Identify monomers from polymer sections
Reverse addition polymerisation by locating the backbone repeat and restoring a C=C bond.
AS Level; Cambridge International Chemistry 9701, syllabus for 2028–2030. The taught examples are addition polymers of alkenes; do not generalise their disposal behaviour to every polymer. Written practice is self-checked
Before you begin
- Recognise C=C bonds and displayed, structural and repeat-unit formulae from Topics 13–14.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What feature identifies an alkene monomer after reversing addition polymerisation?
What you will learn
- Read a polymer backbone segment
- Restore a C=C between adjacent backbone carbons
- Identify the monomer and preserve side groups
Work backwards from the chain
Choose two adjacent carbon atoms in the polymer backbone that came from one alkene monomer. Replace the backbone single bond between them with a C=C bond, then remove the two chain-continuation bonds.
Keep any side group attached to the carbon where it appears. For [–CH₂–CH(Cl)–]ₙ, reversing the process gives CH₂=CHCl, chloroethene.
Do not confuse repeat unit with monomer
The repeat unit is the saturated chain segment inside the polymer. The monomer is the alkene with a C=C bond. A section containing more than one kind of repeat can show more than one monomer; identify each distinct repeat and reverse it separately.
This reasoning reconstructs the monomer from connectivity. It does not reveal the total number of monomers in the original polymer sample.
Pause and explain
How do you handle a side group while recovering the monomer?
Worked example
Identify the monomer for [–CH₂–CH(CH₃)–]ₙ.
Show the worked solution
- Find the two backbone carbons in the repeat.
- Restore a double bond between them.
- Keep CH₃ on the same carbon.
Answer CH₂=CHCH₃, propene.
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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.
Identify the monomer for [–CH₂–CH(Cl)–]ₙ.
Give me a hint
Restore the double bond and retain chlorine on the substituted carbon.
Compare my reasoning
- Select the two-carbon repeat segment.
- Replace its C–C single bond with C=C.
- Keep Cl attached to carbon 2.
CH₂=CHCl, chloroethene.
Look for these in your work
- C=C restored
- Cl retained
- Correct monomer name
A polymer section alternates –CH₂–CH₂– and –CH₂–CH(Cl)– units. Which monomers does it indicate?
Give me a hint
Reverse each distinct repeat unit separately.
Compare my reasoning
- The unsubstituted repeat reverses to ethene.
- The chlorinated repeat reverses to chloroethene.
- List both monomers without inferring their ratio beyond the shown section.
Ethene and chloroethene. The section shows both monomer types; it does not establish the composition of the whole sample.
Look for these in your work
- Both repeat types identified
- Both double bonds restored
- Conclusion limited to the shown section
A student identifies CH₃CH₂Cl as the monomer for [–CH₂–CH(Cl)–]ₙ. Explain the correction.
Give me a hint
The recovered monomer must contain the C=C bond that opened during polymerisation.
Compare my reasoning
- The repeat unit has a two-carbon backbone.
- Restore the backbone C–C bond as C=C.
- Retain Cl on carbon 2.
The monomer is CH₂=CHCl, chloroethene, because addition polymerisation opens that C=C bond.
Look for these in your work
- Unsaturation restored
- Substituent retained
- Correct monomer distinguished from saturated haloalkane
Common mistakes
- Leaving the monomer saturated
- Removing a side group
- Moving a substituent to the other carbon
- Inferring polymer chain length from n in a sample diagram
What can you explain now?
What feature identifies an alkene monomer after reversing addition polymerisation?
Compare with the explanation
A C=C bond between the former backbone carbons
The monomer's double bond opens to form the polymer backbone.
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Make it stick
Which bonds do you reverse to recover an alkene monomer from its polymer 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.