Addition polymerisation joins alkene monomers into long chains

1. A monomer is a small molecule that can join to many other monomer molecules.
2. A polymer is a very large molecule made from many joined monomers.
3. In addition polymerisation, many molecules of an alkene monomer join to make a polymer.
4. Alkenes can do this because they contain the carbon–carbon double bond, $C=C$.
5. Ethene forms poly(ethene) and propene forms poly(propene).
6. You name an addition polymer by putting the monomer's name in brackets after poly.

The double bond opens so monomers link with new single bonds

1. During addition polymerisation, each monomer's $C=C$ bond becomes a $C-C$ single bond.
2. Each carbon from the old double bond forms a new single bond to a carbon in a neighbouring monomer.
3. Repeating this joins many monomers into a long carbon chain.
4. No atoms are removed, so no other molecule is produced.
5. Many ethene molecules join to form poly(ethene):
   1. $n{CH}_2={CH}_2\to {\left[{-CH}_2{-CH}_2-\right]}_n$

Drawing an addition polymer: open the double bond and keep every atom

1. Identify the two carbon atoms joined by the $C=C$ bond in the monomer.
2. Replace the $C=C$ bond with a $C-C$ single bond.
3. Keep every atom or group on the same carbon as in the monomer.
4. Draw a bond from each backbone carbon out through the square brackets.
5. Write $n$ outside the brackets to show the unit repeats many times.
6. For propene the repeating unit is drawn like this:
   1. $n{CH}_2={CHCH}_3\to {\left[{-CH}_2-CH({CH}_3)-\right]}_n$

Telling a monomer and a polymer apart in diagrams

1. An alkene monomer diagram has the $C=C$ functional group and no polymer brackets.
2. An addition polymer diagram has a bracketed repeating unit with single bonds passing through the brackets.
3. The $n$ outside the brackets shows the repeating unit occurs many times.
4. To get the monomer back, remove the brackets, the outward bonds and the $n$ from one repeating unit.
5. Then change the single bond between the two backbone carbons back into a $C=C$ bond.
6. Keep every attached atom or group on the same carbon.

The repeating unit has exactly the same atoms as the monomer

1. The repeating unit is the smallest section of the polymer chain that repeats.
2. In an addition polymer, one repeating unit has the same types and numbers of atoms as one monomer.
3. This is because addition polymerisation makes no other molecule.
4. Only the bonding around the monomer's $C=C$ group changes.
5. Check a diagram by counting each type of atom in the monomer and in one repeating unit.
6. For ethene, ${CH}_2={CH}_2$ becomes ${\left[{-CH}_2{-CH}_2-\right]}_n$.