Shengyi Dong

A Multiresponsive, Shape‐Persistent, and Elastic Supramolecular Polymer Network Gel Constructed by Orthogonal Self‐Assembly

A cross-linked supramolecular polymer network gel is designed and prepared, which shows reversible gel-sol transitions induced by changes in pH, temperature, cation concentration, and metal co-ordination. The gel pore size is controlled by the amount of cross-linker added to the system, and the material can be molded into shape-persistent, free-standing objects with elastic behavior. These features are all due to the dynamically reversible host-guest complexation and good mechanical properties of the cross-linked polymer network. No single organogel has previously been reported to possess all of these features, making this supramolecular gel an unprecedentedly intelligent soft material.

Self‐Healing Supramolecular Gels Formed by Crown Ether Based Host–Guest Interactions

Automatic repair: a polymer with pendent dibenzo[24]crown-8 units (purple in picture) was cross-linked by two bisammonium salts (green) to form two supramolecular gels based on host-guest interactions. These two gels are stimuli-responsive materials that respond to changes of the pH value and are also self-healing materials, as can be seen by eye and as evidenced by rheological data.

Metal Coordination Mediated Reversible Conversion between Linear and Cross-Linked Supramolecular Polymers

The topology of a polymer has a significant influence on its properties and functions, both in bulk and in solution. Therefore, the discovery of efficient methods to control polymer topology is very important. [1] The introduction of non-covalent interactions into traditional covalent polymers represents a novel approach for the control of polymer topologies, and has allowed the incorporation of reversible and switchable functionality into different macromolecular architectures. [2] However, this strategy usually requires the integration of specific molecular recognition motifs into polymer chains; such an approach suffers from problems such as the availability of suitable monomers and the poor efficiency of polymerization techniques that are tolerant to functional groups on the polymer. Conversely, supramolecular polymers that are assembled from low molecular weight monomers by non-covalent interactions, such as hydrogen bonding, [3] metal coordination, [4] and host–guest interactions, [5] have demonstrated traditional polymeric properties and are an important resource in the development of stimuliresponsive dynamic materials. [6] Until now, efforts to control the topology of supramolecular polymers have mainly been concerned with the conversion between the large-sized species and their corresponding monomers/oligomers; comparatively little effort has been devoted to the transformation between supramolecular polymers of different topologies. The desired recognition motifs can be conveniently introduced into the low-molecular-weight-monomers, thus avoiding the problems commonly associated with covalently linked polymer backbones, and thus leading to a more effective method for switching between different architectures. Herein, we present reversible switching between linear and cross-linked supramolecular polymers. That biological systems utilize multiple-interaction selfassembly to afford hierarchical and multifunctional systems [7]

Supramolecular polymers constructed from macrocycle-based host-guest molecular recognition motifs.

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.

A Crown Ether Appended Super Gelator with Multiple Stimulus Responsiveness

A crown ether appended super gelator is designed and synthesized. It can gel a variety of organic solvents and shows excellent gelation properties with both low critical gelation concentration and short gelation time. Due to the introduction of the crown ether moiety and a secondary ammonium unit, the supramolecular gels show reversible gel-sol transitions. The supramolecular gels can also be molded into shape-persistent and free-standing objects.

A pillar[5]arene/imidazolium [2]rotaxane: solvent- and thermo-driven molecular motions and supramolecular gel formation

Based on the pillar[5]arene/imidazolium recognition motif, a [2]rotaxane was effectively prepared. Solvent/temperature triggered molecular motions of the pillar[5]arene ring on the imidazolium axle were successfully realized. By comparison of proton NMR spectra of the [2]rotaxane in different solvents, we found that if we increased the solvent polarity, the pillar[5]arene ring gradually moved away from the imidazolium part. In DMSO, we also could adjust the binding site of the pillar[5]arene ring by changing the temperature. Furthermore, in DMSO, the [2]rotaxane self-assembled to form a supramolecular gel, which showed multiple stimuli-responsiveness.

A Novel Diblock Copolymer with a Supramolecular Polymer Block and a Traditional Polymer Block: Preparation, Controllable Self-Assembly in Water, and Application in Controlled Release

A novel diblock copolymer with a hydrophobic supramolecular polymer block and a hydrophilic traditional polymer block has been prepared. Control over the chain length ratio of the two blocks is obtained by simply changing the concentration proportion of the monomer of the supramolecular polymer block to the traditional polymer block in solution. When the chain length ratio of the two blocks is changed, the formation of various self-assembly morphologies is achieved.

Supramolecular polymer nanofibers via electrospinning of a heteroditopic monomer.

Driven by the benzo-21-crown-7/secondary ammonium salt recognition motif, a linear supramolecular polymer was formed from self-organization of a low-molecular-weight self-complementary monomer in chloroform. From this supramolecular polymer, nanofibers were obtained successfully via electrospinning.

LCST‐Type Phase Behavior Induced by Pillar[5]arene/Ionic Liquid Host–Guest Complexation

The host-guest complex of dipropoxypillar[5]arene and an ionic liquid 1,3-dimethylimidazolium iodide is found to exhibit a lower critical solution temperature (LCST)-type phase transition in chloroform. This LCST-type phase behavior can be conveniently modulated by experimental parameters and can be easily combined with the ionic liquid for potential application in product and educt separation.

A novel pH-responsive supramolecular polymer constructed by pillar[5]arene-based host–guest interactions

A novel pH-responsive supramolecular polymer based on the pillar[5]arene/imidazolium cation recognition motif was successfully prepared. It was demonstrated that the binding strength between the monomer molecules in solution could be reduced by adding base, thereby leading to a decrease in the polymerization degree, while a reverse switch could be achieved by the addition of acid to change the monomer back to the cationic state. These processes were confirmed by 1H NMR, DOSY experiments, viscosity measurements and theoretical calculations. Moreover, a rod-like fiber was drawn from a high concentration solution of the monomer and characterized by SEM, providing direct evidence of the formation of a supramolecular polymer with high molecular weight and high degree of the linear chain extension. In this study, a reversible transition between the high-molecular-weight supramolecular polymer and the relatively low-molecular-weight one, that is, a control of the polymerization degree, was realized by simply altering the solution pH. These results are helpful for the future fabrication of adaptive and smart supramolecular materials with stimuli-responsiveness and special functions.