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 mu-eta(2):eta(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(LR) 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-eHex), 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(LR) 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 ana oxidative polymerization is also discussed.