Topic 5 · page 13 of 15

Addition polymerisation

Many alkene monomers join to make one long saturated chain. The C=C of each monomer becomes C–C, nothing else is formed, and the repeat unit is the part of the chain that repeats.

12 min read5.7

Read first: Adding hydrogen and halogens

Monomers and polymers 5.7

Alkenes can add to each other. A molecule with a C=C can use its π bond to join to another molecule's C=C, and that one to another, until a very long chain is made.

The monomer is the small molecule, such as ethene. The polymer is the long molecule, poly(ethene). Its name is "poly" followed by the name of the monomer in brackets, as in poly(ethene) and poly(propene). The chain can contain thousands of monomer units.

Addition polymerisation of ethene. n molecules of ethene, each with a C=C double bond, join to make poly(ethene). In the polymer the repeat unit, two carbon atoms each with two hydrogen atoms, is inside square brackets with n outside. The bonds that cross the brackets are the extension bonds, drawn in orange. The C=C of the monomer has become a C-C single bond.
n ethene molecules make poly(ethene). The C=C of each monomer becomes a C–C single bond.
n CHX2=CHX2⟶[ −CHX2−CHX2X− ]n n\ \ce{CH2=CH2} \longrightarrow \left[\ \ce{-CH2-CH2-}\ \right]_{n}

What happens to each monomer molecule: the π bond breaks, and each of its two carbons forms a new σ bond to a carbon of a neighbouring monomer. The C=C becomes a C–C single bond. Every atom of every monomer ends up in the polymer.

The repeat unit 5.7

A polymer chain is a pattern that repeats, so it can be described by one piece.

A short section of a poly(ethene) chain: six carbon atoms in a row, each carrying two hydrogen atoms. Orange extension bonds leave both ends of the section. One repeat unit, two neighbouring carbons, is marked with square brackets.
A section of the chain. One repeat unit is bracketed: two carbons from one monomer.

For a monomer with one C=C, the repeat unit has two carbons in the main chain, the two carbons that were in the C=C. Any side groups stay attached to the same carbons.

MethodDrawing a repeat unit from a monomer

  1. Draw the monomer with its C=C and the four groups attached to it.
  2. Change the C=C to a C–C single bond. Keep every group on the carbon it was on.
  3. Draw a bond out of each end of the two carbons (the extension bonds). They are not attached to anything.
  4. Draw square brackets so that the extension bonds cross the brackets, and write n at the bottom right, outside the brackets.

Worked exampleThe mass of a polymer chain

A sample of poly(ethene) has an average relative molecular mass of 56 000. How many ethene molecules joined to make each chain, on average?

  1. MrM_\text{r} of the repeat unit, C₂H₄ = 2 × 12.0 + 4 × 1.0 = 28.0.
  2. n = 56 000 ÷ 28.0 = 2000.

Poly(propene) has the same idea with a different repeat unit: C₃H₆ is 42.0, so a chain with MrM_\text{r} 84 000 has 84 000 ÷ 42.0 = 2000 units.

Check yourself

  1. Define addition polymerisation.
  2. State what happens to the C=C bond when ethene polymerises.
  3. Name the polymer made from propene.
  4. Explain why addition polymerisation has 100% atom economy.
  5. A sample of poly(propene) has an average relative molecular mass of 126 000. Calculate the average number of propene molecules joined to make each chain. (ArA_\text{r}: C 12.0, H 1.0.)
  6. Explain why poly(ethene) is a saturated compound, even though it is made from an unsaturated one.

Answers cover these until you have tried the questions

  1. Addition polymerisation is the joining together of many monomer molecules, each with a C=C double bond, to make one long saturated polymer chain, with no other product.
  2. The π bond breaks and the double bond becomes a C–C single bond; each carbon forms a new bond to a carbon of the neighbouring monomer.
  3. Poly(propene).
  4. Every atom of every monomer ends up in the polymer, and no other product is formed, so the atom economy is 100%.
  5. MrM_\text{r} of propene (C₃H₆) = 3 × 12.0 + 6 × 1.0 = 42.0. n = 126 000 ÷ 42.0 = 3000.
  6. During polymerisation every C=C is turned into a C–C single bond, so the polymer chain contains only single bonds between carbons. It is therefore saturated.