Key Learning Objectives
- Explain how addition polymerisation converts double C=C bonds into single C-C bonds.
- Draw repeating units with open extension bonds passing through square brackets.
- Deduce monomer structures from given polymer repeating units.
Topic Summary & Core Notes
Learn how small alkene monomers join together during addition polymerisation to form long-chain polymers like poly(ethene) and poly(propene).
Key Rules, Formulas & Definitions
Polymerisation Conversion Rule
Monomer (C=C double bond) → Repeating Unit (C-C single bond with open bracket bonds and subscript n)
Atom Economy Rule
Addition polymerisation has 100% atom economy as no byproduct is formed
2 Step-by-Step Worked Examples
Worked Example 1
Worked Example 1: Drawing Poly(ethene) from Ethene
Question: Draw the repeating unit for poly(ethene) formed from ethene (C₂H₄) monomers.
Step-by-Step Solution:
- Step 1: Open the double bond: Change C=C to a single C−C bond.
- Step 2: Draw trailing extension bonds: Extend single bonds out horizontally past the carbons.
- Step 3: Enclose in brackets: Place square brackets through the extension bonds and add a lowercase 'n' at the bottom right.
Final Answer: −[ CH₂ − CH₂ ]ₙ−
Senior Examiner Insight: Extension bonds MUST pierce through the square brackets to get full marks.
Worked Example 2
Worked Example 2: Finding Monomer from a Repeating Unit
Question: Given the repeating unit −[ CH₂ − CH(Cl) ]ₙ−, identify the monomer name and structure.
Step-by-Step Solution:
- Step 1: Identify 2-carbon core: The repeating unit has 2 carbons with 3 hydrogens and 1 chlorine.
- Step 2: Reintroduce double bond: Remove brackets and trailing bonds; place a C=C double bond between carbons: CH₂=CHCl.
- Step 3: Name monomer: Chloroethene.
Final Answer: Chloroethene (CH₂=CHCl)
Senior Examiner Insight: The polymer name is simply 'poly(' + monomer name + ')', e.g. poly(chloroethene) or PVC.
Common Pitfalls & Examiner Warnings
Common Mistake: Drawing double bonds inside polymer repeating units
The double bond OPENS UP into a single bond during polymerisation.
Common Mistake: Extension bonds not crossing the brackets
Extension bonds must clearly pass through the square brackets to represent continuation.
Common Mistake: Forgetting the subscript 'n'
The 'n' indicates thousands of repeating units in the giant molecule.
Interactive Self-Check Quiz (3 Questions)
Question 1 of 3
What happens to the carbon-carbon double bond during addition polymerisation?
Click to Reveal Answer & Mark Scheme
Correct Answer: A) It opens up to form single covalent bonds between adjacent monomers.
Detailed Explanation: The pi bond breaks and electrons form new single sigma bonds with neighboring monomers.
Mark Scheme & Scoring Points: 1 mark for opening up to form single bonds with adjacent units.
Question 2 of 3
What is the starting monomer used to manufacture poly(propene)?
Click to Reveal Answer & Mark Scheme
Correct Answer: B) Propene
Detailed Explanation: Polymers are named after their constituent monomer preceded by 'poly'.
Mark Scheme & Scoring Points: 1 mark for Propene.
Question 3 of 3
Why does addition polymerisation have an atom economy of 100%?
Click to Reveal Answer & Mark Scheme
Correct Answer: A) All reactant atoms end up in the desired single polymer product with zero waste byproducts.
Detailed Explanation: No atoms are lost or discarded; the polymer is the sole product.
Mark Scheme & Scoring Points: 1 mark for no byproducts / all reactant atoms incorporated into product.
Created by Fiaraz Iqbal
Former Headteacher & Senior Science / Maths Examiner
Fiaraz produces structured video walkthroughs, Tier 3 literacy packs, and exam mark scheme breakdowns to help secondary students achieve top grades.
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