Variable quantities of functionalised peroxides bearing carboxylic acid groups were reacted with polypropylene in a twin-screw extruder. Systematic variations in the molecular structure of the peroxides were found to significantly affect the grafting efficiency of the carboxylic acid group onto the polypropylene backbone, as well as affect the polymer degradation process. This behaviour was attributed to the relative reactivities of the different free radicals generated by thermal decomposition of the peroxides. The peroxides which yielded reactive methyl radicals were more efficient at producing polymeric radicals, relative to the peroxides which generated less reactive ethyl radicals. Increased grafting efficiency and PP degradation was attributed to this increase in the quantity of polymeric radicals. In addition, the peroxides which yielded radicals bearing double bonds were found to have a higher grafting efficiency. This behaviour was attributed to an increased reactivity of the alkenyl radicals, relative to the alkyl radicals, and to the potential for the polymeric radicals to add across the double bond to create the graft.