The behaviors of rac-[CH2(3-tert-butyl-1-indenyl)(2)]ZrCl2 (1) and Cp2ZrCl2 (2) activated by methylaluminoxane in ethene/1,4-pentadiene copolymerization are compared. In the presence of 1, inserted methylene-1,3-cyclobutane units, a large number of crosslinks, and a small number of methylene-1,3-cyclohexane units are obtained. Differently, a polyethene containing only 1,3-cyclohexane rings is achieved with 2 as the catalytic precursor. Polymer microstructures are compared with those obtained with 1 and 2 in ethene/1,6-heptadiene copolymerization, which leads only to polyethene containing cyclohexane rings. A tentative rationalization of the experimental data is reported. (c) 2006 Wiley Periodicals, Inc.