Mechanical Properties of Supramolecular Polymeric Materials Formed

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Mechanical Properties of Supramolecular Polymeric Materials Formed by Cyclodextrins as Host Molecules and Cationic Alkyl Guest Molecules on the Polymer Side Chain Yoshinori Takashima,*,†,§ Kohei Otani,† Yuichiro Kobayashi,‡,∥ Hikaru Aramoto,† Masaki Nakahata,† Hiroyasu Yamaguchi,† and Akira Harada*,‡,∥

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Department of Macromolecular Science, Graduate School of Science, and ‡Project Research Center for Fundamental Sciences, Graduate School of Science, Osaka University, 1-1 Machikaneyama-cho, Toyonaka, Osaka 560-0043, Japan § Institute for Advanced Co-Creation Studies, Osaka University, 1-1 Yamadaoka, Suita, Osaka 565-0871, Japan ∥ JST-ImPACT, 5-7, Chiyoda-ku, Tokyo 100-8914, Japan S Supporting Information *

ABSTRACT: Supramolecular polymeric materials have received much attention in recent years due to the fabrication of supramolecular materials with mechanical properties and stimuli-responsive properties in response to polymeric designs based on noncovalent cross-linkers. Host−guest interactions are useful noncovalent interactions that can be employed to realize supramolecular materials. To understand the relationship between the mechanical properties of supramolecular polymeric materials based on host−guest interactions and the molecular structure of guest molecules on the polymer side chain, we prepared supramolecular hydrogels cross-linked by inclusion complexes between cyclodextrin (CD) and cationic alkyl guests on the polymer side chain. The mechanical properties were influenced by an electric barrier due to the cationic group, CD cavity size, length of the guest unit, charge number of the electric barrier, and reduction responsiveness. Although the introduction of an electric barrier to the end of the alkyl guest moiety increased the rupture stress, the rupture strain decreased due to the electric repulsion of the CD unit and the cationic group. The rupture stress of the αCD-cationic alkyl hydrogel was higher than that of the βCD-cationic alkyl hydrogel because electric repulsion between the αCD unit and the cationic group is higher than that of the βCD unit due to the relatively small cavity size of αCD. The fracture energy increased as the alkyl chain length or charge number of the cationic guest group increased. We demonstrated the change in the mechanical properties by reduction stimulus. interactions,16 host−guest interactions,17−19 or a combination of these interactions, have been employed to functionalize materials. The formation and dissociation of the cross-linking points affect the functionality of the supramolecular polymeric materials. We focused on the host−guest interactions between cyclodextrins (CDs) and hydrophobic guest molecules in aqueous media as the reversible cross-linker on a polymer side chain.20 Previously, we reported supramolecular hydrogels cross-linked by an inclusion complex between CDs and aliphatic guest molecules on the polymer side chain. The mechanical properties, such as the rupture stress, rupture strain, and fracture energy, of the resulting supramolecular hydrogels were higher than those of chemically cross-linked materials.21,22 Our previous results indicate that the association constants of CDs with aliphatic guest molecules and the

1. INTRODUCTION Recent studies on supramolecular polymeric materials have reported that noncovalent interactions impart materials with unique physical properties and functions, such as flexibility, toughness, and stimuli-responsiveness.1−4 Supramolecular polymeric materials are primarily classified into two types (i.e., main-chain and side-chain types). The main-chain type is also called a supramolecular polymer, and the monomeric units connect via noncovalent interactions. Therefore, the resulting materials exhibit self-healing and stimuli-responsive properties due to the reversible noncovalent interactions.4−7 The sidechain type polymers are cross-linked via noncovalent interactions, and these polymers exhibit functions that are derived from the main-chain polymer, in addition to selfhealing and stimuli-responsive properties.8,9 In comparison to the main-chain type, side-chain type polymers can be employed to prepare supramolecular materials due to the high designability of the side chain to functionalize materials. Noncovalent interactions, such as ionic interactions,10−12 coordination bonds,13,14 hydrogen bonds,15 hydrophobic © XXXX American Chemical Society

Received: July 2, 2018 Revised: July 26, 2018

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Figure 1. Mechanical properties of the CD-based supramolecular hydrogels affected by CDs and guest molecule on polymer side chain: (a) electric barrier, (b) CD cavity size, (c) length of guest unit, (d) charge number, and (e) reduction responsiveness.

chemical structure of the CDs and the guest unit are closely related to the mechanical properties of the supramolecular hydrogels.23 However, the complex between CDs and the guest unit did not form a pseudo[2]rotaxane structure. The guest unit was composed of a single chemical unit without a stopper or functional unit. If we design supramolecular hydrogels cross-linked by CDs and two chemical guest units, the mechanical properties of the hydrogels will be affected by the introduction of the second guest unit. Herein, we report the use of viologen derivatives as a second unit in the guest molecules on a polymer side chain. The viologen derivatives with cationic properties were not included in the CD cavities due to electric repulsion.24−26 Based on the electric repulsion between CDs and viologens, linear alkyl chain derivatives with viologen at both ends form [2]rotaxanes with CDs. The viologen units at the ends function as electric stoppers to prevent the dissociation of [2]rotaxanes but not as steric bulky stoppers. If applying thermal stress to [2]rotaxanes, the CDs dethread from the viologen-linear alkyl chain, which acts as the axis.24−26 When the pseudo-rotaxane complexes dissociate, the CDs pass through the cationic viologen units as the electric barrier. The dissociation energy of the pseudo-rotaxane complex with the electric barriers at the ends of an axis molecule is higher than that without the electric barriers. The dissociation energy could be easily controlled by changing the size of the CD cavity, alkyl chain length, and number of cationic species. Herein, we report the use of a viologen as the second chemical unit at the end of the guest molecules on the polymer side chain. Although the viologen unit does not function as a sterically bulky stopper, the electric repulsion between the βCD and the viologen units acts as an electric barrier. We

investigated the effects of the viologen at the end of the alkyl chain, the size of CD cavity, the length of the alkyl chain, and the charge number of the viologen units on the mechanical properties of the obtained hydrogels (Figure 1). The fracture energy of the obtained hydrogels (CD-R gels) increased as the alkyl chain length and the cationic charge number of viologen moieties increased.

2. RESULTS AND DISCUSSION Preparation of CD-R Gels. To investigate the relationship between host−guest interactions and the mechanical properties of CD-based supramolecular hydrogels with guest units (Figure 2a), we employed two types of CD monomers with different CD cavities (6-acrylamido-αCD and 6-acrylamidoβCD: αCDAAm and βCDAAm),27 a linear alkyl guest monomer (C12 monomer), and four types of alkyl viologen monomers (Figure 2b) (VC6: Scheme S1, Figures S1 and S2; VC11: Scheme S2, Figures S3 and S4; AVC11: Scheme S3, Figures S5 and S6; VC10VC11 monomer: Scheme S4, Figures S7 and S8). Prior to performing the radical copolymerization, the guest monomer was added to an aqueous solution of the corresponding CD monomer to form the inclusion complex (Figure 2c). The NOESY NMR spectra demonstrated that the inner protons of the α- or βCD were correlated to the protons of the alkyl chain of the guest monomers (VC6, VC11, and VC10VC11 monomers), indicating the formation of the inclusion complex (Figures S9−S16). After the guest monomers (2 mol %) were mixed with the CD monomers (2 mol %), CD-R gels were obtained by radical copolymerization of the inclusion complex with acrylamide (AAm) (96 mol %) at a total monomer concentration of 2 mol/kg, using B

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Figure 2. (a) Chemical structures of the αCD-R gels, βCD-R gels, and VC11 gel. (b) “R” is a guest group. (c) Typical polymerization scheme for the βCD-VC11 gel.

Effect of the Presence of an Electric Barrier on the Mechanical Properties. The mechanical properties of the βCD-C12 and βCD-VC11 gels were investigated in the presence or absence of an electric barrier due to the cationic guest moiety. Figure 3a shows the stress−strain curves of the βCD-C12 and βCD-VC11 gels, which were obtained using a creep meter at a tensile speed of 1 mm/s. The rupture stress of the βCD-VC11 gel (100 ± 38 kPa) was higher than that of the βCD-C12 gel (51 ± 20 kPa). However, the rupture strain of the βCD-VC11 gel (500 ± 63%) was lower than that of the βCD-C12 gel (1200 ± 250%). The association constant (Ka) of βCDAAm with the VC11 monomer (Ka = 6.6 × 103 M−1) is larger than that of βCD with the C12 unit (Ka = 3.5 × 102 M−1).30 The mechanical properties of the supramolecular

ammonium peroxodisulfate (APS) as the initiator and N,N,N′,N′-tetramethylethylenediamine (TEMED) as the cocatalyst (Figure 2c and Tables S1−S7). The VC11 gel with the chemical cross-linker was prepared by the radical copolymerization of VC11 monomer (2 mol %), AAm (96 mol %), and a covalent cross-linker (N,N′-methylenebis(acrylamide), 2 mol %) under the same conditions (Table S8). The Fourier transform infrared (FT-IR) spectra of the CD-R gels exhibited characteristic peaks of both the CD and guest moieties (Figure S17). The solid-state 1H field gradient magic angle spinning (FGMAS) NMR measurements indicated the amount of CD and guest monomer unit ratio in the CD-R gels in accordance with a predefined ratio (Figures S18−S20).21−23,28,29 C

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Figure 3. Stress−strain mechanical properties of the CD-R gels characterized using a creep meter with a 1 mm/s tensile speed. (a) Stress−strain curves and (b) schematic illustrations of the cross-linking point of the βCD-VC11 and βCD-C12 gels. (c) Stress−strain curves and (d) schematic illustrations of the cross-linking point of the βCD-VC11 and αCD-VC11 gels. Stress−strain curves obtained from the cycle tensile test and schematic illustrations of the cross-linking point of (e, f) the αCD-VC11 gel and (g, h) the βCD-VC11 gel.

hydrogels cross-linked with a reversible bond are affected by the molecular structure and Ka of the complex on the polymer side chain.23 For the βCD-C12 and βCD-VC12 gels, the [2]rotaxane structure in the presence or absence of the cationic viologen group greatly influences the mechanical properties. When the βCD-C12 gel was subjected to tensile stress, the [2]rotaxane complex of the βCD and C12 units dissociated due to the movable motion of the βCD unit on the C12 unit in the absence of an electric barrier. However, when the βCDVC11 gel was subjected to tensile stress, the βCD unit must slip over the cation moiety of the viologen unit to dissociate the [2]rotaxane complex of the βCD and VC11 units (Figure 3b). The presence of the viologen group causes an increase in the rupture stress of the βCD-VC11 gel. In general, the rupture stress and rupture strain have a trade-off relationship. Our previous study demonstrated that the chain length of the guest unit is related to the rupture strain.23 Because of the introduction of the viologen group to the end of the long alkyl chain, the rupture stress of the CD-R gels increased. These results indicate that the mechanical properties of the CD-R gels can be controlled by the design of the end of group of the guest unit. Mechanical Properties of CD-VC11 Gels Based on the Cavity Size of the CDs. In a previous section, we discussed the electric barrier of the end group of a linear alkyl guest unit. Here, the relationship between the mechanical properties and

the cavity size of the CDs is discussed based on the VC11 guest unit. Figure 3c shows the stress−strain curves of the αCD-VC11 and βCD-VC11 gels. The rupture stress of the αCD-VC11 gel (153 ± 23 kPa) was higher than that of the βCD-VC11 gel (100 ± 38 kPa). However, the rupture strain of the αCD-VC11 gel (1000 ± 86%) was lower than that of the βCD-VC11 gel (500 ± 63%). Electric repulsion of the αCD unit and the viologen unit is stronger than that of the βCD unit due to small cavity of αCD. The space-filling model of the CDs and viologen demonstrated that the αCD/viologen complex has less space. However, the βCD/viologen complex has larger space (Figure S21). When the αCD-VC11 gel was subjected to tensile stress, the αCD unit required a higher energy to slip over the cation moiety of the viologen unit (Figure 3d). Therefore, the rupture stress of the αCD-VC11 gel increased compared to that of the βCD-VC11 gel. Upon exposure to rupture stress, the [2]rotaxane complex of the CDs unit with the VC11 guest unit on the polymer chain should be dissociated. However, after loading, the CD and VC11 units will re-form the [2]rotaxane complex. The complex re-formation will be observed using a cycle test. Figures 3e and 3g show the cycle test results for the αCDVC11 and βCD-VC11 gels, respectively. The maximum strains were set to 50−250% for the αCD-VC11 gel and 100−500% for the βCD-VC11 gel at a deformation rate of 1 mm/s. The test pieces were continuously stretched and recovered without D

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Figure 4. (a) Stress−strain curves and (b) fracture energies of the βCD-VC6, βCD-VC11, and βCD-VC10VC11 gels characterized using a 1 mm/s tensile speed and a creep meter. (c) Schematic illustrations of the cross-linking point of the βCD-VC6, βCD-VC11, and βCD-VC10VC11 gels.

the βCD-VC6 gel might be higher than that of the βCD-VC11 gel because the rupture stress of the βCD-VC6 gel might be lower than that of the βCD-VC11 gel. In fact, the Ka of βCD with the C6 moiety (Ka = 3.6 × 102 M−1) is smaller than that with the VC11 moiety (Ka = 6.6 × 103 M−1). However, both the rupture stress and strain of the βCD-VC6 gel were smaller than those of the βCD-VC11 gel. Although the same cationic viologen unit was attached to the end group of the guest unit, the rupture stress of the βCD-VC11 gel was higher than that of the βCD-VC6 gel. The fracture energy increased with the movable area of the βCD unit on the alkyl chain of the guest unit (Figure 4c). These results indicate that the chain length of the guest unit is related to the rupture strain and the fracture energy. To increase the fracture energy, the VC10VC11 monomer was prepared as the guest unit. The chain length of the VC10VC11 unit is twice as long as that of the VC11 unit. To obtain water solubility, another viologen unit was introduced into the middle part of the VC10VC11 unit. By introducing the viologen unit, the βCD unit should slip over the one or two viologen units as a functional electric barrier when dissociating the [2]rotaxane complex. Figures 4a and 4b show the stress− strain curve and fracture energy of the βCD-VC10VC11 gel. The rupture stress, rupture strain, and fracture energy of the βCD-VC10VC11 gel (170 ± 38 kPa, 770 ± 110%, and 350 ± 36 kJ/m3) were higher than those of the βCD-VC11 gel (100 ± 38 kPa, 500 ± 63%, and 290 ± 110 kJ/m3) having approximately the same Young’s modulus (βCD-VC11 gel: 41 ± 6.4 kPa; βCD-VC10VC11 gel: 47 ± 3.0 kPa), indicating that

interval. The hysteresis of the αCD-VC11 gel (15%) was larger than that of the βCD-VC11 gel (7%) (Figure S22). The hysteresis difference is related to the re-formation of the [2]rotaxane complex on the polymer side chain. The dethreaded βCDs relatively easily re-form the [2]rotaxane complex under an unloaded state. The larger cavity of the βCD unit can more easily slip through the viologen unit (Figure 3h). However, the smaller cavity of the αCD unit requires longer time to re-form the [2]rotaxane complex because the αCD unit must slip over the viologen unit to recognize the C11 unit (Figure 3f). The recomplexation requires a higher fracture energy in the βCD-VC11 gel than the αCD-VC11 gel. These results indicate that a dynamic equilibrium state exists inside the CD-VC11 gels, which is closely related to the mechanical properties of the CD-VC11 gels. Relationship between the Fracture Energy and the Chain Length of the Guest Molecules. As described in the previous sections, the mechanical properties of the CD-R gels are affected by the molecular design of the [2]rotaxane complexes. Based on these results, the chain length of the guest molecules will affect the rupture stress and strain (i.e., the fracture energy). Figures 4a and 4b show the stress−strain curves and fracture energies of the βCD-VC6, βCD-VC11, and βCD-VC10VC11 gels, respectively. The rupture stress and strain of the βCD-VC11 gel (100 ± 38 kPa, 500 ± 63%) were higher than those of the βCD-VC6 gel (23 ± 4.7 kPa, 360 ± 37%). Similarly, the fracture energy of the βCD-VC11 gel (290 ± 110 kJ/m3) was ≈10 times higher than that of the βCD-VC6 gel (25 ± 3.0 kJ/m3). For the first time, the rupture strain of E

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Figure 5. (a) Stress−strain curves and (b) fracture energies of the βCD-VC11 and βCD-AVC11 gels characterized using a 1 mm/s tensile speed and a creep meter. (c) Schematic illustrations of the cross-linking point of the βCD-VC11 and βCD-AVC11 gels.

required a higher fracture energy than the βCD-VC11 gel because the dissociation of the βCD unit with the AVC11 unit on the polymer side chain is more difficult than that of the βCD unit and the VC11 unit. An increase in the electric repulsion was also suggested by the cycle test. The hysteresis of the βCD-AVC11 gel (24%) was higher than that of the βCD-VC11 gel (7%) (Figure S23). In contrast to the above results, the rupture strain increased despite the increase in the electric repulsion. This result is most likely related to the effect of eliminating entanglement of the polymer chains due to an increase in the cationic charge number. These results indicate that the mechanical properties of the CD-R gels can be improved by increasing the cationic charge number of the electrical barrier. Reduction of the Charge Number of Viologen. Based on the cation number dependency, the mechanical properties of the βCD-VC11 gel can be changed by redox stimuli. When the βCD-VC11 gel was immersed in a 1 M dithionite (Na2S2O4) aqueous solution, the color of the βCD-VC11 gel changed from light yellow to deep purple (Figure 6a). The color change was derived from the radical cation of the viologen moiety due to one-electron reduction.31 UV−vis measurements were employed to characterize the βCD-VC11 gel before and after reduction. After reduction, two peaks were observed at 560 and 605 nm, which corresponded to radical cation dimer and cationic monomer, respectively (Figure S24).31 Figure 6b shows the stress−strain curves for the βCD-VC11 gel before and after reduction. Both the rupture stress and the rupture strain increased due to reduction. Although the rupture stress of the chemically cross-linked VC11 gel increased with reduction stimulus, the rupture strain did not change (Figure 6c). The formation of the radical cation dimer between the

the improvement in the mechanical properties was due to the structure of the guest unit. The increase in the rupture stress is related to the two guest units (C10 and C11 units) and the two viologen units as electric barriers (Figure 4c). These results indicate that controlling the chain length of the guest unit results in control of the stretchable materials with high fracture energies. Effect of the Number of Cationic Charges. In previous sections, we evaluated the effects of the electric barrier, the cavity size, and the chain length of the guest unit on the mechanical properties. Based on these results, the number of cationic moieties of the guest unit should strongly affect the rupture stress. Therefore, we prepared the βCD-AVC11 gel with the three cationic groups by introducing the quaternary ammonium ion to the VC11 units. Figures 5a and 5b show the stress−strain curves and fracture energies of the βCD-VC11 and βCD-AVC11 gels. The rupture stress, rupture strain, and fracture energy of the βCD-AVC11 gel (200 ± 3.2 kPa, 1200 ± 21%, and 1000 ± 53 kJ/m3) were higher than those of the βCD-VC11 gel (100 ± 38 kPa, 500 ± 63%, and 290 ± 110 kJ/ m3). The Young’s modulus values of the βCD-AVC11 and βCD-VC11 gels were nearly the same (βCD-AVC11 gel: 51 ± 1.4 kPa; βCD-VC11 gel: 41 ± 6.4 kPa), indicating that the cross-linking densities of the βCD-AVC11 and βCD-VC11 gels were nearly the same. Of course, the chain length of the alkyl unit (undecane unit: C11) is the same in the AVC11 and VC11 units. These results suggest that the viologen− ethylammonium moiety, which possesses three cationic groups, resulted in the observed mechanical properties of the βCD-AVC11 gel. The electric repulsion between the βCD unit and the viologen−ethylammonium unit is stronger than that of the βCD unit and the viologen unit. When the βCD-AVC11 gel was subjected to tensile stress, the βCD-AVC11 gel F

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Figure 6. (a) Photographs of the βCD-VC11 gel before and after immersion in a 1 M Na2S2O4 aqueous solution. Stress−strain curves of the initial (yellow) and the reduction states (purple) (b) βCD-VC11 gel and (c) VC11 gel. (d) Fracture energies of the initial (yellow) and reduction states (purple) βCD-VC11 gel and VC11 gel. (e) Schematic illustrations of the internal structure of the βCD-VC11 gel before and after reduction.

repulsion between the CD unit and the viologen unit. The rupture stress of the αCD-VC11 gel was higher than that of the βCD-VC11 gel. However, the rupture strain of the αCD-VC11 gel was lower than that of the βCD-VC11 gel because the electric repulsion of the αCD unit with the viologen unit is stronger than that of the βCD unit with the viologen unit due to relatively small cavity of αCD. An increase in the alkyl chain length improved the fracture energy by increasing the movable distance of the CD. Unexpectedly, an increase in the charge number of the electric barrier improved the fracture energy, which may be due to a synergistic effect between an increase in the electric repulsion and a decrease in the entanglement of the polymer chains as the charge number increased. Interestingly, the fracture energy of the βCD-VC11 gel also increased upon reduction. The inclusion complex of βCD and VC11 and the radical cation dimer of the viologen moieties functioned as dual cross-linking points. These promising results should contribute to development in supramolecular chemistry as well as materials science.

viologen units caused the increase in the rupture stress of both the βCD-VC11 and VC11 gels. In addition, the Young’s modulus, which is related to the cross-linking density, increased after reduction. Because the cationic charge number of the viologen unit decreased upon reduction, the electric repulsion of the βCD unit to the viologen unit decreased. Because the βCD unit easily dissociates with the VC11 moiety after reduction, the rupture strain of the βCD-VC11 gel increased upon reduction. These results indicate that the host−guest inclusion complex and radical cation dimer cooperatively functioned as dual cross-linking points, and therefore the mechanical properties of our materials improved (Figures 6d and 6e).

3. CONCLUSION We studied the influence of CDs and cationic alkyl guest molecules on the polymer side chain on the mechanical properties of CD-based supramolecular hydrogels. The mechanical properties were influenced by the electric barrier, CD cavity size, length of the guest unit, charge number, and reduction-responsiveness. The introduction of an electric barrier to the end of the alkyl guest group increased the rupture stress, and the rupture strain decreased due to electric G

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b01410. Experimental details, preparation of the monomers, and preparation and characterization of the gels (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y.T.). *E-mail: [email protected] (A.H.). ORCID

Yoshinori Takashima: 0000-0002-2620-3266 Masaki Nakahata: 0000-0003-0397-5922 Hiroyasu Yamaguchi: 0000-0002-4801-5071 Akira Harada: 0000-0002-9309-5939 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was funded by the ImPACT Program of the Council for Science, Technology and Innovation (Cabinet Office, Government of Japan), a Grant-in-Aid for Scientific Research (B) (No. 26288062) from MEXT of Japan, and a Grant-in-Aid from the Research Grant Program of the Asahi Glass Foundation (2015). We thank Prof. T. Inoue of Osaka University for access to dynamic viscoelastic measurements. We also acknowledge the NMR technical assistance of Dr. N. Inazumi.

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DOI: 10.1021/acs.macromol.8b01410 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules (31) Kosower, E. M.; Cotter, J. L. Stable Free Radicals 0.2. Reduction of 1-Methyl-4-Cyanopyridinium Ion to Methylviologen Cation Radical. J. Am. Chem. Soc. 1964, 86, 5524−5527.

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DOI: 10.1021/acs.macromol.8b01410 Macromolecules XXXX, XXX, XXX−XXX