Enzymatic polycondensation

Vacuum was applied to shift the equilibrium forward by removal of the activated alcohol formed [30, 31, 37, 38]. In the enzymatic polycondensation of bis(2,2,2-trifluoroethyl) sebacate and aliphatic diols, the polymer with Mw of more than 1 x 104 was obtained using lipases CC, MM, PPL, and Pseudomonas cepacia lipase (lipase PC) as catalyst and lipase MM showed the highest catalytic activity [37]. Solvent screening indicated that diphenyl ether and veratrole were suitable for the production of the high molecular weight polyesters under vacuum. In the PPL-catalyzed reaction of bis(2,2,2-trifluoroethyl) glutarate with 1,4-butanediol in veratrole or 1,3-dimethoxybenzene, periodical vacuum method improved the molecular weight (Mw 4 x 104) [38]. 

Galbis et al. described a variety of carbohydrate-based linear polyesters 61 of the poly(alkylene dicarboxylate) type that were obtained by polycondensation reactions of the alditols 2,3,4-tri-(9-methyl-L-arabinitol (9) and 2,3,4-tri-O-methyl-xylitol (10), and the aldaric acids 2,3,4-tri-(9-methyl-L-arabinaric acid (26) and 2,3,4-tri-(9-methyl-xylaric acid (27), butanediol, and adipic acid were also used as comonomers [28]. Copolyesters of the poly(aIkylene-c )-arylene dicarboxylate) type were obtained using bisphenols as comonomers (Scheme 1). Chemical polycondensation reactions were conducted in bulk or in solution. Enzymatic polycondensation reactions of adipic acid with the above-mentioned alditols were carried out successfully using Lipozyme and Novozyme 435. The hydrolytic degradations of some of these polyesters were also described. 

Enzymatic polymerization has been combined with various chemical polymerizations for the synthesis of block copolymers. The choice of chemical polymerization generally depends on the applied strategy for the block copolymer synthesis. These can be divided into three main approaches, as shown in Fig. 4 for the example of enzymatic ROP. It has to be noted that some of these strategies have also been applied for enzymatic polycondensations. 

Sha et al. applied the commercially available dual initiator ATRP-4 for the chemoenzymatic synthesis of block copolymers. In a first series of publications, the group reported the successful synthesis of a block copolymer comprising PCL and polystyrene (PS) blocks [31, 32]. This concept was then further applied for the chemoenzymatic synthesis of amphiphilic block copolymers by macroinitiation of glycidyl methacrylate (GMA) from the ATRP functional PCL [33]. This procedure yielded well-defined block copolymers, which formed micelles in aqueous solution. Sha et al. were also the first to apply the dual enzyme/ATRP initiator concept to an enzymatic polycondensation of 10-hydroxydecanoic acid [34]. This concept was then extended to the ATRP of GMA and the formation of vesicles from the corresponding block copolymer [35]. 

Cellulase was found to be effective in the synthesis of artificial cellulose in a single-step reaction by polycondensation of /J-D-cellobiosyl fluoride (Scheme 13).123 The polymerization is a repetition of the transglycosylation reaction, which became predominant over the hydrolysis reaction when the enzymatic polycondensation was carried out in a mixed solvent of acetonitrile/acetate buffer (5 1, pH 5). This synthesis is therefore kinetically controlled as well as equilibrium controlled. The fi configuration of the Cl fluorine atom is necessary to form a reactive intermediate leading to a / (1—4) product via a double displacement mechanism .124 Thus, this method provided the first successful in vitro synthesis of cellulose, the most abundant biomacromolecules on the earth, the synthesis of which had been unsolved for one-half a century.123... 

Xylan was prepared by enzymatic polycondensation similar to the case of artificial cellulose.129 /i-Xylo-... 

While all previous examples employ enzymatic ROP there are two reports on the block copolymer synthesis using enzymatic polycondensation. The first was published by Gross and Scandola and describes the synthesis and solid-state properties of polyesteramides with poly(dimethylsiloxane) (PDMS) blocks (Figure 12.3) [11]. The polycondensation was carried out with various... 

Figure 12.3 Poly(dimethyl siloxane) polyester amide block copolymers by enzymatic polycondensation of (diaminopropyl)polydimethylsiloxanes, diethyl adipate, and 1,8-octanediol [11].

Hilker et al (44) combined dynamic kinetic resolution with enzymatic polycondensation reactions to synthesize chiral polyesters from dimethyl adipate and racemic secondary diols. The concept offered an efficient route for the one-pot synthesis of chiral polymers from racemic monomers. Palmans at al (18,43) generalized the approach to Iterative Tandem Catalysis (ITC), in which chain growth during polymerization was effected by two or more intrinsically different catalytic processes that were compatible and complementary. 

The enzymatic polycondensation reaction has several advantages over the chemical process (Table 1). It is carried out at lower temperatures (50-110°C... 

Table 1. Comparison of chemical versus enzymatic polycondensation ...

Another approach of enzymatic synthesis of sugar-containing polyesters was demonstrated (123). Lipase CA-catalyzed reaction of sucrose or trehalose with an excess of divinyl adipate produced 6,6 -diacylated product having vinyl esters at both ends, which was employed as monomer in the enzymatic polycondensation with various glycols, jdelding linear polyesters with Mw up to 2.2x 10. ... 

Spicule-forming cells (sclerocytes) into the growing and elongating axial canal. The experiments showed that, around a cell extension that protrudes from a sclerocyte into the axial canal of a given spicule, silicatein molecules are released into the extracellular space of the axial canal (the space between the cell membrane and the inner surface of the siliceous mantel) to catalyze biosilica deposition from the inner surface. This causes the axial canal to narrow from >1 to <0.5 pm. Intracellularly, both the enzyme and its substrate (siUca precursor) are stored in vesicles that have been termed silicasomes (MiiUer et al., 2009a,b). These vesicles are released into the axial canal to allow the enzymatic polycondensation reaction. 

Supercritical carbon dioxide (SCCO2) was employed as solvent for the polycondensation of divinyl adipate and 1,4-butanediol [68]. Quantitative consumption of both monomers was achieved to yield a polyester with a molecular weight of 3.9 X 10, indicating that SCCO2 was a good medium for enzymatic polycondensation. 

Enzymatic polycondensation ttf a dicarboxylic add and a died has been reported using lipase from Candida cyUndracea, Pseudomonas and Porcine pancreas [34]. Competitk>n between linear polye r and macrocydic lactone formation was observed depei ing on the reactitm temperature. 

Supercritical carbon dioxide (SCCO2) was employed for the first time to prepare polyesters via ROP of lactones. Lipase-catalyzed ROP of e-CL proceeded to give a polymer (PCL) with molecular weight higher than 10". Copolymerization of e-CL with DDL afforded a random copolyester. The enzymatic polycondensation between divinyl adipate and 1,4-butane diol also took place to produce the corresponding polyester [73]. Later, a similar study on ROP of e-CL in SCCO2 followed [74]. 

The above enzymatic polycondensation reaction on 100 was reported later on to form the p- and y-CD analogues. 6 -0-Acetylmalotosyl fluoride was also found to undergo the enzymatic reaction to afford 6 , 6, 6 -tri-0-acetylcyclohexaamylose, although in 3% yield." ... 

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