Alkaline Stability of Poly(Phenylene Oxide) Based Anion Exchange Membranes Containing Imidazolium Cations

Two modiﬁed imidazole bases (1-heptyl-2-methyl-imidazole and 1-dodecyl-2-methyl-imidazole) were synthesized, each with a long alkyl chain (7 or 12 carbon) attached to the N-3 position. Anion exchange membranes (AEMs) were prepared with imidazolium cations derived from these bases that were either grafted directly onto the benzyl position of poly(phenylene oxide) (PPO) or afﬁxed to the PPO using a hexyl spacer chain. First, we reacted 1-methylimidazole with brominated PPO and with PPO with a hexyl spacer chain. By comparing the alkaline stability of the resultant AEMs, we demonstrated that a hexyl spacer chain could improve AEM alkaline stability substantially. Second, by comparing the alkaline stability of PPO-based AEMs obtained by the reaction of brominated PPO with 1-methylimidazole and 1,2-dimethylimidazole, we showed that C-2-substituted (with a methyl group) imidazolium-based AEMs were much more stable in alkali than C-2-unsubstituted imidazolium-based AEMs. Finally, by investigating the alkaline stability of AEMs synthesized by reaction of brominated PPO with 1,2-dimethylimidazole and modiﬁed imidazole bases (with 7 or 12 alkyl carbon chain at the N-3 position), we demonstrated that the increase in length of a long alkyl chain afﬁxed to the N-3 position decreases the alkaline stability of the resultant imidazolium-based AEMs. © The 2016. The resul- correlation conﬁrmed the successful synthesis of the modiﬁed imidazole bases and of the AEMs. The IECs of the AEMs synthesized were measured using Volhard titration. The IEC for P1 and P12 was 1.66 ± 0.02 mmol/g and 2.04 ± 0.02 mmol/g, very close to the theoretical IEC; the IEC was signiﬁcantly lower for PC7 and PC12 (0.78 and 0.92 mmol/g) because the bulkier imidazolium bases were less reactive toward the brominated polymer. A similar trend was observed for the AEMs prepared with the pendant chain. The alkaline stability of imidazolium-based AEMs (with and without pendant chains) was evaluated using two independent measure- ments to assure the conclusions were deﬁnitive. We measured the change in IEC after immersion in 1 M KOH at 60 ◦ C for up to 810 mins. By comparing the fraction of retained IEC of P1 and PX1 (27% and 70%), we conﬁrmed that grafting a long alkyl chain as a spacer between PPO backbone and imidazolium cation could increase the alkaline stability to great extent. By comparing the the fraction of retained IEC of P1 and P12 (27% and 74%), we demonstrated that grafting a methyl group at the C-2 site could increase alkaline stability of the imidazolium-based AEMs. By comparing the the fraction of retained IEC of PX12, PXC7 and PXC12 (88%,

There is a growing interest in using anion exchange membranes (AEMs) as separators in alkaline membrane fuel cells (AMFCs) 1,2 and in other energy conversion and storage systems such as redox flow batteries (RFBs), [3][4][5] alkaline water electrolyzers (AWEs) 6,7 and reverse electrodialysis (RED) cells. 8 The most commonly used AEMs reported in the literature contain quaternary ammonium groups, primarily in the form of benzyl trimethylammonium cations. 6, However, it had been shown that quaternary ammonium-based AEMs are sensitive toward Hofmann elimination 10,31 and direct nucleophilic elimination reactions 32,33 that result in loss of ion exchange capacity (IEC) and ionic conductivity. To resolve the stability problem inherent to quaternary-ammonium-group-containing AEMs, several alternative cations, including imidazolium, 31,34,35 benzimidazolium, 36 guanidinium, 37 phosphonium [38][39][40] and metal cations 41 have been proposed and investigated. Imidazolium-based AEMs have drawn researchers' interests mainly because of their high hydroxide-ion conductivities. 42 However, it has been reported that imidazoliumbased AEMs degrade under alkaline conditions at temperatures below 80 • C. Ye and co-workers investigated the degradation mechanism of imidazolium-based AEMs at several relative humidities, temperatures and pH. They found that imidazolium-based AEMs were chemically stable at low temperatures, but relatively unstable at higher temperatures in alkaline conditions (degradation commences at 80 • C when [KOH] >1 M). They postulated the following degradation mechanism: Ring-opening of the imidazolium ring was triggered by the nucleophilic attack of OHon the imidazolium ring at the C-2 position (alpha carbon with respect to both nitrogen atoms). The degradation products were carboxylates and alkoxide salts, and the imidazolium ring was cleaved. 31 Wang and co-workers investigated the activation free energy for the hydroxide-induced nucleophilic substitution reaction on alphacarbon-methyl-substituted imidazolium cations (trimethylimidazolium) and alpha-carbon-methyl-unsubstituted imidazolium cations (dimethylimidazolium) using DFT. They found that the activation free energy barrier (57.1 kJ/mol) for the nucleophilic reaction of the trimethylimidazolium cation with hydroxide ion exceeded the corresponding energy barrier for the alpha-carbon-methyl-unsubstituted dimethylimidazolium cations by about 43 percent. Their results suggested that the alpha-carbon-methyl-substituted imidazolium cations were more stable under alkaline conditions than imidazolium cations containing hydrogen in the alpha position. This was attributed to hyper conjugation between the methyl group at the alpha-carbon position and the imidazole ring, and due to the steric effect of the methyl group. 28 Lin and coworkers grafted a methyl group onto the C-2 position of imidazole (alpha carbon with respect to both nitrogen atoms) to improve the alkaline stability of the imidazolium cation. They reported 48% degradation of C-2 unsubstituted 3-ethyl-1-methylimidazolium bromine exposed to 1 M KOH for 60 hours, while the C-2 substituted 3-ethyl-1,2-dimethylimidazolium bromine did not degrade even in 2 M KOH after exposure for 168 hours. 34 These experimental findings supported the modeling results of Wang and co-workers. 28 Marino and co-workers tested the half-life of imidazolium salts in 6 M sodium hydroxide, and they reported that the half-life of the C-2 unsubstituted 1-methyl-3-octyl-imidazolium was too short to measure at 60 • C, and that the half-life of the C-2 substituted 1,2,3trimethylimidazolium was also too short to measure at 160 • C. 43 So the question of whether grafting a methyl group to the C-2 position of the imidazole ring would improve the alkaline stability of the resultant cation remains controversial.
Gu and co-workers investigated the alkaline stability of C-2 substituted (methyl group) imidazolium cations with various groups attached to the N-3 atom (nitrogen atom that is located farthest from the polymer backbone). They reported 36.4% degradation of 1,2,3trimethylimidazolium iodide exposed to 2 M KOH solution at 80 • C for 432 hours, while the corresponding degradation extent of 1,2dimethyl-3-butylimidazolium bromine was 3.2%. 35 This report suggested that attaching a long alkyl chain to the N-3 position would improve the alkaline stability of imidazolium cations. These prior results make it reasonable to postulate that imidazolium-based AEMs modified by grafting alkyl chains to the C-2 and N-3 positions have the potential to exhibit higher alkaline stability than traditional quaternaryammonium-based AEMs.
Poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) has been extensively studied and shown to be a mechanically and chemically stable backbone for the synthesis of alkaline stable AEMs. 10,14,36,44 Hibbs 45 has shown that poly(phenylene)-based AEMs with quaternary ammonium cations affixed to the benzene rings in the backbone using a six-carbon alkyl chain spacer were considerably more stable than those prepared with benzyl trimethylammonium cations attached directly to the backbone. When these two types of AEMs were immersed in nitrogen-degassed 4 M KOH at 60 • C for 14 days, only a minimal (ca. 5%) loss in IEC was observed in the former case while a 21% loss in IEC was observed in the latter case. This result suggested that the introduction of alkyl spacers separating the cation F825 from the backbone enhanced the alkaline stability of AEMs based on polyphenylene backbones. Dang 46 and co-workers functionalized a PPO backbone with quaternary ammonium groups via flexible alkyl spacers and compared this AEM with to one prepared by reacting PPO directly with quaternary ammonium groups. They reported the quaternary ammonium protons in the 1 H NMR spectrum stayed unchanged when the AEM with spacer was immersed in 1 M NaOH for for 192 hours. However, in the case of the AEM with quaternary ammonium group attached directly to PPO, the proton on quaternary ammonium degraded by 84% after 192 hours in 1 M NaOH. Parrondo and co-workers 47 investigated AEMs prepared with hexyl trimethylammonium cations, separated from the PPO backbone by a hexyl pendant chain. They reported that AEMs prepared with the pendant chains exhibited a 33% loss in IEC after 30 days in 1 M KOH at 60 • C. The degradation rate was very similar for AEMs prepared without pendant chains. This discrepancy in results suggests that the degradation of AEMs prepared with spacers separating the backbone from the cation involves complex interplays between the backbone, the cations affixed and the experimental conditions employed.
The objectives of the current work were: 1) to clarify the role of flexible alkyl spacers in enhancing the alkaline stability of imidazolium-based AEMs; 2) to investigate the effect of grafting alkyl chains at the C-2 and N-3 positions of imidazolium cations on the degradation rate and mechanism (upon exposure to alkali) of the resultant imidazolium-based-AEMs, To this end, AEMs were prepared by reacting several imidazole bases (prepared with methyl and alkyl groups attached to C-2 and N-3 position in imidazole ring) directly with brominated PPO (BrPPO; brominated at the benzyl position of PPO) or with brominated PPO containing the bromide leaving group at the end of a hexyl pendant chain affixed to the PPO backbone (BrPPO-C6Br). The resultant imidazolium-based AEMs were studied to evaluate and compare the alkaline stability of several variants of AEMs: a) those prepared with and without a hexyl pendant chain separating the backbone and cation group, b) those prepared with and without methyl groups grafted onto the C-2 position, and c) those prepared with and without alkyl pendant chains of different lengths grafted onto the N-3 position of the imidazolium group. We functionalized the six-carbon spacer onto PPO using a three steps synthesis process: 1) Friedel-Crafts acylation using 6-bromo-1-hexanoyl chloride; 2) reduction of the ketone in the benzyl carbon attached to the aromatic ring, to improve the alkaline stability; 41 and 3) quaternization by reaction with 1-methylimidazolole or with modified imidazole bases with methyl and/or alkyl groups grafted on C-2 and/or N-3 positions. The alkaline stability was evaluated with time.

Synthesis of 1-heptyl-2-methyl-imidazole (2MI-C7
).-A mixture of 2-methylimidazole (2 g) and of 1-bromoheptane (3.82 mL) was dissolved in 20 mL of 1-methyl-2-pyrrolidinone (NMP) and stirred at 60 • C for 24 h (see Figure 1 for details of the reaction). The product was recovered after evaporation of the solvent and unreacted reagents in an oven at 70 • C overnight. 1 H NMR spectroscopy and 2-D correlation spectroscopy (COSY) were used to verify that the reaction yielded the desired product, and to test the purity of the product (see Figures S1 & S2).

Synthesis of 1-dodecyl-2-methyl-imidazole (2MI-C12
).-A mixture of 2-methylimidazole (2 g) and 1-bromoheptane (5.86 mL) was dissolved in 20 mL of NMP and stirred at 60 • C for 24 h (see Figure 1 for details of the reaction). The product was recovered by removing the solvent in an oven at 70 • C. 1 H NMR and COSY NMR spectroscopy were used to confirm that the reaction occurred as expected and to test the purity of the product (see Figures S3 & S4).
P12: BrPPO (0.5 g) and 1,2DMI (88 μL) were dissolved in 9 mL of NMP, stirred at 60 • C for 24 h, cast onto a glass plate (3.5 × 3.5 ) and the solvent dried in an oven at 70 • C for 12 h (see reaction scheme is in Figure 2 and resulting 1 H NMR spectrum in Figure S7).
PC7: BrPPO (0.5 g) and 2MI-C7 (0.18 g) were dissolved in 9 mL of NMP, stirred at 95 • C for 48 h, cast onto a 3.5 × 3.5 glass plate and the solvent evaporated in an oven at 70 • C for 12 h. (See reaction scheme in Figure 2 and 1 H NMR spectrum in Figure S8).
PC12: BrPPO (0.5 g) and 2MI-C12 (0.25 g) were dissolved in 9 mL of NMP, stirred at 100 • C for 48 h, cast onto a 3.5 × 3.5 glass plate and heated in an oven at 70 • C to dry the solvent. (Reaction scheme shown in Figure 2 and 1 H NMR spectrum of resulting AEM shown in Figure S9).

Addition of a six carbon pendant chain to poly(phenylene oxide): PPO-KC6Br
.-The addition of an alkyl pendant chain to PPO was carried out following a slight modification of the method described by Hibbs. 45 PPO (10 g) was dissolved in 500 mL of chlorobenzene in a round bottom flask. Once PPO was completely dissolved, the flask was placed in an ice bath and allowed to equilibrate for 30 mins. Subsequently, 19.1 mL of 6-bromohexanoyl chloride and 5 g of aluminum (III) chloride (Lewis acid catalyst) were added, the ice bath was removed and the mixture was allowed to react overnight. The resultant brominated polymer was precipitated in 2.5 L methanol, and purified by re-dissolution in chloroform and re-precipitation in methanol. The yield for the acylation reaction was approximately 25% and the degree of functionalization (DF) of the polymer was 0.31. The DF was determined from the 1 H NMR peak area (see 1 H NMR spectrum in Figure S10).

Reduction of the ketone group in PPO-KC6Br (PPO-C6Br
).-PPO-KC6Br (5 g) was dissolved in 400 mL of 1,2-dichloroethane. Once completely dissolved, trifluoroacetic acid (290 ml) and triethylsilane (30 ml) were added, the reaction mixture heated under stirring to 100 • C, and kept reacting under reflux conditions overnight. Subsequently, the polymer solution was washed with 500 mL of 1 M KOH until neutral pH, and poured into 2 L of deionized water and heated until the organic solvent evaporated. The solid precipitate was dried in a vacuum oven at 60 • C and purified by re-dissolution in chloroform and re-precipitation in methanol. The 1 H NMR spectrum of the resultant polymer confirming the reduction of the ketone group is shown in Figure S11. Further details of the synthesis and characterization of the polymer can be found in our previous paper. 47 Synthesis of imidazolium-based AEMs with a six-carbon alkyl spacer: PX1, PX12, PXC7 & PXC12.-PX1: PPO-C6Br (0.5 g) and 1MI (70 μL) were dissolved in 9 mL of NMP and stirred at 60 for 24 h, the solution was cast on a glass plate (3.5 × 3.5 ) and the solvent evaporated in an oven at 70 • C for 12 h. (See Figures 3 and S12 to see the synthesis scheme and 1 H NMR spectrum, respectively). PX12: PPO-C6Br (0.5 g) and 1,2DMI (88 μL) were dissolved in 9 mL of NMP and stirred at 60 • C for 24 h, the solution was cast on a glass plate (3.5 × 3.5 ) and the solvent evaporated in an oven at 70 • C for 12 h. (See Figures 3 and S13 to see the synthesis scheme and 1 H NMR spectrum, respectively).
PXC7: PPO-C6Br (0.5 g) and 2MI-C7 (0.17 g) were dissolved in 9 mL of NMP and allowed to react at 130 • C for 48 h. To get the thin-film membrane, the solution was cast onto a glass plate (3.5 × 3.5 ) and the solvent was evaporated in an oven at 70 • C for 12 h. (See Figures 3 and S14).
PXC12: PPO-C6Br (0.5 g) and 2MI-C12 (0.23 g) were dissolved in 9 mL of NMP, and heated at 110 • C for 48 h. The membrane was obtained by casting the polymer solution as described above (Figures  3 and S15).
Characterization of the AEMs.-The following NMR spectroscopy experiments were performed on a Bruker Avance 360 MHz NMR spectrometer: 1) 1 H NMR spectra (collected at 360 MHz) and 2) correlation spectroscopy (COSY). Additional details regarding the NMR experiments are presented in the electronic supplementary information (ESI) and in our previous papers. 22,47,48 Anion exchange capacity was determined using Volhard titration. The AEMs (dry weight of approximately 0.1 g) were immersed in 20 mL of 1 M sodium nitrate for 48 h. The amount of chloride ions exchanged was determined by back titration using 0.1 M potassium thiocyanate, after addition of 5 mL of 0.1 M silver nitrate. A control sample containing only 20 mL of 1 M sodium nitrate was also titrated F827 as described above. Iron (III) nitrate was used as end point indicator. The anion exchange capacity (for the chloride counter ion) was calculated using the following equation: Where IEC is the anion exchange capacity (mol/g), V C is the volume (mL) of 0.1 M potassium thiocyanate necessary to reach the equivalence point with the control sample, V the volume (mL) of thiocyanate required to reach the equivalence point with the AEM sample and W the dry weight (g) of the AEM sample. Additional details of the application of the method can be found in our previous papers. 3,6,10,14,47 Results and Discussion Figure 1 shows the scheme for the synthesis of modified imidazolium cation groups: 1-heptyl-2-methyl-imidazole (2MI-C7) and 1-dodecyl-2-methyl-imidazole (2MI-C12). The formation of 2MI-C7 was confirmed by the 1 H-NMR and 2D NMR (Correlation Spectroscopy; COSY) spectra as shown in Figures S1 and S2 (Electronic Supplementary Information; ESI). The 1 H NMR spectrum of 1-bromoheptane (in CDCl 3 , 350 MHz) revealed the presence of protons at chemical shifts of ca. 0.9 ppm (3H, triplet), 1.4 ppm (8H, overlapping of several peaks), 1.8 ppm (2H, multiplet) and 3.4 ppm (2H, triplet). The 1 H NMR spectrum of 2-methylimidazole revealed protons at chemical shifts of ca. 2.3 ppm (3H, singlet) and 6.8 ppm (2H, singlet). 49 The protons initially at 3.4 ppm (in the spectrum of 1bromoheptane) shifted, after reaction with 2-methylimidazole, to ca. 4.2 ppm. The disappearance of the proton peak at 3.4 ppm confirmed that the reaction of 2-methylimidazole to 2MI-C7 was complete (see Figure S1). 1 H-1 H COSY NMR spectroscopy ( Figure S2) showed a coupling between protons "4" and "3", but there were no couplings between protons "4" and protons "5" or "6", which were in the aromatic ring and were separated from protons "4" by a nitrogen atom. Please refer to the proton labels and chemical structure in Figure S2. Multiple protonproton couplings in the aliphatic chain "1-4" were also observed. 1 H NMR and COSY spectra thereby confirmed the formation of the desired product (2MI-C7).
The 1 H NMR spectrum of 1-bromododecane (CDCl3, 350 MHz) showed protons at chemical shifts of ca. 0.9 ppm (3H, triplet), 1.4 ppm (18H, overlapping of several peaks), 1.8 ppm (2H, multiplet) and 3.4 ppm (2H, triplet) ppm. As in the previous instance, the protons in 1-bromododecane that initially featured at a chemical shift of 3.4 ppm were altered after the reaction with 2-methylimidazole to a chemical shift of 4.2 ppm. The peak at 3.4 ppm that was initially present in the reactant disappeared, confirming that the reaction of 2-methylimidazole to 2MI-C12 was complete (see Figure S3). The corresponding COSY spectrum ( Figure S4) showed couplings between the protons in the aliphatic chain: "4" and "3", "3" and "2", and "2" and "1". No couplings of these protons were observed with any of the protons in the imidazole ring (the nitrogen atom present in-between impeded any possible couplings). The 1 H NMR and COSY spectra thereby confirmed the formation of the desired product (2MI-C12).
AEMs were synthesized by reacting the BrPPO with 1methylimidazole (1MI), 1,2-dimethylimidazole (1,2DMI), 1-heptyl-2-methyl-imidazole (2MI-C7) and 1-dodecyl-2-methyl-imidazole (2MI-C12). When 1MI was attached on the benzyl position of PPO, the imidazolium-based AEM (P1) was obtained. Following a similar scheme, imidazolium-based AEMs, P12, PC7 and PC12 were obtained. All these AEMs had the cations affixed directly to the benzyl position. Figure 2 shows the scheme for the synthesis of these imidazolium-based AEMs (P1, P12, PC7 and PC12). Note: The N-1 position in the imidazole base and the N-3 position in the imidazolium cation attached to the polymeric backbone are equivalent. The different numbers result from the naming conventions. In the imidazole base, the numbering starts from the nitrogen to which the alkyl group is attached (i.e. heptyl; N-1). The base therefore reacts at the N-3 position under this system. Once the imidazolium cation is be-ing formed, the numbering starts from the nitrogen attached to the backbone, which become the new N-1. BrPPO had a bromomethyl group (benzyl position) and these protons (peak "3") appear at a chemical shift of 4.33 ppm ( Figure S5). The degree of functionalization (DF, mol of bromomethyl groups per polymer repeat unit) obtained by comparison of the peak areas of protons "3" to the protons present in the aromatic rings of the backbone was 0.36. Details of the calculations employed to arrive at this value can be found in our previous papers. 10,47 The shift of the protons present initially in the bromomethyl group (peak "3", initially at 4.33 ppm) to higher frequencies (5.30 ppm) was attributed to the proximate presence of the positively charged cation and demonstrated the formation of the product (P1, see Figure S6). A small shift in the protons of type "6", from ca. 7 ppm to 7.65 ppm was also observed during the reaction with brominated PPO. A similar shift in the protons present in the aromatic ring of imidazole and those present in the bromomethyl groups was observed upon reaction of the other three imidazole bases (1,2DMI, 2MI-C7 and 2MI-C12) and BrPPO (see Figure S7, Figure S8 & Figure S9).
Friedel-Crafts acylation of 6-bromohexanoyl chloride was employed to add a six-carbon spacer to PPO, allowing the cation to be separated from the backbone in an attempt to minimize the possible electronic effects of the backbone/cation on the alkaline stability of the AEM. 45 The following AEMs were synthesized using this modified platform: PX1, PX12, PXC7 and PXC12. Figure 3 shows the synthesis scheme for imidazolium-based AEMs prepared with the spacer chain (PX1, PX12, PXC7 and PXC12). The grafting of the six-carbon pendant chain to the PPO backbone (to yield PPO-KC6Br) was confirmed through 1 H NMR spectroscopy (see Figure S10 under ESI). Peak "5" (3.0 ppm) corresponds to the protons attached to the carbon adjacent to the ketone group. Additional assessment of the presence of the ketone group in the benzyl position was performed using 13 C NMR spectroscopy in our previous paper. 47 The peak at 207 ppm corresponds to the carbon in the ketone group. 43 The 1 H NMR peak at 3.4 ppm (peak "9") corresponds to the protons in the bromomethyl group at the end of the pendant chain. The brominated polymer (PPO-KC6Br) had a degree of bromination (DF) of 0.31 mol of bromine per polymer repeat unit. This DF was selected to prepare AEMs with suitable IEC (of at least 0.7-1.0 mmol (chloride)/g) after addition of the bulky imidazolium bases used in this work.
Hibbs 45 pointed out that the ketone group in PPO-KC6Br promotes rapid AEM degradation in alkaline solutions. To increase the stability of our AEMs, the ketone group in PPO-KC6Br was reduced with triethylsilane in the presence of trifluoroacetic acid in 1,2-dichoroethane ( Figure S11 shows the 1 H NMR spectrum of the reduced polymer, PPO-C6Br). The protons in the aromatic rings functionalized with the pendant chain shifted from 6.1 ppm (peak "2") to ca. 6.0 ppm (peak "2") after reduction of the ketone group. Both peaks were present in the NMR spectrum of the reduced product and their areas were used to calculate the yield of the ketone reduction reaction (Formula: Area 2/ (Area 2 + Area 2 )). The yield for the ketone reduction reaction was between 95% to 98%.
The 1 H NMR spectrum of the product obtained from the reaction between 1-methylimidazole and PPO-C6Br is shown in Figure S12. The protons of the bromomethyl group (at 3.40 ppm) shifted to 4.19 ppm (peak "10") after reaction with the imidazole base. The protons in the aromatic ring of imidazole ("11") shifted from ca. 7.1 ppm to 7.82 ppm after reaction with PPO-C6Br, yielding PX1. Similar changes in NMR spectra were observed for PX12 ( Figure S13), PXC7 ( Figure  S14) and PXC12 ( Figure S15), and these observations confirmed the successful synthesis of the three AEMs.
The IECs of the different AEMs synthesized in this work are shown in Table S1. The theoretical IEC values were calculated from the DF of the corresponding brominated polymers (Formula: IEC (mmol/g) = 1000 × DF/ Equivalent Weight (g/mol)). 47 The theoretical IEC values for P1, P12, PC7 and PC12 were 1.69 mmol/g, 2.15 mmol/g, 1.45 mmol/g and 1.31 mmol/g, respectively. The theoretical IEC values decreased when larger bases were used as a result of the increasing molecular weight of the polymer repeat unit. The experimental IEC values for all AEMs were obtained by the Volhard titration method. The details of the method can be found in our previous paper. 10 The IEC for P1 was very close to the theoretical IEC, confirming that the reaction yield was almost 100%. For P12, PC7 and PC12, the yields for the corresponding reactions were 95%, 54% and 70%, respectively (see Table S1). The long alkyl chain attached to the N-3 position of the imidazole ring made the resultant base bulkier than 1-methylimidazole, and, consequently, more difficult to react with the PPO-C6Br. Similarly, the yields for the reactions between the bases (1MI, 1,2DMI, 2MI-C7 and 2MI-C12) and PPO-C6Br were 93%, 97%, 57% and 72%, respectively. The reasons for the low conversions with 2MI-C7 and 2MI-C12 were the same as described earlier for BrPPO.
Our motivation in studying the alkaline stability of the above AEMs was three-fold. Firstly, we wanted to verify if the finding of Hibbs 45 that the presence of a long alkyl spacer between the PPO backbone and the quaternary ammonium group would improve the alkaline stability of AEM to a great extent would hold for imidazole-based cations in conjunction with the PPO backbone. To verify this finding, we reacted the same imidazolium cation such as 1MI with BrPPO and PPO-C6Br. Then, we compared the alkaline stability of these two AEMs. The second finding that we wanted to verify was that of Lin and coworkers, 34 who proposed that C-2 substituted (with a methyl group) imidazolium-based AEMs were much more stable in alkali when compared with C-2 unsubstituted imidazolium-based AEMs. To verify this finding, we compared the alkaline stability of AEMs made using C-2 substituted imidazole (1,2DMI) and C2 unsubstituted imidazole (1MI). In the third instance, we wanted to verify and extend the work of Ye and coworkers, 31 who reported that N-3 substituted (by adding a long chain alkyl group) imidazolium-based alkaline anion exchange polymerized ionic liquid (poly(1-[(2-methacryloyloxy)ethyl]-3-butylimidazolium hydroxide) is more stable in alkali than N-3 unsubstituted imidazolium-based AEM. We wanted to further investigate if increasing the length of long chain alkyl group in the N-3 position would further enhance the alkaline stability of the resultant AEM. By reacting the bases (1,2DMI, 2MI-C7 and 2MI-C12) with BrPPO, we studied the role played by the long chain alkyl group in the N-3 position on alkaline stability. Figure 4 shows the results of our AEM alkaline stability evaluation experiments. From the experiment results in Figure 4, we could demonstrate three trends. First, the IEC fractions retained after 810 minutes exposure to 1 M KOH at 60 • C were 27% and 70%, for P1 and PX1, respectively. The AEMs prepared with a spacer chain still degraded under these conditions, but the presence of the six-carbon spacer grafted between the PPO backbone and the 1MI cation, substantially improved the stability. This result verified the finding of Hibbs 45 that an alkyl spacer between the BrPPO and cation group would sterically shield the beta-hydrogens and thereby increase stability. Besides, when there was a nonmethyl substitution on the nitrogen atom, there was an increased susceptibility to S N 2 attack. Second, as shown in Figure 4, the IEC fractions retained after 810 minutes of exposure to 1 M KOH for P1 and P12 were 27% and 74%, respectively. The C-2 carbon of 1-methylimidazolium is the most likely place to be attacked by the hydroxide ion. 28 When a C-2 methyl group was grafted, the alkaline stability of the resultant imidazolium cation increased as a consequence of enhanced steric hindrance toward OH − ion attack at the C-2 position of the imidazolium ring. Third, the IEC fractions retained for PX12, PXC7 and PXC12 were 88%, 17% and 11%, respectively. This result demonstrated both that the N-3 substituent lead to worse alkaline stability, and that increasing the length of N-3 alkyl chain did not improve the alkaline stability. Sean and co-workers 50 reacted trimethylamine and N,N-dimethylhexylamine with chloromethylated polystyrene to make two types AEMs. The AEM of N,N-dimethylhexylamine attached to the polystyrene degraded faster under alkaline conditions than trimethylamine attached to the polystyrene. Our alkaline stability test for N-3 substituted with long alkyl chain was consistent with the finding of Sean and coworkers. Even with an alkyl spacer between polymer backbone and imidazolium cation, the long alkyl chain in the N-3 position of imidazolium cation significantly decreased the alkaline stability of the AEM. So the long alkyl chain at the N-3 position of the imidazolium cation decreased both the IEC and the stability of imidazolium-based AEMs. Figure 5 shows the 1 H NMR spectra of PC7 before and after exposure to 1 M KOH (at 60 • C) for 6 hours. After immersion in alkali, we observed the appearance of a new peak [6 * ] at a chemical shift of 3.93 ppm (see red rectangular frame in Spectrum1 in Figure 5). Ye and coworkers 31 have observed that imidazolium-based anion exchange ionic liquids treated with alkali experienced hydroxide ion attack at the C-2 carbon, resulting in a ring-opening reaction. The protons labeled as 6 and 6 (Spectrum 2 in Figure 5) corresponding to the aromatic protons in the imidazolium ring will disappear in the case of a ring opening reaction. The protons will be in an aliphatic chain in the final product [6 * and 6 * ] and will have considerably lower chemical shifts (ca. 4 ppm). We have observed all these changes in our spectra, confirming that the ring opening reaction occurred in PC7 upon exposure to alkali. We have also observed that the peak area for the protons in the methylene bridge (peak 8, linking the backbone to the base) decreased after exposure to alkali. This observation also confirmed the loss of cationic sites and reaffirmed the ring opening reaction in the imidazolium group. Figure 6 shows the 1 H NMR spectrum for PXC12, before and after exposure to 1 M KOH at 60 • C for 6 hours. A new peak (peak 11 * at ca. 4 ppm, in red rectangular frame in the figure) was observed after this AEM was exposed to alkali. The presence of this new peak and the decrease in the area of peak 10 confirmed that there was degradation of the AEM. A ring-opening degradation mechanism of imidazolium cation was posited. Figure 7 shows the proposed degradation mechanism based on the 1 H-NMR spectrum ( Figure 6). The degradation process was triggered by the nucleophilic attack of hydroxide ion at the C-2 position of the cation. A covalent bond was formed between the hydroxide ion and the C-2 carbon (alpha carbon with respect to both nitrogen atoms). Subsequently, there was a ring-opening reaction that resulted in loss of cation sites. The covalent bond between C-2 and N-1 or N-3 was then broken, and, after a rearrangement reaction, formyl groups (-CHO) were formed (steps 2 and 3).

Conclusions
A series of imidazole bases (1MI, 1,2DMI, 2MI-C7 and 2MI-C12) were employed in the synthesis of AEMs based on PPO backbones. The cations were: 1) directly grafted onto the benzyl position of PPO and 2) attached at the end of a hexyl spacer chain previously grafted onto the PPO backbone using Friedel-Crafts acylation. The latter approach was attempted to minimize the electronic effects between the backbone and cation. The AEMs with cations in the benzyl position were synthesized by reaction of the brominated polymers with 1MI, 1,2DMI, 2MI-C7 and 2MI-C12 to yield the following AEMs: P1, P12, PC7 and PC12.
The AEMs with the six-carbon spacer were synthesized by Friedel-Crafts acylation of 6-bromo-1-hexanoyl chloride on PPO, followed by reduction of the ketone and reaction with the imidazole bases. The reaction with 1MI, 1,2DMI, 2MI-C7 and 2MI-C12 resulted in the following AEMs: PX1, PX12, PXC7 and PXC12. 1 H NMR and 2D correlation spectroscopy (COSY) confirmed the successful synthesis of the modified imidazole bases and of the AEMs. The IECs of the AEMs synthesized were measured using Volhard titration. The IEC for P1 and P12 was 1.66 ± 0.02 mmol/g and 2.04 ± 0.02 mmol/g, very close to the theoretical IEC; the IEC was significantly lower for PC7 and PC12 (0.78 and 0.92 mmol/g) because the bulkier imidazolium bases were less reactive toward the brominated polymer. A similar trend was observed for the AEMs prepared with the pendant chain.
The alkaline stability of imidazolium-based AEMs (with and without pendant chains) was evaluated using two independent measurements to assure the conclusions were definitive. We measured the change in IEC after immersion in 1 M KOH at 60 • C for up to 810 mins. By comparing the fraction of retained IEC of P1 and PX1 (27% and 70%), we confirmed that grafting a long alkyl chain as a spacer between PPO backbone and imidazolium cation could increase the alkaline stability to great extent. By comparing the the fraction of retained IEC of P1 and P12 (27% and 74%), we demonstrated that grafting a methyl group at the C-2 site could increase alkaline stability of the imidazolium-based AEMs. By comparing the the fraction of retained IEC of PX12, PXC7 and PXC12 (88%, 17% and 11%), we discovered that substitution of long alkyl chains at the N-3 position would not only decrease the IEC of imidazolium-based AEMs but also lower the alkaline stability of imidazolium-based AEMs.
We also used 1-D NMR spectroscopy to probe the AEM structure to unearth any signs of AEM degradation after exposure to alkali. The predominant degradation process was posited the opening of the imidazole ring. Although more studies are needed to confirm the findings, it is suggested that substituting all the hydrogen atoms on the imidazolium ring will improve the alkaline stability of imidazolium-based AEMs, provided that an appropriate backbone and cation-grafting strategy (use of spacers) is employed.