A real-time study to the evolution of the mechanisms of degradation of both PP powder and PP granular during the mixing process was achieved. A quantitative analysis of combination of the rheological torque-time curves obtained by a Haake rheometer with FTIR spectra was made. The construction of a series of the characteristic functions of the torque-time curve and the FTIR characteristic functions for polypropylene (PP) made the real time and quantitative analysis relatively profound. By means of these characteristic functions, the characters of the mechanisms of thermooxidative degradation, and the mechanism of mechanochemical degradation for PP were investigated. Two criterions for the mechanism of thermooxidative degradation and the mechanism of mechanochemical degradation were obtained. In the thermooxidative treatment process and the mixing process, the necessary condition for the significant occurrence of thermooxidative degradation of PP is Delta h(r) (i) > 0.05, whereas the sufficient condition for the significant occurrence of mechanochemical degradation is Delta h(r) (i) <= 0.05 (D(l) (i) < 0, and i > 1). A detailed description of the evolutions of the degradations for both the PP granules in which an antideterionant was added and the PP powder without any antideterionant was made. The roles and the evolutions of both the mechanism of thermooxidative degradation and the mechanism of mechanochemical degradation were discussed. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 121: 1220-1243, 2011