Abstract: Ethylene copolymers with alpha-olefins utilizable as additives to increase the properties of middle distillates at low temperatures, obtainable by polymerization in the presence of catalysts comprising the reaction product between: 1) a bis-cyclopentadienyl derivative having the general formula: (Cp1Cp2)—M—(L2L3) containing groups with oxygen bound to the transition metal, wherein M is a metal from the IIIb group to the IIb group or of the lanthanides series of the periodic table of the elements; 2) a co-catalyst chosen from alumoxanes or boranes.