Catalysts procatalyst

Methods for identifying additional Ziegler-Natta co-catalysts in a catalyst system containing procatalysts magnesium and titanium are described by Campbell et al. (4). 

Semicrystalline poly(ethylene-co-propylene) was prepared by the authors (1) using a Zr/Si procatalyst, (I), supported on an organic porous substrate having pore sizes of 0.1 and 2 xm. The catalyst was prepared after activating... 

A remarkable complex (33) with a C2-symmetric semicorrin ligand has been recently developed by Pfaltz and coworkers.64 A copper(II) complex was used as a procatalyst, but (33) was shown to be the active cyclopropanation catalyst. As shown in Table 3, this complex resulted in spectacular enantioselecti-vities in the range of 92-97% ee. Once again, the (15,35,4/ )-menthyl group attenuated the selectivity. Unfortunately, even though respectable yields were obtained with dienes and styrenes, the reaction with 1-heptene was rather inefficient. 

Ziegler-Natta catalysts are defined as the products formed in reactions of transition metal compounds of groups 4 to 8 (procatalysts, catalyst precursors) with organometallic compounds or metal hydrides of groups 1 to 4 (activators). These reactions are carried out in an inert medium and under inert (anaerobic) conditions ... 

Finally, most of the MgCl2-supported Ziegler-Natta catalysts for propylene polymerisation also require treatment of the supported procatalyst with... 

Nevertheless, many vanadium-based catalysts and polymerisation systems comprising them have received much academic attention in the hope that they might provide models for heterogeneous catalysts and polymerisation systems, since the problems connected with surface properties and particle size were believed to have been overcome. It must be noted, however, that homogeneous vanadium-based catalysts appeared to be more complex than was thought. There is no decisive evidence on the structure of catalytic sites formed by reaction between the procatalyst and activator. 

In connection with the above characteristic features of metallocene methy-laluminoxane catalysts, it must be emphasised that alternative, potentially cheaper alkylaluminoxanes, such as ethyl or z-butyl derivatives, which are more soluble in aliphatic hydrocarbons than methylaluminoxane, or other alkylaluminium compounds used as activators for metallocene procatalysts, show inferior activity. 

During the last decade, a variety of new catalysts have been presented for the stereospecific polymerisation of a-olefins, based on non-bridged metallocene or stereorigid ansa-metallocene as the procatalyst and a methylaluminoxane activator [29,30,37,105-107,112-114,116-135], Apart from isotactic [118,119,124, 131,132] and syndiotactic [23,118,124,133] polypropylenes and other poly(a-olefin)s [121], hemiisotactic [112,121,124], isoblock [131,132,134], syndioiso-block (stereocopolymer) [127], stereoblock isotactic [135] and stereoblock isotactic atactic [116,128,129] polypropylenes have been obtained using these new catalysts. 

The symmetry of the metallocene and also the kind of procatalyst metal atom, the nature of the catalyst activator and the polymerisation temperature determine the polypropylene tacticity. The general stereoregulation behaviour of metallocene catalysts may be explained in terms of the local chirality, or chirotopicity, of the catalytic sites bonded to the same metal atom. For this analysis, the structure of metallocenes as catalysts should be considered. 

The range of polypropylene microstructures available by procatalyst ligand (and/or metal) variation in the metallocene-methylaluminoxane system is illustrated in Table 3.1 [22,23,101,105,107,112,113,124,127,132,137], The ligands shown in Table 3.1, representative of the particular symmetry and class of the catalyst, are given as examples only. There are a variety of other metallocenes that have been successfully used to obtain polypropylenes of various stereostructures. 

In the case of propylene polymerisation with class III catalysts based on rac.-dimethylsilylenebis[l-(3-methylcyclopentadienyl)]zirconium dichloride [rac.-Me2Si(MeCp)2ZrCl2] (Table 3.1) or rac.-dimethylsilylenebis[l-(3-methyl-cyclopentadienyl)]hafnium dichloride [rac.-Me2Si(MeCp)2HfCl2] as a procatalyst, isoblock polypropylene is obtained [132]. 

For the majority of olefin polymerisations with heterogeneous Ziegler Natta catalysts, the polymerisation rates, Rv, are proportional to the concentrations of procatalyst (MtX ) and monomer (M), but do not depend on the concentration of alkylaluminium activator (A) as long as a threshold concentration is maintained [37] ... 

The Ziegler-Natta catalyst 2,6-diacetylpyridinebisiron(II) chlorotrimethylalu-minium, (I), and procatalyst 2,4- [N-(2,6-dimethylphenyl)]phenylimidoyl 6-methyl pyrimidine iron dichloride, (11), were prepared by Kimberley [1] and Gibson [2], respectively, and used as high-activity 1-olefin polymerization catalysts. 

Variation of the Procatalyst (Metal Component) and the Acetylenic Substrate. The in situ catalysts Co(acac)3-Et2AlCl-phosphine have proven to be well-suited for the synthesis of 4-aryl- and 4-alkyl-substituted deltacyclenes. The catalysts tolerate remote oxygen functionalities in the acetylenic substrate. However, they could not be used with functionalized acetylenes such as propargylic acid derivatives. 

Usually, a catalyst has to be synthesized or conditioned prior to its use in a catalytic reaction. However, there is an alternative to such an isolated or preformed catalyst, the so-called in-situ catalyst. The in-situ catalyst is prepared by mixing the transition metal compound (the procatalyst) and the ligand (the cocatalyst) in the solvent in which the reaction is to be carried out [79]. The use of in-situ catalysts is most appropriate in enantioselective hydrogenation reactions with Wilkinson-type catalysts. The optically active phosphines needed for optical induction have to be synthesized in multi-step syntheses [80, 81]. It is most convenient to combine them directly with the Rh-containing procatalysts. 

The procatalyst [Rh(cod)Cl]2 is an orange, air-stable solid which is commercially available, accessible in one step from RhCls and 1,5-cyclooctadiene [82], The cocatalyst DIOP (Figure 3), the most frequently used optically active phosphine, is also commercially available. A survey of the literature shows that more than half of the numerous studies on the hydrogenation of (Z)-a-acetamido-cinnamic acid have been carried out with in-situ catalysts [79, 81]. 

Procatalyst and cocatalyst combine in solution to give the actual catalyst, a procedure most suitable for routine application. There are no synthetic steps necessary to prepare the catalyst prior to the catalytic reaction to be performed. 

Successful in-situ catalysts for this transfer hydrogenation used BPPM (Figure 3) as a cocatalyst together with either [Rh(cod)Cl]2, Rh2(OAc)4 or RhCla as a procatalyst [84]. The enantioselectivities matched those of the hydrogenation with molecular hydrogen. Replacing triethylamine by (S)-l-phenylethyl-amine, the stereoselectivity (>97 % ee) was even better [85]. 

Both 7 and 9 are procatalysts. The active catalyst, which is likely to be a copper(I) monochelate, is generated by heating in the presence of the diazocarbonyl compound at ca. 60-80 °C for a few minutes, by reduction with phenylhydrazine (9 ) or an alkylhydrazine (7 ), or by treatment with ca. 0.25-0.5 equivalents of diisobutylaluminum hydride. ... 

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