Polymers

This lesson covers polymers and their structure in the context of bonding, structure and properties, as required by the AQA GCSE Combined Science Trilogy specification (8464), section 4.2.2. You need to understand the structure of polymers and how this affects their properties.

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What Are Polymers?

Polymers are very large molecules made up of many small repeating units called monomers that are joined together by covalent bonds. The process of joining monomers together is called polymerisation.

$n \text{ monomers} \xrightarrow{\text{polymerisation}} \text{polymer}$n monomerspolymerisation​polymer

• Monomer — the small molecule that is repeated.
• Polymer — the long-chain molecule made from many monomers.
• The prefix "poly-" means "many."

Examples

Monomer	Polymer	Common Use
Ethene (C₂H₄)	Poly(ethene) / polyethylene	Plastic bags, bottles
Propene (C₃H₆)	Poly(propene) / polypropylene	Food containers, ropes
Chloroethene (C₂H₃Cl)	Poly(chloroethene) / PVC	Pipes, window frames
Styrene	Polystyrene	Packaging, insulation

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Structure of Polymers

Polymers consist of long chains of atoms held together by strong covalent bonds along the chain. Between the polymer chains, there are intermolecular forces that hold the chains next to each other.

graph TD
    A["Polymer Structure"] --> B["Long chains of<br/>covalently bonded<br/>atoms"]
    A --> C["Intermolecular forces<br/>between chains"]
    B --> D["Strong covalent bonds<br/>within the chain<br/>— hard to break"]
    C --> E["Strength depends on<br/>the forces between<br/>chains"]

    style A fill:#2c3e50,color:#fff
    style B fill:#3498db,color:#fff
    style C fill:#e67e22,color:#fff
    style D fill:#3498db,color:#fff
    style E fill:#e67e22,color:#fff

Key Structural Features

1. Covalent bonds within chains — very strong; hold the polymer together.
2. Intermolecular forces between chains — much weaker than the covalent bonds but collectively significant in long-chain molecules.
3. Chain length — polymers are very long molecules, sometimes containing thousands or millions of atoms.
4. Tangling — the long chains tend to become tangled and intertwined, which affects the properties of the material.

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Properties of Polymers

The properties of a polymer depend on:

1. What monomer it is made from — different monomers give different polymers with different properties.
2. The intermolecular forces between chains — stronger forces give harder, more rigid polymers.
3. The arrangement of chains — whether the chains are closely packed or loosely arranged.

General Properties of Polymers

Property	Explanation
Solid at room temperature	The intermolecular forces between the long chains are collectively strong enough to hold them in place
Generally do not conduct electricity	There are no free electrons or mobile ions
Can be flexible or rigid	Depends on the strength of intermolecular forces between chains and how the chains are arranged
Lower melting points than ionic or giant covalent substances	The intermolecular forces between chains, though collectively significant, are weaker than ionic bonds or extensive covalent networks

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Types of Polymer: Thermosoftening and Thermosetting

Thermosoftening Polymers

• The polymer chains are held together by weak intermolecular forces.
• When heated, these forces are overcome and the chains can slide past each other.
• The polymer softens and melts and can be remoulded.
• When cooled, the forces re-form and the polymer hardens again.
• Can be recycled by melting and reshaping.
• Examples: poly(ethene), poly(propene), PVC, polystyrene.

Thermosetting Polymers

• The polymer chains are joined by covalent cross-links (strong covalent bonds between chains).
• When heated, these strong cross-links do not break (until the polymer decomposes).
• The polymer does not soften or melt — it is rigid and hard.
• Cannot be remoulded or recycled by melting.
• Examples: Bakelite, melamine, epoxy resin.

graph LR
    A["Thermosoftening<br/>Polymer"] -->|"Weak intermolecular<br/>forces between chains"| B["Softens when<br/>heated — can<br/>be remoulded"]
    C["Thermosetting<br/>Polymer"] -->|"Strong covalent<br/>cross-links"| D["Does NOT soften<br/>when heated —<br/>cannot be remoulded"]

    style A fill:#3498db,color:#fff
    style B fill:#3498db,color:#fff
    style C fill:#e74c3c,color:#fff
    style D fill:#e74c3c,color:#fff

Exam Tip (AQA 8464): The key distinction is: thermosoftening polymers have weak intermolecular forces between chains (so they can be melted); thermosetting polymers have strong covalent cross-links between chains (so they cannot be melted).

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Comparing Thermosoftening and Thermosetting Polymers

Feature	Thermosoftening	Thermosetting
Forces between chains	Weak intermolecular forces	Strong covalent cross-links
Effect of heating	Softens and melts	Does not soften (decomposes at very high temperature)
Can be remoulded?	Yes	No
Recyclable by melting?	Yes	No
Hardness	Generally softer	Generally harder and more rigid
Examples	Poly(ethene), PVC	Bakelite, melamine

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Polymers in Context: Structure and Properties

Polymers sit between simple molecular substances and giant covalent structures in terms of their properties:

Substance Type	Melting Point	Conductivity	Example
Simple molecular	Low	Does not conduct	Water, methane
Polymer	Moderate	Does not conduct	Poly(ethene)
Giant covalent	Very high	Usually does not conduct	Diamond, SiO₂
Ionic	High	Conducts when molten/dissolved	NaCl
Metallic	High	Conducts	Iron, copper

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Common Mistakes

Mistake	Correction
"Polymers are ionic compounds"	Polymers have covalent bonds within the chains — they are not ionic
"All polymers can be melted and reshaped"	Only thermosoftening polymers can be melted and reshaped; thermosetting polymers cannot
"Thermosetting polymers have weak bonds between chains"	Thermosetting polymers have strong covalent cross-links between chains
"Polymers conduct electricity"	Polymers generally do not conduct electricity — there are no free electrons or ions
Confusing monomer and polymer	The monomer is the small repeating unit; the polymer is the long chain made from many monomers

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Summary

• Polymers are very large molecules made from many repeating monomers joined by covalent bonds.
• They are solid at room temperature and generally do not conduct electricity.
• Thermosoftening polymers have weak intermolecular forces between chains — they soften when heated and can be remoulded.
• Thermosetting polymers have strong covalent cross-links between chains — they do not soften when heated and cannot be remoulded.
• The properties of a polymer depend on the monomer, the intermolecular forces between chains, and whether cross-links are present.

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Worked Example 1 — Explaining the Difference Between Thermosoftening and Thermosetting

Question: Describe, in terms of structure and bonding, the difference between thermosoftening polymers and thermosetting polymers. Use your answer to explain why poly(ethene) melts when heated but Bakelite does not. (6 marks)

Model answer: