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.