Graft copolymers of poly(methyl methacrylate) (PMMA) with spherical, high-porosity polyethylene (PE)/polypropylene (PP) in situ alloys were synthesized by a solid-state reaction. The effects of the amount of the free-radical initiator, the feed ratio of the methyl methacrylate (MMA) monomer, the reaction temperature, and the composition of the alloys on the grafting degree of PMMA were studied. A greater amount of the initiator and a higher reaction temperature led to a higher grafting degree. The grafting degree first increased with the feed ratio of MMA but leveled off at a higher feed ratio. The obtained graft copolymer was fractionated by temperature gradient extraction fractionation into six fractions, and each fraction was characterized by Fourier transform infrared and differential scanning calorimetry. The structures of these fractions were ungrafted ethylene-propylene random copolymers and ethylene-propylene segmented copolymers, ethylene-propylene block copolymers having PE and PP segments of different lengths and grafted by PMMA, PP grafted by PMMA, and nearly pure PP. Most PMMA was grafted on propylene segments, and thus the grafting degree decreased as the content of ethylene units in the alloys increased. (c) 2005 Wiley Periodicals, Inc.