IB Chemistry SLTopic 2 — From Models to MaterialsPaper 1 & 2Organic~12 min read
Addition Polymerisation
This is the friendliest reaction in organic chemistry. One double bond opens, two new single bonds form, and the process repeats a few thousand times. Nothing is lost, nothing is added, and there is only one product. If you can draw an alkene, you can draw its polymer.
📘 What you need to know
Addition polymerisation needs monomers containing a C=C double bond — alkenes and their derivatives.
The weaker pi bond breaks, and each carbon uses that pair of electrons to form a new C–C single bond to the next monomer.
The only product is the polymer. Nothing is given off, so the atom economy is 100%.
The repeat unit is the monomer with the C=C changed to C–C, drawn with bonds passing out through square brackets and a subscript n.
To find the monomer from a polymer, spot the repeating two-carbon section of the backbone and put the double bond back.
Common examples: ethene → poly(ethene); propene → poly(propene); chloroethene → poly(chloroethene), or PVC.
Addition polymers have no reactive links in the chain, so they cannot be hydrolysed and they persist in the environment.
What happens to the double bond
A C=C double bond is made of two different bonds: a strong sigma bond holding the carbons together, and a weaker pi bond sitting above and below it. The pi bond is the loose one, and it is the one that breaks.
When it goes, each carbon is left with an electron available for bonding. Reach out to the carbon of a neighbouring monomer that has done the same thing, and you have a new C–C single bond. Do this along a chain of thousands of monomers and every one of them ends up joined to the two either side of it. The carbon backbone that results is saturated — there are no double bonds left anywhere.
Count the atoms on each side: two carbons and four hydrogens, before and after. Addition polymerisation never loses anything, which is why the two structures look almost identical.
Drawing the repeat unit
This is worth turning into a mechanical routine, because the marks are given for details students rush past.
🧩 From monomer to repeat unit
Draw the two carbons of the old double bond side by side, joined by a single line.
Copy every group across unchanged. Whatever was attached to each carbon stays attached to the same carbon.
Extend a bond outwards from each carbon, to the left and to the right.
Draw square brackets so those two bonds cross them. The bonds must stick out beyond the brackets.
Write n as a subscript at the bottom right of the closing bracket.
Nine times out of ten, a lost mark here is one of two things: the bonds stop at the brackets instead of passing through them, or the n is missing. Both take one second to check.
Going backwards: polymer to monomer
Exams often give you a section of chain and ask what it was made from. The method is the reverse of the one above, and it hangs on one fact: every addition polymer has exactly two carbons in each repeat unit of its backbone, because that is how many were in the original double bond.
A useful check: the methyl groups appear every second carbon. If your chosen repeat unit does not reproduce that pattern when you copy it along, you have cut the chain in the wrong place.
The family of addition polymers
Monomer
Polymer
What changes from ethene
Typical use
Ethene, CH2=CH2
Poly(ethene)
Nothing — this is the simplest case
Bags, bottles, film
Propene, CH2=CHCH3
Poly(propene)
One H replaced by a methyl group
Crates, rope, food containers
Chloroethene, CH2=CHCl
Poly(chloroethene), PVC
One H replaced by chlorine
Pipes, window frames, cable insulation
Tetrafluoroethene, CF2=CF2
Poly(tetrafluoroethene), PTFE
All four H replaced by fluorine
Non-stick coatings, seals
Atom economy: every atom in the monomer ends up in the polymer, so addition polymerisation is 100% atom efficient. That is unusual, and it is a genuine green chemistry point worth making in an extended answer.
Worked examples
WORKED EXAMPLE
Describe the repeat unit formed when chloroethene, CH2=CHCl, polymerises.
Step 1: the two carbons, now single bonded—CH₂—CHCl—Step 2: check the groups have not moved
Two H on the first carbon; one H and one Cl on the second. Same as the monomer.
Step 3: brackets and n
Bonds extend out through both brackets, subscript n outside the closing bracket.
[ —CH₂—CHCl— ]n, called poly(chloroethene) or PVCthe chlorine never moves to the other carbon — a very common slip
WORKED EXAMPLE
A polymer chain is —CF2—CF2—CF2—CF2— repeating. Identify the monomer.
Step 1: cut the backbone into pairs of carbons
Each pair is —CF₂—CF₂—Step 2: put the double bond back between themCF₂=CF₂Step 3: name it
Two carbons with four fluorines: tetrafluoroethene.
Monomer: tetrafluoroethene, CF2=CF2 — the polymer is PTFEevery carbon here is identical, so it does not matter where you make the cut
WORKED EXAMPLE
2.00 mol of propene is polymerised completely. What mass of poly(propene) is produced?
Use Ar(C) = 12.01 and Ar(H) = 1.01.
Step 1: spot the shortcut
Nothing is lost, so the mass of polymer equals the mass of monomer used.
Step 2: mass of propene, C3H6M = (3 × 12.01) + (6 × 1.01) = 42.09 g mol−1m = 2.00 × 42.09 = 84.18 g84.2 g of poly(propene) (3 s.f.)no by-product means no mass lost — the calculation is easier than it looks
💡 Exam tip
Bonds through the brackets, every time. This is the single most frequently dropped mark on the whole topic.
Write n in front of the monomer as well as after the brackets if the question asks for an equation.
If the question says “displayed formula”, you must show every atom and every bond, including all the C–H bonds.
Check your repeat unit has exactly two carbons in the backbone. Three is a sign you have miscounted.
Name addition polymers as poly(monomer name) with brackets: poly(chloroethene) rather than PVC, unless the question uses the trade name first.
If asked why the polymer is unreactive, say the backbone is saturated with strong non-polar C–C and C–H bonds and no double bonds remain.
⚠ Common mix-up
Leaving the double bond in the repeat unit. If a C=C survives, no polymerisation has happened.
Moving side groups to the wrong carbon. Whatever was on a carbon stays on that carbon.
Bonds that stop at the brackets. They must cross them to show the chain continues.
Saying a small molecule is released. That is condensation polymerisation. Addition gives one product only.
Choosing a repeat unit with only one carbon. The double bond had two carbons, so the repeat unit must too.
Confusing addition polymerisation with addition reactions of alkenes. Same bond breaking, but here the alkene reacts with more of itself rather than with bromine or hydrogen.
Up next: Condensation Polymers (HL) — a different way of joining monomers, where each new link throws out a small molecule, and the link that forms can later be broken by water.
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