Crystalline structures and thermal properties of poly (ethylene-co-alpha,omega- nonconjugated diene)s, [diene = 1,5-hexadiene (HD), 1,7-octadiene (OD), and 1,9-decadiene (DD)] have been investigated in relation to insertion mode of the dienes. In the case of poly (ethylene-co-HD), the copolymer containing high cis-1,3-cyclopentane units shows lower melting point depression with increasing the comonomer content than the copolymers containing high trans-1,3-yclopentane units. In the case of poly(ethylene-co-OD) and poly (ethylene-co-DD), the copolymers containing pendant vinyl groups show higher DeltaH(m) than that of the copolymers with cyclic units or branching structures. Thermal degradation of the copolymers has been investigated under nitrogen atmosphere and the degradation of the copolymers containing the cyclic structures begins at lower temperature than the copolymers containing pendant vinyl groups. (C) 2004 Elsevier Ltd. All rights reserved.