PLEASE NOTE: For the exam you need to be able to describe what the steps: Initiation, propagation and termination mean and put them in order (see next slide)

Low Density Polyethylene is manufactured using a free radical mechanism (initiated with a peroxide initiator) under high pressure.

2)Initiation: Initiator added, bonds break and radicals are formed

1)Monomers3) Free radicals + monomers

4)Propagation:monomers join to radical; chain grows.

5) Long chain radicals + monomers

6)Termination:Long chain radicals join. Polymer formed. .

7)Complete polymer

PLEASE NOTE: You DO NOT need to be able to recall the detail on the remaining slides, though it will help you to understand how the process works.

In the initiation stage, radicals are made using an organic peroxide catalyst.

Low Density Polyethylene

The O–O bond is weak and breaks to make R–O radicals (alkoxy radicals).

Low Density Polyethylene: Initiation

An alkoxy radical combines with one of the electrons from the double bond inethylene.

Low Density Polyethylene

An alkoxy radical combines with one of the electrons from the double bond inethylene.

Low Density Polyethylene: Propagation

This forms a new covalent bond and a new radical.

Low Density Polyethylene: Propagation

The new radical reacts with another ethylene molecule.

Low Density Polyethylene: Propagation

This forms another new radical, longer than before.

Low Density Polyethylene: Propagation

The chain grows longer until two radicals react with each other.

Low Density Polyethylene

This is called the termination step.

Low Density Polyethylene: Termination

A growing chain may curl around and attack itself by back-biting.

Low Density Polyethylene: Branched chains

The radical moves to another part of the chain, and allows branched chains to grow from the side.

Low Density Polyethylene: Branched chains