Importance of the Frontal Substituents

To emphasize the importance of fluorenyl s frontal substituents at positions 3 and 6 and to confirm the findings of the computational calculations presented in Sect. 3.2, two new syndioselective catalysts systems are introduced whose ligands 

Stereopentad intensities with polymerization temperature. The data indicate that the syndiotacticity of the polymers produced with the lO/MAO catalyst system at different polymerization temperatures, measured as the concentration of racemic pentads rrrr, are relatively high. The catalysts enantioselectivity, judged by the concentration of the rmmr pentad sequences, are also very high and close to the enantioselectivities measured as rmmr pentads for the catalyst systems consisting of (1, 5, 6 and 9)/MAO. 

The overall stereoregularity and melting points of the polymer are, however, lower for the syndiotactic polymers produced with the lO/MAO catalyst system due to the presence of high concentration of site epimerization related stereo-errors (rrmr pentads). A rather frequent or fast site epimerization rate at higher polymerization temperatures is probably facilitated by the flexibility of the molecular structure of 10 (dynamic umbrella-type reversible inversion at the amido-nitrogen center see [148]) and the rapid same site contact ion-pairing. 

The single-crystal X-ray molecular structure of the complex p-(Me2Si) (3,6di- BuFlu)( BuN)TiCl2 10, is depicted in Fig. 17. It shows a striking similarity to crystal structure of complex 9 with respect to its overall symmetry, despite the exchange of one of the aromatic rings, the cyclopentadienyl, in the molecule and its replacement with an amido group. Complex 10 exemplifies one of the rare examples of a titanium-based syndiotactic-specific metallocene catalyst systems. 

It is in many other aspects different from all the catalyst systems t discussed in previous sections. It is based on a half-sandwich metallocene molecule that cmitains an amido-type N-Ti bond and is a 12-electron system (14-electron system at the most if one considers the contribution of the nitrogen s lone pair of electrons to the overall N-Ti bonding) leaving a cationic 10-electron (maximum 12 including the lone pair electrons of N) electron configuration active site.  

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