Formation of a Supramolecular Polymeric Adhesive via Water

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Formation of a Supramolecular Polymeric Adhesive via Water-Participant Hydrogen Bonding Formation Qiao Zhang, Tao Li, Abing Duan, Shengyi Dong, Wanxiang Zhao, and Peter J. Stang J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b02677 • Publication Date (Web): 08 May 2019 Downloaded from http://pubs.acs.org on May 8, 2019

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Journal of the American Chemical Society

Formation of a Supramolecular Polymeric Adhesive via WaterParticipant Hydrogen Bonding Formation Qiao Zhang,a† Tao Li,a† Abing Duan,b† Shengyi Dong,a* Wanxiang Zhaoa* and Peter J. Stangc* a

State Key Laboratory of Chemo/Biosensing and Chemometrics, College of Chemistry and Chemical Engineering, Hunan University, Changsha, Hunan 410082, P. R. China b

College of Environment Science and Engineering, Hunan University, Changsha, Hunan 410082, P. R. China

cDepartment

of Chemistry, University of Utah, 315 South 1400 East, Room 2020, Salt Lake City, Utah 84112, United States

Supporting Information Placeholder ABSTRACT: A supramolecular polymeric adhesive was prepared from non-viscous, non-polymeric materials by waterparticipant hydrogen bonds. Pt-pyridine coordination and watercrown ether hydrogen bonding combine to affect the supramolecular polymerization. The supramolecular polymeric adhesive displays strong, reversible adhesion to hydrophilic surfaces, a property that forecasts the application of hydrogen bonding in advanced supramolecular materials.

Hydrogen bond-driven self-assembly is a widely acknowledged strategy that combines different building blocks together to fabricate functional supramolecular polymeric materials.1 Many supramolecular polymers prepared via hydrogen bonds, display a range of dynamical properties and functions.2 In supramolecular polymerization, water-involved hydrogen bonds not only stabilize the polymeric structures, but expand the applications of supramolecular polymers.3 However supramolecular polymerization in water purely by hydrogen bonds is difficult, because of the strong competition of the hydrogen bonding formation with water molecules.4 Therefore, structural water, a concept from biological science, has been introduced to supramolecular polymer chemistry.5 Using water molecules as indispensable co-monomers in supramolecular polymerization and polymers on one hand can realize solvent-free supramolecular polymerization.6 On the other hand, it fully demonstrates the versatile property of waterparticipant hydrogen bonding and expands the applications of water in supramolecular chemistry.7 However, the design of water-participant supramolecular polymer is still difficult, because the relationship between water and polymeric repeated monomers is rich with ambiguities.6,7 Encouraged by the acknowledged importance of water-based hydrogen bonds in supramolecular polymerization and the versatile role of water molecular, we describe herein a solvent-free process for the preparation of supramolecular cross-linked polymeric materials by a combination of transitional metal-driven coordination and water-participant hydrogen bonding. The low molecular weight monomer I is a benzo-21-crown-7 (B21C7) functionalized pyridine with B21C7 units at the 2- and 5-positions (Figure 1).8 I was synthesized by an amidation reaction in 44% yield and fully characterized (Figure S1,S2). I is obtained as a white solid and is totally water-insoluble as illustrated in Figure 2a. I exhibits neither water absorption nor adhesion behavior under conditions that include high/low temperature,

changes in relative humidity, and the absence/presence of water or organic solvents. This behavior differs from that of our previous work,6c where we have found that a 1,4-disubstituted crown ether derivative is totally water insoluble, let alone forming hydrogen bonding with water molecules. This structure-induced water insolubility property is due to the strong aggregation of 1,4disubstitued derivatives. Water molecules therefore are expelled to form hydrogen bonding with crown ether units.7b,9 Based on these results, we introduced Pt-coordination to I with the aim to change the geometry structure and avoid the strong aggregation. Reaction of I and Pt(PEt3)2(OTf)2 in dichloromethane for 10 h gives only 1:1 Pt-coordinated product II, even when an excess of I is present. Such a 1:1 Pt-coordination is ascribed to the steric hindrance of the adjacent amide groups. When 3,5-disubstituted III was used, a 1:2 Pt-coordinated product was obtained (Figure S5-S8, S15). The structure of II was characterized by 1H/31PNMR, H-H COSY, 2D NOESY, and high-resolution mass spectrometry (Figure 1 and Figures S1-S4, S9-S14 in Supporting Information).10 As shown in Figure 1a,b, the proton signals of the pyridine ring are shifted downfield, and the protons of the amide groups and their adjacent CH2 groups are split into two sets of sharp peaks. These features support the coordination of the platinum center to the pyridine moiety and the generation of unsymmetrical N-H groups in II.10,11 High-resolution mass spectrometry (FTICR-ESI-MS) reveals a peak with m/z = 1331.5644, which is in full agreement with a 1:1 structure (Figure 1c,d). In its dry state (by overnight drying under vacuum), II is a bright yellow solid without viscous or adhesive properties at room temperature (Figure 2b, middle). This material absorbs water from the air and undergoes a rapid solid-glue transition (Figure 2b, right). Quantitative tests indicate that II absorbs about 1.5 wt% water and reaches a state of equilibrium after roughly 15 min (25 °C, 45 RH%, Figure S17). II-water mixtures are amorphous (Figure S23). After absorbing water, II-water mixtures become highly viscous. Long, flexible, viscous fibers are easily drawn from II-water mixtures (Figure 2c). Rheology tests were performed to provide quantitative information on the role of water in inducing the high viscosity of II (Figure 3a,b and Figures S18-S20). II-water mixtures are viscoelastic liquids with G” values greater than G’ and G” and G’ values independent of frequency. Consistent with their macroscopic performance after water absorption, II-water mixtures exhibit a viscosity of 8 × 105 Pa·s, which is considerably higher than that of many crown ether-based supramolecular polymers.12 II-water mixtures also display good temperature-dependent viscoelasticity

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Figure 1. Synthesis of I and II, and partial 1HNMR spectra in CDCl3 of (a) adhesive material II and (b) non-viscous monomer I. Highresolution mass spectra of II (c) calculated peaks (blue) and (d) observed peaks (red).

Figure 2. Water absorption behavior of (a) I and (b) II, and (c) fibers drawn from II-water mixtures.


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Journal of the American Chemical Society

Figure 3. Reversible temperature-dependent rheological tests: (a) viscosity of II-water mixtures and (b) modulus of II-water mixtures. DFT calculation of (c) I and (d) II (ethyl groups were replaced by methyl groups to simplify the calculation). and reversible cyclical behavior, properties which were established by repeated temperature-dependent cycling (Figure 3a,b).6c Attenuated total reflectance infrared (ATR-IR) spectroscopy was used to obtain a better view of water molecules as comonomers in supramolecular polymerization (Figure S21 in Supporting Information). II-water mixtures show two bands above 3000 cm-1 (3509 and 3297 cm-1), which are assigned to the NH vibrations of amide groups. However we failed to find the band belong to water molecules. A possible explanation is that the band of water molecules is merged either at 3509 cm-1 or at 3297 cm-1. In order to solve D2O, IR spectroscopy of II-D2O mixtures shows two bands similar to those of II-H2O, but with decreased intensity of the 3297 cm-1 band. This result indicates that the band of water molecules is emerged at 3297 cm-1. According to the wavelength of water molecules,13 it is reasonable that water molecules exist in a hydrogen bonding (structural) state rather than as bulk water, which is consistent with our IR studies of water molecules.6c Density functional theory (DFT) calculations were performed to study the relationship between the chemical structure and viscosity (Figure 3c,d, Figure S24, S25). Ions (OTf anions) were also considered during the DFT calculation. Because we have determined the binding mode and strength of hydrogen bonds between water molecules and crown ether units,6c we focus here on the molecular conformation transition from aggregated I to nonaggregated II. The calculated structures and energies of I and Pt(PEt3)2(OTf)2 are shown in Figure 3c; those of II are shown in Figure 3d. The electronic energy and free energy of uncombined Pt(PEt3)2(OTf)2 and I are greater by 36.4 and 14.0 kcal/mol, re-

spectively, than the corresponding energies of complex II. Thus, II is thermodynamically stabilized by Pt-pyridine coordination and intramolecular interactions. Although DFT calculations do not provide a direct explanation for the transition from nonviscous to viscous behavior, it is evident that Pt-coordination exerts a very large influence on the molecular structure. As mentioned before, 1,4-disubstitued crown ether derivative shows strong intermolecular aggregation,6c which quenches its supramolecular polymerization and adhesion behavior. Thus, the prevention of p-diamide-based aggregation is crucial to achieving adhesion.9 Figure 3d shows that coordination of Pt(PEt3)2(OTf)2 promotes obvious changes in molecular conformation. The 2,5diamide pyridine and crown ethers in II become twisted because of Pt-pyridine coordination, compared with the chemical structure of I. Thus, the pyridine unit in II cannot aggregate with pyridine units from adjacent II molecules due to steric hindrance. Once the intermolecular aggregation is forbidden, water molecules can easily form hydrogen bonds with the crown ether rings. Because of the multiple hydrogen bonding sites in the crown ether rings, the incorporated water molecules can act as structural bridges by connecting various crown ether units via multiple crown etherwater hydrogen bonds. Finally, water molecules behave as crosslinkers to connect multiple II molecules together to form a crosslinked supramolecular polymer.6c,9



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Figure 4. Quantitative adhesion experiments involving (a) II-water adhesion at different temperatures and (b) cycling tests of II-water adhesion. (c) Macroscopic adhesion tests of II-water adhesion on glass and paper. The adhesion areas are 7.5 × 2.5 cm2 (glass) and 4.0 × 3.5 cm2 (paper), respectively. We next consider adhesive applications that were suggested by the results of the macroscopic tests and rheological measurements. These experiments confirmed the strong and enduring adhesion properties of II.6c,14 Hydrophilic glass and paper substrates were used as supports for the polymerization reaction that is driven by water-crown ether-based hydrogen bonding (Figure 4). Two glass plates were conjoined as soon as II-water mixtures were coated on their surfaces. No separation or misalignment of glass plates was observed even when weights up to 12 kg were hung from one of the two plates. Similar rapid strong adhesion was observed when scraps of paper were used. The adhesiveness of II-water materials is temperature dependent, and weakens as the temperate increases. Once the temperature exceeds 50 °C, it becomes relatively easy to separate the two glass plates. However, a temperature decrease promptly restores the strong adhesion. Temperaturedependent rheological tests illustrate the temperature-induced changes in adhesion. Temperature-dependent behavior is evident throughout four heating-cooling cycles (between 25 and 50 °C), but no decline in viscosity is observed. Quantitative measurements of adhesion strengths were carried out by a pull-off adhesion method (Figure 4a,b).6c,14,15 At 25 °C and 50% RH, the average pull-off adhesion strength is about 273

± 27 psi for the II-H2O adhesive, which is higher than that of some reported supramolecular polymer adhesives,15 but is less than our previously reported example.6c However this adhesive shows poor adhesive strength to Teflon surfaces, with an average value at 28 ± 7 psi. Variable temperature studies show that elevated temperature weakens adhesion consistent with the macroscopic properties. For example, the adhesion strength of II-H2O at 70 °C is only one-ninth of that at 25 °C. We attribute this change to the disruption of crown ether-water hydrogen bonds and the enhancement of polymer fluidity. At low temperature, this adhesive material loses its fluidity, and shows a weaker stretchability, thus it has a relatively low adhesion strength. Pull-off tests demonstrate that elevated relative humidity does not significantly influence the adhesion behavior, which is an advantage over our previously reported example.6c An increase in relative humidity from 50 to 98% causes only a 7% decay (from 273 ±27 to 254 ±20 psi) in adhesion strength. Temperature cycling experiments demonstrate the regenerative nature of hydrogen bond-based supramolecular polymeric adhesion based on crown ethers, Pt-pyridine centers, and water molecules by revealing no decline in adhesion strength after multiple adhesion-separation cycles. For example, the adhesion strengths of the II-water adhesive after 4 and 8 cy-


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Journal of the American Chemical Society cles are 276 ± 22 and 269 ± 13 psi, respectively. These strengths are almost the same as the initial value. The results completely agree with those of the macroscopic tests and temperaturedependent rheological measurements (Figure 4c). In conclusion, a cross-linked supramolecular polymer with high viscosity and low fluidity was designed and successfully prepared. The conversion of two non-viscous monomers into a supramolecular polymeric adhesive was realized via the introduction of a Ptbased coordination reaction. Incorporation of water molecules is essential in assembling molecules of II into a high-molecularweight supramolecular polymer. Both Pt-based coordination and water-participant hydrogen bonding are crucial in achieving supramolecular polymerization and strong adhesion. Recoverable adhesion behavior, which is sustained by the thermodynamic properties of water-crown ether hydrogen bonding, also is achieved. Supramolecular adhesive materials constructed by crown ethers have been rarely reported,15b compared with other macrocycles. The present work enriches the supramolecular adhesive family. The relationship between monomer structure and adhesion property obtained in this work is helpful in designing crown-ether-involved supramolecular adhesive systems.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Experimental procedures, characterization and NMR spectra for obtained compounds and other materials (PDF)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] [email protected] [email protected] †These authors contributed equally

ORCID Shengyi Dong: 0000-0002-8640-537X Wanxiang Zhao: 0000-0002-6313-399X

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT Financial support from the National Natural Science Foundation of China (Grant No. 21704024 and 21702056), Huxiang Young Talent Program from Hunan Province (2018RS3036)), and the Fundamental Research Funds for the Central Universities are greatly appreciated. We thank the kindly support from Prof. Christoph A. Schalley and Fei Jia for the mass spectrometry measurements.

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Formation of a Supramolecular Polymeric Adhesive via Water

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