Enzymatic Synthesis of Phenolic Polymers

Phenol-formaldehyde resins using prepolymers such as novolaks and resols are widely used in industrial fields. These resins show excellent toughness and thermal-resistant properties, but the general concern over the toxicity of formaldehyde has resulted in limitations on their preparation and use. Therefore, an alternative process for the synthesis of phenolic polymers avoiding the use of formaldehyde is strongly desired. 

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For the last decades, enzymatic synthesis of phenolic polymers has been extensively investigated [1-10]. In living cells, various oxidoreductases play an important role in maintaining the metabolism of living systems. So far, several oxidoreductases—peroxidase, laccase, bilirubin oxidase etc.—have been reported to catalyze an oxidative polymerization of phenol derivatives, and among them, peroxidase is most often used. The enzymatically synthesized phenolic polymers are expected to become an alternative to conventional phenolic resins, which have limitations of their preparation and use due to concerns over the toxicity of formaldehyde. 

A bi-enzymatic system (glucose oxidase -I- HRP) was also used to catalyze the synthesis of phenolic polymers. The polymerization of phenol, albeit in moderate yield, was accomplished in the presence of glucose avoiding the addition of hydrogen peroxide (Scheme 2 ), which was formed in situ by the oxidation of glucose catalyzed by glucose oxidase. 

Enzymes may be classified generally into six groups the details of typical polymers produced via catalysis with respective enzymes are listed in Table 23.1. In the past, the target macromolecules for enzymatic polymerization have included polysaccharides, poly(amino acid)s, polyesters, polycarbonates, phenolic polymers, poly(aniline)s, and vinyl polymers. In this chapter, attention is focused on the enzymatic synthesis of phenohc polymers and polyesters, based on the increasing industrial application of these materials. Notably, most such polymers can be obtained from commercially available, inexpensive monomers by using industrially produced enzymes. Another important point is that the enzymatic process must be regarded as an environmentally benign synthetic pathway. Details of the enzymatic synthesis of other polymers are provided in recent pertinent reviews [3-10]. 

Enzymatic Synthesis of Functional Polymers from Phenols... 

In the previous chapter Synthesis of Phenol Polymers Using Peroxidases , the enzymatic oxidative polymerization of monophenolic derivatives is described. This chapter deals with the enzymatic synthesis and properties of polymers from polyphenols, compounds having more than two hydroxyl groups on the aromatic ring(s). In particular, cured phenolic polymers (artificial urushi) and flavonoid polymers are examined from the standpoint of the enzymatic synthesis of functional materials. 

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. 

Xia Z, Yoshida T, Funaoka M (2003) Enzymatic synthesis of polyphenols from highly phenolic lignin-based polymers (lignophenols). Biotechnol Lett 25 9-12... 

Particles of the enzymatically synthesized phenolic polymers were also formed by reverse micellar polymerization. A thiol-containing polymer was synthesized by peroxidase-catalyzed copolymerization of p-hydroxythiophenol and p-ethylphenol in reverse micelles [70], CdS nanoparticles were attached to the copolymer to give polymer-CdS nanocomposites. The reverse micellar system was also effective for the enzymatic synthesis of poly(2-naphthol) consisting of qui-nonoid structure [71], which showed a fluorescence characteristic of the naphthol chromophore. Amphiphilic higher alkyl ester derivatives were enzymatically polymerized in a micellar solution to give surface-active polymers at the air-water interface [72, 73]. 

The peroxidase-catalyzed oxidative coupling of phenols proceeds rapidly in aqueous solution, giving rise to the formation of oligomeric compounds that, unfortunately, have not well been characterized, as most of them demonstrate a low solubility towards common organic solvents and water. In 1987, the enzymatic synthesis of a new class of phenolic polymer was first reported [15], whereby an oxidative polymerization of p-phenylphenol, using HRP as catalyst, was carried out in a mixture of water and water-miscible solvents such as 1,4-dioxane, acetone. 

The enzymatic reaction kinetics on the HRP-catalyzed oxidation of p-cresol in aqueous 1,4-dioxane or methanol showed that the cataljdic turnover niunber and Michaelis constant were larger than those in water (235). Numerical and Monte Carlo simulations of the peroxidase-catalyzed polymerization of phenols were demonstrated (236). The simulations predicted the monomer reactivity and polymer molecular weight, leading to synthesis of polymers with specific molecular weight and index. In an aqueous 1,4-dioxane, the formation of monomer aggregate was observed (237), which might elucidate the specific polymerization behaviors in such a medium. 

Cardanol was polymerised with SBP in a mixture of acetone buffer (75 25), to form an oily polymer. H-NMR and FTIR analyses indicated that double bonds in the side chain were not affected by polymerisation conditions and polycardanol was a mixture of phenylene and oxyphenylene units (Scheme 12.20) [202]. Epoxide-containing polycardanol was enzymatically synthesised via two routes by two different enzymes, lipase and peroxidase [203]. One route was the synthesis of epoxide-containing cardanol from cardanol, H2O2 and an organic acid in the presence of lipase, followed by the polymerisation of the phenolic functional groups of cardanol using peroxidase. 

An aqueous Friedel-Crafts reaction has also been used in polymer synthesis. The acid-catalyzed polymerization of benzylic alcohol and fluoride functionality in monomeric and polymeric fluorenes was investigated in both organic and aqueous reaction media. Polymeric products are consistent with the generation of benzylic cations that participate in electrophilic aromatic substitution reactions. Similar reactions occurred in a water-insoluble Kraft pine lignin by treatment with aqueous acid. A Bisphenol A-type epoxy resin is readily emulsified in aqueous medium with an ethylene oxide adduct to a Friedel-Crafts reaction product of styrene and 4-(4-cumyl)phenol as emulsifier.Electrophilic substitution reaction of indoles with various aldehydes and ketones proceeded smoothly in water using the hexamethylenetetramine-bromine complex to afford the corresponding Z A(indolyl)methanes in excellent yields.InFs-catalyzed electrophilic substitution reactions of indoles with aldehydes and ketones are carried out in water.Enzymatic Friedel-Crafts-type electrophilic substitution reactions have been reported. ... 

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