"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 cat alyzed 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 co mplex and dioxygen, reacted with phenol to give "controlled" phenoxy radica l-copper(I) intermediate instead of "free" phenoxy radical. The polymerizat ion of 4-phenoxyphenol was performed by the use of the tyrosinase model com plexes, (hydrotris(3,5-diphenyl-1-pyrazolyl)borate)copper (Cu(Tpzb)) chlori de complex and (1,4,7-R-3-1,4,7 triazacyclononane)copper (Cu(L-R): R = isop ropyl (iPr), cyclohexyl (cHex), n-butyl (nBu)) dichloride complexes. The st ructures of these complexes were determined by X-ray crystallography, indic ating 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-iP r), 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 clear ly 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 me lting point, although the polymer contained small amounts of 1,2,4-trioxybe nzene units (ca. 1-5 unit %), However, the polymers obtained in the cases i nvolving the C-C dimer formation showed no clear melting points. The reacti on mechanism of catalytic cycle ana oxidative polymerization is also discus sed.