Abstract: Methylaluminoxane (MAO) remains the most widely used co-catalyst for metallocene and post-metallocene based catalysts. Information on the structure of ¡§cation-like¡¨ species formed upon activation of these catalysts with MAO is crucial in the understanding of polymerization activities. We present here the 1H and 13C NMR spectroscopic study of the cationic intermediates formed upon activation of various zirconium and titanium polymerization catalysts with MAO at varios Al/Zr ratios. The following catalysts were investigated: (Cp-R)2ZrCl2 (R= H, Me, 1,2-Me2, 1,2,3-Me3, 1,2,4-Me3, Me4, Me5, nBu, tBu), rac-C2H4(Ind)2ZrCl2, rac-Me2Si(Ind)2ZrCl2, rac-Me2Si(1-Ind-2-Me)2ZrCl2, rac-Me2Si(2-Me-Benzind)2ZrCl2, rac-Me2Si(1-Ind-2-Me-4-Ph)2ZrCl2, rac-C2H4(1-Ind-4,5,6,7-H4)2ZrCl2, (1-Ind-2-Me)2ZrCl2, Me2C(Cp)(Flu)ZrCl2, Me2C(Cp-3-Me)(Flu)ZrCl2, Me2Si(Flu)2ZrCl2, Si2Me4(2-Ind)ZrCl2, Si2Me4(2-Ind-4,5,6,7-H4)2ZrCl2, Cp2TiCl2, rac-C2H4(Ind)2TiCl2, rac-Me2Si(2-Me-Benzind)2TiCl2, (C5Me5) TiCl3 and [(Me4C5)SiMe2NtBu]TiCl2. Ternary catalytic systems (Cp-R)2ZrCl2/AlMe3 /CPh3+B(C6F5)4ƒ{ (R= Me, 1,2-Me2, 1,2,3-Me3, 1,2,4-Me3, Me4, nBu, tBu) were also studied for comparison of spectroscopic and polymerization data with MAO based systems. It was found that in conditions approaching to real polymerization (Al/Zr or Al/Ti > 200), intermediates [L2Zr(ƒÝ-Me)2AlMe2]+[Me-MAO]ƒ{ (Zr-III), L2ZrMe(+)„]Meƒ{Al( )„kMAO (Zr-IV), [L2Ti(ƒÝ-Me)2AlMe2]+[Me-MAO]ƒ{ (Ti-III) and L2TiMe(+)„]Meƒ{Al( )„kMAO (Ti-IV) are present in the reaction solution. For the more restricted coordination gap aperture metallocenes, heterobinuclear ion pairs of type III strongly dominate in the reaction solution, whereas in the more open half-sandwich, disilyl bridged and constrained-geometry complexes, a closer approach of [Me-MAO] is favored to give mainly ¡¥zwitterion-like¡¦ species of type IV upon activation by MAO. Ethene polymerization activity strongly depends on the Al/Zr ratio (Al/Zr = 200-1000) for the systems (Cp-R)2ZrCl2/MAO (R = H, Me, nBu, tBu), while it changes insignificantly in the same range of Al/Zr ratios for the catalytic systems (Cp-R)2ZrCl2/MAO (R = 1,2-Me2, 1,2,3-Me3, 1,2,4-Me3, Me4 ). These data are interpreted on assumption that complex III is the main precursor of the active centers of polymerization in (Cp-R)2ZrCl2/MAO systems. Using stable nitroxyl radical 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) as a spin probe, Lewis acidity of MAO and a series of aluminoxanes derived from different aluminum alkyls was compared and correlated with their ability to activate (C5Me5)2ZrCl2 for ethene polymerization. The cocatalytic activity of aluminoxanes increases with the increase of their Lewis acidity. The uniqueness of MAO as a co-catalyst is likely related to its optimal Lewis acidity and high stability. The intermediates of olefin polymerization over homogeneous catalysts based on 2,6-bis(imino)pyridine iron(II) chloride (LFeCl2) and AlMe3 or MAO as activators have been studied by 1H and 2H NMR. In conditions approaching to real polymerization, neutral species of the type [LFe(II)(Cl)(ƒÝ-Me)2AlMe2] or [LFe(II)(Me)(ƒÝ-Me)2AlMe2] dominate in the reaction solution in LFeCl2+AlMe3 systems, whereas in LFeCl2/MAO systems, ion pairs [LFe(II)(ƒÝ-Me)(ƒÝ-Cl)AlMe2]+[Me-MAO]- (at Al/Fe < 200) and [LFe(II)(ƒÝ-Me)2AlMe2]+[Me-MAO]- (at Al/Fe> 500) are the predominant species.

Cite: Talsi E.P. , Bryliakov K.P. , Semikolenova N.V. , Zakharov V.A.
MR Studies of the Intermediates of Group 4 Metallocenes and Bis(Imino)Pyridine Iron(II) Catalyzed Olefin Polymerization
In compilation New Developments in Organometallic Chemistry Research. – Nova Science Publishers, Inc.., 2006. – C.151-190. – ISBN 1594548447.

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