The soluble system diethylbis(2,2'-bipyridine)Fe-MAO is very active for the polymerization of isoprene to predominantly 3,4 polymers (ca. 80%). The polymers obtained at 25 degreesC are amorphous by X-ray examination, while those obtained below 10 degreesC are crystalline. The identity period determined from the X-ray fiber spectrum, ca. 5.2 Angstrom, and the NMR analysis show that the polymer contains stereoregular sequences of 3,4 syndiotactic structure. An interpretation is given of the effect of polymerization temperature on stereospecificity. Some differences between the 1,2 polymerization of butadiene and the 3,4 polymerization of isoprene are examined. (C) 2004 Elsevier Ltd. All rights reserved.