Various iron dichloride complexes with aliphatic and aromatic bidentate amine ligands have been synthesized and used, in combination with aluminium-compounds (AlR3 or methylaluminoxane (MAO)), for the polymerization of 1,3-dienes. Catalyst activity, chemo- and stereo selectivity are strongly influenced by the type of ligand; systems based on iron complexes with aromatic bidentate amines (e.g. bypiridine or phenantroline), in particular, exhibit an extremely high activity, giving from butadiene a 1,2-syndiotactic polymer. These systems are also able to polymerize substituted butadienes (e.g. isoprene, (E)-1,3-pentadiene, 2,3-dimethyl-1,3-butadiene) giving polymers with different structure from the different monomers; these results have permitted to confirm some aspects of the polymerization mechanism previously proposed. (C) 2003 Elsevier Science B.V. All rights reserved.