Metallocene polymerized ultralow density polyethylenes (ULDPE) were found to be effective impact modifiers for rigid matrices such as polypropylene (PP). This is mainly due to the very high level of comonomer incorporation with homogeneous short chain branching distributions (SCBD) brought about by the single site metallocene catalysts. However, a very heterogeneous Ziegler Natta copolymer of 0.88 specific gravity was found to be surprisingly effective in impact modification efficiency. A variety of thermal and morphological techniques were used in this study including differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), and low voltage scanning electron microscopy (LVSEM). It was found that the modulus difference between the two phases, degree of coupling as evidenced by morphology at the phase boundaries, and crystallizable fraction of the rubber phase all played an important role in impact modification.