Reversible cross-linking of hydrophilic dynamic covalent polymers with radically exchangeable alkoxyamines in aqueous media

Abstract

Reversible cross-linking of alkoxyamine-containing hydrophilic dynamic covalent polymers in aqueous media has been accomplished. Water-soluble polymers with alkoxyamine units in the side chains were synthesized by radical copolymerization of 2-(dimethylamino)ethyl methacrylate and methacrylic esters with alkoxyamine units, and subsequent protonation of dimethylaminoethyl groups by hydrochloric acid. By heating the polymers in water at 100 °C in a closed system, hydrogels were formed by radical exchange reactions. Because the radical reactions are tolerant of water, the exchange reactions of alkoxyamine units in the side chain caused the cross-linking reaction even in aqueous media. Cross-linking behaviors depended on the amounts of alkoxyamine units in the polymers and the reaction concentrations. A de-cross-linking reaction was also accomplished by radical exchange reaction between the cross-linked polymer and added hydrophilic alkoxyamine compound. This dynamic cross-linking nature in aqueous media provides a foundation for a wide range of polymer reactions, and it can be applied to environmentally benign dynamic materials.