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4.8Organic Chemistry

Synthetic polymers — Edexcel Linear - OL (Chemistry) Revision Notes & Topical Questions

5 min read 8 parts Lesson 14 of 18 Unit quiz

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Lesson summary

Synthetic Polymers: Addition and Condensation — EDEXCEL LINEAR – OL (CHEMISTRY) notes: Explain how addition and condensation polymers form, draw their repeat units and discuss their properties, uses and disposal.A polymer is a long-chain molecule made by joining many small monomer molecules.

  • Polyesters
  • Polyamides
  • Worked example 1
  • Worked example 2
  • Worked example 3
  • Worked example 4

Part 1 of 6Polymers and polymerisation

A polymer is a long-chain molecule made by joining many small monomer molecules. Synthetic polymers are made in industry, mostly from crude oil products. There are two types of polymerisation: addition and condensation.

Part 2 of 6Addition polymers

Monomers are alkenes (or substituted alkenes). The C=C double bond opens up and the molecules join in a long chain, with no other product. Conditions are high pressure and a catalyst. Equation: n CH2=CH2 → [–CH2–CH2–]n.

MonomerPolymerProperties and uses
EthenePoly(ethene)Flexible, light; carrier bags, bottles, films
PropenePoly(propene)Tough, heat-resistant; crates, ropes, carpets
ChloroethenePVCRigid or plasticised; pipes, window frames, cable insulation
Phenylethene (styrene)PolystyreneRigid or foamed; packaging, insulation
TetrafluoroethenePTFENon-stick, low friction, chemically inert

To draw a repeat unit

  • change the C=C to C–C and draw bonds out through brackets on both ends.
  • To deduce a monomer from a repeat unit: take two carbons in the chain and put back the double bond.

Part 3 of 6Condensation polymers

Condensation polymerisation uses two monomers with two reactive groups each (or one monomer with two different groups). Each time two monomers join, a small molecule is released, usually water.

Polyesters
Polyesters such as PET (poly(ethylene terephthalate)): a dicarboxylic acid reacts with a diol, forming ester links.
  • Usesdrinks bottles, polyester clothing fibres.
Polyamides
Polyamides such as nylon: a dicarboxylic acid (or acyl chloride) reacts with a diamine, forming amide links.
  • Usesclothing, ropes, parachutes, carpets.

Natural polyamides include proteins (amino acids). The functional groups can be shown as blocks: HOOC–[ ]–COOH and H2N–[ ]–NH2.

Part 4 of 6Properties, disposal and the environment

Plastics are chosen for their low density, strength, low cost, insulation and resistance to corrosion.

1
These same properties cause problems: addition polymers are non-biodegradable and resist rotting,
  • so waste persists for centuries, and landfill sites fill.
2
Plastic debris in the oceans harms wildlife.
3
Burning produces CO2 and toxic gases such as HCl from PVC.
4
Options: recycling (melt and reform, needs sorted plastics), reuse, incineration with energy recovery, biodegradable polymers made from starch or PLA,
  • and using fewer plastics.

Part 5 of 6Worked monomer and repeat-unit questions

Practice 1: the polymer poly(propene) has repeat unit –CH(CH3)–CH2–.

Worked monomer and repeat-unit questions
To find the monomer, remove the brackets and restore the double bond: CH3CH=CH2.
Practice 2: the section –CH2–CHCl–CH2–CHCl– repeats every two carbons, so the monomer is CH2=CHCl, chloroethene.
Practice 3: a polymer of mass 28 000 g/mol made from ethene (Mr 28) has n = 28 000 ÷ 28 = 1000 monomer units.

Polyester formationa diol HO–(CH2)2–OH and terephthalic acid, HOOC–C6H4–COOH, react to form PET with ester links and release water.

Worked monomer and repeat-unit questions
In a block diagram, the polyester is written as [–O–[ ]–O–CO–[ ]–CO–]n.
For nylon the link is –CO–NH–.
A frequent exam question asks for differences between nylon and a protein: proteins contain different amino acids with the same amide link,
  • while nylon uses two alternating monomers.

Comparison of properties

  • poly(ethene) softens on heating (thermoplastic) because its chains are held by weak forces;
    • thermosetting plastics have cross-links between chains and do not soften, so they are used for pan handles and electrical fittings.
  • Cross-linking also explains why vulcanised rubber is harder than natural rubber.

Part 6 of 6Worked examples

  1. Draw the repeat unit of poly(chloroethene) and name the monomer. [2]2 marks

  2. Explain one difference between addition and condensation polymerisation. [2]2 marks

  3. Why is PTFE used in non-stick pans? [1]1 mark

  4. State two problems with disposal of plastics. [2]2 marks

Key terms

TermMeaning
MonomerSmall repeating molecule
PolymerLong-chain molecule made of monomers
Addition polymerisationAlkene monomers join with C=C opening
Condensation polymerisationMonomers join and release a small molecule
PolyesterPolymer with ester links
PolyamidePolymer with amide links

Avoid these mistakes

1
Writing polymer equations without n.
2
Keeping double bonds in repeat units.
3
Forgetting water is lost in condensation.
4
Thinking polymers are covalent compounds with small molecules.

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