Thermal oxidative degradation behavior of polypropylene (PP) with different tacticities was studied based on the activation energy (Delta E) data obtained by thermogravimetric analysis (TGA). The Delta E value showed a negative proportion to the content of meso pentad fraction (mmmm) in all of isotactic PP (iPP) samples, and that of syndiotactic PP sample considerably deviated from this negative proportion relationship. Since the value of mmmm was directly related to polymer chain conformation, the Delta E value was thought to have close connection with the concentration of 31 helix conformation in the iPPs. The Delta E changes would be caused by the competition between uni- and bimolecular hydroperoxide decomposition, which was controlled by concentration and character of conformations of PPs. (c) 2006 Elsevier Ltd. All rights reserved.