Hafnocene catalysts activities

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

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. 

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

Among ansa-metallocene catalysts with different group IV transition metals, ansa-titanocene-based catalysts (with rare exceptions [6]) lose most of their activity above 0°C, whereas ansa-hafnocene catalysts usually give lower activities than the analogous ansa-zirconocene catalysts [7, 8], which have thus received most research interest. 

With hafnocene catalysts, higher molecular weight and slightly more stereoregular polymers were produced compared to those produced with zirconocene catalysts, although the catalytic activity was very low. 

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). 

Zrrconium(IV) and hafnium(IV) complexes have also been employed as catalysts for the epoxidation of olefins. The general trend is that with TBHP as oxidant, lower yields of the epoxides are obtained compared to titanium(IV) catalyst and therefore these catalysts will not be discussed iu detail. For example, zirconium(IV) alkoxide catalyzes the epoxidation of cyclohexene with TBHP yielding less than 10% of cyclohexene oxide but 60% of (fert-butylperoxo)cyclohexene °. The zirconium and hafnium alkoxides iu combiuatiou with dicyclohexyltartramide and TBHP have been reported by Yamaguchi and coworkers to catalyze the asymmetric epoxidation of homoallylic alcohols . The most active one was the zirconium catalyst (equation 43), giving the corresponding epoxides in yields of 4-38% and enantiomeric excesses of <5-77%. This catalyst showed the same sense of asymmetric induction as titanium. Also, polymer-attached zirconocene and hafnocene chlorides (polymer-Cp2MCl2, polymer-CpMCls M = Zr, Hf) have been developed and investigated for their catalytic activity in the epoxidation of cyclohexene with TBHP as oxidant, which turned out to be lower than that of the immobilized titanocene chlorides . ... 

Hafnocenes are less active polymerization catalysts and metallocene chlorides are almost inactive82,99. However, treatment of [CP2MCI2] (M = Zr, Hf) with n-BuLi provided efficient catalysts for the dehydropolymerization of n-Bu2SnH2101. 

Chiral titanocenes, zirconocenes, and hafnocenes in combination with methylalu-minoxane [A1(CH3)—0] , can lead to highly isotactic propylene. Nonchiral metallocenes like (Cp)2ZrCl2 or other similar compounds produce only pure atactic polypropylenes. Molecular mass of 590,000 for atactic polypropylenes can be achieved by low polymerization r. The activities of these hydrocarbon soluble catalysts are extremely high. Different structures of polypropylenes are obtained when the rr-bonded ligand of the transition metal is varied (Fig. 1). With no other catalyst can atactic, isotactic, stereoblock, isoblock, and syndiotactic polypropylene of such purity be produced. 

Collins and co-workers later developed a series of related C,-symmetric hafnocene and zirconocene catalysts shown in Figure 22.11 that formed elastomeric poljq)ropylene possessing narrow molecular weight distributions (M /M = 1.7-2.1) when activated with MAO at 25 The catalysts containing a silicon bridge produced relatively high... 

If metallocenes, especially zirconocenes but also titanocenes, hafnocenes and other transition metal compounds (Figure 2) are treated with MAO, then catalysts are acquired that allow the polymerization of up to 100 tons of ethene per g of zirconium [151-153]. At such high activities the catalyst can remain in the product. The insertion time (for the insertion of one molecule of ethene into the growing chain) amounts to some 10 s only (Table 6). A comparison with enzymes is not far-fetched. 

With ansa(chiral) titanocenes, zirconocenes, and hafnocenes in combination with methylalumoxane (MAO) it is possible to obtain highly isotactic polypropene [366-374]. When changing the symmetry of the complex, different structures of the polypropene are yielded. The activity of these hydrocarbon soluble catalysts are extremely high. 

Metallocenes, especially zirconocenes but also titanocenes, hafnocenes, and other transition metal complexes treated with MAO are highly active for the polymerization of olefins, diolefins, and styrene. The polymerization activity, which is up to 100 times higher than for classical Ziegler catalysts, as well as the possibility to easily tailor the microstructure of the polymer chain and to obtain polymers with special properties have motivated research groups worldwide to produce thousands of patents and publications in the last 20 years. An overview can be found in selected review articles and books [55-68]. A metallocene/MAO catalyst containing 1 g zirconium produced 40 x 10 g polyethylene in 1 h at 95°C and 8 bar ethene pressure (Table 1). 

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