Stimuli-responsive Gels From Dynamic Covalent Chemistry

Gel，as a kind of "soft matter", is a three-dimensional polymeric crosslinked network, whichabsorbs and retains large amounts of solvents. The gel has very unique physical and chemicalproperties. Gels have pervaded our everyday life in a variety of forms. Scientists have used thegel for tissue engineering, separation and purification, drug delivery, sensor and detection,chemical convertors, catalysis, etc..With the development of smart materials, the gel chemistry is more tending to the "smart"direction. A variety of smart gels, which is capable to responding to external stimuli, aredesigned and studied. These stimuli can be pH, temperature, light, chemicals, electromagnetic ormechanical field, strain, and so on, which can be single, duple, or mutiple. The response may beon the volume (contraction or swelling), also be on state (sol or gel), or physical properties (color,etc.) and so on. Such materials are usually more controllable and easy to be adjusted, so they areeasier to meet the needs of different situations, and are more attractive.After the recent thirty years of research, there has been a great development ofsupramolecular chemistry, but in some cases, the feature of "weak" of the supramolecularinteractions limits its application. The last ten years, some chemists fixated back to covalentbonds, and found that part of covalent bonds are similar to supramolecular interactions, whichwas ignored before. They are the dynamic covalent bonds, which are also thermodynamicallyreversible. But the essence of dynamic covalent bond is covalent bond, more stable than thesupramolecular interactions. Herein, we use the benzoic imine bond, one of dynamic covalentbonds, to build stimuli-responsive gels.1. Dual stimuli-responsive pseudopolyrotaxane hydrogelsThe medium of hydrogels is water, and the majority of the organisms are present in theaqueous medium, therefore, hydrogels generally have a better biocompatibility, and have verybroad applications as biomedical materials. There is a hydrogel which is formed byα-cyclodextrin and polyethylene glycol (PEG) chains. The PEG chain is an axis threadingα-cyclodextrin. The supramolecular inclusion complex, defined as pseudopolyrotaxane, can formhydrogel when the concentration is enough. Many chemists focused on the formation and structure of this hydrogel. A number of bio-pharmaceutical applications have been carried on.However, the stimulus of this hydrogel is primarily the thermal, mechanical, optical. All of thesestimulus is single-stimulus. The design of dual stimuli-responsive pseudopolyrotaxane hydrogelsis necessary.Herein, we designed the polymer of THPP-(PEG2000-BA)4, a four-branched molecule whichhas a core of porphyrin and is end-decorated with benzaldehyde. It can form physicalpseudopolyrotaxane hydrogels in the presence of α-cyclodextrins. Since benzaldehyde can reactwith amine to form a pH-responsive Schiff-base which possesses a dynamic covalent nature, theformation of gels can be modulated by pH. By means of a strongly competitive photoresponsiveguest,1-[p-(phenylazo)benzyl]-bromide (Azo-C1-N+), this hydrogel can be regulated to achievegel-sol transitions by alternation of visible and UV irradiation. Reversible cycles of sol-to-geland gel-to-sol transitions were achieved by means of the synergy of pH-adjustment andUV-visible irradiation. A dual stimuli-responsive pseudopolyrotaxane hydrogels was prepared.2. A pseudopolyrotaxane hydrogels dispersing carbon nanotubeCarbon nanotube is a very attractive nanomaterial, which has a unique structure, mechanicaland electrical properties, including in high electronic and thermal conductivities, greatmechanical strength, and huge specific surface are. There are a wide range of applications in thephysics, materials and many other fields. However, bundled aggregates due to stronginter-tubular van der Waals interactions cause the poor solubility in many conventional solvents,which acts as an obstacle to both the purification and handling of the SWNTs. This seriouslylimits its application. Therefore, all the scientists are meeting and have to solve the problem howto improve its solubility.Conjugated porphyrin macrocycle is capable of forming a J-aggregate composite withcarbon nanotubes through π-π stacking interaction, and can be used to disperse carbon nanotubes.Considering that the polymer of THPP-(PEG2000-BA)4which we previously designed has aporphyrin core, we studied its ability of dispersing carbon nanotubes. Indeed, it is found thatcarbon nanotubes can be dispersed. Meanwhile, after the dispersing carbon nanotubes, it is stillcapable of forming a pseudopolyrotaxane hydrogel. In addition, the pseudopolyrotaxanehydrogel dispersing carbon nanotubes still is dual (pH-and photo-) stimuli-response.3. A pH-responsive self-healing gelSelf-healing is a fascinating property of living creatures, A living body can automaticallyrepair damage by activating the self-healing process and, thus, restore certain destroyedfunctions.Scientists have been simulating this process in the past decade and designed a variety of self-healing materials. They hope that it can improve the safe and prolong the life of thematerials through self-repairing process.Herein, we designed two polymer P(AM25-co-BocEAM1-co-BA1) and P(AM50-co-BocEAM1-co-BA1), which are both modified with amine and benzaldehyde, but with differentcontents. We investigated their ability to gel. We found that the former is capable of forming agel in the DMSO solvent, while the latter not. This is because benzaldehyde can react with amineto form a pH-responsive benzoic imine bond, which plays a role in cross-linking. The differencein the contents of amine and benzaldehyde results in the difference in the degree of cross-linking,further affects the ability to gel. But they are both pH-sensitive. They can maintain the gel orsolution state under weak acidic conditions, both formed the gel under alkaline condition, andboth converted into a sol under strong acid condition. We carried out the sol-gel conversionexperiments by pH-adjusting. In addition, we also found the gel by the latter under alkalinecondition has a good self-healing capability.