“Radical-Controlled” Oxidative Polymerization of 4-Phenoxyphenol by a Tyrosinase Model Complex Catalyst to Poly(1,4-phenylene oxide)

A new concept, “radical-controlled” oxidative polymerization of phenols catalyzed by a tyrosinase model complex, has been proposed. A μ-η 2:η 2 -peroxo dicopper(II) species formed by the reaction between the catalyst complex and dioxygen, reacted with phenol to give “controlled” phenoxy radical−copper(I) intermediate instead of “free” phenoxy radical. The polymerization of 4-phenoxyphenol was performed by the use of the tyrosinase model complexes, (hydrotris(3,5-diphenyl-1-pyrazolyl)borate)copper (Cu(Tpzb)) chloride complex and (1,4,7- R 3 -1,4,7-triazacyclononane)copper (Cu(L R ): R = isopropyl (iPr), cyclohexyl (cHex), n -butyl (nBu)) dichloride complexes. The structures of these complexes were determined by X-ray crystallography, indicating that the order of steric repulsion of the substituents (R) in the Cu(L R ) complexes is cHex > iPr > nBu. Very little of C−C coupling dimers were afforded with the Cu(Tpzb) catalyst in toluene or THF, and with the Cu(L iPr ), Cu(L cHex ), or Cu(L nBu ) catalyst in toluene. The selectivity of para C−O coupling increased with an increase in the steric hindrance of R for the Cu(L R ) catalysts. On the other hand, the formation of C−C dimers was clearly observed in the polymerization catalyzed by a copper/diamine complex or horseradish peroxidase. The selective polymerization almost without the C−C dimer formation produced crystalline poly(1,4-phenylene oxide) having a melting point, although the polymer contained small amounts of 1,2,4-trioxybenzene units (ca. 1−5 unit %). However, the polymers obtained in the cases involving the C−C dimer formation showed no clear melting points. The reaction mechanism of catalytic cycle and oxidative polymerization is also discussed.