The linear viscoelastic behavior of copolymers of ethylene with 1-hexene, 1-octene, 1-decene and 1-octadecene prepared with Et[Ind](2) ZrCl2/MAO is analyzed. The copolymers with alpha -olefin concentration greater than approximately 14 mol% have the simple rheological behavior with temperature that may be associated to linear polyethylenes with short chain branches. On the other hand, the polymers having comonomer concentration lower than approximately 14 mol% are thermo-rheologically complex materials. The rheological behavior of these copolymers may not be explained solely by the presence of long chain branching. A dendritic-like molecular structure with branches-on-branches has to be considered to explain the complexity observed in the case of the ethylene copolymers with low comonomer concentration and ethylene homopolymer. The complex chain structure would be the result of the incorporation of branched macromonomers in the polymers produced by single-site catalysts. The level of branching and complexity of molecular structure increases as the comonomer concentration decreases. No dependence with the length of the alpha -olefin was found in any of the analyzed parameters.