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GCSE Chemistry Revision

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GCSE Chemistry revision

Addition polymerisation

Synthetic and naturally occurring polymers (chemistry only)

AQA 4.7.3.1 Foundation & Higher
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Key knowledge

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What Makes Alkenes Special?

  • Alkenes contain a carbon-carbon double bond (C=C), which makes them unsaturated molecules capable of reacting to form polymers.
  • Because the double bond can break open under the right conditions, the carbon atoms become free to form new bonds with other molecules.

How Addition Polymerisation Works

  • In addition polymerisation, the C=C double bond in each monomer breaks, allowing monomers to join together into one long chain called a polymer.
  • No atoms are lost or gained during addition polymerisation — all atoms from the monomers are incorporated into the polymer.
  • The reaction requires high pressure and a catalyst to proceed.

Monomers and Repeating Units

  • A monomer is the small, individual molecule (e.g. ethene) that reacts repeatedly to build up the polymer chain.
  • A repeating unit is the group of atoms that recurs throughout the polymer and contains exactly the same atoms as the monomer.
  • For ethene (C₂H₄), the repeating unit also contains two carbon atoms and four hydrogen atoms, just bonded differently.

Drawing the Monomer Correctly

  • When drawing a monomer for a polymerisation equation, the bonds on the C=C double bond must be shown pointing straight up and straight down, not at angles.
  • Side groups and hydrogen atoms attached to the double-bonded carbons should be arranged directly above and below those carbons.
  • Large side groups can be simplified into shorthand notation, for example two carbons and five hydrogens is written as C₂H₅.

Drawing the Repeating Unit

  • The repeating unit is drawn almost identically to the monomer, but the C=C double bond is removed and replaced with single bonds extending out to the left and right through the brackets.
  • The bonds extending through the brackets represent the connections to the next repeating unit in the polymer chain.
  • The symbol n is placed outside the bottom-right of the brackets on the repeating unit to indicate how many times the unit repeats.

Writing the Full Polymerisation Equation

  • The monomer (with n written in front of its brackets) is drawn on the left-hand side of the reaction arrow.
  • The repeating unit (with n outside the bottom-right of its brackets) is drawn on the right-hand side of the reaction arrow.
  • The value of n represents the number of monomer molecules reacting and the number of repeating units in the resulting polymer chain.

Example: Polymerisation of But-1-ene

  • But-1-ene is a four-carbon alkene with a double bond between the first and second carbon atoms.
  • When drawing but-1-ene as a monomer, the C₂H₅ group is placed directly above the second double-bonded carbon, with the bond drawn carbon-to-carbon.
  • The resulting polymer is named poly(but-1-ene), following the standard naming convention.

Naming Addition Polymers

  • Addition polymers are named by placing 'poly' in front of the monomer's name, with the monomer's name written in brackets.
  • For example, the polymer made from ethene is called poly(ethene), and the polymer from chloroethene is called poly(chloroethene).

Conditions for Addition Polymerisation

  • Addition polymerisation requires high pressure to force monomer molecules close enough together to react.
  • A catalyst is also needed to initiate and speed up the polymerisation reaction.
  • These conditions (high pressure and catalyst) may be written above or below the reaction arrow in equations.