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Learn7405Paper 2 · Paper 2: Organic chemistryPolymersAddition polymers
Polymers · Subtopic 2 of 2

Addition Polymers

In addition polymerisation, many alkene monomers open their C=C π bonds and join into a long saturated chain; no other product forms. The repeat unit is drawn by turning C=C into C–C with extension bonds through brackets: ethene → poly(ethene), chloroethene → poly(chloroethene) (PVC), propene → poly(propene). Poly(alkenes) are unreactive (saturated, non-polar C–C and C–H bonds), so they're non-biodegradable. Disposal: landfill (lasts centuries), incineration (energy recovery but CO₂ and, for PVC, HCl and dioxins), recycling, or biodegradable alternatives.

Cambridge 9701 Paper 1Cambridge 9701 Paper 2Edexcel YCH11 WCH11AQA 7405 Paper 2≈ 35 minRevision 1 · updated 2026-10-30
Section 1 of 14

Learning objectives

  • Draw repeat units from monomers and vice versa.
  • Explain why poly(alkenes) are unreactive.
  • Evaluate methods of disposal.
Pause and think

Poly(ethene) is an alkane, not an alkene. Why?

Section 2 of 14

Core idea

Start herePlastic that outlives you

A poly(ethene) bag can last hundreds of years in landfill. The same chemistry that makes it useful makes it a problem.

The big idea

Alkene monomers open their C=C bonds and link into saturated chains. The product is as unreactive as an alkane, so it barely degrades.

1. Repeat units

From monomer to repeat unit (propene)
  1. CH2=CH(CH3)\mathrm{CH_2{=}CH(CH_3)}
    Draw the two C=C carbons with all substituents.
  2. −CH2−CH(CH3)−\mathrm{-CH_2-CH(CH_3)-}
    Change C=C to C–C and add a bond on each side.
  3. −[CH2−CH(CH3)]n−\mathrm{-[CH_2-CH(CH_3)]_n-}
    Brackets with extension bonds passing through them; add nn.
Worked exampleChain length

A PVC chain has Mr=62 500M_r=62\,500. How many chloroethene units (MrM_r 62.5) does it contain?

Goal
nn.
Strategy
Divide polymer mass by monomer mass.
  1. Mr(C2H3Cl)=24+3+35.5=62.5M_r(\mathrm{C_2H_3Cl})=24+3+35.5=62.5
    Monomer mass.
  2. n=62 50062.5=1000n=\frac{62\,500}{62.5}=1000
    No by-product, so masses add.
Answer1000 units.
✓ CheckAddition polymer mass = n × monomer ✓.
Disposal options

Each option has a cost.

Summary

  • No by-product.
  • Repeat unit has two main-chain carbons.
  • Inert, non-biodegradable.
Key words
Monomer
Small molecule that joins to form a polymer.
Repeat unit
Smallest section that repeats along the chain.
Biodegradable
Broken down by microorganisms.
HistoryPVC. PVC was discovered accidentally several times in the 1800s as a white solid in flasks of chloroethene left in sunlight. It wasn't useful until plasticisers were added in the 1920s.
Fun factBulletproof. Ultra-high-molecular-weight poly(ethene), with chains millions of units long, is strong enough to be used in body armour.
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