Two series of butadiene-isoprene copolymers with a 1,2 and/or 3,4 structure were prepared at different polymerization temperatures, using CrCl2(dmpe)(2)-MAO as a catalyst system. Copolymerization carried out at higher temperature resulted in polymers in the whole range of monomeric ratio, from the highly crystalline 1,2-syndiotactic polybutadiene to the amorphous 3,4-polyisoprene. The molar composition of the butadiene-isoprene copolymers and the syndiotactic index of the butadiene sequences, represented as molar fraction of the syndiotactic pentads, were evaluated by carbon-13 nuclear magnetic resonance spectroscopy. The thermal behavior of the copolymers was investigated by differential scanning calorimetry. Nonisothermal crystallization kinetics were characterized by Ziabicki and Avrami methods as modified by Jeziorny. The crystallization and melting temperatures and the enthalpy of fusion of the copolymers were in good correlation with the syndiotactic index of butadiene sequences. The index was influenced by polymerization temperature and composition of butadiene-isoprene copolymers. (C) 2003 Wiley Periodicals, Inc.