Polyesters hydrolase-catalyzed

Hydrolase-Catalyzed Modification of Polymers. Terminal hydroxy group of poly(e-CL) was reacted with carboxylic acids using hpase CA catalyst to give end-fimctionalized polyesters (181). Lipase MM catalyzed the regioselec-tive transesterification of the terminal ester group of oligo(methyl methacrylate) with allyl alcohol (201). 

Lipases are enzymes of the hydrolase family and, in nature, hydrolyze fatty acid esters in aqueous environment. It is worth recalling that the hydrolysis of esters is a reversible reaction. Chemists thus often use lipases to catalyze the reverse reaction, i.e., the esterification and the ROP of lactones. In 1993, the groups of Kobayashi [91] and Knani [92] reported independently the hpase-catalyzed ROP of sCL and 8-valerolactone. The aliphatic polyesters were functionalized by a carboxylic group at one chain-end and a hydroxyl group at the other chain-end. Accordingly, the polymerization was initiated and terminated by water present in the reaction media. 

At the time of writing, the applications of biodegradable polymers are confined mostly to the field of agriculture, where they are used in products with limited lifetimes, such as mulch films and pellets for the controlled release of herbicides. The synthetic polyesters used in medical applications, principally polylactide and poly(lactide-co-glycolide), while claimed to be biodegradable, are degraded in the body mainly, if not entirely, by chemical hydrolysis. There is little evidence that the hydrolysis of these polyesters of a-hydroxyacids can be catalyzed by hydrolase or depolymerase enzymes. 

In nature, hydrolases are used to degrade or hydrolyze particular functionalities, e.g., lipases to hydrolyze esters and proteases to hydrolyze peptide (amide) linkages (J). It is well known that under suitable reaction conditions lipase can catalyze reverse reactions to form polyesters 4,5). Some of these methods have relied upon the use of organic solvents, whereas others are solvent-free processes. However, it is not previously known that proteases can catalyze reverse reactions to form amide polymers. 

According to the advantages mentioned above, enzyme-catalyzed polymer syntheses are also of increasing interest. Typical examples of enzymatically produced macromolecules are the synthesis of polysaccharides and polyesters using hydrolases, the production of biopolymers like cellulose, xylan, chitin, or hgnin by hydrolases or peroxidases, and the synthesis of polyaromatic... 

Enzymatic polymerization has emerged in the last few decades as a field of considerable interest and commercial promises. It proceeds with high regio-, enantio-, and chemos-electivity under relatively mild conditions. So far, enzymes have been used to synthesize polyesters, polysaccharides, polycarbonates, polyphenols, polyanilines, vinyl polymers, and poly(amino acid)s. Namely, the lipase B of Candida antarctica (Cal-B, a serine hydrolase) immobilized on polyacrylic resin (Novozyme 435) has proven to be a very versatile catalyst in terms of reaction conditions and acceptance of various substrates. For example, this enzyme has been successfully used to synthesize polyesters. ° However, little has been reported so far on the synthesis of polyamides catalyzed by enzymes. " ... 

Based on the specific reaction that they catalyze, enzymes have been classified into six groups, three of which have been reported to catalyze or induce polymerization in vitro, namely oxidoreductases, transferases and hydrolases. The latter class includes hpases, the natural role of which is the hydrolysis of fatty acid esters at the cell s water-Upid interface. In organic media, hpases can efficiently catalyze ester bond formation, and so have been used extensively in investigations of the in vitro synthesis of polyester by polycondensation or ring-opening polymerization (ROP), without the need for any cocatalyst. One enzyme that deserves special attention when discussing enzymatic ROP is Candida antarctica Lipase B (CALB). 

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