Synthesis and magnetic behavior of poly[6-(N-oxy-tert-butylamino)biphenyl-3,3 -ylene] (2) are described. Polyradical 2 was prepared the Pd-catalyzed polycondensation of N-tert-butyl-2,4-dibromoaniline with 1,3-phenylenebis(trimethylene boronate), followed by oxidation with 3-chloroperoxybenzoic acid. The degree of polycondensation determined by SEC was similar to 18. Polyradical 2 was very stable in both solution and solid state. The spin concentration of 2 determined by ESR was 1.89 x 10(21) spins g(-1) (0.75 spin per repeating unit). The magnetic susceptibility (chi) measurements of 2 carried out with a SQUID magnetometer showed that the magnetic interaction between the impaired electron spins was antiferromagnetic (theta= -0.98 K). No observation of the ferromagnetic interaction was ascribed to twisting of the nitroxide moiety from the pi -conjugated main chain. This twisting was confirmed by the X-ray crystallographic analysis of the monomer model radical 1.