A new class of low density polyolefin elastomers (POE's) prepared using novel constrained geometry catalyst technology has been examined for impact modification of isotactic polypropylene. The resulting thermoplastic polyolefins (TPO's) demonstrate enhanced elastomer dispersion, resulting in improvements in physical properties such as low temperature impact strength, as well as advantages in overall blend processability. The performance of these novel elastomers as impact modifiers has been examined as a function of molecular weight and comonomer content. In addition, physical property comparisons have been made between TPOs prepared utilizing these materials and conventional modifiers such as EPDM and EPR with regard to relative impact efficiency and failure mechanism(s). These data have been correlated to elastomer dispersion using scanning or transmission electron microscopy.