Stimuli-Responsive Supramolecular Polymers in Aqueous Solution

Abstract

CONSPECTUS: Aiming to construct various novel supramolecular polymeric structures in aqueous solution beyond small supramolecular self-assembly molecules and develop functional supramolecular polymeric materials, research interest on functional supramolecular polymers has been prevailing in recent years. Supramolecular polymers are formed by bridging monomers or components together via highly directional noncovalent interactions such as hydrogen bonding, hydrophobic interaction, π-π interaction, metal-ligand coordination, electrostatic interaction, and so forth. They can be easily functionalized by employing diverse building components with specific functions besides the traditional polymeric properties, a number of which are responsive to such external stimuli as pH variance, photoirradiation, chemically or electrochemically redox with the controllable conformation or construction switching, polymerization building and rebuilding, and function adjustment reversibly owing to the reversibility of noncovalent interactions. Supramolecular polymers are "soft matters" and can be functionalized with specific properties such as morphology adjustment, controllable luminescence, shape memory, self-healing, and so forth. Supramolecular polymers constructed based on macrocycle recognition and interlocked structures represent one typical branch of the supramolecular polymer family. Cyclodextrin (CD), cucurbituril (CB), and hydrophilic calixarene derivatives are usually employed to construct hydrophilic supramolecular polymers in aqueous solution. Stimuli-responsive hydrophilic supramolecular polymers, constructed in aqueous solution particularly, can be promising candidates for mimicking biocompatible or vital functional materials. This Account mainly focuses on the recent stimuli-responsive supramolecular polymers based on the host-guest interaction in aqueous solution. We describe the hydrophilic supramolecular polymers constructed via hydrophobic effects, electrostatic interaction, metal-ligand coordination, and multiple combinations of the above noncovalent interactions. The disparate ways to engender stimuli-responsive supramolecular polymers via the hydrophobic effects of α-CD, β-CD, and γ-CD macrocycles are illustrated and discussed. Some recent works on CD-based photoresponsive functional supramolecular polymers are summarized. CB (especially CB[8]) based supramolecular polymers and their pH-responsive and photoresponsive properties are introduced. Hydrophilic calixarene derivative (bis(p-sulfonatocalix[4]arene) typically) based supramolecular polymers via electrostatic interactions are reviewed, and their redox-responsive association/disassociation elaborated in detail. More complicate supramolecular polymers based on multiple noncovalent interactions are illustrated including hydrophobic effect, metal-ligand coordination, and electrostatic interactions and their functional stimuli-responsiveness elaborated as well. Finally, we give perspectives on the strength of these diverse noncovalent interactions to form supramolecular polymers in aqueous solution, on the advantage, disadvantage, efficiency, and reversibility of using certain stimuli in constructing supramolecular polymers and prospect the future function improvement of these polymers as functional materials.