Isoprene Polymerization with Pyrazolylimine Cobalt(II) Complexes: Manipulation of 3,4-Selectivities by Ligand Design and Use of Triphenylphosphine

A series of pyrazolylimine CoCl2 complexes were synthesized and well characterized. The single crystal structures and coordination geometries of these complexes were confirmed by X-ray diffraction, which revealed dimeric and monomeric structures, and thereby distorted trigonal bipyramidal and distorted tetrahedral geometries, respectively. In combination with diethylaluminum chloride (DEAC) and methylaluminoxane (MAO) as cocatalysts, the present cobalt complexes displayed high activity toward isoprene polymerization, affording cis-1,4/3,4-polyisoprenes. Polymerization parameters have significant influence on the polymerization behavior. Additionally, incorporation of the external donor triphenylphosphine led to a switch in selectivity of the system from cis-1,4 to 3,4, providing an effective method to adjust the 3,4-moieties on a large scale.