Addition Polymers

Three short definitions sit at the heart of this topic. Learn them word-for-word — they are easy marks that students throw away by being vague.

• Polymer — a very large molecule made of many small repeating units joined together.
• Monomer — the small molecule that joins, again and again, to build the polymer.
• Polymerisation — the reaction in which the monomers join together.

Think of a polymer like a paper chain: each paper link is a monomer; the finished chain is the polymer; sticking the links together is polymerisation.

A single polymer molecule can contain thousands of monomers, so we never draw it in full. Instead we use the letter n to mean "a very large number", and we draw just one repeat unit (covered later).

Polymers are everywhere — plastic bags, bottles, pipes, ropes, non-stick pans — and almost all of them are made from crude oil products.

Addition polymerisation is the joining of many alkene monomers — molecules that each contain a carbon–carbon double bond (C=C).

What happens, step by step:

1. Each monomer's C=C double bond opens up (one of the two bonds breaks).
2. This leaves each carbon with a spare bond, which joins to the carbon of the next monomer.
3. The monomers link end-to-end into one long chain.
4. No atoms are lost and no other product forms — the single polymer is the only product. The polymer's atoms are exactly the atoms of all the monomers added together.

This is the big idea examiners reward: the C=C opens, the monomers add together, and nothing else is made.

The simplest example is ethene turning into poly(ethene):

$n\,\text{CH}_2{=}\text{CH}_2 \rightarrow {-}(\text{CH}_2{-}\text{CH}_2)_n{-}$

• The n on the left means "many ethene molecules".
• The n outside the brackets on the right means "many repeat units".
• Notice the C=C of ethene (a double bond) has become a single C–C bond in the polymer.

Why only alkenes? Only molecules with a C=C double bond can add together this way. Alkanes (only single bonds) cannot undergo addition polymerisation — this is a common exam trap.

A polymer chain is far too long to draw, so we draw one repeat unit in brackets with a subscript n outside. The bonds extend out of each side of the brackets to show the chain carries on.

From a MONOMER to its repeat unit:

1. Draw the monomer with its C=C.
2. Open the double bond into a single C–C bond.
3. Put the unit in square brackets with n outside.
4. Draw a bond out of each side of the brackets (this shows it joins to its neighbours).
5. Keep all the H atoms and side groups (e.g. the Cl in PVC, the CH₃ in poly(propene)).

From a REPEAT UNIT back to its monomer:

1. Find the two carbons of the main chain inside the brackets.
2. Close the single C–C between them back into a C=C.
3. That gives you the monomer.

Worked example. Repeat unit ─(CH₂–CHCl)ₙ─ → close the C–C to a C=C → monomer is CH₂=CHCl (chloroethene). ✓

Quick reference table:

Monomer	Repeat unit	Polymer name
CH₂=CH₂ (ethene)	─(CH₂–CH₂)ₙ─	poly(ethene)
CH₂=CHCH₃ (propene)	─(CH₂–CH(CH₃))ₙ─	poly(propene)
CH₂=CHCl (chloroethene)	─(CH₂–CHCl)ₙ─	poly(chloroethene) = PVC
CF₂=CF₂ (tetrafluoroethene)	─(CF₂–CF₂)ₙ─	PTFE

The specification names four addition polymers and expects you to recall their uses. Match the property to the use:

Polymer	Made from	Key uses
Poly(ethene) (polythene)	ethene	plastic bags, bottles, films, cling film
Poly(propene) (polypropylene)	propene	containers, ropes, crates, fibres
PVC (poly(chloroethene))	chloroethene	pipes, window frames, cable insulation, flooring
PTFE (poly(tetrafluoroethene))	tetrafluoroethene	non-stick coatings on pans, plumbing tape

A couple of helpful links:

• PVC is rigid and weatherproof → ideal for pipes and window frames (often called uPVC).
• PTFE is extremely slippery and heat-resistant → nothing sticks to it, so it coats non-stick frying pans.
• Poly(propene) is tough and can be drawn into strong threads → ropes and containers.
• Poly(ethene) is cheap, light and flexible → the everyday plastic bag and bottle.

In the exam, you only need one correct use per polymer for the mark — but make sure it matches the right polymer.