Non-fluorous polymers with very high solubility in supercritical CO₂ down to low pressures

Abstract

Liquid and supercritical carbon dioxide have attracted much interest as environmentally benign solvents¹, but their practical use has been limited by the need for high CO₂ pressures to dissolve even small amounts of polar, amphiphilic, organometallic, or high-molecular-mass compounds^2,3,4. So-called ‘CO₂-philes’ efficiently transport insoluble or poorly soluble materials into CO₂ solvent, resulting in the development of a broad range of CO₂-based processes, including homogeneous and heterogeneous polymerization, extraction of proteins and metals, and homogeneous catalysis^{5,6,7,8,9,10,11}. But as the most effective CO₂-philes are expensive fluorocarbons, such as poly(perfluoroether), the commercialization of otherwise promising CO₂-based processes has met with only limited success. Here we show that copolymers can act as efficient, non-fluorous CO₂-philes if their constituent monomers are chosen to optimize the balance between the enthalpy and entropy of solute–copolymer and copolymer–copolymer interactions. Guided by heuristic rules regarding these interactions, we have used inexpensive propylene and CO₂ to synthesize a series of poly(ether-carbonate) copolymers that readily dissolve in CO₂ at low pressures. Even though non-fluorous polymers are generally assumed to be CO₂-phobic, we expect that our design principles can be used to create a wide range of non-fluorous CO₂-philes from low-cost raw materials, thus rendering a variety of CO₂-based processes economically favourable, particularly in cases where recycling of CO₂-philes is difficult.