Dynamic Covalent Hydrazone Supramolecular Polymers toward

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

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Dynamic Covalent Hydrazone Supramolecular Polymers toward Multiresponsive Self-Assembled Nanowire System Kyung-su Kim, Hye Jin Cho, Jookyeong Lee, Seonggyun Ha, Sun Gu Song, Seunghun Kim, Wan Soo Yun, Seong Kyu Kim, Joonsuk Huh,* and Changsik Song* Department of Chemistry, Sungkyunkwan University, 2066 Seobu-ro, Jangan-gu, Suwon-si, Gyeonggi-do 16419, Republic of Korea

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S Supporting Information *

ABSTRACT: Stimuli-responsive polymeric systems are of considerable interest due to their potential applications in environment-adaptive technologies such as smart surfaces. Traditionally, such systems can be constructed either by dynamic noncovalent (supramolecular) or dynamic covalent chemistry, but the use of both chemistries in one system may offer unique opportunities for structural diversity and various controllability. Herein, we report that hydrazone−pyridinum conjugates, which can be dynamically exchanged by transimination, assemble to form one-dimensional nanowires due to direct intermolecular interactions (without metal-ion coordination). The self-assembly process can be controlled not only by dynamic covalent chemistry but also by pH adjustment. The hydrazone−pyridinum conjugates are transformed to merocyanine-type dyes of distinctive negative solvatochromism via deprotonation, which also affects their self-assembly. Such a dual control of the dynamic molecular assembly will provide unique way to develop diverse smart nanomaterials with multistimuli-responsiveness.

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INTRODUCTION Stimuli-controlled or -responsive self-assembly at the molecular scale has been intensively investigated over the past few decades for various applications such as self-healing materials,1,2 biosensing,3−5 drug delivery,6,7 artificial muscles,8−11 and molecular devices.12 To control the assembly behavior at the molecular scale, the molecules or functional groups capable of self-assembly should possess stimuli-responsiveness. Many molecular moieties have been developed for stimuli-responsive assembled materials: spiropyran,13 azobenzene,14 terpyridine,15,16 naphthalene diimide,17 and hydrazone.18 Among them, the hydrazone functional group offers an advantage not only due to its configurational changes via E−Z isomerization induced by light,19,20 pH,21 and metal coordination22 but also due to the dynamics of its reversible CN bond formation,23,24 which enables structural diversity and dynamicity. Hydrazones have been utilized in the construction of stimuli-responsive supramolecular polymers, in which monomeric units are held together with reversible noncovalent bonds. For example, Hanton et al. reported the controlled self-assembly of hydrazone−metal complexes depending on the species of metal ions.18 Lehn et al. reported a hydrazone-based photoand thermoresponsive supramolecular metalloassembly which showed photoinduced potassium release.25 Samori et al. reported the hydrogen-bond assisted metalloassembly of a bis(hydrazone) ligand.26 Precedent research suggests that the hydrazone moieties play a role of either a simple linker or a ligand for metal-ion binding. In the function of the simple linker, supramolecular polymerization is enabled by certain functional groups capable of (multiple) hydrogen bond or host−guest © XXXX American Chemical Society

interactions which are connected by the dynamic hydrazone moiety. On the contrary, in the function of the ligand, the binding of metal ions at the hydrazone moiety may produce supramolecular polymers. The intermolecular interaction that enables the supramolecular polymerization is based on the reversible hydrazone−metal association.27−30 To the best of our knowledge, the direct intermolecular interaction as a driving force for supramolecular polymerization has not yet been reported. Herein, we report novel benzoyl hydrazone−p-pyridinium conjugates that show self-assembly by direct intermolecular interactions. Remarkably, the supramolecular polymerization and absorption properties can be easily controlled by pH and the dynamic covalent hydrazone exchange.

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RESULTS AND DISCUSSION Bis-p-pyridinium benzoyl hydrazone BH1 (Figure 1) can be easily synthesized by a nucleophilic substitution reaction with a high yield from commercially available p-xylene dibromide and (E)-N′-(pyridine-4-ylmethylene)benzohydrazide, which was prepared by the hydrazone formation reaction31 with benzohydrazide and 4-pyridinecarboxaldehyde (Supporting Information). Interestingly, we found that the hydrazone p-pyridinium conjugate BH1 showed pH-responsive color change and helical wire-type self-assembly (see below). In the presence of a base (e.g., diisopropylethylamine), initially transparent BH1 in dimethyl sulfoxide (DMSO) changed to orange-red and became colorless again with the addition of an acid (e.g., trifluoroacetic Received: September 5, 2018

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DOI: 10.1021/acs.macromol.8b01909 Macromolecules XXXX, XXX, XXX−XXX

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Figure 1. (a) Formation of a pH-responsive, merocyanine-type dye from the hydrazone (H), which has a resonance structure of zwitterionic and neutral forms. (b) Molecular structures of synthesized hydrazone−pyridinium conjugates. (c) UV−vis absorption spectra of H1, H2, Hm, and H(Me) in DMSO. (d) SEM images of bis-hydrazones, BHs (10−4 M in DMSO), drop-casted on glass slides, and dipole moments of corresponding monohydrazones, Hs.

peak at 482 nm of H1 was more evident with addition of a base (diisopropylethylamine, Figure S2). Thus, we designated the absorption peak at 482 nm in DMSO as the charge-transfer (CT) band, which cannot be achieved by meta-conjugation and is limited by the methyl substitution. Interestingly, the CT band of the hydrazones showed large negative solvatochromism (hypsochromic shifts) with solvent polarity (Figure S3), which appears to be similar to that of Brooker’s merocyanine.35,36 We assume the dipole moment of the ground state is greater than that of the excited state, resulting in the greater stabilization of the ground state in highly polar solvents. It should be noted that the hypsochromic shift was more pronounced in protic solvents than in aprotic solvents since protic solvents form hydrogen bonds with the zwitterionic resonance structure of H1, promoting more the charge separation in zwitterionic ground state of H1 than the case of aprotic solvents. The self-assembled nanowire formation of the bis-hydrazones (BHs) was also affected by their molecular structures (Figure 1d). Interestingly, the ability to form the CT band seemed to be strongly related to the wire formation. The methyl-substituted BH(Me) and meta-substituted BHm, which were absent in the CT band, produced irregular aggregates rather than selfassembled wires. Importantly, however, the self-assembled wires were not observed with the completely deprotonated merocyanine form of BH1; the presence of amide N−H (protonated) appears to be required in the self-assembly (Figure S4). A different type of self-assembly from bulky BH2 was observed, presumably due to the steric hindrance of the tertbutyl group. To examine the degree to which the charge density of H1 affects dimerization, we calculated the molecular electrostatic potential (MEP) map using Spartan’16 package software (DFT calculation at B3LYP/6-31+G*) (Figure S5). As expected, the most negative MEP value was observed in the region of carbonyl oxygen and the most positive MEP value was observed in the region of nitrogen of the pyridinium ring, generating its molecular dipole moment from the pyridinium

acid) (Figure 1a). We attributed this color change to the formation of a merocyanine-type dye32−34 with deprotonation of cationic BH1. Merocyanine dyes can be described as a resonance hybrid between zwitterionic and quinoid forms and generally show large solvatochromic shifts. In addition, the wiretype supramolecular polymerization of BH1 was observed upon the slow evaporation of the solvent (Figure S1 and Figure 1d), which strongly suggests the presence of intermolecular interaction between the hydrazones. To determine in detail the molecular origins of the absorption change and nanowire formation of the hydrazone, we designed and synthesized the derivatives (BH2, BH(Me), and BHm) of BH1 and their corresponding monohydrazone model compounds (H1, H2, H(Me), and Hm, respectively) (Figure 1b). The bulky tert-butyl was attached to the hydrazone group to specifically investigate the steric effect on the nanowire formation (BH2 and H2). Because the amide hydrogen may play a role in intermolecular assembly, a methyl group was substituted in BH(Me) and H(Me). Finally, m-pyridinium hydrazone conjugate was prepared, instead of the p-pyridinium hydrazone conjugate, to examine the effect of the configuration of π-electrons (BHm and Hm) on the electronic absorption and self-assembly. As revealed by the UV−vis absorption spectra of the monohydrazones (Figure 1c), it was confirmed that the presence of amide N−H and the conjugation in the paradirection of the pyridinium moiety are responsible for the formation of the merocyanine-type dyes from the hydrzones. The similar absorption peak at around 482 nm of H1 and H2 (20 μM in DMSO) disappeared in solutions of the methylsubstituted H(Me) and meta-substituted Hm. These results may be easily understood by examining the resonance canonical structure of BH1 or H1 when deprotonated (Figure 1a); the charge at the amide moiety can be transferred to the electrondeficient pyridinium moiety through a proper π-electron conjugation (i.e., para-direction). Furthermore, the absorption B


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Figure 2. (a) Concentration-dependent UV−vis absorption spectra of H1 in DMSO solutions from 4 μM to 0.7 mM. (b) Mole fractions of the zwitterionic merocyanine form (Z) in the concentration range 6−600 μM (red) with lines of ideal (calculated) Z fractions with various pKa values (lines from left to right: pKa = 8, 7, 6, 5, and 4). (c) Normalized UV−vis absorption spectra of BH1 in DMSO solution (6 μM) and film state. (d) Estimated transition energy (S0 to S1) difference from monomer to dimer calculated from the arrangements of transition dipole vectors. (e) Plot of transition energy (S0 to S1) difference (cm−1) from monomer to dimer in DMSO according to the slip angle of the transition dipoles. (f) Mole fractions of the aggregates (A) (αagg) against KcT determined by the nonlinear regression analysis of UV−vis spectra in (a) with the isodesmic model at 336 nm (blue). Calculated αagg against KcT with different σ (= K/K2) values were also presented in lines. (g) Plausible chemical equations showing the equilibrium of hydrazone-pyridinium conjugate H1 (H) and its aggregates (A) with the deprotonated merocyanine form (Z).

To understand the intermolecular interaction for the supramolecular polymerization of the hydrazone conjugates BHs, the concentration-dependent UV−vis absorption spectra of the model compound H1 was investigated (Figure 2a). The concentration of H1 in DMSO was varied from 0.004 to 0.7 mM, and the resulting spectra were plotted as the molar absorptivity (L/(cm mol)) of H1. We found that (1) the molar absorptivity of the CT band (∼482 nm) decreased and (2) the molar absorptivity of the π−π* band (∼336 nm) increased, as the concentration of H1 increased (similar trend was observed in UV−vis absorption spectra of BH1 as shown in Figure S6); however, these molar absorptivities were expected to remain unchanged, regardless of the concentration if the solution composition does not change. Interestingly, the variations of the molar absorptivities were not dramatic in protic solvent such as water (Figure S7). Following the method suggested by Crampton and Robotham,38 we were able to obtain the acid dissociation constant of H1 (pKa = 11.5) in DMSO buffered with n-butylamine and n-butylamine hydrochloride, furnishing the molar absorptivity of merocyanine Z at 482 nm (ϵZ = 4.98 × 104 L/(cm mol)) and consequently the mole fraction of Z (αZ) at each total concentration (c0) (Figure S8). The observed αZ seemed to be close to the case of pKa ∼ 5 at very low concentration, while it shifted to the less acidic case of pKa (>8)

ring to the benzoyl group. Although our hydrazones did not form self-assembly at their merocyanine form, the dipole−dipole interaction may be responsible for the self-assembled wire formation. Thus, dipole moment values of monohydrazones were estimated by density functional theory (DFT) calculations (B3LYP/6-31+G*). We found that the dipole moments of hydrazone moieties (H1 and H2 for BH1 and BH2) that formed self-assembled wires were higher than those of that did not form self-assembled wires (H(Me) and Hm for BH(Me) and BHm, respectively). However, the least dipole moment value of 11.5 D for H(Me) seems sufficient in the case for dimerization of merocyanine dyes by dipole−dipole interaction.37 Furthermore, we cannot conclude that the tendency to self-assemble appears to increase as the dipole moments of the hydrazone moiety increase, since the dipole moment of Hm (14.1 D) is fairly close to that of H1 (14.4 D). We suspect that the dipole−dipole interaction cannot fully explain the self-assembly behavior of BHs. In addition, the doubly charged bis-hydrazones still formed wires in highly polar solvents such as DMSO (strong hydrogen-bond acceptor), which suggests that the dipole− dipole interaction and the hydrogen bond through amide N−H may be weakened. The above results motivated a detailed investigation of the way in which the hydrazones were dimerized or aggregated to form the self-assembled wires (Figures 2 and 4). C


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Figure 3. (a) ORTEP diagram of H1 (thermal ellipsoids are set at 50% probability, and the iodide anion and methanol are omitted for clarity). (b) Interaction energy of the assembled dimer observed in the solid state of H1 and the distances of interactions (CH−π: 3.451 Å; π−π: 3.561 and 4.565 Å). (c) Hirshfeld surface of H1 with the shape index function showing π−π and CH−π interactions. (d) Hirshfeld surface of H1 with the curvedness function demonstrating good fitness between molecules. (e) Contributions of the specific contacts from the Hirshfeld surface analysis of H1. (f) Schematic representation of supramolecular polymerization of BH1 via hydrazone’s self-assembly.

336 nm, suggesting the possible existence of another species responsible for the absorption at 336 nm. Lastly, as shown in Figure 2d, we derived a model for a centrosymmetric dimer structure of H1, where the distance was 3.599 Å and the slip angle was 48.5° (from the X-ray crystallographic structure of H1; see Figure 3b). With this model and transition dipole vector (21.06 D, TD-DFT calculation at B3LYP/6-31+G*) we could calculate the difference in the transition energy (cm−1) between H and dimer D (Figure 2e). We obtained a fairly small difference (13 cm−1 in DMSO with dielectric constant of 46.7) according to the dipole−dipole approximation,41 which supports the assumption that the bands of H, D, and A may appear in similar locations in the UV−vis spectra (around the π−π* band). After assigning the absorptions of both aggregates A and hydrazone H to the π−π* band, we were able to perform the nonlinear regression analysis of the concentration-dependent UV−vis absorption spectra of H1 using the isodesmic model.42,43 First, the concentration sum (cT) of [H] + [A] was determined easily by subtracting the obtained value of [Z] from the total concentration. The baseline shifting of the bands of H and A (shown in Figure 2a) was corrected prior to the analysis. Then, the isodesmic model was applied to analyze the π−π* band (∼336 nm) (Figure S10) with eq 1.

at higher concentrations (Figure 2b). We suspect that such dramatic decrease of the “apparent” Ka is due to an event at higher concentrations, presumably a certain type of aggregation. Thus, we examined the aggregation model39,40 to explain the concentration behavior of the UV−vis absorption spectra of H1. The band assignment should have been performed prior to the analysis, since the CT band (∼482 nm) was assigned to the absorption of zwitterionic merocyanine Z while the π−π* band (∼336 nm) was initially assigned to the absorption of hydrazone H. The position of the absorption peak of the aggregates A seems rather obscure, but we assume that the aggregates A can also contribute to the π−π* band (∼336 nm) according to the following reasons. First, we observed absorption of BH1 in film state, which shows similar location of absorption with π−π* band derived from the protonated hydrazone (H) (Figure 2c). We can presume safely that all hydrazone moieties of BH1 exist as aggregates (similar to A) in the film state. Second, we measured the temperature-dependent UV−vis spectra of H1 in DMSO (10 and 60 μM), which suggests the existence of additional chemical equilibrium at higher concentrations (Figure S9). At the lower concentration of 10 μM with increasing temperature, the decreasing amount of the absorption at 336 nm was comparable to the increasing amount at 482 nm, suggesting one species converting to the other. However, in the case of the higher concentration of 60 μM, we did not observe the absorbance increase at 482 nm as much as the decrease at D


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Figure 4. (a) Transimination (dynamic covalent chemistry) of hydrazone 1 to BH1 catalyzed by trifluoroacetic acid (TFA) and the formation of pHresponsive, merocyanine-type dye BZ1 from BH1 by triethylamine (TEA). (b) SEM images of transimination reaction mixtures at the initial (left) and after 24 h (middle) and TEA treatment (right), forming zwitterionic merocyanine BZ1. (c) Selected time-dependent nuclear magnetic resonance (NMR) measurements of the transimination (DMSO-d6) from 0 to 21 h. (d) Percent conversion and yield of hydrazone 1 and BH1, respectively, upon proceeding of the transimination reaction.

ϵH + A =

2Kc H + A + 1 −

4Kc H + A + 1 2

2(Kc H + A )

dimerization constant of the tetrasulfonated magnesium phthalocyanine, which is known to undergo cation-induced self-assembly.45 To further investigate the intermolecular interaction of the hydrazone−pyridinium conjugates, Hirshfeld surface analysis was utilized in the crystal network of H146 obtained in a mixture of acetonitrile and methanol. H1 crystallized in the P21/n space group, in which the pyridinium cation was charge-balanced with I− anion (Figure 3a; other atoms are omitted for clarity). Interestingly, we found that the H1 molecules were directionally packed normal to the aromatic rings (Figure S12), and the neighboring molecules were positioned antiparallel to each other or centrosymmetrically, facing the para-hydrogen of the benzoyl group toward the center of the pyridinium ring of the neighboring molecule, with a distance of around 3.5 Å (Figure 3b). This packing mode may explain why sterically bulkysubstituted BH2 showed a different type of self-assembly (Figure 1d); the tert-butyl group may prevent the efficient packing of hydrazone molecules. Figures 3c and 3d show the Hirshfeld surfaces mapped over the shape index and curvedness47 of H1, respectively. In the generated Hirshfeld surface of H1 with the shape index function, we observed “bow-tie” patterns which indicate the presence of π−π stacking,48 and CH−π interactions between benzene and pyridinium ring (Figure 3c). The curvedness map showed flat areas in the contacted surface, which indicates the possibility of a well-matched dimer structure (Figure 3d). The decomposed

(ϵH − ϵA ) + ϵA (1)

where K denotes the aggregation equilibrium constant, cH+A and ϵH+A denote [H] + [A] and the molar absorptivity at 336 nm, respectively, and ϵH and ϵA refer to the molar absorptivity of the hydrazone H and aggregates A, respectively. According to the result of the nonlinear regression analysis of data from Figure 2a at 336 nm with the isodesmic model, the fraction of aggregates A in Figure 2a was obtained. The obtained fraction αagg was plotted against KcT (K: equilibrium constant), with the theoretical lines from the nucleation−elongation model with various σ values (σ = K2/K) from 0.001 to 1000 (Figure 2b).41,44 If the elongation equilibrium constant (K) and the nucleation equilibrium constant (K2) are the same (K2 = K, or σ = 1), it is noncooperative or isodesmic to which our UV−vis data appear to fit very well. The fitted results of the mole fractions of each species (A, H, and Z) were plotted as a function of the total concentration of H1 (c0) (Figure S11). The merocyanine or zwitterionic Z decreases rapidly as the concentration increases. The hydrazone H increases first but then also decreases due to the formation of its aggregates A. In the concentrated solution of H1 (above ∼200 μM), the aggregates A appeared to assume the most abundant portion (>90%). We were also able to obtain the value of the aggregation equilibrium constant K = 8.06 × 104 M−1 , which seems fairly high and comparable to the E


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Macromolecules fingerprint plots indicate that the types of interactions in the packing of H1 in the crystal were van der Waals forces (H−H contacts, 49.6%), CH−π interactions (C−H contacts, 15.2%), and π−π stacking interactions (C−C contacts, 7.4%) (Figure 3e). For the pair of H1 molecules, we were able to calculate the interaction energy using CrystalExplorer3.1 software (B3LYP/ 6-31G(d,p)). The total interaction energy appeared to be −40.7 kcal/mol, which can be further divided in terms of electrostatic (−7.3 kcal/mol), polarization (−3.1 kcal/mol), dispersion (−48.8 kcal/mol), and exchange repulsion (18.5 kcal/mol). In short, the dimerization of H1 appears to be large due to the stabilization by van der Waals, π−π, and CH−π interactions, rather than by electrostatic dipole−dipole interaction only. In addition, since self-assembled wires of BH1 could be formed even in water that can strongly interrupt the intermolecular hydrogen bonding (Figure S13), it can be assumed that direct intermolecular interactions (π−π, CH−π, and van der Waals interactions identified by Hirshfeld surface analysis) have a greater influence than the hydrogen bonding in self-assembly. The morphology of the self-assembled wire of BH1 was investigated by scanning emission microscopy (SEM) and atomic force microscopy (AFM) (Figure S14). The SEM and AFM images of BH1 prepared by drop-casting on glass substrates or silicon wafers displayed self-assembled helical wires that supposedly consisted of individual one-dimensional (1-D) polymers. It appeared that as shown in Figure 3f, the initially formed individual 1-D polymers were combined together progressively, eventually resulting in thick, twisted wires with lengths of up to 10 μm, widths ranging from 5 to 45 nm, and pitches of over 65 nm (Figure S14). Furthermore, it appeared that the two lateral directions of growth were not isotropic; one direction seemed slightly more favorable than the other, resulting in rectangular-type twisted wires (Figure 3f). Although the helicity of the assembled wires seems to be controlled as left-handed (M) in SEM images (Figures 1d and 4b, Figure S4), we identified the right-handed (P) helices also. Currently, the control of the helicity of self-assembled nanowires is under investigation. One of the most important properties of hydrazone-based molecular systems is that the covalent hydrazone bond is also dynamic. Dynamic covalent chemistry (DCvC) has been intensively studied in applications such as adaptive surfaces, polymer actuators, drug delivery, and drug discovery systems.49−53 To investigate the morphology control of supramolecular wires from bis-hydrazones, a hydrazone transimination experiment was performed between hydrazone 117 and competing bis-aldehyde 2 (Figure 4a). Bis-aldehyde 2 (0.5 equiv) was added to the DMSO solution of hydrazone 1 (15 mM) in the presence of catalytic trifluoroacetic acid (TFA), and the mixture was stirred at 100 °C. 1H nuclear magnetic resonance (NMR) spectroscopy confirmed that the compound BH1 was produced as the exchange reaction proceeded, which was also verified by SEM. At the initial stage of the reaction, no self-assembled wire was present, but after a few hours, selfassembled helical wires were observed that were identical to those directly formed from BH1 (Figure 4b). We also further investigated the kinetics of DCvC (transimination) for BH1 by time-dependent NMR spectra (Figure 4c and Figure S15). By assigning the amide proton of BH1 (12.69 ppm) and hydrazone 1 (11.87 ppm), we were able to calculate the molar ratio of hydrazone 1 to BH1, which showed that the equilibrium of transimination from hydrazone 1 to BH1 was established after 21 h at the 80% yield for hydrazone 1 (Figure 4d). The above

results clearly demonstrated that the hydrazone’s reversible or dynamic covalent chemistry controls the morphology of selfassembled nanowires from supramolecular polymerization; the initially nondirectional aggregates of 1 transformed into 1-D self-assembled nanowires of BH1 through the DCvC (transimination) of the hydrazones.

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CONCLUSION We found that “dynamic covalent” hydrazone−pyridinium conjugates (BHs) also underwent “dynamic noncovalent” (supramolecular) polymerization in the formation of stimuliresponsive nanowires. The hydrazone−pyridinium conjugates showed pH-dependent color changes, resulting from their conversion from the protonated hydrazone to the deprotonated merocyanine-type zwitterion. The concentration-dependent UV−vis absorption spectra of the model hydrazone (H1) were analyzed by nonlinear regression analysis using the isodesmic model of aggregation to show that the dimerization (or further aggregation) was prevalent at higher concentration (aggregate fraction >90% above ∼200 μM in DMSO). The intermolecular interactions between the hydrazone−pyridinium conjugates were analyzed with the X-ray crystal structure of H1, indicating that the van der Waals, π−π, and CH−π interactions might work together with the dipole−dipole interaction. The covalently dynamic character of the hydrazones was utilized in demonstrating control of the morphology in self-assembled nanowires; dynamic transimination enabled the conversion of irregular aggregates to twisted wires. We believe that such control of dynamic molecular self-assembly will provide a significant contribution to the development of novel multistimuli-responsive materials for applications such as drug delivery system, actuators, and biosensors.

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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.8b01909. Experimental methods, detailed synthetic procedures, supporting figures, 1H and 13C NMR spectra (PDF) CheckCIF/PLATON report (PDF) X-ray crystal structure of methylpyridinium hydrazone (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*(J.H.) E-mail [email protected]. *(C.S.) E-mail [email protected]. ORCID

Changsik Song: 0000-0003-4754-1843 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Nano Material Development Program (2012M3A7B4049644) and the Small Grant Exploratory Research (SGER) Program (NRF2015R1D1A1A02062095) through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (MEST), Republic of Korea. J.H. also acknowledges support by Basic Science F


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Research Program through the NRF funded by the MEST (NRF-2015R1A6A3A04059773 and 2017R1A4A1015770).

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Dynamic Covalent Hydrazone Supramolecular Polymers toward

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