Tuning Phase Behaviour Of Poly(Ethylene Glycol)(PEG)-functionalized Imidazoles Ionic Liquids For Applications In Cycloaddition Reaction And Proteins Separation

In recent years,the research and application of environmental responsive materials arouse wide attention in academia and industry,among these materials,temperature responsive materials are one of the main foci.Because of their unique physical and chemical properties,ionic liquids?ILs?have been widely applied in organic synthesis,extraction and separation,material preparation,biomass dissolution,gas absorption and among others.However,their strong solubilization capacity makes it extremely difficult to separate ILs from the products in a chemical process and then to recycle the ILs.Poly?ethylene glycol??PEG?is a kind of thermal responsive polymer,which is not toxic,not volatile,and biocompatible.This polymer has the ability to dissolve ILs,to coordinate with metals,to promote organic reaction,and to stablize proteins and metal nanometer materials.Considering the fact that the most interesting feature of ILs is their designability of structures and properties,in this work we designed and synthesized a class of PEG-functionalized ionic liquids which are temperature responsive,by binding different molecular mass of PEG molecules on the imidazolium cations.Then,the phase behaviour of these ILs in water,aliphatic alcohols and alcohol/water mixtures as well as the tuning for the transfer of the ILs between water and organics were investigated.Also,the application of the ILs in cycloaddition reaction and protein separation was explored.The main contents are as follows.1.Five kinds of new PEG-functionalized ionic liquids [PEG_m?mim?₂][NTf₂]₂?m=200,400,600,800,1000?,with bis?trifluoromethanesulfonyl?imide anion and double imidazolium cations,were designed and synthesized,where m is the molecular mass of PEG.The structures of these ILs were characterized by ¹HNMR,¹³C-NMR,mass spectrum?MS?and infrared spectroscopy?IR?.Their glass transition temperature?Tg?and thermal decomposition temperature?Td?were determined by differential scanning calorimetry and themogravimetry technology,respectively.It was shown that these ILs have good low temperature performance?Tg <-46 ??and high thermal stability?Td > 300??.Compared with the methods reported in the literatures,the synthetic scheme of this work has the advantages of cheap raw materials,simple synthetic route,easy purification and high yield.2.Temperature-composition phase diagrams of these ILs with water,ethanol,1-propanol,isopropanoland alcohol/water mixtures were determined,respectively.Then the effects of the molecular weight of PEG middle block and the alkyl chain length of the aliphatic alcohol molecules on the phase transition behaviour were examined.It was found that these ILs exhibited UCST phase transition in aliphatic alcohols but unique LCST phase transition in water.In IL/water binary system,the phase transition temperature increased with the increase of the PEG molecular mass.In IL/ aliphatic alcohol binary system,the phase transition temperature decreased with the increase of the PEG molecular weight and increased with the increase of the alkyl chain length of the alcohol molecules.Most importantly,the phase transition behaviour from UCST to LCST could be observed by the addition of water to the IL/alcohol binary system.Temperature-dependent FTIR and ¹HNMR measurements were performed to examine the possible phase transition mechanism of [PEG_m?mim?₂][NTf₂]₂ in water,ethanol and ethanol/H₂O mixtures,respectively.3.Reversible phase transfer of PEG-functionalized ILs [PEG_m?mim?₂][NTf₂]₂?m=200,400,600,800 and 1000?between water and organics?ethyl acetate,n-butyl alcohol,n-amyl alcohol,n-hexyl alcohol,n-heptyl alcohol and n-octyl alcohol?was found upon change of the temperature.Partition coefficients of these ILs between organics and water were determined at deferent temperatures.Thermodynamic parameters such as changes in standard Gibbs free energ??G⁰?,standard enthalpy??H⁰?and standard entropy??S⁰?were calculated.In addition,DLS and FTIR measurements were performed to probe the possible phase transfer mechanism of [PEG_m?mim?₂][NTf₂]₂ between water and ethyl acetate.With[PEG₁₀₀₀?mim?₂][NTf₂]₂ as an example of the ILs investigated,a highly efficient and controllable CuI-catalyzed cycloaddition reaction of azides and alkynes was achieved in ethyl acetate.Then the IL and catalyst?CuI?were transferred spontaneously to the water phase by tuning the temperature of the system,accomplishing the separation and recycle of the IL and CuI from the reaction system,where the IL was used as activator and entrainer of the catalyst.4.The extraction efficiency of eight kinds of proteins?cytochrome c,myoglobin,hemoglobin,lysozyme,papain,trypsin,bovine serum and peroxidase?from aqueous solution by the ionic liquids[PEG_m?mim?₂][NTf₂]₂ was determined,which have lower critical solution temperature?LCST?phase behaviour in water.The influencing factors such as extraction time,pH value of aqueous phase,PEG molecular mass and water content in the ILs were investigated.It was shown that the single-step extraction efficiency of hemoglobin,myoglobin,cytochrome c,papain and lysozyme by [PEG₈₀₀?mim?₂][NTf₂]₂ washigher than 95% under optimal conditions.However,it was only 8% and 2% for bovine serum and peroxidase,respectively.Based on the significant differences in the extraction efficiency of different proteins by the IL,Cytochrome c was selectively separated from peroxidase.Circular dichroism?CD?and FT-IR spectroscopy were used to understand the structure and conformation change of the proteins before and after extraction.The results suggest that no native secondary structure change was observed for the protein after extraction from water to the ILs.Therefore,these ILs provide a gentle environment for proteins.