A two-step investigation of polypropylene-controlled degradation was performed. First the controlled degradation of polypropylene was investigated by assessing the effects of using dicumyl peroxide (DCP) and 2,5-dimethyl-2,5-di(t-butylperoxy) hexane (DHBP) in different suspensions (neat, calcium carbonate, and organically modified montmorillonite). The second part of this research was performed according to a factorial design at two levels (22 experimental design) with center point, in which peroxide concentration and reaction time were studied. Polypropylene degradation was assessed by melt flow rate (MFR) and parallel plate rheometry. Results showed that DCP presented higher molar efficiency in polypropylene degradation reactions in relation to DHBP; likely due to radical recombination reactions. The results also indicate that clay may catalyze the recombination reactions of polypropylene radicals. (C) 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2012