Urea-Functionalized Swelling Poly(ionic liquid)s as Efficient Catalysts for the Transesterification and Hydrolysis of Ethylene Carbonate

Urea-functionalized poly(ionic liquid)s (PILs) were synthesized through polymerization of urea tethered imidazolium ionic liquid monomers (urea-IL) with sodium acrylate, and N,N '-methylenebisacrylamide (MBA) as a crosslinker. Close-packed and interconnected pores (1-4 mu m ) under swollen state could be observed from the cryogenic scanning electron microscopy (cryo-SEM) images. The promising catalytic activity of the PILs was illustrated for the transesterification reaction of ethylene carbonate with methanol. High activity and selectivity could be achieved by using poly(urea-IL)-n catalysts, which was similar to that of corresponding homogeneous ionic liquid catalysts. The urea tethered imidazolium in PILs acted as hydrogen-bonding donor to activate ethylene carbonate and intermediate 2-hydroxyethyl methyl carbonate (HEMC) for enhancing catalytic activity. The swelling ability of urea-functionalized PILs in methanol enabled active urea sites accessible for substrates. However, the complete conversion of ethylene carbonate was limited by reversible reaction between ethylene carbonate and HEMC. A possible synergistic activation mechanism for the transesterification reaction was proposed and supported by NMR titrations. The catalyst can be reused and recycled five times with stable activity. Furthermore, urea-functionalized swelling PILs also exhibited high catalytic activity for the hydrolysis of ethylene carbonate.