The (E) isomer in mixtures of (E) and (Z) 1,3-hexadiene was polymerized with the system CoCl2((PPrPh2)-Pr-i)(2)-MAO, a highly active and stereospecific catalyst for the preparation of 1,2 syndiotactic polybutadiene. A new crystalline polymer with a melting point of 109 degrees C was obtained. The polymer was characterized by IR, NMR (C-13, H-1 in solution and C-13 in the solid-state), X-ray diffraction, DSC, GPC and it was found to have a trans-1,2 syndiotactic structure with a 5.18 +/- 0.04 angstrom fiber periodicity. Since only the (E) isomer was polymerized, at the end of the reaction we were able to separate the (Z) isomer, which was ultimately polymerized with CpTiCl3-MAO at low temperature, obtaining a low molecular weight, stereoregular polymer that, characterized by IR and NMR methods, was found to exhibit a cis-1,2 syndiotactic structure, never reported before. Molecular mechanics calculations were carried out on the trans-1,2 syndiotactic polymer and structural models consistent with the Xray diffraction data are proposed. (c) 2007 Wiley Periodicals, Inc.