Copolymerizations of ethylene with 1,3-butadiene in the presence of catalytic systems based on some C-2-symmetric zirconocenes (rac-[CH2(3-R-1-indenyl)(2)]ZrCl2 where R = -C(CH3)(3) (1), -CH(CH3)(2) (2), -CH2CH3 (3), -CH3 (4), and -H (5)) are compared. The chemical nature and the relative amount of constitutional comonomer units from butadiene are strongly affected by the bulkiness of the substituent in positions 3 and 3'. DFT calculations indicate that the observed dependence of constitutional units from butadiene on the bulkiness of the alkyl ligand substituent can be easily rationalized by the occurrence of an unusual insertion mechanism for butadiene. This mechanism, involving butadiene eta(2) primary coordination followed by its primary vinyl insertion, becomes largely predominant for catalytic systems based on the zirconocenes (1-4), which present R greater than or equal to CH3. This kind of butadiene insertion mechanism is also able to account for the high stereoregularity which is observed for adjacent methylene-1,2-cyclopropane units.