Hyperbranched polymers have recently received extensive interest for applications as polymer processing aids (PPAs). In this work, we present the first study on the performance of novel hyperbranched polyethylene as a processing aid for a metallocene linear-low-density polyethylene (mLLDPE). Moreover, we investigated for the first time the unique role that chain topology plays in determining the efficiency of the polymers as PPA. Two polymers of drastically different chain topologies, a hyperbranched polyethylene (HBPE) and a linear branched polyethylene (LBPE) having a linear chain topology, were prepared by ethylene polymerization using chain walking Pd-diimine catalysis. The performances of these two polymers of different chain topologies in improving the processability of the mLLDPE were investigated and compared by using two-step capillary extrusion experiments, including constant-shear testing at 200 s(-1) and shear-rate sweep testing. For comparison purposes, a commercial fluoropolymer-based PPA, Viton Free Flow Z100, together with a polyethylene wax sample, were also evaluated for their performances. It was found that, at a concentration >= 3 wt%, the presence of HBPE in mLLDPE significantly reduced the apparent shear stress, improved extrudate surface morphology, and postponed the onset shear ratefor sharkskin instability. However, LBPE did not improve the processability of mLLDPE and it worked merely as a plasticizer, slightly reducing the viscosity of polymer melts. Morphology studies conducted on the cross sections of the blends showed that chain topology greatly affected the polymer additive's miscibility with mLLDPE; the mLLDPE/HBPE blend was a phaseseparated system, whereas LBPE appeared to be miscible with mLLDPE. The hyperbranched polymer, HBPE, forms phase-separated droplets, which can migrate to the die surface and form a lubricating layer promoting extrudate slippage. By identifying the effect of chain topology on the polymers' performance, this work may provide a chain-topology guideline for designing polymerbased processing aids. (C) 2008 The Society of Rheology.