Quick Notes

Quick Notes

Chemistry Chapter 4: Polymer
4.1 Polymer
4.1 Polymer - Chemistry Form 5
CHEMISTRY • Form 5 • Chapter 4: Polymer

4.1Polymer

Polymers are large macromolecular structures found all around us, from the biological proteins and starch inside living organisms to synthetic materials like plastics and nylon. Understanding polymers involves exploring how small repeating units called monomers link together through chemical reactions, how their structures dictate physical properties such as heat resistance and elasticity, and how modern society balances their practical utility with environmental sustainability.

Learning Objectives

  • Explain the meaning of polymer and monomer.
  • Classify polymers based on their source and their thermal/elastic characteristics.
  • Explain polymerisation reactions through addition polymerisation and condensation polymerisation, including laboratory synthesis.
  • Justify the use of polymers in daily life and discuss the environmental impacts and sustainable management of synthetic polymers.

Comparison of Polymer Chain Structures and Thermal Behaviour

How the presence or absence of cross-links between polymer chains determines whether a polymer is a thermoplastic, a thermoset, or an elastomer.

Thermoplastic

Linear or branched chains with no cross-links. The chains slide past one another when heated, so the polymer melts, solidifies when cooled, and can be repeatedly remoulded and recycled.

Examples: polyethene, polyvinyl chloride (PVC), nylon.

Thermosetting

Extensive covalent cross-links between polymer chains form a rigid network. When heated, the polymer disintegrates or burns instead of melting, so it cannot be remoulded or recycled.

Examples: melamine, bakelite.

Elastomer

Coiled polymer chains with minimal cross-links. The chains uncoil when stretched and snap back to their original shape when released, giving high elasticity.

Examples: polyurethane, styrene-butadiene rubber (SBR).

Brief Explanation

Understand the key ideas about polymers, monomers and polymerisation.

A polymer is a long chain molecule that is made from a combination of many repeating basic units. The basic unit of a polymer is known as a monomer. The monomer combination reaction to produce a polymer is called a polymerisation reaction. The term "plastic" is a general name referring to synthetic polymers.

Natural Polymers

Polymers that are naturally available and can be obtained from our surroundings.

  • Starch — monomer: glucose
  • Cellulose — monomer: glucose
  • Protein — monomer: amino acid
  • Natural rubber — monomer: isoprene

Synthetic Polymers

Man-made polymers produced through chemical reactions in laboratories or factories.

  • Polyethene — monomer: ethene
  • Polypropene — monomer: propene
  • Polychloroethene / PVC — monomer: chloroethene / vinyl chloride
  • Polystyrene — monomer: styrene
  • Nylon — synthetic polyamide
  • Terylene — synthetic polyester
Polymerisation type Key feature By-product Examples
Addition polymerisation Monomers with carbon-carbon double bonds, \(\text{C}=\text{C}\), link together without loss of any small molecule. None Polyethene, polypropene, PVC, polystyrene
Condensation polymerisation At least two different monomers, each with two functional groups, link together. Small molecule such as \(\text{H}_2\text{O}\) or \(\text{HCl}\) Terylene, nylon
Addition polymerisation of polyethene: \[ n(\text{CH}_2=\text{CH}_2) \rightarrow -[\text{CH}_2-\text{CH}_2]_n- \] where \(n\) is a large integer. Saturated hydrocarbons cannot undergo addition polymerisation because they lack carbon-carbon double bonds, \(\text{C}=\text{C}\).
Laboratory synthesis of nylon (Laboratory Activity 4A):
  1. Pour Solution B slowly into Solution A using a glass rod to avoid mixing.
  2. A nylon layer forms at the interface between the two immiscible solutions.
  3. Pull the nylon layer using a pair of tweezers and coil it around a glass rod.
  4. Wind slowly to pull out the continuous nylon thread until reagents are exhausted.
  5. Wash the thread with distilled water and dry.

Precautionary steps: Wear rubber gloves, conduct the activity inside a fume chamber, and stir and completely mix the remaining Solutions A and B before discarding.

Try to Answer First

Answer mentally, then check the explanation.

1

State the monomer for natural rubber and classify natural rubber as a natural or synthetic polymer.

Answer: The monomer is isoprene, and natural rubber is a natural polymer.
2

Explain why propene can undergo addition polymerisation while propane cannot.

Answer: Propene contains an unsaturated carbon-carbon double bond, \(\text{C}=\text{C}\), that can break open to link monomer units together, whereas propane is a saturated hydrocarbon with only single covalent bonds.
3

Differentiate between thermoplastic and thermosetting polymers in terms of their recyclability and response to heat.

Answer: Thermoplastic polymers melt upon heating, solidify when cooled, can be repeatedly remoulded, and are recyclable. In contrast, thermosetting polymers have cross-links, cannot be remoulded after heating, disintegrate or burn when heated, and cannot be recycled.

Common Mistakes

Avoid these errors when learning about polymers.

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Confusing the monomers of starch/cellulose (glucose) with the monomer of natural rubber (isoprene) or protein (amino acids).
!
Writing addition polymerisation equations without removing the double bond in the repeating unit, for example mistakenly keeping the \(\text{C}=\text{C}\) inside the polymer brackets.
!
Assuming all plastics can be melted and remoulded; thermosetting polymers decompose and burn instead of melting.
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Forgetting that condensation polymerisation requires two different functional groups and produces a small by-product, \(\text{H}_2\text{O}\) or \(\text{HCl}\), unlike addition polymerisation which yields the polymer as the sole product.
!
Stating that biodegradable plastics break down by light; light breakdown is specifically photodegradable, whereas biodegradable plastics decompose through microbial/bacterial action.

Concept Misunderstandings

Compare each misconception with the correct fact.

Misconception

Polymers only refer to artificial plastics made in factories.

Correct Fact

Many abundant natural materials are polymers, including starch, cellulose, cotton, protein, and natural rubber.

Misconception

Thermosetting polymers can be recycled by heating them at higher temperatures.

Correct Fact

Thermosets cannot be melted; applying high heat breaks chemical bonds and causes them to char, disintegrate, or burn, making them non-recyclable by remoulding.

Misconception

Condensation polymerisation can occur with any alkene.

Correct Fact

Condensation polymerisation requires monomers with reactive functional groups, such as \(-\text{NH}_2\), \(-\text{COCl}\), or \(-\text{COOH}\), not simply carbon-carbon double bonds.

Summary

Key facts to remember.

  • Polymer: Macromolecule composed of repeating monomer units linked via polymerisation.
  • Natural Polymers: Starch (glucose), cellulose (glucose), protein (amino acids), natural rubber (isoprene).
  • Synthetic Polymers: Polyethene, polypropene, PVC, polystyrene, nylon, terylene.
  • Types by Thermal/Elastic Properties: Thermoplastic melts on heating, is remouldable and recyclable; thermosetting disintegrates or burns on heating and is non-recyclable; elastomer stretches and snaps back with high elasticity.
  • Polymerisation Types: Addition polymerisation links monomers with \(\text{C}=\text{C}\) double bonds without by-products; condensation polymerisation links bifunctional monomers with release of small molecules, \(\text{H}_2\text{O}\) or \(\text{HCl}\).
  • Environment & Green Technology: Synthetic polymers are persistent in landfills and oceans; solutions include recycling, single-use plastic reduction, and developing biodegradable and photodegradable plastics.

Short Activity

Complete all parts before checking your score.

A. Objective Quiz

1

Which of the following pairs correctly matches the polymer to its corresponding monomer?

2

Which statement correctly explains why a thermosetting polymer cannot be recycled by melting?

B. Fill in the Blanks

3

Plastics that decompose naturally through the action of bacteria are known as ________ plastics.

4

During the condensation polymerisation between 1,6-hexanediamine and decanedioyl dichloride, nylon and ________ are produced.

C. Match Each Polymer to Its Characteristic or Use

Drag each polymer on desktop. On mobile, tap a polymer and then tap the matching box.

Polymer

Polyethene Polyvinyl chloride (PVC) Polystyrene Polypropene

Characteristic or Use

1Durable and strong polymer commonly used to manufacture plastic bags and plastic packaging
2Strong and hard polymer widely used to make water pipes and electrical insulators
3Lightweight heat insulator widely used for heat insulation and food packaging
4Durable polymer widely used to produce toys, textiles, and medical apparatus
Complete every answer before checking.
 

Keywords

Tap or hover over a term to read its meaning.

Polymer Monomer Polymerisation Thermoplastic Thermoset Elastomer Addition polymerisation Condensation polymerisation Biodegradable Photodegradable