Chemistry Open Paper

Chemistry Coursework

    In Polymerisation the term ‘addition polymerisation' is when there are small unsaturated starting molecules which are called monomers and they join together to form a long saturated polymer that has a double bond this is called a ‘free radical'

eg:   A + A + A + A                 A-A-A-A

    During the Polymerisation of Ethene thousands of ethane molecules join together to form the polymer poly (ethene) the number of ethene molecules that join up is variable but is usually in the region of 2000 – 20000.


Conditions that are needed to polymerise ethene to form ldpe:
Conditions
Temperature: about 200°C
Pressure: about 2000 atmospheres
Initiator: a small amount of oxygen as an impurity

The Mechanism that is used for this process is called the free radical addition. There are three Stages in the polymerisation of ethene:

Stage 1: Initiation

This is where the chain is initiated by free radicals, Ra , produced by reaction between some of the ethene and the oxygen initiator.
Stage 2: Propagation

This is where each time a free radical hits an ethene molecule a new longer free radical is formed.
Stage 3: Termination

This is when the two free radicals hit each other producing a final molecule. The process stops here because no new free radicals are formed.
Structure of Polypropene
There are three different types of polypropene to which group they go into depends ondow the CH groups are arranged.The three different types that there are isotatic, Atactic and Syndrotactic polypropene. The most commonly used version is Isotactic:
Isotactic

Isotactic Polypropene, because it forms a very regular arrangement of the CH groups it is possible for the chains to pack together and therefore this maximises the bonding that is between them. Isotactic polypropene is quite strong this is due to there being a long chain length. This also makes it harder to be pulled apart and it will also be a lot harder to melt hence why it is... [continues]

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