Novel Polymer Synthesis using Enzymatic Catalysis in Nonaqueous Media
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NOVEL POLYMER SYNTHESIS USING ENZYMATIC CATALYSIS IN NONAQUEOUS MEDIA JONATHAN S. DORDICK*, DAMODAR. R. PATIL, KEUNGARP RYU, AND DAVID G. RETHWISCH.
*Department of Chemical and Biochemical Engineering, University of Iowa, Iowa City, IA 52242. ABSTRACT
The use of enzymes in organic solvents has enabled two distinct polymer syntheses to be performed. The first is the enzyme-catalyzed polycondensation of sucrose with an adipic acid derivative in pyridine. Taking advantage of the extremely high regioselectivity of proleather, an alkaline protease, sucrose has been polymerized with 6 and 1' ester linkages. The result is a highly water-soluble product with sucrose an integral part of the polymer backbone. The polyester is degraded by the proleather in aqueous solution, thereby suggesting that the polymer is biodegradable. The second polymer synthesis is the free-radical polymerization of phenols using horseradish peroxidase in dioxane-water mixtures. Molecular weights in excess of 25,000 have been produced. The scope of polymerization can be predicted based on enzyme kinetics in organic media and using a Monte Carlo simulation. Astonishingly accurate simulations have been run that suggest that the high degree of control afforded by enzymatic catalysis over conventional chemical catalysts can be usedpredictively in polymer synthesis. This is of great importance in process control of polymer syntheses. INTRODUCTION The synthesis of commercially useful polymers has generally been outside the realm of biocatalysis. Poor solubilities of both substrates and growing polymer chains in aqueous solutions result in low productivities and high processing costs. Chemical alternatives are cheaper and simpler. Selective (both regio- and stereo-) polymer synthesis is difficult, however, using conventional chemical approaches. We have used enzymes in organic media to overcome several catalytic constraints inherent in the synthesis of sugar-based polyesters and phenolic resins. The former has applications as biodegradable plastics, while the latter has been used as a biocompatible replacement for phenol-formaldehyde plastics. EXAMPLES OF ENZYME-CATALYZED POLYMER SYNTHESIS Enzyme-Catalyzed Synthesis of Sucrose-Based Polyesters The specificity of enzymes in organic solvents is clearly evident in the regioselective acylations of multifunctional molecules. These compounds range from the relatively simple aliphatic glycols to complex oligosaccharides. Chemical syntheses universally lack regiospecificity, giving mixtures of products [ 1,2] and requiring expensive protection and deprotection steps for regiospecific synthesis [3,4]. This is no more apparent than in the synthesis of sugar esters. Chemical methods include basecatalyzed esterifications and acylations with anhydrides or acyl halides. Multifunctional acylations result that reduce the yield of the desired sugar ester. Separation of the desired ester from a mixture of mono-, di, and oligoesters is expensive and tedious. In order to overcome the difficulty associated with selective che
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