Isospecific Oligo-/Polymerization of Styrene with Soluble Cationic Nickel Complexes. The Influence of Phosphorus(III) Ligands

Abstract

The influence of several phosphorus(III) ligands upon the activity and the stereoregulation ability of the soluble organometallic complex [Ni(η3-(2-methylallyl))(η4-COD)]PF6 (COD = 1,5-cyclooctadiene), 1, is studied. There is a general increase of the catalytic activity which can be associated with the P(III) ligand's lability. A detailed analysis of the mass distribution and microstructure is carried out by GPC and 1H and 13C NMR of the solvent-fractionated polymers. In the majority of cases, regioregular low molecular weight polymers with the structure PhCHCH2[CH(Ph)CH2]nCH(Ph)CH3 are obtained. The addition of bulky phosphines, such as P(o-Tol)3 (o-Tol = o-tolyl) and PCy3 (Cy = cyclohexyl), affords highly isotactic oligomers (≈90%). For the latter, the regioselectivity of the chain growth is broken during the insertion of the last monomer, which results in the formation of a tail-to-tail end group, CH2C(C6H5)CH(C6H5)CH2−. The superimposed effects of η3-coordination of the chain end and the large stereochemical influence of these ligands seem to be the reason for the observed isospecifity.