The synthesis, characterization, and some physical properties of a new polyaniline derivative (PHCA), produced by horseradish peroxidase (HRP) initiated oxidative polymerization of 2-hydroxy-chloroaniline (HCA), are presented. The spectroscopic measurements indicate that PHCA consists of a mixture of branched and linear aniline chain units. Data from antioxidant potential assays show that PHCA exhibits moderate 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging activity but excellent metal ion chelating ability. The metal chelating ability of PHCA (82.14 +/- 2.39%) is about 2 times higher than that of EDTA (30.08 +/- 0.81%) at a concentration of 100 mu g/mL. In addition, as a potent antioxidant stabilizer, the influence of PHCA on the thermooxidative degradation of polypropylene (PP) is also investigated. The insertion of PHCA into PP matrix improves significantly both the oxidation induction time (OIT) and the onset oxidation temperature (OIT) of virgin PP.