Poly [4,4-diphenylamine (3,5-di-tert-butyl-4-hydroxy)benzylidene] was synthesized with a dehydrative polycondensation by using H2SO4 as a catalyst. After treatment with DDQ, the polymer was oxidized with PbO2 to generate a stable radical group in the polymer. The colour of the polymer changed from green to dark red upon the oxidation. ESR spectra showed not only a signal with a g value of 2.004, but also a so-called 'half-field' signal with a g value of 4.288. (C) 2004 Elsevier Ltd. All rights reserved.