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Chemistry · Practice book · Senior 6 · Unit 5 of 14

Polymers and polymerisation

Senior 6 Chemistry · 14 questions with answers. Try each question on paper, then tap it to see the answer.

Unit 5 Polymers and polymerisation

Key unit competence: Relate the types of polymers to their structural properties and uses.

Key ideas

  • A monomer is a small molecule that joins with many others to form a polymer, a very large molecule (macromolecule). Polymerisation is the joining process; the degree of polymerisation n is the number of repeat units.
  • Addition polymerisation: unsaturated monomers (C=C) join with no other product: ethene → poly(ethene); chloroethene → PVC; propene → poly(propene); phenylethene → polystyrene; tetrafluoroethene → PTFE (Teflon).
  • Condensation polymerisation: monomers with two functional groups join, losing a small molecule (water or HCl). Nylon-6,6: hexanedioic acid + 1,6-diaminohexane (amide links); Terylene (PET): benzene-1,4-dicarboxylic acid + ethane-1,2-diol (ester links). Natural condensation polymers: proteins, starch, cellulose.
  • Classes: natural (rubber, cellulose, proteins, starch) and synthetic; homopolymers and copolymers; thermoplastics (soften on heating and can be remoulded: polythene, PVC) and thermosets (cross-linked, do not soften: bakelite, melamine); elastomers (rubber) and fibres (nylon).
  • Properties depend on chain length, branching (LDPE is branched, soft; HDPE is less branched, denser, stronger), crystallinity, intermolecular forces and cross-linking. Vulcanisation: heating natural rubber with sulfur forms S–S cross-links, making it harder, more elastic, less sticky and stable over a wider temperature range (tyres).
  • Most synthetic polymers are non-biodegradable: they block drains, harm animals and make soils infertile; burning PVC gives HCl and dioxins. Management: reduce, reuse, recycle, biodegradable polymers. Rwanda banned plastic bags in 2008 and single-use plastics in 2019.

Common misconceptions

  • "Polymerisation of ethene removes the double bond and releases water." No: Addition polymerisation forms no other product; only condensation releases small molecules.
  • "All plastics melt when heated." No: Thermosets are cross-linked and char instead of melting.
  • "A polymer is a mixture of different small molecules." No: A polymer is a very long chain of the same repeat unit (or of two in copolymers).

Worked examples

Example 1. Write equations for the formation of (a) PVC and (b) Terylene.

Solution

(a) n CH2=CHCl → –[CH2–CHCl]n– (addition; the C=C opens, no by-product).

(b) n HOOC–C6H4–COOH + n HO–CH2CH2–OH → –[OC–C6H4–COO–CH2CH2–O]n– + 2n H2O.

Terylene is a polyester: each ester link forms with loss of water.

Uses: PVC pipes and cables; Terylene fibres for clothes and PET bottles.

Example 2. Explain how vulcanisation changes the properties of natural rubber.

Solution

Natural rubber chains (poly(isoprene)) slide over one another: soft, sticky when warm, brittle when cold.

Heating with sulfur (about 140 °C) forms –S–S– cross-links between chains at the C=C positions.

Cross-links pull chains back after stretching (better elasticity), make rubber harder and stronger, less sticky, and stable over a wider temperature range.

More sulfur gives harder rubber (ebonite).

Practice questions

  1. Define polymerisation.

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    The process in which many monomer molecules join to form a polymer.

    Common mistake: Saying it is the breaking of large molecules.

  2. Give two differences between addition and condensation polymerisation.

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    Addition: C=C monomers, no by-product, polymer has same empirical formula as monomer. Condensation: two functional groups, small molecule lost, different formula (any two).

    Common mistake: Saying both release water.

  3. Give one use each of PVC, polystyrene and PTFE.

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    PVC: pipes/cables; polystyrene: packaging/insulation; PTFE: non-stick coatings.

    Common mistake: Swapping the uses.

  4. Why is PVC harder than poly(ethene)?

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    The polar C–Cl bonds give stronger forces between chains.

    Common mistake: Saying PVC is cross-linked.

  5. What is an elastomer?

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    A polymer that can be stretched and returns to its original shape, e.g. rubber.

    Common mistake: Saying any plastic.

  6. State two ways of managing plastic waste.

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    Reduce use, reuse, recycle, use biodegradable polymers, controlled incineration with energy recovery (any two).

    Common mistake: Suggesting open burning.

  7. Draw (describe) the amide link.

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    –CO–NH–.

    Common mistake: Writing –COO–.

  8. Why are condensation polymers like nylon called copolymers?

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    They are made from two different monomers.

    Common mistake: Saying they contain metals.

  9. What is the degree of polymerisation?

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    The number of repeat units in a polymer chain.

    Common mistake: Confusing it with the relative molecular mass.

  10. Give one advantage of plastics over traditional materials like wood.

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    Light, waterproof, durable, cheap, easily moulded, good insulators (any one).

    Common mistake: Saying they decompose quickly.

Matching and drag-and-drop

  1. Matching

    Match each item in column A with the correct answer in column B. Write the letter next to each number.

    Column AColumn B
    1. PVC is made from:a. hydrogen chloride and dioxins
    2. A copolymer is made from:b. two or more different monomers
    3. Vulcanised rubber is:c. chloroethene
    4. Burning PVC is dangerous because it releases:d. harder and more elastic
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    1 → c (chloroethene) · 2 → b (two or more different monomers) · 3 → d (harder and more elastic) · 4 → a (hydrogen chloride and dioxins)

    • Vinyl chloride, CH2=CHCl.
    • e.g. nylon-6,6.
    • Cross-links stop chains sliding.
    • Toxic and acidic gases.

  2. Drag and drop

    Choose the correct word or phrase from the box for each gap. One word in the box is not needed.

    non-stick pans  ·  ethanol  ·  glucose  ·  ethene
    1. Teflon (PTFE) is used for .
    2. Starch is a polymer of .
    3. Poly(ethene) is made from .
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    1: non-stick pans · 2: glucose · 3: ethene

    • Very unreactive and slippery.
    • Condensation polymer.
    • Addition polymerisation of CH2=CH2.

  3. Matching

    Match each item in column A with the correct answer in column B. Write the letter next to each number.

    Column AColumn B
    1. Vulcanisation of rubber uses:a. does not soften on reheating
    2. A thermoplastic:b. sulfur
    3. A thermosetting plastic:c. Cellulose
    4. Which is a natural polymer?d. softens on heating and can be remoulded
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    1 → b (sulfur) · 2 → d (softens on heating and can be remoulded) · 3 → a (does not soften on reheating) · 4 → c (Cellulose)

    • Sulfur cross-links.
    • Weak forces between chains.
    • Cross-links hold the chains.
    • Plant cell walls.

  4. Drag and drop

    Choose the correct word or phrase from the box for each gap. One word in the box is not needed.

    amide (peptide) links  ·  two –OH groups  ·  polyester  ·  a carbon–carbon double bond
    1. Addition polymerisation needs monomers with .
    2. Terylene (PET) is a .
    3. Nylon-6,6 contains .
    Show answer

    1: a carbon–carbon double bond · 2: polyester · 3: amide (peptide) links

    • The C=C opens up to link monomers.
    • Ester links.
    • –CONH– between monomers.

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