A New Thermo‑, pH‑, and CO₂‑Responsive Homopolymer of Poly[<i>N</i>‑[2-(diethylamino)ethyl]acrylamide]: Is the Diethylamino Group Underestimated?

A new multi-stimuli-responsive homopolymer of poly­[<i>N</i>-[2-(diethylamino)­ethyl]­acrylamide] (PDEAEAM), which combines the thermoresponsive and pH/CO₂-responsive moieties of the diethylamino and acrylamide groups, was proposed and synthesized by RAFT polymerization. Well-defined PDEAEAM was synthesized by solution RAFT polymerization as indicated by the linear increase in the polymer molecular weight with the monomer conversion and the narrow molecular weight distribution. The appending diethylamino group in the polymer backbone was found to be crucial to determine the thermoresponse of PDEAEAM in water. The parameters including the polymerization degree, the polymer concentration, the deuterated solvent, the terminal attached on the polymer backbone, the additives of salt and urea, and pH and bubbling CO₂ affecting the thermoresponse of PDEAEAM in aqueous solution at the lower critical solution temperature (LCST) were investigated. The temperature-variable ¹H NMR analysis suggests that the dehydration of PDEAEAM at temperature above LCST is ascribed to the weakened hydrogen bonding between the CONHCH₂ and/or (CH₂N­(CH₂CH₃)₂) moieties with the solvent of water. The proposed multi-stimuli-responsive homopolymer of PDEAEAM has two advantages of (1) the convenient and controllable RAFT synthesis and (2) the pH/CO₂ tunable LCST at ∼36 °C being very close to body temperature