CO2/N2-Switchable Thermoresponsive Ionic Liquid Copolymer

Thermoresponsive random copolymers consisting of poly(N-isopropylacrylamide) (PNIPAM) and polymerized ionic liquid (IL) poly(1,1,3,3-tetramethylguanidine acrylate) (PTMGA) were synthesized via reversible addition-fragmentation chain transfer radical polymerization (RAFT). The reactivity ratios of NIPAM (r(NIPAM) = 2.11) and TMGA (r(TMGA) = 0.56) were determined by the extended Kelen-Todus method. Glass transition temperatures (T-g) of the copolymers were analyzed, which followed the Fox equation very well. The phase transition behaviors of the copolymers in aqueous solution were studied through UV-vis transmission measurements. Their lower critical solution temperature (LCST) ranged from 30.5 to 73.2 degrees C, depending on the hydrophilic IL content. The apparent plc related to LCST was determined, and thus the protonation degree was calculated. The hydrophilicity of the copolymers could be regulated by gas treatments. Bubbling CO2 led to lowering the transition temperature while bubbling N-2 resulted in its recovery. This CO2/N-2 switchability became more profound with higher IL content. With the ability to undergo reversible protonation caused by the change of pH, the system showed good reversibility in LCST when bubbled with CO2 and N-2. SO2 could also be used to lower LCST. However, a basic compound (e.g., NaOH) was required for its recovery. The pH-dependent solution phase transition behavior provided great insight into the LCST regulation mechanism. The widest LCST shifting window (similar to 12 degrees C) was found between pH 5.16 and 5.96, which could be fulfilled by the CO2 regulation approach. This work provides guidance for the design and synthesis of gas-switchable thermoresponsive polymers based on ionic liquids.