Cambridge AS & A Level9701

Addition polymerisation

Chemistry 9701 Chapter Notes

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Addition polymerisation
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In this chapter

  1. 1Introduction to Polymerisation
  2. 2The Addition Polymerisation Reaction
  3. 3Drawing a Polymer from a Monomer
  4. 4Identifying a Monomer from a Polymer
  5. 5Common Addition Polymers and Their Uses

1. Introduction to Polymerisation

Polymers are very large, long-chain molecules, essential to everyday life in the form of plastics. They are built by joining together thousands of small, reactive molecules called monomers in a process called polymerisation. Think of a polymer as a long chain, and each individual link in that chain is a monomer. Addition polymerisation is a specific type where monomers 'add' to each other, with no other substance being produced.

Key term

Polymer: A very large molecule (macromolecule) formed by the chemical bonding of many identical or similar smaller molecules (monomers).

Worked example 12 marks

Define the terms 'monomer' and 'polymerisation'.

  1. 1

    Step 1: Define monomer. A monomer is a small, reactive molecule that can be joined together in repeating units to form a polymer.

  2. 2

    Step 2: Define polymerisation. Polymerisation is the chemical process in which monomers are joined together to form a polymer.

Recap

  • Polymers are long-chain molecules made from repeating units.
  • The small molecules that form these units are called monomers.
  • The process of joining monomers to form a polymer is called polymerisation.
  • Addition polymerisation is one major type of this process.

Quick check

  1. What is the name for the small, reactive molecules that join together to form a polymer?1 mark

2. The Addition Polymerisation Reaction

Addition polymerisation happens with unsaturated monomers, which are molecules containing a carbon-carbon double bond (C=C). The reaction is typically initiated by high temperatures, high pressures, and a catalyst. During the reaction, the weaker pi (π) bond within each C=C double bond breaks. This allows the monomers to link together, forming a long, saturated carbon chain called the polymer backbone. Because the monomers simply 'add' together, the polymer is the only product formed.

n (Monomer) → -(Repeat Unit)-n

n CH₂=CH₂ → -[CH₂-CH₂]-n

Key term

Monomer: A small, reactive molecule, typically containing a C=C double bond for addition polymerisation, that joins with other monomers to form a polymer.

Examiner insight

Examiners look for a clear understanding that the polymer is the only product, which distinguishes addition from condensation polymerisation.

Common pitfall

Simply stating that the 'double bond breaks' is not precise enough for higher marks. You must specify that it is the weaker pi (π) bond that breaks, leaving the sigma (σ) bond intact.

Worked example 13 marks

Explain, in terms of bonding, how ethene molecules polymerise to form poly(ethene).

  1. 1

    Step 1: Identify the key feature of the ethene monomer. Ethene (CH₂=CH₂) is an unsaturated molecule containing a C=C double bond.

  2. 2

    Step 2: Describe the bond breaking. During polymerisation, the weaker pi (π) bond in the C=C double bond of each ethene molecule breaks.

  3. 3

    Step 3: Describe the bond formation. This allows the ethene monomers to join end-to-end, forming new carbon-carbon single (σ) bonds between the units.

  4. 4

    Step 4: State the final structure. A long, saturated hydrocarbon chain, which is the polymer poly(ethene), is formed.

Recap

  • Addition polymerisation requires unsaturated monomers with a C=C bond.
  • The reaction involves the breaking of the pi (π) bond in the double bond.
  • Monomers join to form a long chain with a saturated carbon backbone.
  • The polymer is the only product of an addition polymerisation reaction.

Quick check

  1. What type of bond in an alkene monomer must break for addition polymerisation to occur?1 mark
  2. Why is this process called 'addition' polymerisation?1 mark

3. Drawing a Polymer from a Monomer

A very common exam question asks you to draw the repeating unit of a polymer from a given monomer. It's a simple process if you follow these steps. First, focus only on the two carbons in the C=C double bond. Redraw the monomer so that the C=C bond is horizontal and any other atoms or groups are pointing up or down from these two carbons. Then, change the double bond to a single bond and draw new single bonds extending outwards from each of these two carbons. Finally, place large square brackets around this two-carbon unit and write a subscript 'n' on the outside to show it repeats many times.

Monomer: CH₂=CHX → Repeat Unit: -[CH₂-CHX]-n

Key term

Repeat Unit: The specific arrangement of atoms, derived from the monomer, that occurs repeatedly along the chain of a polymer.

Examiner insight

Examiners specifically award marks for showing the continuation bonds extending through the square brackets, as this indicates the chain continues indefinitely.

Common pitfall

The most common mistake is drawing the polymer backbone with more than two carbon atoms. For example, drawing poly(propene) as -[CH₂-CH-CH₃]-. The CH₃ group is a side branch, not part of the main chain.

Worked example 12 marks

Propene (CH₂=CHCH₃) undergoes addition polymerisation. Draw the repeat unit of the resulting polymer, poly(propene).

  1. 1

    Step 1: Identify the C=C double bond in propene. The atoms involved are CH₂ and CH.

  2. 2

    Step 2: Redraw the monomer focusing on the C=C bond. Place the CH₃ group attached to the CH carbon either up or down. So, H₂C=CH(CH₃).

  3. 3

    Step 3: Change the double bond to a single bond: -H₂C-CH(CH₃)-.

  4. 4

    Step 4: Draw continuation bonds extending out from the two carbons of the original double bond.

  5. 5

    Step 5: Enclose the structure in square brackets and add the subscript 'n'. The final structure is -[CH₂-CH(CH₃)]-n.

Worked example 22 marks

Draw a section of the polymer poly(chloroethene) showing three repeat units joined together. The monomer is chloroethene, CH₂=CHCl.

  1. 1

    Step 1: Determine the repeat unit from the monomer chloroethene. This is -[CH₂-CHCl]-.

  2. 2

    Step 2: Draw this unit three times, joined together by single bonds.

  3. 3

    Step 3: The final structure is -[CH₂-CHCl]-[CH₂-CHCl]-[CH₂-CHCl]-. Ensure the bonds correctly link the carbon atoms of the backbone.

Recap

  • Focus on the two carbons of the C=C double bond.
  • Change the double bond to a single bond in the polymer chain.
  • Draw continuation bonds extending through the brackets of the repeat unit.
  • The repeat unit of an addition polymer always has a two-carbon backbone.
  • Remember to include the square brackets and the subscript 'n'.

Quick check

  1. When drawing the repeat unit for poly(propene), how many carbon atoms are in the main polymer backbone?1 mark

4. Identifying a Monomer from a Polymer

You may also be asked to work backwards: identify the monomer from a given polymer structure. This is the reverse of the previous skill. First, locate the repeating pattern in the polymer chain. For an addition polymer, this will be a unit with two carbon atoms in the main backbone. Isolate one of these repeat units. Then, remove the single continuation bonds that link it to its neighbours and re-form the C=C double bond between the two carbon atoms of the backbone. The resulting alkene molecule is your monomer.

Repeat Unit: -[CR₁R₂-CR₃R₄]-n → Monomer: R₁R₂C=CR₃R₄

Key term

Addition Polymer: A polymer whose backbone consists only of carbon atoms and is formed from unsaturated monomers adding together.

Common pitfall

Forgetting to re-form the C=C double bond is the most frequent error. Students correctly identify the repeat unit but then present it as the monomer, which is incorrect as monomers for addition polymerisation must be unsaturated.

Worked example 12 marks

A section of a polymer chain is shown below. Identify the monomer used to make this polymer. -[CF₂-CF₂]-[CF₂-CF₂]-

  1. 1

    Step 1: Identify the repeating unit in the polymer chain. The unit that repeats is -[CF₂-CF₂]-.

  2. 2

    Step 2: Isolate one repeat unit: F₂C-CF₂.

  3. 3

    Step 3: Remove the continuation bonds and re-form the double bond between the two carbon atoms.

  4. 4

    Step 4: The resulting molecule is CF₂=CF₂. This is the monomer.

  5. 5

    Step 5: Name the monomer: tetrafluoroethene.

Worked example 22 marks

The polymer poly(phenylethene) has the repeat unit -[CH₂-CH(C₆H₅)]-n. Draw the displayed formula of the monomer.

  1. 1

    Step 1: Identify the two-carbon backbone in the repeat unit: -CH₂-CH-.

  2. 2

    Step 2: Note the groups attached: two H atoms on the first carbon, and one H atom and one phenyl group (C₆H₅) on the second carbon.

  3. 3

    Step 3: Re-form the double bond between the two backbone carbons to get CH₂=CH(C₆H₅).

  4. 4

    Step 4: Draw the displayed formula, showing all atoms and bonds. Ensure the phenyl group is correctly attached to a carbon involved in the double bond.

Recap

  • To find the monomer, first identify the two-carbon repeat unit in the polymer chain.
  • Isolate one repeat unit and remove the bonds that link it to the chain.
  • Create a C=C double bond between the two carbon atoms of the backbone.
  • The resulting alkene is the monomer.
  • Always double-check that the groups attached to the carbons are correct.

Quick check

  1. A polymer has the repeat unit -[CH₂-CCl₂]-n. What is the chemical formula of the monomer?1 mark

5. Common Addition Polymers and Their Uses

The properties of an addition polymer depend on the monomer used, which in turn determines its use. Three key examples are:

  1. Poly(ethene): Formed from ethene. It's flexible, cheap, and a good insulator. It's used for plastic bags, cling film, and washing-up liquid bottles.
  2. Poly(propene): Formed from propene. It is stronger, stiffer, and more heat-resistant than poly(ethene). This makes it suitable for ropes, carpets, food containers, and car bumpers.
  3. Poly(chloroethene), or PVC: Formed from chloroethene. It is a very versatile plastic. When rigid, it's used for window frames and drainpipes. With added plasticisers, it becomes flexible and is used for electrical cable insulation and flooring.

Key term

Poly(chloroethene) (PVC): A common and versatile thermoplastic polymer formed from the monomer chloroethene, used in both rigid and flexible forms.

Fun fact

The 'vinyl' in 'polyvinyl chloride' (PVC) comes from the old name for the ethenyl group (CH₂=CH-), which was the 'vinyl group'. This is why vinyl records are made of PVC!

Worked example 12 marks

Poly(propene) is used to make ropes and crates. State one property of poly(propene) that makes it suitable for these uses and explain how this relates to its structure.

  1. 1

    Step 1: State a relevant property. A key property of poly(propene) is its high tensile strength and rigidity.

  2. 2

    Step 2: Relate this property to its use. High strength makes it suitable for making strong ropes that don't easily snap, and rigid crates that hold their shape under load.

  3. 3

    Step 3: Relate to structure. Poly(propene) has long, tightly packed polymer chains with strong intermolecular forces between them, which require a lot of energy to overcome, leading to its strength and higher melting point compared to poly(ethene).

Recap

  • Poly(ethene) is flexible and used for plastic bags and bottles.
  • Poly(propene) is strong and rigid, used for ropes, crates, and carpets.
  • Poly(chloroethene) (PVC) is versatile, used for pipes when rigid and cables when flexible.
  • The properties of a polymer are determined by its monomer structure and the forces between its chains.

Quick check

  1. Which common polymer would be most suitable for insulating electrical wires?1 mark

End-of-chapter exercise

Test yourself on the whole chapter. Work through these before moving on.

  1. Define addition polymerisation, stating the essential structural feature of a suitable monomer.2 marks
  2. The monomer but-2-ene (CH₃CH=CHCH₃) is polymerised. Draw the repeat unit of the resulting polymer.2 marks
  3. A polymer has the following structure: -[CH(CN)-CH₂]-n. Draw the structure of the monomer and state its name.3 marks
  4. Draw a section of poly(ethene) showing three repeat units joined together.2 marks
  5. Explain why poly(propene) is generally stronger and has a higher melting point than poly(ethene).3 marks
  6. State one use for poly(chloroethene) (PVC) and the property that makes it suitable for that use.2 marks
  7. Write a balanced chemical equation, using structural formulae, for the polymerisation of chloroethene (CH₂=CHCl).2 marks
  8. A student incorrectly draws the repeat unit of poly(propene) as -[CH₂-CH-CH₃]-. Identify the two mistakes in their drawing.2 marks
  9. An addition polymer is formed from a monomer with the formula C₃H₅Cl. Draw the structures of two possible monomers and the repeat unit for the polymer formed from one of them.4 marks
  10. Distinguish between a monomer and a repeat unit. Use the polymerisation of ethene as an example in your explanation.3 marks

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