Mechanism of olefin polymerization by a soluble zirconium catalyst

Abstract

A mechanistic study has been carried out on the homogeneous olefin polymerization/oligomerization catalyst formed from Cp 2ZrMe 2 and methylaluminoxane, (MeAlO) x , in toluene. Formal transfer of CH 3 from Zr to Al yields low concentrations of Cp 2ZrMe + solvated by [(Me 2AlO) y (MeAlO) x− y ] y . The cationic Zr species initiates ethylene oligomerization by olefin coordination followed by insertion into the Zr–CH 3 bond. Chain transfer occurs by one of two competing pathways. The predominant one involves exchange of Cp 2Zr–P + (P=growing ethylene oligomer) with Al–CH 3 to produce another Cp 2ZrMe + initiator plus an Al-bound oligomer. Terminal Al–C bonds in the latter are ultimately cleaved on hydrolytic workup to produce materials with saturated end groups. Concomitant chain transfer occurs by sigma bond metathesis of Cp 2Zr–P + with ethylene. Metathesis results in cleavage of the Zr–C bond of the growing oligomer to produce materials also having saturated end groups; and a new initiating species, Cp 2Zr-CHCH 2 +. The two chain transfer pathways afford structurally different oligomers distinguishable by carbon number and end group structure. Oligomers derived from the Cp 2ZrMe + channel are C n ( n=odd) alkanes; those derived from Cp 2Zr–CHCH 2 + are terminally mono-unsaturated C n ( n=even) alkenes. Chain transfer by beta hydride elimination is detectable but relatively insignificant under the conditions employed. Propylene and 1-hexene react similarly but beta hydride elimination is the predominant chain transfer step. The initial Zr-alkyl species produces a Cp 2ZrH + complex that is the principle chain initiator. Chain transfer is fast relative to propagation and the products are low molecular weight oligomers.