Preparation of a Diblock Supramolecular Copolymer via Self-Sorting

Article pubs.acs.org/Macromolecules

Preparation of a Diblock Supramolecular Copolymer via Self-Sorting Organization Shengyi Dong,† Bo Zheng,† Mingming Zhang,† Xuzhou Yan,† Xia Ding,‡ Yihua Yu,‡ and Feihe Huang*,† †

MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Chemistry, Zhejiang University, Hangzhou 310027, P. R. China ‡ Shanghai Key Laboratory of Magnetic Resonance, Department of Physics, East China Normal University, Shanghai 200062, P. R. China S Supporting Information *

ABSTRACT: By incorporating a bis(crown ether) into an otherwise immiscible supramolecular polymer blend prepared from self-sorting organization of two heteroditopic AB-type monomers, a compatible AB diblock supramolecular copolymer was formed. The formation of the supramolecular copolymer was characterized by various techniques including 1 H NMR, DOSY, specific viscosity, and SEM. Because of the existence of the glue-like bis(crown ether), newly formed supramolecular copolymer shows quite different properties from those of the immiscible supramolecular polymer blend in solution, in gel, and in the solid state. The evolution between the blend and the copolymer can be realized reversibly by adding or removing the bis(crown ether). This study provides an efficient and convenient strategy to control phase separation and morphology in supramolecular polymers and to prepare complex and highly ordered supramolecular structures.

1. INTRODUCTION Because of the incompatibility between polymer pairs, most polymer systems have a propensity for phase separation, which is directly related to their physical and mechanical properties. Therefore, to understand and control phase separation and morphology is of great important not only for theoretical studies but also for practical applications.1 Held together by reversible and highly directional noncovalent interactions, such as host−guest interactions, hydrogen bonds, and metal coordination, supramolecular polymers constructed from lowmolecular-weight monomers have become one of the most active frontiers in the past decades and received a great deal of attention.2 Compared with conventional polymers, supramolecular polymers typically show many dynamic or precisely controllable properties arising from dynamic linking between the constituent monomers and have the ability to respond to their environments as adaptive materials.3 In recent years, supramolecular chemistry methods have been applied to stabilize the interfaces between immiscible phases and to control the phase separation in covalently linked traditional polymer blends and block copolymers.4 However, their attention mainly focused on polymeric systems containing high-molecular-weight conventional polymeric backbones, and the study on the control of phase separation and morphology of supramolecular polymers constructed totally from low-molecular-weight monomers via supramolecular interactions has not been explored yet. Herein we want to report the preparation of a compatible AB diblock supramolecular copolymer from an immiscible supramolecular polymer blend and the reversible © 2012 American Chemical Society

evolution between the immiscible supramolecular polymer blend and the compatible AB diblock supramolecular copolymer with the purpose to control the phase separation and morphology in these supramolecular polymer systems. Here we use the term “diblock supramolecular copolymer” instead of “supramolecular diblock copolymer” to show that the blocks in our diblock copolymer are actually noncovalently connected supramolecular polymers instead of covalently connected traditional polymers.

2. EXPERIMENTAL SECTION Materials and Methods. All reagents were commercially available and used as supplied without any further purification. Compounds 1, 2, 4, and 5 were prepared according to the published procedures.5 1 H NMR spectra were collected on a temperature-controlled Bruker 400 MHz spectrometer or Bruker AVIII 500 MHz spectrometer. 13C NMR spectra were recorded on a Bruker Avance DMX-500 spectrometer at 125 MHz. Low-resolution electrospray ionization (LRESI) mass spectra were obtained on a Bruker Esquire 3000 plus mass spectrometer (Bruker-Franzen Analytik GmbH Bremen, Germany) equipped with an ESI interface and an ion trap analyzer. High-resolution electrospray ionization (HRESI) mass spectra were obtained on a Bruker 7 T FT-ICR mass spectrometer equipped with an electrospray source (Billerica, MA). Scanning electron microscopy investigations were carried out on a JEOL 6390LV instrument operating at an energy of 15 or 20 keV. Viscosity measurements were Received: August 4, 2012 Revised: October 23, 2012 Published: November 7, 2012 9070

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Scheme 1. Synthesis of Bis(crown ether) 3

Figure 1. Chemical structures of monomers 1 and 2 and bis(crown ether) 3 and cartoon representation of the formation of a diblock supramolecular copolymer. carried out with a Cannon-Ubbelohde semi-microdilution viscometer (0.45 mm inner diameter) at 25 °C in CH3CN. Synthesis of Compound 3 (Scheme 1). A mixture of 4 (0.49 g, 1.0 mmol), 4-(dimethylamino)pyridine (DMAP, catalytic amount), 1(3′-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride (EDC, 0.60 g, 3.0 mmol), and 5 (0.57 g, 1.0 mmol) in CH2Cl2 (45 mL) was stirred for 48 h at room temperature. The reaction mixture was filtered, and the solvent was removed under vacuum to give a yellow solid. This crude product was purified by flash column chromatog-

raphy (petroleum ether/ethyl acetate, 1:1, v/v) to yield bis(crown ether) 3 as a white solid (0.10 g, 9.6%); mp 102.3−104.9 °C. 1H NMR (400 MHz, CDCl3, room temperature) δ (ppm): 7.63 (dd, J = 3.2 Hz, J = 2.4 Hz, 1H), 7.51 (d, J = 1.6 Hz, 1H), 6.93 (d, J = 1.6 Hz, 1H), 6.86 (dd, J = 3.2 Hz, J = 2.4 Hz, 4H), 6.80 (d, J = 8.4 Hz, 1H), 6.74 (d, J = 8.4 Hz, 1H), 6.71 (m, 5H), 5.32 (s, 2H), 4.00−4.17 (m, 8H), 3.88−4.02 (m, 16H), 3.77−3.82 (m, 16H), 3.62−3.69 (m, 16H). 13C NMR (125 MHz, CDCl3, room temperature) δ (ppm): 61.94, 68.08, 68.12, 68.98, 69.33, 69.41, 69.50, 69.66, 69.81, 69.98, 70.73, 70.77, 9071

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Figure 2. Reversible evolution between supramolecular polymer blends and diblock supramolecular copolymers (both in acetonitrile). Supramolecular polymer blends with different molar ratios of 1 and 2: (a) 1:9; (b) 3:7; (c) 5:5. The corresponding diblock supramolecular copolymers were obtained by (d) adding 3 to sample a; (e) adding 3 to sample b; (f) adding 3 to sample c. The total concentration of monomers 1 and 2 was kept as a constant at 60 mM. The molar ratios of 1/2/3 in samples d/e/f were 1:9:0.03, 3:7:0.09, and 5:5:0.1, respectively. These samples were prepared by cooling down to 278 K and then warming to 298 K. 70.87, 71.31, 71.37, 71.49, 112.02, 113.33, 114.01, 114.38, 114.47, 115.51, 116.03, 121.49, 122.99, 124.17, 126.18, 148.25, 149.91, 150.95, 153.01, 153.08, 168.20. LRESIMS: m/z 1063.8 [M + Na]+ (100%). HRESIMS: m/z calcd for [M + Na]+ C54H72NaO20+, 1063.4509; found: 1063.4521; error 1.1 ppm.

supramolecular copolymers can form from self-assembly of monomers 1 and 2 and bis(crown ether) 3. When monomers 1 and 2 supramolecular polymerize starting from bis(crown ether) 3, the two ends of each of the resultant supramolecular copolymer chains are crown ether units so the second bis(crown ether) 3 molecule cannot be incorporated into this copolymer chain. Therefore, only diblock supramolecular copolymers instead of multiblock supramolecular copolymers can be obtained. Manipulation of Gel Process. Monomer 1 can form a colorless supramolecular polymer gel while monomer 2 forms a red linear supramolecular polymer.5e,8 From our previous work, we have found by progressively increasing the molar ratio of 1 in the blend while their total molar concentration was kept constant at 60 mM, the samples transformed from a dark red homogeneous solution to a mixture of solution and gel.5e By adding bis(crown ether) 3 to the supramolecular polymer blend (in acetonitrile), the gelation process can be manipulated to control a range of gel properties, such as the gelation time, gel transparency, and gel morphology (Figure 2). For example, the critical gel concentration decreased from 4.6 wt % for a supramolecular polymer blend with the molar ratio of 1 and 2 at 1:1 to 1.9 wt % for a block supramolecular copolymer with the molar ratio of 1, 2, and 3 at 1:1:0.02, and the gelating time was shortened. Meanwhile, remarkable changes in gel morphology were also observed. For example, after adding bis(crown ether) 3, sample a became a mixture of solution and gel from the former solution phase, while sample f showed a total gel phase without any solution phase compared with sample c (a mixture of gel and solution). By comparison, we confirmed that the addition of bis(crown ether) could efficiently reduce macroscopic phase separation and obviously affect the morphology of newly formed supramolecular aggregates. On the basis of these observations, we propose that the end of these two different and mutually exclusive supramolecular polymers in the immiscible blend can be connected together to form a compatible diblock supramolecular copolymer.5e,7−10 This successful suppression of the phase separation can be attributed to the unique glue-like property of bis(crown ether) 3. In addition, the reversible gel/ sol−gel transition can be realized by adding the bis(crown ether) or removing the bis(crown ether) (extracted with

3. RESULTS AND DISCUSSION Design. In our previous work,5e we successfully prepared a supramolecular polymer blend containing two different supramolecular polymers via self-sorting organization of two heteroditopic low-molecular-weight monomers, 1 and 2 in Figure 1. Monomer 1 contains a dibenzo-24-crown-8 (DB24C8) unit and a dibenzylammonium salt (DBA) moiety, while monomer 2 is a bis(p-phenylene)-34-crown-10 (BPP34C10)-paraquat-based analogue. It is well-known that DB24C8 and BPP34C10 form 1:1 threaded structures with DBA and paraquat, respectively, in solution and in solid state.6 Furthermore, it has also been demonstrated that the BPP34C10 unit exclusively binds the paraquat moiety while the DB24C8 unit complexes the DBA moiety selectively in a solution of DBA, DB24C8, BPP34C10, and paraquat (self-sorting organization).7 Hence, as a result of this typical self-sorting organization, we found that monomers 1 and 2 could control their self-assembly processes in an independent way and did not interfere with each other, which led to the macroscopic phase separation in the solid state and gel state. It is of considerable interest to investigate whether it is possible to control this kind of phase behavior and morphology via the same self-sorting process in block supramolecular copolymers. Therefore, in order to understand and control phase separation and morphology, our approach is that (a) by adding bis(crown ether) molecule 3 containing both DB24C8 and BPP34C10 units as a glue-like molecule, the supramolecular polymer constructed from monomer 1 can be connected with the linear supramolecular polymer built from monomer 2 to form a diblock supramolecular copolymer (Figure 1) with an objective to control the morphology and reduce the propensity for phase separation, and (b) based on the solubility difference between monomers 1 and 2 and bis(crown ether) 3, the reversible conversion between the immiscible supramolecular polymer blend and the compatible diblock supramolecular copolymer can be easily realized by adding or removing bis(crown ether) 3 with an extraction method. It should be noted that only diblock 9072

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Figure 3. Films cast from a supramolecular polymer blend of equimolar 1 and 2 and the corresponding diblock copolymer with the molar ratio of 1/ 2/3 at 1:1:0.02.

isopropyl ether/hexane with a volume ratio of 1:4 or washed with ethyl acetate/isopropyl ether with a volume ratio of 1:1). Macroscopic Evolution. We also observed the reversible macroscopic evolution between the immiscible supramolecular polymer blend and the compatible AB diblock supramolecular copolymer during the preparation of their films (Figure 3). Thin films of polymer blends and diblock copolymers with different molar ratios of 1, 2, and 3 were cast by slowly evaporating the solvent (acetonitrile) on glass surfaces. As shown in Figure 3, blend films favored independent selfassemblies of monomers 1 and 2 to form their own macroscopic aggregates. After the incorporation of 3, heterogeneous blend films became transparent and homogeneous, indicating the formation of miscible supramolecular polymeric aggregates.4c,f,5e Similar results were also reported by Zimmerman et al. in their hydrogen-bond-based supramolecular polymers.4c Proton NMR Experiments. As the basis for the reversible evolution between the immiscible supramolecular polymer blend and the compatible AB diblock supramolecular copolymer, the self-sorting behavior in solution was investigated later. In our previous work,5e it was demonstrated that monomers 1 and 2 in the supramolecular polymer blend selfsorted to form two different linear supramolecular polymers totally depending on their own host−guest interactions and did not interfere with each other, which could be the basis for both the appearance and suppression of phase separation. By progressively increasing the molar concentration of bis(crown ether) 3 from 0 to 2.0 mM in an equimolar blend of 100 mM 1 and 2, neither new peaks nor obvious chemical shift changes were observed compared with the NMR spectrum of the supramolecular polymer blend (Figure 4), indicating that the self-sorting organization of monomers 1 and 2 was not interfered with by the incorporation of bis(crown ether) 3. On the other hand, after the gradual addition of bis(crown ether) 3, the well-defined signals were no longer observed, along with the broadening of all signals. This observation was consistent with the existence of high-molecular-weight supramolecular aggregates.5e,7−9 This is understandable since previously it was demonstrated that the limited addition of a template containing four corresponding guest units could increase the molecular weight of the supramolecular polymer prepared from the self-organization of a host−guest heteroditopic monomer.10 By removing bis(crown ether) 3 (extracted with isopropyl ether/hexane with a volume ratio of 1:4 or washed with ethyl acetate/isopropyl ether with a volume ratio of 1:1), the broad signals became well dispersed again (Figure S4). Therefore, from these proton NMR results and the wellreported host−guest complexes of DB24C8/DBA and BPP34C10/paraquat, it was reasonable to think that by incorporating bis(crown ether) 3 into the supramolecular

Figure 4. Partial 1H NMR spectra (400 MHz, acetonitrile-d3, 293 K) of diblock supramolecular copolymers obtained by adding bis(crown ether) 3 to a supramolecular polymer blend of 100 mM 1 and 2. The molar concentration of 3: (a) 0, (b) 0.50, (c) 1.0, (d) 1.5, and (e) 2.0 mM.

polymer blend, each end of the two different linear supramolecular polymers (DBA or paraquat group) could be bound by the DB24C8 or BPP34C10 moiety of bis(crown ether) 3, respectively, to form a new linear diblock supramolecular copolymer containing both monomers 1 and 2. We propose that the self-sorting organization in solution may act as a key factor in the suppression of phase separation and controlling the phase behavior and morphology. Specific Viscosity. To further compare the supramolecular polymer blend with the diblock supramolecular copolymer, a double-logarithmic plot of specific viscosity versus the concentration of an equimolar mixture of monomers 1 and 2 in CH3CN was obtained (Figure 5). For the blend, the curve had a slope of 1.08 in the low concentration range, which meant that monomers 1 and 2 mainly formed low-molecularweight cyclic oligomers. As the concentration of the blend increased above the critical supramolecular polymerization concentration, an exponential relationship (slope = 2.36) was observed. After the addition of bis(crown ether) 3 (the molar ratio of 1/2/3 was kept at 1:1:0.02), the curve of the resultant diblock supramolecular copolymer in the high concentration range approached a slope of 3.12, higher than the slopes 2.36 and 2.328 of the polymer blend and monomer 1, respectively. From these viscosity experiments, a slight increase in the slope of the curve was observed after incorporating bis(crown ether) 3 into the mixture of monomers 1 and 2, indicating that the 9073

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corresponding diblock supramolecular copolymer could efficiently eliminate the phase separation and showed an extended and interconnected three-dimensional fibrous network (Figure 6 and Figure S5). These images provided microscopic evidence for the homogeneous mixing. It is also interesting to note that, compared with the fibrillar structure of monomer 1, this AB diblock copolymer showed a more compact three-dimensional fibrillar network structure. These microscopy results indicated that the incompatible supramolecular polymer blend could be affected by the glue-like connector 3 to form a miscible diblock supramolecular copolymer.5e

4. CONCLUSIONS In conclusion, by incorporating a bis(crown ether) into a polymer blend which was prepared via the self-sorting organization of two low-molecular-weight heteroditopic ABtype monomers, a compatible AB diblock supramolecular copolymer was formed. This bis(crown ether) worked as a glue-like molecule that could connect two different and mutually exclusive linear supramolecular polymers together and efficiently reduce the propensity for macroscopic phase separation both in the gelation process and in the solid state. The self-sorting recognition between two different crown ether host units (DB24C8 and BPP34C10) and two different guest moieties (DBA and paraquat) acted as a key factor in the suppression of phase separation and in the control of the phase behavior and morphology. More meaningfully, the transition between the immiscible supramolecular blend and the compatible block copolymer can be realized reversibly by easily adding or removing the bis(crown ether). The work presented here provides an efficient strategy to control phase separation in supramolecular polymers and to prepare complex and highly ordered supramolecular structures with fascinating properties and novel functions.

Figure 5. Specific viscosity of a supramolecular polymer blend with equimolar 1 and 2 versus monomer 1 concentration in acetonitrile at 25 °C and specific viscosity of an acetonitrile solution of the diblock supramolecular copolymer with the addition of bis(crown ether) 3 versus monomer 1 concentration at 25 °C (the molar ratio of 1/2/3 was kept at 1:1:0.02).

diblock supramolecular copolymer has a stronger concentration dependence. Two-Dimensional Diffusion-Ordered NMR (DOSY). We also used two-dimensional diffusion-ordered NMR (DOSY) to investigate the self-assembly process and the difference in diffusion coefficients of the supramolecular blend and diblock supramolecular copolymer. DOSY data were consistent with the above-mentioned 1H NMR results and specific viscosity experiments. When bis(crown ether) 3 was added to a supramolecular polymer blend with 1 and 2 at 100 mM in CD3CN (the molar concentration of 3 was 2.0 mM), a decrease in the measured weighted average diffusion coefficient from 2.51 × 10−10 to 1.20 × 10−10 m2 s−1 was observed, indicating an increase in the average aggregation size due to the transition from the supramolecular polymer blend to the AB diblock supramolecular copolymer with a higher molecular weight.5e,7−10 SEM of Diblock Supramolecular Copolymer. Xerogels, prepared by freeze-drying a supramolecular polymer blend and the corresponding diblock copolymer in CH3CN, were then investigated by SEM, revealing the evolution from the phase separation to miscible phase (Figure 6). As we know, monomer

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Characterizations and other materials. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel +86-571-8795-3189; e-mail [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Jianyang Pan (Pharmaceutical Informatics Institute, Zhejiang University) for performing NMR spectrometry for structure elucidation. The National Natural Science Foundation of China (20834004, 91027006, and 21125417), the Fundamental Research Funds for the Central Universities (2012QNA3013), Zhejiang Provincial Natural Science Foundation of China (R4100009), and Open Project of State Key Laboratory of Supramolecular Structure and Materials are greatly acknowledged for their generous financial support.

Figure 6. SEM images of aggregates: (a) a freeze-dried sample of supramolecular polymer blend with equimolar 1 and 2; (b) a freezedried sample of diblock supramolecular copolymer with the molar ratio of 1/2/3 at 1:1:0.02.

1 can form long fibers and three-dimensional network structures, while monomer 2 favors the formation of a layer of porous structure.5e,8 In the polymer blend, these two independent aggregates showed a typical phase separation with the fiber network assembling at the bottom and the layer of porous structure covering the entire surface.5e Meaningfully, with the unique glue-like property of bis(crown ether) 3, the

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