IB Chemistry SLTopic 2 — From Bonding Models to MaterialsPaper 1 & 2Core skill~11 min read
Addition Polymerisation
This is the reaction behind the entire plastics industry, and it is remarkably simple: take an alkene, open up the C=C double bond, and let the monomers link into a chain. Nothing else is produced — the polymer is the only product.
📚 What you need to know
Addition polymerisation joins monomers containing at least one C=C double bond into a long chain.
The π bond in each C=C breaks, and new C–C single bonds link the monomers together.
The polymer is the only product — there are no by-products.
The repeat unit is the smallest group that repeats along the chain, drawn in square brackets with n outside.
For poly(alkenes), the repeat unit is the same as the monomer except C=C has become C–C.
To find a monomer from a polymer: take two carbons of the chain and put the double bond back.
How the reaction works
An alkene has a C=C double bond, and one of the two bonds in it — the π bond — is relatively easy to break. When it does, each carbon is left with a spare bonding position, and it uses that to bond to the next monomer along.
Repeat this thousands of times and you get one long chain. Because every atom from every monomer ends up in the product, nothing is left over. That is what makes this an addition reaction — exactly like the other alkene addition reactions, just repeated endlessly.
Same atoms on both sides. The only change is that the C=C double bond has become a C–C single bond, freeing each carbon to join the next unit.
Count the atoms and you’ll see nothing is lost. Ethene is C2H4 and the repeat unit is also C2H4. All that has changed is one bond — the double became a single, freeing each carbon to reach out to its neighbour.
Writing it down
There are two ways to represent addition polymerisation, and you should be comfortable with both.
Using general formulae
Quick and compact, useful when the question just wants the equation:
Poly(ethene)
n C2H4 → –[ C2H4 ]–n
Poly(chloroethene), PVC
n H2C=CHCl → –[ H2C–CHCl ]–n
The n in front of the monomer means “a very large number of these”, and the n outside the brackets means the unit inside repeats that many times — up to about 10 000 in a real polymer chain.
Using displayed formulae
This is what most exam questions want, because it shows what happens to the bonds:
💡 Three things to get right when drawing
Draw the square brackets around the repeat unit, with the n at the bottom right, outside them.
Extend a bond through each bracket on both sides — that shows the chain continues.
Change the C=C to a C–C. Leaving the double bond in is the single most common mistake on this topic.
Chloroethene works exactly the same way. The only difference is that one hydrogen has been replaced by a chlorine, and that chlorine simply comes along for the ride:
The chlorine sits exactly where it was in the monomer. Only the bond between the two carbons has changed.
What is a repeat unit?
A repeat unit is the smallest group of atoms that repeats to build the chain. For poly(alkenes) it always contains exactly two carbon atoms in the main chain — the two that came from the original C=C.
That fact is the key to the hardest question type on this topic: being given a section of polymer and asked to work out the monomer.
🧩 Finding the monomer from a polymer
Look along the main chain and find two adjacent carbon atoms where the pattern starts repeating.
Draw just that two-carbon section, keeping every side group exactly where it is.
Change the C–C single bond into a C=C double bond.
Remove the brackets and the n. What is left is the monomer.
Working backwards is just the forward process in reverse: isolate two carbons, restore the double bond, drop the brackets.
WORKED EXAMPLE
A section of polymer chain is –CH(OH)–CH2–CH(OH)–CH2–. Deduce the repeat unit and the monomer.
Find where the pattern repeatsCH(OH) then CH₂, then it starts again — so the repeat is 2 carbons.Repeat unit–[ CH(OH)–CH₂ ]–ₙPut the double bond back between those 2 carbonsmonomer = CH(OH)=CH₂ (ethenol)The polymer is poly(ethenol). The OH stays exactly where it was.
WORKED EXAMPLE
A polymer has the repeating section –CH2–CH(CO2H)–CH2–CH(CO2H)–. Identify the monomer.
The repeat unit has 2 carbons in the main chain–[ CH₂–CH(CO₂H) ]–ₙRestore the C=C between themmonomer = CH₂=CH(CO₂H)That is prop-2-enoic acid. Note the CO₂H group is a side group, NOT part of the main chain.
WORKED EXAMPLE
A polymer chain has an –OH group on every carbon of the backbone. Deduce the monomer.
Take 2 adjacent carbons — each carries one OH and one Hrepeat unit = –[ CH(OH)–CH(OH) ]–ₙPut the double bond backmonomer = CH(OH)=CH(OH), ethene-1,2-diolTwo OH groups on adjacent carbons is the giveaway for a diol monomer.
⚠️ Common mix-up
The structure in square brackets is the repeat unit, not the monomer. They look almost identical, which is exactly why examiners ask.
Don’t leave the C=C in the polymer. The whole point of the reaction is that it became a single bond.
Don’t forget the bonds through the brackets. Without them you have drawn a molecule, not a section of chain.
Repeat units always have 2 carbons in the main chain for poly(alkenes). If you’ve drawn four, you have taken two repeat units.
Addition polymerisation makes no by-product. If your equation has water or HCl on the right, something has gone wrong.
💡 Exam tip
Practise going both ways: monomer → polymer and polymer → monomer. Questions come in both directions.
Naming is mechanical: the polymer of X is poly(X). Ethene → poly(ethene), chloroethene → poly(chloroethene).
Keep every side group in the same position when converting between monomer and repeat unit.
Check your drawing by counting bonds: every carbon must have exactly four.
That completes From Bonding Models to Materials, and with it the whole of Topic 2. You’ve gone from single ions all the way to the materials they build — alloys, plastics and everything in between.
Want this explained one-to-one?
Book a free session with an experienced IB Chemistry tutor and get your trickiest topics made simple.
