What is addition polymerisation in the context of AS-Level Organic Chemistry?

Addition polymerisation is a chemical reaction where monomers with double bonds, typically alkenes, join together to form a polymer. This process involves the breaking of the double bonds and the formation of long chains without the loss of any small molecules. It is a key concept in AS-Level Organic Chemistry under the Cambridge International A Levels curriculum.

How does addition polymerisation differ from condensation polymerisation?

Addition polymerisation involves the joining of monomers with unsaturated bonds, such as alkenes, without the loss of any small molecules. In contrast, condensation polymerisation involves the joining of monomers with the elimination of small molecules like water or methanol. Understanding these differences is crucial for students studying Polymerisation in AS-Level Chemistry.

What are some common examples of polymers formed by addition polymerisation?

Common examples of polymers formed through addition polymerisation include polyethylene, polypropylene, and polystyrene. These polymers are widely used in various industries, and their formation is a fundamental topic in AS-Level Organic Chemistry under the Cambridge International A Levels.

Why is the study of addition polymerisation important in the Cambridge International A Levels Chemistry syllabus?

The study of addition polymerisation is important because it provides insights into the synthesis and properties of polymers, which are essential materials in everyday life. Understanding this process helps students grasp the broader concepts of organic chemistry and material science, which are integral parts of the Cambridge International A Levels Chemistry syllabus.

What role do catalysts play in addition polymerisation reactions?

Catalysts in addition polymerisation, such as Ziegler-Natta catalysts, play a crucial role by increasing the reaction rate and controlling the polymer structure. This allows for the production of polymers with specific properties, which is an important aspect of studying polymerisation in AS-Level Organic Chemistry.

Why is "Omitting extension bonds or n from the repeat unit." a common mistake in Addition polymerisation?

Why it happens: Drawing it like a small molecule. How to avoid it: Always bracket with trailing bonds and a subscript n. Source: 9701 Examiner Reports 2022-2024

Why is "Leaving a C=C in the repeat unit." a common mistake in Addition polymerisation?

Why it happens: Forgetting the double bond opens up. How to avoid it: The backbone is saturated (single bonds) in the polymer.

Why is "Saying a small molecule (e.g. water) is lost." a common mistake in Addition polymerisation?

Why it happens: Confusing addition with condensation polymerisation. How to avoid it: Addition polymerisation makes ONLY the polymer — no by-product.

Why is "Confusing the monomer with the repeat unit." a common mistake in Addition polymerisation?

Why it happens: They look similar. How to avoid it: The monomer has the C=C; the repeat unit has a single C–C backbone with extension bonds.

PolymerisationCambridge International A Level Chemistry 9701

Addition polymerisation

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Addition Polymerisation — Cambridge International AS & A Level Chemistry 9701 (2025-2027 syllabus)

How alkene monomers join by addition to form polymers such as poly(ethene) and PVC, how to deduce the repeat unit and monomer, and the environmental challenges of disposing of poly(alkene)s.

What you’ll learn

Mapped to the Cambridge International A Level 9701 syllabus (2025-2027).

20.1.1 — Describe addition polymerisation as exemplified by poly(ethene) and poly(chloroethene), PVC.
20.1.2 — Deduce the repeat unit of an addition polymer obtained from a given monomer.
20.1.3 — Identify the monomer(s) present in a given section of an addition polymer molecule.
20.1.4 — Recognise the difficulty of disposal of poly(alkene)s, i.e. non-biodegradability and harmful combustion products.

What addition polymerisation is

Alkene monomers add together — the C=C opens to make a long saturated chain, with no by-product.

Addition polymerisation is the joining of many small alkene (unsaturated) monomers into a single long-chain polymer, with no other product formed (unlike condensation polymerisation, which loses a small molecule).

The C=C double bond opens up and the carbons bond to neighbouring monomers, forming the polymer backbone: $n\,\text{CH}_2\text{=CH}_2 \rightarrow {-}\!(\text{CH}_2\text{-CH}_2)_n\!{-}$

Examples:

Ethene → poly(ethene): $-(\text{CH}_2\text{CH}_2)_n-$ .
Chloroethene (CH₂=CHCl) → poly(chloroethene), PVC: $-(\text{CH}_2\text{CHCl})_n-$ .
Propene → poly(propene): $-(\text{CH}_2\text{CH(CH}_3))_n-$ .

Many alkene monomers → one polymer, no by-product.
C=C opens to form the backbone.
Ethene → poly(ethene); chloroethene → PVC.

Repeat units and identifying monomers

Repeat unit = the monomer with the double bond made single, in brackets with extension bonds and n.

Deducing the repeat unit from a monomer:

Draw the monomer with its C=C.
Change the C=C to a single C–C bond.
Put the unit in brackets with a bond extending from each end and a subscript $n$ .

Worked. Monomer propene $\text{CH}_2\text{=CHCH}_3$ → repeat unit $-(\text{CH}_2\text{-CH(CH}_3))_n-$ (the two backbone carbons keep all their side groups; the double bond becomes single).

Identifying the monomer from a polymer section: find the repeating two-carbon unit in the backbone and put the C=C back between those two carbons.

Worked. A section $-\text{CH}_2\text{-CHCl-CH}_2\text{-CHCl}-$ → repeat unit $-(\text{CH}_2\text{CHCl})_n-$ → monomer chloroethene, CH₂=CHCl.

Key rule: the backbone of an addition polymer is always two carbons per repeat unit (from the C=C). Keep the side groups on the same carbons as in the monomer.

Repeat unit: monomer with C=C → single bond, in brackets with extension bonds + n.
Backbone is always 2 carbons per repeat unit.
Monomer from polymer: put the C=C back between the repeating carbons.

Disposal problems of poly(alkene)s

The strong, non-polar C–C backbone makes them inert and non-biodegradable.

Poly(alkene)s like poly(ethene) and PVC have a backbone of strong, non-polar C–C bonds (saturated, like an alkane), which makes them chemically inert. This causes disposal problems:

Non-biodegradable — microorganisms cannot break down the inert C–C chain, so they persist in the environment (landfill, oceans) for a very long time.
Harmful combustion products — burning them can release toxic gases: CO (incomplete combustion), and from PVC, HCl (acidic) and other harmful chlorinated compounds.

Managing the problem: recycling, reuse, and developing biodegradable/degradable polymers (developed further at A Level, Topic 20).

Inert C–C backbone → non-biodegradable, persists in the environment.
Combustion → toxic gases (CO; HCl from PVC).
Solutions: recycle, reuse, biodegradable polymers.

How it’s examined

Drawing the repeat unit from a monomer (and vice versa) is a guaranteed Paper 2 mark, with the environmental disposal points a common short-answer. Examiner reports flag missing extension bonds or n, keeping a C=C in the repeat unit, and a backbone with the wrong number of carbons.

Sources: Cambridge International AS & A Level Chemistry 9701 syllabus (2025-2027), section 20.1; 9701 Examiner Reports 2022-2024; 9701/22 May/Jun 2024 question paper and mark scheme. Last reviewed 2026-05-20.

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Step-by-step worked examples — Addition polymerisation

Step-by-step solutions to past-paper-style questions on addition polymerisation, written exactly the way a tutor would explain them at the board.

1Repeat unit from a monomer (2 marks)
Getting started• repeat unit
Question
Draw the repeat unit of the polymer formed from propene, CH₂=CHCH₃.
Step-by-step solution
1. Step 1
  Open the C=C to a single bond; keep the CH₃ side group.
2. Step 2
  Bracket with extension bonds and n.
  $-(\text{CH}_2\text{-CH(CH}_3))_n-$
Answer
$-(\text{CH}_2\text{CH(CH}_3))_n-$ .
Examiner tip
Include the extension bonds and n; the backbone is two carbons.
2Monomer from a polymer section (2 marks)
Getting started• monomer
Question
A polymer section is $-\text{CH}_2\text{-CF}_2\text{-CH}_2\text{-CF}_2-$ . Identify the monomer.
Step-by-step solution
1. Step 1
  Repeating unit is $-(\text{CH}_2\text{CF}_2)-$ .
2. Step 2
  Put the C=C back between the two carbons.
  $\text{CH}_2\text{=CF}_2$
Answer
1,1-difluoroethene, CH₂=CF₂.
3Polymerisation equation (3 marks)
Building confidence• equation
Question
Write the equation for the polymerisation of chloroethene (CH₂=CHCl) and name the polymer.
Step-by-step solution
1. Step 1
  n monomers → one polymer; open the C=C.
  $n\,\text{CH}_2\text{=CHCl} \rightarrow -(\text{CH}_2\text{-CHCl})_n-$
2. Step 2
  Name: poly(chloroethene), commonly PVC.
Answer
n CH₂=CHCl → –(CH₂CHCl)ₙ– ; poly(chloroethene) / PVC.
Examiner tip
Addition polymerisation forms NO other product (unlike condensation) — atoms are conserved.
4Disposal of poly(alkene)s (3 marks)
Building confidence• disposal
Question
Explain two problems with disposing of poly(alkene)s such as PVC.
Step-by-step solution
1. Step 1
  Non-biodegradable: the inert, non-polar C–C backbone is not broken down by microorganisms → persists in the environment.
2. Step 2
  Harmful combustion products: burning releases toxic gases (CO; HCl from PVC).
Answer
Non-biodegradable (inert C–C backbone) and harmful combustion products (e.g. HCl from PVC).
5Why poly(alkene)s are unreactive (3 marks)
Stretch• properties
Question
Explain why poly(alkene)s such as poly(ethene) are chemically unreactive and non-biodegradable. (3)
Step-by-step solution
1. Step 1
  The backbone is saturated C–C and C–H bonds — strong and essentially non-polar.
2. Step 2
  There are no polar bonds or functional groups for nucleophiles, acids or enzymes to attack.
3. Step 3
  So microorganisms cannot break them down → they are inert and non-biodegradable.
Answer
A saturated, non-polar C–C/C–H backbone with no functional group → nothing to attack → inert/non-biodegradable.
Examiner tip
Link inertness to the lack of polar bonds/functional groups, the same reason alkanes are unreactive.
6Evaluating disposal methods (4 marks)
Stretch• disposal, environment
Question
Compare landfill, incineration and recycling as ways of dealing with waste poly(alkene)s. (4)
Step-by-step solution
1. Step 1
  Landfill: cheap but uses land and the plastic persists (non-biodegradable).
2. Step 2
  Incineration: recovers energy but can release toxic gases (CO, HCl) and CO₂.
3. Step 3
  Recycling: saves crude-oil resources and energy, but waste must be sorted/cleaned (costly).
4. Step 4
  Best practice combines methods — recycle where possible, incinerate with scrubbers for energy.
Answer
Landfill (land use, persists); incineration (energy but toxic gases); recycling (saves resources but needs sorting).
Examiner tip
Give a balanced pro AND con for each method to access the higher marks.

Key Definitions and Keywords — Addition polymerisation

Definitions to memorise and the exact keywords mark schemes credit for addition polymerisation answers — sharpened from recent examiner reports for the 2026 Cambridge International A Level 9701 sitting.

Addition polymerisation
Examiner keyword
The joining of many alkene monomers into a long-chain polymer, with no other product formed.
Monomer
Examiner keyword
A small molecule (an alkene, for addition polymers) that joins to others to form a polymer.
Repeat unit
Examiner keyword
The smallest repeating section of a polymer, drawn in brackets with extension bonds and subscript n.
Non-biodegradable
Examiner keyword
Not broken down by microorganisms; poly(alkene)s persist because their non-polar C–C backbone is inert.

Common Mistakes and Misconceptions — Addition polymerisation

The traps other students keep falling into on addition polymerisation questions — taken from recent Cambridge International A Level 9701 examiner reports and mark schemes — and how to avoid them.

✕Omitting extension bonds or n from the repeat unit.
9701 Examiner Reports 2022-2024
Why it happens
Drawing it like a small molecule.
How to avoid it
Always bracket with trailing bonds and a subscript n.
✕Leaving a C=C in the repeat unit.
Why it happens
Forgetting the double bond opens up.
How to avoid it
The backbone is saturated (single bonds) in the polymer.
✕Saying a small molecule (e.g. water) is lost.
Why it happens
Confusing addition with condensation polymerisation.
How to avoid it
Addition polymerisation makes ONLY the polymer — no by-product.
✕Confusing the monomer with the repeat unit.
Why it happens
They look similar.
How to avoid it
The monomer has the C=C; the repeat unit has a single C–C backbone with extension bonds.

Practice questions

Exam-style questions with step-by-step worked solutions. Try one before checking the method.

Past paper style quiz

Get a report showing which sub-topics you've nailed and which ones still need work.

Addition polymerisation — frequently asked questions

The things students keep getting wrong in this sub-topic, answered.

PolymerisationCambridge International A Level Chemistry 9701

Addition polymerisation

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Short Study Notes — Addition polymerisation

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Addition Polymerisation — Cambridge International AS & A Level Chemistry 9701 (2025-2027 syllabus)

How alkene monomers join by addition to form polymers such as poly(ethene) and PVC, how to deduce the repeat unit and monomer, and the environmental challenges of disposing of poly(alkene)s.

What you’ll learn

Mapped to the Cambridge International A Level 9701 syllabus (2025-2027).

20.1.1 — Describe addition polymerisation as exemplified by poly(ethene) and poly(chloroethene), PVC.
20.1.2 — Deduce the repeat unit of an addition polymer obtained from a given monomer.
20.1.3 — Identify the monomer(s) present in a given section of an addition polymer molecule.
20.1.4 — Recognise the difficulty of disposal of poly(alkene)s, i.e. non-biodegradability and harmful combustion products.

What addition polymerisation is

Alkene monomers add together — the C=C opens to make a long saturated chain, with no by-product.

The C=C double bond opens up and the carbons bond to neighbouring monomers, forming the polymer backbone: $n\,\text{CH}_2\text{=CH}_2 \rightarrow {-}\!(\text{CH}_2\text{-CH}_2)_n\!{-}$

Examples:

Ethene → poly(ethene): $-(\text{CH}_2\text{CH}_2)_n-$ .
Chloroethene (CH₂=CHCl) → poly(chloroethene), PVC: $-(\text{CH}_2\text{CHCl})_n-$ .
Propene → poly(propene): $-(\text{CH}_2\text{CH(CH}_3))_n-$ .

Many alkene monomers → one polymer, no by-product.
C=C opens to form the backbone.
Ethene → poly(ethene); chloroethene → PVC.

Repeat units and identifying monomers

Repeat unit = the monomer with the double bond made single, in brackets with extension bonds and n.

Deducing the repeat unit from a monomer:

Draw the monomer with its C=C.
Change the C=C to a single C–C bond.
Put the unit in brackets with a bond extending from each end and a subscript $n$ .

Worked. Monomer propene $\text{CH}_2\text{=CHCH}_3$ → repeat unit $-(\text{CH}_2\text{-CH(CH}_3))_n-$ (the two backbone carbons keep all their side groups; the double bond becomes single).

Identifying the monomer from a polymer section: find the repeating two-carbon unit in the backbone and put the C=C back between those two carbons.

Worked. A section $-\text{CH}_2\text{-CHCl-CH}_2\text{-CHCl}-$ → repeat unit $-(\text{CH}_2\text{CHCl})_n-$ → monomer chloroethene, CH₂=CHCl.

Key rule: the backbone of an addition polymer is always two carbons per repeat unit (from the C=C). Keep the side groups on the same carbons as in the monomer.

Repeat unit: monomer with C=C → single bond, in brackets with extension bonds + n.
Backbone is always 2 carbons per repeat unit.
Monomer from polymer: put the C=C back between the repeating carbons.

Disposal problems of poly(alkene)s

The strong, non-polar C–C backbone makes them inert and non-biodegradable.

Poly(alkene)s like poly(ethene) and PVC have a backbone of strong, non-polar C–C bonds (saturated, like an alkane), which makes them chemically inert. This causes disposal problems:

Non-biodegradable — microorganisms cannot break down the inert C–C chain, so they persist in the environment (landfill, oceans) for a very long time.
Harmful combustion products — burning them can release toxic gases: CO (incomplete combustion), and from PVC, HCl (acidic) and other harmful chlorinated compounds.

Managing the problem: recycling, reuse, and developing biodegradable/degradable polymers (developed further at A Level, Topic 20).

Inert C–C backbone → non-biodegradable, persists in the environment.
Combustion → toxic gases (CO; HCl from PVC).
Solutions: recycle, reuse, biodegradable polymers.

How it’s examined

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Step-by-step worked examples — Addition polymerisation

Step-by-step solutions to past-paper-style questions on addition polymerisation, written exactly the way a tutor would explain them at the board.

1Repeat unit from a monomer (2 marks)
Getting started• repeat unit
Question
Draw the repeat unit of the polymer formed from propene, CH₂=CHCH₃.
Step-by-step solution
1. Step 1
  Open the C=C to a single bond; keep the CH₃ side group.
2. Step 2
  Bracket with extension bonds and n.
  $-(\text{CH}_2\text{-CH(CH}_3))_n-$
Answer
$-(\text{CH}_2\text{CH(CH}_3))_n-$ .
Examiner tip
Include the extension bonds and n; the backbone is two carbons.
2Monomer from a polymer section (2 marks)
Getting started• monomer
Question
A polymer section is $-\text{CH}_2\text{-CF}_2\text{-CH}_2\text{-CF}_2-$ . Identify the monomer.
Step-by-step solution
1. Step 1
  Repeating unit is $-(\text{CH}_2\text{CF}_2)-$ .
2. Step 2
  Put the C=C back between the two carbons.
  $\text{CH}_2\text{=CF}_2$
Answer
1,1-difluoroethene, CH₂=CF₂.
3Polymerisation equation (3 marks)
Building confidence• equation
Question
Write the equation for the polymerisation of chloroethene (CH₂=CHCl) and name the polymer.
Step-by-step solution
1. Step 1
  n monomers → one polymer; open the C=C.
  $n\,\text{CH}_2\text{=CHCl} \rightarrow -(\text{CH}_2\text{-CHCl})_n-$
2. Step 2
  Name: poly(chloroethene), commonly PVC.
Answer
n CH₂=CHCl → –(CH₂CHCl)ₙ– ; poly(chloroethene) / PVC.
Examiner tip
Addition polymerisation forms NO other product (unlike condensation) — atoms are conserved.
4Disposal of poly(alkene)s (3 marks)
Building confidence• disposal
Question
Explain two problems with disposing of poly(alkene)s such as PVC.
Step-by-step solution
1. Step 1
  Non-biodegradable: the inert, non-polar C–C backbone is not broken down by microorganisms → persists in the environment.
2. Step 2
  Harmful combustion products: burning releases toxic gases (CO; HCl from PVC).
Answer
Non-biodegradable (inert C–C backbone) and harmful combustion products (e.g. HCl from PVC).
5Why poly(alkene)s are unreactive (3 marks)
Stretch• properties
Question
Explain why poly(alkene)s such as poly(ethene) are chemically unreactive and non-biodegradable. (3)
Step-by-step solution
1. Step 1
  The backbone is saturated C–C and C–H bonds — strong and essentially non-polar.
2. Step 2
  There are no polar bonds or functional groups for nucleophiles, acids or enzymes to attack.
3. Step 3
  So microorganisms cannot break them down → they are inert and non-biodegradable.
Answer
A saturated, non-polar C–C/C–H backbone with no functional group → nothing to attack → inert/non-biodegradable.
Examiner tip
Link inertness to the lack of polar bonds/functional groups, the same reason alkanes are unreactive.
6Evaluating disposal methods (4 marks)
Stretch• disposal, environment
Question
Compare landfill, incineration and recycling as ways of dealing with waste poly(alkene)s. (4)
Step-by-step solution
1. Step 1
  Landfill: cheap but uses land and the plastic persists (non-biodegradable).
2. Step 2
  Incineration: recovers energy but can release toxic gases (CO, HCl) and CO₂.
3. Step 3
  Recycling: saves crude-oil resources and energy, but waste must be sorted/cleaned (costly).
4. Step 4
  Best practice combines methods — recycle where possible, incinerate with scrubbers for energy.
Answer
Landfill (land use, persists); incineration (energy but toxic gases); recycling (saves resources but needs sorting).
Examiner tip
Give a balanced pro AND con for each method to access the higher marks.

Key Definitions and Keywords — Addition polymerisation

Addition polymerisation
Examiner keyword
The joining of many alkene monomers into a long-chain polymer, with no other product formed.
Monomer
Examiner keyword
A small molecule (an alkene, for addition polymers) that joins to others to form a polymer.
Repeat unit
Examiner keyword
The smallest repeating section of a polymer, drawn in brackets with extension bonds and subscript n.
Non-biodegradable
Examiner keyword
Not broken down by microorganisms; poly(alkene)s persist because their non-polar C–C backbone is inert.

Common Mistakes and Misconceptions — Addition polymerisation

The traps other students keep falling into on addition polymerisation questions — taken from recent Cambridge International A Level 9701 examiner reports and mark schemes — and how to avoid them.

✕Omitting extension bonds or n from the repeat unit.
9701 Examiner Reports 2022-2024
Why it happens
Drawing it like a small molecule.
How to avoid it
Always bracket with trailing bonds and a subscript n.
✕Leaving a C=C in the repeat unit.
Why it happens
Forgetting the double bond opens up.
How to avoid it
The backbone is saturated (single bonds) in the polymer.
✕Saying a small molecule (e.g. water) is lost.
Why it happens
Confusing addition with condensation polymerisation.
How to avoid it
Addition polymerisation makes ONLY the polymer — no by-product.
✕Confusing the monomer with the repeat unit.
Why it happens
They look similar.
How to avoid it
The monomer has the C=C; the repeat unit has a single C–C backbone with extension bonds.

Practice questions

Exam-style questions with step-by-step worked solutions. Try one before checking the method.

Past paper style quiz

Get a report showing which sub-topics you've nailed and which ones still need work.

Addition polymerisation — frequently asked questions

The things students keep getting wrong in this sub-topic, answered.

Addition polymerisation

Short Study Notes in the form of Slides

Short Study Notes — Addition polymerisation

Detailed Notes

What you’ll learn

How it’s examined

1Repeat unit from a monomer (2 marks)

2Monomer from a polymer section (2 marks)

3Polymerisation equation (3 marks)

4Disposal of poly(alkene)s (3 marks)

5Why poly(alkene)s are unreactive (3 marks)

6Evaluating disposal methods (4 marks)

Addition polymerisation

Monomer

Repeat unit

Non-biodegradable

✕Omitting extension bonds or n from the repeat unit.

✕Leaving a C=C in the repeat unit.

✕Saying a small molecule (e.g. water) is lost.

✕Confusing the monomer with the repeat unit.

Practice questions

Past paper style quiz

Assess your understanding

Addition polymerisation — frequently asked questions

Addition polymerisation

Short Study Notes in the form of Slides

Short Study Notes — Addition polymerisation

Detailed Notes

What you’ll learn

How it’s examined

1Repeat unit from a monomer (2 marks)

2Monomer from a polymer section (2 marks)

3Polymerisation equation (3 marks)

4Disposal of poly(alkene)s (3 marks)

5Why poly(alkene)s are unreactive (3 marks)

6Evaluating disposal methods (4 marks)

Addition polymerisation

Monomer

Repeat unit

Non-biodegradable

✕Omitting extension bonds or n from the repeat unit.

✕Leaving a C=C in the repeat unit.

✕Saying a small molecule (e.g. water) is lost.

✕Confusing the monomer with the repeat unit.

Practice questions

Past paper style quiz

Assess your understanding

Addition polymerisation — frequently asked questions