Role of the thermoresponsive segment in determining the redox properties of phenothiazine-labeled poly(ethoxyethyl glycidyl ether)-block-poly(ethylene oxide)

This study demonstrates that the position of the phenothiazine (PT) group relative to the poly(ethoxyethyl glycidyl ether) (PEEGE) segment is an important factor in determining the phase transition behavior and redox properties of diblock copolymers consisting of the thermoresponsive PEEGE segment, a poly(ethylene oxide) (PEO) segment, and a redox-active hydrophobic PT group attached to the chain-end of the copolymer. In the case of PT-PEEGE-b-PEO, where PT is adjacent to the PEEGE segment, water-soluble core-shell micelles are formed at temperatures higher than the coil-globule transition temperature of the PEEGE segment (T_c). However, in the case of PT-PEO-b-PEEGE, where PT is not adjacent to the PEEGE segment, insoluble aggregates are formed in an aqueous solution in the temperature range from 40 to 50 °C. The oxidation potential of PT in PT-PEEGE-b-PEO is more positive than that of PT in PT-PEO-b-PEEGE at temperatures lower than T_c; this suggests that the PT group of the former is incorporated in a hydrophobic atmosphere formed by the PEEGE segment even at temperatures lower than T_c. When the temperature increases, the rate of the temperature-dependent shift in the oxidation potential of PT increases significantly and the anodic current starts to decrease around T_c; this result shows that the microscopic atmosphere around PT groups changes into a hydrophobic atmosphere, accompanying the formation and growth of core-shell micelles. In the case of PT-PEO-b-PEEGE, a PT group remains in a hydrophilic atmosphere even at temperatures higher than T_c where aggregates are formed, as indicated by the constant rate of the potential shift in the temperatures around T_c.