Edexcel IGCSE Chemistry · Spec 4.44-4.47
Addition Polymers & Their Disposal
How addition polymers form from alkenes, and the problems of disposing of them.
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Addition Polymers & Their Disposal
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Explained
Building an addition polymer, and the problem of getting rid of it
A polymer is an enormous molecule made by joining thousands of small molecules called monomers. In addition polymerisation the monomers are alkenes, and nothing else is produced.
The carbon to carbon double bond opens, leaving each carbon with a spare bond, and those spare bonds link to the neighbouring units. Every atom in the monomer ends up in the polymer, which is what the word addition means here.
Drawing the repeat unit
Take the monomer, change the double bond to a single bond, and add a bond extending from each end. Everything attached to the carbons stays exactly where it was.
Ethene gives poly(ethene). Chloroethene gives poly(chloroethene), better known as PVC, with a chlorine on one of the two carbons. Propene gives poly(propene), with a CH3 group.
Going backwards is the same steps reversed. Remove the extending bonds and put the double bond back between the two backbone carbons.
Why disposal is difficult
The backbone is a chain of carbon to carbon single bonds, which are strong and unreactive. There is nothing for water, enzymes or microorganisms to attack, so the polymer does not break down. That is exactly the property that makes plastics useful and exactly the property that makes them a problem afterwards.
Landfill takes up space, and because the plastic is inert it remains there for a very long time.
Burning releases energy that can be used, but it produces carbon dioxide, and polymers containing chlorine produce hydrogen chloride, which is toxic and forms acid rain. Incomplete combustion also produces carbon monoxide.
Recycling avoids both, but the plastics have to be sorted by type first, which is difficult and expensive, and each cycle degrades the material a little.
What the mark scheme accepts and rejects
An Edexcel International GCSE Chemistry mark scheme for a four mark disposal question builds it as two dependent pairs. The first mark is for naming landfill, and the second, which depends on it, is for saying that it takes up space, takes a very long time to decompose, or that the polymers are inert. The third is for naming burning, and the fourth, which depends on that, is for hydrogen chloride or toxic gases being produced.
Then comes the instruction: ignore non biodegradable. The term states the problem rather than explaining it, and the explanation is what carries the mark. Say that it is not broken down by microorganisms, or that it takes a very long time to decompose.
The dependency is worth noticing too. Naming a method with no consequence earns one mark of the pair; giving a consequence without naming the method earns neither.
On drawing, two mark schemes from the same series differ in a way that is not a contradiction. Where the question asked only for the repeat unit, the notes allow it with or without extension bonds and ignore brackets and the letter n. Where the question asked for the full equation, the first mark requires the correct repeat unit with extension bonds and the second requires the equation to include the monomer, the brackets and the n.
So read what is being asked for. A repeat unit on its own is marked leniently; a polymerisation equation is marked in full.
Where the monomers come from
Alkenes come from cracking, which breaks long chain molecules from crude oil into shorter ones and always produces at least one alkene.
So the plastics industry depends on crude oil, which is finite, and that is a further argument for recycling alongside the disposal problem.
Spec 4.44-4.47
What you need to know
- Explain addition polymerisation
- Draw a repeat unit from a monomer
- Discuss the problems of disposing of plastics
Active recall
Quick check
Answer each question before opening the answer.
How does addition polymerisation work?
Many small monomers containing C=C double bonds join together to form one long polymer, with no other product.
Why is disposing of addition polymers a problem?
They are non-biodegradable (do not rot) and release toxic gases if burned.
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