Covalent bonding: Polymers can be represented in the form:

The idea

Polymers are very large covalent molecules built from many small repeating units called monomers. In a displayed formula, the polymer is shown with brackets around the repeating unit and the letter n written outside the brackets. The bonds at the two ends of the repeating unit pass through the brackets, showing that the unit joins to identical units on both sides. For example, poly(ethene) is represented with the repeating unit —CH₂—CH₂— inside brackets and n outside. The value of n is not fixed for a sample; it shows the number of monomer units joined together. This representation is a shorthand that avoids drawing thousands of atoms and lets chemists see the repeating pattern clearly.

What earns the marks

• A polymer is a large covalent molecule formed when many small monomer molecules join together by covalent bonds.
• The repeating unit is the smallest section of the polymer chain that repeats many times.
• In the shorthand representation, the repeating unit is placed inside brackets.
• The letter n is written outside the brackets to show that the unit repeats a large number of times.
• The bonds at each end of the repeating unit cross the brackets, showing continuation of the chain in both directions.
• For addition polymers such as poly(ethene), the repeating unit has the same atoms as the monomer but with the double bond opened to form single bonds.

Common mistakes and how to put them right

• Drawing the repeating unit without brackets or without n outside.

  Put it right by enclosing the repeating unit in brackets and writing n outside.

• Putting n inside the brackets.

  Put it right by placing n outside the brackets, because n counts how many repeating units are joined.

• Showing a double bond inside the repeating unit of an addition polymer.

  Put it right by drawing single bonds only, because the double bond opens when monomers join.

Exam tips

• When asked to draw a polymer, draw the repeating unit with its two free bonds crossing the brackets, then write n outside.
• Check that the atoms in the repeating unit match the monomer for an addition polymer, with the C=C changed to C—C.
• If asked what n represents, state that it is a large number of repeating units, not a fixed small number.