Drawing Repeat Units

AQA expects you to work in both directions between monomers and polymers: draw the repeating unit from the monomers, draw it from a section of chain, and draw the monomers from a section of chain.From monomers to repeating unit
1. Draw one molecule of each monomer side by side.
2. Remove -OH from the acid group (or -Cl from an acyl chloride) and -H from the -OH or -NH₂ group of the other monomer.
3. Join the carbonyl carbon to the O (ester link) or N (amide link).
4. Leave an open bond at each end, put the unit in square brackets and add n.A repeating unit made from a diacid and a diol or diamine always contains one of each monomer. A polymer from a single amino acid has a repeating unit of one residue: [-NH-CHR-CO-]_n.From a chain to the repeating unit
Find the ester or amide links, then count along the chain until the pattern starts to repeat.From a chain to the monomers
Cut every ester or amide link at the C-O or C-N single bond. Add -OH to each carbonyl carbon (giving -COOH) and -H to each O or N (giving -OH or -NH₂).Worked example. A section of chain is -O-CH₂CH₂-O-CO-CH₂-CO-O-CH₂CH₂-O-CO-CH₂-CO-. The links are ester links. Cutting them and adding -OH and -H gives HO-CH₂CH₂-OH (ethane-1,2-diol) and HOOC-CH₂-COOH (propanedioic acid). The repeating unit is [-O-CH₂CH₂-O-CO-CH₂-CO-]_n.