Polymers and polyethene: many monomers, one long chain
Explain the monomer-to-polymer relationship and show how ethene forms a chain with single carbon–carbon backbone bonds.
Core and Extended · Chemistry 0620, 2026–2028. Deducing other repeat units, condensation structures and proteins follows in the Extended lessons.
Before you begin
- Draw ethene's C=C bond and recognise ordinary carbon's four bonds.
- Distinguish a chemical reaction from physically mixing substances.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What is a monomer in this context?
What you will learn
- Define polymers as large molecules built from many smaller monomer molecules.
- Describe poly(ethene) formation by addition polymerisation of ethene.
Join molecules rather than pile them together
A polymer is a large molecule built up from many smaller molecules called monomers. Poly(ethene), also written polyethene, forms from many ethene monomers by addition polymerisation. The C=C bond in each ethene becomes a carbon–carbon single bond in the chain, while new single bonds join the carbon framework to neighbouring units. The starting monomer is ethene, not ethane.
The chain pattern is [–CH₂–CH₂–]ₙ. Each repeat contains the two carbon atoms and four hydrogens of the ethene monomer. Addition polymerisation retains those atoms in the polymer pattern without eliminating water as a second product. Simply placing separate ethene molecules next to each other would not create the new covalent chain bonds.
Read the repeat brackets as continuing bonds
The diagram shows every H in one repeat unit, with continuation bonds crossing both brackets. Those bonds lead to adjacent units; they are not extra hydrogen atoms, a dangling oxygen atom or a claim that the two-carbon drawing is the entire polymer molecule. n indicates many repeated units.
For example, 20 copies of the repeat pattern contribute 40 C and 80 H atoms to the repeated part. This count describes the repeat contribution, not the complete end groups of an isolated finite chain. A fully ended molecule needs its actual end-group information; a repeat diagram deliberately leaves the continuation open. The flat drawing shows connectivity rather than a measured three-dimensional shape.
Pause and explain
Which chain pattern correctly comes from ethene?
Worked example
Explain how ethene produces the poly(ethene) repeat and why each carbon has four bonds in the continued chain.
Show the worked solution
- Ethene CH₂=CH₂ supplies two C and four H atoms per repeat.
- Its carbon–carbon bond becomes single and chain bonds join to adjacent units.
- Each chain C has two C–H bonds and two single backbone connections, including a continuation where the repeat drawing ends.
Answer [–CH₂–CH₂–]ₙ: the C=C becomes single and every backbone carbon has two H bonds plus two backbone connections.
Try a topic test
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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.
Define polymer and monomer, then name the monomer for poly(ethene).
Give me a hint
Start with the size relationship and chemical joining.
Compare my reasoning
- A polymer is a large molecule built from many smaller monomer molecules.
- The monomer for poly(ethene) is ethene, CH₂=CH₂.
- Addition polymerisation changes the C=C and forms single chain bonds joining the repeated carbon framework.
Polymer: large molecule built from many smaller monomers. Poly(ethene)'s monomer is ethene CH₂=CH₂.
Look for these in your work
- The size and building relationship is clear.
- Ethene is distinguished from ethane.
Draw one complete displayed poly(ethene) repeat, showing every H and both continuation bonds.
Give me a hint
Each carbon needs two H and two single backbone connections.
Compare my reasoning
- Draw two C linked by one single bond, with two separate H atoms bonded to each C.
- Extend one single bond from each end across the repeat brackets.
- Label n and explain that these bonds continue into neighbouring units rather than terminate as extra atoms.
Displayed [–CH₂–CH₂–]ₙ with every H separate, a single C–C bond and one continuation at each end.
Look for these in your work
- Every explicit atom and required bond appears.
- Brackets and n represent repetition and continuing connections.
A repeat diagram represents 20 poly(ethene) units. Count the C and H contributions and assess a claim that this alone gives the complete formula of any isolated finite chain.
Give me a hint
Multiply repeat counts, then consider what the diagram leaves unspecified.
Compare my reasoning
- Twenty repeats contribute 20×2=40 C atoms.
- They contribute 20×4=80 H atoms to the repeated part.
- Actual finite chain end groups are not specified, so the contribution C₄₀H₈₀ is not automatically a complete isolated-chain formula.
Repeated-part contribution C₄₀H₈₀; the complete finite-chain formula still needs its end-group information.
Look for these in your work
- Both multiplied atom counts are correct.
- Repeat contribution and complete molecule are distinguished.
Common mistakes
- Starting from ethane rather than ethene.
- Keeping C=C while adding chain bonds and exceeding carbon's valence.
- Calling repeat continuation lines extra atoms or complete chain end caps.
- Producing water as a required second product of this addition polymerisation.
What can you explain now?
Which monomer forms poly(ethene)?
Compare with the explanation
Ethene, CH₂=CH₂.
The alkene double bond changes as many monomers join into the single-bonded chain.
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Make it stick
Draw ethene and the full displayed repeat side by side. Explain the bond change, retained atoms and meaning of the continuation lines.
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.