Repeat Units & Monomers (Cambridge International A-Level Chemistry)

Deducing the Repeat Unit of a Condensation Polymer

Repeat units for condensation polymers

• Remember we can tell the type of polymerisation by identifying the linking between the monomers

  • If a chain of carbon atoms is present, the polymer is an addition polymer
  • If there is an ester link, the polymer is a polyester (formed by condensation polymerisation)

Example polyester structure

Polyesters contain the ester link

• If there is an amide link, the polymer is a polyamide (formed by condensation polymerisation)

Example polyamide structure

Polyamides contain the amide or peptide link

• In condensation polymerisation, the monomers either contain:

  • 2 monomers each with the same functional group, such as a diamine with a dicarboxylic acid or
  • One single monomer that has both of the functional groups needed for polymerisation, such as an aminocarboxylic acid

Exam Hint

• Remember: in condensation polymerisation, a small molecule is expelled as a result of the 2 monomers joining together.

• When a dioic acid and diamine polymerise, a water molecule is expelled

  • OH from acid and H from the amine

• When a dioyl chloride and diamine are polymerised, a hydrochloric acid molecule is expelled

  • Cl from the chloride and H from the amine

Worked Example Draw the repeating unit and identify the monomers used to make the following polymers

Answer:

Identifying Monomers in Condensation Polymers

• When a section of polymer is presented, the monomers can be identified by considering the small molecules expelled from the monomers
• If a water molecule is expelled, the -OH must have been from an acid group
• The hydrogen atom may be from an amine group of a monomer.
• If the molecule was hydrochloric acid (HCl), a dioyl chloride monomer may have been used