Ziegler – Natta Catalyst

In chemistry, a catalyst is any substance that accelerates a process without being consumed. Many vital metabolic reactions are catalyzed by enzymes, which are naturally occurring catalysts.

Metals and their oxides, sulphides, and halides, as well as the semi-metallic elements boron, aluminum, and silicon, make up the majority of solid catalysts. Solid catalysts are usually disseminated in other substances known as catalyst supports; gaseous and liquid catalysts are commonly utilized in their pure form or in combination with suitable carriers or solvents.

A catalyst is a substance that either increases or decreases the rate of reaction without taking part in that reaction. Ziegler-Natta catalyst is used in the synthesis of polymers of 1-alkenes (also known as alpha-olefins). The Ziegler-Natta catalyst also increases the rate of polymerization. It is named after German Chemist Karl Ziegler and Italian Chemist Giulio Natta. 

Ziegler- Natta catalyst contains two parts – a transition metal compound and an organoaluminum compound. It contains transition metal from group IV such as Ti, Zr, Hf, etc. Organoaluminum compounds contain bonds between aluminum and carbon atoms. Examples of Ziegler-Natta catalysts include TiCl₄+Et₃Al and TiCl₃+AlEt₂Cl. So, if we want to write the chemical formula of one of the Ziegler-Natta catalysts then it can be represented as TiCl₄-Al(CH₃)₂(CH₂)₂Cl.

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Ziegler Natta catalyst is used in the polymerization of -olefins. A general reaction can be represented as follows – 

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Discovery of Ziegler – Natta Catalyst 

The Ziegler Natta catalyst was discovered by Karl Ziegler. He got the Nobel Prize in chemistry in 1963 for the discovery of titanium-based catalysts. The Giulio Natta prepared stereoregular polymers from propylene. On the basis of that the catalyst is named as Ziegler-Natta Catalyst.  

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Preparation of Ziegler-Natta Catalyst 

Ziegler-Natta catalyst is formed by the reaction of transition metal compounds of group IV – VIII (derivatives of alkyl, aryl of alkoxide or halide) of the periodic table with an alkyl metal halide or alkyl halide of groups I to III. Compounds of transition metal-containing TI, V, Cr, Co, Ni, etc. are majorly used for the preparation of Ziegler- Natta catalyst. 

The reaction involved can be represented as follows -

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Classes of Ziegler Natta Catalyst 

To create or get transition metal halides belonging to groups IV-VIII in the current periodic table, they are often reacted with organometallic compounds belonging to groups I – III. Catalysts by Ziegler-Natta. A classic example is a combination of titanium tetrachloride (TiCl₄) and trimethylaluminum (Al(C₂H₅)₃). Catalysts have proven to be quite advantageous.

Ziegler – Natta catalysts can be divided into following classes – 

• Heterogeneous catalysts 

• Homogeneous catalysts 

Heterogeneous Catalysts - Heterogeneous catalysts are those catalysts which are based on titanium compounds and then are combined with organometallic compounds. 

In polymerization operations, heterogeneous supported catalysts based on titanium compounds are utilised in conjunction with cocatalysts, organoaluminum compounds such as triethylaluminum, Al(C₂H₅)₃. This type of catalyst is the most common in the business.

Homogeneous Catalysts - Homogeneous catalysts are based on the compounds of Hf and Zr. They contain metallocenes (compounds that consist of two cyclopentadienyl anions) and multidentate oxygen-based ligands. These are soluble in the reaction medium. 

These are commonly composed of titanium, zirconium, or hafnium compounds. They're generally combined with methyl aluminoxane, a distinct organoaluminum cocatalyst (or methylalumoxane, MAO). Metallocenes are commonly used in these catalysts, but they also include multidentate oxygen and nitrogen-based ligands.

Mechanism of Ziegler – Natta Catalytic Polymerization 

We are explaining the mechanism with respect to TiCl₄ + AlEt₃. Mechanism of polymerization of Ziegler – Natta catalyst can be explained by following four steps - 

Step 1. Activation of Ziegler - Natta Catalyst 

Titanium atoms are coordinated with 6 chlorine atoms and this compound has a crystal structure. When it reacts with AlEt₃, it gets one ethyl group from AlEt₃. Aluminum gets attached to a chlorine atom. While one chlorine atom gets removed from titanium compounds. Thus, now the catalyst has an empty orbital on the surface. Now the catalyst is activated by the coordination of AlEt3 and titanium. 

Reaction can be written as follows

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 Step 2. Initiation 

This polymerization is initiated by formation of alkene metal complexes.

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 Step 3. Propagation 

Availability of free propylene molecules in the reaction propagates the reaction. When more propylene molecules keep coming, the process takes place again and again and gives linear polypropylene. 

Reaction can be written as follows :

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Step 4. Termination 

It is the final step of the chain reaction. After this step, we get the desired product. In Ziegler Natta catalytic-polymerization termination may take place by several approaches.

Application of Ziegler-Natta Catalyst

A versatile and important polymerization process is the Ziegler-Natta catalyst polymerization reaction. The following are some of the most common uses for this catalyst:

• They're used to make high-density and low-density polyethylene.

• Thermoplastic polyolefins, polybutylene, crystalline polypropylene, and carbon nanotube nanocomposites are all manufactured.

A Ziegler–Natta catalyst is a catalyst used in the synthesis of 1-alkene polymers. It is named after Karl Ziegler and Giulio Natta (alpha-olefins). There are two types of Ziegler–Natta catalysts used, each with its own solubility. Transition metal halides, such as titanium, vanadium, chromium, and zirconium, as well as organic non-transition metal derivatives, particularly alkyl aluminum compounds, are commonly used in today's Ziegler-Natta catalysts.

In coordination polymerizations, the Ziegler-Natta catalyst is utilized, and it involves complexes generated between a transition metal and the electrons of a monomer. The insertion of monomers at the end of the expanding chain, where the transition metal ions are attached, is typically how polymerization is performed. The entering monomers are all coordinated at the same moment at unoccupied orbital locations, resulting in lengthy polymer chains. The C=C bond is also included in the TiC bond at the active center.

Finally, the chain-growth polymerization reaches the termination stage, which produces "dead" polymers (the intended result). Anionic polymerization, which creates linear and stereo-regular polymers, is comparable to these processes.