Topic 5 · page 14 of 15

From monomer to repeat unit, and back

Draw the repeat unit by turning the C=C into C–C and keeping the side groups in place. Work backwards by finding the two carbons of one unit and putting the C=C back.

13 min read5.7

Read first: Addition polymerisation

From monomer to repeat unit 5.7

The rules from the last page are the same for every alkene monomer. The side groups stay on the carbon they were on, and the two carbons become the main chain.

Three monomers and the repeat unit of the polymer each makes. Propene, CH2=CH(CH3), gives poly(propene), repeat unit CH2 then CH(CH3) in brackets. Chloroethene, CH2=CHCl, gives poly(chloroethene), also called PVC, repeat unit CH2 then CHCl. Tetrafluoroethene, CF2=CF2, gives poly(tetrafluoroethene), repeat unit CF2 then CF2. In every case the C=C becomes a C-C single bond and the side groups stay on the same carbons.
Three monomers and their repeat units. The side groups (CH₃, Cl, F) stay on the carbons where they began.
Monomer Formula Polymer Repeat unit
ethene CH₂=CH₂ poly(ethene) –CH₂–CH₂–
propene CH₂=CHCH₃ poly(propene) –CH₂–CH(CH₃)–
chloroethene CH₂=CHCl poly(chloroethene), PVC –CH₂–CHCl–
tetrafluoroethene CF₂=CF₂ poly(tetrafluoroethene) –CF₂–CF₂–

In poly(propene) the CH₃ is a side group: it hangs off the carbon chain. The main chain of the repeat unit has only two carbons, even though the monomer has three. The same is true of chloroethene: the Cl is on a main-chain carbon.

Worked examplePoly(but-1-ene)

Draw the repeat unit of poly(but-1-ene).

  1. But-1-ene is CH₂=CHCH₂CH₃. The C=C is at the end, so the ethyl group, C₂H₅, is the side group.
  2. Make the C=C into C–C: the two chain carbons are CH₂ and CH.
  3. Keep C₂H₅ on the CH carbon: –CH₂–CH(C₂H₅)–.
  4. Put brackets round it so the extension bonds cross them, and write n outside.

From repeat unit to monomer 5.7

Going backwards, you are given a section of a polymer chain, or its repeat unit, and must name the monomer.

A section of a polymer chain with four carbon atoms in a row. Every carbon carries one CH3 group above and one hydrogen atom below. Extension bonds leave both ends.
Which monomer made this chain?

MethodFinding the monomer

  1. Find the two carbons that make one repeat unit. They are two neighbouring main-chain carbons. Their side groups are the ones that stay.
  2. Take away the extension bonds.
  3. Put a C=C back between the two carbons.
  4. Name the alkene.
The same chain with one repeat unit marked in square brackets: two neighbouring carbons, each with a CH3 group and a hydrogen atom. An arrow leads to the monomer, but-2-ene, CH3CH=CHCH3, with the C=C put back between the two bracketed carbons.
One repeat unit is –CH(CH₃)–CH(CH₃)–. Put the C=C back to give but-2-ene.

Worked exampleThe monomer of this chain

  1. Each main-chain carbon has a CH₃ and an H. A repeat unit is two carbons: –CH(CH₃)–CH(CH₃)–.
  2. Put the C=C back: CH₃CH=CHCH₃.
  3. The monomer is but-2-ene.
The repeat unit CH2 then CHCl, in square brackets with extension bonds, and the monomer it comes from: chloroethene, CH2=CHCl, with the C=C put back between the two carbons.
The repeat unit –CH₂–CHCl– comes from chloroethene.

Check yourself

  1. Draw the repeat unit of poly(propene).
  2. A polymer has the repeat unit –CH₂–CHCl–. Name and draw its monomer.
  3. Give the repeat unit of poly(tetrafluoroethene) and the formula of its monomer.
  4. Draw the repeat unit of poly(2-methylpropene).
  5. A section of a polymer chain is –CH₂–CH(CH₃)–CH₂–CH(CH₃)–. Name the monomer.
  6. A student draws the repeat unit of poly(propene) as –CH₂–CH₂–CH₂–. Give two errors.

Answers cover these until you have tried the questions

  1. –CH₂–CH(CH₃)–, in square brackets, with the extension bonds crossing the brackets and n outside: [–CH₂–CH(CH₃)–]ₙ. The CH₃ is a side group on the second carbon.
  2. Put the C=C back between the two carbons: CH₂=CHCl, chloroethene.
  3. –CF₂–CF₂–. The monomer is CF₂=CF₂, tetrafluoroethene.
  4. 2-methylpropene is (CH₃)₂C=CH₂. The two carbons of the C=C become the main chain, and both CH₃ groups stay on the same carbon: –CH₂–C(CH₃)₂–.
  5. The repeat unit is –CH₂–CH(CH₃)– (two main-chain carbons, with CH₃ on one). Put the C=C back: CH₂=CH(CH₃) = propene.
  6. (1) The repeat unit has three main-chain carbons: the CH₃ carbon has been put into the main chain. A monomer with one C=C gives a repeat unit with two main-chain carbons, and the CH₃ must be a side group: –CH₂–CH(CH₃)–. (2) Brackets and extension bonds are missing.