Polyethylene made with a nickel-diimine catalyst was studied in order to understand the relationship between molecular structure and dynamic mechanical properties. Different branch lengths, from methyl to longer, and their contents in these ethylene homopolymers, dramatically affects the crystallinity of these poIyethylene samples, as shown by differential scanning calorimetry (DSC) and crystallization analysis fractionation (CRYSTAF). T-m and CRYSTAF profiles are related not only to the branch content but also to the distribution on the chain. The dynamic mechanical thermal analysis showed three different groups of materials, where the stiffness Varies according to the short-chain branching (SCB) content. Polyethylenes with high SCB showed a strong p-transition (tan delta > 1) despite the lack of any detectable crystallinity. (C) 2001 EIsevier Science Ltd. All rights reserved.