Addition Polymers And Macromolecules (O level Chemistry)

 

A polymer is a substance made of very large molecules, formed when lots of small repeating molecules join together. The small repeating molecules that make up a polymer are known as monomers.

 

The molecules of monomers join to form a macromolecule. The macromolecules group together to form a polymer. So, we can say: a polymer is a substance made of macromolecules.

Types of polymerisation

Polymerisation is the process by which thousands of small molecules join to form a large molecule called a macromolecule. There are basically two types of polymerisation:

  • addition polymerisation
  • condensation polymerisation

Addition polymerisation

 

Addition polymerisation is when small molecules join together to form a larger macromolecule which no loss of atoms. In addition polymerisation, double bonds in molecules break and the molecules add on to each other. For addition polymerisation to take place, the monomers must have C=C double bonds.

 

Addition polymerisation is an example of an addition reaction and it occurs when the monomers have double bonds. This means that alkenes undergo addition polymerisation. For example:

  • ethene molecules join together to form poly(ethene), also known a polythene.
  • propene molecules join together to produce poly(propene).

Poly- means many. A polymer molecule, a very large molecule or a macromolecule, usually consists of many thousands of monomer units.


Formation of polythene

Polythene is a plastic that almost everyone uses everyday. It is made by heating ethene to a high temperature under a high pressure in the presence of a catalyst. The catalyst for the reaction is usually a mixture of titanium and aluminium compounds.The double bond in each of the ethene molecules break to form single bonds with two adjacent ethene molecules. The ethene molecules join together to form a very long hydrocarbon molecule called polythene or poly(ethene).

Formation of polythene

The reaction is represented as follows:

n(C2H4) → (C2H4)n

Polythene

where n is a large number, usually many thousands.

Polythene is produced in three main forms:

  • low density poly(ethene) (LDPE)
  • linear low density poly(ethene) (LLDPE)
  • high density poly(ethene) (HDPE).

Uses of polythene

Poly(ethene) has many useful properties:

  • it is easy to mould
  • it is an excellent electrical insulator
  • it is resistant to corrosion
  • it is tough
  • it is durable.

Examples of addition polymers

PolymerMonomerPropertiesUses
Poly(ethene)CH2=CH2Tough, durableCarrier bags, bowls, buckets, packaging
Poly(propene)CH3CH=CH2Tough, durableRopes, packaging
Poly(chloroethene)) or polyvinyl chloride (PVC)CH2=CHClStrong, hardPipes, electrical insulation, guttering
poly(tetrafluoroethene) (PTFE)CF2=CF2Non-stick surface, withstands high temperaturesNon-stick frying pans, soles of irons
Poly(phenylethene) or PolystyreneCH2=CHC6H5Light, poor conductor of heatThermal insulation, packaging (especially as foam)
PerspexCH2=C(CO2CH3)CH3Transparent, brittleUsed as a glass substitute

Poly(tetrafluoroethene) (PTFE)


Perspex

Poly(phenylethene) also known as polystyrene

Poly(chloroethene) also known as polyvinyl chloride (PVC)

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