Ziegler – Natta Catalyst

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

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 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. 

Reaction involved can be represented as follows -

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Classes of Ziegler Natta Catalyst 

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. 

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

 

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 atom is coordinated with 6 chlorine atoms and this compound has 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.

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