Cyclopentadienyl metallocenes

Further simulations have been performed. In contrast to what was observed for bis-cyclopentadienyl metallocenes, mono-cyclopentadienyl systems did reveal a significant barrier to insertion [lOj. However, for all these systems it turned out that insertion only proceeded after the formation of a relatively stable agostic interaction, an observation that clearly supports the Brookhart-Green mechanism. 

Both the permethylmetallocene and the cyclopentadienyl metallocene, low-valent titanium and zirconium systems are extremely reactive, primarily because of their electron-deficient carbenoid character (Section I,B). Generally speaking, there are a number of important differences vis-a-vis the chemistry of the [t7-C5(CH3)5]2 M and (77-C5 Hs)2M (M = Ti, Zr) systems. 

Unbridged (fluorenyl) (cyclopentadienyl) metallocene dichloride complexes are potential catalyst precursors for the production of syndiotactic polypropylene. There are three prerequisites the catalyst precursors have to fulfill Cg symmetry and a ligand ensemble with one ligand being much smaller (Cp) than the other (Flu). Moreover, the hindered rotation... 

In the USA Exxon Chemical and Dow Plastics were the leaders in the metallocene technology. While Exxon explored both mono- and bis-cyclopentadienyl metallocenes, Dow focused on constrained geometry catalysts based on Ti-monocyclopentadienyl metallocenes. Exxon first produced metallocene-based polymers with its Exxpol catalysts in 1991. Dow uses its INSITE technology to make ethylene-octene copolymers, introduced in 1993. Copolymers with up to 20 wt% octene are sold as AFFINITY plastomers, competing with specialty polymers in packaging, medical devices, and other applications. Dow, producing its own catalyst, considers that it leads to the uniform introduction of comonomers and long-chain branches that improve processability of otherwise linear polymers. 

Yamazaki reported meso-selective formation of binuclear fx-oxo-ansa-metallocenes [75]. This can be a convenient method for separating meso and racemo-isomers of ansa-bis(substituted cyclopentadienyl)metallocenes, although it might be less effective for non-substituted ansa-bisindenyl complexes [76]. 

---