To improve the compatibility and properties of blends based on high-density polyethylene (HDPE) and the ethylene-propylene copolymer (EPR), the functionalization of both through grafting with an itaconic acid derivative, monomethyl itaconate (MMI), was investigated. The grafting reaction was performed at 180degreesC in a Brabender Plasticorder using an initial monomer concentration of 3 phr in the case of HDPE and 5 phr in the case of EPR. 2,5-Dimethyl-2,5-bis(tert-butylperoxy)hexane was used as a radical initiator for the functionalization of HDPE and dicumyl peroxide was used as a radical initiator for the modification of EPR. The degree of grafting was 1.56% by weight for HDPE and 0.8% by weight for EPR. The effect of grafting on the processability, morphology, and thermal and mechanical properties of the blends are of particular interest. The results show that the grafting reaction increases the toughness and elongation at break of all tested blends and they retained their strength and stiffness. Moreover, the grafted polymers behaved as nucleating agents, accelerating the HDPE crystallization. These results are particularly relevant when both polymeric phases are modified. Morphological studies are in concordance with the mechanical characterization, showing a reduction of the rubber particle size and a better interfacial adhesion when both polymers are functionalized with MMI. (C) 2003 Wiley Periodicals, Inc.