Addition Polymerisation

This guide covers subtopic 20.1, Addition polymerisation, from Topic 20 of Cambridge International AS & A Level Chemistry 9701, 2025–2027 series. This is AS Level content.

Before studying this

This resource assumes Hydrocarbons: Alkanes and Alkenes, where addition polymerisation was already introduced briefly as one of alkenes’ reactions (subtopic 14.2) — this page is that idea developed into its own topic: deducing repeat units and monomers, and the environmental consequences of these polymers.

Syllabus coverage

CAMBRIDGE INTERNATIONAL AS & A LEVEL CHEMISTRY 9701 — AS Level, Topic 20

20.1 Addition polymerisation — describing addition polymerisation as exemplified by poly(ethene) and poly(chloroethene) (PVC); deducing the repeat unit of an addition polymer from a given monomer; identifying the monomer(s) present in a given section of an addition polymer molecule; recognising the difficulty of disposing of poly(alkene)s — non-biodegradability and harmful combustion products.

Addition polymerisation

An addition polymer forms when many monomer molecules — each containing a C=C double bond — join together, opening their double bonds and linking end to end, with no other product formed (unlike condensation polymerisation, which isn’t part of this syllabus point).

Poly(ethene): n CH₂=CH₂ → –[CH₂–CH₂]ₙ–

Poly(chloroethene), PVC: n CH₂=CHCl → –[CH₂–CHCl]ₙ–

Deducing a repeat unit from a monomer

The repeat unit is simply the monomer with its C=C double bond “opened out” into two single bonds extending to the neighbouring repeat units, written in square brackets with a subscript n.

Worked example. Deduce the repeat unit of the polymer formed from propene, CH₂=CHCH₃.

Open the double bond into two single bonds, keeping every atom and the CH₃ side-group exactly where it was:

–[CH₂–CH(CH₃)]ₙ–

Deducing a monomer from a polymer section

Run the process in reverse: identify one repeating unit in the given structure, then close its two open (dangling) bonds back into a C=C double bond.

Worked example. A section of an addition polymer is drawn as …–CH₂–CCl₂–CH₂–CCl₂–CH₂–CCl₂–… Deduce the monomer.

The repeating unit is –[CH₂–CCl₂]–. Closing the two dangling bonds back into a double bond gives the monomer CH₂=CCl₂ (1,1-dichloroethene).

The disposal problem

Addition polymers share the same essential unreactivity as the alkanes they’re built from — a saturated carbon backbone, held together by strong, non-polar C–C and C–H bonds that most everyday reagents can’t attack. This is exactly why they’re useful (durable, resistant to rot and corrosion), and exactly why they cause a disposal problem: they are not biodegradable, and can persist in landfill for a very long time.

Combustion adds a second problem, and it depends on what’s in the polymer besides carbon and hydrogen. Burning a plain poly(alkene) like poly(ethene) produces the same products as burning any hydrocarbon (CO₂, CO and soot depending on how complete the combustion is). Burning PVC is worse, because it contains chlorine: combustion releases toxic and corrosive hydrogen chloride gas, and incomplete or poorly controlled combustion of chlorinated plastics can also produce other harmful chlorinated by-products.

Common mistakes

• Drawing a repeat unit with the double bond still present. The whole point of addition polymerisation is that the double bond opens up to form the new single bonds linking monomers together — a repeat unit is fully saturated.
• Losing or moving a side-group when deducing a repeat unit or monomer. Every atom in the monomer, including substituents like Cl or CH₃, must appear in exactly the same relative position in the repeat unit.
• Assuming all addition polymers cause the same combustion problem as PVC. The HCl (and related) hazard is specific to chlorine-containing polymers — plain hydrocarbon-only polymers don’t share it.
• Confusing non-biodegradability with combustion hazard as if they were the same issue. They’re two separate disposal problems: one is about what happens if the polymer is simply discarded, the other about what happens if it’s burned.

Quick revision checklist

• Addition polymerisation: monomer’s C=C opens to form single bonds; no by-product
• Poly(ethene) and PVC as the two named examples
• Deducing a repeat unit from a monomer, and a monomer from a repeat unit
• Non-biodegradability, from the same unreactive C–C/C–H bonding as alkanes
• PVC’s chlorine content causing HCl (and other harmful products) on combustion

Related resources

• Hydrocarbons: Alkanes and Alkenes — where addition polymerisation was first introduced, alongside alkenes’ other reactions
• Organic Synthesis: Planning Multi-Step Routes — applying every AS organic reaction, including this one, to plan a route
• Cambridge AS & A Level Chemistry hub

Written against Cambridge International AS & A Level Chemistry 9701, 2025–2027 series. Always check the current syllabus for your examination year.