CrCl2.(dmpe)(2)-MAO gives crystalline 1,2 syndiotactic polymers from butadiene and 3,4 atactic polymers from isoprene. Butadiene-isoprene copolymerization gives copolymers in which butadiene and isoprene units have respectively a 1,2 and a 3,4 structure. Copolymers with less than 20% isoprene are crystalline, their melting point decreasing from ca. 160degreesC to ca. 50degreesC with increasing isoprene content. This makes possible to obtain 1,2 syndiotactic polybutadiene with controlled melting point. Copolymers with more than 20% isoprene are amorphous, with a glass transition temperature in the range 2divided by5degreesC. Copolymer microstructure was determined by C-13 NMR.