Ziegler type catalysis

Although Ziegler-type catalysts have been widely investigated for the homogeneous hydrogenation of polymers, their catalytic mechanism remains unknown. One possible reason for this may be the complexity of the coordination catalysis and the instability of the catalysts. Metallocene catalysts are highly sensitive to impurities, and consequently it is very difficult to obtain reproducible experimental data providing reliable kinetic and mechanistic information. 

A two-step mechanism for catalysis is widely accepted (1) adsorption of the monomer, which may be activated, with the configuration established in this step, and (2) insertion of the activated monomer into a metal-carbon bond. This sequence places Ziegler-type polymerization in the context of what Nature accomplishes with enzymes. 

As is showm in Figure 2, methane is formed over nickel and ruthenium catalysts, especially at low pressures (atmospheric up to 10 bar) and elevated temperatures. Paraffins and olefins are produced over nickel and cobalt catalysis at mild temperatures (< 200 0) and pressures of I -10 bar. With iron catalysts, olefins,parafllns.and minor amounts of alcohols are formed at medium pressures (10 100 bar) and temperatures of 210--340 C. Ruthenium catalysts give, at elevated pressures (150-1000 bar) and low temperatures (100-180 C), poly methylene with a molecular weiglii of up to I 000000. This polymer has similar properties as Ziegler-type low pressure polyethylene. 

USE Friedel-Crafts catalyst- Component of Ziegler-type catalysis in the condensation of ethylene. Starting material in the synthesis of a number of organic derivs of zirconium, such as alkoxides and zircocene. The alkoxides have been shown to be of value in the curing of silicone plastic films. The alkoxyzirconium carboxylates are said to be useful in the water-repellent treatment of textiles and other fibrous materials. Review Blumenthal, J. Chem. Ed. 39, 604-610 (1962). 

Ziegler-Natta type catalysis is one of two methods used commercially to produce high density polyethylene, the other being metal oxide catalysis. Ziegler-Natta catalysis is very flexible the variety of catalyst systems that fall into this family is immense. In addition to ethylene, many other alkenes may also be polymerized, to produce either homopolymers when reacted in isolation or copolymers when... 

Sometimes a lower density polyethylene is made with both this type of catalysis or Ziegler-Natta. Branching is controlled by the addition of small amounts of 1-alkenes added to the ethylene. 1-Hexene would give a C4 branch, 1-octene a Ce branch, etc. If enough 1-alkene is used the polymer is called linear low-density polyethylene (LLDPE). It is made by a high-density polyethylene process but branching gives a lower density. 

Finally, one last type of natural polymer is natural rubber, obtained from the rubber tree and having the all cw-l,4-polyisoprene structure. This structure has been duplicated in the laboratory and is called synthetic rubber, made with the use of Ziegler-Natta catalysis. 

Ziegler catalysis involves rapid polymerization of ethylene and a-ole-fins with the aid of catalysts based on transition-element compounds, normally formed by reaction of a transition-element halide or alkoxide or alkyl or aryl derivative with a main-group element alkyl or alkyl halide (1,2). Catalysts of this type operate at low pressures (up to 30 atm), but often at 8-10 atm, and, in special cases, even under reduced pressure, and at temperatures up to 120°C, but often as low as 20-50°C. Approximately 2,200,000 tons of polyethylene and 2,900,000 tons of polypropylene are produced per year with the aid of such catalysts. The polyeth-... 

Applications of HT-type catalysts, prepared by the above methods, have been reported in recent years for basic catalysis (polymerization of alkene oxides, aldol condensation), steam reforming of methane or naphtha, CO hydrogenation as in methanol and higher-alcohol synthesis, conversion of syngas to alkanes and alkenes, hydrogenation of nitrobenzene, oxidation reactions, and as a support for Ziegler-Natta catalysts (Table 2). 

The pentamethyl cyclopentadiene lanthanide complexes containing hydrocarbyl substituents have been studied extensively for their applications in homogeneous catalysis and C-H activation. The well-known catalyst of the Ziegler-Nutta type Cp2LnMe(Et20) is typical of the large number of compounds [155] that have been studied. Solvent-free electrophilic alkyl derivatives serve as precursors of the majority of the compounds which have been studied. 

Many examples of such eliminations have now been seen for the f-block and for d metals. This type of /3-aIkyl elimination is recognized as an important chain transfer step in Ziegler-Natta and metallocene polymerization catalysis. When it occurs the polymer chain terminates in a C=C bond (equation 2) and in certain cases the aUcene chain end can undergo reinsertion and get back into the polymer growth... 

In conclusion, the theories based on the plurality of the catalytic active species appear more convincing than those based only on physical phenomena in explaining MWD. Together with some general principles, only a better knowlegde of number and types of polymerization centres and of the relevant kinetic constants could lead to a more effective MWD control. This should represent one of the future trends of research and development in Ziegler-Natta catalysis. 

As well known, the type of transition metal can deeply affect many fundamental aspects of the Ziegler-Natta catalysis (e.g. activity, stereochemical and molecular weight control). Thus a certain influence on polymer polydispersity could be expected. 

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