ESR studies on vanadocene/cocatalyst systems for ethylene polymerization

Three vanadocene-cocatalyst catalytic systems for ethylene polymerization-Cp 2 ZrCl 2 /Al 2 Et 3 Cl 3 [dichlorobis(η-cyclopentadienyl)vanadium/ethylaluminumsesquichloride], Cp 2 ZrCl 2 /MAO [dichlorobis(η-cyclopentadienyl)vanadium/methylaluminoxane] and Cp 2 ZrCl 2 /AlEt 3 [dichlorobis(η-cyclopentadienyl)vanadium/triethylaluminum]-were monitored by electron spin resonance (ESR) spectroscopy. It was found that at least a certain kind of vanadium complex is formed after mixing dichlorobis(η-cyclopentadienyl)vanadium with ethylaluminumsesquichloride. After introducing ethylene, a new kind of vanadium complex is detected by ESR. Ethylene can be polymerized by using a dichlorobis(η-cyclopentadienyl)vanadium/ethylaluminumsesquichloride catalytic system. These results possibly indicate that the vanadium complex exists in the forms ClCp 2 VClEtAlCl 2 and ClCp 2 VClEtAlClEt, which are responsible for forming active centers. The Dichlorobis(η-cyclopentadienyl)vanadium/methylaluminoxane and dichlorobis(η-cyclopentadienyl)vanadium/triethylaluminum systems also were monitored by ESR. Completely different spectra were recorded compared to those of the dichlorobis(η-cyclopentadienyl) vanadium/ethylaluminumsesquichloride system. Ethylene almost cannot be polymerized by the dichlorobis(η-cyclopentadienyl)vanadium/ methylaluminoxane and dichlorobis(η-cyclopentadienyl)vanadium/triethylaluminum catalytic systems, showing that resultant vanadium complexes of dichlorobis(η-cyclopentadienyl)vanadium/methylaluminoxane and dichlorobis(η-cyclopentadienyl)vanadium/triethylaluminum catalytic systems are different from that arising from the dichlorobis(η-cyclopentadienyl)vanadium/ethylaluminumsesquichloride system. Plausible mechanisms are suggested for this.