Design and Synthesis of Thermoresponsive Ionic Liquid Polymer in Acetonitrile as a Reusable Extractant for Separation of Tocopherol Homologues

Abstract

We report the design and development of a series of novel ionic liquid polymers (PILs) which possess thermoresponsive properties in organic solvent. The PILs were synthesized via reversible addition–fragmentation chain transfer (RAFT) copolymerization of 1-vinyl-3-butylimidazolium bromide and N-isopropylacrylamide, followed by anion exchange of bromide to amino acid d-alanine. The PILs possessed 0.36–0.76 molar fraction of ionic liquid with number-average molecular weight of 2.70–8.17 kg/mol and polydispersity index ranging between 1.12 and 1.25. The copolymerizations followed first-order reaction kinetics, and they were well-controlled, as indicated by the linear increase of molecular weight with monomer conversion. The PILs were thermoresponsive in acetonitrile with upper critical solution temperatures (UCST) varying from 25.7 to 34.8 °C, owing to the introduction of anion of amino acid. The PILs could be completely precipitated out by lowering the solution temperature. The PIL/acetonitrile solutions were used as extract phase for separation of tocopherol homologues in hexane. The distribution coefficient of δ-tocopherol between the extract and raffinate phases (Dδ) and the selectivity coefficient of δ-tocopherol to α-tocopherol (Sδ/α) reached as high as 7.86 and 13.0, respectively, while Dβ&γ was 3.63 and Sβ&γ/α was 6.0. The mole ratio of α-tocopherol in the raffinate phase increased from 0.08 to 0.27 after one stage extraction. The PILs could be reused for multiple extraction cycles with negligible change in the tocopherol distribution and selectivity coefficients. The thermoresponsivity of the PILs is of great benefit by eliminating the normally required back extraction steps. This work demonstrates the potential of thermoresponsive polymers for use in high performance separation of natural products.