In this work, five branched polyethylenes with different branching units were synthesized using bidentate nickel (II) catalyst containing alpha-diimine ligands. For comparison, one linear polyethylene was also prepared using tridentate iron (II) catalyst containing alpha-diimine ligand. The crystalline structure of the polyethylenes was investigated using X-ray diffraction (XRD) and polarized optical microscope. The crystalline properties were also measured by differential scanning calorimeter (DSC). Viscoelastic properties of the polyethylenes were investigated using rheometric dynamic analyzer. The DSC and XRD results showed that highly branched polyethylenes exhibit no melting points and no predominating crystalline forms, while the linear polyethylene exhibits clear orthorhombic (110) and (200) reflections on XRD pattern and a clear melting point at 118 degreesC. The viscoelastic properties of the branched polyethylenes were very complicated due to the combined effect of the molecular weight difference and the degree of chain branching as well as the branching structure. (C) 2003 Elsevier Ltd. All rights reserved.