The shape of the lesson
By the end of the lesson, learners can
- Polyesters: esterification over and over: Watch the diacid and diol join in the simulator and note the water that leaves.
- Polyamides, Kevlar and proteins: The amide link forms when an NH 2 group reacts with a COOH or COCl group.
- Diol + diacid (or diacyl dichloride): polyester, –COO–.
- Diamine + diacid (or diacyl dichloride): polyamide, –CONH–.
- Lost: H2O from COOH, HCl from COCl.
- Repeat unit Mr = monomers − small molecules lost.
One picture every idea returns to
In condensation polymerisation monomers join with the loss of a small molecule at every link: H2O when a carboxylic acid is used, HCl when an acyl chloride is used. Each monomer needs two reactive groups. A diol plus a dicarboxylic acid (or diacyl dichloride) gives a polyester, for example PET (Terylene) from ethane-1,2-diol and benzene-1,4-dicarboxylic acid; a single hydroxycarboxylic acid such as lactic acid can also polymerise on its own, giving poly(lactic acid). A diamine plus a dicarboxylic acid or diacyl dichloride gives a polyamide, for example nylon-6,6 from hexane-1,6-diamine and hexanedioic acid, or Kevlar from aromatic monomers. Proteins are natural polyamides made from amino acids, joined by peptide (amide) links. The repeat unit contains one of each monomer residue, so its Mr is the sum of the monomers minus the small molecules lost: 116 + 146 − 2(18) = 226 for nylon-6,6.
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, Join the ends, lose the water.. Two monomers, each with a reactive group at both ends, join head to tail and spit out a small molecule at every link. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 17 min | Teach and check: Polyesters: esterification over and over | Watch the diacid and diol join in the simulator and note the water that leaves. Project tab 1 of the lab (Condensation polymers lab, polyester from monomers) and let a student drive it while the class predicts. | Whole class, then pairs |
| 17 to 30 min | Teach and check: Polyamides, Kevlar and proteins | The amide link forms when an NH 2 group reacts with a COOH or COCl group. Project tab 2 of the lab (Condensation polymers lab, polyamides) and let a student drive it while the class predicts. | Cooperative Concept Mapping |
| 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, 14 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 Polyamides, Kevlar and proteins, 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.
Cooperative Concept Mapping
Teams receive the key terms on cards and arrange them on a large sheet, drawing and labelling the links. Compare maps between teams before showing the model map.
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 |
|---|---|
| In a condensation polymer the repeat unit contains one residue of each monomer and the links inside the chain: do not leave H on the N end and OH on the C=O end of a repeat unit. | Every link releases one small molecule, so forming n repeat units of nylon-6,6 releases about 2n H 2 O, not n. With a diacyl dichloride the molecule lost is HCl, not water. |
Condensation polymerisation: worksheet
6 questions, 14 marks. Written in the style of the papers; no past paper question is reproduced.
PET is made from ethane-1,2-diol and benzene-1,4-dicarboxylic acid. (a) Name the type of polymerisation. (b) Name the linkage formed. (c) Draw the repeat unit.
Write the structure of the repeat unit of nylon-6,6 and state the small molecule lost when it is made from hexanedioyl dichloride.
Calculate the Mr of the repeat unit of Kevlar, C14H10N2O2, and the number of repeat units in a chain of Mr 47 600.
900 g of lactic acid (Mr 90.0) is fully polymerised to poly(lactic acid). Calculate the mass of polymer and the mass of water formed.
Explain why proteins are described as polyamides.
Explain why ethanol and ethanoic acid cannot form a polymer, but ethane-1,2-diol and hexanedioic acid can.
Answer key and marking notes
- (a) Condensation polymerisation.
- (b) Ester linkage.
- (c) –O–CH2CH2–O–CO–C6H4–CO–, with open bonds at both ends.
- –NH(CH2)6NHCO(CH2)4CO–.
- Hydrogen chloride, HCl.
- Mr = 14(12.0) + 10(1.0) + 2(14.0) + 2(16.0) = 238.
- 47 600 ÷ 238 = 200 repeat units.
- 10.0 mol of lactic acid gives 10.0 mol of repeat units, C3H4O2, Mr 72.0: 720 g of polymer.
- Water = 900 − 720 = 180 g (10.0 mol, one per link).
- Proteins are made from amino acids, each containing an NH2 group and a COOH group.
- The NH2 of one amino acid reacts with the COOH of the next, losing water.
- This forms an amide (peptide) link, –CONH–, repeated along the chain, so a protein is a condensation polyamide.
- Ethanol and ethanoic acid each have only one reactive group, so once the ester forms there is no free group left to react.
- The diol and diacid each have two reactive groups, so ester links form at both ends and the chain keeps growing.
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.
Which statement correctly describes condensation polymerisation?
- Amonomers with two reactive groups join together, and a small molecule such as water or HCl is eliminated at each link
- Bmonomers containing C=C bonds join together with no other product
- Ca polymer is broken down into monomers by adding water
- Dmonomers join together only when they are identical
PET (Terylene) is made from ethane-1,2-diol and benzene-1,4-dicarboxylic acid. Which is the repeat unit of PET?
- A–OCH2CH2OOCC6H4CO–
- B–OCH2CH2OH–HOOCC6H4COOH–
- C–CH2CH2–C6H4–
- D–NHCH2CH2NHCOC6H4CO–
Nylon-6,6 is made from hexane-1,6-diamine and hexanedioyl dichloride, ClOC(CH2)4COCl. Which small molecule is eliminated?
- Ahydrogen chloride, HCl
- Bwater, H2O
- Cammonia, NH3
- Dchlorine, Cl2
What is the Mr of the repeat unit of nylon-6,6, –NH(CH2)6NHCO(CH2)4CO–? (Ar: C 12.0, H 1.0, N 14.0, O 16.0)
- A226
- B262
- C244
- D113
Which compound can form a condensation polymer on its own, without a second monomer?
- A2-aminopropanoic acid, H2NCH(CH3)COOH
- Bethane-1,2-diol, HOCH2CH2OH
- Chexanedioic acid, HOOC(CH2)4COOH
- Dpropanoic acid, CH3CH2COOH
Monomers A and B react at room temperature to form polymer P, which forms strong, rigid fibres. Which row identifies P and the other product?
- AKevlar, a polyamide; HCl
- BKevlar, a polyamide; H2O
- CPET, a polyester; HCl
- Dnylon-6,6, a polyamide; HCl
A sample of nylon-6,6 (repeat unit Mr 226) has an average Mr of 33 900. How many molecules of water are eliminated in making one average chain from the dicarboxylic acid and diamine?
- Aabout 300
- Babout 150
- Cabout 75
- Dabout 600
Poly(lactic acid) is made from lactic acid, CH3CH(OH)COOH (Mr 90.0). What mass of polymer can be made from 450 g of lactic acid?
- A360 g
- B450 g
- C540 g
- D270 g
Quiz key
In condensation polymerisation each new link (ester or amide) releases a small molecule: water from a carboxylic acid group, HCl from an acyl chloride group. Joining C=C monomers with no by-product is addition polymerisation. Breaking a polymer with water is hydrolysis, the reverse process. Condensation polymers can form from one monomer (lactic acid, amino acids) or from two different monomers (a diol and a diacid), so identical monomers are not required.
Each ester link forms from an OH of the diol and a COOH of the acid, losing H2O, so the repeat unit is –OCH2CH2OOCC6H4CO–, containing one residue of each monomer. Keeping the full OH and COOH groups ignores the water that is eliminated. A chain of only CH2 and C6H4 has lost the oxygen atoms entirely. The NH version would be a polyamide made from a diamine, not a diol.
An acyl chloride reacts with an amine: the Cl from COCl and an H from NH2 leave as HCl, forming the amide link –CONH–. Water is eliminated only when the dicarboxylic acid, HOOC(CH2)4COOH, is used. Ammonia would mean losing the N, but the N stays in the amide link. Chlorine gas is not formed: Cl leaves bonded to H.
C12H22N2O2: 12(12.0) + 22(1.0) + 2(14.0) + 2(16.0) = 226. Equivalently 116 + 146 − 2(18) = 226, since two amide links form per repeat unit. 262 is the sum of the monomers with no water removed. 244 removes only one water. 113 is the repeat unit of nylon-6, half of 226.
An amino acid has NH2 at one end and COOH at the other, so molecules join head to tail by amide (peptide) links: this is how proteins form. Ethane-1,2-diol has two OH groups, which do not react with each other to form a polymer; it needs a diacid. Hexanedioic acid has two COOH groups and needs a diol or diamine. Propanoic acid has only one reactive group, so it cannot form a chain.
A is benzene-1,4-diamine and B is benzene-1,4-dicarbonyl dichloride. NH2 and COCl give amide links with loss of HCl, forming the aromatic polyamide Kevlar; its rigid, aligned chains hydrogen bond strongly, giving very strong fibres. Water is lost only if the dicarboxylic acid is used. PET is a polyester made from a diol, and there is no OH here. Nylon-6,6 is made from aliphatic monomers with (CH2) chains, not benzene rings.
33 900 ÷ 226 = 150 repeat units. Each repeat unit contains two amide links, and each link releases one H2O, so about 2 × 150 = 300 water molecules (strictly 299 for a chain with acid and amine ends). 150 counts one water per repeat unit, the classic mistake. 75 halves instead of doubling. 600 comes from dividing by 113 (half the repeat unit) and then doubling.
450 ÷ 90.0 = 5.00 mol of lactic acid gives 5.00 mol of repeat units, C3H4O2, Mr 72.0: 5.00 × 72.0 = 360 g, and 90.0 g of water is released. 450 g ignores the water lost. 540 g adds the water on instead of removing it. 270 g subtracts two waters per monomer, but each lactic acid forms only one new ester link.
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.