The relationship between the structure and mechanical properties has been established for several copolymers of ethylene and 5,7-dimethylocta-1,6-diene synthesized with a metallocene catalyst. A dependence on the composition and polymerization temperature has been found. The branches cannot be incorporated into the orthorhombic crystal lattice, and consequently, structural parameters such as the crystallinity and crystal size are considerably affected as the 5,7-dimethylocta-1,6-diene content increases in the copolymers. The viscoelastic relaxations have been analyzed and compared with those exhibited by high-density polyethylene (HDPE). The beta relaxation does not appear in HDPE and is exclusively seen in the copolymers. As the 5,7-dimethylocta-1,6-diene content rises, the intensity of this process is increased, and its location is shifted to a lower temperature up to comonomer contents of approximately 6-8 mol % in the copolymers. On the other hand, the a mechanism associated with motion within the crystalline regions is also moved to a lower temperature and its intensity is diminished as the 5,7-dimethylocta-1,6-diene molar fraction increases in the copolymers. (C) 2004 Wiley Periodicals, Inc.