Article pubs.acs.org/Macromolecules
Multiresponsive Supramolecular Gel Based on Pillararene-Containing Polymers Junxia Chang, Qiuhua Zhao, Le Kang, Haimei Li, Meiran Xie, and Xiaojuan Liao* School of Chemistry and Molecular Engineering, East China Normal University, Shanghai, 200241, China S Supporting Information *
ABSTRACT: A multiresponsive supramolecular gel was constructed based on a bis(pyridinium) dication guest and a copolymer with pillararenes as the pendant groups, which was synthesized by free radical copolymerization of methacrylatefunctionalized pillararenes and methyl methacrylate. The mechanism of gel formation was explored by the intensive study. Upon addition of competitive host or guest molecules, pillararene-based gel could be transferred into sol due to the competition of host−guest complexation. Surprisingly, the ordered stacking of pillararenes was indispensable to obtain the supramolecular gel, which endowed the system with response to temperature change.
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field, Prof. Harada constructed various multiresponsive supramolecular polymer materials via cyclodextrin−guest interactions.24,25 Therefore, host−guest interactions could enable the system multiresponsivities and consequently provide new methods to prepare functional materials. Pillar[n]arenes,26,27 a new family of macrocyclic host molecules, forms the symmetrical pillar architecture and πrich cavity which affords the possibility of their highly effective binding to a wide variety of guest molecules28−32 and extensive applications in the construction of novel supramolecular architectures.33−37 So far, researches about pillararenes are mainly concerned about the pillararene monomers which are lack of linkages between each other and behave in a monotonous way.38−42 Therefore, it is still a challenge to construct supramolecular systems by polymers bearing pillarenes, which were rarely studied.5,43−46 In our previous work45 and Prof. Woisel’s work,46 polymers containing pillararenes which lie on the end of polymer chain were successfully synthesized through the reversible addition− fragmentation chain transfer polymerization. Besides, Prof. Wang also reported a gel-like supramolecular network based on a ferrocenium-functionalized copolymer and a pillar[6]arene copolymer.5 However, to the best of our knowledge, multiresponsive supramolecular gel based on pillar[5]arene-containing polymers has not been reported. It aroused our interest whether polymers bearing pillarenes could form supramolecular gel, which were generally reported by employing small molecular pillararenes.47−49 In particular, the mechanism of gel formation triggered us to start the in-depth study.
INTRODUCTION Stimulus-responsive properties, one of the most fascinating features, have been widely used in the construction of functional materials.1−5 Recently, many stimulus-responsive systems have been developed to fabricate smart materials, among which responsive polymeric gels that can reversibly switch between free-flowing liquid and free-standing gel states have aroused increasing interest due to their applications in controlled drug delivery and sensors.6−10 For example, Tong et al. developed a gel−sol transition system with an aqueous poly(acrylic acid) (PAA) solution containing an Fe(III)−citrate complex.11 This provides a strategy to fabricate a redoxresponsive gel−sol transition in polymer systems. However, only redox-response was realized in this system, which may limit its application. To construct multiresponsive polymeric gels, many methods were proposed, including using block copolymers to build the network or cross-linking several polymers with different stimuli-responsivenesses,12 which are time-consuming and expensive. Therefore, it is necessary to develop strategies combining other universal techniques, especially the techniques which endowed systems with multiresponsivities. Host−guest interactions,13−17 a kind of fascinating noncovalent driving forces, are susceptive to be destroyed by environmental changes so that they can provide supramolecular systems with stimuli-responsive properties.18−21 For example, Prof. Jiang reported a dual stimuli-responsive hydrogel composed of an azobenzene end functionalized block copolymer and β-cyclodextrin modified CdS quantum dot.22 Besides this, based on the multiple stimuli-responsiveness of complexation between dibenzo[24]crown-8 and secondary ammonium salt, Prof. Huang fabricated a supramolecular network, which can serve as a cation sensor, an anion sensor, a pH sensor, and a temperature sensor.23 As a pioneer in this © XXXX American Chemical Society
Received: February 4, 2016 Revised: March 11, 2016
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DOI: 10.1021/acs.macromol.6b00270 Macromolecules XXXX, XXX, XXX−XXX
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Scheme 1. Representation of the Multi-Responsive Supramolecular Gel Constructed by Copolymer pP[5] and Bifunctional Guest G
Scheme 2. Synthetic route of the Copolymer pP[5] and Bifunctional Guest G
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Herein, we reported the facile method to synthesize a pillar[5]arene derivative with polymerizable group (mP[5]), which was further applied to obtain a copolymer with pendant pillar[5]arene (pP[5]) via free radical polymerization. Concerning that pyridinium salts were widely used as fluorescent material,50 ionic liquids,51 phase transfer catalyst52 and facile to be synthesized, a bis(pyridinium) dication G was selected as the guest molecule. On the basis of the strong complexation between pP[5] and this bifunctional guest molecule, as well as the ordered stacking of pillararenes, supramolecular gel formed after mixing pP[5] and G (Scheme 1). Such a supramolecular gel can be transformed to sol after increasing temperature, or adding competitive host (ethylpillar[5]arene, EtP5A) or guest (butanedinitrile, G1) molecules.
RESULTS AND DISCUSSION
Design and Synthesis of Pillar[5]arene-Containing Copolymer pP[5]. As reported, Prof. Wang synthesized acrylate functionalized pillar[6]arene by etherification of monodeprotected per-butylated pillar[6]arene and a tosylated acrylate derivatives.5 However, three pillararene derivatives were demanded to be synthesized, which was time-consuming, due to the great difficulty in synthesis of pillararene derivatives as compared with other small molecules. Therefore, to obtain pillar[5]arene-containing copolymer pP[5], a functionalized pillar[5]arene monomer (mP[5]) with polymerizable group (methacrylate) was first synthesized via the cyclization of dimethoxybenzene and methacrylate derivatives in 1,2dichloroethane (Scheme 2a, characterization in Figures S1− S7), using BF3·OEt2 as the catalyst. In this method, only one B
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Macromolecules pillararene derivatives was synthesized, which was much convenient. Then free radical polymerization of methyl methacrylate and mP[5] was carried out to obtain the copolymer with pillar[5]arene as the pendant groups, using 2,2-azobis(isobutyronitrile) (AIBN) as the initiator. According to the integration area of proton H1 owing to pillar[5]arene aromatic ring and proton H12 attributed to the methoxy group of methyl methacrylate moieties in the 1H NMR spectrum of pP[5] (Figure S8), a copolymer with 16% pendant pillar[5]arene groups (Figure S9, Mn= 96 kDa, PDI = 1.65) was successfully prepared under the feed ratio of methyl methacrylate to mP[5] with 5:1. The component proportion in the copolymer is in good agreement with the feed ratio due to that both of the monomers possess the same polymerizable group, leading to the similar reactivity. To the best of our knowledge, this is the first copolymer with pillar[5]arene as the pendant groups. On the other hand, a bis(pyridinium) dication guest (G) with two long alkyl chains linked by benzene was designed to increase the solubility in chloroform (Scheme 2b, characterization in Figures S10 and S11). Host−Guest Complexation Studies. First, an attempt to investigate the interaction between the bifunctional guest G and pillararene monomer mP[5] was carried out by 1H NMR characterization in CDCl3 (Figure S12, Supporting Information). After 1 equiv of mP[5] was added, the signals of protons from guest molecules changed as follows: the signal owing to proton Hc disappeared, and the signal of proton Hb and Hd broadened and shifted upfield after complexation due to the shielding effect of the electron-rich cavities of pillararenes, whereas the signal of proton Ha broadened and shifted downfield. However, no obvious change was observed for the protons Hk and Hi of G. All these phenomenons provided critical evidence that pillararene was threaded by the pyridinium moieties of G with the protons Hb‑d in the cavities of pillararenes and other protons Ha and Hf‑i out of the cavities, in agreement with other pillararene/guest systems.34,38,44 On the basis of the 1H NMR titration experiment in CDCl3 (Figure S13) and nonlinear curve-fitting method (Figure S14),53 the binding constant between mP[5] and G was calculated as (905 ± 89) M−1. The binding behavior of mP[5] with G provides an opportunity for copolymer pP[5] to form complexes with G, which was further investigated (Figure 1, curve b). To our delight, despite the steric hindrance of the copolymer backbone, the pendant pillar[5]arene group remained its good host−guest recognition property to pyridinium moiety, which showed a similar behavior to the free monomer mP[5]. It is noteworthy that rare studies concerning the host−guest behavior of pillararenes with dication guests were performed in CHCl3,54 which is convenient for the essential 1H NMR characterization of inclusion complexation. Formation of the Supramolecular Gel. Since the pendant pillar[5]arene groups on pP[5] could bind with the pyridinium moiety of G as demonstrated above, a supramolecular network based on pillar[5]arene-pyridinium recognition motifs could be constructed by the random copolymer pP[5] and bifunctionalized guest G. Simply mixing pP[5] with G in chloroform, a slight yellow solution with high viscosity was obtained despite both copolymer and guest solution were colorless and nonviscous, which showed that a strong complexation occurred. After ultrasounding for 0.5 h and standing overnight in the refrigerator at 4 °C, a sample of gel was prepared. As well-known, a gel forms when elastic (or
Figure 1. 1H NMR spectra (500 MHz, CDCl3, 293 K): 2 mM G (a), 7 mM pP[5] + 2 mM G (b), 7 mM pP[5] + 2 mM G + 4 mM G1 (c), 7 mM pP[5] (d), and 4 mM G1 (e) (here concentration of copolymer refers to concentration of pendant pillararenes).
storage) modulus G′ surpasses viscous (or loss) modulus G″, which can be characterized effectively by rheology measurement.55 Therefore, an oscillatory experiment of the mixture of G with pP[5] was carried out. As shown in Figure 2, the storage
Figure 2. Rheology results of gel (G′ in black square, G″ in red circle, η in blue triangle).
modulus G′ was higher than the loss modulus G″ in the whole experimental range of 0.1 to 10 Hz, demonstrating the formation of a gel, which is quite different from the gel-like supramolecular network as reported by Prof. Wang, where G″ surpassed G′ at low frequency.5 Meanwhile, the viscosity sharply decreased with the increasement of frequency, showing their well-known shear-thinning properties, which are the base for their potential applications in nanomedicine.56,57 Furthermore, the SEM image of the freeze-dried gel sample also exhibited a porous network structure (Figure 3), giving the microstructure of the supramolecular gel. Stimuli-Responsiveness of the Supramolecular Gel. Since the gel formed through host−guest interaction between the pendant pillararenes and pyridinium groups, which was featured by stimulus-response, the gel could be dissociated (Figure 4). First, dissociation of such gel by competitive guest molecules was investigated. In this work, butanedinitrile G1 was selected as the competitive guest due to that G1 could strongly bind with pillar[5]arene, as reported by Prof. Li.58 The C
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Figure 3. SEM image of the freeze-dried gel sample. Figure 5. Viscosity of pP[5]/G supramolecular gel at 0 °C (curve a), pP[5] /G/G1 sol (curve b), pP[5]/G/EtP5A sol (curve c), and pP[5]/ G supramolecular gel at 50 °C (curve d).
Figure 4. Reversible gel−sol transition of supramolecular gel.
competition of complexation among mP[5], G and G1 was first studied by 1H NMR (Figure S15). When equiv of G1 was added in the dilute mixture of mP[5] and G, the signals of protons Hc of G reappeared, and the signals owing to complexed protons (Ha, Hb and Hd) recovered to free uncomplexed state, demonstrating that butanedinitrile could form a stronger complex with pillar[5]arene and consequently push pyridinium moiety out of pillarenes’ cavities. It was dissappointed that an attempt to study the complexation constant by 1H NMR titration experiment in CDCl3 failed because despite 0.1 equiv of G1 was added, the proton Ha upshifted even to −1.30 ppm where Ha upshifted when 1 equiv. G1 was added, and we can not determine the complexation constant by the change of the chemical shift of Ha (Figure S16), which is in agreement with the reported phenomenon.37,59 On the basis of the strong binding ability between mP[5] and G1, the complexation behavior among pP[5], G1 and G by 1H NMR was also investigated (Figure 1, curve c). As expected, protons Ha‑d displayed similar behavior, which demonstrated that butanedinitrile G1 could push pyridinium moiety out of the pendant pillararenes’ cavities, revealing that the polymer backbone did not retard the competition of complexation. Then excess amount of butanedinitrile G1 was added in the supramolecular gel, which turned to be sol and the viscosity experienced significant decrease compared with the original gel (Figure 5, curves a and b). Next, dissociation of the supramolecular gel by competitive host molecules was investigated. As well-known, free small molecules always form stronger complexes than polymers which is retarded by the slow mobility of macromolecular chains.60 Therefore, when free EtP5A was added into the gel, it became a solution and the viscosity dramatically decreased (Figure 5, curve c). To monitor this dissociation process, dynamic laser light scattering (DLS) was performed in chloroform (Figure 6). The concentration of polymer was 0.008 g/mL, much lower than that used to form the gel, so the complexation existed but no gelation took place. As shown in Figure 6, curve a, the solution of polymer alone shows a relatively wide hydrodynamic radius distribution with a peak value of 12 nm, which is attributed to the copolymer with rigid
Figure 6. DLS results for pP[5] (curve a in black line), pP[5]/G (curve b in red), and pP[5]/G/EtP5A (curve c in blue) in chloroform.
pillar[5]arenes as the pendant groups. Mixing this polymer solution with G causes a remarkable change in the distribution curve (Figure 6, curve b), that is, three peaks appear. The middle one is the same as that of Figure 6, curve a, which is obviously associated with the polymer. The right-hand peak, with a size ranging from about 70 to 500 nm, is attributed to the formation of pyridinium-pP[5] supramolecular network. In addition, a small peak with a size of 2 nm is observed, which is no doubt from the free guest G. Finally, Figure 6, curve c clearly shows the great change caused by adding EtP5A to the solution of polymer and G: the peak associated with the supramolecular network completely disappears. This reflects the dissociation of network was caused by pulling the complexed pyridinium molecules out of the cavities of pillararenes from polymer chains, driven by the strong complexation of free EtP5A and pyridinium moiety. The presence of the small molecular complex is indicated by the small peak located at about 2 nm. Therefore, in the ternary system, only free polymer chains and the complex of EtP5A and G exist. Both viscosity experiment and DLS results indicated that addition of EtP5A into the supramolecular gel could achieve the transition from gel to sol. As shown in Figure 4, after heating to 50 °C, the gel was dissociated into a transparent solution immediately. Reformation of the gel was achieved by cooling to 4 °C and standing overnight. Viscometry experiments were performed to prove the temperature stimulus response (Figure 5, curve d). The viscosity of solution heating at 50 °C experienced a sharp D
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ization. Moreover, a supramolecular polymeric gel based on the pyridinium-pillar[5]arene motif was fabricated for the first time by mixing a bifunctionalized guest G with the copolymer pP[5], expanding the field of pillararene studies. On the basis of the competition of host−guest interactions, such a gel could be transformed to sol by addition of competitive host (EtP5A) or guest molecules (butanedinitrile). Besides host−guest interactions, ordered stacking of pillararenes played an important role in construction of this supramolecular gel, which could realize the gel−sol transition by increasing temperature and the reverse process by decreasing temperature. This study provide a novel strategy to prepare multiresponsive materials and new insight into the mechanism of pillararene-based gel formation. Further studies on copolymers with pillararenes as the pendant groups and application of this multiresponsive gel as smart actuators and adaptive coatings are currently underway in our laboratory.
decrease compared with the sample of gel. Although some pillararene based gels were responsive to temperature, the mechanism was rarely explored.61 In the present work, we tried to investigate the mechanism by variable-temperature 1H NMR (Figure S17). However, the spectra of mixture of G with pP[5] were almost the same in the range of 25−50 °C, except for the peak at −1 ppm shifted downfield when heating at 50 °C, revealing that the complexation was slightly broken. Inspired by the formation of pseudopolyrotaxane hydrogel, we speculated that some microcrystal domains may exist in this pillararene gel, which could be characterized by wide-angle X-ray scattering (WAXS).62 As shown in Figure 7, compared with the polymer
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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.6b00270. Experimental procedures and characterization data for new compounds and analytical data including 1H NMR titration spectra and GPC results (PDF)
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AUTHOR INFORMATION
Corresponding Author
*Telephone/Fax: +86-21-54340105. E-mail:
[email protected]. edu.cn (X.L.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS National Natural Science Foundation of China (No. 21204022), Research Fund for the Doctoral Program of Higher Education of China (No. 20120076120005), the Fundamental Research Funds for the Central Universities, and the Large Instruments Open Fundation of East China Normal University (No. 201409251425) are acknowledged for their financial supports.
Figure 7. Wide angle X-ray scattering for freeze-dried gel (curve a in black), solution heating at 50 °C (curve b in blue), and copolymer pP[5] (curve c in red).
(Figure 7, curve c) and sol heating at 50 °C (Figure 7, curve b), freeze-dried gel (Figure 7, curve a) showed a characteristic peak at 2θ = 25.2°, which was attributed to the ordered structure formed by complexes of pendant pillararene with G, as well as ordered stacking of pillararenes caused by π−π interaction. The corresponding spacing distance d was 0.353 nm according to the equation λ = 2dsin θ. Compared with the ordered structure (2θ = 19.8°) formed by α-cyclodextrin and poly(ethylene glycol) (PEG), the spacing distance of the pillararene-ordered structure is smaller, suggesting that the supramolecular network is supposed to be more transparent than those constructed by cyclodextrin and PEG, which is always opaque.63 When heating to 50 °C, the intensity of the peak decreased sharply, revealing that the orderd structure had been destroyed, which presented a temperature response. Therefore, we put forward that besides of host−guest interaction, ordered stacking of pillararenes is essential to construct this supramolecular gel.
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CONCLUSIONS In conclusion, we have successfully developed a facile method to synthesize pillar[5]arene monomer with polymerizable group, which was further applied to obtain a copolymer with pillar[5]arenes as the pendant groups by free radical polymerE
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DOI: 10.1021/acs.macromol.6b00270 Macromolecules XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.macromol.6b00270 Macromolecules XXXX, XXX, XXX−XXX
