Polymers (Higher tier)
A polymer is a long-chain molecule made by joining many small monomer units together. Two main types of polymerisation are studied at GCSE: addition and condensation.
Addition polymerisation
- Monomers must contain a C=C double bond (alkenes).
- The double bond opens up; many monomers join end-to-end with no other product.
Examples
- Ethene → poly(ethene): n CH₂=CH₂ → –(CH₂–CH₂)ₙ–
- Propene → poly(propene): n CH₂=CH(CH₃) → –(CH₂–CH(CH₃))ₙ–
- Chloroethene (vinyl chloride) → PVC: n CH₂=CHCl → –(CH₂–CHCl)ₙ–
The repeating unit is shown in square brackets with the n subscript: [–CH₂–CH₂–]ₙ.
Condensation polymerisation
- Monomers each have two functional groups.
- When they join, a small molecule (usually water) is released for each link formed.
Examples
- Polyester (e.g. PET): diol + dicarboxylic acid → ester linkage + nH₂O. Used in drinks bottles, fabrics.
- Nylon: diamine + dicarboxylic acid → amide linkage + nH₂O. Used in clothing, ropes.
A simple monomer pair: HOOC–R–COOH + HO–R'–OH → –(OC–R–COO–R'–O)ₙ– + nH₂O.
Naturally occurring polymers
Many essential biological molecules are condensation polymers:
| Polymer | Monomer | Role |
|---|---|---|
| Starch / Cellulose / Glycogen | Glucose | Energy storage / structure |
| Proteins | Amino acids | Enzymes, structure, transport |
| DNA | Nucleotides | Genetic material |
Each is made by condensation (water released at each link).
Properties of synthetic polymers
- Thermosoftening (e.g. polyethene, polystyrene): chains held by weak intermolecular forces; melt easily; recyclable.
- Thermosetting (e.g. melamine, Bakelite): chains crosslinked by strong covalent bonds; do NOT melt; rigid; cannot be re-melted/recycled.
✦Worked example— Worked example — drawing a polymer
Show the polymer of propene.
Propene = CH₃–CH=CH₂.
The C=C opens; long chain forms:
–CH(CH₃)–CH₂–CH(CH₃)–CH₂– …
Repeat unit: [–CH(CH₃)–CH₂–]ₙ.
⚠Common mistakes
- Drawing C=C in the polymer — it should be a single bond after addition.
- Forgetting the n for repeat units.
- Saying nylon is an addition polymer — it's a condensation polymer.
- Confusing thermosoftening with thermosetting — first softens with heat (recyclable), second doesn't (rigid).
Links
Builds on C7.4 (alkenes), C7.6 (carboxylic acids). Sets up C10.9 (polymers as materials, recycling).
AI-generated · claude-opus-4-7 · v3-deep-chemistry