20. Polymerisation
- Syllabus
- 9701–2028–2029
- Section
- 20
- Level
- AS
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Topic 20.1
20.1 Addition polymerisation
Objectives in this topic
Addition polymerisation joins alkene monomers without eliminating a small molecule
In addition polymerisation, many alkene molecules open their C=C bonds and join into a saturated carbon-chain backbone. No small molecule is eliminated.
Draw the repeat unit by replacing the double bond with single bonds to neighbouring units, then put the repeat unit in brackets with n. The substituent on the monomer remains attached to the backbone.
nCH₂=CH₂ → [–CH₂–CH₂–]ₙ poly(ethene); nCH₂=CHCl → [–CH₂–CHCl–]ₙ PVC.
Do not leave a C=C in the repeat unit or add water as a product; those features belong to different polymerisation chemistry.
Recover an addition-polymer repeat unit by reopening the monomer double bond
In an addition polymer, the alkene C=C becomes a C–C link in the backbone. The repeat unit is the monomer with the double bond opened and free valencies shown at both ends.
Put the repeat unit in brackets with n. Keep substituents attached to the same carbon positions as in the monomer and do not add a small molecule.
CH₂=CHCl gives [–CH₂–CHCl–]ₙ, poly(chloroethene). CH₂=CHCH₃ gives [–CH₂–CH(CH₃)–]ₙ.
Do not leave C=C inside the repeat unit or move a substituent to the wrong backbone carbon.
Identify an addition-polymer monomer by restoring the C=C at the repeat-unit ends
To find the monomer, cut the polymer backbone at a repeat-unit boundary and replace the single bond between the two backbone carbons with a C=C. Preserve all substituents.
Check that the reconstructed monomer has the correct valency and that repeating it would reproduce the polymer. This is a structure-reversal task, not a naming guess.
[–CH₂–CH(CH₃)–]ₙ comes from propene, CH₂=CHCH₃. [–CH₂–CHCl–]ₙ comes from chloroethene.
The brackets do not represent a separate molecule to be hydrolysed; they indicate a repeating section of a chain.
Poly(alkene) disposal is difficult because the chains are persistent and combustion can pollute
Many poly(alkenes) are not readily biodegradable because their carbon backbones resist biological breakdown. Disposal by burning can produce CO₂ and, if combustion is incomplete or substituents are present, other harmful products.
The environmental issue is a life-cycle trade-off: collection, mechanical recycling, chemical recycling, energy recovery and landfill have different costs and emissions.
PVC disposal needs special care because chlorine-containing material can form acidic or chlorinated products if burned poorly; controlled conditions and treatment matter.
“Non-biodegradable” does not mean impossible to recycle, and recycling is not impact-free.