Addition Polymerisation
Summary: Addition polymerisation is the process where many small unsaturated monomer molecules (containing C=C double bonds) join together to form a long polymer chain without producing any byproducts. Tags: igcse chemistry organic-chem polymers Created: 2026-07-14 Last Updated: 2026-07-23
Definition
Addition polymerisation is the reaction in which many small unsaturated monomers (alkenes with C=C double bonds) join together to form a very long saturated polymer chain. No other products are formed — all the atoms from the monomer end up in the polymer.
The process requires:
- High pressure
- A catalyst (often a Ziegler-Natta catalyst or an organic peroxide initiator)
- Heat (moderate temperature)
What Happens
- The C=C double bond opens up
- Monomers link together by forming single covalent bonds between carbon atoms
- The polymer backbone is a chain of carbon-carbon single bonds
- The reaction continues until the chain is terminated
Key point: The double bond breaks open → single bonds form between monomers. Think of it as “unzipping” the double bond to “zip” monomers together.
Addition Polymers You Need to Know
Poly(ethene) — commonly called polythene
- Monomer: Ethene (C₂H₄), CH₂=CH₂
- Repeating unit: —CH₂—CH₂—
- Properties: Flexible, low density (LDPE) or rigid, high density (HDPE)
- Uses: Plastic bags (LDPE), bottles and pipes (HDPE), cling film
Poly(propene) — commonly called polypropylene
- Monomer: Propene (C₃H₆), CH₂=CH—CH₃
- Repeating unit: —CH₂—CH(CH₃)—
- Properties: Tough, flexible, higher melting point than poly(ethene)
- Uses: Rope, carpet fibres, food containers, medical syringes, car bumpers
Poly(chloroethene) — commonly called PVC
- Monomer: Chloroethene (C₂H₃Cl), CH₂=CHCl
- Repeating unit: —CH₂—CHCl—
- Properties: Rigid (uPVC) or flexible (PVC with plasticiser added)
- Uses: Window frames, drainpipes (uPVC), electrical cable insulation (flexible PVC)
PTFE — Poly(tetrafluoroethene)
- Monomer: Tetrafluoroethene (C₂F₄), CF₂=CF₂
- Repeating unit: —CF₂—CF₂—
- Properties: Non-stick, heat resistant, very low friction
- Uses: Non-stick pan coating, plumber’s tape, chemical container linings
From Monomer to Polymer
General equation for addition polymerisation of any alkene:
n(CH₂=CHX) → —(CH₂—CHX)—ₙ
Where:
- n = a very large number (typically 10,000–100,000+)
- X = H (ethene), CH₃ (propene), Cl (chloroethene), F (tetrafluoroethene)
Drawing Addition Polymers at IGCSE
The examiner wants to see:
- The repeating unit shown in brackets with extension bonds: [—CH₂—CHX—]ₙ
- Extension bonds through the brackets
- No double bonds in the polymer backbone
- The ‘n’ subscript outside the brackets
Example — Poly(ethene):
H H
| |
—[—C—C—]—ₙ
| |
H H
Comparison: Addition vs Condensation Polymerisation
| Feature | Addition | Condensation |
|---|---|---|
| Monomer | Unsaturated (has C=C) | Two functional groups per monomer |
| Byproduct? | None | Small molecule (H₂O, HCl) eliminated |
| Polymer backbone | C—C chain only | May contain O, N in backbone |
| Examples | Poly(ethene), PVC, PTFE | Nylon, PET, proteins |
| Mass of polymer | = mass of all monomers | < mass of all monomers (byproduct lost) |
Key Facts
- Monomers must have C=C double bonds (alkenes)
- Double bond opens up → monomers join by single covalent bonds
- No byproducts — all atoms end up in the polymer
- Polymer is saturated (no double bonds in backbone)
- Repeating unit formula = same as monomer (atoms just rearranged)
- Examples: poly(ethene), poly(propene), PVC (poly(chloroethene)), PTFE
- Addition polymers are non-biodegradable → environmental/pollution concerns