Linear and cyclic poly(2-isopropyl-2-oxazoline)s for fine control of thermoresponsiveness

Linear and cyclic poly(2-isopropyl-2-oxazoline)s (POx)s with the same average molar mass and molar mass distribution were synthesized by living cationic ring-opening polymerization of 2-isopropyl-2-oxazoline (Ox) and a successive intramolecular Cu(I)-catalyzed click reaction. The synthesis of linear poly(2-isopropyl-2-oxazoline) (l-POx) and cyclic poly(2-isopropyl-2-oxazoline) (c-POx) was characterized by ¹H NMR, FT-IR, MALDI-TOF-MS and analytical SEC. Both POxs exhibited a sharp thermal transition upon temperature changes in physiological saline (150 mM NaCl) phosphate buffered solution (PBS, 10 mM, pH 7.4). Interestingly, the thermal transition temperature of the c-POx was much higher than that of the l-POx. The large difference of thermal transition temperatures between c-POx and l-POx was utilized for the fine tuning of the thermal transition temperature. By adjusting the mixing ratio of the two polymers, the thermal transition temperatures were varied from 44 to 57 °C.