Ulearngo

Formation of Polymers Continued

Initiation, propagation and termination

There are three stages in the process of addition polymerisation. Initiation refers to a chemical reaction that triggers the first reaction. In other words, initiation is the starting point of the polymerisation reaction. Chain propagation is the part where monomers are continually added to form a longer and longer polymer chain.

During chain propagation it is the reactive end groups of the polymer chain that react in each propagation step to add a new monomer to the chain.

Once a monomer has been added the reactive part of the polymer is now in this last monomer unit so that propagation will continue. Termination refers to a chemical reaction that destroys the reactive part of the polymer chain so that propagation stops.

It is important to be able to identify the monomer used to produce a polymer from the repeat unit of the chain.

Example: Identifying Monomers

Question

Which monomer was used to make this polymer (give the name and draw the structure):

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Identify the polymer through functional groups

This polymer contains a chlorine atom. Which polymers contain chlorine? Polyvinyl chloride contains chlorine.

Consider what you know of this polymer

PVC forms through an addition reaction. There are two carbon atoms in the repeat unit (ethane) and the polymer contains chlorine atoms.

Apply this knowledge to the monomer

An addition reaction indicates that the monomer must contain a double bond. As the polymer contains units of ethane the monomer must be an ethene. The monomer must contain a chlorine atom.

Name the monomer

The monomer must be chloroethene also known as vinyl chloride

Draw the structure of this monomer

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Example: Identifying Monomers

Question

Which monomer was used to make this polymer (give the name and draw the structure):

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Identify the polymer through functional groups

This polymer contains a benzene ring connected to two carbon atoms. Which polymers contain a benzene ring? Polystyrene contains a benzene ring connected to two carbon atoms.

Consider what you know of this polymer

Polystyrene forms through an addition reaction. There are two carbon atoms attached to the benzene ring.

Apply this knowledge to the monomer

An addition reaction means that the monomer must contain a double bond. The polymer contains two carbon atoms attached to the benzene ring which must be connected by a double bond in the monomer.

Name the monomer

The monomer must be styrene.

Draw the structure of this monomer

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Example: Identifying Monomers

Question

Give the structure of the monomer used to make polyvinyl acetate:

Draw the repeat unit structure of polyvinyl acetate

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Consider what you know of this polymer

Polyvinyl acetate must form from vinyl acetate. The IUPAC name of vinyl acetate is ethenyl ethanoate. The polymer forms through an addition reaction.

Apply this knowledge to the monomer

An addition reaction means that the monomer must contain a double bond. The only place this bond can occur is between carbon 1 and carbon 2:

2121be3600348b79106697267beced62.png

Draw the structure of the monomer

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Example: Identifying Polymers

Question

Which polymer is formed from propene? Give the name and repeat structural unit.

Draw the structural representation of the monomer

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Consider what you about this monomer

Propene contains a double bond between the first and second carbon atoms.

Apply this knowledge to the polymer

The polymer must form through an addition reaction by breaking the double bond. There must be a methyl group on every second carbon atom.

Name the polymer

The polymer must be polypropene (polypropylene).

Draw the repeat structural unit of this polymer

260428f4810b23a191fa4bd7e1aea745.png

Example: Identifying Types of Polymerisation Reactions

Question

Was an addition or a condensation reaction used to make this polymer?

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Look at the types of bonds and functional groups in the polymer repeat unit

There are only single carbon-carbon bonds, there are no functional groups.

What type of monomer could this polymer have formed from?

As there are no oxygen atoms in the repeat unit, this polymer could only have formed from a monomer containing a double bond.

Apply this knowledge to determine the type of reaction

This polymer must have formed through an addition reaction.

(This is polyethene and was formed by the addition of ethene monomers)

Example: Identifying Types of Polymerisation Reactions

Question

Was an addition or a condensation reaction used to make this polymer?

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Look at the types of bonds in the polymer repeat unit

There are carbon-carbon single bonds. If you expand the repeating unit you can see that there is an ester linkage in the structural chain.

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What type of monomer could this polymer have formed from?

The ester linkage requires an alcohol and a carboxylic acid monomer or a monomer that contains both a hydroxyl and carboxyl functional group.

Apply this knowledge to determine the type of reaction

The reaction of an alcohol and carboxylic acid to form an ester linkage (and resulting in the loss of water) is a condensation reaction.

(This is polylactic acid and was formed by the condensation reaction of lactic acid monomers)

Optional Experiment: Making polymer slime

Aim

To compare the differences in slime made from polyvinyl alcohol and polyvinyl acetate.

Apparatus

  • 4% aqueous polyvinyl alcohol solution, 4% aqueous borax (hydrated sodium tetraborate) solution, white glue (polyvinyl acetate), food colourant

  • a beaker, two disposable plastic cups, a metal spatula, disposable plastic gloves

Method

  1. Place \(\text{40}\) \(\text{cm$^{3}$}\) of the polyvinyl alcohol solution in one plastic cup and label it A.

  2. Place \(\text{20}\) \(\text{cm$^{3}$}\) of the white glue in the second cup. Add \(\text{20}\) \(\text{cm$^{3}$}\) of water and stir. Label this cup B.

  3. Add one drop of food colourant to each cup and stir well.

  4. Measure out \(\text{10}\) \(\text{cm$^{3}$}\) of the borax solution into the beaker, then add to cup A.

  5. Repeat step 4 with cup B.

  6. Stir until gelling is complete (i.e. the slime).

  7. Put on the disposable plastic gloves, take the slime out of cup A and roll it around in your hand to remove air bubbles, repeat with the slime in cup B.

Questions

  1. Pull each slime apart slowly, what happens?

  2. Pull each slime apart quickly, what happens?

  3. Roll each slime into a ball and drop it onto the bench, what happens?

  4. Place a small bit of each slime on the bench and hit it hard with your hand, what happens?

  5. What are the similarities and differences between the two slimes?

Discussion and Conclusions

You should find that the slime does not break under slow stress, but will break if you pull it apart too quickly. This is because the slime can expand under stress. It should even bounce slightly if you throw it onto a hard surface.

This lesson is part of:

Organic Molecules

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