Designing Ionic Liquids For Enzymatic Ring-opening Polymerization Ofε-Caprolactone

Ionic Liquids (ILs) have been promoted as "green solvents" due to their low volatility, good chemical and thermal stability, a wide range of solubility for various compounds and also high designability. In recent years, ILs have been popularly used in organic synthesis and catalyst, separation and extraction, nanotechnology, biotechnological and engineering processes. On the other hand, enzymatic polymerization has been receiving more and more attention as a new environmentally friendly method of polymer synthesis. Novozyme-435, Candida antarctica lipase B immobilized on macroporous polyacrylate resin, is one of the extensively studied enzyme for polyester synthesis. Recently, some authors have reported increased stability and activity of enzymes in ILs compared with common organic solvents. In the first chapter of this thesis, recent developments of ILs and enzymatic reactions in ILs are briefly reviewed.Poly(s-caprolactone)(PCL) is one of the most important environmentally biodegradable polymers due to its facile accessibility and good biodegradability. The ring-opening polymerization (ROP) of the cyclic monomer ε-caprolactone (ε-CL) is the most popular pathway of PCL synthesis. In this thesis, designing imidazole-based ILs were synthesized and used as solvents for enzymatic ring-opening polymerization of ε-CL using Novozyme-435as the catalyst. The structures of ILs determined their physicochemical properties, while further resulted in their different properties as the reaction media.In the second chapter, monocationic ionic liquids (MILs)[CnMIm][X](X=BF4、PF6、 NTf2)(n=2,6,12) were synthesized and characterized by’H-NMR,13C-NMR, IR and EA methods. MILs with different alkyl chain length at the3-position of the imidazolium ring were then evaluated as the media for the enzymatic ROP of ε-CL No polymers could be obtained within [BF4]-based MILs, while better results could be reached within [NTf2]-based MILs. At reaction temperature, with the increase of the alkyl chain length from2to6and12, the viscosity of MILs obviously increase, which has a positive effect on enzyme activity but also a negative effect on mass-transfer of rate. Among the three kinds of [CnMIm][X](n=2,6,12),[C6MIm]-based MILs would be the best solvent for the enzymatic synthesis of PCLIn the third chapter, dicationic ionic liquids(DILs)[C4(CnIm)2][X]2(X=BF、PF6、NTf2)(n=2,6,12) were designed and successfully synthesized. At the same temperature, the viscosities of DILs are obviously higher than that of MILs. Novozyme-435also couldn’t catalyze ε-CL polymerization within [BF4]-based DILs, while relatively high molecular weight PCL could be obtained within [PF6]-based DILs. Compared with MILs, high viscosity and low anion molar concentration of DILs would help to maintain enzyme stability as well as activity.In the fourth chapter, DILs functionalized with poly(ethylene glycol)(PEG) linkages were synthesized and characterized. Compared with MILs and DILs synthesized in chapter2and chapter3, enzymatic polymerization of ε-CL could be effectively performed at lower temperature within [PEG(MIm)2][NTf2]2, especially in th case of [PEG200(MIm)2][NTf2]2. In additon, after5cycles,[PEG200(MIm)2][NTf2]2could be still an effective solvent for PCL synthesis.In the fifth chapter, ILs-coated Novozyme-435were prepared and charactered by Scanning Electron Microscopy (SEM) and Energy Dispersive X-Ray Spectroscopy (EDX). Solvent free polymerization catalyzed by ILs-coated Novozyme-435was compared with the use of ILs catalyzed by Novozyme-435, while the former showed a high catalytic activity. Moreover, in organic solvent (toluene), ILs-coated Novozyme-435also showed both high activity and good recyclability than Novozyme-435.