Enzymatic synthesis of functional phenolic polymers

Poly(2,6-dimethyl-1,4-oxyphenylene) (poly(phenylene oxide), PPO) is a material widely used as high-performance engineering plastics, thanks to its excellent chemical and physical properties, e.g., a high T (ca. 210°C) and mechanically tough property.151 PPO was first prepared from 2,6-dimethylphenol monomer using a copper/amine catalyst system. 2,6-Dimethylphenol was also polymerized via HRP catalysis to give a polymer exclusively consisting of 1,4-oxyphenylene unit,177 while small amounts of Mannich-base and 3,5,3',5'-tetramethyl-4,4'-diphenoquinone units are always contained in the chemically prepared PPO.

Laccase (PCL) as well as peroxidases (HRP and SBP) induced a new type of oxidative polymerization of the 4-hydroxybenzoic acid derivatives, 3,5-dimethoxy-4-hydroxybenzoic acid (syringic acid) and 3,5-dimethyl-4-hydroxybenzoic acid. The polymerization involved elimination of carbon dioxide and hydrogen from the monomer to give PPO derivatives with molecular weight up to 1.8 x 104 (Scheme 22).178-179

a-Hydroxy-w-hydroxyoligo(1,4-oxyphenylene)s were formed in the HRP-catalyzed oxidative polymerization of 4,4'-oxybisphenol in an aqueous methanol.180 During the reaction, hydroquinone was formed. Scheme 23 shows the postulated mechanism of the trimer formation; the redistribution and/or rearrangement of the quinone-ketal intermediate takes place, involving the elimination of hydroquinone.

New positive-type photoresist systems based on enzymatically synthesized phenolic polymers were developed.181 The polymers from the bisphenol monomers

1) : Peroxidase + H202,-H20rC02

1) : Peroxidase + H202,-H20rC02

Scheme 22

hrp/h2o2

Direct hrp/h2o2

Direct

Scheme 23

exhibited high photosensitivity, comparable with a conventional cresol novolak. Furthermore, the present photoresist showed excellent etching resistance. Nano-scale polymer patterning was reported to be fabricated by the enzymatic oxidative polymerization with dip-pen nanolithography technique.182

Scheme 24

HRP catalysis induced a chemoselective polymerization of a phenol derivative having a methacryloyl group.183 Only the phenol moiety was polymerized to give a polymer having the methacryloyl group in the side chain. The resulting polymer was readily cured thermally and photochemically (Scheme 24). A phenol with an acetylenic substituent in the meta position was also chemoselectively polymerized by HRP to give a polymer bearing acetylenic groups (Scheme 25).184 For comparison, the reaction of the monomer using a copper/amine catalyst, a conventional catalyst for oxidative coupling, was performed, producing a diacetylene derivative exclusively. The resulting polymer was converted to a carbon polymer in much higher yields than enzymatically synthesized poly(m-cresol). Therefore, it might have a large potential as precursor of functional carbon materials.

Hydroquinone mono-PEG ether was polymerized by HRP in aqueous 1,4-dioxane.185 High ionic conductivities (4 x 10-5 Scm-1) were found in the film consisting of the lithiated phenolic polymer and PEG. A thiol-containing polymer was synthesized by peroxidase-catalyzed copolymerization of p-hydroxythiophenol and p-ethylphenol in reverse micelles.186 CdS nanoparticles were attached to the copolymer to give polymer-CdS nanocomposites. The reverse micellar system was also effective for the enzymatic synthesis of a poly(2-naphthol) consisting of quinonoid structure.187 It showed a fluorescence characteristic of the naphthol chromophore. A novel photoactive azopolymer, poly(4-phenylazophenol), was synthesized using HRP catalyst.188 A reversible trans to cis photoisomerization of the azobenzene group with long relaxation time was observed.

Phenolic copolymers containing fluorophores (fluoroscein and calcein) were synthesized by SBP catalysis and used as array-based metal-ion sensor.189 Selectivity and sensitivity for metal ions could be controlled by changing the polymer components. Combinatorial approach was made for efficient screening of specific sensing of the metals.

A natural phenolglucoside, 4-hydroxyphenyl ^-D-glucopyranoside (arbutin), was subjected to regioselective oxidative polymerization using a peroxidase catalyst in a buffer solution, yielding the water-soluble polymer consisting of 2,6-phenylene units, in turn converted to poly(hydroquinone) by acidic deglycosy-lation (Scheme 26).190 The resulting polymer was used for a glucose sensor exploiting its good redox properties.191 Another route for the chemoenzymatic synthesis of poly(hydroquinone) was the SBP-catalyzed polymerization of 4-hydroxyphenyl benzoate, followed by alkaline hydrolysis.192

A polynucleoside with an unnatural polymeric backbone was synthesized by SBP-catalyzed oxidative polymerization of thymidine 5'-p-hydroxyphenylacetate.193 Chemoenzymatic synthesis of a new class of poly(amino acid), poly(tyrosine) containing no peptide bonds, was achieved by the peroxidase-catalyzed oxidative polymerization of tyrosine ethyl esters, followed by alkaline hydrolysis.194 Amphiphile higher alkyl ester derivatives were also polymerized in

micellar solution to give macromolecules showing surface activity at the air-water interface.195-196

The molecular weight of the enzymatically synthesized phenolic polymers was in the range of several thousands. Ultrahigh molecular weight polymers were synthesized by the oxidative coupling of the phenolic polymers using Fe-salen as a catalyst, which can be regarded as a mimic of peroxidases.197 198 The oxidative coupling of the precursor polymers with molecular weight of several thousands produced DMF-soluble polymers with Mw > 106 quantitatively, where a crosslinking is suppressed by selection of reaction conditions. The intermolecular coupling of the polymer efficiently proceeded at the initial stage of the reaction, leading to the rapid increase of the molecular weight. On the other hand, the molecular weight of the polymer obtained from the monomer was much lower. This result provides a new synthetic method of ultrahigh molecular weight polymeric materials via oxidative couplings. Furthermore, the oxidative coupling behaviors between the precursor polymers and the corresponding phenolic monomers were examined. The structure of the starting monomers had a great influence on the oxidative coupling of the precursor polymers. The polymers from o- and m-substituted monophenols were subjected to efficient oxidative coupling to give high molecular weight polymers quantitatively; on the other hand, the oxidative coupling of the polymers from p-substituted monophenols scarcely proceeded. Polymers from bisphenol and triphenol were also oxidatively polymerized, leading to the quantitative production of the high molecular weight polymers under the appropriate reaction conditions.

We prepared a phenol-containing hyaluronan derivative, which was inter-molecularly coupled by HRP to yield a crosslinked hydrogel (Scheme 27).199 The sequential injection of this hyaluronan derivative and peroxidase formed

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biodegradable hydrogels in vivo, offering high potential as a promising biomaterial for drug delivery and tissue engineering.

Morphology of the enzymatically synthesized phenolic polymers was controlled under the selected reaction conditions. Monodisperse polymer particles in the sub-micron range were produced by HRP-catalyzed dispersion polymerization of phenol in 1,4-dioxane-phosphate buffer (3:2 v/v) using poly(vinyl methyl ether) as stabilizer.200-202 The particle size could be controlled by the stabilizer concentration and solvent composition. Thermal treatment of these particles afforded uniform carbon particles. The particles could be obtained from various phenol monomers such as m-cresol and p-phenylphenol.

Blends of enzymatically synthesized poly(bisphenol-A) and poly(p-t-butylphenol) with poly(e-CL) were examined.203 FT-IR analysis showed the expected strong intermolecular hydrogen-bonding interaction between the phenolic polymer with poly(e-CL). A single T was observed for the blend, and the value increased as a function of the polymer content, indicating their good miscibility in the amorphous state. In the blend of enzymatically synthesized poly(4,4'-oxybisphenol) with poly(e-CL), both polymers were miscible in the amorphous phase also.204 The crystallinity of poly(e-CL) decreased by poly(4,4'-oxybisphenol).

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