It is very challenging to obtain very large amounts of vinyl monomers grafted onto polymer backbones in the melt through a free-radical mechanism. The objective of this study was to develop strategies that would allow one to maximize the amount of 3-isopropenyl-alpha,alpha-dimethylbenzene isocyanate (TMI) grafted onto polypropylene (PP) by reactive extrusion processes. For that purpose, an internal batch mixer was used to simulate potential reactive extrusion processes. Two strategies were studied: The first one was to apply the comonomer concept developed in previous studies. More specifically, styrene (St) was used as a comonomer to control the grafting yield and the molar mass of TMI-grafted PP. The second strategy delt with the feeding mode of the reactants. Two feeding modes were investigated: (a) one-pot feeding, that is, the total amount of TMI, St, and a peroxide were premixed with PP and the whole mixture was then charged to the reactor at once; (b) stepwise feeding, that is, the total amount of TMI, St, and the peroxide was divided in several equal fractions. The first fraction of the mixture was premixed with PP and then charged to the reactor. After a certain reaction time, the other fractions were charged to the reactor one after another in certain time intervals. Both strategies were shown to be very good at maximizing free-radical grafting reaction yields. They can be easily adopted if free-radical grafting is to be carried out by reactive extrusion in a screw extruder. (C) 2003 Wiley Periodicals, Inc.