Ziegler-Natta catalyst system structures

Polymers containing 90-98% of a c 5-1,4-structure can be produced using Ziegler-Natta catalyst systems based on titanium, cobalt or nickel compounds in conjuction with reducing agents such as aluminium alkyls or alkyl halides. Useful rubbers may also be obtained by using lithium alkyl catalysts but in which the cis content is as low as 44%. 

Recently, Doi152) speculated on the presence of two types of bimetallic active centers, based on 13C NMR analysis of the structure and stereochemistry of polypropylene fractions obtained with different Ziegler-Natta catalyst systems (see Fig. 44). Site A produces highly isotactic polypropylene, site B atactic polypropylene consisting of isotactic and syndiotactic stereoblocks. The formation of the latter fraction would be due to the reversible migration of the aluminum alkyl, made... 

Metallocene catalysts for the polymerizations of olefins have been known since early 1957 when Natta and co-workers first reacted triethyl aluminum (AlEts) and bis(cyclopentadienyl) titanium dichloride ( -C5H5)2TiCl2 to form a complex that polymerized ethylene. The structure of this complex was described and the polymerization results reported. With ethylene they reported to have made 7 g of crystalline polyethylene in about 8 h at 95°C with 40 atm ethylene pressure in n-heptane. Later in the same year, Breslow and co-workers repeated Natta s experiments. They found that the blue complex described by Natta was a somewhat poor catalyst (in agreement with Natta s findings), but discovered that small amoimts of oxygen in the ethylene boosted polymerization activity. When compared to the heterogeneous Ziegler-Natta catalyst system, these metallocene catalysts were poor with respect to polymerization activity. They were used essentially for mechanistic studies because of their simplicity and ease of structure elucidation. 

Haszeldine et al. [478] studied the effect of vanadium compounds together with zinc and aluminum alkyls in tetrahydrofuran (THF). The most satisfactory modified Ziegler-Natta catalyst system is obtained when vanadium oxytrichloride and triisobutyl aluminum react together in the presence of an excess THF. The initiation mechanism and the structure of the active complexes is studied in a further publication of Haszeldine. The results demonstrate that the active centers in the catalyst system VOCl3/Al(iBu)3/THF are formed by interaction of vanadium oxydichloride tetrahydrofuranate and triisobutylalu-minum in the presence of THF ... 

Sodium-catalyzed polybutadienes contain a preponderance of 1,2-structures but since there are also significant quantities of other microstructures the products are not stereoregular. Since the discovery of the Ziegler-Natta catalyst systems both syndiotactic and isotactic 1,2-polybutadienes have been prepared. The syndiotactic polymers are obtained by the use of aluminium triethyl and halogen-free compounds of vanadium, molybdenum and cobalt, particularly the acetyl acetonates. 

The most important characteristic of the Ziegler-Natta catalyst system is its ability to stereoregulate the polymer. The configuration of the resultant polymer depends on the choice of the catalyst system and its crystalline structure. Stereoregulation is believed to occur as follows ... 

Special catalyst system (Ziegler-Natta) required Three structures ... 

Supports used for obtaining Ziegler-Natta catalysts can differ essentially from one another. Some of the supports may contain reactive surface groups (such as hydroxyl groups present in specially prepared metal oxides) while others do not contain such reactive functional groups (such as pure anhydrous metal chlorides). Therefore, the term supported catalyst is used in a very wide sense. Supported catalysts comprise not only systems in which the transition metal compound is linked to the support by means of a chemical covalent bond but also systems in which the transition metal atom may occupy a position in a lattice structure, or where complexation, absorption or even occlusion may take place [28]. The transition metal may also be anchored to the support via a Lewis base in such a case the metal complexes the base, which is coordinatively fixed on the support surface [53,54]. 

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