Analyses of Propene and 1-Hexene Oligomers from Zirconocene/MAO Catalysts – Mechanistic Implications by NMR, SEC, and MALDI-TOF MS

A series of Cp′(C₅H₅)ZrCl₂ and Cp′₂ZrCl₂ pre-catalysts (Cp′ = C₅HMe₄, C₄Me₄P, C₅Me₅, C₅H₄^tBu, C₅H₃-1,3-^tBu₂, C₅H₂-1,2,4-^tBu₃) together with (C₅H₅)₂ZrCl₂ was used for the directed oligomerization of propene and 1-hexene in comparative experiments. Oligomer characterization was carried out by ¹H NMR, SEC (GPC), MALDI-TOF MS, cryoscopy and Raman spectroscopy. From ¹H NMR the nature and relative ratio of the double-bond end group is determined together with M̄_n if every oligomer molecule contains such a double-bond end group. Normally vinylidene double bonds (from β-hydrogen elimination) are found. From ¹H NMR and MALDI-TOF MS also vinyl end groups (from β-methyl elimination) are observed in the case of oligopropenes with (C₄Me₄P)- (C₅H₅)ZrCl₂ and with the symmetrical methyl containing Cp′₂ZrCl₂ pre-catalysts. The vinylidene/vinyl ratio depends on the ligand and increases from C₅HMe₄ (65/35) over C₄Me₄P (61/39) to C₅Me₅ (9/91). A comparison of M̄_n from ¹H NMR and SEC together with MALDI-TOF MS shows that the phospholyl-zirconocenes and (C₅HMe₄)₂ZrCl₂ also exhibit chain transfer to aluminium, thereby giving saturated oligomers.