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 200oC

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 the most commonly used form of polypore. isotactic polypropene is of a crystalline structure

Atactic

.

Atactic Polyprope is when they when the carbon groups are arranged in a random order along the carbon cahin so therefore atactic polypropene is amorphous irregular. The lack of regularity makes it impossible for the chains to lie closely to one another so therefore the polymer is soft a flexible and has a low density and has a low density and a low softening temperature because of these properties it is only used to make things such as waterproof coatings etc..

Syndiotactic

This is another regularly arranged version of polypropene.In this case every alternate CH group is oriented in the same way. The regularity of the syndiotatic polypropene means that there are the chains are quite long and therefore they will also be very strong and will also have a high tensile strength and will have greater intermolecular force's it will be more difficult to pull apart HOWEVER syndiotactic polypropene is still not as strong as isotactic as the syndiotactic is softer.

The structure of polyethene

There are two different structures of polyethene hdpe and ldpe (high density and low density polyethene.

LDPE

In ldpe the polymer chains are extensively branched and therefore would not be very strong as they would be unable to be packed closely together and it would be