C2-sym. zirconocenes activated by methylaluminoxane were utilized as catalysts in the polymn. of 1,3-diolefins. The results indicate that the most crowded catalytic precursor rac[CH2(3-tert-butyl-1-indenyl)2]ZrCl2 (1) is also the most active one, giving 1,4-polymn. of 1,3-butadiene and (Z)-1,3-pentadiene and 1,2-polymn. of (E)-1,3-pentadiene and 4-methyl-1,3-pentadiene. Probably, the different behavior of 1 with respect to other C2-sym. zirconocenes utilized is due to the different stability of the bond between the last inserted monomer unit and the metal, as well as to the coordination of incoming monomer.