AQA · A-Level · Chemistry

    Course version: 7405 · Version from Sept 2016

    Addition polymers: The repeating unit of addition polymers.

    The repeating unit is the smallest section of chain that, repeated end to end, builds the whole polymer.

    Checked against the AQA 7405 specification (opens in a new tab) on .

    • Point 2 of 6
    • 4 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    The repeating unit is the smallest section of chain that, repeated end to end, builds the whole polymer. For an addition polymer it contains exactly the same atoms as the monomer, with the former double bond now a single bond and two bonds drawn out through the brackets to show where the next unit joins. Going forwards, take the monomer, open the double bond, and enclose the two carbons. Going backwards from a section of chain, find the shortest length that repeats, cut the backbone at equivalent points, and then put the double bond back between the two backbone carbons to recover the monomer. Keep any substituent on the same carbon it occupied in the monomer, or the repeat will not match the drawn chain.

    What earns the marks

    • A repeating unit containing the same atoms as the monomer.
    • The two continuation bonds drawn through the brackets.
    • A single carbon–carbon bond in the backbone with n outside the bracket.
    • Restoring the C=C in the correct position when deducing a monomer from a section of chain.

    Common mistakes and how to put them right

    • Drawing the monomer, complete with its double bond, when the repeating unit was asked for.

    • Leaving out the continuation bonds so the unit looks like an isolated molecule.

    • Choosing two monomer lengths as the repeating unit when one would do.

    • Moving a substituent onto the wrong backbone carbon so the repeat does not reproduce the chain.

    Exam tips

    • Test your answer by drawing the unit twice side by side; it should reproduce the chain in the question.
    • Repeating unit and monomer are different answers, so read which one the question wants.
    • Extend the bonds clearly past the brackets; a bond that stops short loses the mark.

    Try the lesson

    The start of the Addition polymers: The repeating unit of addition polymers. lesson, made from checked teaching notes for this specification point.

    Have a go first

    Which structure correctly represents the repeating unit of the addition polymer formed from propene, CH₂=CHCH₃?

    Listen

    The repeating unit is the smallest section of chain that, repeated end to end, builds the whole polymer. For an addition polymer it contains exactly the same atoms as the monomer, with the former double bond now a single bond and two bonds drawn out through the brackets to show where the next unit joins. Going forwards, take the monomer, open the double bond, and enclose the two carbons. Going backwards from a section of chain, find the shortest length that repeats, cut the backbone at equivalent points, and then put the double bond back between the two backbone carbons to recover the monomer. Keep any substituent on the same carbon it occupied in the monomer, or the repeat will not match the drawn chain. A strong answer earns marks for these points. a repeating unit containing the same atoms as the monomer. the two continuation bonds drawn through the brackets. a single carbon–carbon bond in the backbone with n outside the bracket. restoring the C=C in the correct position when deducing a monomer from a section of chain. Exam tip: Test your answer by drawing the unit twice side by side; it should reproduce the chain in the question.

    See it

    A top-down flowchart. The first node is 'Monomer with C=C', which leads to 'Open the double bond', then 'Enclose the two carbons in brackets', then 'Draw two continuation bonds through the brackets', then 'Repeating unit with single C-C backbone and n outside'. From that repeating unit one branch goes to 'Check: draw the unit twice side by side', which leads to 'Should reproduce the chain'. A second branch goes to 'Going backwards: find shortest length that repeats', then 'Cut the backbone at equivalent points', then 'Restore the C=C between the two backbone carbons', ending at 'Monomer recovered'.
    It shows the steps for turning a monomer into an addition polymer repeating unit and the reverse steps for deducing a monomer from a section of chain.

    Key words

    Say what each word means before you turn the card.

    repeating unit

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    continuation bonds

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    monomer to polymer

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    polymer to monomer

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    See every definition
    repeating unit
    The smallest section of chain that, repeated end to end, builds the whole polymer.
    continuation bonds
    Two bonds drawn out through the brackets to show where the next unit joins.
    monomer to polymer
    Take the monomer, open the double bond, and enclose the two carbons.
    polymer to monomer
    Find the shortest length that repeats, cut the backbone at equivalent points, and put the double bond back between the two backbone carbons.

    Label the diagram

    Some boxes have lost their names. Pick the right name for each number.

    The diagram again, with 3 boxes numbered instead of named. A top-down flowchart. The first node is 'Monomer with C=C', which leads to 'Open the double bond', then 'Enclose the two carbons in brackets', then 'Draw two continuation bonds through the brackets', then 'Repeating unit with single C-C backbone and n outside'. From that repeating unit one branch goes to 'Check: draw the unit twice side by side', which leads to 'Should reproduce the chain'. A second branch goes to 'Going backwards: find shortest length that repeats', then 'Cut the backbone at equivalent points', then 'Restore the C=C between the two backbone carbons', ending at 'Monomer recovered'.

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

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