Sustainable, Naringenin-Based Thermosets Show Reversible

Letter Cite This: ACS Macro Lett. 2019, 8, 239−244

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Sustainable, Naringenin-Based Thermosets Show Reversible Macroscopic Shape Changes and Enable Modular Recycling Yuree Oh,† Kyoung Min Lee,†,‡ Doyoung Jung,† Ji Ae Chae,† Hea Ji Kim,† Mincheol Chang,† Jong-Jin Park,† and Hyungwoo Kim*,† †

School of Polymer Science and Engineering, Chonnam National University, 77 Yongbong-ro, Buk-gu, Gwangju 61186, Korea Department of Materials Science and Engineering, College of Engineering, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, Korea

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‡

S Supporting Information *

ABSTRACT: A sustainable biobased thermoset exhibiting shape-memory behavior and modular recycling capabilities has been developed herein. The prepared thermoset consists of naringenin and biocompatible polymer components. Naringenin, which has three phenolic moieties, has been converted to a multifunctional monomer containing glycidyl groups and readily formed a thermosetting network via epoxide ring opening reaction with a poly(ethylene glycol) diacid under solvent-free conditions. The resulting material is malleable yet as strong as articular cartilage and selectively absorbs water when compared with n-dodecane oil. Moreover, the thermoset can be physically reused. After being crumpled, stretched, or coiled, the initial shape of the material is restored in response to heat or water. Furthermore, the material is amenable to chemical recycling in a bulk state via transesterification, and its components can be recovered on a molecular level after degradation under benign conditions, as was confirmed using a model compound.

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synthesis of the thermosets. Recently, more elaborate chemical reactions have also been explored involving transcarbamoylation,9,10 transcarbonation,11 thiol−ene reaction,12,13 transalkylation,14,15 trithiocarbonate exchange,16 boronate exchange,17−19 and olefin metathesis.20,21 Such polymers have been mainly introduced as vitrimers,22 or covalent adaptable networks,23 and have been applied to the environmental fields. In addition, biomolecules have been used as monomeric units and incorporated into thermosets via a diverse range of polymerization reactions.24,25 In particular, some of these biomolecules but a few rendered the thermosets reusable or recyclable as well as biocompatible and cost-competitive. Key examples include eugenol26,27 and limonene28 that were functionalized and incorporated in thermosets as well as vanillin-based monomers that formed a reversible polyimine network.29

lastics are widely, practically, and efficiently used in contemporary applications, but serious questions have been raised relating to their health or environmental issues (e.g., microplastics). When carelessly discarded at the end of life, the materials can reduce the productivity of entire ecosystems. Thus, many renewable polymers and recycling processes for polymeric materials now have received significant attention with growing concerns regarding plastic contamination.1 Among common plastics, thermosets are extensively used because of their thermal stability, mechanical durability, and chemical resistance. However, their recyclability remains challenging compared to that of thermoplastics. On the other hand, thermosets with dynamic covalent bonds2 are promising candidates for a greener use of plastics. These types of chemical bonds have high bonding energy, but exhibit reversible bond exchange, which allows the prepared materials to be used in a more sustainable manner. From the viewpoint of polymer chemistry, transesterification,3−5 Diels−Alder reactions,6,7 and condensation8 are commonly used for the © XXXX American Chemical Society

Received: January 5, 2019 Accepted: February 15, 2019

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mold. The curing reaction was then completed in the mold at 120 °C for 12 h. Therefore, thermoset 3 can be cast in diverse shapes in a large scale. As an example, a yellow strip (2 cm × 0.6 cm × 0.3 mm) from 3 is shown in the inset of Scheme 1. We investigated thermal transition of thermoset 3 using differential scanning calorimetry (DSC), as shown in Figure 1a.

Herein, a novel sustainable thermoset using naringenin has been prepared. Naringenin is a bioflavonoid that is abundant in grapefruits and exhibits chemo-preventive functions toward certain cancers, Alzheimer’s disease, inflammation, depression, and adipogenesis. The molecule can readily be converted to a multifunctional epoxide monomer, yielding an epoxy network after combination with a biocompatible polymer containing carboxylic acid functionalities. The resultant covalent bonds in the network are reversible via transesterification reaction, as proven to be effective in epoxy systems before.30 The physical properties of the prepared material were also investigated. For example, the thermoset was found to be soft, but as strong as cartilage or rubber, and selectively absorbed water. Notably, the thermoset is physically reusable; the material can rapidly restore the original shape by entropic force after deformation when exposed to heat or water (i.e., shape-memory behavior). Moreover, the thermoset is chemically recyclable. The material can be reproduced through enthalpy-driven bond exchange in the bulk state, or the incorporated components can be recollected on a molecular level. Therefore, the biobased thermoset is capable of (i) being physically reused, (ii) chemically reprocessed through bulk recycling, and (iii) molecular recycling to recover individual components. We designed a naringenin-based thermosetting network 3 following Scheme 1. Naringenin contains three phenolic

Figure 1. (a) DSC heating thermograms of 2 during the second heating scan (blue), 2 with 1 and TBD during the first (sky blue) and second (gray) heating scans, and 3 during the second heating scan (black). (b) The glass transition of 3 monitored by DMA testing. The change in storage modulus (black) and tan δ value (purple) as a function of temperature are shown. (c) Representative tensile stress− strain curve of 3. (d) Change in the swelling degree of 3 when exposed to deionized water or n-dodecane over the course of 4.5 h at 25 °C. The data points were obtained in triplicate, and the average is shown with standard deviation. The sample was swelled after soaking in water, as shown in the inset.

Scheme 1. Synthetic Procedures for Bio-Based Thermoset 3 from Naringenin

Polymer 2 showed an endothermic, enantiotropic peak at 50 °C with a phase transition enthalpy of 94.0 J/g corresponding to a melting event. After additional incorporation of 1 and TBD, a new and broad exothermic peak was observed at approximately 150 °C (enthalpy value of 52.2 J/g) along with an endothermic peak (81.7 J/g) during the first heating cycle. However, in the second heating cycle, the exothermic peak disappeared. In addition, the endothermic peak shifted to 52 °C and the fusion enthalpy decreased by 20% (64.2 J/g), which can be attributed to the addition reaction of 1 and 2. Thermoset 3 only displayed a broad endothermic peak with an enthalpy of 55.2 J/g because of the partial fusion of the PEG component in its cross-linked network. Thermal behavior of 3 was investigated by dynamic mechanical analysis (DMA) in a tensile film mode (Figure 1b). The DMA profile as a function of temperature showed that the storage modulus rapidly decreased at >50 °C without breaking, indicating the good malleability of the material. The change in the tan δ value of 3 further indicated that the glass transition occurred at 58 °C (Tg). In addition, the thermal gravimetric analysis (TGA) traces of 1, 2, and 3 are shown in Figure S3. The monomeric component 1 and 2 exhibited a 10% weight loss (T10%) at 236 and 327 °C, respectively. However, the T10% value of 3 increased to 360 °C after crosslinking. Tensile stress−strain curves were measured to examine the mechanical properties of 3. The network of 3 exhibited a

moieties that could be tethered with three epoxides using epichlorohydrin and triethylbenzylammonium chloride (TEBAC), resulting in a biobased epoxide 1. The trifunctional 1 was further reacted with oxidized poly(ethylene glycol) (PEG-diacid, 2). Here, 2 containing terminal carboxylic acid groups on both ends was obtained from poly(ethylene glycol) (PEG-diol; Mn, 3000) using Jones reagent and analyzed by gel permeation chromatography (GPC; Figure S1) and proton nuclear magnetic resonance spectroscopy (1H NMR; Figure S2). Stoichiometric addition of 1 and 2 (a molar ratio of 1:1.5) provided the cross-linked thermoset 3 after epoxide ring opening reaction under solvent-free, neat conditions in the presence of a triazabicyclodecene (TBD) catalyst (0.4 wt %). In detail, we briefly polymerized 1 and 2 to yield a prepolymer resin at 80 °C for 1 h, which was then transferred into a Teflon 240

DOI: 10.1021/acsmacrolett.9b00008 ACS Macro Lett. 2019, 8, 239−244

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ACS Macro Letters Young’s modulus of 40 ± 2 MPa, similar to the values of articular cartilages.31 A representative tensile curve (Figure 1c) and photographs of the elongated specimen (Figure S4a) are shown each. Furthermore, length of the polymer linker affected properties of the thermosetting network. For example, we incorporated another PEG-diacid (Mn, 600) instead of 2 and measured the properties of the resulting material under identical conditions. The resulting network revealed a much smaller tensile Young’s modulus of 2.2 ± 0.1 MPa (Figure S4b), implying that the longer one would provide more physical entanglement.32 Here, the mechanical properties of the network were briefly altered by the molecular weight of the polymer component, but can be potentially fine-tuned on demand. Interestingly, 3 selectively absorbs water. We monitored rapid change in swelling degree of 3 when immersed in water (Figure 1d). The sample volumetrically swelled by a factor of approximately 8 after immersion for 4.5 h, as shown in the inset. On the contrary, a model oil, n-dodecane, was not absorbed at all. We further evaluated the rate constant for water absorbency of 3 using a pseudo-second-order kinetic model following eq 1 below: t 1 t = + 2 St S k 2Se e

Figure 2. Shape-memory behavior of the thermoset 3. The material was programmed at 60 °C and fixed by quenching to 18 °C using nhexane. After re-exposure to the temperature over 50 °C, 3 spontaneously recovered the original shape. The material (thickness, 1 mm) was able to be (a) deformed with ripples or (b) stretched by hand, but completely returned to the previous dimension. (c) Dualresponsive behavior using water as well as heat. The coiled 3 (size, 17 mm × 3 mm × 0.3 mm) returned to the initial state after dripping a small water droplet and drying.

(1)

where Se and St are the absorbed amount of water (g/g) at equilibrium and each time interval (t, min), respectively, and k2 (g g−1 min−1) is the rate constant of the model. Here, the constant k2 for the absorption was calculated to be 0.02 min−1 (Figure S5), which is comparable to those of other PEGcontaining materials.33 Therefore, 3 can potentially be used for the selective removal of an aqueous phase from heterogeneous mixtures. Figure 2 displays shape-memory behavior of 3 exhibiting macroscopic, programmed responses. We prepared a sample of 3 in the mold, deformed with ripples above the Tg, and quenched the temperature to fix its shape. The deformed material rapidly recovered its original shape when exposed to gentle heating to approximately 50 °C (Figure 2a). The material is also stretchable. Drawn by hand and fixed, deformed 3 reverted back to its original shape soon after reheating (Figure 2b). The network slightly became transparent because the internal molecular orientation could change while drawing.34 Furthermore, the entire shape of 3 could be controlled using water in addition to heat. We programmed the coiled shape above the Tg; once a trace amount of water (5 μL) was applied to the curl, the deformed material rapidly restored its original shape without reheating. Thus, we obtained the initial state of 3 after brief drying (Figure 2c). The partial wetting presumably alleviated the local internal strain in 3 when swelled and caused unfolding. The dualresponsive properties would be further applied to induce biological-like, water-driven shape changes35,36 or cooperative deformation in a hierarchical structure.37,38 Thermoset 3 can be recycled in its bulk state. For bulk recycling, a pristine disc of 3 was mechanically ground into powder, then reprocessed to form a new disc after heating at 140 °C for 5 h at 2000 psi using a hot press machine. We measured the tensile stress−strain curves of the as-prepared 3 and the recycled material and compared the initial regions of the curves in Figure 3a. The recycled 3 showed a Young’s modulus of 38 ± 4 MPa, indicating 95% restoration of the

initial modulus. Photographs in the inset display the polymer disc that underwent bulk recycling for comparison. We observed the dynamic mechanical properties of 3 by tensile stress relaxation experiments at elevated temperatures ranging from 45 to 60 °C (Figure 3b). As the temperature increased, the network relaxed the internal stress more rapidly. The characteristic relaxation time, τ*, linearly decreased from 430 to 24 s, and was defined as the time required for the G/G0 ratio to reach 1/e. The obtained values were also well fitted by the Arrhenius equation to estimate the Arrhenius activation energy (Ea) of stress relaxation. The inset in Figure 3b shows that the activation energy Ea could be obtained from the slope and was estimated to be 115.3 kJ mol−1, which is in good agreement with the transesterification of epoxy networks.39 We further demonstrated the molecular recycling of 3. The thermoset was easily degraded when immersed in 1:9 water− MeOH at 70 °C for 1 d in the presence of 0.4 wt % TBD catalyst, resulting in 2 and the concomitant hydroxyl product 4. After purification by dialysis in water using a tubing membrane (MWCO, 2 kDa) for 4 d at rt, polymer 2 was isolated in an average of 40% yield (Figure 4a). Figure 4b,c provides characterization data of the recovered 2. The GPC chromatograms of PEG-diol, 2, and the recovered 2 showed a peak that appeared at identical retention times of 15.9 min, indicating that the polymers have a similar molecular weight (Figure 4b). In addition, 1H NMR spectroscopy revealed αhydrogens adjacent to carbonyl group developed at 4.17 ppm in 2 after oxidation, which was also observed in the recovered 2 (Figure 4c). Although we only recovered 40% of 2, the use of 241

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other membranes with smaller pore sizes or chromatographic methods can increase the total recovery yield. To corroborate the synthesis and recycling process described above, we prepared a model compound 6 and performed the hydrolysis reaction (Figure 4d). Compound 6 was synthesized from 5 under the same solvent-free conditions, which supports the facile preparation of 3. Then, 6 was completely hydrolyzed under the same conditions as used previously and wholly converted to n-heptanoic acid and 7. After removing the heptanoate by extraction, we found that the 1,2-diol group remained on 7 without further extensive purification, as determined by 1H NMR (Figure S6) and high-resolution mass spectrometry (HRMS; Figure S7). In summary, we newly designed and synthesized the sustainable thermoset based on the naringenin biomolecule, investigated its physical properties and demonstrated promising recycling strategies. The multifunctional, naringenin-based monomer was polymerized with a biocompatible polymer linker via epoxy ring-opening reaction under solvent-free conditions, which readily provided the thermosetting network. The resulting material showed a similar elastic modulus (approximately, 40 MPa) as articular cartilage or rubber, and selectively absorbed water to swell by >400%. The shapememory behavior of the prepared material was also investigated. The material could be physically deformed but rapidly restored its initial shape in response to heat and water as well. Furthermore, the thermoset could be chemically recycled in its bulk state via transesterification. Thus, the material was reused after grinding and heating. In addition, we recollected and separated the components on a molecular level. Herein, we demonstrated hypothetical strategies, but the material has the potential to replace general thermosets once achieving the desired properties by copolymerization with other monomers or additional processing. In addition, the use of autonomous mechanisms such as molecular-machineassisted catalysis,40,41 self-propagating reactions,42,43 and head-to-tail depolymerization44 would potentially offer noble polymeric materials that exhibit more sophisticated responses for use in advanced sustainable or healthcare applications.

Figure 3. (a) Initial tensile stress−strain curve of pristine 3 (solid line) and that measured after bulk recycling (dotted line). Necking occurred after the marked points near a strain of 0.3%. The discshaped 3 (thickness, 1 mm) that underwent recycling is shown in the inset. The dimensions of each grid square were 2 mm × 2 mm. (b) Stress relaxation tests for 3 at various temperatures. The dotted line indicates the characteristic relaxation time (τ*). An Arrhenius plot showed a linear correlation (inset) and an activation energy of 115.3 kJ mol−1.

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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.9b00008. Additional synthetic procedures, experimental details, TGA data, tensile stress−strain curves, supplementary pseudo second order plot, and characterization data (PDF).

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

Figure 4. Recovery of the PEG-diacid 2 from the thermoset 3 after use. (a) Thermoset 3 was decross-linked in 1:9 water−MeOH using TBD catalyst, and then 2 was recovered by dialysis. (b, c) Characterization data of PEG-diol (gray), 2 (blue), and 2 after recovery (black) when measured by (b) GPC and (c) 1H NMR spectroscopy (asterisk, CDCl3; orange arrow, carboxymethyl group). (d) Synthesis of model compound 6 from 5 and n-heptanoic acid under neat conditions, which supports the ring-opening reaction and subsequent hydrolysis that resulted in the diol 7.

ORCID

Mincheol Chang: 0000-0002-1090-4930 Jong-Jin Park: 0000-0002-5958-4050 Hyungwoo Kim: 0000-0003-1958-3587 Notes

The authors declare no competing financial interest. 242

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(19) Cromwell, O. R.; Chung, J.; Guan, Z. Malleable and SelfHealing Covalent Polymer Networks through Tunable Dynamic Boronic Ester Bonds. J. Am. Chem. Soc. 2015, 137, 6492−6495. (20) Lu, Y. X.; Guan, Z. Olefin Metathesis for Effective Polymer Healing via Dynamic Exchange of Strong Carbon−Carbon Double Bonds. J. Am. Chem. Soc. 2012, 134, 14226−14231. (21) Lu, Y. X.; Tournilhac, F.; Leibler, L.; Guan, Z. Making Insoluble Polymer Networks Malleable via Olefin Metathesis. J. Am. Chem. Soc. 2012, 134, 8424−8427. (22) Montarnal, D.; Capelot, M.; Tournilhac, F.; Leibler, L. SilicaLike Malleable Materials from Permanent Organic Networks. Science 2011, 334, 965−968. (23) Bowman, C. N.; Kloxin, C. J. Covalent Adaptable Networks: Reversible Bond Structures Incorporated in Polymer Networks. Angew. Chem., Int. Ed. 2012, 51, 4272−4274. (24) Faye, I.; Decostanzi, M.; Ecochard, Y.; Caillol, S. Eugenol BioBased Epoxy Thermosets: From Cloves to Applied Materials. Green Chem. 2017, 19, 5236−5242. (25) Jiang, Y.; Ding, D.; Zhao, S.; Zhu, H.; Kenttämaa, H. I.; AbuOmar, M. M. Renewable Thermoset Polymers Based on Lignin and Carbohydrate Derived Monomers. Green Chem. 2018, 20, 1131− 1138. (26) Liu, T.; Hao, C.; Wang, L.; Li, Y.; Liu, W.; Xin, J.; Zhang, J. Eugenol-Derived Biobased Epoxy: Shape Memory, Repairing, and Recyclability. Macromolecules 2017, 50, 8588−8597. (27) Gazzotti, S.; Hakkarainen, M.; Adolfsson, K. H.; Ortenzi, M. A.; Farina, H.; Lesma, G.; Silvani, A. One-Pot Synthesis of Sustainable High-Performance Thermoset by Exploiting Eugenol Functionalized 1,3-Dioxolan-4-one. ACS Sustainable Chem. Eng. 2018, 6, 15201− 15211. (28) Li, C.; Sablong, R. J.; van Benthem, R. A. T. M.; Koning, C. E. Unique Base-Initiated Depolymerization of Limonene-Derived Polycarbonates. ACS Macro Lett. 2017, 6, 684−688. (29) Geng, H.; Wang, Y.; Yu, Q.; Gu, S.; Zhou, Y.; Xu, W.; Zhang, X.; Ye, D. Vanillin-Based Polyschiff Vitrimers: Reprocessability and Chemical Recyclability. ACS Sustainable Chem. Eng. 2018, 6, 15463− 15470. (30) Liu, T.; Guo, X.; Liu, W.; Hao, C.; Wang, L.; Hiscox, W. C.; Liu, C.; Jin, C.; Xin, J.; Zhang, J. Selective Cleavage of Ester Linkages of Anhydride-Cured Epoxy Using a Benign Method and Reuse of the Decomposed Polymer in New Epoxy Preparation. Green Chem. 2017, 19, 4364−4372. (31) Liang, X.; Duan, P.; Gao, J.; Guo, R.; Qu, Z.; Li, X.; He, Y.; Yao, H.; Ding, J. Bilayered PLGA/PLGA-HAp Composite Scaffold for Osteochondral Tissue Engineering and Tissue Regeneration. ACS Biomater. Sci. Eng. 2018, 4, 3506−3521. (32) Cai, J.; Liu, C.; Cai, M.; Zhu, J.; Zuo, F.; Hsiao, B. S.; Gross, R. A. Effects of Molecular Weight on Poly(ω-pentadecalactone) Mechanical and Thermal Properties. Polymer 2010, 51, 1088−1099. (33) Kesavan, M. P.; Ayyanaar, S.; Vijayakumar, V.; Raja, J. D.; Annaraj, J.; Sakthipandi, K.; Rajesh, J. Magnetic Iron Oxide Nanoparticles (MIONs) Cross-Linked Natural Polymer-Based Hybrid Gel Beads: Controlled Nano Anti-TB Drug Delivery Application. J. Biomed. Mater. Res., Part A 2018, 106, 1039−1050. (34) Vincent, P. I. The Tough-Brittle Transition in Thermoplastics. Polymer 1960, 1, 7−13. (35) Shibasaki, Y.; Mori, T.; Fujimori, A.; Jikei, M.; Sawada, H.; Oishi, Y. Poly(amide−ether) Thermoplastic Elastomers Based on Monodisperse Aromatic Amide Hard Segments as Shape-Memory and Moisture-Responsive Materials. Macromolecules 2018, 51, 9430− 9441. (36) Lee, K. M.; Kim, H. J.; Jung, D.; Oh, Y.; Lee, H.; Han, C.; Chang, J. Y.; Kim, H. Rapid Accessible Fabrication and Engineering of Bilayered Hydrogels: Revisiting the Cross-Linking Effect on Superabsorbent Poly(acrylic acid). ACS Omega 2018, 3, 3096−3103. (37) Jung, D.; Lee, K. M.; Chang, J. Y.; Yun, M.; Choi, H.-J.; Kim, Y. A.; Yoon, H.; Kim, H. Selective De-Cross-Linking of Transformable, Double-Network Hydrogels: Preparation, Structural Conversion, and

ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT; No. 2017R1C1B5017785). This study was also financially supported by Chonnam National University (Grant No. 2017-2779).

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REFERENCES

(1) Ignatyev, I. A.; Thielemans, W.; Beke, B. V. Recycling of Polymers: A Review. ChemSusChem 2014, 7, 1579−1593. (2) Jin, Y.; Yu, C.; Denman, R. J.; Zhang, W. Recent Advances in Dynamic Covalent Chemistry. Chem. Soc. Rev. 2013, 42, 6634−6654. (3) Shi, Q.; Yu, K.; Kuang, X.; Mu, X.; Dunn, C. K.; Dunn, M. L.; Wang, T.; Qi, H. J. Recyclable 3D Printing of Vitrimer Epoxy. Mater. Horiz. 2017, 4, 598−607. (4) Lu, L.; Pan, J.; Li, G. Recyclable High-Performance Epoxy Based on Transesterification Reaction. J. Mater. Chem. A 2017, 5, 21505− 21513. (5) Qiu, M.; Wu, S.; Fang, S.; Tang, Z.; Guo, B. Sustainable, Recyclable and Robust Elastomers Enabled by Exchangeable Interfacial Cross-Linking. J. Mater. Chem. A 2018, 6, 13607−13612. (6) Zhang, G.; Zhao, Q.; Yang, L.; Zou, W.; Xi, X.; Xie, T. Exploring Dynamic Equilibrium of Diels−Alder Reaction for Solid State Plasticity in Remoldable Shape Memory Polymer Network. ACS Macro Lett. 2016, 5, 805−808. (7) Jin, K.; Kim, S.-S.; Xu, J.; Bates, F. S.; Ellison, C. J. Melt-Blown Cross-Linked Fibers from Thermally Reversible Diels−Alder Polymer Networks. ACS Macro Lett. 2018, 7, 1339−1345. (8) Taynton, P.; Yu, K.; Shoemaker, R. K.; Jin, Y.; Qi, H. J.; Zhang, W. Heat- or Water-Driven Malleability in a Highly Recyclable Covalent Network Polymer. Adv. Mater. 2014, 26, 3938−3942. (9) Fortman, D. J.; Brutman, J. P.; Cramer, C. J.; Hillmyer, M. A.; Dichtel, W. R. Mechanically Activated, Catalyst-Free Polyhydroxyurethane Vitrimers. J. Am. Chem. Soc. 2015, 137, 14019−14022. (10) Zheng, N.; Hou, J.; Xu, Y.; Fang, Z.; Zou, W.; Zhao, Q.; Xie, T. Catalyst-Free Thermoset Polyurethane with Permanent Shape Reconfigurability and Highly Tunable Triple-Shape Memory Performance. ACS Macro Lett. 2017, 6, 326−330. (11) Snyder, R. L.; Fortman, D. J.; De Hoe, G. X.; Hillmyer, M. A.; Dichtel, W. R. Reprocessable Acid-Degradable Polycarbonate Vitrimers. Macromolecules 2018, 51, 389−397. (12) Scott, T. F.; Schneider, A. D.; Cook, W. D.; Bowman, C. N. Photoinduced Plasticity in Cross-Linked Polymers. Science 2005, 308, 1615−1617. (13) Zhang, B.; Digby, Z. A.; Flum, J. A.; Chakma, P.; Saul, J. M.; Sparks, J. L.; Konkolewicz, D. Dynamic Thiol−Michael Chemistry for Thermoresponsive Rehealable and Malleable Networks. Macromolecules 2016, 49, 6871−6878. (14) Xiang, D.; Liu, H.; Yang, L.; Liang, Y. T.; Zhu, J.; Lu, Z. Y.; Hou, Y.; Yang, M. From Molecular-Level Organization to Nanoscale Positioning: Synergetic Ligand Effect on the Synthesis of Hybrid Nanostructures. Adv. Funct. Mater. 2017, 27, 1703006. (15) Hendriks, B.; Waelkens, J.; Winne, J. M.; Du Prez, F. E. Poly(thioether) Vitrimers via Transalkylation of Trialkylsulfonium Salts. ACS Macro Lett. 2017, 6, 930−934. (16) Amamoto, Y.; Kamada, J.; Otsuka, H.; Takahara, A.; Matyjaszewski, K. Repeatable Photoinduced Self-Healing of Covalently Cross-Linked Polymers through Reshuffling of Trithiocarbonate Units. Angew. Chem., Int. Ed. 2011, 50, 1660−1663. (17) Rottger, M.; Domenech, T.; van der Weegen, R.; Breuillac, A.; Nicolay, R.; Leibler, L. High-Performance Vitrimers from Commodity Thermoplastics through Dioxaborolane Metathesis. Science 2017, 356, 62−65. (18) Ogden, W. A.; Guan, Z. Recyclable, Strong, and Highly Malleable Thermosets Based on Boroxine Networks. J. Am. Chem. Soc. 2018, 140, 6217−6220. 243

DOI: 10.1021/acsmacrolett.9b00008 ACS Macro Lett. 2019, 8, 239−244

Letter

ACS Macro Letters Controlled Release. ACS Appl. Mater. Interfaces 2018, 10, 42985− 42991. (38) Kim, H.; Mohapatra, H.; Phillips, S. T. Rapid, On-Command Debonding of Stimuli-Responsive Cross-Linked Adhesives by Continuous, Sequential Quinone Methide Elimination Reactions. Angew. Chem., Int. Ed. 2015, 54, 13063−13067. (39) He, C.; Shi, S.; Wu, X.; Russell, T. P.; Wang, D. Atomic Force Microscopy Nanomechanical Mapping Visualizes Interfacial Broadening between Networks Due to Chemical Exchange Reactions. J. Am. Chem. Soc. 2018, 140, 6793−6796. (40) Erbas-Cakmak, S.; Leigh, D. A.; McTernan, C. T.; Nussbaumer, A. L. Artificial Molecular Machines. Chem. Rev. 2015, 115, 10081− 10206. (41) Pan, T.; Liu, J. Catalysts Encapsulated in Molecular Machines. ChemPhysChem 2016, 17, 1752−1758. (42) Kim, H.; Baker, M. S.; Phillips, S. T. Polymeric Materials that Convert Local Fleeting Signals into Global Macroscopic Responses. Chem. Sci. 2015, 6, 3388−3392. (43) Mohapatra, H.; Kim, H.; Phillips, S. T. Stimuli-Responsive Polymer Film that Autonomously Translates a Molecular Detection Event into a Macroscopic Change in Its Optical Properties via a Continuous, Thiol-Mediated Self-Propagating Reaction. J. Am. Chem. Soc. 2015, 137, 12498−12501. (44) Baker, M. S.; Kim, H.; Olah, M. G.; Lewis, G. G.; Phillips, S. T. Depolymerizable Poly(Benzyl Ether)-Based Materials for Selective Room Temperature Recycling. Green Chem. 2015, 17, 4541−4545.

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