Photoreversible Smart Polymers Based on 2Ï + 2Ï ... - ACS Publications

Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

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Photoreversible Smart Polymers Based on 2π + 2π Cycloaddition Reactions: Nanofilms to Self-Healing Films Mustafa Abdallh,† Chiaki Yoshikawa,§ Milton T. W. Hearn,† George P. Simon,*,‡ and Kei Saito*,† †

School of Chemistry and ‡Department of Materials Science and Engineering, Monash University, Clayton, VIC 3800, Australia § International Center for Materials Nanoarchitectonics, National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan

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S Supporting Information *

ABSTRACT: A simple nanostructured, photoresponsive film made from a coumarin-modified tetrafunctional monomer, which is both photodegradable and photoreproducible, was prepared using a simple spin-coating process and UV irradiation. The film produced from this system self-healed scratches using only UV light, with no need for catalyst, heat, or other stimuli. The photoreversible mechanism was investigated, and a range of techniques were used to characterize the resultant photoproducts after the polymerization and depolymerization processes. Infrared spectroscopy was used to determine the optimal energy for a complete reversible polymerization reaction, and the mechanism was further confirmed by UV−vis spectroscopy which was able to monitor key structural changes. GPC analysis was used to track the molecular weight changes after the depolymerization reaction, which showed that the polymer was able to be converted back to monomers and oligomers, demonstrating the highly reversible polymerization and suggesting a potential for recyclability. Microhardness measurements of neat and irradiated samples were also used to determine the changes in mechanical properties before and after cleavage of the polymer network, and following the recovery of its structure, the latter showed a recovery of up to 91% of its mechanical properties.

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INTRODUCTION Smart materials which have the ability to change their molecular structure and subsequent properties when exposed to external stimuli such as light, heat, and pH are of much interest.1−6 These materials have been used in different applications, such as self-cleaning surfaces,7 drug release,8,9 shape memory,10 sensors,11 separation,6 liquid crystals,12,13 recycling,14 3D printing,15 and self-healing polymers.16 Incorporation of labile bonds into cross-linked polymer networks converts them into more sustainable and suitable polymers for applications requiring recyclability or reuse.17 Different dynamic reactions such as isomerization,18 coordination interactions,19 Diels−Alder cyclization,20,21 disulfide exchange,22,23 transesterification,24 imine formation,25 and photodimerization reactions26−28 have also been applied to synthesize reversible polymers that can polymerize and depolymerize due to an external stimulus. Recently, research interest has focused on using light as a stimuli for chemical reactions. Among these reactions, 2π + 2π cycloaddition reactions can be used for the control of chemical structure of materials and to allow their physical properties to vary in response to light.28,29 A select few molecules are known to possess the ability to undergo this type of reaction such as cinnamine,30 stilbene,31 thymine,32 coumarin,33 and styrylpyrene.34 These reactions are considered as sustainable, since the © XXXX American Chemical Society

forward and backward reactions require light rather than the use of chemicals.35 The 2π + 2π cycloaddition dynamic moieties can be incorporated into the polymer backbone or pendant structures linked to main chain units of the polymer network to create new materials. For example, 2D nanosheets were prepared using topochemical polymerization via 4π + 2π and 2π + 2π cycloaddition reactions.36,37 Vittal et al. have reported the 2π + 2π cycloaddition reaction within the molecular organic framework (MOF) to create photoresponsive MOFs.38 However, most of these systems require special conditions, with the reaction only proceeding in the crystalline state, or do not focus on the reversible reaction to create reversible polymers per se. Recently, our group has developed photoreversible linear polymers using the topochemical polymerization via the 2π + 2π cycloaddition reaction of thymines,32 which also required a crystalline state for the reaction. More recently, Barner-Kowllik et al. have used 2π + 2π reversible dimerization reaction of styrylpyrene as a polymer ligation in visible light.39 Coumarin moieties undergo photoreversible dimerization via 2π + 2π cycloaddition reactions. When irradiated with UVA Received: August 10, 2018 Revised: February 22, 2019

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DOI: 10.1021/acs.macromol.8b01729 Macromolecules XXXX, XXX, XXX−XXX

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Scheme 1. Schematic Illustration of the Synthesized Monomers 1 and 2 and the Suggested Network Structure for the Produced Polymers 3 and 4

We report the introduction of the dynamic moiety, coumarin, as a key feature of monomer design for photoreversible polymers for smart film applications. The dynamic moieties were placed on the terminal of tetrafunctional monomers. The concept is that the tetrafunctional monomers, when exposed to light (365 nm), will form polymer networks via the [2π + 2π] cycloaddition reaction. Exposure to 254 nm UV light reverses the reaction and cleaves the network structure, causing the polymeric materials to re-form monomeric or low molecular weight fragments (Scheme 1). This article describes the synthesis of photoreversible polymer networks that can be exploited in producing such nanofilms and self-healing films.

light, they form photodimerized products, while irradiation with UVC light results in reversible cleavage of the cyclobutane ring.16,27,40,41 Studies on photo-cross-linking and chain extension of polymer chains using coumarin have been increasingly reported. For example, Huyck et al. have studied the photo-cross-linking of substituted coumarin acrylates,42 while Trenor et al. have used coumarin to generate extended PEG polymer chains.43 The coumarin functional unit has also been reported in hydrogels; for example, Bettinger et al.44 and Forsythe et al.33 have used coumarins as photocleavage pendant groups in PEG hydrogels. Faust et al. reported the use of coumarin with polyisobutelene to form networks as a sealant for photovoltaics.45 Self-healing cross-linked polymers as smart films have recently also been studied. Most of the research in the selfhealing field uses heat (i.e., Diels−Alder reactions) or uses light combined with heat to trigger the healing process.46 Previous research has shown that healing at ambient conditions usually makes use of gel systems15,47 or highly flexible polymers that have poor mechanical properties. Corner et al. discovered the doubly dynamic self-healing hydrogel based on oxime click dynamic reaction increased the mechanical properties of the hydrogel.47 The system we present in this work heals using light only, with no need for a catalyst, heat, or other stimuli and exhibits properties well-suited to their application in films and coatings not hydrogels. We describe the formation of a simple, nanoscale film using coumarin-modified tetrafunctional monomer, with the film produced using a simple spin-coating plus UV irradiation method.

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RESULTS AND DISCUSSION Polymer Synthesis. The synthesis of the tetrafunctional monomers with coumarin photoactive sites is shown in Scheme 2. To synthesize the polymer with the desired capability of self-healing, the first step condensation involved the reaction between pentaerythritol and 11-bromoundecanoic acid to produce an extended tetrafunctional compound which represents the precursor for monomer 1. The second step involved the attachment of the coumarin to produce monomer 1 with the photoactive polymerizable sites. Similarly, monomer 2, which has shorter arms, were synthesized by the reaction of 7-(4-bromobutoxy)-2H-chromen-2-one with pentaerythritol tetrakis(3-mercaptopropionate) (Supporting Information, Figure S1). By irradiation of monomers 1 and 2 with 365 nm UV light, the dimerization reaction between the coumarins resulted B


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Macromolecules Scheme 2. Synthetic Route for Monomer 1

UV light rapidly led to regaining the peak at 325 nm (Figure 1b). The absorption of this peak increased with increasing illumination dose. The cycloconversion of the coumarin unit in the polymeric structure can be calculated using the following equation:

in the formation of a cross-linked polymer of 3 and 4 (Scheme 1). Reversibility. To examine the formation of the photocross-linked polymers and to test the reversibility of their structures, the monomers were cast as films from a concentrated solution of the monomers on a glass substrate and irradiated with 365 nm light, followed by ATR-IR measurements. Irradiation of monomer 1 with 365 nm light resulted in the formation of the cross-linked polymer (3), as indicated by the shifts in CO stretching frequency to higher frequency, the shift of the 1620 cm−1 stretching frequency, and the appearance of a new aromatic peak at 1580 cm−1 due to the breaking of the CC bonds. In contrast, irradiation of the cross-linked polymer 3 with 254 nm UV light initiated the decross-linking cleavage reaction with the recovery of the bonds to almost its original position due to the regeneration of the CC moiety. This study thus found that the UV exposure required for maximum polymerization is 16.66 J cm−2 and 5.41 J cm−2 for the reversible reaction (Figure S2). The photoreversibility of the produced polymers was also monitored by UV−vis spectroscopy. A solution (0.2 M) of monomer 1 in chloroform was spin-coated onto a quartz substrate and irradiated to produce a thin film of polymer 3. The film was left for 1−2 h to dry at room temperature. Upon irradiation with 365 nm light, the absorption at 325 nm was significantly reduced. The illumination with 365 nm light results in the conversion of the double bond of the coumarin to form the cyclobutane ring and hence the formation of the polymer (Figure 1a). Conversely, illumination with 254 nm

% conversion = (1 − Ad /A 0) × 100

(1)

where Ad is an absorbance at a certain dose of exposure and A0 is the absorbance before irradiation. The changes in conversion percentages are shown in Figure S4. Around 79% of the coumarin units underwent photocyclization reaction to form the polymer and almost completed after exposure to ca. 8 J cm−2 of irradiation at 365 nm. The process of photocleavage of the dimerized coumarin units was initiated by exposing the polymeric film to UV light (254 nm). The polymer was de-cross-linked ca. 97%, largely returning to the monomeric state after exposure to only 0.5 J cm−2. The photopolymerization and depolymerization cycles of the resultant cross-linked polymer 3 were measured using UV spectroscopy measurements. The polymerized, cross-linked structure was prepared by irradiation of the polymer film using 365 nm light with 8 J cm−2. Subsequently, the film was irradiated by 254 nm light with 0.5 J cm−2. The process was repeated for three cycles with reversion of ∼92% (Figure 2). The solubility of the cross-linked polymers after irradiation with 254 nm light was probed to investigate the reversible reaction of the system. The cross-linked samples were initially placed in THF (30 mg/50 mL) for 30 min without any irradiation, where it was found that only 7% of the sample was C


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different irradiation doses. The results showed that more monomeric and oligomeric units were obtained as the illumination dose was increased. These findings indicate that a reversible reaction takes place, and the cyclobutane rings are cleaved to form more small units which are soluble in THF. Figure 3 shows that after exposure to 1.2 J cm−2 dose of

Figure 3. Weight change due to induced solubility of polymer 3 after illumination with 254 nm UV light at different energy exposure intervals.

irradiation, 75% of the cross-linked polymer could be dissolved, and it was completely dissolved after exposure to a 3.33 J cm−2 dose of irradiation. GPC was used to analyze the de-cross-linked polymer. After evaporation of THF, the samples were dissolved in DMF, which was then used as the GPC solvent. It was clear from the solubility data that as the illumination dose increases, the degree of dissociation of the cross-linked polymer also increased. GPC chromatograms showed two peaks (Figure 4). A sharp peak at a higher retention time indicating the reverse reaction proceeded completely back to monomeric state, and a broad peak at lower retention time represented the larger oligomeric units. The Mn of the peak around 2000 g/mol with a dispersity of 1.05 represent the smaller monomeric fragments, while the Mn of the peaks relating to longer retention times of about 6000 g/

Figure 1. UV study of changes after irradiation of monomer 1 with (a) 365 nm light to form polymer 3 and (b) subsequent irradiation of 3 with 254 nm light.

Figure 2. Reversible cycloaddition reaction of polymerization and depolymerization process of polymer 3 after illumination with 365 nm followed by 254 nm for three cycles.

soluble (unreacted monomer and small oligomeric units), and thus 93% of the monomer was converted to the polymeric state. The sample was illuminated with 254 nm light for

Figure 4. GPC traces of polymer 3 once solubilized after illumination with 254 nm UV light at different doses of irradiation exposure. D


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Figure 5. AFM images polymer 3 films: (a) 3D image and (b) cross section of film spin-coated from a solution of 0.2 M showing the thickness of the polymer is about 400 nm; (c) 3D image and (d) cross section of film spin-coated from a solution of 4 × 10−3 M showing the thickness of the polymer is about 50−100 nm.

thermal properties of the resultant polymers. DSC was used to determine the glass transition temperature, Tg, of the samples before and after irradiation. Tg is an important polymer characteristic and aids in understanding the mechanism of selfhealing. It is known that the healing process involves two stages. Initially, the network cleaves and de-cross-linking occurs, which leads to small, mobile fragments that can readily flow and fill and scratches. The next step is re-cross-linking and recovery of the original cross-linked properties. The results showed that the Tg of pristine samples exposed to 254 nm UV light reduced from 74.5 and 76.3 °C (before exposure) to 37.5 and 50.9 °C (after exposure) for polymers 3 and 4, respectively. This effect is due to cleavage of the cyclobutane ring of the cross-linked network and formation of smaller more mobile units. Subsequent irradiation with 365 nm UV light triggered the reverse 2π + 2π cycloaddition reaction leading to the recovery of the cross-linked network structure. As a result of the reverse reaction, the Tg increased to 71.7 and 68.3 °C for polymers 3 and 4. To test the self-healing ability of the polymers, samples of the different monomers were cast and cured on a glass substrate. The ability of a scratched surface to heal was measured using optical microscopy. A blade razor was used to make the scratches on the surface of the polymer 3 sample. The scratches ranged in size from 30 to 40 μm in width. The sample was illuminated with 254 nm UV light with an intensity of 5.41 J cm−2 to stimulate the chains session to form small monomeric and oligomeric units (as shown from the GPC data) and hence enable the mobility of the chain, leading to flow and repair of the scratches. The samples were again irradiated with 365 nm light at 16.66 J cm−2 to recover the cross-linked structure of the polymer (Figure 7). A similar procedure was followed to examine the ability of the scratched sample to heal similar damage with polymer 4 using a scratch with 30 μm width. It was observed that the scratch was reduced in size but not completely healed, even though the polymer showed high reversibility as observed using UV and IR (Figure S8). This might be due to the shorter chain length of polymer 4 in comparison to polymer 3, which

mol correspond to materials of a range of tetramers to pentamers. The reduction in the extent of cross-linkage and the formation of these smaller units with greater mobility and ability to flow play a key role in the healing process. The tetrafunctional monomer 1 can easily form a film with a thickness of nanodimension by simple casting and spin coating of the monomer with concentration of 0.2 M on a glass substrate, followed by irradiation of the specimen with 365 nm to trigger the cross-linking reaction. The thickness of the resulted film was measured using atomic force microscopy (AFM).48−50 The film was scraped off the glass substrate using a needle. The results showed that the film thickness was of the order of 400 nm (Figure 5a,b). It was possible to control the thickness of the produced polymer film in nanoscale by changing the concentration of the monomer solution used in spin-coating process; i.e., spin-coating of 4 × 10−3 M of monomer solution resulted in formation of a film of 50−100 nm thickness (Figure 5c,d). The resulting film can be obtained as free-standing film by swelling in THF and followed by adding water. Self-Healing Properties. Prior to testing the self-healing ability of the produced polymers, it is important to test the

Figure 6. Free-standing transparent polymer 3 film. E


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Figure 7. Self-healing of polymer (3). The figure shows: a) scratched sample and b) sample with scratch healed by irradiation with UV light only. Also shown in panel (a) and panel (b) is the ink mark which was employed to permit the orientation of the film to be readily determined.

resulted in a higher Tg (50.9 °C) with less flowable fragments affecting the healing process. To further validate that the healing process is due to the dynamic bonds within the designed system, a specimen was made and cast on a glass substrate in the same manner as the healed sample. A scratch was made on the surface of the new specimen with a razor blade. This time the sample was not exposed to UV light (254 nm) to break the cyclobutane linkers in the networks but was only exposed to heat where its network structure remained intact. The sample was maintained at 70 °C for the same period required to achieve healing with light. The obtained results showed that there was no change on the surface of the damaged sample (Figure S9). This result proves that the healing needs to be triggered by light due to the cleavage of the cross-linked structure, not due to the heat generated by the light. The self-healing properties were further investigated by producing a free-standing film. The film was cut into two pieces; then those pieces were partially covered with a mask, allowing only a specific portion of the material to be irradiated with 254 nm UV light to promote the cleavage of the network structure and form a softer film with free coumarin units on the surface of the film. The resulting films with the coumarin units were brought close to each other. Subsequent irradiation of the film at 365 nm UV light resulted in a reaction of the two noncross-linked surfaces, and they fused to become one piece and were no longer able to be separated. Figure 8 illustrates the healing process of such films cut into two pieces and subsequently rejoined. Hardness Measurements. The mechanical properties of the produced films were assessed based on hardness tests of the virgin and irradiated samples. Hardness measurements were achieved using a Duramin A-300 microhardness tester. A pyramidal diamond was used to make indentations into the film surface with a load of 0.2 kg for 10 s. Microscopic examination with a ×100 objective was used to monitor and measure the indents to calculate the hardness. The indentation process was repeated three times, and the average value determined. The results showed that there is a decrease in the hardness properties of the irradiated sample with 254 nm flux in comparison to pristine sample. The reduction in hardness is due to the cleavage of the cyclobutane rings in the polymer network, resulting in the formation of smaller units (as

independently confirmed by GPC and UV results) leading to the materials becoming softer. Subsequent exposure to higher wavelength light, e.g., 365 nm, resulted in increased hardness of up to 91.4% of its original hardness value. This behavior resulted from the re-formation of the cyclobutane rings and recovery of the network structure. The results of pristine, cleaved, and recovered samples are presented in Table 1. It is clear from data in Table 1 that the recovery of hardness is not complete. This behavior can be explained by the motion of the smaller fragment formed by irradiation with 254 nm light. As a result, the networks lose the necessary molecular alignment for subsequent, full 2π + 2π cycloaddition reaction.50 Vickers hardness values from pristine and recovered samples were comparable with the value of the poly(methyl methacrylate) (19−21) and show the potential of this material as a coating material.

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CONCLUSION A tetrafunctional monomer containing terminal coumarin units was successfully synthesized and polymerized using irradiation of 365 nm UV light. The reversibility of the resultant polymers was studied by IR, UV, and GPC. The resulting polymer film thickness was easy to control by simple variation of the concentration of the spin-coated solution. The self-healing capability of the polymers was studied in terms of the removal and healing of scratches on the damaged surface. Upon irradiation with 254 nm UV light, cleavage of the cross-linked network occurs, lowering the Tg and leading to smaller molecular units with the ability to flow and heal the cracks. The healability of polymer 3 was studied in terms of the ability to reconnect two distinct pieces to form one. A soft film was created as a result of 254 nm irradiation. These soft samples were brought together by pressing them between glass slides. Subsequent irradiation with 365 nm UV light initiated the reverse reaction and led to the re-formation of the original networks. Hardness testing was used to assess the mechanical properties of the polymers before and after irradiation. When the polymer was exposed to 254 nm light, the cross-linked structure cleaved to form smaller units with low Tg (37.5 °C) and poor mechanical properties. Subsequent exposure to 365 nm resulted in recovery much of its characteristics such as Tg (71.7 °C) and hardness. F


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Figure 8. Self-healing process of polymer 3. (a) The polymer film is cut into two pieces. (b) The film is partially covered prior to irradiation with 254 nm light. (c) The film is sandwiched between glass slides prior irradiation with 365 nm light. (d) The sample is covered, except for the targeted area prior to irradiation with 365 nm light. (e) A healed sample after irradiation with 365 nm light under elongation constraint. (f) The resultant transparent healed film.

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Dibromobutane was supplied by Fluka (Buchs, Switzerland) while the other chemicals were supplied by Merck (Billerica, MA) All chemicals were used without further purification. Characterization. 1H NMR spectra were obtained with a Bruker DRX 400 instrument, and chemical shifts were calibrated by CDCl3 (δ = 7.26). 13C NMR spectra were recorded on a Bruker DRX 400,

EXPERIMENTAL SECTION

Materials. Pentaerythritol, 11-bromoundecanoic acid, 7-hydroxycoumarin, pentaerythritol tetrakis(3-mercaptopropionate), tetrabutylammonium iodide, and anhydrous potassium carbonate were all supplied by Sigma-Aldrich (Sydney, Australia). p-Toluenesulfonic acid was supplied from Ajax Chemicals (Sydney, Australia). 1,4G


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Synthesis of 7-(4-Bromobutoxy)-2H-chromen-2-one. 7-Hydrocoumarin (5 g, 30 mmol) and 1,4-dibromobutane (20 g, 92 mmol) were mixed in acetone (100 mL). Anhydrous potassium carbonate (6.7 g, 48 mmol) was then added to the above mixture, and the suspension refluxed at 60 °C for 24 h. A precipitate was removed by filtration and washed with acetone (50 mL). Water (50 mL) was added to the residue, and the mixture was extracted with dichloromethane. The extract was dried over anhydrous MgSO4, and the solvent was removed under reduced pressure. The resulting oil was poured into ethanol (150 mL). A precipitate formed and was removed by filtration. The residual solvent was then removed from the recovered organic phase under reduced pressure. The resulting oil was poured into hexane (100 mL) to yield the product. Yield = 73%. 1 H NMR (400 MHz, CDCl3): 1.99−2.10 (m, 4H), 3.51 (t, 2H), 4.07 (t, 2H), 6.26 (d, 1H), 6.82−6.86 (m, 2H), 7.37 (d, 1H), 7.64 (d, 1H). 13 C NMR (400 MHz, CDCl3): δc 27.6 (C−C−O), 29.3 (Br−C−C), 33.1 (Br−C), 67.5 (O−C), 101.3 (C9), 112.6 (C7), 112.8 (C5), 113.1 (C3), 128.7 (C6), 143.3 (C4), 155.9 (C10), 161.2 (C8), 162.0 (C2). IR (cm−1): 1703 s, 1619 s, 1510 m, 1398, 1289 s, 1235 m, 1129 m, 939 m, 719 m. Synthesis of 2,2-Bis(((3-((4-((2-oxo-2H-chromen-7-yl)oxy)butyl)thio)propanoyl)oxy)methyl)propane-1,3-diyl Bis(3-((4-((2-oxo-2Hchromen-7-yl)oxy)butyl) thio)propanoate) (2). Pentaerythritol tetrakis(3-mercaptopropionate) (0.2 g, 0.4 mmol) and anhydrous potassium carbonate (0.47 g, 3 mmol) were dissolved in dry DMF (40 mL), and 7-(4-bromobutoxy)-2H-chromen-2-one (1.4 g, 4 mmol) and a catalytic amount of tetrabutylammonium iodide (TBAI) (0.24 g, 0.6 mmol) were added to the above reaction mixture. The reaction was stirred for 48 h at room temperature. The mixture was then filtrated, and the filtrate was extracted with chloroform (50 mL) and washed with distilled water (50 mL). The solution was then dried over anhydrous MgSO4 and filtered to remove the MgSO4, and the chloroform was removed under reduced pressure to produce a yellowish oil. The crude product was purified via column chromatography (silica gel 60 (0.040−0.63 mm)) (hexane/ethyl acetate 1:1). Further purification of the product was done using a second column chromatography step (silica gel 60 (0.040−0.63 mm)) (chloroform/methanol 98:2). Yield = 33%. 1H NMR (400 MHz, CDCl3): 4.04 (t, 8H), 4.20 (m, 8H), 6.24 (d, 4H), 6.81−6.86 (m, 8H), 1.80 (m, 8H), 1.92 (m, 8H), 2.61 (m, 16H), 2.80 (t, 7.36 (d, 4H), 7.63 (d, 4H). 13C NMR (400 MHz, CDCl3): δc 25.9 (S−C−C), 26.8 (C−S), 31.4 (S−C), 34 6 (CCOCO), 36.4 (CO−C), 62.4 (COCO), 67.9 (C8O−C), 101.3 (C9), 112.5 (C7), 112.9 (C5), 113.0 (C3), 128.7 (C6), 143.4 (C4), 155.9 (C10), 161.2 (C8), 162.2 (C2). IR (cm−1): 729 s, 1612 s, 1508 m, 1389 m, 1350 m, 1280 m, 995 m, 724 s. Synthesis of the Cross-Linked Polymers 3 and 4. The products 1 and 2 were individually dissolved in chloroform and then casted onto glass slides by a drop-casting for the IR study and a spin-coating for the nanofilm formation and the UV−vis study. The reaction mixture was left for 2 h before irradiation with 365 nm UV light to produce the final cross-linked polymers.

Table 1. Vickers Hardness Values of Polymers 3 and 4 before and after Irradiation Vickers hardness (HV) polymer

pristine

irradiated with 254 nm

irradiated with 365 nm

3 4

21.1 25.1

13.9 18.5

19.3 23.8

and chemical shifts were calibrated by the signal of CDCl3 (δ = 77.16). An ATR sampler, Agilent Cary 630, was used to measure IR frequencies. The operation condition was 32 scans at a resolution of 4 cm−1. UV−vis spectra were recorded on a UV-1800 Shimadzu spectrophotometer. Irradiation was performed using a CL1000 M UV-cross-linker lamp (UVP, LLC, Clyton, Australia) that provided polychromatic light centered at 365 and 254 nm. The glass transition temperature (Tg) was obtained with a PerkinElmer DSC-8000 using a nitrogen purge and aluminum pans. The images of the scratched and healed samples were obtained using an Olympus BX60 microscope and a Canon Legria HFS20 camera. A Duramin A-300 hardness tester was used to determine the microhardness of the polymer samples before and after healing. TGA curves were obtained using a Mettler TGA/DSC1 star system at a heating rate of 10 °C min−1. The film thickness was measured using an atomic force microscope (AFM) (NanoWizard II, JPK Instruments AG, Berlin, Germany) by scanning the surface image across the boundary between the scraped and unscraped regions of the film with a cantilever (FORT-20, Applied Nanostructures Inc., Mountain View, CA, k = 1.2−6.4). Synthesis of 2,2-Bis(((11-bromoundecanoyl)oxy)methyl)propane-1,3-diyl Bis(11-bromoundecanoate). Pentaerthritol (0.2 g, 1.4 mmol), 11-bromoundecanoic acid (1.95 g, 7.3 mmol), and ptoluenesulfonic acid (76 mg, 0.4 mmol) were mixed together in the solid state. The reaction mixture was heated to 120 °C for 48 h. The mixture was then dissolved in dichloromethane and washed with saturated NaHCO3 (15 mL) twice, followed by saturated brine (10 mL). The organic layer was recovered and removed under vacuum. The resultant oil was purified by column chromatography (silica gel 60 (0.040−0.63 mm)) (1:1 of hexane:ethyl acetate) to give the product. Yield = 56%. 1H NMR (400 MHz, CDCl3): 1.30 (br, 48H), 1.42 (m, 8H), 1.64 (m, 8H), 1.85 (m, 8H), 2.32 (t, 8H), 3.42 (t, 8H), 4.13 (s, 8H). 13C NMR (400 MHz, CDCl3): 24.8 (COCC), 28.1 (BrCCC), 28.7 (BrCCCC), 29.1 (COCCC), 29.2 (COCCCC), 29.3 (COCCCCC), 29.3 (BrCCCCC), 32.8 (BrCC), 34.0 (BrC), 34.0 (COC), 41.8 (CCOCO), 62.1 (COCO), 173.2 (CO). IR (cm−1): 2915, 2851, 1737 s, 1469 m, 1163 s, 721 s. Synthesis of 2,2-Bis(((11-((2-oxo-2H-chromen-7-yl)oxy)undecanoyl)oxy)methyl)propane-1,3-diyl Bis(11-((2-oxo-2H-chromen-7-yl)oxy)undecanoate) (1). 7-Hydroxycoumarin (0.4 g, 2.4 mmol) was dissolved in acetone (10 mL), and potassium carbonate (0.5 g, 3 mmol) was added to the above solution. The 2,2-bis(((11bromoundecanoyl)oxy)methyl)propane-1,3-diyl bis(11-bromoundecanoate) (0.2 g, 0.17 mmol) was dissolved in acetone (10 mL) and added to the above reaction mixture over 6 h. The reaction mixture was then refluxed for 48 h. The solvent was removed using a rotary evaporator, and the residue was extracted with dichloromethane (50 mL) and water (50 mL). The organic layer was recovered and dried over MgSO4. The solvent was then removed under reduced pressure. The resulting oil was purified by column chromatography (silica gel 60 (0.040−0.63 mm)) (1:1 of hexane:ethyl acetate) to give 1. Yield = 49%. 1H NMR (400 MHz, CDCl3): 1.31 (br, 48H), 1.47 (m, 8H), 1.60 (m, 8H), 1.81 (m, 8H), 2.17 (t, 8H), 3.99 (t, 8H), 4.12 (s, 8H), 6.22 (d, 4H), 6.79 (m 8H), 7.35 (d, 4H), 7.62 (d, 4H). 13C NMR (400 MHz, CDCl3): 24.8 (C OCC), 25.9 (C8OCCC), 28.9 (COCCC), 29.1 (COCCCC), 29.2 (COCCCCC), 29.3 (COCCCCCC), 29.3 (C8OCCCC), 29.4 (C8OCC), 34.0 (COC), 41.8 (CCOCO), 62.1 (COC O), 62.1 (COCO), 68.6 (C8OC), 103.3 (C9), 112.3 (C7), 112.8 (C5), 113.0 (C3), 129.1 (C6), 143.5 (C4), 155.9 (C10), 161.3 (C8), 162.4 (C2), 173.2 (CO). IR (cm−1): 2915, 2851, 1731, 1614, 1508, 1145, 1020, 832.

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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/acs.macromol.8b01729.

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Synthetic route of monomer 2, the suggested network of polymers 3 and 4, IR and UV reversibility study of 4; TGA analysis and self-healing images of polymers 3 and 4 (PDF)

AUTHOR INFORMATION

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ORCID

Chiaki Yoshikawa: 0000-0002-6589-387X Kei Saito: 0000-0002-5726-8775 Author Contributions

M.A. performed all the experiment except AFM analysis. AFM analysis was performed by C.Y. M.H. provided the feedback to shape the research. G.S. and K.S. devised the project, the main conceptual ideas, and proof outline. All authors discussed the results and contributed to the final manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by PRESO, JST, JPMHPR1515, and Monash University interdisciplinary research project seed funding program.

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