Thermally Stable Radical-Type Mechanochromic Polymers Based on

Letter Cite This: ACS Macro Lett. 2018, 7, 1359−1363

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Thermally Stable Radical-Type Mechanochromic Polymers Based on Difluorenylsuccinonitrile Hio Sakai, Toshikazu Sumi, Daisuke Aoki, Raita Goseki, and Hideyuki Otsuka* Department of Chemical Science and Engineering, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8550, Japan

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S Supporting Information *

ABSTRACT: Mechanochromism, a color change induced by mechanical force, has attracted much attention in materials science, as it can be used to create stress- and damagedetecting sensors. In particular, radical-type mechanochromic molecules (mechanochromophores), which produce colored radicals upon exposure to mechanical force, enable the qualitative visualization of mechanical stress and the quantitative evaluation of the generated radical species by electron paramagnetic resonance spectroscopy. However, the sensitivity of radical-type mechanochromophores to thermal stimuli limits their range of applications. Herein, we report the radical-type mechanochromophore difluorenylsuccinonitrile (DFSN), which can be used to synthesize mechanochromic polymers via living radical polymerization techniques, as its central carbon−carbon bond exhibits high thermal stability. The obtained DFSN-centered polymers show mechanochromism and desirably high thermal resistance.

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thermal resistance. Specifically, radical-type mechanochromophores cannot be used in combination with radical polymerization techniques, which would allow an easy introduction of mechanochromophores into polymer chains, given that the thus generated radical species work as polymerization inhibitors. Accordingly, click reactions, which do not require heating, have predominantly been employed to introduce radical-type mechanochromophores into polymer midchains.24,25,27 In this paper, we report the design and synthesis of the novel mechanochromophore difluorenylsuccinonitrile (DFSN), whose central carbon−carbon bond exhibits high thermal resistance while remaining sensitive to mechanical stimuli (Figure 1). In order to suppress the homolytic cleavage of the central C−C bond in radical-type mechanochromophores upon mild heating, we focused on the 9,9′-bifluorene skeleton reported by Neumann and Stapel.28 The central C−C bond of 9,9′bifluorene dissociates at relatively high temperature (110 °C) and generates the corresponding radical species. The thermal stability of 9,9′-bifluorene is thus higher than that of previously reported radical-type mechanochromophores, whose central C−C bonds are reversibly cleaved even at room temperature.16,22−27 In addition, the cyanofluorene radical, which is expected to be generated from the DFSN skeleton, exhibits high tolerance toward oxygen (Scheme 1a).29 These reports

echanoresponsive materials have recently received increasing interest.1−7 In particular, mechanochromic polymers, which change color upon exposure to mechanical force, have attracted much attention in materials science, given that they enable detecting stressed and damaged parts in materials.8−27 One promising strategy for the preparation of mechanochromic polymers is the incorporation of mechanochromic molecules (mechanochromophores) into polymer chains. In the very first report on this concept, Sottos and coworkers demonstrated that poly(methyl acrylate) with a mechanochromic spiropyran moiety at the central position exhibited a drastic color change in the bulk in response to tensile stress.1 Furthermore, they also demonstrated that the mechanochemical reactions accelerate when mechanochromic units are incorporated into the middle of the polymer chain.1 A subtype of mechanochromophores are radical-type mechanochromophores, which generate relatively stable radical species upon mechanical stimulation and change color based on the absorption of light by the generated radicals.22−27 The main advantage of using radical-type mechanochromophores is that a quantitative evaluation of the generated radical species can be accomplished by electron paramagnetic resonance (EPR) spectroscopy both in solution and in the bulk. We have already reported radical-type mechanochromophores, which undergo homolytic cleavage of the central C−C bonds upon exposure to mechanical force to afford colored radical species and introduced them into mechanochromic polymers to furnish functional polymeric materials.22−27 However, the range of applications of radical-type mechanochromophores is severely limited by their weak © XXXX American Chemical Society

Received: September 29, 2018 Accepted: October 23, 2018

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DOI: 10.1021/acsmacrolett.8b00755 ACS Macro Lett. 2018, 7, 1359−1363

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Figure 1. Schematic illustration of thermally stable radical-type mechanochromic polymers.

salt of NH2OH in ethanol generated oxime 4, which was dehydrated by a treatment with thionyl chloride to afford 5. Subsequently, 5 was deprotected with tetrabutylammonium fluoride to furnish 6. Finally, an oxidative coupling of 6 afforded DFSN-diol as a white powder. The chemical structure of DFSN-diol was confirmed by 1H and 13C NMR as well as FT-IR spectroscopy measurements in combination with fast atom bombardment mass spectrometry (FAB-MS) (Figures S1 and S2). To confirm the thermal stability of DFSN-diol, the thermal dissociation behavior of acetylated DFSN-diol (7; Scheme 1d), which was prepared for the EPR measurements given the poor solubility of DFSN-diol, was investigated. EPR measurements of 7 in anisole at elevated temperatures showed signals that are comparable to those of typical carbon radicals (g = 2.003; Figure S8). The dissociation enthalpy (23.3 kcal/ mol) was estimated from the slope of the van’t Hoff plots of the corresponding dissociation equilibrium constants as a function of the temperature (Figure S9, Table S1). The corresponding value is similar to those of TASN and DABBF and is much lower than those of general C−C bonds (e.g., 90.0 kcal/mol for ethane).24,26 It should be noted that the dissociation ratio of the radical generated from 7 (6.0 × 10−4%) was ∼1/250 of that of the TASN derivatives (1.6 × 10−1%) at 100 °C (Figure 2b).24 While TASN (10 mM in anisole) changes to pink at 100 °C, due to the thermally generated radical species (Figure S7b),24 DFSN-diol (10 mM in anisole) does not change its color, not even at 130 °C (Figure 2a), which demonstrates the excellent thermal stability of the DFSN skeleton. This outstanding thermal resistance of the DFSN skeleton can be used in radical polymerizations, as the number of free radicals generated upon heating DFSN is quite low. To attach polymer chains onto the DFSN skeletons via living radical polymerizations, DFSN-dibromide 8, which bears the bifunctional initiator moieties for atom transfer radical polymerizations (ATRPs),31 was prepared by functionalizing the hydroxy groups with 2-bromoisobutyryl bromide. ATRP of methyl methacrylate (MMA; MMA/8/ligand = 400/1/1) was conducted in 50% (v/v) anisole at 50 °C (Scheme 1e). The

inspired us to investigate the mechanochromic properties of DFSN derivatives, given their outstanding anticipated potential as novel radical-type mechanochromophores with high thermal stability. As the chemical structure of DFSN is similar to that of tetraarylsuccinonitrile (TASN; Scheme 1b), which is a radicaltype mechanochromophore we have already reported,24 an investigation into the thermo- and mechanoresponsive properties of DFSN should provide further knowledge on radical-type mechanochromophores and mechanochromic polymers. DFSN was synthesized according to a previously reported oxidative coupling of 9-cyanofluorene.30 To investigate the thermal properties of DFSN, variable-temperature EPR measurements were carried out in anisole. The results revealed that the dissociation ratio of DFSN (2.7 × 10−4%) was ∼1/600 compared to that of TASN derivatives (1.6 × 10−1%) at 100 °C (Figure S7a), confirming the expected good thermostability of DFSN. Subsequently, we investigated the mechanochromic properties of DFSN using a grinding test and EPR measurements. Although coloration of the ground DFSN was not observed (Figure S10a), a weak but nevertheless increased EPR signal relative to that before grinding was observed (Figure S11a). The observed g value (2.003) supports a homolytic bond cleavage of the central C−C bond in DFSN upon exposure to shear forces, demonstrating that DFSN works as a mechanochromophore. However, its sensitivity to heating is substantially diminished compared to the hitherto reported radical-type mechanochromophores TASN and diarylbibenzofuranone (DABBF).22−27 These results motivated us to attach polymer chains onto DFSN, given that the mechanoresponsiveness of mechanochromophores can be amplified upon incorporation into a polymer chain. As shown in Scheme 1c, DFSN-diol, which has two hydroxy groups for the potential attachment of polymer chains, was newly designed and synthesized. The starting material fluorene-2-carboxaldehyde was reduced to 2-hydroxymethylfluorene 1 using sodium borohydride. The hydroxy group in 1 was then protected by a tert-butyldimethylsilyl group to furnish 2, followed by a formylation with ethyl formate under basic conditions to afford 3. Treatment of 3 with the hydrochloride 1360

DOI: 10.1021/acsmacrolett.8b00755 ACS Macro Lett. 2018, 7, 1359−1363

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ACS Macro Letters Scheme 1. (a) Equilibrium between DFSN and Its Radical Species, (b) Equilibrium between TASN and Its Radical Species, (c) Synthetic Route to DFSN-diol, (d) Synthetic Route to Acetylated DFSN-diol (7), and (e) Synthetic Route to DFSN-Centered Poly(methyl methacrylate) (PMMA-DFSN-PMMA)

Figure 2. (a) Photographs of DFSN-diol in anisole at r.t. and 130 °C. (b) EPR-derived dissociation ratio of TASN and 7 as a function of temperature.

Figure 3. SEC traces of the ATRP reaction solution of MMA initiated by 8 after (a) 15, (b) 30, and (c) 45 min.

with a broad peak, which was attributed to the radicalscavenging performance of TASN, was obtained (Figure S5). After purification, the resulting polymer was characterized by SEC (Mn = 13 200; Mw/Mn = 1.14; Figure S4). To demonstrate that the central C−C bond in the DFSN skeleton remains intact during the radical polymerization, an exchange reaction between PMMA-DFSN-PMMA and excess DFSN (50 equiv) was carried out in anisole at 110 °C (2 h). The SEC curve of the polymer shifted to lower molecular weight after the exchange (Figure S6), and the Mn of the resulting polymer was almost half of that of the original polymer, which supports that the DFSN skeleton is located at the center of the polymer chain and that an exchange reaction with DFSN occurred. These results revealed that 8 effectively initiates the ATRP of MMA and that the polymerization proceeded in a living fashion under retention of the DFSN skeleton. The important point to note here is that the exchange reaction of the DFSN skeleton can be easily induced despite its high thermal stability, which is one of the advantages of the dynamic nature of the DFSN skeleton. Actually, the amount of

polymerization reaction was monitored by SEC, which revealed increasing molecular weight during the progress of the reaction, whereby a significant change in Mw/Mn was not observed (Figure 3). When the polymerization reaction was carried out using TASN-dibromide 9 instead of 8, a polymer 1361

DOI: 10.1021/acsmacrolett.8b00755 ACS Macro Lett. 2018, 7, 1359−1363

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DFSN moieties in the middle of polymer chains. As it was revealed that mechanical stress is effectively transferred to the DFSN unit through the polymer chains in the DFSNcontaining polymer system, additionally the pink color of PMMA-DFSN-PMMA after the grinding went back to the original white color when THF was added (Figure S12). This color change was caused by the recombination of the generated carbon radicals. The thermal dissociation properties of PMMA-DFSN-PMMA in the solid state were examined by EPR measurements at 150 °C. However, at that temperature, neither EPR signals nor a significant color change that could potentially be associated with the formation of cyanofluorene radicals was observed (Figure 4d, Figure S11b), which suggests high thermal stability. Thus, the DFSN-based polymers not only are activated by mechanical force but also show appreciably high thermal resistance. In conclusion, we have successfully demonstrated that the DFSN skeleton can serve as a thermally stable radical-type mechanochromophore, which can be applied to living radical polymerization systems in order to synthesize well-defined polymers. The activation of DFSN-based polymers is selective toward the exposure to mechanical force, which generates colored radicals. This behavior was confirmed by heating tests in solution and in the bulk, grinding tests, as well as solid-state UV−vis spectroscopy and EPR spectroscopy. Although the detailed mechanism and reasons for the thermal stability of the central C−C bond in DFSN remain under investigation, such a selective activation of radical-type mechanochromophores is unprecedented. The results disseminated in this paper thus represent a significant advance for fundamental research on mechanochemistry and its applications in material science.

radical generated from PMMA-DFSN-PMMA was too low to accurately quantify. In order to investigate its mechanochromic properties, PMMA-DFSN-PMMA, a white solid, was ground at room temperature using a mortar. Although the color of DFSN-diol in the absence of a polymer chain did not change upon exposure to mechanical stress (Figure S10b), the color of the PMMA-DFSN-PMMA sample changed color from white to pink upon grinding (Figure 4a). This color change was

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.8b00755. Experimental details, 1H and 13C NMR spectra, SEC profiles, and EPR spectra for all synthesized compounds and polymers (PDF)

Figure 4. (a) Photographs of PMMA-DFSN-PMMA before and after grinding. (b) Solid-state UV−vis spectra of PMMA-DFSN-PMMA before and after grinding. (c) EPR spectra of PMMA-DFSN-PMMA and DFSN-diol before and after grinding. (d) Photographs of PMMADFSN-PMMA after heating and grinding.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

examined by solid-state UV−vis spectroscopy. The pink coloration after grinding was attributed to the new peaks at 542 and 526 nm (Figure 4b). To confirm the generation of radical species during the grinding process, EPR spectroscopic measurements were carried out on PMMA-DFSN-PMMA samples before and after grinding. Figure 4c shows the EPR spectra of PMMA-DFSN-PMMA and DFSN-diol before and after grinding. The intensity of the signal for PMMA-DFSNPMMA after grinding was much stronger than that for DFSNdiol. The estimated g values (2.003) were identical and consistent with the generation of carbon-centered radicals, indicating that the central C−C bond in the DFSN skeleton is homolytically cleaved in response to mechanical stress, which is commensurate with the generation of the corresponding radical species. The dissociation ratio of the DFSN skeleton in PMMA-DFSN-PMMA after grinding (3.2 × 10−1%) was estimated to be at least 3 orders of magnitude higher than that of DFSN-diol (4.0 × 10−4%), which reflects an enhancement of the mechanochromic properties upon the introduction of

ORCID

Hideyuki Otsuka: 0000-0002-1512-671X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a KAKENHI grant 17H01205 (H.O.) from the Japan Society for the Promotion of Science (JSPS), as well as by the ImPACT Program of the Council for Science, Technology, and Innovation (Cabinet Office, Government of Japan).

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REFERENCES

(1) Davis, D. A.; Hamilton, A.; Yang, J.; Cremar, L. D.; Van Gough, D.; Potisek, S. L.; Ong, M. T.; Braun, P. V.; Martínez, T. J.; White, S. R.; Moore, J. S.; Sottos, N. R. Force-Induced Activation of Covalent Bonds in Mechanoresponsive Polymeric Materials. Nature 2009, 459 (7243), 68−72. 1362

DOI: 10.1021/acsmacrolett.8b00755 ACS Macro Lett. 2018, 7, 1359−1363

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ACS Macro Letters (2) Diesendruck, C. E.; Steinberg, B. D.; Sugai, N.; Silberstein, M. N.; Sottos, N. R.; White, S. R.; Braun, P. V.; Moore, J. S. ProtonCoupled Mechanochemical Transduction: A Mechanogenerated Acid. J. Am. Chem. Soc. 2012, 134 (30), 12446−12449. (3) Jakobs, R. T. M.; Ma, S.; Sijbesma, R. P. Mechanocatalytic Polymerization and Cross-Linking in a Polymeric Matrix. ACS Macro Lett. 2013, 2 (7), 613−616. (4) Ramirez, A. L. B.; Kean, Z. S.; Orlicki, J. A.; Champhekar, M.; Elsakr, S. M.; Krause, W. E.; Craig, S. L. Mechanochemical Strengthening of a Synthetic Polymer in Response to Typically Destructive Shear Forces. Nat. Chem. 2013, 5 (9), 757−761. (5) Boulatov, R. Mechanochemistry: Demonstrated Leverage. Nat. Chem. 2013, 5 (2), 84−86. (6) Kida, J.; Imato, K.; Aoki, D.; Goseki, R.; Morimoto, M.; Otsuka, H. The Photoregulation of a Mechanochemical Polymer Scission. Nat. Commun. 2018, 9, 3504. (7) Li, J.; Nagamani, C.; Moore, J. S. Polymer Mechanochemistry: From Destructive to Productive. Acc. Chem. Res. 2015, 48 (8), 2181− 2190. (8) Imato, K.; Kanehara, T.; Ohishi, T.; Nishihara, M.; Yajima, H.; Ito, M.; Takahara, A.; Otsuka, H. Mechanochromic Dynamic Covalent Elastomers: Quantitative Stress Evaluation and Autonomous Recovery. ACS Macro Lett. 2015, 4 (11), 1307−1311. (9) Zhang, H.; Li, X.; Lin, Y.; Gao, F.; Tang, Z.; Su, P.; Zhang, W.; Xu, Y.; Weng, W.; Boulatov, R. Multi-Modal Mechanophores Based on Cinnamate Dimers. Nat. Commun. 2017, 8, 1147. (10) Celestine, A. D. N.; Beiermann, B. A.; May, P. A.; Moore, J. S.; Sottos, N. R.; White, S. R. Fracture-Induced Activation in Mechanophore-Linked, Rubber Toughened PMMA. Polymer 2014, 55 (16), 4164−4171. (11) Chen, Y.; Sijbesma, R. P. Dioxetanes as Mechanoluminescent Probes in Thermoplastic Elastomers. Macromolecules 2014, 47 (12), 3797−3805. (12) Chen, Y.; Spiering, A. J. H.; Karthikeyan, S.; Peters, G. W. M.; Meijer, E. W.; Sijbesma, R. P. Mechanically Induced Chemiluminescence from Polymers Incorporating a 1,2-Dioxetane Unit in the Main Chain. Nat. Chem. 2012, 4 (7), 559−562. (13) Caruso, M. M.; Davis, D. A.; Shen, Q.; Odom, S. A.; Sottos, N. R.; White, S. R.; Moore, J. S. Mechanically-Induced Chemical Changes in Polymeric Materials. Chem. Rev. 2009, 109 (11), 5755− 5798. (14) Cho, S. Y.; Kim, J. G.; Chung, C. M. A Fluorescent Crack Sensor Based on Cyclobutane-Containing Crosslinked Polymers of Tricinnamates. Sens. Actuators, B 2008, 134 (2), 822−825. (15) Crenshaw, B. R.; Weder, C. Deformation-Induced Color Changes in Melt-Processed Photoluminescent Polymer Blends. Chem. Mater. 2003, 15 (25), 4717−4724. (16) Kobashi, T.; Sakamaki, D.; Seki, S. N-Substituted Dicyanomethylphenyl Radicals: Dynamic Covalent Properties and Formation of Stimuli-Responsive Cyclophanes by Self-Assembly. Angew. Chem., Int. Ed. 2016, 55 (30), 8634−8638. (17) Verstraeten, F.; Göstl, R.; Sijbesma, R. P. Stress-Induced Colouration and Crosslinking of Polymeric Materials by Mechanochemical Formation of Triphenylimidazolyl Radicals. Chem. Commun. 2016, 52 (55), 8608−8611. (18) Göstl, R.; Sijbesma, R. P. Π-Extended Anthracenes As Sensitive Probes for Mechanical Stress. Chem. Sci. 2015, 7 (1), 370−375. (19) Chen, Y.; Zhang, H.; Fang, X.; Lin, Y.; Xu, Y.; Weng, W. Mechanical Activation of Mechanophore Enhanced by Strong Hydrogen Bonding Interactions. ACS Macro Lett. 2014, 3 (2), 141−145. (20) Peterson, G. I.; Larsen, M. B.; Ganter, M. A.; Storti, D. W.; Boydston, A. J. 3D-Printed Mechanochromic Materials. ACS Appl. Mater. Interfaces 2015, 7 (1), 577−583. (21) Beyer, M. K.; Clausen-Schaumann, H. Mechanochemistry: The Mechanical Activation of Covalent Bonds. Chem. Rev. 2005, 105 (8), 2921−2948. (22) Imato, K.; Natterodt, J. C.; Sapkota, J.; Goseki, R.; Weder, C.; Takahara, A.; Otsuka, H. Dynamic Covalent Diarylbibenzofuranone-

Modified Nanocellulose: Mechanochromic Behaviour and Application in Self-Healing Polymer Composites. Polym. Chem. 2017, 8 (13), 2115−2122. (23) Kosuge, T.; Imato, K.; Goseki, R.; Otsuka, H. PolymerInorganic Composites with Dynamic Covalent Mechanochromophore. Macromolecules 2016, 49, 5903−5911. (24) Sumi, T.; Goseki, R.; Otsuka, H. Tetraarylsuccinonitriles as Mechanochromophores to Generate Highly Stable Luminescent Carbon-Centered Radicals. Chem. Commun. 2017, 53, 11885−11888. (25) Oka, H.; Imato, K.; Sato, T.; Ohishi, T.; Goseki, R.; Otsuka, H. Enhancing Mechanochemical Activation in the Bulk State by Designing Polymer Architectures. ACS Macro Lett. 2016, 5 (10), 1124−1127. (26) Imato, K.; Irie, A.; Kosuge, T.; Ohishi, T.; Nishihara, M.; Takahara, A.; Otsuka, H. Mechanophores with a Reversible Radical System and Freezing-Induced Mechanochemistry in Polymer Solutions and Gels. Angew. Chem., Int. Ed. 2015, 54 (21), 6168−6172. (27) Ishizuki, K.; Oka, H.; Aoki, D.; Goseki, R.; Otsuka, H. Mechanochromic Polymers That Turn Green Upon the Dissociation of Diarylbibenzothiophenonyl: The Missing Piece toward Rainbow Mechanochromism. Chem. - Eur. J. 2018, 24, 3170−3173. (28) Neumann, W. P.; Stapel, R. Zur Existenz von Tetraphenylbernsteinsaure Und Ihrer Ester Sowie Ü ber Die Struktur Der Dimeren Der Diarylmethyl-Radikale. Chem. Ber. 1986, 119, 3422−3431. (29) Font-Sanchis, E.; Aliaga, C.; Focsaneanu, K.-S.; Scaiano, J. C. Greatly Attenuated Reactivity of Nitrile-Derived Carbon-Centered Radicals toward Oxygen. Chem. Commun. 2002, No. 15, 1576−1577. (30) Hou, X.; Ge, Z.; Wang, T.; Guo, W.; Wu, J.; Cui, J.; Lai, C.; Li, R. Synthesis and Structure-Activity Relationships of A Novel Class of Dithiocarbamic Acid Esters as Anticancer Agent. Arch. Pharm. (Weinheim, Ger.) 2011, 344 (5), 320−332. (31) Matyjaszewski, K.; Xia, J. Atom Transfer Radical Polymerization. Chem. Rev. 2001, 101 (9), 2921−2990.

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