Deducing monomer from polymer (and vice versa)

From ethene to poly(ethene), one bond at a time

Step through in order. The double bond has to open before the brackets mean anything.

1. 1Spot the C=CStart with the monomer: ethene, CH₂=CH₂. See the double bond between the carbons? A monomer for an addition polymer has a C=C, and that one bond is where all the action happens.+
2. 2Open the double bondNow open it. The C=C turns into a single bond, and each carbon gets a bond reaching out sideways: –CH₂–CH₂–. Count the atoms: still 2 carbons and 4 hydrogens. Nothing added, nothing lost. Only the bond changed.+
3. 3Add the bracketsPut brackets around the repeating unit and let the open bonds run through them: [–CH₂–CH₂–]. The brackets say 'this section, repeated'.+
4. 4Write n outsideAdd n outside the bracket: [–CH₂–CH₂–]ₙ. n monomers went in, so the chain has n repeating units. n can be any number, and in a real polymer chain it is often hundreds of thousands.+
5. 5Name itWrite poly, then the monomer's name in brackets: poly(ethene). Now look again at [–CH₂–CH₂–]ₙ. Where did the C=C go? It has opened. The name keeps the monomer; the structure doesn't keep its double bond.+

The polymerisation equation, with propene

Tap the monomer or the polymer to see what each part of the equation is saying.

What you need to know

• A polymer is made of small repeating units. Monomers are the small molecules that join to make it.
• Addition polymers are made from monomers with a C=C double bond, such as alkenes.
• In addition polymerisation the C=C opens up so the monomers join, and the polymer is the only product.
• Have a goYour classmate says, 'When ethene turns into a polymer, a second product comes off too, like steam from a kettle.' Are they right?

  No. Only one product forms: the polymer.

  The double bond opens and the monomers simply join together, so nothing else is made.

• The repeating unit is the section that repeats. It is drawn with open bonds on either side.
• Monomer to polymer: open the C=C to get the repeating unit, then add brackets and n.
• Have a goQuick draw: ethene is CH₂=CH₂. Write down its repeating unit before you look.

  –CH₂–CH₂–, with an open bond on each side.

  Opening the C=C leaves a single bond between the carbons and the open bonds show the unit is bonded to more units.

• Polymer to monomer: find the repeating unit, then reform the carbon–carbon double bond.
• In an equation, n goes before the monomer. It means any number of monomer units, often hundreds of thousands.
• Name a polymer by writing poly, then the monomer's name in brackets. Ethene gives poly(ethene).
• Same rule for propene, chloroethene and tetrafluoroethene. Styrene gives polystyrene, with no brackets.
• Have a goTetrafluoroethene is the monomer. What is the polymer called?

  poly(tetrafluoroethene)

  The name is poly followed by the monomer's full name in brackets, so the monomer is easy to read back.

• The polymer's name keeps the monomer's name, but the monomer's C=C has opened into single bonds.

The big picture

Addition polymers are built from monomers that contain a C=C. To go from monomer to polymer, open the C=C to get the repeating unit, add brackets and n, and name it poly(monomer). To go back, find the repeating unit and reform the C=C.

⚠ Watch out

Leaving the C=C in the repeating unit because the polymer's name contains the monomer's name. The name only records the monomer. In the structure the C=C has opened into a single bond, with an open bond on each side.

Memory hook

Open the C=C and you get the polymer. Reform the C=C and you get the monomer back. One bond, two directions.

Check yourself

Styrene forms polystyrene. A friend says every unit still has styrene's C=C. What happened to that bond, and how would you get styrene back from a section of chain?