20.1 Addition polymerisation
- Syllabus
- 9701–2028–2029
- Topic
- 20.1
- Level
- AS
Learning objectives
20.1.1Addition polymerisation• Describe addition polymerisation as exemplified by poly(ethene) and poly(chloroethene), PVC20.1.2Repeat units of addition polymers• Deduce the repeat unit of an addition polymer obtained from a given monomer20.1.3Monomer(s) present• Identify the monomer(s) present in a given section of an addition polymer molecule20.1.4The difficulty of the disposal• Recognise the difficulty of the disposal of poly(alkene)s, i.e. non-biodegradability and harmful combustion products
Addition polymerisation opens C=C and joins many alkene monomers
In addition polymerisation, many alkene monomers open their C=C π bonds and form new C–C σ bonds to neighbouring monomers. The product is one long saturated carbon-chain polymer; no small molecule is eliminated.
nCHX2=CHX2[−CHX2−CHX2X−]Xn
nCHX2=CHCl[−CHX2−CHClX−]Xn
Ethene forms poly(ethene). Chloroethene forms poly(chloroethene), commonly called PVC; every Cl substituent remains attached to the same backbone carbon position inherited from its monomer.
A repeat unit shows continuation bonds through the brackets and n outside. Do not leave C=C in the backbone or write water as a coproduct: both contradict addition polymerisation.
Convert an alkene monomer into its repeat unit
- Identify the two atoms of the monomer C=C. 2. Replace C=C with C–C. 3. Keep every substituent on its original carbon. 4. Draw a continuation bond from each of the two backbone carbons through enclosing brackets. 5. Place n outside the brackets.
CHX2=CHCl[−CHX2−CHClX−]Xn
CHX2=CHCHX3[−CHX2−CH(CHX3)X−]Xn
Each backbone carbon must still have valency four, and repeating the bracketed section end-to-end must reproduce the chain without moving or losing a substituent.
The two continuation bonds are not extra atoms and are not joined to each other inside the brackets. The repeat unit represents connectivity, not a complete isolated molecule.
Recover the alkene monomer from a polymer section
- Find the shortest backbone pattern that repeats. 2. Select its two carbon atoms that came from one alkene C=C. 3. Remove their continuation bonds to neighbouring repeat units. 4. Change the C–C bond between them into C=C. 5. Preserve all attached substituents and check carbon valency four.
| Polymer repeat unit | Recovered monomer |
|---|---|
| [–CH₂–CH₂–]ₙ | CH₂=CH₂, ethene |
| [–CH₂–CHCl–]ₙ | CH₂=CHCl, chloroethene |
| [–CH₂–CH(CH₃)–]ₙ | CH₂=CHCH₃, propene |
When a longer polymer section is supplied without brackets, mark the repeating substitution pattern first. A boundary may be drawn in more than one equivalent place along a uniform chain, but the reconstructed monomer connectivity must be the same.
Do not hydrolyse the chain or add atoms that are not shown. Reconstructing the monomer reverses the change in bond order; it does not reverse the polymer by writing separate saturated fragments.
Poly(alkene)s persist, while burning can create harmful products
| Disposal route / material | Chemical difficulty | Consequence |
|---|---|---|
| landfill or litter | poly(alkene) carbon backbones are not readily biodegraded | material persists and accumulates |
| complete combustion of hydrocarbon polymers | carbon is converted to CO₂ | greenhouse-gas release |
| incomplete combustion | limited oxygen produces CO and soot | CO is toxic; particulates harm air quality |
| combustion of PVC | chlorine in the polymer can form HCl(g) | corrosive, acidic and harmful gas requires controlled treatment |
Recycling can reduce demand for new feedstock and delay disposal, but mixed or contaminated plastics are difficult to separate and recycle. Controlled incineration may recover energy, yet it still needs oxygen control and flue-gas treatment.
Non-biodegradable does not mean chemically impossible to recycle, and burning does not simply make the waste disappear. Product identity depends on polymer composition and combustion conditions.