Identify the repeat units and/or linkages in addition polymers and in condensation polymers

Organic Chemistry – Polymers (Cambridge IGCSE 0620, Section 11.8)

1. What is a polymer?

• A polymer is a very large molecule (macromolecule) formed by linking many small molecules called monomers.
• Most everyday plastics are solid polymeric products, so plastics are made from polymers.

2. Two ways in which polymers are formed

2.1 Chain‑growth (addition) polymerisation – stages

1. Initiation – formation of an active centre (free radical, cation or anion) by heat, light or a catalyst.
2. Propagation – the active centre attacks the C=C bond of another monomer, adding it to the chain and regenerating the active centre.
3. Termination – two active chains combine or the active centre is quenched, stopping growth.

2.2 Step‑growth (condensation) polymerisation – stages

1. Activation – functional groups on the two monomers become reactive (often by heating).
2. Condensation – the groups combine, forming a new linkage and releasing a small molecule.
3. Growth – the process repeats, building up long chains; high molecular weight is only reached near the end of the reaction.

3. Identifying repeat units and linkages

• First decide whether the polymer is formed by addition or condensation (look for C=C in the monomer or for complementary functional groups).
• “Cut” the polymer chain at the points where the original monomers joined.
• Write the smallest repeating fragment – the repeat unit – and highlight the bond that links one unit to the next.
  • In addition polymers the linkage is always a single C–C bond.
  • In condensation polymers the linkage is the functional group formed (‑CO‑O‑ for esters, ‑CO‑NH‑ for amides, ‑O‑CO‑O‑ for carbonates, etc.).

4. Common addition polymers – repeat units and linkages

5. Common condensation polymers – repeat units, linkages and small molecules lost

6. Everyday plastics derived from the polymers above

• Polyethylene (PE) – LDPE/HDPE bags, bottles, containers.
• Polypropylene (PP) – food‑wrap, bottle caps, automotive parts.
• Polyvinyl chloride (PVC) – pipes, window frames, cable insulation.
• Polystyrene (PS) – disposable cups, packaging foam, CD cases.
• Poly(methyl methacrylate) (PMMA) – acrylic sheets, lenses, signage.
• Polytetrafluoroethylene (PTFE) – non‑stick cookware, gaskets.
• Polyethylene terephthalate (PET) – beverage bottles, textile fibres.
• Nylon 6,6 – strong fibres for clothing, ropes, engineering plastics.
• Polycarbonate (PC) – safety glasses, compact discs, electronic housings.

7. Why disposal of plastics is a problem

• Both addition and condensation polymers contain strong covalent linkages (C–C, ester, amide, carbonate) that are resistant to chemical attack.
• Most microorganisms cannot break these bonds efficiently → low biodegradability.
• Consequences:
  • Long‑term accumulation in landfills and marine environments.
  • Potential ingestion by wildlife.
  • Incineration releases CO₂ and, for some polymers (e.g. PVC), toxic gases such as HCl.
• Recycling (mechanical or chemical) reduces waste but requires careful sorting because different polymers have different melting points and chemical behaviours.

8. Key points to remember for the exam

1. Identify the polymerisation type:
   • Addition → look for a C=C in the monomer; repeat units are linked by single C–C bonds.
   • Condensation → look for complementary functional groups (‑OH, ‑COOH, ‑NH₂, etc.); repeat unit contains the new functional group (ester, amide, carbonate…) and a small molecule is lost.
2. When drawing the repeat unit, cut the chain at the original monomer‑junction points.
3. Show the linkage clearly (e.g., –CO‑O‑ for an ester, –CO‑NH‑ for an amide).
4. Remember that the same repeat unit can be written in two orientations; the functional group is the decisive feature.
5. For condensation polymers always state the small molecule eliminated (H₂O, HCl, CH₃OH…).

9. Practice questions (with brief answers)

1. Write the repeat unit for polypropylene and indicate the type of linkage.
   Repeat unit: ‑[CH₂‑CH(CH₃)]ₙ‑.
   Linkage: single C–C bond (addition polymer).
2. Polyester fibres are made from terephthalic acid and ethylene glycol. Show the repeat unit and state the small molecule eliminated.
   Repeat unit: ‑[OCH₂CH₂O‑CO‑C₆H₄‑CO]ₙ‑.
   Small molecule lost: H₂O.
3. Identify whether the polymer formed from styrene is an addition or condensation polymer and justify your answer.
   It is an addition polymer because the monomer (styrene) contains a C=C double bond and no small molecule is released during polymerisation.