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How addition polymers form from alkenes, how condensation polymers form, and the natural polymers in proteins, starch and DNA.
Plastic bags, nylon clothes, the protein in your muscles and the DNA in your cells have something in common. They are all polymers. A polymer is a very long molecule made by joining many small molecules together.
This matters because polymers are everywhere. Some are made in factories and some are made by living things. Knowing how they form helps us explain their properties and why plastic waste is a problem.
By the end you will be able to describe addition polymerisation and draw a repeating unit. You will describe how plastics are disposed of. If you are doing the Higher tier, you will also describe condensation polymers and natural polymers.
A polymer is a long chain of small molecules joined together. The small molecules are called monomers. Think of paper clips linked into a long chain.
In addition polymerisation the monomers are alkenes. An alkene has a carbon-carbon double bond. The double bond opens up, and each carbon joins to the next monomer. No other substance is made, so every atom ends up in the polymer.
The name is poly(name of monomer). Ethene makes poly(ethene). Chloroethene makes poly(chloroethene), also called PVC. Tetrafluoroethene makes PTFE.
The repeating unit is the part of the chain that repeats, like one link in a bracelet. To draw it, redraw the carbon-carbon double bond as a single bond. Then add a bond at each end that goes out through the brackets.
For ethene, the repeating unit is –CH₂–CH₂–. For propene, it is –CH₂–CH(CH₃)–. The repeating unit has the same atoms as the monomer.
This lets you work out the mass of a chain. The relative formula mass of ethene is 28. A polymer of 1000 ethene molecules has a relative formula mass of 1000 × 28 = 28 000.
The atoms in a polymer chain are joined by strong covalent bonds. The forces between separate chains are weaker. When poly(ethene) is heated, these forces are overcome and it softens. It sets again on cooling. This is called thermosoftening.
Poly(ethene) is used for bags and bottles. PVC is used for pipes and window frames. PTFE is non-stick, so it coats frying pans.
Most addition polymers are not biodegradable, so microorganisms cannot break them down. They last for centuries in landfill. Burning can release toxic gases, such as hydrogen chloride from PVC. Recycling saves crude oil, but it needs sorting and uses energy.
Higher tier. In condensation polymerisation, each monomer has a functional group at both ends. A functional group is the part of a molecule that decides how it reacts. When two monomers join, a small molecule is lost, often water.
A polyester is made from two monomers. One is a dicarboxylic acid, with –COOH at each end. The other is a diol, with –OH at each end. Each join is an ester link.
A polyamide, such as nylon, is made from a dicarboxylic acid and a diamine, with –NH₂ at each end. Each join is an amide link. Water is lost every time a link forms.
Amino acids are natural monomers. Each has two functional groups: an amine group, –NH₂, and a carboxylic acid group, –COOH. Different amino acids have different side groups. The simplest is glycine.
Amino acids join by condensation polymerisation, and water is lost at each link. The polymer is called a polypeptide, which is a protein.
A protein has many amino acids in a specific order. Think of letters in a word. The same letters in a different order make a different word. The order decides how the protein folds and what it does.
DNA is a polymer made from four different monomers called nucleotides. Each nucleotide has one of four bases: A, C, G or T. DNA has two strands twisted into a double helix.
The order of the bases carries the genetic code. Think of a recipe written with only four letters.
Starch and cellulose are polymers of glucose. Starch stores energy in plants. Cellulose forms strong cell walls. Natural polymers like these are made in living things by condensation reactions.
Addition polymers form when alkene monomers open their double bonds and join. No other product is made.
The repeating unit has the same atoms as the monomer. Most addition polymers are not biodegradable.
Condensation polymers lose a small molecule, often water. Polyesters and polyamides are examples.
Proteins, DNA, starch and cellulose are natural polymers.
Name the polymer made from ethene.
Answer: poly(ethene)
Describe what happens to the monomer molecules when propene forms poly(propene).
Answer: The double bond in each propene opens and the molecules join in a long chain with no other product.
Explain how condensation polymerisation differs from addition polymerisation.
Answer: Condensation monomers have two functional groups and lose a small molecule such as water. Addition polymers are made from alkenes and make no other product.
Polymers practice questions for GCSE Combined Science Chemistry. Try each one, then open the answer. Easy questions come first.
What is the name for the small molecules that join to make a polymer?
Answer: monomers
Name the polymer made from chloroethene.
Answer: poly(chloroethene)
Which type of hydrocarbon is the monomer for an addition polymer?
Answer: an alkene
Name one reason why plastic waste is a problem.
Answer: it is not biodegradable
Name the polymer made from tetrafluoroethene.
Answer: poly(tetrafluoroethene)
Is poly(ethene) biodegradable, yes or no?
Answer: no
A polymer chain contains 500 ethene units. How many carbon atoms are in the chain?
Answer: 1000
The relative formula mass of ethene is 28. Calculate the relative formula mass of a poly(ethene) molecule made from 1000 ethene molecules.
Answer: 28000
Longer questions in the style of a written paper, with the marking points. These are Yumio practice questions.
Propene, C₃H₆, can form an addition polymer. (a) Name the polymer. [1 mark] (b) Explain what happens to the propene molecules when the polymer forms. [2 marks] (c) State one problem with disposing of poly(propene). [1 mark]
Answer: (a) poly(propene) (b) The double bond in each molecule opens and the molecules join together in a long chain, with no other product. (c) It is not biodegradable so it stays in landfill.
Compare addition polymerisation with condensation polymerisation, using the formation of poly(ethene) and a polyester as examples. [6 marks]
Answer: In addition polymerisation, alkene monomers such as ethene, each with a C=C double bond, join together. The double bond opens and no other product forms, so the polymer has the same atoms as the monomers, as in poly(ethene). In condensation polymerisation, the monomers have two functional groups, such as a dicarboxylic acid with two –COOH groups and a diol with two –OH groups. When they join, an ester link forms and a small molecule, water, is lost each time. The polyester that forms is different in composition from the monomers because of the water that is lost.
Saying addition polymerisation makes water as a by-product
Addition polymerisation only makes the polymer. Water is lost in condensation polymerisation.
Saying the monomer in addition polymerisation is an alkane
The monomer is an alkene, because it needs the C=C bond.
Leaving the open bonds off the repeating unit
Show a bond extending out of the brackets at both ends.
Saying polymers are all biodegradable
Most plastics are not. They last for a long time in landfill.
Polymers is on both Foundation and Higher papers for GCSE Combined Science Chemistry, aimed at around grade 7.
Saying addition polymerisation makes water as a by-product. Addition polymerisation only makes the polymer. Water is lost in condensation polymerisation.
Open Show answer under each question on this page. The questions are Yumio practice questions, not past exam questions.
See how the papers are set: AQA Combined Science papers and Edexcel Combined Science papers.
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