Ligand and solvent effects on the catalytic activity and lifetime of zwitterionic Nickel(II) catalysts for alternating CO-Ethylene copolymerization

A series of zwitterionic nickel(II) compounds with anionic diphosphine ligands having varied substituents have been synthesized and studied as catalysts for CO-ethylene copolymerization. The electron-donating abilities of the ligands are assessed on the basis of the infrared absorptions of the CO-stretching vibrations of their Ni(0) carbonyl compounds. The equilibriums between Ni(II)-methyl and Ni(II)-acetyl and 4-coordinate and 5-coordinate Ni(II)-acetyls are studied using NMR and IR spectroscopies. With respect to the catalytic performances, a certain degree of steric hindrance around the Ni(II) center is required to achieve a high polymerization activity. The steric hindrance likely disfavors the 5-coordinate catalytically dormant state. Further increased steric congestion decreases the catalytic activity. The electron-donating ability of the ligand on the catalytic activity is modest within the relatively narrow range of variation of the present study. The catalyst lifetime and overall productivity are markedly improved by the presence of o-methoxy ancillaries on the phenyl group of the ligand although the o-methoxy decreases the activity of the catalyst. A comparison of the o-methoxy- and o-methyl-substituted ligands attributes the stabilization to the coordinating ability of the o-methoxy group rather than a steric or inductive electronic origin. The present catalysts are several times faster and more productive in THF than in toluene at room temperature. The increased activity is attributable to the coordinating ability of THF as opposed to polarity. The polymer products are strictly alternating copolymers of CO and ethylene.