Knowledge Organiser: Polymers
Key Terms
- Polymer: long chain molecule made of many monomers
- Monomer: small molecule with C=C that joins to form polymer
- Addition polymerisation: C=C opens, monomers join, no atoms lost
- Repeat unit: smallest repeating section of polymer chain
- Intermolecular forces: weak forces BETWEEN polymer chains
- Thermosoftening: no cross-links, melts on heating, can be recycled
- Thermosetting: covalent cross-links, cannot melt, decomposes on heating
Must-Know Facts
- Strong covalent bonds WITHIN chains
- Weak intermolecular forces BETWEEN chains
- Low MP — only intermolecular forces between chains broken
- Do NOT conduct — no free electrons or ions
- Ethene (CH₂=CH₂) → poly(ethene)
- Naming: poly(monomer name)
- Thermosoftening: poly(ethene), PVC, polystyrene — can be remoulded
- Thermosetting: Bakelite, melamine, epoxy resin — permanent shape
- Cross-links prevent chain movement → thermosetting stays rigid when heated
Key Equations
- n(CH₂=CH₂) → (−CH₂−CH₂−)ₙ (addition polymerisation of ethene)
- Monomer must contain C=C double bond for addition polymerisation
- Repeat unit = monomer with double bond replaced by single bond and brackets with n
Common Mistakes
- Drawing the repeat unit with a double bond: The C=C opens during polymerisation — the repeat unit has only single bonds
- Saying polymers have high melting points: Thermosoftening polymers have relatively low melting points — only intermolecular forces between chains break
- Confusing thermosoftening and thermosetting: Thermosoftening = no cross-links, can be remelted; thermosetting = covalent cross-links, cannot be remelted
- Forgetting the subscript n: The repeat unit must show n outside the brackets to indicate many repeating units
Memory Aid
- MONO = one, POLY = many — like Monopoly! The board game Monopoly means "one seller" — mono (one) + poly (many properties, wait that doesn't quite work, but the mono/poly part sticks!). Monomer = one unit, Polymer = many units.
- Addition polymerisation = ADD everything together: In addition polymerisation, ALL atoms are added into the chain — nothing is lost. All atoms from the monomers end up in the polymer.
- Naming polymers: "poly" brackets + monomer name. Ethene → poly(ethene). Propene → poly(propene). If you forget which comes first, remember you ADD the poly AT THE START.
- Drawing repeat units in exams: Start from the monomer (CH₂=CH₂), change the double bond to single (CH₂-CH₂), put brackets around it, and add little bonds sticking out at each end (—[CH₂-CH₂]ₙ—).