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Chemistry 3% exam weight

Polymers and Synthetic Polymers

Part of the NECO SSCE study roadmap. Chemistry topic chem-15 of Chemistry.

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Polymers and Synthetic Polymers

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

A polymer is a giant molecule built from many small monomer units joined by covalent bonds. Synthetic polymers are made in factories from petrochemicals such as ethene, vinyl chloride, and styrene. The number of repeat units, called the degree of polymerisation (n), equals the polymer’s molar mass divided by the monomer’s molar mass.

Two reactions dominate NECO questions:

  • Addition (chain-growth) — monomers add together with no loss of atoms (e.g., polyethene from ethene).

  • Condensation (step-growth) — each linkage expels a small molecule, usually water or HCl (e.g., nylon-6,6).

  • High-yield pointers:

    • Write repeat units inside square brackets with a continuation bond (─) and subscript n, e.g., ─[CH₂─CH₂]ₙ─.
    • Know thermosetting vs thermoplastic: thermosets (Bakelite) cannot be remelted; thermoplastics (PVC, polyethene) can.
    • Vulcanisation with sulphur cross-links rubber chains, raising its elasticity.

🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Definitions and Classification

A monomer is the small reactive unit; the repeat unit is the structural segment that reappears along the chain. Synthetic polymers are manufactured through controlled polymerisation of petrochemical feedstocks, unlike natural polymers such as cellulose, starch, proteins, and natural rubber. Polymers are classified by source (natural vs synthetic), structure (linear, branched, cross-linked), response to heat (thermoplastic vs thermosetting), and chain uniformity (homopolymer vs copolymer).

Addition Polymerisation

In addition polymerisation, unsaturated monomers (containing C=C) link directly without losing atoms. The free-radical mechanism proceeds in three stages — initiation (e.g., benzoyl peroxide forms radicals), propagation (radical adds across the double bond, creating a new radical), and termination (two radicals couple or disproportionate). Polyethene, PVC, polystyrene, PTFE, and Perspex are classic examples.

Condensation Polymerisation

In condensation polymerisation, each bond formed releases a small molecule such as H₂O or HCl. Nylon-6,6 forms from hexamethylenediamine + adipic acid, while Terylene (a polyester) forms from ethylene glycol + terephthalic acid. The degree of polymerisation relates to the extent of reaction p via the Carothers equation: DP = 1 / (1 − p).

PolymerMonomer(s)TypeKey Use
PolyetheneEthene (CH₂=CH₂)AdditionPlastic bags, bottles
PVCVinyl chloride (CH₂=CHCl)AdditionPipes, cable insulation
Nylon-6,6Hexamethylenediamine + adipic acidCondensationTextiles, ropes
TeryleneEthylene glycol + terephthalic acidCondensationFabrics, bottles
BakelitePhenol + formaldehydeCondensationSwitches, handles
  • Study pointers:
    • Memorise the repeat-unit drawings with brackets and the continuation bond.
    • Nylon-6 (one monomer, caprolactam) differs from nylon-6,6 (two monomers) — examiners test this.
    • Recycling codes 1 (PET), 2 (HDPE), 3 (PVC), 4 (LDPE), 5 (PP), 6 (PS) are frequent one-mark items.

🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Worked Example: Molar Mass Calculation

A polyethene sample has M(polymer) = 84,000 g/mol. Ethene’s molar mass is 28 g/mol, so: n = 84,000 ÷ 28 = 3,000 repeat units. Mass of polymer formed from 5 mol ethene = 5 × 28 = 140 g.

Degree of Polymerisation and Molar Mass

The number-average molar mass Mn = Σ(Ni × Mi) / Σ(Ni) and weight-average molar mass Mw = Σ(Ni × Mi²) / Σ(Ni × Mi). The polydispersity index PDI = Mw / Mn equals 1 for perfectly uniform chains and rises with chain-length distribution.

Cross-linking and Rubber

Vulcanisation introduces short sulphur bridges between polyisoprene chains. This converts sticky natural rubber into an elastomer that regains shape after stretching. Cross-links also make thermosetting polymers (Bakelite, melamine) rigid and infusible.

Common Mistakes and Edge Cases

  • Confusing addition with condensation — addition releases no small molecule.
  • Drawing ─CH=CH─ instead of ─CH₂─CH₂─ for polyethene.
  • Mixing up thermoplastic (softens, recyclable) with thermosetting (decomposes on heating, not recyclable).
  • Believing all plastics biodegrade; C–C backbones resist microbial attack, hence recycling codes matter.

Exam Strategy (3% weightage)

NECO SSCE Chemistry typically asks 3–5 mark structured questions. Expected tasks:

  1. Name a polymer and write its repeat unit with subscript n.
  2. State the monomer(s) and classify the reaction type.
  3. Differentiate thermoplastic from thermosetting with one example each.
  4. Explain vulcanisation or recycling using a named polymer.

Allocate about 90 seconds per part; chemical equations take most of that time, so practise drawing repeat units from memory.

Practice Prompts

  1. Write the equation for the formation of PVC from vinyl chloride, showing three repeat units and the continuation bond.
  2. A polyester is made from ethane-1,2-diol and benzene-1,4-dicarboxylic acid. Name the polymer, state the byproduct, and calculate the mass of water released when 1 mol of each monomer reacts completely.

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