Immobilized Enzymes:Preparations And Applications For Polyester And Polycarbonate Synthesis In Ionic Liquids

Polyester and polycarbonate are two main kinds of biodegradable and biocompatible polymers which have been extensively researched for medical applications. Their most common and effective synthetic method is ring-opening polymerization. As an attractive new methodology that provides biodegradable polymer synthesis without the use of toxic catalysts, enzymatic polymerization has been receiving more and more attention. Enzyme immobilization is the most important route that can offer enzyme many favorable advantages such as increased enzyme activity and improved stability. The activity and stability of the immobilized enzymes are closely related to the properties of supports, such as pore size, chemical durability, capacity for ligand binding, etc. On the other hand, as green solvents, ionic liquids (ILs) have been widely studied due to their low volatility, chemical stability as well as good solubility of many organic or inorganic compounds. Some researchers have presented that enzymes show both excellent operational and thermal stability in anhydrous ILs. In the first chapter of this thesis, recent developments of the immobilized enzymes and polymerization reactions in ILs are briefly reviewed.In the second chapter, porous silica particles (PSP) were modified through silanization process with different silane coupling agents, such as (3-aminopropyl)triethoxysilane (APTES) and (y-glycidoxypropyl)trimethoxysilane (GPTMS). Furthermore,(n-dodecyl)trimethoxysilane (DTMS) and low molecular weight polyethylenimine (PEI) were also employed to modify the surface properties of PSP. Then, PPL was immobilized on these five kinds of modified PSP. The comparison between different chemically immobilized enzymes was done in terms of functional group, enzyme loading and catalytic performance using a standard hydrolytic assay (hydrolysis of olive oil). Introducing long chain alkyl groups on the surface of carrier resulted in the higher hydrophobicity and lower accessibility of carrier, while a suitable hydrophilic modification would be favorable to improve lipase activity.Poly(ε-caprolactone)(PCL) is one of the most important environmentally biodegradable polyester due to its facile accessibility and good biodegradability. In the third chapter, the five kinds of IPPL mentioned above were employed to perform the ring-opening polymerization of s-caprolactone (ε-CL) in ionic liquid,1-butyl-3-methylimidazolium hexaflurophospate (abbreviated as BMIMPF6). IPPL showed much higher activity in ILs for enzymatic ring-opening polymerization of ε-CL compared to that in bulk. However, only a slight difference between the five types of immobilized enzymes could be found regarding polymerization reaction yield and the molecular weight of resulting PCL. The results also suggested that good combination of immobilization and ionic liquids effect led to the successful enzymatic synthesis of PCL at higher temperature. Poly(5,5-dimethyl-1,3-dioxan-2-one)(PDTC) is a kind of important biodegradable polycarbonate. In the fourth chapter, PDTC was successfully synthesized by the five different kinds of IPPL. IPPL-2showed the highest catalytic activity for the ring-opening polymerization of DTC. In comparison with ε-CL, the higher interligand distance within IPPL-2would make a great contribution to the enzymatic polymerization process, considering steric hindrance of the side groups of DTC monomer. Moreover, IPPL-1lost its good catalytic activity towards DTC, while IPPL-3showed the lowest catalytic activity towards DTC. The dodecyl groups on the IPPL-3’surface resulted in a much lower accessibility of substrate (monomer) molecules to enzyme.In the fifth chapter, five kinds of IPPL with different functional groups and also different groups distribution were also used to catalyze the polymerization of another kind of cyclic carbonate with a protected functional group,5-methyl-5-benzyloxycarbonyl-trimethylene carbonate (MBC). IPPL-1showed better catalytic activity towards MBC. IPPL-2and IPPL-3showed low activity because of higher steric hindrance caused by the dodecyl groups and benzyloxycarbonyl groups. IPPL-4and IPPL-5presented poor activity considering hydrophobic-hydrophilic mismatch between immobilized enzyme and substrate (MBC).In the sixth chapter, considering good stability of enzymes in ILs, a novel IL with silyl groups ([TMSP-MIm][PF6]) was successfully synthesized for the modification of PSP together with APTES. Then PPL was covalently immobilized on this new modified PSP using glutaraldehyde as the crosslinking agent. The resulting IPPL (IPPL-6) was also evaluated as the catalyst for the enzymatic polymerization of ε-CL, DTC and MBC. Compared with other five kinds of IPPL, IPPL-6presented the highest catalytic activity for PCL synthesis. However, for PDTC synthesis, IPPL-6only showed almost the same activity as IPPL-3. In our experiments, IPPL-6could not catalyze the ring-opening polymerization of MBC. The ILs molecules on the IPPL-6’surface would contribute to increasing the enzyme stability, while also result in the higher steric hindrance and lower accessibility.