Condensation Polymerisation

Full Notes

Condensation polymerisation occurs when monomers with two functional groups react together to form a long-chain polymer and eliminate a small molecule, typically H₂O or HCl, with each link.

This is different from addition polymerisation, which forms polymers from alkenes without losing any atoms (see here for more detail).

Formation of Polyesters

Polyesters are made when monomers containing alcohol (–OH) and carboxylic acid (–COOH) groups react.

Diol + Dicarboxylic Acid or Dioyl Chloride

A diol (–OH group at both ends) reacts with a dicarboxylic acid (or dioyl chloride).

Each ester bond forms with loss of H₂O (or HCl if using acyl chlorides).

Hydroxycarboxylic Acid

A single monomer with both –OH and –COOH can polymerise with itself.

The –OH of one molecule reacts with –COOH of another, forming an ester bond.

Formation of Polyamides

Polyamides are made when amine (–NH₂) and carboxylic acid (–COOH) groups react to form amide bonds (–CONH–) that join repeating units.

Diamine + Dicarboxylic Acid or Dioyl Chloride

Each end of the diamine and dicarboxylic acid reacts to form an amide link and water/HCl.

Aminocarboxylic Acid

A molecule with both –NH₂ and –COOH groups can polymerise with itself to form a polyamide.

Amino Acids

Amino acids naturally polymerise to form proteins (polypeptides).

The amine group of one amino acid bonds to the acid group of another, forming peptide bonds.

Deducing Repeat Units and Monomers

Repeat Unit:

• The smallest section of a polymer that repeats.
• Usually includes part of both monomers (e.g. from each end in a polyester or polyamide).
• Always show two linkage bonds to show continuation of the chain.

Identifying Monomers:

• Look at the repeat unit and:
  • Identify the linking group (e.g. ester: –COO–, amide: –CONH–).
  • Split the unit at these points.
  • Reconstruct the functional groups on each side (–COOH or –OH, –NH₂).

Summary

• Condensation polymers form by joining monomers with loss of small molecules like H₂O or HCl.
• Polyesters are formed from diols with dicarboxylic acids or dioyl chlorides, or from hydroxycarboxylic acids.
• Polyamides are formed from diamines with dicarboxylic acids/dioyl chlorides, aminocarboxylic acids, or amino acids.
• Amino acids form proteins via peptide bonds.
• Repeat units show the smallest repeating section and monomers can be deduced by identifying and splitting at linkages.