Tertiary-Amine-Containing Thermo- and pH-Sensitive Hydrophilic ABA Triblock Copolymers: Effect of Different Tertiary Amines on Thermally Induced Sol-Gel Transitions

This Article reports on the synthesis of a series of well-defined, tertiary-amine-containing ABA triblock copolymers, composed of a poly(ethylene oxide) (PEO) central block and thermo- and pH-sensitive outer blocks, and the study of the effect of different tertiary amines on thermally induced solgel transition temperatures (T-solgel) of their 10 wt % aqueous solutions. The doubly responsive ABA triblock copolymers were prepared from a difunctional PEO macroinitiator by atom transfer radical polymerization of methoxydi(ethylene glycol) methacrylate and ethoxydi(ethylene glycol) methacrylate at a feed molar ratio of 30:70 with similar to 5 mol % of either N,N-diethylaminoethyl methacrylate (DEAEMA), N,N-diisopropylaminoethyl methacrylate, or N,N-di(n-butyl)aminoethyl methacrylate. The chain lengths of thermosensitive outer blocks and the molar contents of tertiary amines were very similar for all copolymers. Using rheological measurements, we determined the pH dependences of T-solgel of 10 wt % aqueous solutions of these copolymers in a phosphate buffer. The T-solgel versus pH curves of all polymers exhibited a sigmoidal shape. The T-solgel increased with decreasing pH; the changes were small on both high and low pH sides. At a specific pH, the T-solgel decreased with increasing the hydrophobicity of the tertiary amine, and upon decreasing pH the onset pH value for the Tsolgel to begin to increase noticeably was lower for the more hydrophobic tertiary amine-containing copolymer. In addition, we studied the effect of different tertiary amines on the release behavior of FITC-dextran from 10 wt % micellar gels in an acidic medium at 37 and 27 degrees C. The release profiles for three studied hydrogels at 37 degrees C were essentially the same, suggesting that the release was dominated by the diffusion of FITC-dextran. At 27 degrees C, the release was significantly faster for the DEAEMA-containing copolymer, indicating that both diffusion and gel dissolution contributed to the release at this temperature.