Hafnocene, polymerization

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... 

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. 

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. 

Tilley, T. D. Woo, H.-G. Catalytic Dehydrogenative Polymerization of Silanes to Polysilanes by Zirconocene and Hafnocene Catalysts. A New polymerization Mechanism, (H. F. Harrod, R. M. Laine, Eds.) Inorganic and Organometallic Oligomers and Polymers, Kluwer Academic Publishers, Netherlands, 1991, p. 3. 

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. 

Under the same conditions, syndiospecihc (Cs-symmetric) metallocenes are more effective in inserting a-olefins into an ethylene copolymer than isospecific (C2-symmetric) metallocenes or unbridged metallocenes. In particular, hafnocenes are more efficient than zirconocenes. An interesting effect is observed for the polymerization with ethylene(bisindenyl)zirconium dichloride and some other metallocenes. The catalytic activity for the homopolymerization of ethylene is very high, and it increases when copolymerization with propylene occurs (114) (Fig. 12). Munoz-Escalona et al. (125) observed similar effects in the copolymerization of ethylene with 1-hexene. 

In polymerization at low temperatures, the time required to form one polymer chain is long enough to consume one monomer fully and allow the subsequent addition of another one. Thus, it becomes possible to synthesize block copolymers, provided that the polymerization (especially when it is catalyzed by hafnocenes) starts with propylene and, after the propylene is nearly consumed, continues with ethylene. 

In an effort to enhance the activity and syndiospecificity of 6 -symmetric ansa-Cp-Flu metallocene complexes in the MAO-co-catalyzed polymerization of propylene, a large number of fluorenyl-substituted ansa-Cp-Flu metallocene complexes have been prepared,879-881 including the di-/z r/-butyl-substituted derivative 1129.882 The hafnocene... 

Alkene complexes of Ti, Zr and Hf have been intensively investigated with regard to the nature of bonding and the close relation to olefin oligomerization and polymerization. Alkene complexes of zirconocene and hafnocene are isolated as the trimethylphosphine adduct, Cp2M(T -alkene)(PMe3) (33) [92-94]. Cp 2Ti(CH2=CH2) (34) is a 16 electron ethylene complex with a rich reaction chemistry as summarized in Scheme 6.4 [95-99]. The reaction profile of 34 indicates that the metallacyclopropane canonical form makes an important contribution [100]. 

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. 

The dehydrogenating polymerization of hydrosilanes was found to be catalyzed by Group 4 metallocene alkyl and sUyl derivatives (equation 26). For zirconocene and hafnocene catalysts, the reaction was found to proceed via a-bond metathesis steps. ... 

The important influence of propene concentration and polymerization temperature on the regioregu-larity and end group structure of metallocene i-PP have been realized only recently, thanks to detailed H and NMR analysis of the polymers made with some prototypical zirconocenes. No data are available on the corresponding hafnocenes or titanocenes. With the moderately isospecific rac-C2H4(l-Ind)2ZrCl2/ MAO catalyst, the total amount of secondary insertions does not depend on monomer concentration, while it shows a dependence on polymerization temperature. On the other hand, the chemical structure of the chain fragment generated by an isolated secondary unit does depend on both polymerization temperature and monomer concentration. 

FIGURE 2.20 Metallocenes used for the preparation of isotactic-hemiisotactic polypropylene. The [m] dyad fractions are for MAO-cocatalyzed polymerizations performed in liquid propylene at 0 °C. Hafnocene [m] values are given in parentheses. 

Braunschweig reported that his R2NB-bridged ansa-zirconocene complexes are approxinoately seven times more active than their hafnocene analogues toward ethylene polymerization, while the hafnocene complexes produce higher molecular weight polymer (Table 5.1, entries 12 and 15). This is consistent with the alkene polymerization behavior of other zirconocene and hafnocene systems. 

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. 

CATALYTIC DEHYDROGENATIVE POLYMERIZATION OF SILANES TO POLYSILANES BY ZIRCONOCENE AND HAFNOCENE CATALYSTS. A NEW POLYMERIZATION MECHANISM. 

Figure 3. Proposed mechanism for dehydrogenative silane polymerization by zirconocene and hafnocene catalysis.

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