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K326 / K328 / 2027
Organic Chemistry

Lesson 6 of 7

Addition polymers: open the double bond, keep the substituents

A repeat unit shows the pattern within a long chain.

O-Level 5086 / 5088 (2026) / SEC G3 K326 / K328 (2027)

  • I can describe ethene reactions.Syllabus

    Syllabus K326 / K328, 11.2(i). Describe ethene reactions

  • I can describe polymers and monomers.Syllabus

    Syllabus K326 / K328, 11.4(a). Describe polymers and monomers

  • I can explain poly(ethene) formation.Syllabus

    Syllabus K326 / K328, 11.4(b). Explain poly(ethene) formation

  • I can identify poly(ethene) uses.Syllabus

    Syllabus K326 / K328, 11.4(c). Identify poly(ethene) uses

  • I can deduce addition-polymer structures.Syllabus

    Syllabus K326 / K328, 11.4(d). Deduce addition-polymer structures

Addition polymers: open the double bond, keep the substituents

A repeat unit shows the pattern within a long chain.

Make a guess. You are not marked.

Thousands of ethene molecules, CH2=CH2, join to make poly(ethene). What happens to each C=C double bond?

  1. It opens to a single bond and links to the next unit
  2. It breaks fully and the carbon atoms separate
  3. It stays as a double bond along the whole chain
Show the answer

It opens to a single bond and links to the next unit

Addition polymerisation uses one bond of each double bond to join the monomers, so the chain has only single carbon-carbon bonds.

A polymer is a large molecule built from many smaller monomer molecules. Different monomers give different repeating units and properties. In addition polymerisation, the C=C bonds open to link monomers into a chain; no small molecule is eliminated. Poly(ethene) forms from ethene and is used in plastic bags and clingfilm.

Draw an addition polymer
  1. Start with the alkene

    Identify the two carbon atoms of C=C.

  2. Change the double bond to a single bond

    Give each of those carbons a new bond to the next unit.

  3. Preserve every attached group

    Do not detach or lose hydrogens, chlorine atoms or side chains.

  4. Show repetition

    Enclose a repeat unit in brackets, with backbone bonds extending through the brackets and n outside.

Ethene gives the repeat unit [-CH2-CH2-]n. Propene, CH2=CH-CH3, gives [-CH2-CH(CH3)-]n. The CH3 side group stays attached; it does not become a third carbon in the two-carbon backbone repeat.

Poly(ethene): bonds through the brackets

Two backbone carbon atoms have a single bond between them and open bonds passing through the brackets to neighbouring units. Each carbon has two hydrogen atoms.

The n means many repeats. The bonds crossing the brackets join neighbouring units; they are not additional hydrogen atoms. Restoring C=C between the two backbone carbons gives ethene.

Poly(propene): keep the side group

Two backbone carbon atoms have a single bond between them and open bonds passing through the brackets to neighbouring units. A complete CH3 side group remains attached to the second carbon.

The n means many repeats. The bonds crossing the brackets join neighbouring units; they are not additional hydrogen atoms. Restoring C=C between the two backbone carbons gives propene.

Two repeat units of poly(propene)

A chain of four carbon atoms joined by single bonds, with bonds continuing off both ends. Every carbon has one hydrogen on top. Below, the first and third carbons have a hydrogen and the second and fourth carbons have a CH3 group.

When asked for two repeat units, draw four backbone carbons, all with single bonds, plus a bond sticking out at each end to show the chain continues. Each propene unit keeps its CH3 group, on every second carbon.

Worked exampleRecover a monomer

A polymer repeat unit is [-CH2-CHCl-]n. Deduce its monomer.

Try it first. Then show the worked stepsHide the worked steps
  1. Identify the two backbone carbons.
  2. Remove their links to neighbouring repeat units and restore a C=C double bond.
  3. Keep the chlorine attached to the same carbon.
Answer

CH2=CHCl. An unfamiliar monomer can be deduced from bonding without memorising its name.

Check your understanding

Should the repeat unit of poly(ethene) keep the C=C double bond of ethene?

  1. No. The C=C becomes a triple bond so that each carbon can join two chains.
  2. No. The C=C opens to a single bond so each carbon can join the chain.
  3. Yes. The C=C bond is what holds the repeat units together along the polymer chain.
Show the answer

No. The C=C opens to a single bond so each carbon can join the chain.

One bond of C=C breaks so that each carbon can join the next unit. Keeping the double bond as well would give each carbon five bonds.

Check your understanding

Chloroethene is CH2=CHCl. Which describes two repeat units of its addition polymer?

  1. -CH2-CHCl-CH2-CHCl-: four carbons, all single bonds, Cl on alternate carbons
  2. -CH=CCl-CH=CCl-: four carbons, double bonds kept, Cl on alternate carbons
  3. -CHCl-CHCl-CH2-CH2-: four carbons, all single bonds, Cl atoms side by side
Show the answer

-CH2-CHCl-CH2-CHCl-: four carbons, all single bonds, Cl on alternate carbons

Each C=C opens and the units join head to tail. Show a continuation bond at each end. The polymer is poly(chloroethene), also called PVC.

Check your understanding

A polymer has the repeat unit [-CF2-CF2-]n. What is its monomer?

  1. CF2=CF2
  2. CHF=CHF
  3. CF3-CF3
  4. CF2=CF-CF3
Show the answer

CF2=CF2

Remove the bonds to the next units and put back the C=C double bond. Each carbon keeps its two F atoms.

Exam-style question

Ethene is the monomer of poly(ethene).
(a) Name the type of polymerisation, and describe what happens to the ethene molecules. [2]
(b) State one use of poly(ethene). [1] [3]

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