Synthesis and characterization of responsive poly(anionic liquid) microgels

Abstract

We report a class of poly(anionic liquid) microgels that undergo stimuli-responsive volume phase transitions. Such a microgel is synthesized by free radical precipitation polymerization of a tetrabutylphosphonium 4-styrenesulfonate monomer and a cross-linker N,N’-methylenebisacrylamide. These microgels can not only undergo reversible volume phase transitions in response to changes in temperature in water, methanol, or water/methanol mixtures, but also undergo re-entrant swelling–shrinking–swelling transitions as the methanol content increases in water/methanol mixtures at a set temperature in our experimental temperature window of 25.0–64.0 °C. Such microgels can be post functionalized, e.g., via ion-exchange treatment with HSO₃CF₃ to partially transform sulfonate to the catalytic Brønsted acidic –SO₃H, whilst the yielded microgels can inherit the responsive properties. With both responsive and catalytic properties simultaneously harnessed on the same object, the functionalized microgels potentially can be used as a highly efficient catalyst for the esterification reaction of palmitic acid and the transesterification reaction of tripalmitin with methanol (as model reactions) at 65.0 °C (temperature of an oil bath), and allow the catalytic activity to be modulated to a certain extent in a non-monotonous way, making it possible to boost the reactions at a relatively lower temperature (e.g., 42.0 °C), while maintaining considerable catalytic activity. These features underlie the feasible use of the microgels in biodiesel production (with waste cooking oil as a model feedstock).