Enhancing Mechanochemical Activation in the Bulk State by

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Enhancing Mechanochemical Activation in the Bulk State by Designing Polymer Architectures Hironori Oka, Keiichi Imato, Tomoya Sato, Tomoyuki Ohishi, Raita Goseki, and Hideyuki Otsuka* Department of Organic and Polymeric Materials, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8550, Japan S Supporting Information *

ABSTRACT: Mechanoresponsive polymers can have attractive functions; however, the relationship between polymer architecture and mechanoresponsiveness in the bulk state is still poorly understood. Here, we designed well-defined linear and star polymers with a mechanophore at the center of each architecture, and investigated the effect of molecular weight and branched structures on mechanoresponsiveness in the solid state. Diarylbibenzofuranone, which can undergo homolytic cleavage of the central C−C bond by mechanical force to form blue-colored radicals, was used as a mechanophore because the cleaved radicals could be evaluated quantitatively using electron paramagnetic resonance measurements. We confirmed that longer polymer chains induce mechanochemical activation more effectively and found that, in the bulk state, the star polymers have higher sensitivity to mechanical stress compared with a linear polymer having similar molecular weight arm segment.

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chain entanglements, it remains poorly understood. Because mechanoresponsiveness in bulk is more important for practical applications than that in solution, elucidation of this relationship in the bulk state may contribute to the rational design of mechanoresponsive materials. In the present study, we designed and synthesized welldefined linear and star polymers with a mechanochromophore functionality at the center of each polymer structure to prepare mechanoresponsive materials with high sensitivity and demonstrate the relationship between polymer architecture and mechanoresponsiveness in the bulk state. Diarylbibenzofuranone (DABBF), which can undergo homolytic cleavage of the central C−C bond by mechanical force to form bluecolored radicals (Figure 1a), was selected as the mechanochromophore. Notably, the blue-colored radicals obtained by cleavage of DABBF are relatively stable, even in air, and can be evaluated quantitatively using electron paramagnetic resonance (EPR) measurements.17 Recently, we have revealed that the mechanoresponsiveness of DABBF mechanophore is enhanced by incorporation of the mechanophore into rigid silica networks.18 In addition, it was also found that the rigid network can allow suppression of the recombination of activated radicals from the cleavage of DABBF.18 Based on these previous results, to investigate the relationship between polymer architecture and mechanores-

echanoresponsive materials, which have properties that are altered in response to external force, have been of great interest over the past several years.1 In particular, mechanochromic polymers, which change color with mechanical stress, have attracted much attention because they can detect damage inflicted on a material, enabling it to be replaced or repaired before failure.2−7 The main strategy for building mechanochromic polymers involves incorporation of mechanochromophores (optically mechanoresponsive units) into polymer chains. Sottos and co-workers first reported that poly(methyl acrylate) with a mechanochromic spiropyran moiety in the main chain changes from yellow to red in the bulk state in response to tensile stress.2 Furthermore, they demonstrated that the mechanochemical reactions are accelerated when mechanochromic units are incorporated in the polymer midchain instead of at the chain terminus or as a pendant group. This behavior is consistent with the common belief that the shear forces reach a maximum in the middle of a polymer chain, in both solution and solid states.8 Similar to the effect of mechanophore location, the polymer architecture, such as comb, star, and loop, should affect mechanochemical reactions significantly because these structures have smaller hydrodynamic radii compared with their linear counterparts. To date, the mechanochemistry of topologically complex polymers has been extensively investigated in solution, and these polymers were revealed to be less likely to experience shear force in solution than their linear analogues.9−16 Although the relationship between polymer architecture and mechanoresponsiveness in bulk is expected to differ from that in solution owing to the effect of intermolecular © XXXX American Chemical Society

Received: July 11, 2016 Accepted: September 16, 2016

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DOI: 10.1021/acsmacrolett.6b00529 ACS Macro Lett. 2016, 5, 1124−1127

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ACS Macro Letters

Table 1. Molecular Weights and Molecular Weight Distributions of Linear and Four- and Eight-Arm Star Polystyrenes with DABBF at the Center (Core) of the Polymer Structures polymer

Mna (g mol−1)

Mw/Mna

Tgb (°C)

linear PS1 linear PS2 linear PS3 linear PS4 four-arm star PS eight-arm star PS

13100 24700 42400 95000 38600 (44300c) 52100 (81600c)

1.05 1.04 1.04 1.03 1.05 1.03

100 103 105 106 104 103

a Estimated by GPC calibrating with polystyrene standard. bDetermined by DSC measurement. cDetermined by GPC equipped with triple detectors.

but react with highly reactive radicals.20,21 Therefore, to confirm the structure of the obtained polymers, selective scission of the incorporated DABBF units by treatment with a radical generator was performed. The GPC curves of the polymers were shifted to the lower molecular weight region after reaction with 2,2′-azobis(4-methoxy-2,4-dimethylvaleronitrile) (V-70) at 30 °C for 24 h (Figure S6). The molecular weights of the resulting polymers were almost half those of the original polymers. In addition, quantitative incorporation of DABBF units into the polymer chains was confirmed by 1H NMR spectroscopy (Figure S4). Thus, DABBF functionalities were successfully incorporated into the center of most of the chains in the linear polymers. Similarly, four- and eight-arm star polystyrenes with DABBF at the core were synthesized by click reactions between an azide-terminated polystyrene (Mn = 11600 g mol−1) or in-chain azide-functionalized polystyrenes (Mn = 20200 g mol−1) and a DABBF-tetraalkyne derivative. This derivative was prepared in 85% yield in the abovementioned manner using tetrahydroxy DABBF instead of dihydroxy DABBF (Figure 1). The molecular weights of the obtained four- and eight-arm star polystyrenes with DABBF were determined by GPC-RALLS (Mn = 44300, 81600 g mol−1) in good agreement with the calculated values (Mn calcd = 46400 and 82000 g mol−1) and narrow molecular weight distributions (1.05 and 1.03) (Table 1). The selective scission of the core DABBFs in the four- and eight-arm star polymers resulted in decreases in the molecular weights (Figures S7 and S8). In particular, the molecular weight after chain scission of the four-arm star polymer was 22000 g mol−1, which is corresponding to the half of determined for the original polymer. The mechanochromic properties of the obtained polymers were evaluated using grinding tests.18 In the bulk state, in addition to the chain mobility plays a key role for propagating the mechanical force to mechanophore,22 the rapid recombination reactions has to be considered in the DABBF-containing polymer systems.17 Therefore, simple operating at room temperature, grinding test, was employed as preliminary accounts. To reduce experimental errors, an autogrinding machine was used in this study. The powdered sample (100 mg) was placed in the mortar and ground for 10 min at 100 rpm. Both the linear and star polymers showed a drastic color change from white to blue after grinding for 10 min (Figure 2). On the other hand, with thermal treatment, no color change was observed even above the glass transition temperature, though the ratio of the dissociated DABBF was barely increased (Figure S9). Thus, the multiplex mechanical stresses (such as

Figure 1. (a) Chemical structure and equilibrium of diarylbibenzofuranone (DABBF), and chemical structures of (b) linear polystyrenes with DABBF in the center of polymer chains and (c) four- and (d) eight-arm star polystyrenes with DABBF at the core of the structures.

ponsiveness, we designed linear and four- and eight-arm star glassy polystyrenes with DABBF functionality in the center of the polymer structures (Figure 1b−d). These desired polymers were synthesized by using a combination of controlled/living polymerization techniques (atom transfer radical polymerization (ATRP) and anionic polymerization) and Cu-catalyzed alkyne/azide click reaction.19 To prepare the linear polystyrenes, a DABBF-dialkyne derivative was synthesized in 96% yield by esterification of dihydroxy DABBF and 5-hexynoyl chloride. Then, azide-terminated polystyrenes with different molecular weights (Mn = 4900, 11600, 16800, and 45200 g mol−1) were prepared by ATRP or anionic polymerization, followed by substitution of each terminal bromine atom with an azide functionality. Finally, the DABBF-dialkyne derivative and the azide-terminated polystyrenes were coupled via a click reaction to form center-functionalized linear polystyrenes 1, 2, 3, and 4 (Mn = 13100, 24700, 42400, and 95000 g mol−1, respectively) with narrow molecular weight distributions (Mw/ Mn = 1.03−1.05; Table 1). DABBF is known to be in equilibrium with its dissociated radicals, which are quite stable 1125

DOI: 10.1021/acsmacrolett.6b00529 ACS Macro Lett. 2016, 5, 1124−1127

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ACS Macro Letters

Figure 3. Ratio of dissociated DABBF in dihydroxy DABBF, linear (blue bars), four-arm star (red bar), and eight-arm star (red bar) polystyrenes with DABBF in the center (core) of polymer structures.

To quantitatively evaluate the effect of polymer chain topology on mechanochemical activation, grinding tests were also performed for the four- and eight-arm star polymers (Figure 3). Surprisingly, the percentage of dissociated DABBF in the four- and eight-arm star polymers (Mn = 44300 and 81600 g mol−1) were approximately 26 and 46%, which are much higher than the values for the corresponding linear polymers having similar total molecular weight (3, Mn = 42400 g mol−1, 17%; and 4, Mn = 95000 g mol−1, 26%). Moreover, the ratio of dissociated DABBF was dramatically increased with increasing the numbers of the arms with almost same lengths (Mn = 10000 g mol−1) attached on DABBF (two-arm 2 (Mn = 24700 g mol−1), 14%; four-arm, 26%; and eight-arm, 46%). These results indicate that the mechanical force accumulates at the cores as a result of efficient transfer via the polymer chains in the star architectures. On the other hand, the chain mobility of the activated ABF radicals from the star polymers would be much lower than those of the linear polymers due to different locations (linear polymer, chain-end; and star polymer, midchain), though the glass transition temperatures of prepared polymers were almost comparable (Table 1). In addition, the recombination reaction of midchain localized radical might be disturbed by steric hindrance from the branched-arms. As a result, it was considered that the star polymers showed high dissociation ratio to mechanical stress. In contrast, as described above, the molecular weight dependence in the bulk state is consistent with the expected trend in solution, whereas the relationship between polymer architecture and mechanoresponsiveness in the bulk state differs completely from that observed in solution systems, where an increase in the branch number decreases the reactivity. The shear stability of star polymers in solution may be attributed to a lower shear force arising from a smaller hydrodynamic radius.10−12 Therefore, the development of mechanoresponsiveness in the bulk state could result from an increase of intermolecular entanglements owing to the multiple branch arms in the star polymers and the abovementioned architectural effect. In conclusion, we achieved an extraordinary enhancement of the mechanochemical activation of DABBF mechanophores by introducing them into polymer chains. In addition, the relationship between the polymer architecture and mechanoresponsiveness in the bulk state was revealed. The findings in this study can provide a new strategy for designing mechanoresponsive bulk materials. A systematic study of the

Figure 2. Photographs of center-functionalized linear polystyrene 1 (a) before and (b) after grinding. (c) EPR spectra of centerfunctionalized linear polystyrene 1 before (red line) and after (blue line) grinding.

shearing, elongating, and compressing stress) generated during grinding process could induce the DABBF activation. The g value (2.003) obtained from the EPR spectra of the ground polymers was in good agreement with that of activated DABBF in previous reports.17 These results reveal that the central C−C bonds of the DABBF functionalities incorporated in the polymer structures were cleaved in response to the grinding stimulus, and blue-colored arylbenzofuranone (ABF) radicals were generated. The blue color of the ground polymers was fairly stable in air but immediately disappeared by addition of good solvents, such as tetrahydrofuran (THF; Movie S1). Solvent addition probably caused an increase in polymer chain mobility, resulting in recombination of the ABF radicals, which are in equilibrium with DABBF at room temperature in solution. 17,21,23 Furthermore, the GPC curves of the recombined ground polymers corresponded to those of the intact samples (Figure S10). These results indicate that the mechanical site-selective scission of the central C−C bonds of the DABBF units in the center of the polymer chains occurred. The amount of ABF radicals (as a percentage of dissociated DABBF) generated by grinding could be estimated from the EPR measurements. The ratio of dissociated DABBF in linear polymer 1 after grinding for 10 min was 7.5%, which is more than 50× as high as that of the dihydroxy DABBF derivative (0.14%) under the same grinding conditions. In contrast, no peak was observed in EPR spectrum of ground PS homopolymer (57100 g mol−1) as a control sample. These results indicate that the mechanical force was efficiently transferred to the central DABBF units via the polymer chains, even in the bulk state (Figure 3). The ratio of dissociated DABBF increased with increasing molecular weight of the linear polymers (14, 17, and 26% for linear polymers 2, 3, and 4, respectively). This relationship between chain length and mechanoresponsiveness in bulk corresponds to that observed in solution.24−27 1126

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(14) Ramirez, A. L. B.; Schmitt, A. K.; Mahanthappa, M. K.; Craig, S. L. Faraday Discuss. 2014, 170, 337−344. (15) Li, Y.; Nese, A.; Lebedeva, N. V.; Davis, T.; Matyjaszewski, K.; Sheiko, S. S. J. Am. Chem. Soc. 2011, 133, 17479−17484. (16) Hong, G.; Zhang, H.; Lin, Y.; Chen, Y.; Xu, Y.; Weng, W.; Xia, H. Macromolecules 2013, 46, 8649−8656. (17) (a) Imato, K.; Irie, A.; Kosuge, T.; Ohishi, T.; Nishihara, M.; Takahara, A.; Otsuka, H. Angew. Chem., Int. Ed. 2015, 54, 6168−6172. (b) Imato, K.; Kanehara, T.; Ohishi, T.; Nishihara, M.; Yajima, H.; Ito, M.; Takahara, A.; Otsuka, H. ACS Macro Lett. 2015, 4, 1307−1311. (c) Imato, K.; Kanehara, T.; Nojima, S.; Ohishi, T.; Higaki, Y.; Takahara, A.; Otsuka, H. Chem. Commun. 2016, 52, 10482−10485. (18) Kosuge, T.; Imato, K.; Goseki, R.; Otsuka, H. Macromolecules 2016, 49, 5903−5911. (19) (a) Gao, H.; Matyjaszewski, K. Macromolecules 2006, 39, 4960− 4965. (b) Paraskeva, S.; Hadjichristidis, N. J. Polym. Sci., Part A: Polym. Chem. 2000, 38, 931−935. (c) Nunns, A.; Ross, C. A.; Manners, I. Macromolecules 2013, 46, 2628−2635. (20) Frenette, M.; MacLean, P. D.; Barclay, L. R. C.; Scaiano, J. C. J. Am. Chem. Soc. 2006, 128, 16432−16433. (21) Frenette, M.; Aliaga, C.; Font-Sanchis, E.; Scaiano, J. C. Org. Lett. 2004, 6, 2579−2582. (22) Beiermann, B. A.; Davis, D. A.; Kramer, S. L. B.; Moore, J. S.; Sottos, N. R.; White, S. R. J. Mater. Chem. 2011, 21, 8443−8447. (23) Imato, K.; Ohishi, T.; Nishihara, M.; Takahara, A.; Otsuka, H. J. Am. Chem. Soc. 2014, 136, 11839−11845. (24) Price, G. J.; Smith, P. F. Polymer 1993, 34, 4111−4117. (25) Hickenboth, C. R.; Moore, J. S.; White, S. R.; Sottos, N. R.; Baudry, J.; Wilson, S. R. Nature 2007, 446, 423−427. (26) Potisek, S. L.; Davis, D. A.; Sottos, N. R.; White, S. R.; Moore, J. S. J. Am. Chem. Soc. 2007, 129, 13808−13809. (27) Kryger, M. J.; Munaretto, A. M.; Moore, J. S. J. Am. Chem. Soc. 2011, 133, 18992−18998.

mechanochemical activation of various DABBF-containing polymers is now underway.

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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.6b00529. Experimental section and 1H NMR spectra, GPC profiles, and EPR spectra of the synthesized compounds and polymers (Figures S1−10; PDF). Recombination behavior of dissociated DABBF with polymer chains (Movie S1; MPG).

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partly supported by a Grant-in-Aid for Scientific Research from the Japan Society of the Promotion of Science (No. 26288057 for H.O. and No. 15K17907 for R.G.). A part of this work was also supported by the ImPACT Program of Council for Science, Technology and Innovation (Cabinet Office, Government of Japan). We thank Prof. Takashi Ishizone (Tokyo Institute of Technology, Japan) for measuring GPC with a light-scattering detector.

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REFERENCES

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DOI: 10.1021/acsmacrolett.6b00529 ACS Macro Lett. 2016, 5, 1124−1127