Zirconium-Allyl Complexes as Intermediates in Zirconocene-Catalyzed Olefin Polymerizations
Final Report Abstract
UV-vis spectral changes accompanying the addition of excess 1-hexene to the catalyst precursor SBIZr(µ-Me)2AlMe2+ B(C6F5)4− (SBI = rac-dimethylsilyl-bis(1-indenyl)) in benzene or toluene solution at 23°C are recorded and ambiguities in their interpretation limited through additional data obtained from catalyst re-activation by a second addition of 1-hexene. The results indicate that the onset of polymerization catalysis is accompanied by concurrent formation of two distinct zirconocene species. One of these is observed only while monomer is still present, and is proposed to consist of SBIZr-σ−polyhexyl cations arising by repeated olefin insertions from SBIZr-Me+, which is released from SBIZr(µ- Me)2AlMe2+ by dissociation of AlMe3. The other one of the initially formed species is proposed, based on its UV-vis absorbance band at 560 nm, to consist of SBIZr-π-allyl cations formed by a σ-bond metathesis reaction between SBIZr-Me+ and 1-hexene under release of methane and, hence, probably of composition SBIZr-π-(1-R-C3H4)+ (R = n-propyl). At later reaction stages all zirconocene-σ−polymeryl cations appear to decay by a unimolecular reaction to yet another SBIZr-allyl species, presumably with doubly substituted allyl ligands, i.e. of cations of the type SBIZr-π-(x-R-(3-x)-pol-C3H3)+ (pol = i-polyhexyl, x = 1 or 2). After another addition of excess 1-hexene, monomer insertion into the polymer-containing Zr-allyl moiety of these Zr-allyl cations seems to convert this species back, within a few seconds, to catalytically active SBIZr-σ−polymeryl cations, while the initially formed, less crowded allyl cations SBIZr-π-(1-R-C3H4)+ are left unchanged. Implications of these observations with regard to the kinetics of zirconocenecatalyzed olefin polymerization are discussed.
