Polymer Technology

1.      Natural – Synthetic

Based on the origin of the material, whether natural or synthesized.

 

2.      Organic – Inorganic

Organic polymers have carbon backbone. PE, PP, PS etc. Inorganic polymers do not contain carbon backbone. Glass, Silicone polymers

 

3.      Thermoplastic – Thermosetting Thermoplastics soften on heating.

Do not soften or melt on heating. Cross-linked chains.

 

4.      Plastics, elastomers, fibers, resins

Classified as per the use of polymeric material.

 

5.      Addition polymers – Condensation polymers Based on manufacturing process.

The classification based on manufacturing process is important for the course, ‘Industrial Polymerization’.

 

Addition Polymerization: Also called as Chain Polymerization, Chain growth Polymerization, Chain Reaction Polymerization.

Condensation Polymerization: Also called as Step Polymerization, Step Growth Polymerization, Step Reaction Polymerization.

 

Examples:

Addition polymers: Polyethylene, Polypropylene, PVC, Polystyrene

Condensation polymers: Polyesters, Nylon 66, Polycarbonates, Polyurethanes, Epoxy resins.

 

There are other types of addition polymerization which do not fit into  the categories of polymerization that were mentioned before. It is not within the scope of this course to mention all of the types available.

However, one of the most important types of catalyst is the so-called "Ziegler-Natta" catalyst discovered by Karl Ziegler in 1953.

 

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These catalysts can be prepared by mixing

• Alkyl
• Aryl

of an element from Groups 1 – 3

with a halide (Group 7) of a transition element, as below

Before this new catalyst had been invented polyethylene had been produced by free radical polymerization at

• high pressures (1000-3000 atmospheres)
• high temperatures (250°C).

However, this method had resulted in branched polymers being formed.

Question – Why do you think the free radical type of polymerization produces branched polymers?

 

Giulio Natta realized the potential of this new type of polymerization for the production of stereospecific polymers. This means that polymer chains could now be produced that had high degrees of regularity in their tacticity. How? Let us look at the mechanism.

 

Taking propylene as an example, the monomer forms a TT-complex with the titanium active site (a vacant d-orbital).  

 

• low pressures
• ambient temperatures

After the coordination of the monomer at the active site, there is a

transition state with a ring structure involving Ti and C atoms. The new polymer chain then moves back to its original position, which means that the stereochemistry is kept the same as the polymer grows.

 

 

 In summary, the propagation step can be written as

 

 

This discovery had enormous academic and industrial importance and in 1963 Ziegler and Natta were jointly awarded the Nobel Prize for Chemistry.

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