Tuning Micellization and Dissociation Transitions of Thermo- and pH-Sensitive Poly(ethylene oxide)-b-poly(methoxydi(ethylene glycol) methacrylate-co-methacrylic acid) in Aqueous Solution by Combining Temperature and pH Triggers

We report in this article on the active control of multiple micellization and dissociation transitions of thermo- and pH-sensitive water-soluble block copolymers, poly(ethylene oxide)-b-poly(methoxydi(ethylene glycol) methacrylate-co-methacrylic acid) (PEO-b-P(DEGMMA-co-MAA)), in aqueous solutions by combining temperature and pH triggers. Two block copolymers and one random copolymer P(DEGMMA-co-MAA) were prepared by atom transfer radical polymerization of a mixture of methoxydi(ethylene glycol) methacrylate (DEGMMA) and tert-butyl methacrylate (t-BMA) with a molar ratio of 100: 13 from a PEO macroinitiator and a small molecule initiator, respectively, and subsequent removal of t-butyl group. PDEGMMA is a thermosensitive water-soluble polymer exhibiting a lower critical solution temperature in water at 25 °C. The cloud point of P(DEGMMA-co-MAA) in water can be readily tuned from 24 to 60 °C by changing the solution pH. The block copolymers dissolved molecularly in cold aqueous buffer solutions with a pH of 4.0 and underwent micellization when the temperature was raised to a critical point. Upon judiciously changing the solution pH by injection of a predetermined amount of a KOH solution, the block copolymer micelles were dissociated into the unimers, which subsequently reformed micelles at the same temperature in response to the addition of a predetermined amount of a HCl solution or heating to a higher temperature. These temperature- and pH-induced micellization/dissociation transitions were reversible. By continuously increasing temperature from 20 to 57 °C and simultaneously controlling the pH, three cycles of micellization and disassembly transitions of these block copolymers in aqueous solutions were realized, demonstrating the possibility of achieving on-demand micellization and dissociation transitions by combining temperature and pH triggers. Fluorescence spectroscopy studies showed that a hydrophobic fluorescence dye, Nile Red, can be repeatedly encapsulated by block copolymer molecules upon micellization and released upon dissociation of micelles under different combinations of pH and temperature.