Photoinitiated Copper(I)-Catalyzed Azide–Alkyne Cycloaddition

Letter Cite This: ACS Macro Lett. 2019, 8, 795−799

pubs.acs.org/macroletters

Photoinitiated Copper(I)-Catalyzed Azide−Alkyne Cycloaddition Reaction for Ion Conductive Networks Bassil El-Zaatari,†,‡ Andrew C. Tibbits,†,‡ Yushan Yan,† and Christopher J. Kloxin*,†,§ Departments of †Chemical and Biomolecular Engineering and §Materials Science and Engineering, University of Delaware, Newark, Delaware 19716, United States

Downloaded via BUFFALO STATE on July 19, 2019 at 02:59:05 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

S Supporting Information *

ABSTRACT: The photoinitiated copper(I)-catalyzed azide−alkyne cycloaddition (photo-CuAAC) is a “click” reaction that enables spatially and temporally controlled polymerizations. The solventless photopolymerization of multifunctional azide and cationic alkyne monomers results in the rapid formation of a charged polymer network. Full conversion of these monomers is achieved within 30 min under mild, blue-light irradiation conditions (470 nm light at 30 mW/cm2). The modulus of the material is readily tuned by controlling the ratio of di- and trifunctional alkyne monomers. Facile exchange of the hydrophobic bistriflimide counterion for a hydroxide anion yields an ion conductive polymer network with photopatternable charged regions. The spatiotemporal nature of the ionic photo-CuAAC reaction coupled with the chemical stability and mechanical flexibility suggests this chemistry as a facile and novel approach for ion-containing material synthesis (e.g., alkaline fuel cell components).

P

olymeric ionic liquids (PILs) contain at least one ionic repeat unit that imparts the polymers with electrolytic properties, specifically ionic conductivity, while maintaining their mechanical properties and stability. PIL properties are typically tunable by ionic liquid monomer composition as well as anion and cation structure.1−3 PILs are often highly hygroscopic and, thus, susceptible to dissolution in waterbased applications (e.g., fuel cell membranes). The synthesis of cross-linked ionic networks has been employed to enhance mechanical stability.4 Click chemistry offers a facile and robust monomer crosslinking strategy to synthesize charged polymer networks.5 Click reactions, such as the copper(I)-catalyzed azide−alkyne cycloaddition (CuAAC) reaction,6,7 are extremely selective and high yielding organic reactions that can be implemented over a wide range of conditions.8 The copper(I)-catalyzed reaction between an azide and an alkyne functional group has already shown potential in the creation of PILs using two different synthetic routes: (1) the CuAAC polyaddition reaction and subsequent quaternization of the triazole ring9−17 and (2) the direct reaction of azide-side groups with alkyne-functionalized ionic liquids.18,19 Here, we demonstrate the rapid formation of a polymer network exhibiting ion conductivity by incorporating multifunctional alkyne ionic monomers through photochemistry. Photoinitiation is achieved by converting copper(II) to copper(I) in the presence of a photoinitiating system (Figure 1A), enabling spatial and temporal control with implications for the design and synthesis of functional polymeric materials as well as photopatterning. A schematic showing the use of photoCuAAC polymerizations for charged networks is depicted in Figure 1B. © 2019 American Chemical Society

Figure 1. (A) Schematic of the photo-CuAAC reaction. Copper(II) is reduced to copper(I) via a photoinitiated radical species (PI), which then catalyzes the cycloaddition reaction between an alkyne (blue) and an azide (red) functional group to produce a 1,2,3-triazole product (purple). (B) Schematic of the charged network formation using photo-CuAAC chemistry. As illustrated, a difunctional azide monomer reacts with a difunctional alkyne ionic monomer and a trifunctional alkyne cross-linker in the presence of light, copper(II), and a photoinitiator, yielding a charged photopolymerized network.

Received: May 15, 2019 Accepted: June 13, 2019 Published: June 17, 2019 795

DOI: 10.1021/acsmacrolett.9b00324 ACS Macro Lett. 2019, 8, 795−799

Letter

ACS Macro Letters

The ionic liquid monomer counteranion is important to the successful implementation of the photo-CuAAC polymerization. The presence of the bromide counterion in YNE-2 not only resulted in decreased solubility in the CuAAC resin but also significantly slowed the reaction kinetics. This kinetic effect is exemplified in the reaction between a monofunctional azide and an imidizolium alkyne having two different counterions, bromide and bis(trifluoromethane)sulfonimide (TFSI) (shown in Figure 3A). While the TFSI-based propargyl imidazolium reaction reaches near full conversion within 3 min, the bromide-based imidazolium shows negligible reactivity over an hour of irradiation. The inhibitory nature of high concentrations of the bromide ion to CuAAC chemistries has been documented in the literature and is attributed to the strong affinity of bromide ions, when used in excess, to bind to the copper(I) centers in organic media.26−28 The TFSI-based YNE-2 was hence utilized in the network polymerizations, which exhibited excellent solubility in the resin formulation while possessing rapid reaction kinetics. The difunctional alkyne ionic liquid monomer (YNE-2) with the TFSI counteranion, the azide monomer (AZ) and the alkyne cross-linker (YNE-1) were photopolymerized to form a network structure as depicted in Figure 1B. For these photopolymerizations, blue light (470 nm) was utilized in the presence of camphorquinone, as a photosensitizer, and 4,N,N-trimethylaniline, as a co-initiator. The visible light system was used to mitigate azide decomposition into nitrogen gas and nitrenes, which can cause defects in the polymer network in the form of bubbles, and where the loss of azide could shift the stoichiometry of the resin.29 All photopolymerizations were performed on optically thin films, having a geometric thickness of 0.2 mm, except for the films used in the ionic conductivity experiments, which were 0.11 mm. All networks are formulated with a 1:1 stoichiometric ratio between azide and alkyne functional groups; however, the cross-link densities were tuned by controlling the ratio of trifunctional (YNE-1) and difunctional (YNE-2) alkynes in the formulation (e.g., 50:50 YNE-2/YNE-1 indicates 50 mol % of the alkyne functionality are from the cross-linker, YNE-1, and 50 mol % are from the ionic monomer, YNE-2). The photopolymerization of the YNE-2/YNE-1 and AZ formulation reaches full conversion in less than 30 min when irradiated at 30 mW/cm2 of 470 nm light. Interestingly, the polymerization kinetics were found to be independent of the cross-link density (Figure 3B). The mechanical properties of the photopolymerized specimens were examined using dynamic mechanical analysis (DMA). As more YNE-2 is added to the formulation, a decrease in room temperature modulus in the rubbery regime is observed as expected (Figure 3C), since the elastic modulus is directly proportional to the network cross-link density. Surprisingly, however, all ion containing polymers possess comparable Tgs ranging between −1 and 3.5 °C (Figure 3C). While the Tg of a cross-linked network is mainly influenced by molecular mobility, the near-uniformity in the Tg values at different YNE-2/YNE-1 ratios suggests that, mechanically, an increase in charge density balances the decrease in cross-link density. When replacing YNE-2 with a noncharged difunctional alkyne with similar molecular weight (YNE-3), the Tg decreases from 3 °C to −15 °C in a 50:50 (from YNE-2 to YNE-3)/YNE-1 network. This result supports that an increase in charge density counteracts the decrease in cross-link density.

Previous polymer networks synthesized via the photoCuAAC reaction have exhibited narrow glass transition temperatures and desirable thermomechanical properties at room temperature owing to the step-growth mechanism and presence of 1,2,3-triazole linkages, respectively.20−22 These triazole linkages are rigid in nature and possess hydrogen bonding and π−π stacking interactions that are hypothesized to impart unique mechanical properties, such as enhanced toughness.22 The photo-CuAAC polymerization shows rapid kinetics when compared with typical CuAAC reactions under optimized conditions.23,24 The work herein describes the synthesis, characterization and implementation of ion-conductive polymers from the photo-CuAAC reaction. The PILs are synthetically facile and exhibit rapid conversions. We furthermore assess the ability of the PILs to form hydroxide exchange membranes for potential applications in alkaline fuel cells. The backbone flexibility of the comonomers is critical in enabling molecular mobility, which facilitates ionic conductivity of the network structure in its hydrated state. A hydrophilic poly(ethylene glycol) (PEG)-containing difunctional azide (AZ) was selected as the azide monomer (Figure 2). The

Figure 2. Monomers, photoinitiators, and model triazole used in the current work.

use of a PEG backbone is hypothesized to enhance ionic conductivity properties as they have been shown to act as multivalent ligands.25 The increased chain mobility, associated with low glass transition (Tg) polymer networks, compared with bulkier, aromatic difunctional azides (Figure S1), should facilitate water uptake and increase hydroxide conductivity. Charge was incorporated into the polymer backbone via a difunctional alkyne imidazolium ionic liquid (YNE-2), which was selected as the ionic backbone due to its ease of synthesis and increased solubility in the CuAAC resin formulation. Finally, the inclusion of a trifunctional alkyne monomer (YNE1) into the formulation provided chemical cross-linking and dimensional stability. 796

DOI: 10.1021/acsmacrolett.9b00324 ACS Macro Lett. 2019, 8, 795−799

Letter

ACS Macro Letters

The polymer films using various YNE-2/YNE-1 ratios were tested for ionic conductivity under a fully hydrated state; however, the TFSI-form exhibited ionic conductivities similar to the baseline conductivity. The TFSI counterion was exchanged with a more hydrophilic counteranion, hydroxide, for potential implementation as a hydrated anion exchange membrane. Formulations with YNE-2/YNE-1 ratios of 25:75, 50:50, and 75:25 were converted to the hydroxide form by immersion of the TFSI-form network in 1 M KOH for 48 h, after leaching out the copper(II). The removal of copper was confirmed by UV−vis spectroscopy (Figure S2). Hydroxide conductivity of networks did not significantly change from 2 days to 10 days immersion in 1 M KOH, suggesting that ion exchange is complete after 2 days. The hydroxide conductivities of (5.1 ± 1.3), (16 ± 3), and (5.8 ± 1.0) mS/cm were measured for the 25:75, 50:50, and 75:25 YNE-2/YNE-1 ratio formulations. The decrease in hydroxide conductivity with increased ionic content for the 75:25 YNE-2/YNE-1 formulation is attributed to decreased cross-link density resulting in excess water swelling of ionconductive sites. This trend is commonly observed in hydroxide-coordinated networks and is indicative of the need to balance charge density, hydroxide conductivity, and water management.4,30 While these hydroxide conductivities are modest relative to commercially available hydroxide exchange membranes, such as Tokuyama A201 (∼46 mS/cm at room temperature),31 this work demonstrates a general photoCuAAC approach to facilely incorporating charge functionality into a chemically stable triazole-based polymer network. The tensile strength and strain were measured on dried networks. The materials containing TFSI or hydroxide showed very little difference in tensile properties after 2 days in water or 1 M KOH, respectively, at room temperature (Figure 4).

Figure 3. (A) Conversion for the reaction between methyl-2azidoacetate and a monofunctional alkyne ionic liquid monomer as a function of counterion: TFSI (closed square) and bromide (open square). Reaction conditions: 50 mM azide and alkyne, 10 mM CuSO4·5H2O, and 10 mM I819 photoinitiator in DMF with 405 nm visible light at 20 mW/cm2 and a 0.2 mm path length. (B) Conversion for the photopolymerization of YNE-2/YNE-1 with an equimolar amount of AZ, 1.5 wt % CuCl2/PMDETA, 0.7 wt % camphorquinone, and 0.7 wt % trimethylaniline using 470 nm light at 30 mW/cm2 for 0.2 mm samples. The reaction was not irradiated until 5 min after starting the experiment, as represented by the gray box. (C) Elastic modulus values when different ratios of the charged alkyne (YNE-2) and the noncharged alkyne (YNE-1) are used as measured for 0.2 mm samples. The modulus decreases as more YNE-2 is added to the formulation.

Figure 4. Tensile properties of the TFSI form CuAAC networks (50:50 YNE-2/YNE-1 ratio) remain unchanged before and after immersion in 1 M KOH for 48 h for 0.2 mm thick samples. The strain at break increased when the sample was heated at 60 °C for 7 days, indicating a loss in the mechanical integrity of the network.

Both TFSI and hydroxide containing networks had a modulus of ∼4 MPa and withstood strains of over 30% without breaking. To better assess the long-term stability of these networks under alkaline conditions, specimens were immersed in 1 M KOH at elevated temperatures (60 °C). The elastic modulus and tensile properties of the materials were measured 797

DOI: 10.1021/acsmacrolett.9b00324 ACS Macro Lett. 2019, 8, 795−799

Letter

ACS Macro Letters

kinetics and Tg are largely independent of the composition of comonomers in the network, while the rubbery modulus decreases with increasing ionic monomer content. The ion exchange from bistriflimide to hydroxide converts the networks from hydrophobic to hydrophilic and imparts networks with measurable ionic conductivity. While the hydroxide conductivities obtained in our 50:50 YNE-2/YNE-1 system are relatively low as compared with commercial benchmarks reported in the literature, the results in the hydroxide form are promising and shows a proof-of-concept approach toward synthesizing hydroxide exchange membranes using a one-pot cross-linked photo-CuAAC polymerization approach. The network cross-link density decreased under elevated temperatures in basic media. This was attributed to the imidazolium functional group, rather than the stable triazole resulting from the CuAAC polymerization. Stabilizing the imidazolium, via C4 and C5 substitutions, as shown by the Coates group,32 for example, could further enhance the stability of the photoCuAAC PIL under basic conditions and elevated temperatures. Nonetheless, directed synthesis toward morphologies and microstructures conducive to optimizing both conductivity and mechanical robustness are being targeted to further enhance these materials as alkaline fuel cell membranes.33

after 7 days. Compared with materials immersed in 1 M KOH for 2 days, there was a 2-fold decrease in the modulus and an increase in the strain percent at break, from 30% to 100% strain. To rule out the potential degradation of the triazole crosslink as the reason for the decrease in modulus, a model 1,4disubstituted 1,2,3-triazole, BPOHD (Figure 2), was synthesized and immersed under basic conditions. Specifically, BPOHD was introduced to a solution of 1 and 5 M KOH in CD3OH and heated to 80 °C. The 1H NMR spectra of BPOHD was monitored as a function of time (from 0 to 8 days) using a hydroxide suppression technique32 to monitor the degradation. The NMR spectra did not change over the 8 days in either concentration (Figures S3 and S4), demonstrating triazole stability in basic media. Thus, the main mechanism for degradation was hypothesized to be the imidazolium cation. This functional group is known to exhibit susceptibility to degradation under various alkaline conditions.32 When YNE-2 was placed in 1 M KOH in CD3OH at 80 °C, full degradation of the imidazole was observed in less than 12 h (Figure S5). Thus, the loss in mechanical properties of this photo-CuAAC PIL is attributed to the imidazolium degradation in the polymer backbone. A significant benefit of creating a charged polymer network via the photo-CuAAC reaction is the potential to rapidly create membranes in a spatio-specific manner using masked irradiation. As a simple demonstration, a noncharged photoCuAAC network containing YNE-1, AZ, initiator (camphorquinone, 4-N,N-trimethylaniline), and a base indicator (thymol blue) was photopatterned between two glass slides, leaving a “J” shape of unreacted monomer, as shown in Figure 5. The

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.9b00324. Experimental details (materials, synthesis, sample preparation, methods, and characterization), 1NMR spectra for synthesized compounds, degradation studies, glass transition temperature comparisons, and UV−vis of copper leaching (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: 302-831-8670. Fax: 302-8311048. ORCID

Bassil El-Zaatari: 0000-0002-2447-9250 Yushan Yan: 0000-0001-6616-4575 Christopher J. Kloxin: 0000-0002-1679-0022 Author Contributions ‡

These authors contributed equally.

Figure 5. Photopatterning of the charged photo-CuAAC polymer network within a noncharged CuAAC polymer network.

Notes

The authors declare no competing financial interest.

■

unreacted monomer was removed and a resin containing the charged ionic monomer (i.e., 50:50 YNE-2/YNE-1, AZ, initiator and base indicator) was used to fill the space and photopolymerized. The material was then placed in 1 M KOH solution at room temperature for 24 h. While both networks contained thymol blue, only the “J” shape changed color to greenish-blue, owing to the flux of hydroxide ions into the pattern. This demonstration highlights the spatial control over the CuAAC reaction and selectivity of the hydroxide exchange. In summary, the covalent incorporation of an alkynefunctionalized ionic liquid into a polymer network was achieved. The resultant charged networks were formed from the single cross-linking step proceeding to quantitative conversion in less than 30 min. The network polymerization

ACKNOWLEDGMENTS This research was supported by NSF CBET Grant Award No. 1264503, EPSCoR Grant No. IIA-1301765, NIH-NIDCR (U01 DE023774), and the State of Delaware. The authors would like to thank Brian Setzler for his help with the ion conductivity experiments and analysis.

■

REFERENCES

(1) Mecerreyes, D. Polymeric Ionic Liquids: Broadening the Properties and Applications of Polyelectrolytes. Prog. Polym. Sci. 2011, 36, 1629−1648. (2) Yuan, J.; Antonietti, M. Poly(Ionic Liquid)s: Polymers Expanding Classical Property Profiles. Polymer 2011, 52, 1469−1482.

798

DOI: 10.1021/acsmacrolett.9b00324 ACS Macro Lett. 2019, 8, 795−799

Letter

ACS Macro Letters (3) Yuan, J.; Mecerreyes, D.; Antonietti, M. Poly(Ionic Liquid)s: An Update. Prog. Polym. Sci. 2013, 38, 1009−1036. (4) Merle, G.; Wessling, M.; Nijmeijer, K. Anion Exchange Membranes for Alkaline Fuel Cells: A Review. J. Membr. Sci. 2011, 377, 1−35. (5) Mirjafari, A. Ionic Liquid Syntheses via Click Chemistry: Expeditious Routes toward Versatile Functional Materials. Chem. Commun. 2018, 54, 2944−2961. (6) Rostovtsev, V. V.; Green, L. G.; Fokin, V. V.; Sharpless, K. B. A Stepwise Huisgen Cycloaddition Process: Copper(I)-Catalyzed Regioselective “Ligation” of Azides and Terminal Alkynes. Angew. Chem., Int. Ed. 2002, 41, 2596−2599. (7) Tornøe, C. W.; Christensen, C.; Meldal, M. Peptidotriazoles on Solid Phase: [1,2,3]-Triazoles by Regiospecific Copper(I)-Catalyzed 1,3-Dipolar Cycloadditions of Terminal Alkynes to Azides. J. Org. Chem. 2002, 67, 3057−3064. (8) Xi, W.; Scott, T. F.; Kloxin, C. J.; Bowman, C. N. Click Chemistry in Materials Science. Adv. Funct. Mater. 2014, 24, 2572− 2590. (9) Dimitrov-Raytchev, P.; Beghdadi, S.; Serghei, A.; Drockenmuller, E. Main-Chain 1,2,3-Triazolium-Based Poly(Ionic Liquid)s Issued from AB + AB Click Chemistry Polyaddition. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 34−38. (10) Zhou, X.; Obadia, M. M.; Venna, S. R.; Roth, E. A.; Serghei, A.; Luebke, D. R.; Myers, C.; Chang, Z.; Enick, R.; Drockenmuller, E.; Nulwala, H. B. Highly Cross-Linked Polyether-Based 1,2,3-Triazolium Ion Conducting Membranes with Enhanced Gas Separation Properties. Eur. Polym. J. 2016, 84, 65−76. (11) Luo, Y.; Montarnal, D.; Treat, N. J.; Hustad, P. D.; Christianson, M. D.; Kramer, E. J.; Fredrickson, G. H.; Hawker, C. J. Enhanced Block Copolymer Phase Separation Using Click Chemistry and Ionic Junctions. ACS Macro Lett. 2015, 4, 1332−1336. (12) Mudraboyina, B. P.; Obadia, M. M.; Allaoua, I.; Sood, R.; Serghei, A.; Drockenmuller, E. 1,2,3-Triazolium-Based Poly(Ionic Liquid)s with Enhanced Ion Conducting Properties Obtained through a Click Chemistry Polyaddition Strategy. Chem. Mater. 2014, 26, 1720−1726. (13) Liu, L.; He, S.; Zhang, S.; Zhang, M.; Guiver, M. D.; Li, N. 1,2,3-Triazolium-Based Poly(2,6-Dimethyl Phenylene Oxide) Copolymers as Anion Exchange Membranes. ACS Appl. Mater. Interfaces 2016, 8, 4651−4660. (14) Jourdain, A.; Serghei, A.; Drockenmuller, E. Enhanced Ionic Conductivity of a 1,2,3-Triazolium-Based Poly(Siloxane Ionic Liquid) Homopolymer. ACS Macro Lett. 2016, 5, 1283−1286. (15) Obadia, M. M.; Drockenmuller, E. Poly(1,2,3-Triazolium)s: A New Class of Functional Polymer Electrolytes. Chem. Commun. 2016, 52, 2433−2450. (16) Zare, P.; Mahrova, M.; Tojo, E.; Stojanovic, A.; Binder, W. H. Ethylene Glycol-Based Ionic Liquids via Azide/Alkyne Click Chemistry. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 190−202. (17) Obadia, M. M.; Crépet, A.; Serghei, A.; Montarnal, D.; Drockenmuller, E. Expanding the Structural Variety of Poly(1,2,3Triazolium)s Obtained by Simultaneous 1,3-Dipolar Huisgen Polyaddition and N-Alkylation. Polymer 2015, 79, 309−315. (18) Ge, Q. Q.; Ran, J.; Miao, J. B.; Yang, Z. J.; Xu, T. W. Click Chemistry Finds Its Way in Constructing an Ionic Highway in AnionExchange Membrane. ACS Appl. Mater. Interfaces 2015, 7, 28545− 28553. (19) Li, N. W.; Guiver, M. D.; Binder, W. H. Towards High Conductivity in Anion-Exchange Membranes for Alkaline Fuel Cells. ChemSusChem 2013, 6, 1376−1383. (20) Adzima, B. J.; Tao, Y.; Kloxin, C. J.; DeForest, C. A.; Anseth, K. S.; Bowman, C. N. Spatial and Temporal Control of the Alkyne-Azide Cycloaddition by Photoinitiated Cu(II) Reduction. Nat. Chem. 2011, 3, 256−259. (21) Gong, T.; Adzima, B. J.; Baker, N. H.; Bowman, C. N. Photopolymerization Reactions Using the Photoinitiated Copper (I)Catalyzed Azide-Alkyne Cycloaddition (CuAAC) Reaction. Adv. Mater. 2013, 25, 2024−2028.

(22) Baranek, A.; Song, H. B.; McBride, M.; Finnegan, P.; Bowman, C. N. Thermomechanical Formation-Structure-Property Relationships in Photopolymerized Copper-Catalyzed Azide-Alkyne (CuAAC) Networks. Macromolecules 2016, 49, 1191−1200. (23) El-Zaatari, B. M.; Shete, A. U.; Adzima, B. J.; Kloxin, C. J. Towards Understanding the Kinetic Behaviour and Limitations in Photo-Induced Copper(I) Catalyzed Azide−alkyne Cycloaddition (CuAAC) Reactions. Phys. Chem. Chem. Phys. 2016, 18, 25504− 25511. (24) Song, H. B.; Baranek, A.; Bowman, C. N. Kinetics of Bulk Photo-Initiated Copper(I)-Catalyzed Azide−alkyne Cycloaddition (CuAAC) Polymerizations. Polym. Chem. 2016, 7, 603−612. (25) Agrawal, R. C.; Pandey, G. P. Solid Polymer Electrolytes: Materials Designing and All-Solid-State Battery Applications: An Overview. J. Phys. D: Appl. Phys. 2008, 41, 223001. (26) Lal, S.; Díez-González, S. [CuBr(PPh3)3] for Azide-Alkyne Cycloaddition Reactions under Strict Click Conditions. J. Org. Chem. 2011, 76, 2367−2373. (27) Berg, R.; Straub, B. F. Advancements in the Mechanistic Understanding of the Copper-Catalyzed Azide-Alkyne Cycloaddition. Beilstein J. Org. Chem. 2013, 9, 2715−2750. (28) Moorman, R. M.; Collier, M. B.; Frohock, B. H.; Womble, M. D.; Chalker, J. M. Halide Inhibition of the Copper-Catalysed AzideAlkyne Cycloaddition. Org. Biomol. Chem. 2015, 13, 1974−1978. (29) Shete, A. U.; El-Zaatari, B. M.; French, J. M.; Kloxin, C. J. BlueLight Activated Rapid Polymerization for Defect-Free Bulk Cu(I)Catalyzed Azide-Alkyne Cycloaddition (CuAAC) Crosslinked Networks. Chem. Commun. 2016, 52, 10574−10577. (30) Tibbits, A. C.; Yan, Y. S.; Kloxin, C. J. Covalent Incorporation of Ionic Liquid into Ion-Conductive Networks via Thiol−Ene Photopolymerization. Macromol. Rapid Commun. 2017, 38, 1700113. (31) Ren, X. M.; Price, S. C.; Jackson, A. C.; Pomerantz, N.; Beyer, F. L. Highly Conductive Anion Exchange Membrane for High Power Density Fuel-Cell Performance. ACS Appl. Mater. Interfaces 2014, 6, 13330−13333. (32) Hugar, K. M.; Kostalik, H. A.; Coates, G. W. Imidazolium Cations with Exceptional Alkaline Stability: A Systematic Study of Structure-Stability Relationships. J. Am. Chem. Soc. 2015, 137, 8730− 8737. (33) El-Zaatari, B. M. Photo-induced Copper(I) Catalyzed AzideAlkyne Cycloaddition Polymerization: Fundamentals and Applications. Ph.D. Dissertation, University of Delaware, Newark, DE, 2018.

799

DOI: 10.1021/acsmacrolett.9b00324 ACS Macro Lett. 2019, 8, 795−799