Elastomeric anion exchange membranes (AEMs) were prepared by acid-catalyzed Friedel–Crafts alkylation of the polystyrene block of polystyrene-b-poly­(ethylene-co-butylene)-b-polystyrene (SEBS) using bromoalkylated tertiary alcohols and triflic acid as a catalyst, followed by amination with trimethylamine. This simple one-step bromoalkylation allowed convenient control of both the degree of functionalization and cation tether length by changing the molar ratio and the structure of the bromoalkylated tertiary alcohol. The resulting quaternary ammonium-functionalized ionic triblock SEBS copolymers showed a microphase-separated morphology on the 35 nm length scale. A series of AEMs with different ion exchange capacities and ion tether lengths were systematically investigated by comparing swelling and anion conductivity. Because the SEBS AEMs showed high swelling and low dimensional stability in water due to the rubbery nature of SEBS, the hard segment PS units were cross-linked by 1,6-hexanediamine for practical use. The cross-linking of SEBS AEMs reduced water uptake significantly (e.g., 155% vs 28%) and enhanced their mechanical properties. Because the backbone of the SEBS AEMs are composed of all carbon–carbon bonds, they showed good alkaline stability, preserving their IEC and OH– conductivity after testing in a 1 M NaOH solution at 80 °C for 500 h. Alkaline membrane fuel cell performance was evaluated with the cross-linked SEBS AEM, and a peak power density of 520 mW/cm2 was achieved at 60 °C under H2/O2 conditions.

Cyclohexane-1,4-dimethanol/DMC Condensation Polymerization. The first step was conducted with cyclohexane-1,4-dimethanol (10.0 g, 69 mmol, a mixture of cisand transisomers in 3:7 ratio), NaH (3.3 mg, 0.14 mmol) and DMC (10.3 g, 114 mmol), using identical conditions and procedures to those used for DB/DMC condensation polymerization. In the second step, the condensation reaction was conducted at 180 °C and 380 mmHg for 1 h, at 210 °C and 0.3 mmHg for 1 h, and finally at 240 °C and 0.3 mmHg for 3 h. The catalyst was neutralized with phthaloyl dichloride using the same procedures and conditions as those used for DB/DMC condensation polymerization. 1 H NMR (CDCl 3 ):  4.04 (d, J = 7.2 Hz, 1.2H, 2 CH 2 OC(O) in cis-isomer), 3.94 (d, J = 6.4 Hz, 2.8H, CH 2 OC(O) in trans-isomer), 2.10-0.80 (m, 10H) ppm.

The performance of anion exchange membrane fuel cells (AEMFCs) employing Pt or PtRu electrocatalyst and ionomers with different polyaromatic backbones is correlated with the density functional theory (DFT)-calculated adsorption energies of the ionomer fragments on the metal surfaces. The performance of the AEMFCs tested in this work significantly changes depending on the backbone structure of polyaromatic ionomer or the type of the catalyst used at the anode. For the same anode catalyst, the performance decreases in the order poly­(fluorene) > poly­(p-biphenyl alkylene) > poly­(terphenyl alkylene)­s, which is in excellent agreement with the decrease in the DFT-calculated interaction energies between the catalyst surface and the corresponding ionomer fragment. Namely, DFT-calculated adsorption energies decrease in the order: p-terphenyl ≥ m-terphenyl > biphenyl > diphenyl ether > benzene ≥ o-terphenyl > 9,9-dimethyl fluorene. The trend in the adsorption energies is explained on the basis of the structural and conformational features of the ionomer fragments. Namely, strong adsorption of the polyaromatic ionomer fragments correlates with the number of benzene rings with a low rotational barrier that can bind parallel to the metal surfaces, leading to strong interaction and hybridization of the aromatic π-orbitals with metal electronic states. The results of this work suggest, therefore, that the interaction between the ionomer and electrocatalyst should be taken into account when designing high-performing ionomers even before considering other factors such as hydroxide conductivity, gas permeability, and water uptake of the ionomeric binder.

CONSPECTUS: Anion exchange membranes (AEMs) based on hydroxide-conducting polymers (HCPs) are a key component for anion-based electrochemical energy technology such as fuel cells, electrolyzers, and advanced batteries. Although these alkaline electrochemical applications offer a promising alternative to acidic proton exchange membrane electrochemical devices, access to alkaline-stable and high-performing polymer electrolyte materials has remained elusive until now. Despite vigorous research of AEM polymer design, literature examples of high-performance polymers with good alkaline stability at an elevated temperature are uncommon. Traditional aromatic polymers used in AEM applications contain a heteroatomic backbone linkage, such as an aryl ether bond, which is prone to degradation via nucleophilic attack by hydroxide ion. In this Account, we highlight some of the progress our group has made in the development of advanced HCPs for applications in AEMs and electrode ionomers. We propose that a synthetic polymer design with an all C−C bond backbone and a flexible chain-tethered quaternary ammonium group provides an effective solution to the problem of alkaline stability. Because of the critical demand for such a polymer system, we have established new synthetic strategies for polymer functionalization and polycondensation using an acid catalyst. The first approach is to graft a cationic tethered alkyl group to pre-existing, commercially available styrene-based block copolymers. The second approach is to synthesize high-molecular-weight aromatic backbone polymers using acid-catalyzed polycondensation of arene monomers and a functionalized trifluoromethyl ketone substrate. Both strategies involve a simple two-step reaction process and avoid the use of expensive metal-based catalysts and toxic chemicals, thereby making the synthetic processes easily scalable to large industrial quantities. Both polymer systems were found to have excellent alkaline stability, confirmed by the preservation of ion exchange capacity and ion conductivity of the membrane after an alkaline test under conditions of 1 M NaOH at 80−95 °C. In addition, the advantage of good solvent processability and convenient scalability of the reaction process generates considerable interest in these polymers as commercial standard AEM candidates. AEM fuel cell and electrolyzer tests of some of the developed polymer membranes showed excellent performance, suggesting that this new class of HCPs opens a new avenue to electrochemical devices with real-world applications.

SummaryThe (salen)Co(III) complex 1 tethering four quaternary ammonium salts, which is a highly active catalyst in CO2/epoxide copolymerizations, shows high activity for propylene oxide/phthalic anhydride (PO/PA) copolymerizations and PO/CO2/PA terpolymerizations. In the PO/PA copolymerizations, full conversion of PA was achieved within 5 h, and strictly alternating copolymers of poly(1,2-propylene phthalate)s were afforded without any formation of ether linkages. In the PO/CO2/PA terpolymerizations, full conversion of PA was also achieved within 4 h. The resulting polymers were gradient poly(1,2-propylene carbonate-co-phthalate)s because of the drift in the PA concentration during the terpolymerization. Both polymerizations showed immortal polymerization character; therefore, the molecular weights were determined by the activity (g/mol-1) and the number of chain-growing sites per 1 [anions in 1 (5) + water (present as impurity) + ethanol (deliberately fed)], and the molecular weight distributions were narrow (M w/M n, 1.05–1.5). Because of the extremely high activity of 1, high-molecular-weight polymers were generated (M n up to 170,000 and 350,000 for the PO/PA copolymerization and PO/CO2/PA terpolymerization, respectively). The terpolymers bearing a substantial number of PA units (f PA, 0.23) showed a higher glass-transition temperature (48 °C) than the CO2/PO alternating copolymer (40 °C).

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Low-molecular-weight poly(propylene carbonate)s bearing -OH groups at both ends (PPC-diols) are prepared by feeding protic chain-transfer agents (1,2-propanediol, terephthalic acid, 2,6naphthalenedicarboxylic acid, and phenylphosphonic acid) in the CO 2 /propylene oxide copolymerization catalyzed by a highly active Salen-Co(III) complex tethered by four quaternary ammonium salts. The generated low-molecular-weight PPC-diols are used in situ for the formation of thermoplastic polyurethanes through subsequent feeding of diisocyanates (4,4 0 -methylenebis(phenyl isocyanate), 1,4-phenylene diisocyanate, and toluene 2,4-diisocyanate). The formation of polyurethanes is confirmed by 1 H NMR spectroscopy and GPC studies. By varying the structure of the fed diisocyanate and chain-transfer agent, the glass transition temperature of the polyurethane can be tuned in the range 40-60 C. A high glass transition temperature of up to 60 C, which is 20 C higher than that of high-molecular-weight PPC itself (40 C), is attained when 2,6-naphthalenedicarboxylic acid (as the chain-transfer agent) and 4,4 0 -methylenebis(phenyl isocyanate) are employed. In addition, flame-retarding polyurethanes are generated by using an organophosphorus-based chain-transfer agent.

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