Temperature-induced self-assembly and metal-ion stabilization of histidine functional block copolymers

Histidine functional block copolymers are thermally self-assembled into polymer micelles with poly-N-isopropylacrylamide in the core and the histidine functionality in the corona. The thermally induced self-assemblies are reversible until treated with Cu²⁺ ions at 50 °C. Upon treatment with 0.5 equivalents of Cu²⁺ relative to the histidine moieties, metal-ion coordination locks the self-assemblies. The self-assembly behavior of histidine functional block copolymers is explored at different values of pH using DLS and ¹H NMR. Metal-ion coordination locking of the histidine functional micelles is also explored at different pH values, with stable micelles forming at pH 9, observed by DLS and imaged by atomic force microscopy. The thermal self-assembly of glycine functional block copolymers at pH 5, 7, and 9 is similar to the histidine functional materials; however, the self-assemblies do not become stable after the addition of Cu²⁺, indicating that the imidazole plays a crucial role in metal-ion coordination that locks the micelles. The reversibility of the histidine-copper complex locking mechanism is demonstrated by the addition of acid to protonate the imidazole and destabilize the polymer self-assemblies.