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Transcript of Polymerisation
Production of Materials
identify that ethylene serves as a monomer from which polymers are made
identify polyethylene as an addition polymer and explain the meaning of this term
outline the steps in the production of polyethylene as an example of a commercially and industrially important polymer
Polymerisation is a chemical reaction where many identical small molecules called monomer are joined together to form a larger molecule called a polymer
Importance of Ethylene
Ethylene is a very useful monomer because it can undergo addition polymerisation to form a very long chain.
A polyethylene molecule can be between 300 to 3000 units long
Low Density Polyethylene (LDPE)
High pressure (1000 to 3000 times atmospheric)
High temperature (300 C)
Significant chain branching. (Hydrogen side groups replaced by alkyl group)
High Density Polyethylene (HDPE)
Lower pressure (between 1 and 20 atmosphere)
Lower temperature (60 C)
No side branching
In both cases the reaction is started by an initiator chemical.
An organic peroxide (LDPE)
Titanium (III) chloride and trialkylaluminium (HDPE)
Average Molecular Weight
Because of the way polymerisation occurs, the length of the chain varies.
Generally melting point and hardness increase with chain length.
Uses of LDPE
Lots of chain branching causes it to be relatively soft and flexible
It is used as:
disposable shopping bags
Uses of HDPE
No side branching means that the chains are packed very tightly. This polymer is harder and stronger with higher melting point.
It is used in:
Polyvinyl Chloride (PVC)
PVC is made from the polymer vinyl chloride.
It is often used as electrical insulation, garden hoses and pipes.
The monomer of polystyrene is styrene. A hydrogen of ethylene is replaced by a benzene ring.
Uses of polystyrene are tool handles, foam packing material, disposable drink cups.
In chain branching, the polymer is unable to pack tightly together. This forms a amorphous structure where there is low density, low melting point, greater flexibility and softness.
Where a hydrogen side group is replaced by a larger side group such as benzene. This makes the polymer much more rigid because the larger side groups are unable to move past each other.
Cross Linking and Thermoset
Cross linking occur when the linear chains of polymer are joined together. This forms a massive molecule which is very hard and has very high melting point.
The melting point is so high that it will decompose before it melts (Thermoset)
Plastics that can be melted and remolded are called thermoplastics
Designing a Polymer
Because of the number of different ways that carbon can be bonded. Polymer can be designed with a specific function in mind.
This is why there are so many plastics out there are so are tailor made for their specific function
Substitution of Ethylene