Polyethylene Manufacture by Ziegler Method

Introduction:
The Ziegler-Natta method is a widely used industrial process for the manufacture of polyethylene. It involves the use of a catalyst system that allows for the controlled polymerization of ethylene, resulting in the production of high-density polyethylene (HDPE) or low-density polyethylene (LDPE). The catalyst used in this process plays a crucial role in determining the properties of the final polymer product.

The Catalyst:
The catalyst used in the Ziegler-Natta method for polyethylene manufacture consists of two main components: a transition metal compound and an organoaluminum compound. These two compounds work together synergistically to initiate and control the polymerization reaction.

Components of the Catalyst:
The two most commonly used compounds in the Ziegler-Natta catalyst system for polyethylene manufacture are:

1. Titanium Tetrachloride (TiCl4):
- Titanium tetrachloride is a transition metal compound that acts as the active site for the polymerization reaction.
- It is capable of coordinating with the double bond of the ethylene monomer, initiating the polymerization process.
- TiCl4 is highly reactive and can readily react with water or air, making it unstable in its pure form. Therefore, it is typically used in a complexed form in the catalyst system.

2. Triethyl Aluminum (Al(C2H5)3):
- Triethyl aluminum is an organoaluminum compound that acts as a cocatalyst in the Ziegler-Natta catalyst system.
- It reacts with the titanium tetrachloride to form a Lewis acid-base pair, which stabilizes the active site and controls the polymerization reaction.
- Triethyl aluminum also helps in the activation of the titanium tetrachloride by removing any chloride ions that may inhibit the reaction.

Role of the Catalyst:
The titanium tetrachloride and triethyl aluminum catalyst system functions as follows:

1. Activation of the Catalyst:
- The triethyl aluminum reacts with the titanium tetrachloride to form a stable complex, which activates the titanium compound for the polymerization reaction.
- This activation step is crucial for the catalyst to initiate the polymerization process and control the chain growth.

2. Polymerization:
- The activated catalyst coordinates with the ethylene monomer to form a metal-alkyl intermediate.
- This intermediate can undergo chain growth through repeated coordination with additional ethylene molecules.
- The polymerization reaction continues until the desired molecular weight or polymer chain length is achieved.

Conclusion:
In the manufacture of polyethylene by the Ziegler method, the catalyst system consists of titanium tetrachloride and triethyl aluminum. These two compounds work together to initiate and control the polymerization reaction, resulting in the production of polyethylene with specific properties. The titanium tetrachloride acts as the active site for the polymerization, while the triethyl aluminum functions as a cocatalyst, stabilizing the active site and controlling the reaction.