Enzymatic Polyester Synthesis

Finally, polyesters can also be synthesised via enzymatic processes. This type of polymerisations possesses some advantages over the earlier described chemical synthesis routes, including mild reaction conditions, a higher selectivity, a high tolerance of functional groups and the synthesis of pure reaction products which are metal free [79, 80]. However, reaction times are longer and yields lower compared to chain growth polymerisations. 

Poly(jS-hydroxyalkanoates) (PHAs) are a class of poly(esters) which are synthesised by many bacteria as energy reserve and carbon source when exposed to an excess of carbon under unbalanced growth conditions [82]. The produced PHAs can account for up to 80% of the bacterium s dry weight. This has led to the biotechnological production of PHAs. Depending on the substrates provided to the bacteria, different monomers can be produced which results in different PHA (co)polymers [83]. 

Poly[3-hydroxybutyrate] was the first PHA to be produced on an industrial scale, but its brittle nature, its poor mechanical properties and its high production cost limited its application potential. In the early 1990s, Imperial Chemical Industries [ICI] started the production of poly[3-hydroxybutyrate-co-3-hydroxyvalerate] [P3HB3HV] under the trade name Biopol . This material showed lower degrees of crystallinity and superior mechanical properties. Later on, the production of Biopol was continued by Monsanto and subsequently followed up by Metabolix. PHAs were originally intended as bio-based alternatives for polyolefins used in plastic containers, films and bottles. Despite the large interest in PHAs, their application remains, however, limited due to their narrow processing window [84, 85]. 

In addition to the research being done on the incorporation of a larger monomer variety in /3-hydroxyalkanoates, a mutant form of the PHA synthase enzyme was observed to enable the polymerisation of 2-hydroxyacids and the incorporation of lactic acid in the polymer backbone [80]. The properties of these materials were found to be distinctly different from those of the PHAs synthesised starting from 3-hydroxyacids. 

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Kim D-Y, Dordick JS (2001) Combinatorial array-based enzymatic polyester synthesis. Biotechnol Bioeng 76 200-206... 

Marcilla R, De Geus M, Mecerreyes D et al (2006) Enzymatic polyester synthesis in ionic liquids. Eur Polym J 42 1215-1221... 

In the present chapter, the current status of enzymatic polyester synthesis is described. For information on the enzymatic synthesis of chiral polyesters and polyester block copolymers using enzymatic polymerizations please refer to Chapters 11 and 12 respectively. 

Various substituted lactones were used for enzymatic polyester synthesis via ROP ( )-a-methyl- 3-propiolactone [101], P-methyl-P-propiolactone [78], a-decenyl-P-propiolactone [27], a-dodecenyl-P-propiolactone [46],benzyl-P-D,L-malonolactonate [104], a-methyl-e-caprolactone [96], oc-methyl-8-valerolactone [96], l,4-dioxane-2-one [91], and others (see also Table 4.2). 

The most extensively studied enzymatic polymerization system is that of polyesters via polycondensations or ring-opening polymerizations. The state of the art of in vitro enzymatic polyester synthesis is reviewed in Chapter 4. 

In polyester synthesis via ring-opening polymerizations, metal catalysts are often used. For medical applications of polyesters, however, there has been concern about harmful effects of the metallic residues. Enzymatic synthesis of a metal-free polyester was demonstrated by the polymerization of l,4-dioxan-2-one using Candida antarctica lipase (lipase CA). Under appropriate reaction conditions, the high molecular weight polymer (molecular weight = 4.1 x 10" ) was obtained. 

In the case of polyester synthesis from divinyl esters, hydrolysis of the vinyl end group partly took place, resulting in the limitation of the polymer growth.201 A mathematical model showing the kinetics of the polymerization predicts the product composition. On the basis of these data, a batch-stirred reactor was designed to minimize temperature and mass-transfer effects.202 The efficient enzymatic production of polyesters was achieved using this reactor poly(l,4-butylene adipate) with Mn 2 x 104 was synthesized in 1 h at 60 °C. 

Enzymatic polymerizations are a promising strategy under study by many groups throughout the world to develop environmental friendly processes for polyester synthesis. 

AcetoacetylCoA thiolase (E.C. 2.3.1.9), acetoacetylCoA reductase (E.C. 1.1.1.36), and polyhydroxybutyrate synthetase12471 are the enzymes involved in polyester synthesis. AcetoacetylCoA thiolase catalyzes the head-to-tail Claisen condensation of two acetylCoA molecules. In this reaction, the active site cysteine attacks acetylCoA to form a thioester enzyme intermediate, which then reacts with the enolate derived from enzymatic deprotonation of the other acetylCoA. Mechanistic studies have been performed on this enzyme from Zooglea ramigera, which has been cloned and overexpressed12471. It has been established that the thiolase will form acyl enzyme intermediates with a number of acylCoA substrates, but will only accept acetylCoA as the nucleophile. After subsequent reduction, this results in all polymer units possessing a P-hydroxy group. These polymers are also useful sources of (R)-P-hydroxy acids[2481. 

In recent years, the enzymatic synthesis of biodegradable polyesters was focused on the polycondensation method (22,23). Among the very few successful example of enzymatic ring-opening polymerization for polyesters synthesis, Novozyme-435 (immobilized lipase B from Candida antartica) has been proved an effective catalyst for polycaprolactone (PCL) synthesis in toluene (24). Considering the low cost and high recyclablity of IPPL, we also... 

On the other hand, highly selective enzymatic catalysis has been used to modify sugars containing multiple hydroxyl groups in microaqueous media. Enzymes are non-harmful catalysts which catalyze under mild conditions. Recent advances in enzymatic catalysts would be useful tools not only for polyester synthesis but also for vinyl monomer synthesis (96). 

Another field of enzymatic polymer synthesis is the enzyme-catalyzed modification of preformed polymers by esterification or transesterification. Thereby, it is possible to either introduce functional side groups into an existing polymer with a stable backbone (no polyester) to synthesize functional homopolymers as well as random copolymers or to generate multiblock copolymers by enzymatic transesterification between two different homopolymers. 

Kobayashi S, Uyama H (2002), In vitro polyester synthesis via enzymatic polymerization , Curr Org Chem, 6, 209-22. 

More recently, Heise and coworkers have shown that DKR can be combined with enzymatic polymerization for the synthesis of chiral polyesters from racemic secondary diols in one pot [34] (Figure 4.12). 

Nagata M, Kiyotsukuri T, Ibuki H, Tsutsumi N, and Sakai W. Synthesis and enzymatic degradation of regular network aliphatic polyesters. React Fund Polym, 1996, 30, 165-171. 

Enzymes are generally classified into six groups. Table 1 shows typical polymers produced with catalysis by respective enzymes. The target macromolecules for the enzymatic polymerization have been polysaccharides, poly(amino acid)s, polyesters, polycarbonates, phenolic polymers, poly(aniline)s, vinyl polymers, etc. In the standpoint of potential industrial applications, this chapter deals with recent topics on enzymatic synthesis of polyesters and phenolic polymers by using enzymes as catalyst. 

Enzymatic synthesis of aliphatic polyesters was also achieved by the ringopening polymerization of cyclic diesters. Lactide was not enzymatically polymerized under mild reaction conditions however, poly(lacfic acid) with the molecular weight higher than 1 x 10" was formed using lipase BC as catalyst at higher temperatures (80-130°C). Protease (proteinase K) also induced the polymerization however, the catalytic activity was relatively low. 

The enzymatic synthesis of polyesters from activated diesters was achieved under mild reaction conditions. The polymerization of bis(2,2,2-trichloroethyl) glutarate and 1,4-butanediol proceeded in the presence of PPL at room temperature in diethyl ether to produce the polyesters with molecular weight of 8.2 x 10. Vacuum was applied to shift the equilibrium forward by removal of the activated alcohol formed, leading to the production of high molecular weight polyesters. The polycondensation of bis(2,2,2-trifluoroethyl) sebacate and aliphatic diols took place using lipases BC, CR, MM, and PPL as catalyst in diphenyl ether. Under the... 

Lipase-catalyzed synthesis of polyesters from cyclic anhydrides and oxi-ranes was reported. The polymerization took place by PPL catalyst and the molecular weight reached 1 x 10" under the selected reaction conditions. During the polymerizahon, the enzymatically formed acid group from the anhydride may open the oxirane ring to give a glycol, which is then reacted with the anhydride or acid by lipase catalysis, yielding the polyesters. 

Epoxide-containing polyesters were enzymatically synthesized via two routes using unsaturated fatty acids as starting substrate (Scheme 11)." Lipase catalysis was used for both polycondensation and epoxidation of unsaturated fatty acid group. One route was synthesis of aliphatic polyesters containing an... 

Chemoenzymatic synthesis of alkyds (oil-based polyester resins) was demonstrated. PPL-catalyzed transesterification of triglycerides with an excess of 1,4-cyclohexanedimethanol mainly produced 2-monoglycerides, followed by thermal polymerization with phthalic anhydride to give the alkyd resins with molecular weight of several thousands. The reaction of the enzymatically obtained alcoholysis product with toluene diisocyanate produced the alkyd-urethane. 

The enzymatic polymerization of 12-hydroxydodecanoic acid in the presence of 11-methacryloylaminoundecanoic acid conveniently produced the methacrylamide-type polyester macromonomer. Lipases CA and CC were active for the macromonomer synthesis. Enzymatic selective monosubstitution of a hydroxy-functional dendrimer was demonstrated. Lipase CA-catalyzed polymerization of 8-CL in the presence of the first generation dendrimer gave the poly(8-CL)-monosubstituted dendrimer. 

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