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Multistimuli Responsive Supramolecular Cross-Linked Networks On the Basis of the Benzo-21-Crown-7/Secondary Ammonium Salt Recognition Motif Long Chen, Yu-Kui Tian, Yue Ding, Yu-Jing Tian, and Feng Wang* Key Laboratory of Soft Matter Chemistry, Department of Polymer Science and Engineering, University of Science and Technology of China, Hefei, Anhui 230026, P. R. China S Supporting Information *

ABSTRACT: A facile route is demonstrated to realize the supramolecular cross-linked networks based on the benzo-21crown-7/secondary ammonium salt recognition motif, which involves the interchain host−guest interactions between the secondary ammonium salt-functionalized polystyrene and the complementary homoditopic benzo-21-crown-7 cross-linker. Reversible addition−fragmentation chain transfer polymerization is utilized to prepare the well-defined secondary ammonium salt-functionalized graft polymer. As determined by 1H NMR and GPC, the molecular weight Mn is 13.3 kDa with polydispersity value of 1.10, suggesting that 15.9 repeating units of secondary ammonium salt moieties exist in a single polymer chain. The properties of the resulting supramolecular cross-linked networks are characterized in solution by means of 1H NMR titration and viscosity measurements, which indicate the growth of entangled polymer chains after the efficient host−guest complexation. At high concentrations of acetonitrile, the interpenetrating three-dimensional networks could entrap large amounts of solvent and thereby lead to the formation of supramolecular gels, which exhibit multistimuli (thermo-, pH-, and chemo-) responsive sol−gel transition behaviors. Such a strategy will benefit for the further development of intelligent supramolecular materials with desired functionalities.

1. INTRODUCTION Supramolecular cross-linked networks, a fascinating class of soft materials, have attracted increasing attention in recent years because of their intrinsic scientific interests as well as technological applications.1 As compared with the conventional polymer networks interconnected by covalent bonds,2 supramolecular cross-linked networks exhibit much superior properties, ascribed to the incorporation of noncovalent bonds into cross-linkage sites. Specifically, on account of their threedimensional character, supramolecular cross-linked networks could immobilize the solvent molecules on the macroscopic scale, leading to the formation of physical gels with the desirable shear-thinning property, which have the potential applications in the fields of molecular sensing, actuation, and controlled release.3 Second, the reversibility of the noncovalent interactions would allow for the construction of adaptive and intelligent materials, for which the macroscopic properties could be turned on and off by external stimuli such as temperature, pH, solvent, light, and redox reactions.4 The reversible cross-linkages of the supramolecular polymeric networks are also advantageous for the development of selfhealing and shape-memory materials.5,6 Up to now, various noncovalent bonds, such as hydrogen bonding,7 hydrophobic,8 π−π stacking,9 and metal−ligand coordination interactions,10 have been employed to construct the desired supramolecular cross-linked networks. Host−guest © 2012 American Chemical Society

interactions, on the other hand, represent another kind of noncovalent cross-linkages highly suitable for the fabrication of such supramolecular networks. For example, Harada et al. reported that cyclodextrin-based host−guest systems could direct the macroscopic self-assembly and furnish the healable supramolecular hydrogels.11 It should be noted that the host− guest linkages assign some unique properties to the resulting cross-linked networks. First, complexation of the host−guest moieties affords the pseudorotaxane or rotaxane structures with inherent reversibility, thereby addressing the stimuli-responsiveness properties to the supramolecular networks. Second, placing the host−guest interactions as the cross-linkages would result in the unusual mobility and unique mechanical property for the supramolecular cross-linked networks, further leading to the prominent viscoelastic behavior.12 Crown ethers are considered as the first generation of supramolecular macrocyclic hosts with specific structures and functions. Although a variety of supramolecular architectures such as linear, hyperbranched, star and dendronized polymers have been realized with the utilization of crown ether moieties,13 the incorporation of crown ether based host− guest systems into supramolecular cross-linked networks have Received: August 5, 2012 Revised: September 16, 2012 Published: October 10, 2012 8412

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Scheme 1. Formation of Supramolecular Cross-Linked Networks 3 from the Secondary Ammonium Salt-Functionalized Polymer 1 and the Homoditopic B21C7 Host 2

functionalized styrene monomer 5, B21C7 aldehyde 6, and S-1dodecyl-S′-(α,α′-dimethyl-α′-acetic acid) trithiocarbonate (DDACT) were synthesized according to the literature procedures.18−20 THF was dried according to the typical procedures described in the literature. The other reagents and solvents were employed as purchased. NMR spectra were collected on a Varian Unity INOVA-300 spectrometer with TMS as the internal standard. Electrospray ionization mass spectra (ESI-MS) were obtained on a Bruker Esquire 3000 plus mass spectrometer (Bruker-Franzen Analytik GmbH, Bremen, Germany) equipped with an ESI interface and ion trap analyzer. Viscosity measurements were carried out with Ubbelohde dilution viscometers (Shanghai Liangjing Glass Instrument Factory, 0.55 mm inner diameter) at 25 °C in chloroform/acetonitrile (2/1, v/ v). The relative molecular weight and its distribution were determined on a Waters 150C GPC equipped with Waters 1515 HPLC pump, Waters 2414 differentia refractive index detector and three Ultrastyragel columns (500, 103 and 104 Å in series) at 40 °C, using monodispersed polystyrene as calibration standard. DMF was used as the eluent at a flow rate of 1.0 mL/min. Rheological characterization was performed by using a TA AR2000ex stress-controlled rheometer with 40 mm parallel plate geometry. Oscillatory dynamic shear experiments were performed in the frequency range of 1−100 rad/s at 30 °C,, using a constant strain (0.2%) determined with a strain sweep to lie within the linear viscoelastic regime. The evolution of moduli (G′ and G″) vs time was tested at 25 °C, with a frequency of 1 Hz and a strain of 0.2%. Sample Synthesis. Synthesis of copolymer 1: Styrene 4 (7.00 g, 67.3 mmol), secondary ammonium salt-functionalized styrene monomer 5 (3.00 g, 9.00 mmol), AIBN (5.70 mg, 0.034 mmol), and DDACT (90.0 mg, 0.25 mmol) were dissolved in THF (20 mL). The obtained solution was added into a polymerization tube equipped with a magnetic stirring bar. The mixture was degassed by three freeze−pump−thaw cycles. After the tube was sealed under vacuum, the polymerization reaction proceeded under stirring at 80 °C for 48 h. The tube was opened and the reaction mixture was precipitated into hexane. Secondary ammonium salt-functionalized polymer 1 (3.00 g, 30%) was obtained after drying under vacuum at 25 °C for 24 h. The chemical structure of copolymers was determined by 1H NMR spectroscopy (Figure S4, Supporting Information). The numberaverage molecular weight (Mn = 13.3 kDa) and polydispersity value (PDI = 1.10) could be calculated from the GPC data of copolymer 9. Synthesis of Copolymer 9. A solution of copolymer 1 (0.50 g, 0.036 mmol), (Boc)2O (1.00 g, 4.58 mmol), and a catalytic amount of DMAP in dry THF (10 mL) was stirred for 8 h at room temperature. The solution was then evaporated and the residue was precipitated in hexane for three times to afford copolymer 9 as a white solid (0.40 g, 85%). The chemical structure of the desired copolymer was

been far-less achieved. Few of the previously reported examples were exclusively focused on the dibenzo-24-crown-8 (DB24C8)/dibenzyl-ammonium salt recognition motif; meanwhile, they also suffered from the problems of tedious multistep synthesis toward A2B2- or A2B4-type complementary monomers.14 Hence, researchers are still in keen pursuit to exploit a more convenient and versatile route to the desired supramolecular cross-linked networks with novel crown ether-based building blocks. Benzo-21-crown-7 (B21C7)/secondary ammonium salt recognition motif is an appealing choice, which exhibits the enhanced binding property (association constant: ∼103 M−1 in acetone at 25 °C) and the easier availability, as compared with the DB24C8/dibenzylammonium salt motif.15 In recent years the B21C7/secondary ammonium salt recognition motif has been widely utilized for the fabrication of interlocked structures as well as linear supramolecular polymers.16,17 Therefore, in this work we hope to demonstrate a facile route to realize the desired supramolecular cross-linked networks 3 based on such recognition motif (Scheme 1). Our idea involves the interchain host−guest interactions between the secondary ammonium salt-functionalized graft polymer 1 and the complementary homoditopic benzo-21-crown-7 cross-linking agent 2. It is worthy of note that such strategy could significantly simplify the synthetic procedures, by avoiding the stepwise incorporation of polytopic host (guest) moieties onto the scaffold. The efficient complexation of the multivalent host−guest moieties would contribute to the formation of supramolecular gels with stimuli-responsive behaviors. To the best of our knowledge, this work represents the first report to achieve the supramolecular cross-linked networks from the B21C7/secondary ammonium salt recognition motif. It is highly anticipated that the elaborate design of the building blocks and the elucidation of supramolecular cross-linking process would benefit for the further development of smart materials with tunable responsiveness.

2. EXPERIMENTAL SECTION Materials and Methods. Terephthalic acid, N-(3(dimethylamino)propyl)-N′-ethylcarbodiimide hydrochloride (EDC·HCl), 4-dimethylamino pyridine (DMAP), lithium aluminum hydride (LiAlH4), azobis(isobutyronitrile) (AIBN), and styrene were reagent grade and used as received. Secondary ammonium salt8413

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Scheme 2. Synthetic Routes to the Secondary Ammonium Salt-Functionalized Copolymer 1 and the Homoditopic B21C7 Host 2

Figure 1. Proton NMR spectra (300 MHz, d-chloroform/d3-acetonitrile (2/1, v/v), 25 °C) of (a) 2.00 mM 8, (b) 1:2 equimolar mixture of 1.00 mM 2 and 2.00 mM 8, and (c) 1.00 mM 2. All of these chemical shift changes indicate that the B21C7 rings of 2 are complexed by the secondary ammonium salt axles to form [3]pseudorotaxanes. Peaks associated with the complexed and uncomplexed crown ether and salt are designated by (c) and (uc), respectively. Signals affiliated with solvents are denoted by star symbols. 8414

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Figure 2. Proton NMR spectra (300 MHz, d-chloroform/d3-acetonitrile (2/1, v/v), 25 °C) of 5.00 mM 1 upon addition of (a) 0, (b) 7.39, (c) 15.5, (d) 24.7, (e) 33.2, (f) 42.1, and (g) 58.5 mM of 2. Peaks associated with the complexed and uncomplexed crown ether and salt are designated by (c) and (uc), respectively. Signals affiliated with solvents are denoted by star symbols. determined by 1H NMR spectroscopy (Figure S5, Supporting Information) and GPC measurements (Figure S6, Supporting Information, Mn = 12.6 kDa, PDI = 1.10). Synthesis of 7. Compound 6 (1.00 g, 2.59 mmol) in THF (10 mL) was added to a solution of LiAlH4 (0.30 g, 7.77 mmol) in anhydrous THF (10 mL) at 0 °C. The mixture was stirred for 24 h at room temperature. Excess LiAlH4 was destroyed with H2O. THF and H2O were then removed with a rotary evaporator and the residue was purified by chromatography (dichloromethane/methanol, 10:1 v/v as the eluent) to give 7 as a white solid (0.47 g, 46%). 1H NMR (300 MHz, CDCl3, room temperature) δ (ppm): 6.93 (s, 1H), 6.87 (m, 2H), 4.60 (s, 2H), 4.17 (m, 4H), 3.91 (m, 4H) 3.67−3.80 (m, 16H). 13 C NMR (75 MHz, CDCl3, room temperature) δ (ppm): 149.1, 148.4, 134.3, 120.0, 114.2, 113.3, 71.1, 71.0, 69.8, 69.4, 69.2, 65.2. ESI−MS m/z: [M − OH]+ calcd for C19H29O7, 369.1908; found, 369.1901; error, 1.9 ppm. Synthesis of 2. A solution of terephthalic acid (0.065 g, 0.39 mmol), EDC·HCl (0.24 g, 1.25 mmol), and DMAP (0.038 g,0.31 mmol) in dichloromethane (10 mL) was stirred for 1 h at room temperature, and then compound 7 (0.35 g, 0.91 mmol) was added. The reaction mixture was stirred for 24 h at room temperature, filtered, washed with brine and then dried on MgSO4. The solvent was removed in vacuo and the crude product was purified by flash column chromatography (dichloromethane/methanol, 20:1 v/v as the eluent) to provide 2 as a white solid (0.20 g, 48%). 1H NMR (300 MHz, CDCl3, room temperature) δ (ppm): 8.10 (s, 4H), 7.00 (m, 4H), 6.88 (d, J = 7.6 Hz, 2H), 5.28 (s, 4H), 4.17 (m, 8H), 3.92 (m, 8H), 3.56−3.83 (m, 32H). 13 C NMR (75 MHz, CDCl3, room temperature) δ (ppm): 165.6, 149.2, 149.0, 134.1, 129.6, 128.7, 121.9, 114.8, 114.0, 71.1, 71.0, 70.6, 69.7, 69.4, 69.3, 67.1. ESI−MS m/z: [M + NH4]+ calcd for C46H66NO18, 920.4274; found, 920.4265; error, 1.0 ppm.

3. RESULTS AND DISCUSSION Synthesis. The synthetic routes to the desired compounds 1 and 2 are quite straightforward (Scheme 2). Reversible

Figure 3. Specific viscosity (chloroform/acetonitrile (2/1, v/v), 25 °C) of copolymer 1 (Δ), homoditopic B21C7 host 2 (○), and supramolecular cross-linking polymers 3 (■) versus the B21C7 or secondary ammonium salt unit concentration.

addition−fragmentation chain transfer (RAFT) polymerization is one of the living polymerizations to efficiently prepare the polymers with well-defined structures.21 Therefore, we anchor the secondary ammonium salt moieties onto a polystyrene backbone by copolymerizing monomers 4 and 5 with the utilization of RAFT polymerization, in which the nonfunctionalized styrene monomer 4 serves as the spacer to increase the solubility of the copolymers in organic solvents. 8415

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As shown in Figure 1, the 1H NMR spectrum of 1: 2 mixture of 2 and 8 exhibits two sets of signals for the protons H2−16, corresponding to the complexed and uncomplexed molecules, which stands for the slow-exchange complexation between the B21C7 and secondary ammonium salt moieties on the 1H NMR time scale. After the formation of [3]pesudorotaxane, the complexed aromatic protons H3−5 and the ethyleneoxy protons H6−11 on 2, as well as the methylene protons H12−13 on 8, move downfield, while the complexed aromatic protons H14−16 on 8 shift upfield due to the shielding effect. The obvious proton chemical shifts definitely support that the secondary ammonium salt moiety NH2+ is located in the center of the B21C7 macrocyclic ring. After the tendencies of chemical shifts changes in the model systems were clearly elucidated, we proceeded to cross-link the secondary ammonium salt-functionalized copolymer 1 through host−guest complexation, with the utilization of the ditopic B21C7 host 2 (Figure 2). Specifically, the feed-ratio effect of the cross-linker 2 on the formation of supramolecular crosslinked networks 3 was examined, by gradual titration of 2 into copolymer 1 at the concentration of 5.00 mM (corresponding to 79.5 mM secondary ammonium salt concentration). Upon addition of 2, the 1H NMR spectra display two sets of signals for the methylene protons Ha,b on 1, demonstrating the slowexchange host−guest complexation in the supramolecular cross-linked networks. Increasing the amount of 2 favors the cross-linking process, as manifested by the fact that the original uncomplexed methylene signals progressively decrease, while the newly formed complexed ones strengthen. When 42.1 mM of 2 (Figure 2f, corresponding to 84.2 mM B21C7 unit concentration) is added into the solution, representing that the B21C7 and secondary ammonium salt units achieved almost the same molar ratio, the complete disappearance of the original uncomplexed signals could be observed. Such phenomenon is also observed in the 1H NMR titration measurements between 1 and the monotopic B21C7 host 6 (Figure S1, Supporting Information), suggesting the involvement of efficient host−guest complexation within the polymer matrix. Capillary Viscosity Measurements. Since the supramolecular cross-linked networks will lead to a significant size increase, we continued to investigate the self-assembly behavior in solution by means of capillary viscosity measurements, which is a convenient technique to characterize the growth of polymer networks. Therefore, the variation of specific viscosity as a function of polymer concentration for the mixture of 1 and 2 were performed in chloroform/acetonitrile (2/1, v/v) at 25 °C (Figure 3). As a comparison, the specific viscosity of the individual 1 or 2 was also plotted. In the B21C7/secondary ammonium salt unit concentration range of 10−55 mM, specific viscosity changes almost linearly with the concentration for the individual compound 1 or 2, indicating that no significant physical entanglements occurred. By contrast, for the solution mixture of 1 and 2 (0.93 equiv of the B21C7 versus the secondary ammonium salt), the specific viscosity changes exponentially with the monomer concentration, directly supporting the formation of the cross-linking supramolecular polymer networks 3. Hence, it is evident that the introduction of complementary host−guest interactions as the cross-linkages could considerably affect the viscosity of the polymer solution. Gelation and Rheological Measurements. At high monomer concentration, mixing 1 and 2 in acetonitrile, accompanying with the subsequent heating and cooling

Figure 4. Elastic modulus G′ (●) and the viscous modulus G″ (□) values of the gels (10 mmol/L of copolymer 1 and 79 mmol/L of 2) derived from the supramolecular cross-linked networks 3 as a function of (a) oscillation frequency at 30 °C with a strain of 0.2% and (b) time at 25 °C with a frequency of 1 Hz and a strain of 0.2%.

The mole percentage of secondary ammonium salt unit in copolymer 1, estimated from the 1H NMR integration ratio of the benzylic peak Ha (4.00 ppm) and the aromatic peaks (6.53 and 7.07 ppm), is around 18% (Figure S4, Supporting Information). The calculated value is a little higher than the monomer feed ratio (12%), probably resulting from the higher polymerization reactivity of the functionalized monomer 5 due to the charge-density and ion-pairing effects. Since the ionic copolymer 1 is not suitable for the GPC measurement, we managed to transform 1 to the neutral state copolymer 9, through addition of excess amount of di-tert-butyl dicarbonate. Determined by the GPC measurement, the number-average molecular weight (Mn) of the resulting neutral copolymer 9 is around 12.6 kDa while the polydispersity index (PDI) value is 1.10 (Figure S6, Supporting Information). Considering the highly efficient conversion rate of secondary ammonium salt into the corresponding N-tert-butoxycarbonyl (t-Boc) group, the average number of secondary ammonium salt groups per chain (15.9) could be calculated from the Mn and the mole percentage of the functional unit on the polymer. On the other hand, the homoditopic B21C7 host 2, prepared by the reaction of compound 7 and terephthalic acid in the presence of EDC·HCl and DMAP, is confirmed by the NMR and ESI−MS spectra. Proton NMR Characterization. Proton NMR spectra give important insights into the host−guest complexation behavior in solution. The spectra of the desired supramolecular crosslinked networks 3 would be very complicated because of the multivalent complexation properties of the B21C7/secondary ammonium salt recognition motifs. Therefore, the host−guest binding studies were first performed between the homoditopic B21C7 host 2 and the monotopic secondary ammonium salt 8. 8416

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Figure 5. Reversible sol−gel transitions of the supramolecular gel induced with a variety of external stimuli.

procedures, resulted in the formation of supramolecular gels (Figure 5, left) (critical gelation concentration: 7 mmol/L of copolymer 1 and 55 mmol/L of 2 at 25 °C). Such phenomena could be ascribed to the effect of the multivalent host−guest interactions, which induce the formation of an interpenetrating 3-D network and thereby entrap a large amount of solvent. Rheological experiments on the resulting gels were carried out to investigate the viscoelastic properties. The frequency sweep curve (Figure 4a) further confirms the properties of the true organogel. Specifically, the elastic modulus G′ and the viscous modulus G″ values are not dependent on the oscillation frequency in the frequency range of 1−100 rad/s, meanwhile the G′ value is much higher than G″ value at all tested frequencies, which indicate that the supramolecular gel is elastically strong and dominates the viscous property. The variation of dynamic moduli (G′ and G″) versus time at 25 °C is shown in Figure 4b, in which the modulus values are almost unchanged within a long time, suggesting the stability of the gel networks once they are formed. Stimuli-Responsiveness of the Supramolecular Gel. Then, we would like to explore the sol−gel transition behaviors of the resulting supramolecular gel, by manipulating the binding strength of the B21C7/secondary ammonium salt recognition motif with a variety of external stimuli (Figure 5). As we know, host−guest complexation is sensitive to the temperature. Therefore, heating of the resultant supramolecular gels to 80 °C weakened the host−guest binding strength and led to the formation of a fluid solution. Subsequent cooling to the room

temperature restored the gel state immediately, demonstrating the thermo-responsive sol−gel transition behavior for the supramolecular cross-linked networks 3. Furthermore, the sol−gel transition was triggered by chemical and pH stimuli. Since the crown ether moiety shows remarkable binding affinity toward potassium cation K+,17a adding 1.5 equiv of KPF6 to the supramolecular gel would destroy the B21C7/secondary ammonium salt interactions, accompanying with the immediate gel collapse. The competitive host, benzo-18-crown-6 (B18C6), which exhibits higher affinity toward K+ than the B21C7 moiety, would trap K+ and thereby realize the reversible gel−sol transition. Therefore, in our experiment adding 1.5 equiv of B18C6 could restore the gel state after a few minutes. On the other hand, the secondary ammonium salt unit could also be modified. As soon as 1.5 equiv of triethylamine was added, the elevated pH value gave rise to the deprotonation of the secondary ammonium salt, further resulting in the immediate gel disruption and formation of a white precipitate. After the addition of 1.7 equiv of trifluoroacetic acid, the newly formed secondary amine turned back to the charged state and hence reformed the gel, proving the pH-responsive sol−gel transition property. Moreover, motivated by the efficient transformation of the secondary ammonium salt-functionalized copolymer 1 to the neutral t-Boc protected copolymer 9, the reversible sol−gel transition could be easily realized by the successive addition of di-tert-butyl dicarbonate ((Boc)2O) and acetic acid. Specifically, adding 10 equiv of (Boc)2O resulted in 8417

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the collapse of the supramolecular gel after 5 min. The subsequent addition of acetic acid restored the gel state after a few hours. Therefore, it provides a convenient strategy for the supramolecular gel to exhibit multistimuli (thermo-, pH-, and chemo-) responsive sol−gel transition behaviors.

4. CONCLUSION In summary, a facile route is demonstrated to realize the supramolecular cross-linked networks based on the novel benzo-21-crown-7/secondary ammonium salt recognition motif, which involves the interchain multivalent host−guest interactions between the secondary ammonium salt-functionalized polystyrene and the complementary homoditopic benzo21-crown-7 cross-linker. Such strategy could display the simplicity and versatility for the supramolecular structure formation. At relatively high monomer concentration in acetonitrile, the resulting supramolecular cross-linked networks could immobilize the solvent and lead to the formation of supramolecular gel, which exhibits thermo-, pH-, and chemoresponsive sol−gel transition behaviors. Therefore, the current study paves the way for the establishment of structure− property relationships for such supramolecular cross-linked networks. Moreover, with the embedded multistimuli responsiveness features, it would be an ideal candidate for the development of intelligent materials with desired functionalities.

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ASSOCIATED CONTENT

S Supporting Information *

Synthesis, characterization, 1H NMR titration data, and stimuliresponsiveness studies of the supramolecular gel. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Fax: (+86) 551 3606 095. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS F.W. acknowledges support from the Faculty Startup Fund at the University of Science and Technology of China. This work was supported by the National Natural Science Foundation of China (21274139), and Anhui Provincial Natural Science Foundation. We thank Prof. Haiyang Yang (USTC) for the help with the rheological measurements.

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