Mechanism of olefin polymerization by a soluble zirconium catalyst

A mechanistic study has been carried out on the homogeneous olefin polymerization/oligomerization catalyst formed from Cp₂ZrMe₂ and methylaluminoxane, (MeAlO)_x, in toluene. Formal transfer of CH₃ from Zr to Al yields low concentrations of Cp₂ZrMe⁺ solvated by [(Me₂AlO)_y(MeAlO)_x−y]_y. The cationic Zr species initiates ethylene oligomerization by olefin coordination followed by insertion into the Zr–CH₃ bond. Chain transfer occurs by one of two competing pathways. The predominant one involves exchange of Cp₂Zr–P⁺ (P=growing ethylene oligomer) with Al–CH₃ to produce another Cp₂ZrMe⁺ 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 Cp₂Zr–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, Cp₂Zr-CHCH₂⁺. The two chain transfer pathways afford structurally different oligomers distinguishable by carbon number and end group structure. Oligomers derived from the Cp₂ZrMe⁺ channel are C_n (n=odd) alkanes; those derived from Cp₂Zr–CHCH₂⁺ 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 Cp₂ZrH⁺ complex that is the principle chain initiator. Chain transfer is fast relative to propagation and the products are low molecular weight oligomers.