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Organic Chemistry

Topic 7 of 9

Addition polymers: open the double bond, keep the substituents

A repeat unit shows the pattern within a long chain.

O-Level 6092 (2026) / SEC G3 K324 (2027)

Addition polymers: open the double bond, keep the substituents

A repeat unit shows the pattern within a long chain.

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.

Worked example

Recover a monomer

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

  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 understandingShould a poly(ethene) repeat unit retain the original C=C double bond?Think it through, then reveal the answer
No. The double bond changes to a single bond, allowing new bonds along the chain. Keeping both the double bond and the new chain bonds would give each carbon too many bonds.