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Addition Polymers

 

This is part of the HSC Chemistry course under the topic Polymers. There are two types of polymers: addition and condensation polymers.

HSC Chemistry Syllabus

  • model and compare the structure, properties and uses of addition polymers of ethylene and related monomers, for example:

– polyethylene (PE)
– polyvinyl chloride (PVC)
– polystyrene (PS)
– polytetrafluoroethylene (PTFE) (ACSCH136)
 

Addition Polymers: HDPE, LDPE, PVC, PS and PTFE

This video introduces the concept of addition polymers. The video discusses the structure, properties and uses of various addition polymers including:

  • Polyethylene (PE)
  • Polyvinylchloride (PVC)
  • Polystyrene (PS)
  • Polytetrafluoroethylene  (PTFE)

 


How are Addition Polymers Produced?

  • Addition polymers are produced using monomers with double carbon-to-carbon bonds (C=C).

 

Polymerisation of ethylene produces polyethylene

  • Reactive C=C of alkene molecules can ‘open up’ to form single carbon-carbon bonds with other alkene molecules. This results in the formation of a long, saturated polymer chain.

 

 Step 1: Initiation

  • An initiator molecule e.g. hydrogen peroxide (H2O2) produces free radical OH species with a highly reactive unpaired electron.
  • The unpaired electron forms a new covalent bond with one of the electrons in the C=C bond in the monomer. This produces another radical molecule with an unpaired electron that was formerly in the C=C bond.

Step 2: Elongation

  • The radical molecule formed in initiation is able to react with another monomer via the same mechanism, that is the unpaired electron forms a covalent bond with one of the electrons in the C=C bond.
  • This reaction joins the two organic molecules to produce a larger molecule that also contains an unpaired electron.

 

  • As the reaction between the radical molecule and monomers continues, the molecule grows in length

 

 Step 3: Termination

  • Elongation of the molecule stops when two radical molecules (with unpaired electrons) react to form a covalent bond. This terminates the polymerisation as the product no longer has an unpaired electron.
  • Termination can occur with hydroxyl radical molecules produced from hydrogen peroxide or another large polymer that has an unpaired electron. 

  • Two relatively large radical molecules react to produce a final product (polymer)


High-density Polyethylene (HDPE)

  • Structure: HDPE consists of straight, linear chains of polyethylene with no or minimal branching.

 

Properties

Uses
  • The chains produced are linear and can pack closely together – high density.
  • Straight-chained HDPE have greater dispersion force than LDPE. Therefore, they have higher melting points, harder and are crystalline in

 

 
  • Plastic utensils
  • Plastic bottles
  • Plastic toys

 

Low-density Polyethylene (LDPE)

  • Structure: polyethylene chains with numerous branches.

 

Properties

Uses
  • Branched structure of LDPE causes it to have reduced area of contact and dispersion forces between polymer chains. This results in a lower melting point than HDPE. 
  • Branched structure gives rise to LDPE’s amorphous nature i.e. the shape of LDPE can be easily changed.

 
  • Plastic bags 
  • Films
  • Plastic packaging

Polyvinyl chloride (PVC)

  • Structure: PVC is produced from polymerisation of chloroethene (vinyl chloride). 
  • Compared to ethylene, chloroethene has one hydrogen substituted by a chlorine.

 

 

Properties

Uses
  • Presence of Cl atom causes PVC to have much greater molecular mass than LDPE and HDPE. Cl’s electronegativity provides a permanent dipole-dipole force between polymer chains. This leads to stronger intermolecular force. PVC has higher melting points than HDPE and LDPE.
  • PVC can be manufactured to be either rigid or quite flexible, depending on its production method and other added chemicals. 
  • PVC is durable, particularly resistant to weathering (suitable for outdoor uses).
  • PVC is resistant to acids. 
  • Like most polymers, PVC is a poor conductor of electricity and so acts as a good electrical insulation material.

 

Rigid PVC

  • Gas pipes and water drainage pipes because it is durable and rigid.
  • Window frames

 

Flexible PVC

  • Cling wrap
  • Insulation for cables and wires
  • Inflatables

 

Polystyrene (PS)

  • Structure: PS is produced from polymerization of ethenylbenzene (styrene).
  • An ethyenylbenzene monomer is an ethene molecule with one of the hydrogen atoms replaced by a large and bulky benzene ring.
  • A benzene ring is a six-carbon membered ring with alternating C=C bonds.

 

 

 

Properties of PS

  • Extensive dispersion forces are present between adjacent chains of polystyrene. However, the bulkiness of benzene rings causes PS to be brittle.
  • Transparent
  • Light weight and low density
  • Thermally insulating 
  • Electrically insulating

 

Uses of PS

  • In gas-expanded (aerated) form, polystyrene is used as Styrofoam in disposable containers and packaging
  • Transparency and light weight properties are useful in cassette cases.
  • Light weight and electrically insulating properties are useful in handles of screwdrivers.

 

Polytetrafluoroethylene (PTFE)

  • Structure: PTFE is produced from polymerisation of tetrafluoroethene. Each monomer is essentially an ethene molecule with all 4 hydrogen atoms replaced by fluorine atoms.

 

Properties of PTFE

  • Extensive dispersion and dipole-dipole forces between chains of PTFE gives rise to its high melting point. Thermally stable at temperatures up to 250 ºC.
  • Almost totally chemically inert.
  • Insoluble in most solvents, resistant to water
  • Flame resistant 
  • Acid resistant 
  • Very low coefficient of friction

 

Uses of PTFE

  • Non-stick cooking pans
  • Low frictional resistance warrants its use in many areas of engineering e.g. gaskets.

 

 

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