This article discusses a new chemical route to prepare maleic anhydride (MA) grafted polyethylene and polypropylene polymers with controlled molecular structure, that is, MA grafted content and polymer molecular weight and composition distributions. The chemistry involves a free radical graft reaction of maleic anhydride with poly(ethylene-co-p-methylstyrene) and poly(propylene-co-p-methylstyrene) copolymers. Under a suspension reaction condition, the grafting reaction takes place selectively on the p-methylstyrene units in the copolymer, due to high reactivity of p-methyl group and favorable mixing between p-methylstyrene units and chemical reagents in the swollen amorphous phases. The resulting polymer shows no detectable molecular weight change during the reaction, and the MA grafted content increases with the increase of initiator and p-methylstyrene concentrations.