Several novel poly(propylene)-graft-poly ethylene-co-propylene) copolymers with isotactic poly(propylene) (PP) backbones and ethylene/propylene rubber (EPR) branches were synthesized. The thermomechanical properties of these samples were investigated using a dynamic mechanical analyzer. There appeared to be a critical EPR molecular weight above which a two-phase system developed with EPR domains dispersed in a PP matrix. This domain formation gave an enhanced loss modulus compared to a commercial high impact PP product below 40degreesC.