Immobilized Catalysts
It may be moreaccurate to refer to many of them as catalysts attached to polymers. Such catalysts can be inorganic compounds, like, for instance, Lewis acids attached to organic polymers. They can also be organic or biochemical catalysts. Perhaps the biggest group among such attached catalysts are immobilized enzymes. They are used in industrial processes as well as in research laboratories. Immobilization often improves stability, and in some rare cases, activity over a broader range of pH and temperatures. Another advantage is elimination of enzyme contamination of waste streams. On the other hand, immobilized enzymes can often be less active after immobilization.
Immobilized Enzymes Several major techniques of enzyme immobilization are used. One important one is covalent bonding of the enzyme to a support material. Such attachment usually consists of reacting some functional group of the enzyme, not active in the enzymatic process, with a functional group on another polymer that is the carrier. Hydrophilic groups are preferred for reactions with enzymes in aqueous media. An immobilization of an enzyme on cellulose with azide groups [42] attached can serve as an illustration. Carboxymethyl cellulose is the starting material:
In other techniques, the protein may be bound by some copolymer of maleic anhydride, where the anhydride groups react with some available amine groups on the enzyme [42]. Other techniques may utilize cyanuric chloride attached to polysaccharides for immobilization [42]:
Polyaminostyrene can be diazotized or treated with thiophosgene and then used in enzyme immobilization [43]:
Because epoxy groups on carrier molecules are capable of reacting with an amine or a carboxylic acid groups of the enzymes, they can be used in enzyme immobilization. A variation on the technique is to react a vinyl monomer that contains an epoxy groups, like glycidyl methacrylate or glycidyl acrylate, with the enzyme first. The product is then polymerized or copolymerized through the vinyl portion [44]. As stated earlier, in many cases, immobilization of enzymes is accompanied by some loss of activity. In some instance, the loss in activity can be severe. A special technique, however, was developed [45], where the enzyme called protease is immobilized on a polymer with amino glucose units to form covalently bonded carbohydrate-protein conjugates. In aqueous solution, the conjugated enzymes show about the same catalytic activity as native enzymes. At elevated temperatures, however, they exhibit enhanced stability. In addition, they are capable of catalyzing reactions in organic solvents that denature and inactivate the native enzymes [45].
Hiroshi et al. reported [46] a high-performance immobilized lipase catalyst for polyester synthesis. A porous polypropylene was found to be a good support for immobilization of Candida antarctica lipase (enzyme). The immobilized lipase on polypropylene efficiently catalyzes ring opening polymerization of 15-pentadecanolide, polycondensation of divinyl sebacate, and 1,8-octanediol.
