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Condensation polymerisation – polyesters (PET), nylon-6,6 and proteins

ChemistryBiomolecules & PolymersAges 17–18

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Join monomers with two functional groups into a polymer chain: a diacid and a diol give PET (ester links), a diamine and a diacid or diacid chloride give nylon-6,6 (amide links), and amino acids give a peptide chain. Each new link releases one H₂O or HCl molecule. The simulation shows the repeat unit and the chain's molar mass, compares it with the addition polymerisation of ethylene, and hydrolyses the chain back to monomers.

Lesson: Condensation polymerisation: polyesters, polyamides and peptides; comparison with addition polymerisation; hydrolysis of polymers

What it shows

In condensation polymerisation, monomers with two functional groups join end to end and a small molecule is released at every link. A dicarboxylic acid and a diol form a polyester such as PET, joined by ester links; a diamine and a dicarboxylic acid or its acid chloride form a polyamide such as nylon-6,6, joined by amide links; amino acids form peptide links in proteins. The by-product is water or hydrogen chloride, so the polymer weighs less than the monomers used. Because ester and amide links can be hydrolysed, these polymers can be broken back down into their monomers, unlike addition polymers with C–C backbones.

How to use

Choose a Polymer, set the Number of monomers in the chain and the Joining speed, then watch each monomer join while H₂O or HCl leaves. Tap a floating monomer to add it yourself. Press Hydrolyse to let water cut the links, Build polymer to start again and Reset to restore the settings. Compare the condensation and addition columns of the table.

Parameters you can change

  • Polymer PET (polyester): diacid + diol, Nylon-6,6: diamine + diacid, Nylon-6,6: diamine + diacid chloride, Protein: amino acids → peptide chain, Compare: ethylene addition → PE
  • Number of monomers in the chain 4–16
  • Joining speed 0.5–3 monomers/s

Questions to explore

  1. What structural feature must a monomer have to take part in condensation polymerisation rather than addition polymerisation?
  2. How many water molecules are released when 8 monomers form one PET chain, and what is its molar mass?
  3. Why does hydrolysing nylon-6,6 give adipic acid rather than adipoyl chloride?

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