Peroxidase-catalyzed oxidative coupling of phenols proceeds fast in aqueous solutions, giving rise to the formation of oligomeric compounds. However, the resulting oligomers have not been well characterized, since most of them show low solubility towards common organic solvents and water. In 1987, enzymatic synthesis of a new class of phenolic polymers was first reported.150 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, DMF, and methyl formate. The polymerization proceeded at room temperature and during this process, powdery polymers were precipitated. The reaction medium composition greatly affected the molecular weight, and the polymer with the highest molecular weight (2.6 x 104) was obtained in 85% 1,4-dioxane. Afterwards, peroxidase-catalyzed
oxidative polymerization of various phenol derivatives has been examined to produce functional and useful phenolic polymers.148149
Phenol, the simplest and industrially more important phenolic compound, is a multifunctional monomer when considered as a substrate for oxidative polymerizations, and hence conventional polymerization catalysts afford insoluble macromolecular products with non-controlled structure.151 Phenol was subjected to oxidative polymerization using HRP or soybean peroxidase (SBP) as catalyst in an aqueous-dioxane mixture, yielding a polymer consisting of phenylene and oxyphenylene units (Scheme 19). The polymer showed low solubility; it was partly soluble in DMF and dimethyl sulfoxide (DMSO) and insoluble in other common organic solvents.152153
When a water-soluble alcohol and a buffer were used as the solvent, the product showed an improved solubility toward DMF and DMSO.154 155 For instance, in an equivolume mixture of methanol and phosphate buffer (pH 7), a DMF-soluble polymer was obtained in good yields. The solubility of the resulting polymer strongly depended on the buffer pH and content of the mixed solvent. The molecular weight of the polymer was in the range of several thousand Daltons. The resulting phenolic polymer showed relatively high thermal stability and no clear glass transition temperature (Tg) was observed below 300°C. Molecular weight control of the polymer was achieved by the copolymerization with 2,4-dimethylphenol to give a soluble oligomer with a molecular weight of 500.156
The control of the polymer structure was achieved by solvent engineering. The ratio of phenylene and oxyphenylene units was strongly dependent on the solvent composition. In the HRP-catalyzed polymerization of phenol in a mixture of methanol and buffer, the oxyphenylene unit increased by increasing the methanol content, while the buffer pH scarcely influenced the polymer structure.154155
The proposed polymerization mechanism is shown in Scheme 20. A phenoxy free radical is first formed, then two molecules of the radical species dimerize via coupling. Since peroxidase often does not recognize larger molecules, a radical transfer reaction between a monomeric phenoxy radical and a phenolic polymer takes place to give the polymeric radical species. In the propagation step, such propagating radicals are subjected to oxidative coupling, producing polymers of higher molecular weight.
The polymerization outcome depended on the monomer structure as well as on the enzyme origin. For instance, using HRP and p-n-alkylphenols, the
Radical transfer reaction
Radical transfer reaction
polymer yield increased by increasing the chain length of the alkyl group from one to five in an aqueous 1,4-dioxane as medium, while the amount of polymer obtained with hexyl- or heptylphenol was almost the same as that obtained with the pentyl derivative.157 158 The polymer formation by HRP catalysis was observed with all the cresol isomers investigated.159 A polymer was obtained in high yields using p-i-propylphenol, whereas o- and m-isomers were not polymerized under similar reaction conditions. Poly(p-n-alkylphenol)s prepared in the aqueous 1,4-dioxane showed low solubility toward organic solvents. On the other hand, soluble oligomers with molecular weights lower than 1000 were formed from p-ethylphenol using aqueous DMF as a solvent.160 Poly(p-t-butylphenol) enzymatically synthesized in water-dioxane showed Tg and melting point (Tm) at 182 and 244° C, respectively.
When the HRP-catalyzed polymerization of p-substituted phenols was conducted in an equivolume mixture of organic solvent and pH 7 phosphate buffer, the regioselectivity was influenced by the monomer substituents and the solvent nature.161162 The hydrophobic nature of the monomer substituent and of the organic solvent (evaluated as n and log P, respectively) strongly affected the polymer structure. A significative first-order correlation between these parameters and the polymeric structure was observed. The structures of these macromolecules covered a wide range of the unit ratio between a phenylene and an oxyphenylene (from 94/6 to 4/96), indicating that the regioselectivity can be controlled by varying the solvent and substituent nature, yielding poly(phenylene) or poly(oxyphenylene).
The peroxidase-catalyzed polymerization of m-alkyl substituted phenols in aqueous methanol produced soluble phenolic polymers.163 The mixed ratio of buffer and methanol greatly affected the yields and the molecular weight of the polymer. The enzyme source greatly affected the polymerization pattern of m-substituted monomers. Using SBP catalyst, the polymer yield increased as a function of the bulkiness of the substituent, whereas the opposite tendency was observed when HRP was the catalyst.
In order to better understand the solvent effect on the enzymatic polymerization of phenols, the self-association of m-cresol in water-organic solvent mixtures was examined.164 Clustering of m-cresol in these solvents was observed by UV absorption spectroscopy and mass spectrometry for clusters. The pattern of the clustering formation in the solvents of different composition was significantly related to the results of the enzymatic polymerization in these mixed solvents, suggesting that the clustering of the phenol monomer in water-organic solvent mixtures affords the phenolic polymer more efficiently than that in the absence of the organic solvent.
Fluorinated phenols, 3- and 4-fluorophenols, and 2,6-difluorophenol, were subjected to peroxidase-catalyzed polymerization in an aqueous organic solvent, yielding fluorine-containing polymers.165 Elimination of fluorine atom partly took place during the polymerization to give polymers with complicated structures.
Various bisphenol derivatives were also polymerized by peroxidase under selected reaction conditions, yielding soluble phenolic polymers. Bisphenol-A was polymerized by peroxidase catalyst to give a polymer soluble in acetone, DMF, DMSO, and methanol.166 The polymer was produced in higher yields using SBP as a catalyst. This polymer showed a molecular weight of 4 x 103 and a Tg at 154°C. The HRP-catalyzed polymerization of 4,4'-biphenol produced a polymer showing high thermal stability.167
Peroxidase also induced the polymerization of an industrial product, bisphenol-F, consisting of 2,2'-, 2,4'-, and 4,4'-dihydroxydiphenylmethanes.168 Under the selected reaction conditions, the quantitative formation of a soluble phenolic polymer from bisphenol-F was achieved. Among the isomers, 2,4'- and 4,4'-dihydroxydiphenylmethanes were polymerized to give the corresponding polymers in high yields, whereas no polymerization of the 2,2'-isomer occurred, suggesting the frequent occurrence of a radical transfer reaction between a phenoxy radical of the enzymatically polymerizable monomer and a phenol moiety of the enzymatically non-polymerizable monomer. In the case of 4,4'-dihydroxyphenyl monomers, the bridge structure enormously affected the polymerization behaviors and the thermal properties of the resulting polymers.
Laccase is a protein containing copper as its active site and uses oxygen as an oxidizing agent. An oxidative polymerization of phenol and its derivatives was performed using laccase as catalyst without hydrogen peroxide in aqueous organic solvents at room temperature under air.169 Laccase derived from Pycnoporus coc-cineus (PCL) efficiently induced the polymerization to produce phenolic polymers consisting of a mixture of phenylene and oxyphenylene units. The unit ratio of the polymer could be precisely controlled by selection of the solvent nature and the monomer substituent.
A bi-enzymatic system (glucose oxidase + 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 21),170 which was formed in situ by the oxidation of glucose catalyzed by glucose oxidase.
As described above, the enzymatic polymerization of phenols was often carried out in a mixture of a water-miscible organic solvent and a buffer. By adding 2,6-di-O-methyl-P-cyclodextrin (DM-^-CD), the enzymatic polymerization of water-insoluble m-substituted phenols proceeded in buffer.171 The water-soluble complex of the monomer and DM-^-CD was formed and was polymerized by HRP to give a soluble polymer. In the case of phenol, the polymerization took place in the presence of 2,6-di-O-methyl-a-cyclodextrin (DM-a-CD) in a buffer.172 Only a catalytic amount of DM-a-CD was necessary to induce the polymerization efficiently. Coniferyl alcohol was oxidatively polymerized in the presence of a-CD in an aqueous solution.173
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