The shape of the lesson
By the end of the lesson, learners can
- From monomer to repeat unit: Pick a monomer in the simulator and watch the π bonds open and the molecules join end to end.
- Count the monomers: Use the counting view of the simulator to divide the mass of a chain by the mass of one monomer.
- Why poly(alkenes) last: The properties that make poly(alkenes) so useful, being tough, waterproof and chemically inert, are the same ones that cause the disposal problem.
- Repeat unit: C=C becomes C–C, brackets, bonds through, n.
- Monomer: split into 2-carbon units, put C=C back.
- n = Mr(chain) ÷ Mr(monomer).
- Disposal: non-biodegradable; PVC burns to give HCl.
One picture every idea returns to
In addition polymerisation many alkene molecules (the monomers) join to form one very long saturated chain (the polymer). The weaker π bond of each C=C breaks and the electrons form new C–C σ bonds to neighbouring monomers, so nothing else is produced and the polymer has the same empirical formula as the monomer. The repeat unit is the monomer with its C=C turned into C–C and a bond sticking out from each end of the 2-carbon backbone, written in brackets with a subscript n. To find the monomer from a chain, mark off 2-carbon units along the backbone and put the double bond back. Poly(alkenes) are saturated with strong, non-polar C–C and C–H bonds, so they are unreactive and non-biodegradable: they persist in landfill, and burning them releases CO, CO2 and, from PVC, toxic HCl.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, Thousands of alkenes, one long chain.. Open the C=C double bond of an alkene and the molecules can join end to end, thousands at a time. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 13 min | Teach and check: From monomer to repeat unit | Pick a monomer in the simulator and watch the π bonds open and the molecules join end to end. Project tab 1 of the lab (Addition polymerisation lab, monomer to polymer) and let a student drive it while the class predicts. | Whole class, then pairs |
| 13 to 21 min | Teach and check: Count the monomers | Use the counting view of the simulator to divide the mass of a chain by the mass of one monomer. Project tab 3 of the lab (Addition polymerisation lab, count the monomers) and let a student drive it while the class predicts. | Pairs |
| 21 to 30 min | Teach and check: Why poly(alkenes) last | The properties that make poly(alkenes) so useful, being tough, waterproof and chemically inert, are the same ones that cause the disposal problem. Project tab 4 of the lab (Addition polymerisation lab, why poly(alkenes) last) and let a student drive it while the class predicts. | Frayer Model |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 15 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in Why poly(alkenes) last, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Frayer Model
Pairs fill a four-box grid for the key term: definition, characteristics, examples and non-examples. The non-examples box is where the misconceptions surface, so give it the most time.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| Poly(ethene) is saturated, so never draw a C=C inside a repeat unit, and never write “polyethene contains double bonds”. | Only the two C=C carbons form the backbone; side groups such as CH 3 or Cl hang off it, so the repeat unit always has a 2-carbon backbone (per monomer) with bonds through the brackets and an n. Addition polymerisation produces no by-product such as water. Poly(alkenes) are non-biodegradable because they are unreactive, not because they are toxic, and it is specifically PVC that gives HCl on burning. |
Addition polymerisation: worksheet
6 questions, 15 marks. Written in the style of the papers; no past paper question is reproduced.
Write the repeat unit of the addition polymer formed from (a) ethene, (b) chloroethene, (c) tetrafluoroethene.
A section of an addition polymer is –CH2–CH(CN)–CH2–CH(CN)–CH2–CH(CN)–. Give the structural formula of the monomer and draw the repeat unit.
A sample of poly(chloroethene) has an average Mr of 1.25 × 105. Calculate the average number of monomer units per chain, and explain why the repeat unit has the same Mr as the monomer. (Ar: C 12.0, H 1.0, Cl 35.5)
Explain why poly(ethene) is chemically unreactive and why this makes it non-biodegradable.
1.00 kg of PVC waste is burned completely. Write an equation per repeat unit and calculate the mass and the volume at room conditions of hydrogen chloride produced. (Mr C2H3Cl 62.5, HCl 36.5; molar gas volume 24.0 dm3 mol−1)
Discuss the problems of disposing of poly(alkene) waste by landfill, by incineration and by recycling.
Answer key and marking notes
- (a) [–CH2–CH2–]n.
- (b) [–CH2–CHCl–]n.
- (c) [–CF2–CF2–]n. In each, the C=C has become C–C and a bond extends through each bracket.
- The repeating 2-carbon unit is CH2–CH(CN), so the monomer is CH2=CHCN (propenenitrile).
- Repeat unit: [–CH2–CH(CN)–]n.
- Mr(C2H3Cl) = 24.0 + 3.0 + 35.5 = 62.5, so n = 1.25 × 105 ÷ 62.5 = 2.00 × 103.
- Addition polymerisation forms no by-product: only the π bond rearranges, so the repeat unit contains exactly the atoms of one monomer.
- Poly(ethene) is saturated: it has only strong, non-polar C–C and C–H σ bonds and no C=C or polar groups to attack.
- Microorganisms and enzymes cannot break these bonds, so the polymer is not broken down and persists in landfill.
- C2H3Cl + 2.5O2 → 2CO2 + H2O + HCl.
- n(repeat units) = 1000 ÷ 62.5 = 16.0 mol, so n(HCl) = 16.0 mol and mass = 16.0 × 36.5 = 584 g.
- Volume = 16.0 × 24.0 = 384 dm3.
- Landfill: poly(alkenes) are non-biodegradable, so they persist for centuries and use up land.
- Incineration: releases CO2 (global warming) and toxic CO if combustion is incomplete; PVC also releases toxic, acidic HCl that must be removed from the waste gases.
- Recycling: plastics must be collected and sorted by type, which is costly, and the polymer degrades with each cycle.
8 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
The diagram shows a section of an addition polymer. What is the monomer?
- A2-methylpropene, CH2=C(CH3)2
- Bpropene, CH2=CHCH3
- Cbut-2-ene, CH3CH=CHCH3
- D2-methylpropane, CH(CH3)3
Which is the repeat unit of poly(chloroethene), PVC?
- A[–CH2–CHCl–]n
- B[–CH2=CHCl–]n
- C[–CHCl–CHCl–]n
- D[–CH2–CH2–Cl–]n
Why are poly(alkenes) such as poly(ethene) non-biodegradable?
- Athey are saturated, with only strong non-polar C-C and C-H bonds, so they are chemically unreactive
- Bthey still contain C=C double bonds, which microorganisms cannot attack
- Cthey are giant ionic lattices with very strong electrostatic forces
- Dthey are toxic, so they kill the bacteria that would break them down
A sample of poly(propene) has an average Mr of 2.10 × 105. How many propene units are in an average chain? (Ar: C 12.0, H 1.0)
- A5.00 × 103
- B7.50 × 103
- C1.50 × 104
- D2.50 × 103
Poly(chloroethene), PVC, is burned in a waste incinerator. Which harmful gas is produced from PVC that is NOT produced when poly(ethene) burns?
- Ahydrogen chloride, a toxic and acidic gas
- Bcarbon monoxide, a toxic gas
- Csulfur dioxide, which causes acid rain
- Dnone, because PVC is saturated and burns cleanly
The diagram shows the monomer. Which is the repeat unit of the addition polymer it forms?
- A[–CH(CH3)–CH(CH3)–]n
- B[–CH2–CH=CH–CH2–]n
- C[–CH2–CH(CH3)–]n
- D[–CH(CH3)=CH(CH3)–]n
A section of an addition polymer made from two different monomers is: -CH2-CH2-CH2-CH(CH3)-CH2-CH2-CH2-CH(CH3)-. Which two monomers were used?
- Aethene and propene
- Bethene and but-1-ene
- Cpropene and but-2-ene
- Dethene and 2-methylpropene
1.00 kg of PVC waste, [-CH2-CHCl-]n, is burned completely. What volume of hydrogen chloride, measured at room conditions, is produced? (Mr: C2H3Cl 62.5; molar gas volume 24.0 dm3 mol−1)
- A384 dm3
- B192 dm3
- C768 dm3
- D16.0 dm3
Quiz key
Each 2-carbon unit is CH2–C(CH3)2; restoring the double bond gives CH2=C(CH3)2, 2-methylpropene. Propene would put only one CH3 on every second carbon, with an H opposite it. But-2-ene would give one CH3 on every backbone carbon, not two on alternate carbons. 2-methylpropane is an alkane: it has no C=C, so it cannot undergo addition polymerisation at all.
In addition polymerisation the π bond of C=C breaks and the two carbons become singly bonded, with a bond extending through each bracket to the next unit; the Cl stays on its own carbon as a side group: [–CH2–CHCl–]n. Keeping C=C in the repeat unit is wrong because polymers are saturated. [–CHCl–CHCl–]n would need the monomer CHCl=CHCl, 1,2-dichloroethene. Putting Cl in the backbone as -CH2-CH2-Cl- is impossible: only the two C=C carbons form the chain, and Cl forms only one bond.
Polymerisation uses up every C=C, so a poly(alkene) is essentially a very long alkane. Its C-C and C-H bonds are strong and non-polar, giving no site for enzymes or reagents to attack, so it persists for centuries in landfill. Poly(alkenes) have no C=C bonds left (they do not decolourise bromine water). They are covalent molecules held by intermolecular forces, not ionic lattices. Their persistence comes from unreactivity, not toxicity: poly(ethene) is used for food packaging precisely because it is inert.
Mr(C3H6) = 3(12.0) + 6(1.0) = 42.0. No atoms are lost in addition polymerisation, so n = 2.10 × 105 ÷ 42.0 = 5.00 × 103. 7.50 × 103 divides by 28.0, the Mr of ethene, the wrong monomer. 1.50 × 104 divides by 14.0, the CH2 empirical unit, which counts CH2 groups, not monomers. 2.50 × 103 divides by 84.0, wrongly treating two monomers as one repeat unit.
PVC contains chlorine: C2H3Cl + 2.5O2 → 2CO2 + H2O + HCl per repeat unit. The HCl is toxic and forms acid in moist air, so incinerators must scrub it out with a base. Carbon monoxide is harmful, but any poly(alkene) gives CO when oxygen is limited, so it is not special to PVC. Neither polymer contains sulfur, so no SO2 forms. Being saturated makes PVC unreactive at room temperature; it does not stop it burning or producing HCl.
The π bond of but-2-ene breaks, the two C=C carbons become joined by a single bond, and each keeps its H and its CH3 as side groups: [–CH(CH3)–CH(CH3)–]n, a 2-carbon backbone with a methyl branch on each carbon. [–CH2–CH=CH–CH2–]n wrongly puts the methyl carbons into the chain and keeps a double bond; that is the unit of poly(buta-1,3-diene), not of an alkene with one C=C. [–CH2–CH(CH3)–]n is poly(propene), which has only one methyl group per unit. Keeping the double bond in the repeat unit is wrong because the polymer is saturated.
Splitting the backbone into pairs gives CH2-CH2 (from ethene, CH2=CH2) alternating with CH2-CH(CH3) (from propene, CH2=CHCH3). But-1-ene would give units of CH2-CH(C2H5), with an ethyl branch, not a methyl. Propene and but-2-ene would leave no unbranched CH2-CH2 pairs, and but-2-ene units, CH(CH3)-CH(CH3), have a methyl on both carbons, which never occurs here. 2-methylpropene would give C(CH3)2, two methyls on one carbon, which does not appear.
C2H3Cl + 2.5O2 → 2CO2 + H2O + HCl. Amount of repeat units = 1000 ÷ 62.5 = 16.0 mol, so 16.0 mol of HCl forms, and 16.0 × 24.0 = 384 dm3. 192 dm3 halves the amount, as if the chlorine left as Cl2 (one Cl2 per two units). 768 dm3 doubles it, as if each unit had two Cl atoms. 16.0 dm3 is the amount in moles mistakenly written as a volume.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.