AQA GCSE Chemistry Foundation (8462), Foundation tier · Organic chemistry › Synthetic and naturally occurring polymers
Practise Addition polymerisation. 8 exam-style questions plus unlimited generated ones on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
How alkenes join together to make addition polymers such as poly(ethene) and poly(propene), how to draw the repeating unit from a monomer, and how to work back from a polymer to its monomer. GCSE Chemistry only. Expect drawing questions worth 1 to 3 marks and short explanations.
Key facts
Monomer = small molecule; polymer = very large molecule made from many monomers
Addition polymer monomers contain a C=C
Addition polymerisation: C=C opens to C–C; no other product
Repeating unit has the same atoms as the monomer
n CH2=CH2 → –(CH2–CH2)n–
Ethene → poly(ethene); propene → poly(propene)
Repeating unit: 2 chain carbons, single bond, side groups, bonds through brackets, n
Mr of polymer = n × Mr of monomer
Notes
Monomers and polymers
diagram
Polymers are very large molecules made when many small molecules, called monomers, join together.
In addition polymerisation, the monomers are alkenes (or other molecules with a C=C double bond).
The C=C in each monomer opens up to a single bond, and the monomers join into a long chain.
n ethene → poly(ethene): each C=C opens up and links to the next monomer.
No other product forms, so the repeating unit has exactly the same atoms as the monomer.
Examples: ethene makes poly(ethene); propene makes poly(propene).
Drawing the repeating unit
diagram
1. Draw the two carbon atoms of the C=C, now joined by a single bond.
2. Keep every other atom or group on those carbons exactly as in the monomer.
3. Draw a bond out from each of the two carbons, through a bracket on each side, and write n after the brackets.
Equation: n CH2=CH2 → –(CH2–CH2)n–
Repeating unit of poly(ethene):
In poly(propene), the CH3 group is a branch. Only the two carbons of the C=C are in the main chain:
Working backwards
To find the monomer, take the two chain carbons of the repeating unit, join them with a double bond, and remove the brackets, the n and the bonds that stick out.
In a section of chain, the pattern repeats every two carbon atoms: that two-carbon section is the repeating unit.
n is large (often thousands), and the relative formula mass of the polymer = n × the relative formula mass of the monomer.
Don’t lose marks
Leaving the C=C in the repeating unit. It must be a single bond.
Forgetting the bonds through the brackets, or the n.
Putting the CH3 of propene into the main chain, making a three-carbon backbone.
Saying water or another small molecule forms in addition polymerisation. Nothing else forms.
Drawing the extending bonds from a side group instead of from the two chain carbons.
More tips
Memory tricks
Open the door and hold hands: the C=C opens, and each carbon reaches out to link with the next monomer.
Repeating unit = the monomer with its double bond made single and two 'arms' through the brackets.
Side groups such as CH3 or Cl hang off the chain; they never go into the main chain.
Naming: 'poly' + the monomer's name in brackets.
Exam technique
Draw the brackets tall enough that the extending bonds clearly pass through them.
Check that your repeating unit has the same number of each type of atom as the monomer.
Use the words 'monomer', 'polymer', 'repeating unit' and 'double bond' in explanations.
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
Grade 3
Define monomer and polymerMonomers are small molecules that join together in large numbers to make a very large molecule, a polymer.
Grade 4
Name the polymer made from an alkeneWrite 'poly' and put the monomer's name in brackets: propene gives poly(propene).
Grade 5
Recognise monomers that form addition polymersThe monomer must contain a C=C double bond.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
What is a polymer?
A very large molecule made from many small molecules (monomers) joined together.
What name is given to the small molecules that join together to make a polymer?
Monomers
What is the name of the addition polymer made from chloroethene?
Poly(chloroethene)
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy4 marks
(a) What is a polymer?[1]
(b) Name the polymer made from propene.[1]
(c) Which functional group must a monomer have to form an addition polymer? Tick (✓) one box.[1]
C=C
–OH
–COOH
–NH2
(d) In addition polymerisation, what else is formed as well as the polymer? Tick (✓) one box.[1]
Nothing else
Water
Hydrogen
Carbon dioxide
Show the answer and mark scheme
(a)Answer: A very large molecule made from many small molecules (monomers) joined together.
a very large molecule made from many small molecules (monomers) joined together
(b)Answer: Poly(propene)
poly(propene)
(c)Answer: C=C
(d)Answer: Nothing else
Question 2Medium5 marks
Poly(ethene) is made from ethene. n CH2=CH2 → –(CH2–CH2)n–
(a) Explain why the repeating unit of poly(ethene) has the same atoms as the monomer.[1]
(b) A sample of poly(propene) has a relative formula mass (Mr) of 126 000. Calculate the number of propene molecules, C3H6, that joined to make one molecule of this polymer. Relative atomic masses (Ar): H = 1 C = 12[2]
(c) Ethene decolourises bromine water but poly(ethene) does not. Explain why.[2]
Show the answer and mark scheme
(a)Answer: No other product is formed in addition polymerisation.
no other molecule / product is formed in addition polymerisation
(b)Answer: 3000
Mr of C3H6 = 42
126 000 ÷ 42 = 3000
(c)Answer: Ethene has a C=C double bond; in poly(ethene) the double bonds have become single bonds.
ethene has a C=C double bond (is unsaturated)
in poly(ethene) the double bonds have become single bonds (it is saturated), so bromine cannot add