In the presence of rac-[CH2(3-tert-butyl-1-indenyl)2]ZrCl2 (1) activated by methylaluminoxane (MAO) 1,4-pentadiene (1,4-PD), if copolymerized with ethene (E), gives unprecedented formation of methylene-1,3-cyclobutane units and a large amount of cross-links. Instead, in the presence of the same catalytic system, copolymerization of ethene with 1,3-butadiene (1,3-BD) or with 1,5-hexadiene (1,5-HD) gives exclusively a cyclization reaction, producing cyclopropane and cyclopentane units. The experimental results were rationalized by a theoretical approach, comparing the minimum energy pathways for the ring formation, after a diene insertion, with the competing ethene insertion reaction.