ESR studies on vanadocene/cocatalyst systems for ethylene polymerization

Three vanadocene-cocatalyst catalytic systems for ethylene polymerization-C p2ZrCl2/Al2Et3Cl3 [dichlorobis(eta -cyclopentadienyl)vanadium/ethylaluminum sesquichloride], Cp(2)ZrCl(2)MAO [dichlorobis(eta -cyclopentadienyl)vanadiu m/methylaluminoxane] and Cp2ZrCl2AlEt3 [dichlorobis(eta -cyclopentadienyl)v anadium/triethylaluminum]-were monitored by electron spin resonance (ESR) s pectroscopy. It was found that at least a certain kind of vanadium complex is formed after mixing dichlorobis(eta -cyclopentadienyl)vanadium with ethy laluminumsesquichloride. After introducing ethylene, a new kind of vanadium complex is detected by ESR. Ethylene can be polymerized by using a dichlor obis(eta -cyclopentadienyl)vanadium/ethylaluminumsesquichloride catalytic s ystem. These results possibly indicate that the vanadium complex exists in the forms ClCp2VClEtAlCl2 and ClCp2VClEtAlClEt, which are responsible for f orming active centers. The Dichlorobis(eta -cyclopentadienyl)vanadium and d ichlorobis (eta -cyclopentadienyl)vanadium/triethylaluminum systems also we re monitored by ESR. Completely different spectra were recorded compared to those of the dichlorobis(eta -cyclopentadienyl) vanadium/ethylaluminumsesq uichloride system. Ethylene almost cannot he polymerized by the dichlorobis (eta -cyclopentadienyl)vanadium/methylaluminoxane and dichlorobis(eta -cycl opentadienyl)vanadium/triethylaluminum catalytic systems, showing that resu ltant vanadium complexes of dichlorobis(eta -cydopentadienyl)vanadium/methy laluminoxane and dichlorobis(eta -cyclopentadienyl)vanadium/triethylaluminu m catalytic systems are different from that arising from the dichlorobis(et a -cyclopentadienyl)vanadium/ethylaluminesquichloride system. Plausible mec hanisms are suggested for this. (C) 2000 John Wiley & Sons, Inc.