for alkaline stability studies; details of DFT calculations (PDF) K

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