Chapter 14: Macromolecules – Chemistry 2nd Year Notes

This chapter covers Macromolecules from the 2nd Year (FSc Part-II) Chemistry syllabus of the Punjab Curriculum and Textbook Board (PTB/PCTB). Macromolecules are very large molecules built by joining many small repeating units. The most important macromolecules are polymers, formed when hundreds or thousands of small molecules called monomers link together.

Here you will learn how polymers form, the two main types of polymerisation, the difference between thermoplastics and thermosetting plastics, and the uses and environmental effects of these materials.

Learning Objectives

• Define macromolecule, polymer and monomer.
• Distinguish addition and condensation polymerisation.
• Give examples of natural and synthetic polymers.
• Distinguish thermoplastic and thermosetting plastics.
• Describe important polymers such as polythene, PVC, nylon and Bakelite.
• Discuss the uses and environmental impact of plastics.

Key Concepts

Polymers and Monomers

A polymer is a large molecule made of many repeating units joined together; the small molecules that join up are called monomers, and the process is called polymerisation. For example, thousands of ethene molecules join to form polythene. Because polymers are so large, they are also called macromolecules.

Two Types of Polymerisation

In addition polymerisation, monomers that contain a carbon to carbon double bond simply add to one another with no other product formed; examples are polythene (from ethene), PVC (from vinyl chloride) and polystyrene. In condensation polymerisation, monomers join with the loss of a small molecule, usually water, at each link; examples are nylon and polyester (each from two monomers) and Bakelite.

Natural and Synthetic Polymers

Natural polymers are made by living things: starch and cellulose (polymers of glucose), proteins (polymers of amino acids), natural rubber and DNA. Synthetic polymers are made in industry from petroleum products: polythene, PVC, nylon, polyester, Bakelite and synthetic rubber. Synthetic polymers can be designed to have almost any property.

Thermoplastic and Thermosetting Plastics

Thermoplastics such as polythene and PVC soften on heating and can be melted and remoulded again and again, because their chains are only weakly held together. Thermosetting plastics such as Bakelite set permanently on first heating and cannot be softened again, because their chains become joined by strong cross-links into a rigid network.

Important Definitions

Formulas & Rules

Item	Fact
Polymer	many monomers; polymerisation is the joining process
Addition	unsaturated monomers, no by-product (polythene, PVC, polystyrene)
Condensation	loss of a small molecule (nylon, polyester, Bakelite)
Thermoplastic vs thermosetting	remouldable vs set permanently
Natural	starch, cellulose, protein, rubber, DNA

Diagrams & Illustrations

Monomer to polymer: a diagram showing many small monomer units joining to form a long polymer chain (a macromolecule).

Addition versus condensation: a comparison showing addition polymerisation (monomers with double bonds join, no by-product) and condensation polymerisation (monomers join with loss of water).

Solved Examples & Numericals

Example 1: Type of polymerisation

Is the formation of polythene addition or condensation polymerisation? It is addition polymerisation, because ethene molecules simply add to one another with no small molecule lost.

Example 2: Classifying a plastic

Why can polythene be recycled but Bakelite cannot? Polythene is a thermoplastic whose chains are weakly held, so it softens on heating and can be remoulded; Bakelite is thermosetting with cross-linked chains that do not soften.

Example 3: Natural polymer

Name the monomer of starch and of protein. Starch is a polymer of glucose; proteins are polymers of amino acids.

Short Questions & Answers

Long Questions & Answers

Q1: Distinguish between addition and condensation polymerisation with examples.

Polymers are formed by joining many monomers, and this can happen in two ways. In addition polymerisation the monomers contain a carbon to carbon double bond and simply add to one another when the double bond opens; no other molecule is produced, so the polymer has the same empirical formula as the monomer. Common examples are polythene (from ethene), PVC (from vinyl chloride) and polystyrene. In condensation polymerisation the monomers usually contain two reactive groups and join with the elimination of a small molecule, most often water, at every link; because a small molecule is lost, the polymer does not have the same formula as the monomers. Common examples are nylon and polyester, each made from two monomers, and Bakelite. Thus the key difference is whether a small molecule is given off.

Q2: Compare thermoplastic and thermosetting plastics.

Plastics fall into two classes that behave differently on heating. Thermoplastics are made of long chains held together only by weak forces; when heated they soften and can be remoulded, and this can be repeated many times, which makes them easy to recycle. Familiar thermoplastics are polythene, PVC and polystyrene. Thermosetting plastics form strong chemical cross-links between their chains during their first heating and moulding, so they set into a hard, rigid network; once set they cannot be softened or remoulded and on strong heating they char. Familiar thermosetting plastics are Bakelite, melamine and epoxy resins. Because of these differences, thermoplastics are used where reshaping and recycling are useful, while thermosetting plastics are used where permanent hardness and heat resistance are needed, such as in electrical fittings.

Q3: Discuss the importance and environmental impact of polymers.

Polymers are important because they are cheap, light, strong, easily shaped and resistant to water and corrosion. They are used in packaging, building, clothing, electrical insulation, medical equipment and countless everyday objects, often replacing heavier or costlier materials. However, most synthetic polymers are non-biodegradable, so discarded plastics remain in the environment for a very long time, litter the land and oceans and harm wildlife, and burning them can release toxic gases. The impact can be reduced by using plastics responsibly, recycling thermoplastics, and developing biodegradable and plant-based plastics. Balancing the great benefits of polymers against their environmental cost is one of the important challenges of chemistry today.

MCQs with Answers

Quick Revision Summary

• Polymer = many monomers joined; macromolecule = giant molecule.
• Addition polymerisation: unsaturated monomers, no by-product (polythene, PVC).
• Condensation polymerisation: loses a small molecule (nylon, polyester, Bakelite).
• Natural polymers: starch, cellulose, protein, rubber; synthetic: plastics and fibres.
• Thermoplastic = remouldable; thermosetting = set permanently.
• Plastics are useful but mostly non-biodegradable, causing pollution.

Exam Tips

• Define monomer and polymer precisely with one example each.
• Contrast addition versus condensation by whether a small molecule is lost.
• Learn the thermoplastic versus thermosetting comparison; a common question.
• Know the monomers of polythene (ethene) and PVC (vinyl chloride).
• Mention non-biodegradable when discussing environmental impact.