MECHANISM OF OLEFIN POLYMERIZATION BY A SOLUBLE ZIRCONIUM CATALYST

A mechanistic study has been carried out on the homogeneous olefin pol ymerization/oligomerization catalyst formed from Cp2ZrMe2 and methylal uminoxane, (MeAlO)(x), in toluene. Formal transfer of CH3 from Zr to A l 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. Ch ain 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 oligome r. Terminal Al-C bonds in the latter are ultimately cleaved on hydroly tic workup to produce materials with saturated end groups. Concomitant chain transfer occurs by sigma bond metathesis of Cp2Zr-P+ with ethyl ene. Metathesis results in cleavage of the Zr-C bond of the growing ol igomer to produce materials also having saturated end groups; and a ne w initiating species, Cp2Zr-CH=CH2+. The two chain transfer pathways a fford structurally different oligomers distinguishable by carbon numbe r and end group structure. Oligomers derived from the Cp2ZrMe+ channel are C-n (n = odd) alkanes; those derived from Cp2Zr-CH=CH2+ are termi nally mono-unsaturated C-n (n = even) alkenes. Chain transfer by beta hydride elimination is detectable but relatively insignificant under t he conditions employed. Propylene and 1-hexene react similarly but bet a hydride elimination is the predominant chain transfer step. The init ial Zr-alkyl species produces a Cp2ZrH+ complex that is the principle chain initiator. Chain transfer is fast relative to propagation and th e products are low molecular weight oligomers. (C) 1998 Elsevier Scien ce B.V.