Construction of supramolecular polymer gels cross-linked by two types of discrete well-defined metallacycles through self-sorting

SummaryThe precise sequence control of polymer chain is an important research topic of polymer chemistry. Although some methods such as iterative synthesis and supramolecular polymerization have been developed to fabricate sequence-controllable polymer, it is still a great challenge to consecutively prepare multiple supramolecular polymers with different sequence structures. In this work, through the reasonable utilization of assembly motifs, we integrated multiple host-guest recognitions and metal coordination interactions to prepare different sequence-controlled supramolecular polymers by a multistep assembly strategy. This research provides inspiration for the design and preparation of supramolecular polymers with different sequence structures.

Triptycene-derived macrotricyclic polyether was proved to be an efficient "molecular glue" for cross-linking the copolymer containing dibenzylammonium salts to form supramolecular polymer networks via host–guest complexation, which was evidenced by 1H NMR spectroscopy, solution viscometry, and formation of transparent and elastic supramolecular polymer gels. Interestingly, a well-defined porous structure of the supramolecular polymer gel could be observed at the fully cross-linked density, and its mechanical properties could be modulated by the amount of the macrotricyclic host added to the system. Moreover, the supramolecular polymer gel showed acid/base- and thermo-induced reversible gel–sol transitions. Additionally, the supramolecular polymer gel could also be employed for the encapsulation and controllable release of squaraine dyes, which might find potential applications in materials science.

Supramolecular polymer formed by non-covalent interactions between complementary building blocks entraps solvents and develops supramolecular polymer gel. A supramolecular polymer gel was prepared by the heating-cooling cycle of β-cyclodextrin (β-CD) and naphthalenedimide (NDI) solution in N,N-dimethylformamide (DMF). The host-guest inclusion complex of β-CD and NDI 1 containing dodecyl amine forms the supramolecular polymer and gel in DMF. However, β-CD and NDI 2, having glutamic acid, fail to form the supramolecular polymer and gel under the same condition. X-ray crystallography shows that the alkyl chains of NDI 1 are complementary to the hydrophobic cavity of the two β-CD units. From rheology, the storage modulus was approximately 1.5 orders of magnitude larger than the loss modulus, which indicates the physical crosslink and elastic nature of the thermo-responsive gel. FE-SEM images of the supramolecular polymer gel exhibit flake-like morphology and a dense flake network. The flakes developed from the assembly of smaller rods. Photophysical studies show that the host-guest complex formation and gelation have significantly enhanced emission intensity with a new hump at 550 nm. Upon excitation by a 366 nm UV-light, NDI 1 and β-CD gel in DMF shows white light emission. The gel has the potential for the fabrication of organic electronic devices.

Supramolecular hydrogelation via host-guest anion recognition: Lamellar hydrogel materials for the release of cationic cargo

Supramolecular polymer gel with multi stimuli responsive, self-healing and erasable properties generated by host–guest interactions

The creation of complex and highly-ordered structures with desired properties and novel functions has been fundamental to supramolecular chemistry and material science, and plays an essential role in high-tech and biorelated fields, such as drug delivery systems, molecular devices and photovoltaic applications. Nature, as the best example of precise and efficient self-assembly processes, promotes chemists to engage in developing highly-ordered artificial supramolecular assemblies, such as supramolecular polymers through the utilization of noncovalent interactions. Held together by reversible and highly dimensional interactions, such as host–guest interactions, hydrogen bonds, and metal coordination, supramolecular polymers have become one of the most active frontiers in the past two decades and have received a great deal of attention. 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 environment as adaptive materials. The interest in supramolecular polymers has expanded in recent years, not only for their potential properties as functional materials, but also for their intriguing architectures and topologies that act as a basic aspect of potential applications. The simple blending of different polymers has produced new polymer blend materials with controllable and unique properties. Polymer blends have been widely studied in polymer chemistry and materials science, and as such supramolecular polymer blends, defined as mixtures of two or more different and mutually exclusive supramolecular polymers, are an attractive topic. There are beautiful examples of miscible polymer blends based on either multiple hydrogen-bonding, p-stacking, carboxy–amine interactions, or crown ether functionlized polymers with ammoniumor paraquat-functionalized polymers. However, these blends were composed of high-molecular-weight, conventional covalently bonded polymers instead of noncovalently connected supramolecular polymers. It is still a big challenge for chemists to design and prepare supramolecular polymer blends completely from low-molecularweight monomers without any conventional polymeric backbones. Self-sorting systems, whereby different molecules or molecular aggregates can assemble themselves with corresponding recognition units, display a critical ability to efficiently distinguish between different recognition units even in a complex mixture or in a system with similar recognition units. For example, Harada et al. found that multiple noncovalent interactions including hydrophobic interactions, p– p stacking and hydrogen-bonding interactions can create a social-self-sorting system cooperatively. Crown ethers, as the first generation of supramolecular macrocyclic hosts, have been widely used as building blocks to construct various functional assemblies with different guest molecules. Based on the differences in binding affinity and binding selectivity between two crown ethers, dibenzo[24]crown-8 (DB24C8) and bis(p-phenylene)[34]crown-10 (BPP34C10), and their complementary guest moieties, dibenzylammonium salts (DBA) and paraquat derivatives, our group has successfully prepared an alternating supramolecular polymer by means of the self-sorting organization of two AB-type heteroditopic monomers. It is of considerable interest to investigate whether it is possible to combine a binary system consisting of a pair of supramolecular polymers through a self-sorting process into a supramolecular polymer blend from low-molecular-weight monomers. For example, if a supramolecular polymer gel is blended with a linear supramolecular polymer, the gelation and other properties of the supramolecular polymer gel can be tuned. Herein we report on a supramolecular polymer blend, which is formed due to the self-sorting organization of two heteroditopic AB-type monomers, 1 and 2 (Scheme 1). Monomer 1 has a DB24C8 moiety and a DBA group, which are linked together by a long and flexible alkyl chain. Monomer 2 contains a BPP34C10-paraquat-based analogue and has [a] S. Dong, X. Yan, B. Zheng, J. Chen, M. Zhang, Prof. Dr. F. Huang Department of Chemistry, Zhejiang University Hangzhou, Zhejiang 310027 (P.R. China) Fax: (+86)571-87953189 E-mail : fhuang@zju.edu.cn [b] X. Ding, Prof. Dr. Y. Yu Shanghai Key Laboratory of Magnetic Resonance Department of Physics, East China Normal University Shanghai 200062 (P.R. China) [c] Dr. D. Xu Changchun Institute of Applied Chemistry Chinese Academy of Sciences Changchun 130022 (P. R. China) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201200016.

Supramolecular polymer gels are precisely designed physical gels brought together by reversible secondary interactions to form three dimensional networks of melt macromolecules. Generally, they differ from supramolecular gels because they are comprised of polymers instead of low molecular weight compounds. Recently, much effort has focused on designing supramolecular polymer gels and related materials with excellent properties; indeed, improvements have been made in their supramolecular interactions, complementarity in the non-covalent bonding units, the nature of the macromolecular building blocks, and strand elasticity of supramolecular polymer networks. Owing to the precise molecular design, they represent nanophase separation and characteristic viscoelasticity. Here, we review supramolecular polymer gels in terms of molecular design, morphology, and rheology. We also discuss future directions in practical application of supramolecular polymer gels.

Here, we present a method for the building of new bicyclic heterometallic cross-linked supramolecular polymers by hierarchical unification of three types of orthogonal noncovalent interactions, including platinum(II)-pyridine coordination-driven self-assembly, zinc-terpyridine complex, and host-guest interactions. The platinum-pyridine coordination provides the primary driving force to form discrete rhomboidal metallacycles. The assembly does not interfere with the zinc-terpyridine complexes, which link the discrete metallacycles into linear supramolecular polymers, and the conjugation length is extended upon the formation of the zinc-terpyridine complexes, which red-shifts the absorption and emission spectra. Finally, host-guest interactions via bis-ammonium salt binding to the benzo-21-crown-7 (B21C7) groups on the platinum acceptors afford the cross-linked supramolecular polymers. By continuous increase of the concentration of the supramolecular polymer to a relatively high level, supramolecular polymer gel is obtained, which exhibits self-healing properties and reversible gel-sol transitions stimulated by various external stimuli, including temperature, K+, and cyclen. Moreover, the photophysical properties of the supramolecular polymers could be effectively tuned by varying the substituents of the precursor ligands.

Main observation and conclusionPillar[5]arene‐based supramolecular polymer gels (SPGs) show broad application prospects. To investigate the influence of the supramolecular monomers’ structure on the assembly and properties of corresponding pillar[5]arene‐based SPGs, a series of monomers based on different functionalized pillar[5]arene derivatives with various structures were synthesized. There are per‐methylated pillar[5]arene (H1), bromobutane‐functionalized pillar[5]arene (H2), 4‐hydroxybenzaldehyde‐functionalized pillar[5]arene (H3), ethyl thioglycolate‐functionalized pillar[5]arene (H4), thioacetylhydrazine‐functionalized pillar[5]arene (H5), bola‐type bis‐pillar[5]arene (H6) and tripodal‐type tri‐pillar[5]arene (H7). Meanwhile, a neutral tripodal‐guest TG was also employed to co‐assemble with these pillar[5]arene‐based monomers by host‐guest interactions. As a result, under the same conditions (10%, DMSO‐H2O, w/v, 10 mg·mL–1 = 1%), H1 and H2 cannot assemble into SPGs with TG. Interestingly, mono‐p[5] derivatives H3—H5 could assemble into SPGs with TG. More importantly, bis‐p[5] H6 and tri‐p[5] H7 could assemble into supramolecular polymer network gel (SPNG) and supramolecular polymer organic framework gel (SOFG) with TG, respectively. These gels all show blue aggregation‐induced emission (AIE) properties. Among these SPGs, the SPNG shows the best viscoelastic behavior and self‐healing properties. The result is attributed to the flexible network structure of SPNGs. In addition, the xerogels of SOFG and SPNG have shown nice adsorption and separation properties for organic dyes in water solution.

CONSPECTUS: Supramolecular polymers, fabricated via the combination of supramolecular chemistry and polymer science, are polymeric arrays of repeating units held together by reversible, relatively weak noncovalent interactions. The introduction of noncovalent interactions, such as hydrogen bonding, aromatic stacking interactions, metal coordination, and host-guest interactions, endows supramolecular polymers with unique stimuli responsiveness and self-adjusting abilities. As a result, diverse monomer structures have been designed and synthesized to construct various types of supramolecular polymers. By changing the noncovalent interaction types, numbers, or chemical structures of functional groups in these monomers, supramolecular polymeric materials can be prepared with tailored chemical and physical properties. In recent years, the interest in supramolecular polymers has been extended from the preparation of intriguing topological structures to the discoveries of potential applications as functional materials. Compared with traditional polymers, supramolecular polymers show some advantages in the fabrication of reversible or responsive materials. The development of supramolecular polymers also offers a platform to construct complex and sophisticated materials with a bottom-up approach. Macrocylic hosts, including crown ethers, cyclodextrins, calixarenes, cucurbiturils, and pillararenes, are the most commonly used building blocks in the fabrication of host-guest interaction-based supramolecular polymers. With the introduction of complementary guest molecules, macrocylic hosts demonstrate selective and stimuli-responsive host-guest complexation behaviors. By elaborate molecular design, the resultant supramolecular polymers can exhibit diverse structures based on the self-selectivity of host-guest interactions. The introduction of reversible host-guest interactions can further endow these supramolecular polymers with interesting and fascinating chemical/physical properties, including stimuli responsiveness, self-healing, and environmental adaptation. It has been reported that macrocycle-based supramolecular polymers can respond to pH change, photoirradition, anions, cations, temperature, and solvent. Macrocycle-based supramolecular polymers have been prepared in solution, in gel, and in the solid state. Furthermore, the solvent has a very important influence on the formation of these supramolecular polymers. Crown ether- and pillararene-based supramolecular polymers have mainly formed in organic solvents, such as chloroform, acetone, and acetonitrile, while cyclodextrin- and cucurbituril-based supramolecular polymerizations have been usually observed in aqueous solutions. For calixarenes, both organic solvents and water have been used as suitable media for supramolecular polymerization. With the development of supramolecular chemistry and polymer science, various methods, such as nuclear magnetic resonance spectroscopy, X-ray techniques, electron microscopies, and theoretical calculation and computer simulation, have been applied for characterizing supramolecular polymers. The fabrication of macrocycle-based supramolecular polymers has become a currently hot research topic. In this Account, we summarize recent results in the investigation of supramolecular polymers constructed from macrocycle-based host-guest molecular recognition motifs. These supramolecular polymers are classified based on the different macrocycles used in them. Their monomer design, structure control, stimuli-responsiveness, and applications in various areas are discussed, and future research directions are proposed. It is expected that the development of supramolecular polymers will not only change the way we live and work but also exert significant influence on scientific research.

Lost circulation during drilling has significantly hindered the safe and efficient development of oil and gas resources. Supramolecular polymer gel-based lost circulation materials have shown significant potential for application due to their unique molecular structures and superior performance. Herein, a high-performance supramolecular polymer gel was developed, and the influence of reservoir conditions on the performance of the supramolecular polymer gel was investigated in detail. The results identified an optimal formulation for the preparation of supramolecular polymer gel comprising 15 wt% acrylamide, 3 wt% 2-acrylamide-2-methylpropanesulfonic acid, 2.6 wt% divinylbenzene, 5 wt% polyvinyl alcohol, 0.30 wt% cellulose nanofibers, and 3 wt% laponite. The performance of the gel-forming suspension and the resulting supramolecular polymer gel was influenced by various factors, including temperature, density, pH, and the intrusion of drilling fluid, saltwater, and crude oil. Nevertheless, the supramolecular polymer gels consistently exhibited high strength under diverse environmental conditions, as confirmed by rheological measurements. Moreover, the gels exhibited strong plugging performance across various fracture widths and in permeable formations, with maximum breakthrough pressures exceeding 6 MPa. These findings establish a theoretical foundation and practical approach for the field application of supramolecular polymer gels in complex geological formations, demonstrating their effectiveness in controlling lost circulation under challenging downhole conditions.

Review: 56 refs.

This paper reports a stimuli-responsive designer supramolecular polymer gel in dimethylsulphoxide (DMSO)/water (1:2) based on a dipeptide amphiphile and β-cyclodextrin (β-CD) The dipeptide amphiphile contains caproic acid at the N terminus and methyl ester at the C terminus. From X-ray single crystal diffraction, the amphiphile adopts a kink-like conformation. The amphiphile self-assembled to form a parallel sheet-like structure stabilized by multiple intermolecular hydrogen bonds. Moreover, the parallel sheet-like structure is also stabilized by edge-to-edge π–π stacking interactions. In higher-order packing, it forms a corrugated sheet-like structure stabilized by hydrophobic interactions. The dipeptide amphiphile interacts with β-cyclodextrin and forms gel through supramolecular polymer formation in (DMSO)/water (1:2) by a simple heating-cooling cycle. The sol-to-gel transformation is because of a host–guest complex between compound 1 and β-CD and the formation of supramolecular polymer accompanied by microstructure changes from nanofibers to microrods. The gel is temperature responsive with a Tgel of 70 °C. The supramolecular polymer gel is also responsive to stimuli such aspicric acid and HCl. The extensive spectroscopic studies show that the aromatic hydrophobic side chain of compound 1 forms a host–guest complex with β-CD. These results will be helpful for the design of advanced programable eco-friendly functional materials.

Herein, we report the preparation of a multifunctional metallacage-core supramolecular gel by orthogonal metal coordination and host-guest interactions. A tetragonal prismatic cage with four appended 21-crown-7 (21C7) moieties in its pillar parts was first prepared via the metal-coordination-driven self-assembly of cis-Pt(PEt3)2(OTf)2, tetraphenylethene (TPE)-based sodium benzoate ligands and linear dipyridyl ligands. Further addition of a bisammonium linker to the cage delivered a supramolecular polymer network via the host-guest interactions between the 21C7 moieties and ammonium salts, which formed a supramolecular gel at relatively higher concentrations. Due to the incorporation of a TPE derivative as the fluorophore, the gel shows emission properties. Multiple stimuli responsiveness and good self-healing properties were also observed because of the dynamic metal coordination and host-guest interactions used to stabilize the whole network structure. Moreover, the storage and loss moduli of the gel are 10-fold those of the gel without the metallacage cores, indicating that the rigid metallacage plays a significant role in enhancing the stiffness of the gel. The studies described herein not only enrich the functionalization of fluorescent metallacages via elegant ligand design but also provide a way to prepare stimuli-responsive and self-healing supramolecular gels as robust and smart materials.

This article summarizes the basic concepts and synthetic strategies leading to various types of supramolecular polymers with chelated units, including linear, branched, cross-linked, and heterometallic polymers. Particular attention is paid to such new synthetic approaches to supramolecular polymers as hierarchical and orthogonal self-assembly based on a combination of metal–ligand interaction with hydrogen bonds and host–guest interactions. Metallosupramolecular polyelectrolytes, supramolecular polymer gels, self-assembled metallosupramolecular monolayers, and supramolecular metal chelate dendrimers are analyzed. The stimuli-responsive, self-healing, and shape memory supramolecular polymers with chelated units are considered. The bibliography includes articles published over the past five years.