Diethylbis(2,2'-bipyridine)Fe/MAO is an extremely active catalyst for the polymerization of 1,3-dienes. Polymers with a 1,2 or 3,4 structures are formed from butadiene, isoprene, (E)-1,3-pentadiene and 3-methyl-1,3-pentadiene, while cis-1,4 polymers are derived from 2,3-dimethyl-1,3-butadienes. The 1,2(3,4) polymers obtained at 25degreesC are amorphous, while those obtained below 0degreesC are crystalline, as was determined by means of X-ray diffraction. Mechanistic implications of the results are briefly discussed.