Chemoselectivity and Stereospecificity of Chromium(II) Catalysts for 1,3-Diene Polymerization

1,3-Butadiene, isoprene, ( E )- and ( Z )-1,3-pentadiene were polymerized with catalyst systems based on CrCl 2 (dmpe) 2 and Cr(CH 3 ) 2 (dmpe) 2 (dmpe = 1,2-bis(dimethylphosphino)ethane) and Al(C 2 H 5 ) 3 or methylaluminoxane (MAO). The chemo- and stereoselectivity of these systems are strongly influenced by monomer structure. They give 1,2-syndiotactic polybutadiene, 3,4-amorphous polyisoprene, cis -1,4-isotactic poly[( E )-pentadiene] and cis -1,4-atactic poly[( Z )-pentadiene]. The factors influencing chemo- and stereoselectivity are discussed.