The effects of chemical structure and synthesis method on the photodegradation behavior of polypropylene (PP) were investigated in injection-molded samples exposed to ultraviolet radiation (UV) at 60degreesC. For this purpose, three PP samples with different chemical structures were chosen: two homopolymerized PP samples (H2P, synthesized by bulk polymerization; whereas H2P was synthesized by Ziegler-Natta catalyst) and copolymerized PP sample (CP). The photodegradation was characterized by melt flow rate and mechanical properties and Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and scanning electron microscopy. The results showed that CP possesses the most superior resistance to UV-irradiation, followed by H2P and then H1P, which indicates that copolymerization with a small amount of ethylene monomer is an effective approach to obtain high stability of PP to UV-irradiation, and synthesis methods of PP play an important role in the resistance to UV-irradiation. Moreover, the effect of photodegradation on the thermal behaviors of H,P was also investigated using X-ray diffraction, differential scanning calorimetry, and dynamical mechanical thermal analysis. (C) 2004 Wiley Periodicals, Inc.