Deduce polyester repeat units from monomers
Preserve each monomer residue and join them through ester links to write a condensation-polymer repeat.
A Level; Cambridge International Chemistry 9701, 2028–2030 syllabus, Topic 35. Track the functional groups, repeat-unit connectivity and small-molecule by-products. Written practice is self-checked
Before you begin
- Recognise alkenes, carboxylic acids, acyl chlorides, alcohols, amines, amides and ester linkages.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
When deducing a polyester repeat, which atom from the diol appears in the ester linkage?
What you will learn
- Locate the reactive ends on monomers
- Construct an ester-linked repeat unit
- Check atom retention and eliminated small molecules
Mark the bond-forming atoms first
For a diol and dicarboxylic acid, connect the diol oxygen to a carboxyl carbon to make –O–CO–. Repeat the pattern so the repeat contains one residue from each monomer.
For a hydroxycarboxylic acid, join its hydroxyl oxygen to another molecule's carboxyl carbon. The repeat is [–O–R–CO–]ₙ when the monomer is HO–R–COOH.
Check the repeat against the monomers
The carbon skeletons and substituents stay in the polymer. At each link, water or HCl is eliminated depending on whether the carbonyl monomer is a carboxylic acid or acyl chloride.
The bracketed repeat is a connectivity pattern that repeats; it is not a complete molecule with capped ends. Write the by-product separately rather than inserting it into the chain.
Pause and explain
Which check helps detect an incorrect polyester repeat unit?
Worked example
Deduce a polyester repeat from HO(CH₂)₃OH and ClCOC₆H₄COCl.
Show the worked solution
- Mark O–H on the diol and C–Cl on each acyl chloride end.
- Connect through –O–CO– and preserve the three-carbon chain and aromatic residue.
- Identify HCl from the leaving groups.
Answer [–O(CH₂)₃OCOC₆H₄CO–]ₙ is an equivalent repeat; HCl is eliminated as ester links form.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Deduce an idealised polyester repeat from HOCH₂CH₂OH and ClCOCH₂COCl.
Give me a hint
Alternate the ethane-1,2-diol and dioyl-chloride residues.
Compare my reasoning
- Keep –OCH₂CH₂O–.
- Keep –COCH₂CO–.
- Join through –O–CO– and list HCl separately.
[–OCH₂CH₂OCOCH₂CO–]ₙ; HCl is eliminated at the ester-forming links.
Look for these in your work
- Residues retained
- Ester connectivity
- By-product correct
Deduce the repeat for HO–CH₂–COOH self-condensation.
Give me a hint
This molecule has one hydroxyl and one carboxyl end.
Compare my reasoning
- Keep the CH₂ residue.
- Link O of one molecule to the carbonyl C of another.
- Use the –OCH₂CO– repeat pattern.
[–OCH₂CO–]ₙ, with water eliminated as ester links form.
Look for these in your work
- One monomer type used
- Link order correct
- Water noted
An answer writes [–CO–O–CH₂CH₂–O–CO–]ₙ for a diol/diacid polymer. What should you verify before accepting it?
Give me a hint
Compare atom connectivity with both monomer residues and the repeat boundaries.
Compare my reasoning
- Check that the diol O atoms remain bonded to its carbon skeleton.
- Check that each diacid carbonyl is connected through ester oxygen to a neighbouring residue.
- Allow equivalent repeat-unit orientation if connectivity and atom counts match.
Accept only a repeat with the correct diol and diacid residues joined as –O–CO– ester links; an equivalent orientation is fine if connectivity and atom count are preserved.
Look for these in your work
- Both residues checked
- Ester direction checked
- Equivalent orientation considered
Common mistakes
- Reversing the carbonyl and oxygen order
- Deleting a substituent or carbon skeleton
- Adding water to a dioyl-chloride route
- Showing chlorine inside the repeat unit
What can you explain now?
When deducing a polyester repeat, which atom from the diol appears in the ester linkage?
Compare with the explanation
The diol oxygen
The alcohol oxygen bonds to a carbonyl carbon.
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Make it stick
What three structural checks help you deduce and verify a polyester repeat unit?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.