Condensation polymerisation — common mistakes

When drawing a repeating unit, a foolproof method is to first draw the two monomers side-by-side. Use a pencil to circle the atoms that will be eliminated (e.g., H from an –OH group and OH from a –COOH group to make H₂O). Then, redraw the remaining fragments, connecting them with a new bond. Finally, place the unit in square brackets, ensuring the bonds at each end pass through the brackets to indicate the chain continues.

In addition polymerisation, monomers add to each other so that the polymer contains all the atoms of the monomer units. The empirical formula of the polymer is the same as the monomer. In condensation polymerisation, monomers join with the elimination of a small molecule (like water), so the polymer does not contain all the atoms of the original monomers.

The numbers refer to the number of carbon atoms in each of the two monomers. Nylon 6,6 is made from 1,6-diaminohexane (which has 6 carbon atoms) and hexanedioic acid (which also has 6 carbon atoms). Similarly, Nylon 6,10 is made from 1,6-diaminohexane (6 carbons) and decanedioic acid (10 carbons).

Yes, if the monomer molecule is bifunctional, meaning it has two different functional groups that can react with each other. A perfect example is an amino acid, which has both an amine group (–NH₂) and a carboxylic acid group (–COOH). Amino acids polymerise to form polyamides, which we know as proteins or polypeptides.

Some are, particularly those with linkages that can be hydrolysed. Polyesters and polyamides can be broken down by hydrolysis (reaction with water), often catalysed by acids or alkalis. This is essentially the reverse of the condensation reaction. This susceptibility to hydrolysis makes them more biodegradable than addition polymers like poly(ethene), which have a non-polar, unreactive carbon-carbon backbone.