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Metal‐containing ionic liquid‐based, uncharged–charged diblock copolymers that form ordered, phase‐separated microstructures and reversibly coordinate small protic molecules

2017; Wiley; Volume: 55; Issue: 18 Linguagem: Inglês

10.1002/pola.28623

1099-0518

Zhangxing Shi, Alyssa W. May, Yuki Kohno, Travis S. Bailey, Douglas L. Gin,

Covalent Organic Framework Applications

Journal of Polymer Science Part A: Polymer ChemistryVolume 55, Issue 18 p. 2961-2965 Rapid CommunicationFree Access Metal-containing ionic liquid-based, uncharged–charged diblock copolymers that form ordered, phase-separated microstructures and reversibly coordinate small protic molecules Zhangxing Shi, Zhangxing Shi Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado, 80309Search for more papers by this authorAlyssa W. May, Alyssa W. May Department of Chemistry, Colorado State University, Fort Collins, Colorado, 80523Search for more papers by this authorYuki Kohno, Yuki Kohno Department of Chemical and Biological Engineering, University of Colorado, Boulder, Colorado, 80309Search for more papers by this authorTravis S. Bailey, Travis S. Bailey Department of Chemistry, Colorado State University, Fort Collins, Colorado, 80523 Department of Chemical and Biological Engineering, Colorado State University, Fort Collins, Colorado, 80523Search for more papers by this authorDouglas L. Gin, Corresponding Author Douglas L. Gin douglas.gin@colorado.edu Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado, 80309 Department of Chemical and Biological Engineering, University of Colorado, Boulder, Colorado, 80309Correspondence to: D. L. Gin (E-mail: douglas.gin@colorado.edu)Search for more papers by this author Zhangxing Shi, Zhangxing Shi Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado, 80309Search for more papers by this authorAlyssa W. May, Alyssa W. May Department of Chemistry, Colorado State University, Fort Collins, Colorado, 80523Search for more papers by this authorYuki Kohno, Yuki Kohno Department of Chemical and Biological Engineering, University of Colorado, Boulder, Colorado, 80309Search for more papers by this authorTravis S. Bailey, Travis S. Bailey Department of Chemistry, Colorado State University, Fort Collins, Colorado, 80523 Department of Chemical and Biological Engineering, Colorado State University, Fort Collins, Colorado, 80523Search for more papers by this authorDouglas L. Gin, Corresponding Author Douglas L. Gin douglas.gin@colorado.edu Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado, 80309 Department of Chemical and Biological Engineering, University of Colorado, Boulder, Colorado, 80309Correspondence to: D. L. Gin (E-mail: douglas.gin@colorado.edu)Search for more papers by this author First published: 04 May 2017 https://doi.org/10.1002/pola.28623Citations: 12AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Graphical Abstract A metal-containing ionic liquid-based, uncharged-charged block copolymer (BCP) system made via RAFT polymerization is presented. Small-angle X-ray scattering studies show these Co(II) bis(salicylate) anion-containing BCPs can form ordered microstructures (including a gyroid phase) in their neat states. Additionally, these BCPs can reversibly coordinate H2O or small alcohols with a noticeable color change. INTRODUCTION Polymerized ionic liquids (poly(IL)s)1 are macromolecules with charged repeat units prepared by polymerization of ionic liquid (IL) monomers (i.e., monomers that are molten salts at ≤100 °C),1(a,b) or that contain IL-like moieties if prepared by other methods.1(c) As poly(IL)s have the properties of polymers and many of the desired features of ILs (e.g., negligible vapor pressure, ion conductivity, and high solubility for certain gases), poly(IL)s have been shown to be useful for many applications (e.g., as gas separation membranes, solid-state ion conductors, etc.).1 IL-based block copolymers (BCPs) are a distinct and relatively new class of BCPs that contain at least one poly(IL) segment.2 By combining the unique properties of poly(IL)s with the ability of BCPs to phase-separate into ordered microstructures, IL-based BCPs have shown promise as new functional materials.2 Over the past few years, a variety of IL-based BCPs have been prepared by either sequential controlled/living polymerization of an IL monomer and an uncharged comonomer; or by post-polymerization functionalization of uncharged BCPs containing reactive units to generate charged moieties on the polymer.2 Although many IL-based BCPs have been studied as functional materials,2 only a small subset of them has been reported to phase-separate into periodic, nanostructured morphologies in the neat melt state.3 One recent method for introducing new functional properties into ILs has been to incorporate a metal complex in the IL. These metal-containing ILs (MCILs) are a relative new class of functional ILs with metal-based magnetic,4 optical,4, 5 catalytic,4, 6 or molecular binding properties.4, 7 Consequently, MCIL-based BCPs with such properties and the ability to form ordered microstructures would be desirable as new functional materials. Whereas uncharged metal-containing polymers are fairly well known in the literature, MCIL-based poly(IL) homopolymers are rare,8 and MCIL-based BCPs are unprecedented to our knowledge. The closest reported materials with charged blocks are metallocene-based BCPs that are not true IL-based BCPs.9 These charged metallocene-based BCPs were made by copolymerizing uncharged monomers with charged monomers that are not molten salts and do not have typical IL structures.9 Herein, we present the first example of a MCIL-based, uncharged–charged BCP platform (1) that exhibits ordered, phase-separated microstructures in the neat state and can reversibly coordinate protic small molecules with an accompanying color change. This MCIL-based BCP system was made by reversible addition-fragmentation chain-transfer (RAFT) polymerization of first butyl methacrylate to form uncharged poly(butyl methacrylate) (PBMA) macro-chain-transfer agents (macroCTAs) of controlled length (2), and then copolymerization of a styrenic phosphonium IL monomer with a cobalt(II) bis(salicylate) anion (3) (Fig. 1). Short BCPs of this system (1a–f) ranging from 35-b-35 to 60-b-10 (uncharged-b-charged block ratios) show a range of ordered nanostructures including lamellar (LAM), columnar hexagonal (HEX), and gyroid (GYR) by small-angle X-ray scattering (SAXS) after annealing in their neat states. These MCIL-based BCPs were also found to reversibly coordinate to H2O and small alcohols in their vapor form with a noticeable color change. This combination of properties makes this new BCP platform unique and potentially useful for applications development. Figure 1Open in figure viewerPowerPoint Synthesis and structures of the MCIL-based BCPs in this study. [Color figure can be viewed at wileyonlinelibrary.com] As shown in Figure 1, MCIL-based BCPs 1a–f were synthesized via sequential RAFT polymerization of butyl methacrylate and MCIL monomer 3 using 2-cyano-2-propyl benzodithioate (CPBD) as the chain-transfer agent, azobis(isobutyronitrile) (AIBN) as the radical initiator, and chlorobenzene as the polymerization solvent (see Supporting Information for details). MCIL monomer 3 is a new compound that was synthesized using a procedure based on one previously reported by our group.7 In our sequential RAFT copolymerization, reactive PBMA blocks 2 with controlled lengths and low PDI values were first synthesized and then used as RAFT macroCTAs to attach the subsequent poly(MCIL) block via addition of the appropriate amount of monomer 3. This polymerization sequence was chosen because BCPs made via RAFT are typically synthesized by polymerizing the monomer with the better propagating radical leaving group first.10 RAFT polymerizations of methacrylates and styrenic monomers are well established in literature,11 but RAFT polymerization of monomers containing a Co(II) bis(salicylate) complex is unprecedented. Consequently, kinetics studies of the copolymerization of butyl methacrylate and 3 were performed to confirm controlled polymerization (see Supporting Information). The absolute lengths and block composition ratios of BCPs 1a–f were confirmed by 1H NMR analysis: The block lengths of the PBMA macroCTAs 2a–f were determined by 1H NMR end-group analysis using the aromatic protons on the CPBD as an integration reference.12 The PBMA:poly(3) block ratios for each BCP were determined by integrating and comparing distinct 1H NMR signals indicative of each block. The poly(MCIL) block lengths were then calculated based on the PBMA block lengths and the block composition ratios.12 These results were further confirmed by monitoring the degree of conversion and monomer-to-initiator ratios. Then, the absolute Mn values for 1a–f were calculated by multiplying the absolute block lengths (from 1H NMR spectroscopy) by the molecular weights of the repeat units (see Supporting Information). Unfortunately, GPC13 and other conventional polymer MW determination techniques could not be used to confirm the MW, PDI, or block structure of 1a–f because of the unusual solubility and other physical properties of these uncharged–charged MCIL-based BCPs.3(h) Instead, a combination of alternative methods (i.e., surfactant behavior and solubility analysis, diffusion-ordered NMR spectroscopy, SAXS studies) was used to verify the block architectures of 1a–f and differentiate their behavior from that of a physical blend of PBMA and poly(3) homopolymers, as described previously for characterizing IL-based BCPs3(h) (see Supporting Information). SAXS was particularly effective in demonstrating the block connectivity, as well as the MW and composition control, afforded by the CPBD chain-transfer agent. As depicted in Figure 2 and summarized in Table 1, careful control of the relative sizes of each block in 1a–f permitted synthesis of a series of macromolecules collectively displaying characteristics representative of each of the classic BCP phases (LAM, GYR, HEX, and a weakly ordered sphere phase (S)). Notably, sample 1d may be the very first example of an IL-based BCP exhibiting the GYR phase. In previous investigations of imidazolium-based IL-BCPs based on styrenic3(f,g) and norbornene3(h,i) monomer derivatives, the GYR phase was noticeably absent, with systems preferring to produce (presumably metastable but persistent) regions of LAM/HEX coexistence. MCIL-BCP 1d exhibited behavior prototypical of many non-ionic BCPs, quickly transitioning from a metastable HEX phase to a persistent GYR phase with minimal thermal annealing.14-16 SAXS data for 1a–f as a function of temperature during heating, annealing, and cooling are provided in the Supporting Information. Figure 2Open in figure viewerPowerPoint SAXS patterns (175 °C) for MCIL-based BCPs 1a–f after annealing in vacuo for 2 h. Inverted triangles designate the expected reflection locations for the indicated morphologies based on the position of q*. [Color figure can be viewed at wileyonlinelibrary.com] Table 1. Morphological Characteristics of MCIL-Based BCPs 1a–f BCP n m d*/{hkl}* (nm) Morphology Observed q*/q100 1a 35 35 14.8/{110} (S) Weakly-ordered spheres 1b 40 30 14.8/{100} HEX √1, (√3), √4, √7 1c 45 25 14.2/{100} HEX √1, √3, √4, √7 1d 50 20 14.4/{211} GYR (HEX) √6, √8, √14, √16, (√20), √22, √24, √26) 1e 55 15 13.5/{100} LAM √1, √4, √9 1f 60 10 12.8/{100} LAM √1, √4, (√9) Notably, the sequence of morphologies and their compositional distribution with respect to volume fraction in IL-BCPs has proven to mimic that of traditional uncharged BCP systems.3 However, one unique trait exhibited by these charged MCIL-based BCPs and shared with the previously studied styrenic-3(f,g) and norbornene3(h,i)-based IL-BCP systems is a clear disparity in repeat unit volumes and its role in determining the selection of morphology. The data in Table 1 reveal that even at uncharged-to-charged repeat unit ratios as high as 60:10 (1f), the relative volumes occupied by each block are likely similar, promoting the adoption of a LAM phase. As this ratio decreases toward unity (1a), the charged block continues to occupy greater fractions of the overall BCP volume, and the adopted phases follow the prototypical path toward morphologies with increasing average mean curvature.17 Under that observation, we suspect that 1a, for which no higher order reflections are evident, is likely adopting a weakly-ordered sphere or micelle-like phase. Such phases tend to persist at the edges of the phase diagram, with the evolution of a more ordered lattice often constrained kinetically.3(i),18 Finally, it is notable that these MCIL-based BCPs are able to adopt ordered morphologies at such small numbers of repeat units. Clearly, the Flory-like interaction parameter, χ, for this combination of blocks is significant; however, direct χ measurement was beyond the scope of this initial work. MCIL-based BCPs 1a–f were also found to selectively and reversibly coordinative to small protic molecules (e.g., H2O, small alcohols such as MeOH, EtOH, etc.) with a distinct color change. After exposure to the vapor of these small protic molecules, 1a–f undergo a color change from dark blue to light purple. The original dark blue color can be restored by mild heating or in vacuo treatment of the coordinated BCPs (e.g., Fig. 3). This reversible colorimetric coordination behavior has been observed previously with MCILs containing the same Co(II) bis(salicylate) anion upon exposure to H2O or alcohols.7 The proposed mechanism for this behavior is described in a previous report.7 However, the vapor of aprotic small molecules (e.g., Et2O, acetone, and ethyl acetate) will not trigger the reversible color change described above, indicating no coordination between 1a–f and these aprotic molecules (see Supporting Information). Interestingly, preliminary SAXS of 1d suggests that water vapor coordination does not appear to affect its GYR morphology but may slightly increase domain spacing (see Supporting Information). Figure 3Open in figure viewerPowerPoint Reversible color change of 1d upon coordination with water vapor. [Color figure can be viewed at wileyonlinelibrary.com] In summary, new MCIL-based BCPs 1a–f have been made that are capable of forming ordered nanostructures (including the GYR phase) in their neat states and that can also reversibly coordinate with H2O and small alcohols with an associated color change. We are currently investigating the morphological behavior and phase stability of this MCIL-BCP system as a function of the extent of H2O and small alcohol coordination. Future work will be focused on exploring on whether phase changes can be induced upon reversible water or alcohol coordination to allow these MCIL-based BCPs to be used in responsive applications. EXPERIMENTAL Synthesis of Bis[tributyl(4-vinylbenzyl)phosphonium] [Cobalt(II) bis(salicylate)] (3) Tributyl(4-vinylbenzyl)phosphonium chloride (see Supporting Information) (1.000 g, 2.82 mmol) was dissolved in H2O (5 mL). Subsequently, individual aqueous aliquots (2.5 mL) of lithium salicylate (0.812 g, 5.64 mmol) and cobalt(II) chloride hexahydrate (0.335 g, 1.41 mmol) were prepared and then added dropwise. A deep purple liquid formed immediately and was extracted using CH2Cl2. The CH2Cl2 layer was then repeatedly washed with water until no halides were detected by the silver nitrate test. This solution was then dried over anhydrous Na2SO4, filtered, and then concentrated. The resulting liquid was dissolved in MeOH, stirred at room temperature (RT) for 24 h, filtered, and finally concentrated in vacuo to give 3 as a dark blue liquid (yield: 1.24 g, 91%). 1H NMR (300 MHz, CD3OD): δ 8.04 (s, 1H), 7.44 (d, J = 8.0 Hz, 2H), 7.33–7.17 (m, 4H), 7.10 (s, 1H), 6.69 (dd, J = 17.6, 10.9 Hz, 1H), 5.77 (dt, J = 17.6, 0.9 Hz, 1H), 5.23 (dt, J = 10.9, 1.0 Hz, 1H), 3.65 (d, J = 14.8 Hz, 2H), 2.17–1.98 (m, 6H), 1.51–1.33 (m, 12H), 1.00–0.82 (m, 9H). 13C NMR (75 MHz, CD3OD): δ 138.00 (d, J = 3.9 Hz), 135.64 (d, J = 2.2 Hz), 133.06, 129.80 (d, J = 5.1 Hz), 127.59 (d, J = 8.9 Hz), 126.84 (d, J = 3.3 Hz), 120.78, 118.86, 113.95 (d, J = 1.8 Hz), 25.41 (d, J = 45.4 Hz), 23.45 (d, J = 15.7 Hz), 22.77 (d, J = 4.7 Hz), 17.56 (d, J = 47.3 Hz), 12.15 (d, J = 0.9 Hz). The 13C signals of the phosphonium cation were split into doublets by the 31P nucleus. The number of 13C signals for the cobalt anion is less than expected due to the interference of the paramagnetic Co(II) center.7 Anal. calcd. for C56H80CoO6P2: C 69.33, H 8.31, N 0; found: C 69.34, H 8.02, N 0. The full 1H and 13C NMR spectra for isolated 3 are provided in Supporting Information to help confirm its purity. Example: Synthesis of PBMA macroCTA 2d Butyl methacrylate (1.50 g, 10.5 mmol), CPBD (46.7 mg, 0.211 mmol), chlorobenzene (1.2 mL), and AIBN (3.50 mg, 0.0213 mmol) were added to a flame-dried Schlenk flask and degassed by 3 freeze–pump–thaw cycles. The flask was then allowed to warm to RT and back-filled with Ar. The resulting mixture was then stirred at 70 °C for 24 h. Upon complete consumption of butyl methacrylate (as verified by 1H NMR analysis), the contents of the flask were cooled to RT, diluted with THF, precipitated by adding into MeOH, and the precipitate recovered by filtration to give the desired PBMA macroCTA 2d as a pink solid (yield: 1.24 g, 80%). DP = 50; PDI = 1.04; Mn = 7331 g/mol (calculated using 1H NMR polymer end-group analysis. See Supporting Information for details on how the DP and absolute Mn were determined using 1H NMR analysis). The synthesis and characterization details for the rest of the PBMA macroCTAs used are provided in Supporting Information. Example: Synthesis of MCIL-Based BCP 1d Monomer 3 (274 mg, 0.565 mmol), 2d (207 mg, 0.0282 mmol), chlorobenzene (1.5 mL), and AIBN (0.930 mg, 0.00566 mmol) were added to a flame-dried Schlenk flask and degassed by three freeze-pump-thaw cycles. The flask was then allowed to warm to RT and back-filled with Ar. The resulting mixture was then stirred at 90 °C for 48 h. Upon complete consumption of 3 (as verified by 1H NMR analysis), the contents of the flask were cooled to RT, diluted with ethyl acetate, precipitated by adding into hexane/ethyl acetate (4/1 (v/v)) mixture. The resulting precipitate was recovered by filtration to give the MCIL-based BCP 1d as a dark blue solid (yield: 300 mg, 62%). Block repeat units molar ratio = 2.5:1 (butyl methacrylate:3); block length composition = 50-b-20 (PBMA-b-poly(3)); Mn = 17,032 g/mol (calculated based on 1H NMR analysis. See Supporting Information for details on how the copolymer block composition, block lengths, and Mn were determined). ACKNOWLEDGMENTS Financial support for the work performed at CU Boulder was provided by the U.S. Department of Energy ARPA-E program (grant: DE-AR0000343) and matching funds from Total, S.A. Financial support for the work performed at CSU was provided by the CSU Energy Institute and the OVPR Catalyst for Innovative Partnerships Program. Supporting Information Additional Supporting Information may be found in the online version of this article. Filename Description pola28623-sup-0001-suppinfo01.pdf1.4 MB Supporting Information Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article. REFERENCES AND NOTES 1For recent reviews on poly(IL)s, see (a) N. Nishimura, H. Ohno, Polymer. 2014, 55, 3289– 3297; (b) D. Mecerreyes, Prog. Polym. Sci. 2011, 36, 1629– 1648; (c) J. Yuan, M. Antonietti, Polymer. 2011, 52, 1469– 1482. 2For recent reviews on IL-based BCPs, see: (a) K. M. Meek, Y. A. Elabd, J. Mater. Chem. A. 2015, 3, 24187– 24194; (b) J. Yuan, D. Mecerreyes, M. Antonietti, Prog. Polym. Sci. 2013, 38, 1009– 1036. 3For recent examples, see: (a) S. Liu, T. Xu, Macromolecules 2016, 49, 6075– 6083; (b) G. E. Sanoja, B. C. Popere, B. S. Beckingham, C. M. Evans, N. A. Lynd, R. A. Segalman, Macromolecules 2016, 49, 2216– 2223; (c) E. Margaretta, G. B. Fahs, D. 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Mater. 2010, 22, 5992– 6000. 15 G. Floudas, B. Vazaiou, F. Schipper, R. Ulrich, U. Wiesner, H. Iatrou, N. Hadjichristidis, Macromolecules 2001, 34, 2947– 2957. 16 M. A. Hillmyer, F. S. Bates, K. Almdal, K. Mortensen, A. J. Ryan, J. P. A. Fairclough, Science 1996, 271, 976– 978. 17 M. W. Matsen, F. S. Bates, Macromolecules 1996, 29, 7641– 7644. 18 V. F. Scalfani, T. S. Bailey, Macromolecules 2011, 44, 6557– 6567. Citing Literature Volume55, Issue18Special Issue: Special Issue in Honor of Professor Robert H. GrubbsSeptember 15, 2017Pages 2961-2965 FiguresReferencesRelatedInformation