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Tunicate-Inspired Gallol Polymers for Underwater Adhesive: A Comparative Study of Catechol and Gallol Kan Zhan,† Chaehoon Kim,† Kyungmo Sung,† Hirotaka Ejima,*,‡ and Naoko Yoshie*,† †

Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505, Japan Department of Materials Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan

‡

S Supporting Information *

ABSTRACT: Man-made glues often fail to stick in wet environments because of hydration-induced softening and dissolution. The wound healing process of a tunicate inspired the synthesis of gallol-functionalized copolymers as underwater adhesive. Copolymers bearing three types of phenolic groups, namely, phenol, catechol, and gallol, were synthesized via the methoxymethyl protection/deprotection route. Surprisingly, the newly synthesized copolymers bearing gallol groups exhibited stronger adhesive performances (typically 7× stronger in water) than the widely used catechol-functionalized copolymers under all tested conditions (in air, water, seawater, or phosphate-buffered saline solution). The higher binding strength was ascribed to the tridentate-related interfacial interaction and chemical cross-linking. Moreover, gallol-functionalized copolymers adhered to all tested surfaces including plastic, glass, metal, and biological material. In seawater, the performance of gallol-functionalized copolymer even exceeds the commercially available isocyanate-based glue. The insights from this study are expected to help in the design of biomimetic materials containing gallol groups that may be utilized as potential bioadhesives and for other applications. The results from such a kind of comparable study among phenol, catechol, and gallol suggests that tridentate structure should be better than bidentate structure for bonding to the surface.

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for applications as medical adhesives.19 Kuroda et al. have prepared the catechol-functionalized polymer poly(dopamine methacrylate-co-2-methoxyethyl acrylate) (P(DMA-co-MEA)), which can be used as a dental adhesive. Compared to the commercially available dental adhesive resins, the shear strength of the P(DMA-co-MEA) increased when it was treated with a small amount of water, while that of commercial adhesive decreased under the same treatment. The authors have attributed these results to the extended expansion of polymer chains onto the surface of the substrates.20 Tunicate-inspired wound healing mechanisms also offer an effective way for bioadhesive applications.21−24 Tunicate, a marine invertebrate organism, has a tough and flexible body called tunic, which is composed of cellulose nanofiber and peptides with gallol-functionalized amino acids (trihydroxyphenylalanine, TOPA).22,25 Tunicates can heal an injury under the seawater environment by using adhesive proteins that include the gallol moieties.24,26 The gallol groups in TOPA can form covalent cross-links with the functional groups from the other proteins. Additionally, they can form coordination complexes with metal ions, and these cross-links act as an adhesive for

INTRODUCTION Adhesives play a nonsubstitutable role in our daily life, being widely used in many fields including housing construction, packaging, labeling, and aerospace engineering.1−3 Adhesives that work under water need to be explored because biobased adhesives used in medical applications such as dental cements, tissue glues, and surgical adhesives are in great demand.4,5 However, underwater adhesives present several technical challenges. Adhesives themselves generally interact with water instead of forming adhesive bonds to the surfaces or cohesive bonds within the bulk materials when applied to the submerged substrates.6−8 Most man-made adhesives do not work well when tasked with such underwater bonding. In contrast, mussels attach to wet rocks by depositing a mixture of proteins containing an amino acid, 3,4-dihydroxyphenylalanine (DOPA),9−12 which is central to the cross-linking reactions of cohesive curing and adhesive surface bonding.9,13 Incorporating DOPA-like chemistry into synthetic polymers is being pursued for developing adhesives that work under water.14−18 Wilker et al. have synthesized poly[(3,4-dihydroxystyrene)-co-(p-vinyltolyltriethylammonium chloride)-co-styrene] terpolymers inspired from the polyphenolic proteins in mussel byssal thread. The resulting polymers showed better underwater adhesion compared to selected commercial glues, indicating that the synthetic adhesives can be potentially used © XXXX American Chemical Society

Received: June 30, 2017 Revised: August 17, 2017

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Figure 1. Synthetic route of the gallol-functionalized copolymer P(VGal-co-BA): (a) K2CO3, CH3OCH2Cl, acetone, r.t. to reflux, 12 h; (b) (Ph)3PCH3Br, n-BuLi, THF, 0 °C to r.t., 12 h; (c) AIBN, THF, 60 °C, 24 h; (d) HCl, butanol (18%), r.t, 12 h.

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healing injuries in the tunicate.27 Therefore, gallol derivatives are potential candidates for use as underwater bioadhesives. Although several studies related to the catechol-functionalized adhesives have been reported in literature, there are only a few26,28 that discuss the adhesive applications of synthetic gallol-functionalized materials. Ahn et al. have prepared a tunicate-inspired gallic acid/metal ion complex by simply mixing aqueous gallic acid and metal ion solution, which has been suggested as an anesthetic solution for the treatment of dentin hypersensitivity. However, these complexes might dissolve and detach from the surfaces when immersed in artificial saliva, owing to the usage of the small molecule, gallic acid.28 Hwang et al. have synthesized the gallol-functionalized chitin nanofiber.26 The resulting hydrogel adhesives have improved wet adhesion properties. However, it is difficult to be certain about the number of gallol groups in the polymer chains by following this synthetic method. Moreover, the above materials need cross-linkers to compensate their insufficient cohesive strength. This work describes a new synthetic route using the methoxymethyl (MOM) protecting group, which can be easily removed under acidic condition, to synthesize a family of poly(vinylgallol-co-n-butyl acrylate) [P(VGal-co-BA)] copolymers (Figures 1 and S1−S4). This method was also adapted for the synthesis of poly(vinylcatechol-co-n-butyl acrylate) (P(VCat-co-BA), Figure S5), poly(vinylphenol-co-BA) (P(VPh-coBA), Figure S6), and poly(styrene-co-n-butyl acrylate) (P(St-coBA), Figure S7). The adhesive performance of P(VGal-co-BA) was then investigated without adding any cross-linkers in air, water, seawater, and phosphate-buffered saline (PBS) solution and compared with the adhesive performance of other phenolic copolymers under identical conditions. A similar systematic and detailed study has not been reported before with any other mussel-mimetic polymer system. Additionally, the adhesion performances of these copolymers were also studied on an array of substrates: poly(tetrafluoroethylene) (PTFE), poly(vinyl chloride) (PVC), polished aluminum, and glass. The P(VGal-co-BA) copolymers strongly adhered even to pig skin in water, seawater, and PBS buffer solution, which makes gallolfunctionalized polymers a promising candidate for use as potential bioadhesives for surgical and dental applications. Based on this work, the previous Bell theory that bidentate interactions have an advantage over monodentate interactions9,29−31 can be further proved. Interestingly, the tridentate interactions from gallol groups could be even better than the excellent bidentate interaction.

EXPERIMENTAL SECTION

Materials. 3,4,5-Trihydroxybenzaldehyde (THB) was purchased from Tokyo Chemical Industry and used as received. 3,4Dihydroxybenzaldehyde, 4-hydroxybenzaldehyde, chloromethyl methyl ether, potassium carbonate, methyl triphenylphosphonium bromide, n-butyllithium, n-butyl acrylate, anhydrous acetone, anhydrous tetrahydrofuran (THF), n-butanol, and hydrochloric acid (36%) were purchased from Wako Pure Chemical Industry and used as received. 2,2′-Azobis(isobutyronitrile) (AIBN) was also purchased from Tokyo Chemical Industry and recrystallized from methanol before use. Styrene and n-butyl acrylate were purified by passing them through basic alumina before use. Phosphate-buffered saline (PBS, pH 7.4) buffer containing 1370 mM NaCl, 80 mM Na2HPO4, 26.8 mM KCl, and 14.7 mM K2HPO4 was purchased from the Wako Pure Chemical. Synthesis of 3,4,5-Tris(methoxymethoxy)benzaldehyde (TMMB). This product was synthesized according to the procedure detailed in a previous report.32 Briefly, under a nitrogen atmosphere at room temperature, 5 g (32.5 mmol) of 3,4,5-trihydroxybenzaldehyde (THB) was added to a suspension of potassium carbonate (45.0 g, 325.0 mmol) in 300 mL of acetone. After the reaction mixture was cooled to 10 °C, chloromethyl methyl ether (15.2 g, 195.0 mmol) was added to the flask and the resulting solution was allowed to reflux overnight. After refluxing, the reaction mass was filtered, concentrated under vacuum, and extracted with 50 mL of ethyl acetate three times. The organic layer was washed with 50 mL of brine three times, dried over MgSO4, filtered, evaporated, and purified by column chromatography (silica, 2:1 = hexane/ethyl acetate) to yield a colorless oil (65%). 1 H NMR (400 MHz, CDCl3): (δ, ppm) 9.87 (s, H), 7.40 (s, 2H), 5.28 (s, 4H), 5.25 (s, 2H), 3.62 (s, 3H), 3.52 (s, 6H). Synthesis of 3,4,5-Tris(methoxymethoxy)styrene (TMMS). In a typical reaction, under a nitrogen atmosphere at 0 °C, 12 mL (19.2 mmol) of n-butyllithium (1.6 M in hexane) was added to a suspension of methyl triphenylphosphonium bromide (7.0 g, 19.2 mmol) in 100 mL of THF. After the reaction mixture was warmed to room temperature, TMMB (4.0 g, 14.2 mmol) was added to the flask. The resulting solution was stirred overnight at 45 °C and then poured into 10 mL of deionized water, extracted with diethyl ether, dried over MgSO4, filtered, evaporated, and purified by column chromatography (silica, 2:1 = hexane/ethyl acetate) to yield a white solid (64%). 1H NMR (400 MHz, CDCl3): (δ, ppm) 6.90 (s, 2H), 6.62 (dd, 1H, J = 17.5 Hz, J′ = 10.8 Hz), 5.66 (dd, 1H, J = 17.5 Hz, J′ = 0.8 Hz), 5.21 (dd, 1H, J = 10.8 Hz, J′ = 0.8 Hz), 5.20 (s, 4H), 5.12 (s, 2H), 3.60 (s, 3H), 3.50 (s, 6H). 13C NMR (100 MHz, CDCl3): (δ, ppm) 151.2, 136.5, 136.4, 134.0, 113.8, 108.4, 98.7, 95.4, 57.2, 56.3. FAB-MS: calcd, 284.1; found, 284.1. Synthesis of Poly(3,4,5-tris(methoxymethoxy)styrene-co-nbutyl acrylate) P(TMMS-co-BA). In a typical polymerization reaction, a Schlenk flask equipped with a magnetic stirring bar was charged with TMMS (1.0 g, 3.5 mmol), BA (1.8 g, 14 mmol), AIBN (28.7 mg, 0.18 mmol), and THF (10 mL). Subsequently, the flask was B

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Biomacromolecules sealed after three freeze−pump−thaw cycles and placed in an oil bath at 60 °C. After polymerization for 24 h, the reaction mixture was cooled to ∼0 °C with liquid nitrogen. The product was precipitated from the cooled solution by adding hexane. The resulting white viscous solid was collected and dried in vacuum. The final yield was 74%. 1H NMR (400 MHz, CDCl3): (δ, ppm) 6.58−6.23 (2H, Ar−H), 5.28−4.96 (6H, O−CH2−O), 4.11−3.70 (4H, COO−CH2−CH2− CH2−CH3), 3.62−3.34 (9H, O−CH3), 2.47−2.06 (2H, CH2−COO− CH2−CH2−CH2−CH3), 2.06−1.06 (aliphatic region), 1.00−0.70 (3H, COO−CH2−CH2−CH2−CH3). Deprotection of P(TMMS-co-BA) to Poly(3,4,5-trihydroxystyrene-co-n-butyl acrylate), Poly(vinylgallol-co-BA) (P(VGal-coBA)). P(TMMS-co-BA) (1.0 g) was dissolved in 30 mL of n-butanol in a 100 mL round-bottom flask equipped with a magnetic stirring bar under a nitrogen atmosphere. The solution was stirred for 1 h before 0.5 mL of an HCl−butanol solution (18%) was added dropwise and the reaction was left to stir overnight. The resulting polymer solution was precipitated in 200 mL of hexane and subsequently collected by centrifugation. This precipitate was dissolved in THF and then precipitated again in hexane (0.45 g, 60%). 1H NMR (400 MHz, DMSO-d6; δ, ppm): 8.70−7.67 (OH), 6.01−5.75 (2H, Ar−H), 4.11− 3.70 (4H, COO−CH2−CH2−CH2−CH3), 2.47−2.06 (2H, CH2− COO−CH2−CH2−CH2−CH3), 2.06−1.06 (aliphatic region), 1.00− 0.70 (3H, COO−CH2−CH2−CH2−CH3). Characterization. 1H and 13C NMR spectra were recorded using a JEOL JNM-AL400 spectrometer (400 MHz, JEOL). The numberaverage molecular weight (Mn), and weight-average molecular weight (Mw) of the polymers were measured using size-exclusion chromatography (SEC, HLC-8220GPC, Tosoh) using polystyrene standards for calibration. Chloroform (HPLC grade) was used as the eluent at a flow rate of 1 mL min−1. Ultraviolet−visible (UV−vis) spectroscopy was carried out using a UV−vis spectrophotometer (U−3010, Hitachi). Attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectra were obtained using a Thermo Scientific Nicolet iS10 FT-IR spectrometer equipped with a Thermo Scientific Smart iTR ATR apparatus with a diamond crystal. FAB−MS measurements were performed on a JEOL JMS-600H spectrometer using 3-nitrobenzyl alcohol as a matrix. Tensile tests were conducted by SHIMADZU AGS-X 100 N and the dimensions of dumbbell-shaped sample used in Figure S8. Tests were performed at a strain rate of 50 mm min−1 under ambient condition (25 °C, 60% of relative humidity). Each measurement was repeated at least three times.

is too brittle to be used as an adhesive, it was copolymerized with n-butyl acrylate (BA; Figures 1 and S1−S4). Figure 1 shows the synthetic scheme. First, 3,4,5-tris(methoxymethoxy)benzaldehyde (TMMB) was obtained by protecting the hydroxyl groups in 3,4,5-trihydroxybenzaldehyde, followed by Wittig reaction to synthesize the monomer 3,4,5-tris(methoxymethoxy)styrene (TMMS). TMMS was then copolymerized with BA via free radical polymerization. Other phenolic polymers with similar compositions were also prepared following the same method (Figures S5 and S6), and P(St-co-BA) copolymers (Figure S7) were synthesized via free radical polymerization without protection. Table S1 summarizes the aromatic monomer content in copolymers such as poly(tris(methoxymethoxystyrene)-co-n-butyl acrylate) (P(TMMS-co-BA)) and other aromatic polymers. Sizeexclusion chromatography (SEC) was carried out in order to further characterize the synthesized copolymers. Given the adhesive nature of these copolymers, the molecular weight was obtained for the protected polymers.2 This approach prevented adhesion onto the high surface area SEC column. The SEC data given in Table S1 indicates the molecular weight of the copolymers in the range of ∼8300 to 24000 g mol−1 for each polymer. Polydispersity indices (PDI) have values between 1.4 and 2.1. The final P(VGal-co-BA) copolymers were obtained by deprotecting the MOM groups under acidic conditions. According to the 1H NMR spectrum (Figures S3 and S4), broad peaks of OH groups at 7.8 and 8.5 ppm were new, while the methyl peaks at 3.5 and 3.6 ppm, and methylene peaks at 5.1 and 5.2 ppm from the protection groups disappeared after deprotection. These results suggest that complete deprotection of MOM groups and successful conversion of P(TMMS-co-BA) to P(VGal-co-BA) had taken place. Three P(VGal-co-BA) copolymers of varied compositions (Table S2) were prepared in order to examine the influence of gallol cross-linking chemistry upon adhesion. Bonding Strength and Mechanism Studies. A total of 12 copolymer samples were synthesized with different ratios of gallol-functionalized, catecholic, phenolic, and aromatic groups from the MOM-protected copolymers (Tables S1 and S2). Their bonding strengths were measured by employing them to bond together polished aluminum substrates by lap shear adhesion tests. Aluminum is a high-energy surface and a common substrate in the aviation and automotive industries.38 Lap shear bonding is the most widely used method for quantifying adhesion.18 Adhesion of each copolymer in air (Figure 2) was examined without the addition of a cross-linking agent in order to directly compare the bonding strength of gallol-functionalized copolymers to that of catechol-functionalized copolymers. Samples containing gallol groups exhibited greater bonding strength, while P(VPh-co-BA) and P(St-co-BA) copolymers showed nearly no bonding strength (Figure 2) under identical conditions. P(VGal-co-BA) copolymer with 26% content of gallol groups showed the highest bonding strength due to the interfacial and cohesive interactions. In order to investigate the gallol-functionalized copolymers for potential bioadhesive applications, their bonding strengths were studied under water, seawater, and PBS environments (Figure 3). First, the effect of curing time on underwater bonding strength (Figure 3a) was investigated. The bonding strength of P(VGal26%-co-BA74%) under water increased as the curing time proceeded and reached saturation at 1.01 ± 0.26 MPa after approximately 6 days (Figure 3a). In contrast, the bonding strength of P(VCat27%-co-BA73%) was relatively low

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RESULTS AND DISCUSSION Synthesis and Structural Characterization of Poly(vinylgallol-co-BA), P(VGal-co-BA). Phenolic groups act as radical scavengers and thus need to be protected during radical polymerization. While methoxy groups have often been employed for this purpose,13,33,34 the deprotection step requires harsh reagents such as boron tribromide (BBr3), which can cleave ester and ether groups owing to its strong reactivity. This harsh deprotection step makes the copolymerization reaction with acrylates and methacrylates impossible. Silyl protecting groups have also been utilized for the synthesis of catecholbased polymers.19,35−37 However, the homopolymerization of tert-butyldimethylsilyl (TBDMS) group-protected gallol monomer yielded an oligomer (Mn = 2000 g mol−1). Thus, it is difficult to synthesize polymers that contain a large number of gallol groups. Therefore, a new route of methoxy methyl (MOM) protection has been developed for the preparation of gallol-functionalized polymers. Homopolymerization of MOMprotected gallol monomers was carried out first to compare with the previously reported silyl protecting method. The molecular weight (Mn = 12200 g mol−1) of the obtained products indicates a successful polymerization reaction owing to the lesser amount of steric interactions of MOM groups compared with TBDMS groups. As poly(vinylgallol) (PVGal) C

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buffer solution was 1.35 ± 0.17 MPa (Figure 3d), which is 13× greater than the performance of the P(VCat27%-co-BA73%) copolymer. The above-mentioned bonding strengths under different tested conditions indicate that gallol-functionalized copolymers offer stronger adhesive performances than catecholfunctionalized copolymers, and other phenolic and polystyrenebased copolymers. Bulk adhesion is the cumulative outcome of interfacial interactions and cohesive interactions.7,9,39 The former category includes H-bonding, electrostatic attraction, hydrophobic interaction, and coordination between adhesives and substrates, and can be estimated by work of adhesion (WA).9,14 WA is derived from the summation of surface energy components that arise from dispersion and polar interactions.40,41 The water contact angles of P(VGal-co-BA) copolymers are smaller than those of P(VCat-co-BA) (Table S3), indicating that the former copolymers are more hydrophilic than the latter. These results also suggest that the gallol groups exhibit higher interfacial interactions compared to catechol groups that are utilized for adhesive action with the immersed substrates. The WA of P(VGal-co-BA) copolymers is also larger than the corresponding values of P(VCat-co-BA) and P(VPh-co-BA) copolymers in air condition (Table S3), indicating that the former copolymers have stronger tridentate-related interfacial interactions than the latter bidentate and monodentate interactions based on the previous discussions on Bell theory.9,29−31 After immersing in water for 6 days (Table S4), the WA of P(VGal-co-BA) are still the highest among copolymers, also suggesting that the tridentate interaction is more stable than the bidentate and monodentate interactions in wet condition. Cohesive interactions including H-bonding, cation-π interaction, electrostatic attraction, hydrophobic interaction, and cross-linking inside polymers also contribute to the bonding

Figure 2. Bonding strengths of P(VGal-co-BA), P(VCat-co-BA), P(VPh-co-BA), and P(St-co-BA) under air. Bonding strength was in a lap shear configuration with aluminum substrates. Error bars indicate standard deviation, n ≥ 5.

under water and reached 0.14 ± 0.08 MPa after 6 days of curing. Thus, the optimal curing time of 6 days was chosen for the subsequent adhesion tests in wet conditions. The bonding strengths of P(VGal-co-BA) copolymers under water were greater than those of P(VCat-co-BA) copolymers in the entire tested composition range. Figure S9 shows that both P(VGalco-BA) and P(VCat-co-BA) failed cohesively. Typically, the bonding strength of P(VGal26%-co-BA74%) is 7× and 12× stronger than that of P(VCat27%-co-BA73%) in water and seawater, respectively. P(VGal26%-co-BA74%) displays a bonding strength of 1.34 ± 0.43 MPa in seawater, which is one of the highest values among previously published reports.17−19 Next, the adhesion properties of the copolymers were investigated in PBS solution because its ingredients and ion concentrations are similar to those of the body fluids. The bonding strength of P(VGal26%-co-BA74%) copolymer in PBS

Figure 3. Adhesion strength under wet conditions. (a) Effect of curing time on bonding strength with P(VGal26%-co-BA74%) and P(VCat27%-coBA73%) under water. Bonding strengths of P(VGal-co-BA), P(VCat-co-BA), P(VPh-co-BA), and P(St-co-BA) after 6 days under (b) water, (c) seawater, and (d) PBS conditions. Error bars indicate standard deviation, n ≥ 5. D

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without any drying procedures (Figure S11 and Table S6). These results can be attributed to the low swelling properties of P(VGal16%-co-BA84%) copolymers (Table S7). After 6 days of immersing in water, the swelling ratio of copolymers was determined to be 6.9 ± 0.6 wt %. This ratio was still low after 12 days, suggesting that the underwater bonding strength of P(VGal-co-BA) was still strong after this time period (Figure 3a). In contrast, the toughness of P(VCat-co-BA) copolymer was decreased to ∼0 MJ m−3 after 3 days in water (Figure S12 and Table S8). The swelling ratio of P(VCat-co-BA) was less than that of P(VGal-co-BA) in water for 12 days (Table S7), indicating that catechol-functionalized copolymer is more hydrophobic than gallol-functionalized copolymers, which is consistent with the previous result that gallol-functionalized copolymers are more hydrophilic. In order to test the hypothesis that oxidation-mediated crosslinking gives rise to strong cohesive interactions inside P(VGalco-BA), the attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectrum of the copolymer P(VGal16%co-BA84%) was measured after exposure to air for 6 days. Figure S13 shows that the intensity of the absorption peak at 1710 cm−1 increased in this time period, indicating that some gallol groups turned into galloquinone structures.26,42,43 However, in Figure S14, the absorption peak of P(VCat19%-co-BA81%) at 1710 cm−1 did not change before and after exposure to air, indicating that the catechol group was less prone to oxidation than the gallol group. The UV−vis absorption spectrum of P(VGal16%-co-BA84%) was also compared to that of P(VCat19%co-BA81%) (Figure S15). In the case of P(VGal16%-co-BA84%), the absorptions at 320 and 370 nm that can be attributed to the gallolquinone formation26,42 increased in intensity, while P(VCat19%-co-BA81%) did not show any absorbance. 1H NMR spectra (Figures S16 and S17) show that the P(VGal-co-BA) has higher oxidation ratio (14.5%) than that of P(VCat-co-BA) (3.1%) after 6 days. These results are consistent with the previous reports on the oxidation of phenolic groups.26,43 Thus, it could be concluded that cross-linking takes place between gallolquinone groups once they are formed (Figure S18).

strength. In order to better understand the cohesive interactions of P(VGal-co-BA) copolymers, the mechanical properties of P(VGal16%-co-BA84%) (P(VGal26%-co-BA74%) is too brittle to be a free-standing film) and P(VCat19%-co-BA81%) copolymers were compared by tensile strength tests of dumbbell-shaped films (Figure S8). P(VCat19%-co-BA81%) was a viscous polymer and did not break even at 2000% strain (Figure 4). In contrast, the P(VGal16%-co-BA84%) exhibited

Figure 4. Tensile test curves of P(VGal16%-co-BA84%) and P(VCat19%co-BA81%) copolymers.

higher Young’s modulus (7.5 ± 1.6 MPa), break strength value (3.5 ± 0.7 MPa), and toughness (17.6 ± 6.5 MJ m−3), but lower strain at break values (777.0 ± 135.3%; Figure 4 and Table S5). This is likely due to the formation of the crosslinking network in the polymer via oxidation of the gallol groups. After exposure to air, the polymer became partially insoluble (Figure S10). Oxidation-mediated cross-linking has been discussed in a later section. The mechanical properties of P(VGal16%-co-BA84%) and P(VCat19%-co-BA81%) were then investigated in water. The mechanical properties of P(VGal16%-co-BA84%) were nearly unchanged even after immersing in water for 3 and 6 days

Figure 5. Schematic illustration of lap shear adhesive bonding for P(VGal26%-co-BA74%) copolymer on Al, glass, PVC, PTFE, and pig skin substrates in water (a). Bonding strength of P(VGal26%-co-BA74%) copolymer on glass, PVC, PTFE, and pig skin substrates in water (b), seawater (c), and PBS (d). E

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polymer-state. The polymerization of isocyanates is affected by the surrounding environment such as its ionic strength.44 The ionic strength (I) (Table S9) in seawater (I = 0.68 M) is much higher than that of PBS (I = 0.17 M). The bonding strength performance under seawater is the most challenging condition compared with that in water and PBS.9

Recently, Lee et al. have investigated the cross-linking of pyrogallol.42 Figure S18 details the structure (Figure S18d) that is formed via the oxidative intermediates of galloquinones (Figure S18b,c), followed by intergallol cross-linking to generate structure (Figure S18e) with an additional fivemembered ring structure. Then, it turns out to be a structure (Figure S18f) with a proton. The water molecule reacts with the newly formed carbonyl group in the five-membered ring, resulting in a ring-opening reaction (Figure S18g). The final structure (Figure S18h) is formed on reaction with a proton. Thus, gallol groups form cross-links with each other via covalent bonds, which strongly promotes the cohesive interactions of P(VGal-co-BA). Collectively, P(VGal-co-BA) copolymers show higher bonding strength than P(VCat-co-BA) copolymers owing to the slightly higher interfacial interaction between the polymer and the substrate and greater cohesive interactions inside the polymers. Bonding Strength on Different Substrates. The different substrates onto which the adhesive adhere significantly affect the adhesion strength. These substrates can range from low-energy plastics to high-energy metals and from smooth to rough surfaces. In general, the high-energy surfaces are easier to stick with adhesives and smooth plastics are challenging substrates for adhesion. P(VGal-co-BA) has been identified as a strong adhesive for being able to bind to polished aluminum (Figures 2 and 3). Its adhesive performances on other substrates have been assessed as well (Figure 5). In addition to aluminum, glass, PVC, and PTFE were tested as substrates. These substrates exhibit a range of surface energies, roughness, and industrial applications. Additionally, pig skin was selected as a substrate to check the potential of the synthesized copolymers to act as a potential bioadhesive (Figure 5). Pairs of each substrate were joined together using P(VGal26%-co-BA74%). Figure 5 shows the adhesive performance on the different substrates. Aluminum provided the strongest bonding for P(VGal26%-co-BA74%) because of the strong coordination between the surface and adhesive. A glass surface offered lower bonding strengths because it was smooth and weaker interfacial interactions such as H-bonding were prevalent. Interestingly, P(VGal26%-co-BA74%) was able to stick to the most challenging, low-fouling substrates, including PVC and PTFE under all conditions (Figure 5). The gallol-functionalized copolymers could also attach well onto pig skin. Comparison to Commercial Glue. The maximum bonding strengths of P(VGal26%-co-BA74%) on aluminum were quite appreciable at 1.01 ± 0.26, 1.34 ± 0.43, and 1.35 ± 0.17 MPa under water, seawater, and PBS conditions, respectively. This performance was benchmarked against established commercial adhesives under identical conditions. The substrate, quantity of glue, and curing conditions were held constant. One of the most common underwater adhesives was chosen for comparison: Gorilla glue. This glue is isocyanate-based and performs very well under water (≥2.0 MPa) and has a similar strength (1.28 ± 0.10 MPa) as P(VGal26%-co-BA74%) in PBS buffer solution. However, its bonding strength decreases to 0.34 ± 0.09 MPa under seawater (Figure S19), which is much lower than that of P(VGal26%-co-BA74%) copolymers under identical conditions. Even though the adhesion behavior of P(VGal26%co-BA74%) copolymer was comparable to that of isocyanates, their chemistry differed dramatically. Isocyanate adhesives are applied to surfaces in the form of a flowing monomer followed by polymerization.2 In contrast, the gallol-functionalized copolymer is deposited onto the substrate already in its

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CONCLUSIONS In summary, gallol-functionalized copolymers were first synthesized via the MOM-protected method, and the method was then adapted for the synthesis of other phenolic copolymers. This allowed a comparison between the bonding strengths of the synthesized gallol-functionalized copolymers and that of the widely used catechol-functionalized copolymers under identical conditions. Gallol-functionalized copolymers showed better performance under all conditions compared with the catechol-functionalized copolymers. Bonding strengths were quantified on a variety of substrates, ranging from lowenergy, smooth plastic to high energy, rough metal. The gallolfunctionalized new glue also performed well in attaching pig skin together and is, thus, promising for use as a potential bioadhesive. These results help attest to the value of using blueprints from the tunicate when designing new materials. Such biomimetic design of polymers may aid the development of similar underwater adhesives for industrial or biomedical applications including surgical reattachment of soft tissues and dental cements. This kind of comparative study will promote more researchers to pay attention to gallol-functionalized copolymers.

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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.biomac.7b00921. Synthesis of DMMB, MMB, DMMS, MMS, P(DMMSco-BA), P(VCat-co-BA), P(MMS-co-BA), P(VPh-co-BA), and P(St-co-BA). Detailed experimental protocols for lap shear adhesion tests, swelling tests, work of adhesion, contact angle, film preparation, and ion strength. Dimension of the dumbbell-shaped sample, solubility of P(VGal16%-co-BA84%) copolymer solution, mechanical properties of P(VGal16%-co-BA84%), and P(VCat19%-coBA81%) copolymer versus immersion time in water, ATRFTIR spectra of the P(VGal16%-co-BA84%) and P(VCat19%-co-BA81%) copolymer, UV−vis absorbance for solutions containing P(VGal 16% -co-BA 84% ) and P(VCat19%-co-BA81%), possible mechanisms of oxidationmediated cross-linking of gallol-functionalized copolymers, and bonding strengths of the P(VGal26%-co-BA74%) copolymer as compared to Gorilla glue. Characterization of P(TMMS-co-BA), P(DMMS-co-BA), and P(MMS-coBA) and P(St-co-BA), summary of the contact angles and work of adhesion, summary of mechanical properties of P(VGal16%-co-BA84%) and P(VCat19%-co-BA81%), effect of immersion time in water on the mechanical properties of P(VGal16%-co-BA84%) and P(VCat19%-co-BA81%) copolymer, swelling ratio of P(VGal16%-co-BA84%) and P(VCat19%-co-BA81% copolymer underwater condition, and ionic strengths of water, seawater, and PBS solution (PDF). F

DOI: 10.1021/acs.biomac.7b00921 Biomacromolecules XXXX, XXX, XXX−XXX

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Hirotaka Ejima: 0000-0002-4965-9493 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Koyanagi-foundation, Leading Initiative for Excellent Young Researcher and JSPS KAKENHI Grant Number 15K17440. K.Z. is grateful for the support by the China Scholarship Council (CSC). We would like to thank Prof. Waite for fruitful discussion. We thank Xiaohai Jin and Zidi Wang for taking photos.

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DOI: 10.1021/acs.biomac.7b00921 Biomacromolecules XXXX, XXX, XXX−XXX