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 CrCl2(dmpe)(2) and Cr(CH3)(2)(dmpe)(2) (dmpe = 1, 2-bis(dimethylphosphino)ethane) and Al(C2H5)(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-amorpho us polyisoprene, cis-1,4-isotactic poly [(E)-pentadiene] and cis-1,4-atacti c poly [(Z)-pentadienel. The factors influencing chemo- and stereoselectivi ty are discussed.