Enzyme-Catalyzed Direct Polyesterification - ACS Symposium Series

Chapter 13

Enzyme-Catalyzed Direct Polyesterification 1

2

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on November 5, 2015 | http://pubs.acs.org Publication Date: December 10, 2002 | doi: 10.1021/bk-2003-0840.ch013

Kurt F. Brandstadt , John C. Saam , and Ajit Sharma

3

1

Dow Corning Corporation, P.O. Box 994, Midland, MI 48686 Michigan Molecular Institute, 1910 West St. Andrews, Midland, MI 48640 Central Michigan University, Mt. Pleasant, MI 48859

2

3

The ability of an inexpensive, commercially available enzyme, porcine pancreatic crude type II lipase, to catalyze the direct polyesterification of a primary (12-hydroxydodecanoic acid) and, to a lesser extent, a secondary (12-hydroxystearic acid) hydroxy acid under mild reaction conditions was demonstrated. Porcine pancreatic lipase powder was used without modification to catalyze the heterogeneous reactions in hydrophobic solvents. The enzyme was determined to selectively catalyze the esterification of the primary hydroxy acid in comparison to the secondary hydroxy acid. The removal of water formed in the reactions dramatically promoted the polyesterifications suggesting the role of reversibility. The significance of temperature, enzyme reuse and hydration, the monomer to enzyme weight ratio, surfactants, sonication, and molecular structure were evaluated. The process promises to be an environmentally compatible synthesis that would result in a 'green' material through a reduction of waste stream products, as well as an ability to recycle solvents and the catalyst.

© 2003 American Chemical Society

In Biocatalysis in Polymer Science; Gross, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2002.

141


142 Typically, polyester coating resins are synthesized in a one-step process at high temperatures (> 200°C) for long periods (/). Although these reaction conditions favor the equilibrium of polycondensation, they also promote uncontrolled side reactions: redistribution of monomer sequences, cross-linking, and broad molecular weight distributions. Since resin properties are presently achieved on a trial and error basis, the ability to control the resin structure is essentially lost. In 1986, the synthesis of stereoregular polyester materials with enzyme catalysts was documented (2). Candida rugosa and Chromobacterium viscosum lipase were used to polymerize 12-hydroxystearic acid, 12-hydroxy-c/s-9octadecenoic acid, 16-hydroxyhexadecanoic acid, and 12-hydroxydodecanoic acid with Mn values equal to 600-1,307 in different media including water and organic solvents. The reactions were performed at 35°C for three days with recovery of the product and catalyst. It was observed that hydroxy acids with secondary hydroxy groups polymerized quickly to yield oligomers with wide molecular weight distributions. In contrast, hydroxy acids with primary hydroxy groups polymerized slowly to yield oligomers with narrow molecular weight distributions. Since the reported enzymatic hydroxy selectivity contrasted with classical alcohol reactivity (primary > secondary > tertiary), mis advantageous observation was identified for evaluation in order to potentially promote slower esterifications with secondary alcohols. Given the ability to synthesize structurally defined materials, the use of an enzyme to strategically catalyze a polyester material directly under mild reaction conditions was evaluated. The catalytic ability of an inexpensive, commercially available enzyme, porcine pancreatic crude type II lipase, to directly polymerize a primary (12-hydroxydodecanoic acid) and a secondary (12-hydroxystearic acid) hydroxy acid under mild reaction conditions was explored.

Background Based on biological systems, enzyme-catalyzed reactions have been historically studied in aqueous media in an attempt to maintain the catalytically active tertiary configuration (3). The belief that a hydrophobic environment would dramatically alter the configuration and decrease the activity of the enzyme persisted and inhibited exploration for decades. Fortunately, this notion was not true. In fact, enzymes have been observed to function in two-phase, reverse micelles, one-phase, and anhydrous organic media (4). The use of organic solvents as media have led to the following attributes (3,4):



143 • • • • • • • •

increased enzymatic stability and activity. the ability to perform reactions not possible in water. increased solubility of a variety of substrates. different substrate specificities. decreased substrate or product inhibition. suppression of side reactions. simplified recovery of the catalyst and product. ability to recycle the catalyst and solvent.

Commercially available enzymes have been applied directly as a suspension, immobilized, chemically functionalized, or genetically engineered to catalyze well-defined enantio- and regioselective molecules under mild reaction conditions (4). Conversely, reaction times are typically long and yields may be low. Economically, the availability, specific activity, and operating lifetime influence the potential application(s) of an enzyme (5). In industry, enzymes have been used in large scale conversions of porcine to human insulin and in the production of high-fructose corn syrup, aspartic acid, malic acid, and guanosine 5'-phosphate (GMP) (5). Interestingly, given the low cost, high stability, and broad substrate specificity of lipase, Whitesides stated, "chemists comfortably use platinum as a catalyst; they will eventually use lipase (an enzyme) with no more hesitation (5)." The following examples illustrate successful enzymatic syntheses of novel materials. Thermoplastic elastomers, poly(P-hydroxyalkanoates) (PHA), have been produced by bacterial fermentation under specific experimental conditions ((5). Figure 1 illustrates the polymeric repeat unit of a PHA.

Figure 1: Stereochemical Repeat Unit of Poly (β-hydroxyalkanoate).

The thermoplastic polyester, poly(P-hydroxybutyrate) (PHB), has been accumulated in various bacteria as an intracellular polymeric carbon reserve/energy source. In the absence of an essential nutrient, select bacteria trigger the accumulation of a polymeric reserve in the presence of an excessive amount of carbon, a food source. With limited amounts of ammonium as a



144 nutrient, 80% yields of PHB in the bacterium, Alcaligenes eutrophus, were reported (6) based on the dry weight of carbon biomass (glucose). Different carbon sources have led to the production of different PHAs. Based on microstructure, molecular weight, and the results of other physical property tests, the bacterium, Psuedomonas oleovorans, has produced highly reproducible PHAsfromdifferent sodium alkanoate molecules. In application, 80 to 100,000 dalton materials have been produced under mild experimental conditions in 1520 hours. In industry, Monsanto uses this technology to produce a tough, flexible copolymer commercially sold under the Biopol brand, poly(phydroxybutyrate-co-p-hydroxyvalerate) (7). Regardless, the technology has the following limitations: (1) the polymers are restricted to the β-hydroxyalkanoate repeat unit, and (2) the polymer must be extractedfromthe biomass with solvent in a separate step. In 1991, the chemoenzymatic synthesis of a novel sucrose-containing polymer was documented (8). Proleather, an alkaline proteasefromBacillus sp. was used to synthesize a regioselective monomer which was polymerized using a conventional chemical catalyst. This was an effective approach to synthesize poly(sucrose adipamide) as illustrated in Figure 2. 9

The research group commented that, "it occurred to us that a far more efficient approach would be to use enzymes only for the highly selective step(s) in polymer synthesis (such as monomer preparation) and to employ conventional chemical catalysts for the bulk polymer synthesis (

Enzyme-Catalyzed Direct Polyesterification - ACS Symposium Series

Recommend Documents