Graft and star copolymers having poly(methacrylate) backbone and ethylene-propylene random copolymer (EPR) branches were successfully synthesized by radical copolymerization of an EPR macromonomer with methyl methacrylate (MMA). EPR macromonomers were prepared by sequential functionalization of vinylidene chain-end group in EPR via hydroalumination, oxidation, and esterification reactions. Their copolymerizations with MMA were carried out with monofunctional and tetrafunctional initiators by atom transfer radical polymerization (ATRP). Gel-permeation chromatography and NMR analyses confirmed that poly(methyl methacrylate) (PMMA)-g-EPR graft copolymers and four-arm (PMMA-g-EPR) star copolymers could be synthesized by controlling EPR contents in a range of 8.6-38.1 wt % and EPR branch numbers in a range of 1-14 branches. Transmission electron microscopy of these copolymers demonstrated well-dispersed morphologies between PMMA and EPR, which could be controlled by the dispersion of both segments in the range between 10 nm and less than 1 nm. Moreover, the differentiated thermal properties of these copolymers were demonstrated by differential scanning calorimetry analysis. On the other hand, the copolymerization of EPR macromonomer with MMA by conventional free radical polymerization with 2,2'-azobis(isobutyronitrile) also gave PMMA-g-EPR graft copolymers. However, their morphology and thermal property remarkably differed from those of the graft copolymers obtained by ATRP. (c) 2005 Wiley Periodicals, Inc.