Enzyme-Catalyzed Polycondensations

Although there are notable exceptions as given below, the most common lipase-catalyst used for polyester synthesis is Candida antarctica lipase B (CALB) (please refer to Chapter 14 for more information on the structure and reaction mechanisms of CALB). The immobilized CALB catalyst that has been primarily used is Novozym 435, manufactured by Novozymes (Bagsvaerd, Denmark). Novozym 435 consists of CALB physically adsorbed within the macroporous resin Lewatit VPOC 1600 (poly[methyl methacrylate-co-butyl methacrylate], supplied by Bayer) (please refer to Chapter 3 for more information on Novozym 435). 

1) polycondensation between a carboxyl group and an alcohol group (following route (i) or route (ii)), and 

The enzymatic condensation reaction to form an ester using enzymes is composed of four modes of elemental reactions (i) dehydration (ii) alcoholysis  

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Rodney, R.L., Allinson, B.T., Beckman, E.J., and Russell, A.). (1999) Enzyme-catalyzed polycondensation reactions for the synthesis of aromatic polycarbonates and polyesters. Biotechnol. Bioeng., 65 (4), 485-489. 

Important progress has been made on the use of immobilized enzyme catalyzed polycondensation reactions to prepare a wide range of polyesters (2,83). In this book, Hunsen et al (20) describe how an inunobilized catalyst from Humilica insolens has excellent activity for polycondensation reactions between diols and diacids. The natural role of cutinases is hydrolysis of the outer cutin polyester layer on plant leaves. Cutin is made of long chain... 

Figure 3. Other polyamides that can be synthesized via enzyme-catalyzed polycondensation. TETA = triethylene tetraamine,...

A study has been presented that describes the use of an epoxy functional fatty acid, 9,10-epoxy-18-hydroxyoctadecanoic acid, extracted from birch Betula pendula) outer bark to produce renewable thermosets (93). An epoxy fatty acid was polymerized by enzyme catalyzed polycondensation utilizing Candida antarctica lipase B to form oligomers with degrees low of pol5merization up to 9. The crosslinked polyesters form insoluble, amorphous, and transparent films. So it was demonstrated that thermoset films with tailored properties can be effectively made using forest products. 

In this work lipase was found to be an effective catalyst for the polycondensation of a diester with a diamine to form a polyamide. The reaction is relatively mild and can be achieved at a high yield at relatively low temperatures. The resulting polyamide is amenable to derivatization reactions, thereby generating structures that can impart additional properties to the polymer. This enzyme-catalyzed polymerization reaction is applicable to a wide range of water-soluble polyamides, including several that cannot be previously prepared via chemical methods. 

PEAs have been synthesized by ring-opening polymerization and polycondensation methods. The first ones were mainly employed to get copolymers of a-hydroxy acids and a-amino acids (i.e., polydepsipeptides) and reported in the literature [4]. Recent works are focused to the use of enzymes (e.g., lipases) as new efficient catalysts for reaction of these morpholine-2,5-diones [5]. It has been demonstrated that the configuration of the a-amino acid moiety did not affect the enzyme-catalyzed polymerization, but in contrast, the configuration of the a-hydroxy acid moiety strongly in-flnenced the polymerization behavior. Unfortunately, ra-cemization of both units was demonstrated to take place dnring polymerization. 

Polymer Production. The high selectivity of enzymes allows the production of polymers with high chemical, regio- or stereoselectivity (61-63). Polyesters can be obtained through lipase-catalyzed polycondensation of hydroxyacids or of diols with diacids, as shown in Figure 12. Lipases can also catalyze the ringopening polymerization of lactones of various ring sizes. Lipases can also be used to produce monomers, difficult to obtain chemically, that are then polymerized. For instance, various monosaccharides were esterified with vinyl acrylate. 

Keywords synthesis, PLA, polycondensation, azeotropic dehydrative polycondensation, ring-opening polymerization, enzyme-catalyzed polymerization, molecular weight, kinetics, mechanism... 

S. Kobayashi, H. Ritter, D. Kaplan, H. Uyama and S. Kobayashi, Enzymatic Synthesis of Polyesters via Polycondensation in Enzyme-Catalyzed Synthesis of Polymers, Springer Berlin/Heidelberg, 2006, p. 133. 

FIGURE 2. Glassy silica needles (2 mm x 30 gm) produced in copious amount by a marine sponge. Each needle contains an occluded axial filament comprised of silicateins, enzyme-like proteins that catalyze and spatially direct the polycondensation of silicon alkoxides and silicic acid at neutral pjj41,42 Reprinted from Reference 3, copyright (1999), with permission from Elsevier Science... 

Inspired by the fmding that HiC is active for polycondensation polymerizations, studies were performed to assess HiC activity for lactone ringopening polymerizations (Scheme 2). HiC-catalyzed ROP was carried out in either bulk or toluene. To determine the relationship between reaction temperature and enzyme activity for lactone ring-opening polymerizations, e-caprolactone (1) in bulk was taken as the model system. 

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. 

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