Free-radical melt grafting of the multi-monomer system of maleic anhydride (MAH)/styrene (St) onto polypropylene (PP) was studied using a single-screw extruder. The effects of St and initiator concentrations on the grafting reaction were investigated. It was shown that the addition of St to the melt-grafting system as a comonomer could significantly enhance MAH graft degree onto PP. The maximum MAH graft degree was obtained when the molar ratio of MAH to St was approximately 1:1. However, the melt flow rate (MFR) value of the grafted PP was the highest at this ratio. This implies that the interaction and reaction between MAH and St monomers plays an important role in the grafting reaction. St improves the grafting reactivity of MAH and also reacts with MAH to form the St-MAH copolymer (SMA) before the two monomers graft onto PP. Crafting of SMA onto PP greatly enhanced the graft degree of MAH. When the molar ratio of MAH to St is 1:1, the main grafting reaction in the system is the grafting of SMA to PP; therefore, the graft degree of the grafted PP is the highest. When the concentration of MAH is higher than that of St, some MAH monomer reacts with St to form SMA, but others can directly graft onto PP macroradicals. When the amount of St added is higher than that of MAH, part of the St monomer may preferentially react with PP macroradicals to form more stable styryl macroradicals, while others copolymerize with MAH to yield SMA. The MFR value of the grafted PP is lower in this case. (C) 2001 Elsevier Science Ltd. All rights reserved.