Hafnocenes

The dichlorides of hafnocene and zirconocene were found by Hillman, Weiss and Hahne to have retentions of 15—60% and to show very strong isotope effects. The isotope effects were even larger in benzene solution than in the crystalline targets, reaching a value of 4.13 for the ratio of the retentions of "" Hf and Hf. 

While Cp2Hf (CO)(PPh3) (62) was found to be more reactive toward acetylenes than Cp2Hf(CO)2 (3), no monocarbonyl-rj2-acetylene complexes of hafnocene were observed, in contrast to the reaction of acetylenes with Cp2Ti(CO)(PPh3) (42) (50). Instead, the reaction of 62 with RfeCR (R = Et, Ph) led to the respective hafnacyclopentadienes (55). 

IR and H-NMR Spectral Properties for Hafnocene Monocarbonyl-Phosphine Complexes... 

Characterization of the various monocarbonyl-phosphine complexes of hafnocene was achieved by IR and H-NMR spectroscopy. These data appear in Table V. 

Abstract Zirconocenes have been used for a long time in the field of olefin polymerization using MAO as cocatalyst. The equivalent hafnocenes were seldom used due to a lack of productivity while using MAO activation. In the last few years borane and borate activation has come into the focus of research for olefin polymerization. A variety of different hafnocenes were used to investigate the polymerization mechanism and the different cocatalysts. 

Keywords Borate activation, Hafnocene catalysts, Olefin polymerization, Ultrahigh Mw polyolefins... 

In a time when prices for oil are continuously rising, hafnocene catalysts enable production of high-tech materials in a relatively cheap way. Hafnocenes are the key to a family of high and ultrahigh molecular weight polypropylenes whose material properties can be individually and simply varied. To help comprehend the importance... 

Collins et al. reported in 1995 that catalysts based on hafnium are desirable for the production of elastomeric polypropylene in that they polymerize propylene to a high molecular weight polymer and are indefinitely stable under typical polymerization conditions [8], Based on the theory that hafnium as a catalytic center leads to a significant increase of molecular weight in propene polymerization compared with the zirconium-based catalyst, Rieger et al. searched for hafnocene systems to obtain polymers with new properties. 

To find more information about the influence of steric effects on the polymerization performance, catalyst 2 (5,6 triptycene indenyl hafnocene dichloride) was developed. 

Since plastomeric polypropylenes were insufficiently investigated, further improvement of catalyst activity correlated with an easy synthetic approach was required. Therefore, the two asymmetric hafnocene dichloride complexes, each bearing a 2,5,7- and 2,4,6-trimethyl substituted indenyl moiety (4a, 4b) were developed. 

As inactive species, a binuclear complex with a CH3 bridge between the metal and A1 center is existent (Fig. 10) [20], For the regeneration of the binuclear inactive products to the active hafnocene cations, alkyl-exchange reactions are assumed between the M-CH2-A1- unit and MAO. 

The experimental data of the polymerization reactions of propylene performed with the hafnocene compounds 1-4 after MAO or [(C6H5)3C+][(C6F5)4B ] activation are summarized in Table 1. 

The catalyst 4b/borate was tested under similar conditions as in the case of 4a (toluene solution and liquid propylene) in propylene polymerization experiments after preactivation with TIBA (Table 1). According to the data from Table 1, the catalytic properties of 4b are inferior to those of 4a. The behavior of 4b is similar to that of asymmetric catalysts with a forward orientation of the 4-substituted indene unit [10]. The effect of the substitution position is remarkable. While the 5,7-substituted hafnocene 4a shows higher activities (up to 3.2 x 105 kg PP mol 1 Hf h 1 at 40 °C) with increasing temperatures, substantially lower or almost no activities were found for the 4,6-substituted hafnocene 4b at the same temperature (Fig. 13). 

The combination of structural studies and polymerization experiments of a series of hafnocene catalysts has provided greater insight into the polymerization mechanism and the possibilities of tailoring polymer microstructures. 

Depending on the hafnocene catalyst, it is possible to obtain polypropylenes with different tacticities in a range from 10 to 80%. Taking into consideration the... 

Based on the current situation that oil prices are continuously rising and the industrial process of natural gas to methanol, methanol to propylene, propylene to polypropylene becomes more important, it will be interesting to see what importance hafnocene-catalyzed polypropylenes will have in future daily life. 

Rieger B, Troll C, Preuschen J (2002) Ultrahigh molecular weight polypropylene elastomers by high activity dual-side hafnocene catalysts. Macromolecules 35 5742-5743... 

Rieger B, Troll C, Hild S, Cobzaru C, Deisenhofer S, Hearley A (2005) Novel high and ult-rahigh molecular weight poly(propylene) plastomers by asymmetric hafnocene catalysts. Macromol Chem Phys 206 1231-1240... 

Deisenhofer S (2002) Entwicklung neuer Zirkonocen- und Hafnocen-Katalysatorarchitekturen und deren Verwendung in der metallocenkatalysierten Olefinpolymerisation. Dissertation, Ulm... 

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