UNIT 2 CHEMISTRY · PEARSON AAQ · E1.8.3
Addition Polymerisation
FREE CHEMISTRY REVISION · TOPIC 65
Addition Polymerisation
Pearson BTEC Level 3 AAQ Applied Science
Unit 2: Principles and Applications of Chemistry · E1.8.3
What you will learn
- Convert alkene monomers into addition-polymer repeating units.
- Recognise all five specified polymers and connect structures to properties and uses.
- Explain why a C=C becomes a single bond in a saturated polymer backbone.
1. Many monomers, one long chain
A polymer is a large molecule containing many repeating units. In addition polymerisation, many unsaturated monomer molecules join without elimination of a small molecule. For these examples, the alkene C=C opens: the π bond is replaced by new σ bonds linking neighbouring monomer units. The original two carbon atoms become adjacent atoms in the polymer backbone; their connecting bond is now single.
Ethene, CH₂=CH₂, gives poly(ethene), [–CH₂–CH₂–]ₙ. The repeating unit has the same atoms as one monomer. A chain contains many such units; it is not one ethene molecule merely drawn inside brackets. Suitable initiators/catalysts, temperature and pressure depend on the polymer process; they are not identical for all polymers.
2. Draw repeating units accurately
- Draw the two former C=C carbons as a C–C single bond.
- Keep each substituent on its original carbon. Do not remove Cl, F, CH₃ or C₆H₅.
- Give each backbone carbon four bonds: two along the chain and two to side groups.
- Draw continuation bonds through the brackets, with n outside. Never leave a C=C in the saturated backbone.
- To recover an alkene monomer, isolate one two-carbon repeat, remove continuation bonds and restore C=C.
3. poly(ethene)
Flexible grades for bags and films; tougher grades for bottles and pipes. Non-polar chains resist water and are good electrical insulators. Branching and crystallinity affect density and stiffness.
4. poly(propene)
The methyl side groups influence packing and chain movement. Often tough, low-density and resistant to repeated flexing; used for living hinges, food containers and ropes.
5. poly(styrene)
Bulky phenyl side groups restrict chain movement: ordinary polystyrene is rigid and can be brittle. Used in packaging and rigid housings; expanded foam traps gas for thermal insulation and cushioning.
6. poly(chloroethene), PVC
Polar C–Cl bonds contribute to useful intermolecular attractions. Unplasticised PVC is rigid for pipes and window frames; plasticisers increase flexibility for cable covering and flooring.
7. poly(tetrafluoroethene), PTFE
Strong C–F bonds and a fluorine-covered chain give high chemical resistance and a low-friction surface. Used for non-stick coatings, seals and chemical-resistant apparatus; use within specified temperature limits.
8. Compare structures and applications
| Polymer | Monomer / condensed repeat | Useful application |
|---|---|---|
| Poly(ethene) | CH₂=CH₂ → [–CH₂–CH₂–]ₙ | Packaging films or bottles, depending on grade |
| Poly(propene) | CH₂=CHCH₃ → [–CH₂–CH(CH₃)–]ₙ | Hinges, containers, fibres |
| Poly(styrene) | CH₂=CH(C₆H₅) → [–CH₂–CH(C₆H₅)–]ₙ | Rigid packaging or expanded insulating foam |
| PVC | CH₂=CHCl → [–CH₂–CHCl–]ₙ | Rigid pipes; plasticised cable insulation |
| PTFE | CF₂=CF₂ → [–CF₂–CF₂–]ₙ | Low-friction, chemically resistant coatings/seals |
Properties depend on chain length, branching, crystallinity, additives and processing as well as monomer identity. These named polymers are generally persistent in ordinary environments; useful durability can become a waste problem. Electrical insulation arises from the absence of freely moving charge carriers, not from a lack of electrons.
Worked and applied examples
Example 1 · Ethene equation
nCH₂=CH₂ → [–CH₂–CH₂–]ₙ. Each repeat contains C₂H₄. For n incorporated monomers, the idealised repeat representation conserves 2n carbon and 4n hydrogen atoms. Initiator fragments and chain ends are omitted from this conventional equation.
Example 2 · Propene side group
CH₂=CH–CH₃ gives [–CH₂–CH(CH₃)–]ₙ. The methyl group stays attached to the second backbone carbon. It is a side group, so the repeat still has three carbons overall, not three carbons in the main backbone segment.
Example 3 · Recover the PVC monomer
From [–CH₂–CHCl–]ₙ, restore the double bond between the two backbone carbons: CH₂=CHCl, chloroethene. Chlorine remains attached; no HCl is eliminated during addition polymerisation.
Example 4 · Recover the PTFE monomer
[–CF₂–CF₂–]ₙ comes from CF₂=CF₂. Each backbone carbon has two fluorines. Replacing only one H with F would describe a different monomer and polymer.
Example 5 · Choose a polystyrene application
Expanded polystyrene is useful insulation because trapped gas reduces heat transfer. Ordinary solid polystyrene is not automatically a soft foam: expansion and physical form matter.
Example 6 · Choose a PVC grade
A drainpipe needs rigid, durable PVC. A flexible cable cover uses plasticised PVC. Saying all PVC is flexible overlooks the role of additives.
Common misconceptions and examination tips
- Addition polymerisation does not release water or HCl.
- Styrene has a phenyl side group, C₆H₅; it is not lost or converted into a methyl group.
- PTFE has four F atoms per two-carbon repeat. PVC has one Cl per two-carbon repeat.
- Keep side-group bonds distinct from continuation bonds. Reversing a repeat is valid if connectivity is unchanged.
- For a use question, link one property to the required function; do not just list applications.
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