Addition Polymerisation

⬢ CHEMISTRY ONLY ⬢

SPEC 4.7.3.1 • PAPER 2

How alkenes (C=C) join end-to-end to make polymers like poly(ethene). One mechanism, five drawing rules, and one rule that loses students more marks than any other in this topic.

What is addition polymerisation?

A chemical reaction where many small molecules — monomers — join together to form one very large molecule — a polymer.

The defining feature

In addition polymerisation, NO ATOMS ARE LOST. Every atom in the monomer ends up in the polymer.

This is why the spec says: "the repeating unit has the same atoms as the monomer because no other molecule is formed in the reaction".

Monomer

A small molecule that joins with others to make a polymer. For addition polymerisation, the monomer must contain a C=C double bond.

Polymer

A very large molecule made of many monomers joined together. Long-chain structure with a repeating unit.

Naming rule

The polymer is named "poly(monomer name)" — e.g. ethene → poly(ethene), propene → poly(propene), chloroethene → poly(chloroethene). Brackets included.

The monomer must have C=C

For a molecule to undergo addition polymerisation, it must contain a C=C double bond — that's the alkene functional group.

Examiner alert — circle the C=C correctly

The functional group is the C=C double bond plus the two carbon atoms it connects. Verbatim from the Jun23 P2 examiner report: "A common mistake for those who didn't gain the mark was identifying the C-Cl bond or to circle the double bond alone without including the carbon atoms." Always include the two C atoms in the circle.

The mechanism — C=C opens up

Under the right conditions, the C=C double bond opens up to become a single C–C bond. Each carbon now has a "spare" bond available — and that's how the monomers join together.

What happens next

Each spare bond joins to a spare bond on the next monomer. Repeat thousands of times — and you have a long polymer chain made entirely of single C–C bonds.

Poly( ethene ) — the canonical equation

Take n ethene molecules. The C=C bond in each opens up, and the molecules join end-to-end into a long chain. The portion shown in brackets is the repeat unit.

Read the equation

"n" means "many" — typically thousands. The brackets enclose one repeat unit. The "n" outside the bracket says "this unit repeats n times". The lines extending through the brackets are the extension bonds showing the chain continues both ways.

Five rules for drawing a repeat unit

Almost every mark lost on this topic comes from breaking one of these five rules. Drill them.

1

Single C–C bond inside the unit. The C=C opened up, so the repeat unit has a single C–C bond between the two carbons. Drawing C=C in the polymer is the most common error.

2

Keep all the side groups. Every atom that was on the monomer must appear on the same carbon in the repeat unit. No atoms are added; no atoms are removed.

3

Brackets around the unit. Round brackets ( … ) or square brackets [ … ] — both accepted by AQA. The brackets show "this is one repeating section".

4

Extension bonds out both sides. Two single lines must extend through the brackets — left and right. They indicate the chain continues. Nothing is attached to the end of these lines.

5

"n" outside the closing bracket, as a subscript. The "n" tells you the unit repeats n times along the chain. It must be outside the bracket and written as a subscript (small, at the bottom).

Rule 4 — the extension bond trap

Verbatim from the Jun24 P2 examiner report on this exact question style: "A minority of students added atoms or groups to the extension bonds, which contradicted marking point 1." Extension bonds must be plain lines with nothing on the ends.

Worked example — poly( propene )

Propene has the formula CH₂=CH–CH₃. The C=C is between the two left-hand carbons. The methyl group (–CH₃) is the side group on the right-hand carbon — it stays put when the C=C opens.

What changed

The C=C in the monomer became a single C–C in the repeat unit. The methyl group (–CH₃) is still attached to the same carbon. Same atoms, same arrangement — just the double bond opened.

Worked example — poly( chloroethene )

Chloroethene is ethene with one H replaced by Cl: CH₂=CHCl. It's the monomer for the polymer commonly known as PVC.

Spot the pattern

Three C–H bonds and one C–Cl bond on the two carbons in the monomer. Same three C–H and one C–Cl in the repeat unit. Atoms and side groups preserved exactly — only the double bond opens.

Examiner alert: the equation as a whole

Two examiner reports cover this — both find the same gaps year after year.

Verbatim — P2 Nov21 Q04.1 examiner report

"Around half of the students either drew five single bonds or wrote 'n'. Around one third provided a completely correct response." Translation: students get partial credit (1 of 2 marks) for showing five single bonds OR for writing the "n" — but only one third do both. The mark scheme awards 2 marks for the full equation: correct repeat unit (single C–C, side groups, extension bonds) and the "n".

Verbatim — P2 Jun23 Q08.2 examiner report

"Incorrect answers often lacked extended bonds or failed to extend them far enough or failed to replace the C=C bond by C-C." Three failure modes flagged: (1) no extension bonds at all, (2) extension bonds drawn too short to clearly extend through the bracket, (3) keeping the C=C double bond in the repeat unit. All three lose marks.

Pre-flight check (do this before moving on)

Before you submit a polymerisation equation, run through the five rules:

✅ Single C–C inside the brackets (not C=C)

✅ All side groups present on the correct carbons

✅ Brackets enclose the repeat unit

✅ Extension bonds clearly extending through the brackets

✅ n outside the closing bracket (subscript)

Going backwards — monomer from repeat unit

Just as common in past papers: you're shown a polymer's repeat unit and asked to draw the monomer. Reverse the rules.

The reverse procedure

Take the two carbons inside the repeat unit (the ones connected by the single C–C bond).

Replace the single C–C with a double C=C bond.

Remove the extension bonds — they belonged to the polymer, not to the standalone monomer.

Keep all side groups on the same carbons they came from.

Drop the brackets and the "n" — the monomer is one molecule, not a repeating unit.

Sanity check

If your monomer has a C=C double bond and the same atoms as the repeat unit, you've done it correctly. The molecular formula of the monomer should match the formula of one repeat unit.

Worked example — monomer from repeat unit

Suppose the polymer has the repeat unit shown on the left below — find the monomer.

Step-by-step

Identify the two carbons inside the brackets — they will form the C=C in the monomer.

Replace the single C–C with C=C (draw two parallel lines instead of one).

Erase the extension bonds — they are gone.

Keep the H atoms and the –CH₃ side group on the same carbons.

Drop the brackets and the "n".

Result: CH₂=CH–CH₃, propene. ✓

Recognising addition polymers

The spec asks you to recognise addition polymers and their monomers from diagrams. Two diagnostic checks tell you immediately:

Is it an addition monomer?

✅ Look for a C=C double bond (the alkene functional group). If C=C is present and the molecule is small, it's an addition monomer.

❌ If there's no C=C, it cannot polymerise by addition.

Is it an addition polymer?

✅ Look for a repeating unit with single C–C bonds and side groups, with extension bonds extending out both sides through brackets, and an "n" subscript.

❌ If small molecules (water, HCl, etc.) appear as by-products, it's a condensation polymer, not addition (covered in lesson 7.07).

A note on more complex monomers

AQA past papers have used poly(ethene), poly(propene), and poly(chloroethene) — and sometimes monomers carrying additional side groups beyond H, halogens, or methyl. The rule is the same: find the C=C, open it to a single C–C, and treat everything else on the two carbons as side groups that stay put in the repeat unit. The C=C is always what drives the addition polymerisation.

Common errors — a checklist

Every error here costs marks. Self-check after every drawing.

Common errorWhy it loses marksThe fix

C=C kept in the polymer repeat unitContradicts the addition mechanism — the C=C opens upAlways single C–C between the two carbons in the repeat unit

Atoms / groups added to extension bondsImplies extra atoms join the chain — contradicts "no atoms lost"Extension bonds = plain lines, nothing on the ends

Forgotten brackets, or "n" inside bracketsThe unit is no longer marked as repeatingBrackets around the repeat unit; "n" outside, as subscript

Side groups dropped from the repeat unitAtoms have been "lost" — but no atoms can be lost in addition polymerisationEvery side group on the monomer must reappear on the same carbon in the repeat unit

Circling C=C without including the carbon atomsNot the full functional groupInclude both Cs and the C=C in the circle

Practice question

3 MARKS

Bromoethene has the structural formula CH₂=CHBr.

(a) Name the type of polymerisation that bromoethene can undergo and explain why bromoethene is suitable for this type of polymerisation. 2 MARKS

(b) State the name of the polymer formed. 1 MARK

(a)

Type of polymerisation: Bromoethene undergoes addition polymerisation. (1 mark)

(a)

Why bromoethene is suitable: Because bromoethene contains a C=C double bond (the alkene functional group). The C=C is what allows monomers to join — under the right conditions, it opens to a single C–C bond and links to the next monomer in the chain. (1 mark)

(b)

Polymer name: The polymer is named "poly(monomer name)", so the polymer formed is poly(bromoethene). (1 mark)

Lesson summary

Definition

In addition polymerisation, many alkene monomers (each containing a C=C bond) join end-to-end via single C–C bonds to form a long-chain polymer. The repeat unit has the same atoms as the monomer because no atoms are lost and no other molecule is formed.

The five drawing rules

1

Single C–C inside the unit

2

Keep all side groups

3

Brackets around the unit

4

Extension bonds through the brackets

5

"n" outside, as a subscript

Three named polymers

Ethene → poly(ethene)

Propene → poly(propene)

Chloroethene → poly(chloroethene)

The single biggest mark-loser

Adding atoms or groups to the extension bonds. Extension bonds are plain continuation lines — nothing sits on the end. Memorise this; the examiner has flagged it twice in two years.

Next lesson

Lesson 7.07 — Condensation Polymerisation, Amino Acids, and Naturally Occurring Polymers [chemistry only]. The contrasting mechanism: two functional groups, two monomers, water released, ester or peptide bonds formed. Plus DNA and proteins.

Checkpoint

Before moving on, you should be able to:

Self-check

Define addition polymerisation, including the key fact that no atoms are lost.

Identify an addition monomer by the presence of the C=C functional group, and circle the C=C correctly (including both carbon atoms).

Draw the polymerisation equation from a given alkene monomer, applying all five rules: single C–C, side groups, brackets, extension bonds, "n" subscript.

Draw the monomer when given a repeat unit, by reversing the rules — replace single C–C with C=C, drop extension bonds, drop brackets, drop "n".

Recognise common addition polymers: poly(ethene), poly(propene), poly(chloroethene).

Avoid the five most common errors — especially the extension-bond trap.

One sentence to remember

"The C=C opens, the monomers link, no atoms are lost." If you can explain that sentence with a worked diagram, you can answer almost every addition polymerisation question on this paper.

Final review — equation in five seconds

Treat every polymerisation equation as a five-second checklist. If any one of these fails, you've lost a mark.

Look at...Check for

Left-hand side" n " before the monomer; the monomer has a clear C=C double bond

ArrowSingle forward arrow (not equilibrium)

Right-hand side, opening bracketBracket with an extension bond passing through it horizontally

Inside the bracketSingle C–C bond between the two carbons; all side groups present

Right-hand side, closing bracketA second extension bond passing through the closing bracket

After the bracket"n" as a subscript on the outside

If you can tick all six...

...you've earned every mark on the equation question. Move on.