Thermo- and pH-Responsive Nanogel Particles Bearing Secondary Amine Functionalities for Reversible Carbon Dioxide Capture and Release

We report the synthesis of temperature- and pH-responsive nanogel particles (NPs) consisting of <i>N</i>-isopropylacrylamide (NIPAM) and <i>N</i>-(2,2,6,6-tetra­methyl­piperidin-4-yl)­methacryl­amide (TMPMA). NPs can reversibly capture and release carbon dioxide via temperature-induced volume phase transition and changes in pH. These stimuli-responsive particles contain sterically hindered secondary amine functionalities and exhibit a volume phase transition temperature (VPTT) in aqueous solution. The fully reversible VPTT behavior involves a precise shrinkage to 40% of the initial particle size along with a large change in pH from 10.25 to 7.65 upon increasing temperature. We could reversibly release 35 mL (1.4 mmol) of CO₂ per gram of polymer in very short heating times, thereby significantly increasing the amount of CO₂ with respect to the regeneration time. This behavior could be repeated for various cycles at moderate temperatures (85 °C)