These revision notes cover every content standard of the Form 5 Polymers chapter: natural and synthetic polymers, the two types of polymerisation, natural rubber and vulcanisation, and a balanced view of the uses and environmental issues of synthetic polymers, set out clearly for reliable revision.
These notes work through the Polymers chapter one content standard at a time, so you can revise each part and check you have the definitions the marking scheme expects. Keep them beside past-paper questions and use them to confirm your answers. The four skills the exam keeps testing are: identify a monomer from a polymer (and the reverse), tell the two types of polymerisation apart, explain vulcanisation as a change in structure that changes properties, and argue both sides of the environmental question.
12.1 Natural and synthetic polymers
A polymer is a very large molecule (a macromolecule) built from many small repeating units joined by covalent bonds. Each small unit is a monomer, and the reaction that joins them is polymerisation. Because the same unit repeats hundreds or thousands of times, we draw a polymer as a repeating unit inside brackets with a subscript n.
Natural polymers occur in living things. Examples you must know are natural rubber (from the latex of the rubber tree, whose monomer is isoprene), proteins (monomers are amino acids), carbohydrates such as starch and cellulose (monomer is glucose), and natural silk and wool. Synthetic polymers are made in industry from petroleum products. Common examples are polythene (from ethene), polypropene (from propene), poly(vinyl chloride) or PVC (from chloroethene), polystyrene (from phenylethene), Perspex, nylon, terylene and Teflon.
Key definitions. A monomer is the small molecule that repeats; a polymer is the long chain it forms; polymerisation is the process. Always name the monomer as a single small molecule, never as a fragment of the chain.
12.2 Types of polymerisation
There are two types, and telling them apart is a very common question.
- Addition polymerisation. The monomers are unsaturated, each has a carbon–carbon double bond (C=C). The double bonds open and the monomers add directly to one another, so no other molecule is released. Ethene → polythene, propene → polypropene, chloroethene → PVC and tetrafluoroethene → Teflon are all addition polymers.
- Condensation polymerisation. Each monomer has two functional groups (for example –COOH and –OH, or –COOH and –NH₂). As the monomers join, a small molecule is eliminated each time a bond forms, usually water. Nylon and terylene (a polyester) are made this way, and so are natural polymers such as protein from amino acids.
Exam tip. The quick test: if the monomer has a C=C double bond and nothing is given off, it is addition; if two functional groups react and a small molecule (usually water) is removed, it is condensation.
12.3 Natural rubber and vulcanisation
Natural rubber is obtained from latex, a colloid in which rubber particles are dispersed in water. Each particle is wrapped in a membrane that carries a negative charge, so the particles repel one another and do not clump. Adding an acid (such as methanoic or ethanoic acid) neutralises the charge, the particles collide and join, and the latex coagulates. Adding a little alkali (ammonia solution) keeps the charge and stops premature coagulation, which is how latex is preserved for transport.
An unvulcanised rubber molecule is a long, coiled chain of poly(isoprene) with double bonds along it. The chains are held to one another only by weak forces, so they slip past each other easily: the rubber is soft, not very strong, and loses its springiness quickly.
Vulcanisation is heating rubber with sulfur. Sulfur atoms form cross-links (sulfur bridges) between the chains at the double bonds. These cross-links tie the chains together so they can no longer slide far apart. The result is rubber that is harder, stronger, more elastic, and more resistant to heat and to chemical attack (oxidation), with a higher elasticity limit. This is the chapter’s clearest example of structure deciding property.
12.4 Uses and environmental issues of polymers
Synthetic polymers are useful because they are light, cheap, strong, waterproof, resistant to corrosion, easily moulded into shape, and good insulators of heat and electricity. That is why they replace metal, glass and wood in packaging, pipes, containers, clothing and electrical fittings.
The environmental problem is that most synthetic polymers are non-biodegradable: microorganisms cannot break them down, so waste accumulates on land and in the sea for a very long time. Open burning makes matters worse, burning PVC releases hydrogen chloride, an acidic and corrosive gas, and incomplete burning of any plastic releases poisonous carbon monoxide and soot.
The responses are to reduce, reuse and recycle, to develop biodegradable and photodegradable plastics, and to dispose of plastics properly rather than by open burning. A good exam answer balances the real usefulness of polymers against these real costs.
How to use these notes
Revise one content standard at a time and test yourself by explaining it aloud without looking. For 12.1, practise naming the monomer of each named polymer. For 12.2, sort a list of monomers into addition and condensation. For 12.3, draw the cross-links and list four property changes. For 12.4, prepare one advantage, one problem and one solution. Then attempt the practice questions for this chapter and mark your answers against the definitions above. A one-to-one teacher can check that your monomer identification and your vulcanisation answer are precise, the two places students most often lose easy marks. Because these ideas run through the whole of SPM Chemistry, securing them now pays back across the paper.
Worked detail: identifying the monomer
Suppose a polymer is drawn with the repeating unit –CH₂–CHCl– inside brackets, subscript n. To find the monomer, take one repeating unit and put the double bond back between the two carbons: the monomer is CH₂=CHCl, chloroethene. The same method works in reverse: given the monomer CH₂=CH₂ (ethene), the repeating unit of the polymer is –CH₂–CH₂–. The single most common error is to write the monomer with a single bond, or to leave a stray bond hanging; the monomer must be a complete, neutral molecule with the double bond restored.
Vulcanisation: structure decides property
Learn vulcanisation as a cause-and-effect chain, not a list. Cause: sulfur forms cross-links between chains. Consequence for movement: the chains can no longer slide freely and are pulled back to shape. Consequence for properties: harder, stronger, more elastic, higher elasticity limit, better resistance to heat and oxidation. If a question asks why vulcanised rubber is more elastic, the mark is for the cross-links restricting the chains, the structural reason, not just for saying it is “better”.
A balanced environmental answer
When a question asks you to discuss the environmental impact of polymers, give both sides. State two or three genuine advantages (light, durable, cheap, versatile), then two or three genuine problems (non-biodegradable, pollute land and sea, release toxic gases such as hydrogen chloride when burned), and finish with a workable solution (reduce, reuse, recycle; use biodegradable polymers; avoid open burning). An answer that is all praise, or all complaint, is incomplete, and the balance itself is what the marking scheme rewards.
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