Nylon in ropes, PET in drinks bottles and the proteins in your muscles are all built the same way: two functional groups react, a small molecule of water is released, and a link forms. Spot the link and you can work out the monomers.
Extended. In condensation polymerisation two different functional groups react to form a link and a small molecule, usually water, is released each time. Nylon is a polyamide made from a dicarboxylic acid and a diamine; the monomers join by amide linkages, –CONH–. PET is a polyester made from a dicarboxylic acid and a diol; the monomers join by ester linkages, –COO–. PET can be converted back into its monomers and re-polymerised, so it can be recycled. Proteins are natural polyamides made from amino acids, H2N–CHR–COOH, joined by the same amide link; hydrolysis breaks them back into amino acids.
Extended. Exam questions draw the carbon chains of the monomers as boxes. Only the functional groups at the ends matter.
| Nylon (polyamide) | PET (polyester) | |
|---|---|---|
| monomer 1 | dicarboxylic acid, HOOC–□–COOH | dicarboxylic acid, HOOC–□–COOH |
| monomer 2 | diamine, H2N–▢–NH2 | diol, HO–▢–OH |
| linkage | amide, –CO–NH– | ester, –CO–O– |
| small molecule lost | water (one per link) | water (one per link) |
| repeat unit | –[CO–□–CO–NH–▢–NH]– | –[CO–□–CO–O–▢–O]– |
| uses | clothing, ropes, fishing line | drinks bottles, polyester fibres for clothing |
Four quick checks. Each correct answer earns XP and records this skill on your profile.
(Extended) What small molecule is released when nylon is formed?
(Extended) Which type of linkage is found in PET?
(Extended) Which pair of monomers forms a polyamide such as nylon?
(Extended) What are the monomers of proteins?
Extended. The two types of polymerisation differ in the monomers they use and the products they make.
| Addition | Condensation | |
|---|---|---|
| monomers | alkenes, with a C=C bond | two functional groups per monomer (e.g. –COOH, –NH2, –OH) |
| number of products | one: the polymer only | two: the polymer and a small molecule (water) |
| linkage in chain | C–C single bonds only | amide or ester linkages |
| examples | poly(ethene), PVC | nylon, PET, proteins |
Extended. Living things make polyamides from about twenty different amino acids.
The amide link is –CO–NH– and the ester link is –CO–O–: check whether the atom after the C=O is N or O. In a repeat unit, the water has already gone, so do not leave –OH or –H on the ends of the bracketed unit. Hydrolysis is the reverse of condensation: water is used, not released.
Unlocks once the four checks above are done. Worth more XP, written to IGCSE Paper 2 and Paper 4 standard.
(Extended) How does condensation polymerisation differ from addition polymerisation?
(Extended) A polymer contains the linkage –CO–O–. Which pair of monomers could form it?
(Extended) What happens when a protein is hydrolysed?
(Extended) Four amino acid molecules join to form a single chain. How many molecules of water are released?
This skill is now recorded on your profile. Keep the chain going.