C-2-symmetric zirconocenes activated by methylaluminoxane were utilized as catalysts in the polymerization 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-polymerization of 1,3- butadiene and (Z)-1,3-pentadiene and 1,2 polymerization of (E)-1,3 pentadiene. and 4-methyl-1,3-pentadiene. Probably, the different behaviour of 1 with respect to other C-2-symmetric 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. [GRAPHICS] rac[CH2(3-tert-butyl-1-indenyl)(2)]ZrCl2.