📘 IB Understanding
Addition polymers form by the breaking of a double bond in each monomer. The π bond breaks and the freed electrons form new C–C σ bonds to adjacent monomers. No atoms are lost – everything in the monomer appears in the polymer.
How Addition Polymerisation Works
Every alkene monomer contains a C=C double bond. This bond consists of one strong σ bond and one weaker π bond. During polymerisation:
- The weaker π bond in each monomer breaks open
- The freed electrons form new C–C single bonds to adjacent monomers
- This repeats thousands of times, producing a long saturated chain
Addition Polymerisation of Ethene
Common Addition Polymers
| Monomer | Polymer | Key Property | Uses |
|---|---|---|---|
| Ethene (CH₂=CH₂) | Poly(ethene) / PE | Flexible, cheap, waterproof | Bags, bottles, packaging |
| Propene (CH₂=CHCH₃) | Poly(propene) / PP | Tough, heat-resistant | Ropes, crates, car bumpers |
| Chloroethene (CH₂=CHCl) | PVC | Rigid or flexible, durable | Pipes, window frames, flooring |
| Tetrafluoroethene (CF₂=CF₂) | PTFE (Teflon) | Very low friction, chemically inert | Non-stick coatings, seals |
| Phenylethene (CH₂=CHC₆H₅) | Polystyrene / PS | Lightweight, good insulator | Packaging foam, insulation |
📋 Exam Note
You do not need to memorise monomer structures. They will be given to you in the exam. You do need to be able to convert between monomer and repeat unit in both directions.
The Skill of Drawing Repeat Units
This is one of the most commonly examined skills in the polymers topic. You must be able to:
- Monomer → Polymer: Given a monomer, draw the repeat unit of the polymer
- Polymer → Monomer: Given a section of polymer chain, deduce the monomer
Monomer → Repeat Unit
- Take the monomer structure
- Break the C=C into a C–C single bond
- Keep all side groups attached to their original carbon
- Add extension bonds (dashes) at each end
- Enclose in square brackets with subscript n
Repeat Unit → Monomer
- Identify two adjacent carbons in the backbone
- Replace the C–C single bond with a C=C double bond
- Remove the square brackets and subscript n
- Remove the extension bonds
Drawing a Repeat Unit from Chloroethene
⚠️ Common Mistakes
- The repeat unit must show single bonds only – no C=C should remain in the polymer backbone
- You must include the extension bonds (dashes) extending outside the brackets
- You must include the subscript n – missing either the extension bonds or n loses the mark
- Side groups (e.g. Cl, CH₃, C₆H₅) must stay attached to the same carbon they were on in the monomer
100% Atom Economy
Addition polymerisation has 100% atom economy because every atom in the monomer ends up in the polymer. No by-products (like water) are formed. This is a key distinction from condensation polymerisation.
🔗 Link to R2.1 (Atom Economy)
The IB syllabus explicitly connects addition polymerisation to atom economy. In an exam, you should be able to explain:
\[\text{Atom economy} = \frac{\text{M}_r \text{ of desired product}}{\sum \text{M}_r \text{ of all products}} \times 100\%\]
For addition polymerisation: there is only one product (the polymer), so atom economy = 100%
Environmental Impact
Because the polymer backbone consists entirely of strong, saturated C–C and C–H σ bonds, addition polymers are:
❌ Non-biodegradable
- Microorganisms cannot break the saturated C–C backbone
- Persist in landfill for hundreds of years
- Cause pollution in oceans and ecosystems
✅ Solutions
- Recycling – melting and remoulding (mechanical recycling)
- Incineration (energy recovery, but produces CO₂)
- Biodegradable alternatives – using modified polymers with hydrolysable linkages
💬 Deep Think
Addition polymerisation achieves 100% atom economy (maximum efficiency), yet the product is non-biodegradable (maximum environmental persistence). This tension between chemical efficiency and environmental sustainability is a central theme in green chemistry and could form the basis of an IB extended-response question.
Addition vs Condensation – Quick Comparison
| Addition | Condensation (HL) | |
|---|---|---|
| Monomer requirement | Must contain C=C | Two functional groups (bifunctional) |
| By-product | None | H₂O (or HCl) |
| Atom economy | 100% | < 100% |
| Backbone | C–C only | C–O or C–N linkages |
| Biodegradable? | No (chemically inert) | Potentially (hydrolysable bonds) |
🔑 Exam Strategy
When asked to identify whether a polymer is formed by addition or condensation:
- Look at the backbone – if it contains only C–C bonds, it is addition
- If the backbone contains ester (C–O–C=O) or amide (C–N–C=O) linkages, it is condensation
- If unsure, check: is there a small molecule by-product? No = addition. Yes = condensation