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Questions on condensation polymers and the disposal of polymers

Condensation polymerisation compared with addition polymerisation; polyesters, including Terylene (PET) from benzene-1,4-dicarboxylic acid and ethane-1,2-diol and poly(lactic acid); polyamides, including nylon-6,6, nylon-6 and Kevlar; drawing repeating units and finding monomers; hydrogen bonding and other forces between chains and the properties they give; hydrolysis of ester and amide links by acid and alkali; biodegradability compared with polyalkenes; disposal by landfill, incineration and recycling, including chemical recycling of PET; and, for the boards that set them, predicting the type of polymerisation from a monomer and photodegradable polymers.

19 original questions · 61 marks · the condensation polymers and the disposal of polymers notes · Carbonyls and carboxylic acids · Flashcards for this unit

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• 6 marks · Evaluate · CPD-Q12

  Evaluate a supermarket's plan to replace its poly(ethene) carrier bags with bags made from a polyester, with reference to the bonds in each polymer and to how each can be disposed of.

  Mark scheme

  Indicative content

  1. poly(ethene) has a chain of C–C and C–H bonds only, strong and non-polar, so it is chemically inert and is not biodegraded; in landfill it persists for centuries
  2. the polyester chain contains ester links, whose δ+ carbonyl carbon is attacked by water, acids and alkalis, so the polymer can be hydrolysed
  3. hydrolysis breaks the polyester back into its diol and its dicarboxylic acid, or its salt in alkali, so it can be chemically recycled to monomer or biodegraded
  4. in a dry, oxygen-poor landfill even a polyester breaks down slowly, so the gain depends on where the used bags are sent
  5. the hydrolysis that helps disposal can weaken a polyester bag in use, in contact with damp, acidic or alkaline goods, while poly(ethene) is unaffected by them
  6. both polymers can be burned to release energy, giving carbon dioxide, so a justified conclusion favours the polyester only if its waste is hydrolysed or composted rather than buried

  Levels

  • Level 3 (5–6 marks) the inert C–C chain of poly(ethene) and the hydrolysable ester link of the polyester are both explained, the hydrolysis products are named, and the two bags are compared in landfill, incineration and recycling; the judgement allows for how slowly even a polyester breaks down when buried, or for the weakening of the bag in use
  • Level 2 (3–4 marks) the ester link is said to be hydrolysed and poly(ethene) to be non-biodegradable, and some disposal routes are compared; the answer usually stops short of the products of hydrolysis or of any drawback of the polyester, so its verdict is one-sided
  • Level 1 (1–2 marks) the polyester bag is preferred because it is 'natural' or 'rots', or poly(ethene) is said to break down slowly, with no mention of the ester link or of the bonds in either chain

• 4 marks · Give · CPD-Q3

  Give the names and formulae of the two monomers used to make nylon-6,6, and the products of its complete hydrolysis by hot dilute hydrochloric acid.

  Mark scheme

  1. B1 hexanedioic acid, HOOC(CH₂)₄COOH; allow hexanedioyl dichloride, ClOC(CH₂)₄COCl
  2. B1 the other monomer is hexane-1,6-diamine, H₂N(CH₂)₆NH₂
  3. B1 hydrolysis gives hexanedioic acid, HOOC(CH₂)₄COOH
  4. B1 and the hexane-1,6-diammonium ion, as its chloride, because the amine groups are protonated by the acid

• 4 marks · Draw · CPD-Q4

  Draw the repeating unit of the polymer formed from ethane-1,2-diol and butanedioic acid, and give the type of polymer and the small molecule lost.

  Mark scheme

  1. B1 –OCCH₂CH₂COOCH₂CH₂O–, one residue of each monomer
  2. B1 the ester link drawn in full with C=O displayed, and an open bond at each end; ends closed with H or OH are not credited
  3. B1 the polymer is a polyester
  4. B1 the small molecule lost is water, two per repeating unit, one per ester link

• 4 marks · Compare · CPD-Q11

  Compare addition polymerisation with condensation polymerisation.

  Mark scheme

  1. B1 addition monomers have a C=C bond, while condensation monomers have two functional groups each
  2. B1 addition gives no other product, while condensation loses a small molecule such as water or HCl at each link
  3. B1 an addition polymer's main chain is carbon only, while a condensation polymer's chain contains ester or amide links with O or N
  4. B1 condensation polymers can be hydrolysed and are biodegradable, while addition polymers are not

• 4 marks · Suggest · CPD-Q19 CAIE

  Suggest the type of polymerisation shown by propenamide, CH₂=CHCONH₂, and by 4-aminobutanoic acid, H₂N(CH₂)₃COOH, and give the repeating unit of each polymer.

  Mark scheme

  1. B1 propenamide undergoes addition polymerisation, through its C=C bond
  2. B1 its repeating unit is –CH₂CH(CONH₂)–, with the amide group as a side chain
  3. B1 4-aminobutanoic acid undergoes condensation polymerisation, its NH₂ reacting with the COOH of the next molecule to give a polyamide
  4. B1 its repeating unit is –NH(CH₂)₃CO–, and water is lost at each link

• 3 marks · State · CPD-Q2

  State what is meant by condensation polymerisation, and state the type of link, with its structure, in a polyester and in a polyamide.

  Mark scheme

  1. B1 monomers with two functional groups join into a polymer with the loss of a small molecule, such as water, at each link
  2. B1 a polyester has ester links, –COO–
  3. B1 a polyamide has amide links, –CONH–

• 3 marks · Identify · CPD-Q5

  Identify the two monomers that form the polymer with repeating unit –OC(CH₂)₃COOCH₂CH₂O–, and the type of polymer.

  Mark scheme

  1. B1 pentanedioic acid, HOOC(CH₂)₃COOH; allow its diacyl chloride
  2. B1 the other monomer is ethane-1,2-diol, HOCH₂CH₂OH
  3. B1 the polymer is a polyester, because the links are ester links, –COO–

• 3 marks · Explain · CPD-Q6

  Explain why Kevlar fibres are very strong.

  Mark scheme

  1. B1 Kevlar is a polyamide whose chains contain N–H and C=O groups in the amide links
  2. B1 hydrogen bonds form between the N–H of one chain and the C=O oxygen of a neighbouring chain
  3. B1 the chains are straight and rigid because of the 1,4-linked benzene rings, so they pack closely and many hydrogen bonds form along their length

• 3 marks · Explain · CPD-Q7

  Explain why polyesters are biodegradable but poly(ethene) is not.

  Mark scheme

  1. B1 the ester links in a polyester contain a polar C=O bond with a δ+ carbon
  2. B1 water or hydroxide ions attack that carbon as nucleophiles, so the links can be hydrolysed, catalysed by enzymes from microorganisms
  3. B1 poly(ethene) has only strong, non-polar C–C and C–H bonds, so there is no site for attack

• 3 marks · Calculate · CPD-Q9

  Calculate the mean number of repeating units in a Terylene chain of Mr 38 400. The repeating unit is –OCC₆H₄COOCH₂CH₂O–.

  Mark scheme

  1. M1 the repeating unit is C₁₀H₈O₄
  2. M1 Mr of the unit = (10 × 12.0) + (8 × 1.0) + (4 × 16.0) = 192.0
  3. A1 number of units = 38 400 ÷ 192.0 = 200, a whole number with no unit; allow 2.00 × 10²

• 3 marks · Calculate · CPD-Q10

  Calculate the mass of water released when 2.00 kg of nylon-6,6 is made from hexanedioic acid and hexane-1,6-diamine. The repeating unit has Mr 226.0 and contains two amide links.

  Mark scheme

  1. M1 amount of repeating units = 2000 ÷ 226.0 = 8.850 mol
  2. M1 two water molecules are lost per repeating unit, so 17.70 mol of water
  3. A1 mass of water = 17.70 × 18.0 = 319 g, to 3 significant figures

• 3 marks · Suggest · CPD-Q14

  Suggest the type of polymer formed by 4-hydroxybutanoic acid, HOCH₂CH₂CH₂COOH, on its own, its repeating unit, and whether it is biodegradable.

  Mark scheme

  1. B1 it is a polyester, because the OH of one molecule reacts with the COOH of the next
  2. B1 the repeating unit is –OCH₂CH₂CH₂CO–
  3. B1 it is biodegradable, because its ester links can be hydrolysed

• 3 marks · Describe · CPD-Q16

  Describe the products of heating Terylene with aqueous sodium hydroxide, and how benzene-1,4-dicarboxylic acid is then recovered.

  Mark scheme

  1. B1 one product is ethane-1,2-diol, HOCH₂CH₂OH
  2. B1 the other is the sodium salt of benzene-1,4-dicarboxylic acid, NaOOCC₆H₄COONa
  3. B1 adding a strong acid protonates the carboxylate ions, giving benzene-1,4-dicarboxylic acid

• 3 marks · Explain · CPD-Q18 OCR A · CAIE

  Explain how a photodegradable polyalkene breaks down, and why a photodegradable bag buried in landfill does not.

  Mark scheme

  1. B1 the chains contain a few C=O groups, which absorb ultraviolet light
  2. B1 the energy absorbed breaks chain bonds next to the C=O groups, so the polymer breaks into small fragments
  3. B1 buried in landfill, no light reaches the bag, so the bonds are not broken

• 3 marks · Calculate · CPD-Q20

  Calculate the maximum mass of ethane-1,2-diol, Mr 62.0, recovered when 2.40 kg of PET, repeating unit –OCC₆H₄COOCH₂CH₂O– of Mr 192.0, is completely hydrolysed by aqueous sodium hydroxide.

  Mark scheme

  1. M1 amount of repeating units = 2400 ÷ 192.0 = 12.5 mol
  2. M1 each repeating unit gives one molecule of ethane-1,2-diol, so 12.5 mol of diol forms
  3. A1 mass = 12.5 × 62.0 = 775 g, to 3 significant figures

• 3 marks · Justify · CPD-Q21

  Justify the description of the polymer whose chain is –NH(CH₂)₅CO–NH(CH₂)₅CO–NH(CH₂)₅CO– as a polyamide made by condensation from a single monomer, and give the monomer.

  Mark scheme

  1. B1 the link that repeats along the chain is –CONH–, the amide link, so the polymer is a polyamide
  2. B1 the repeating unit –NH(CH₂)₅CO– holds one residue only, with the N at one end and the C=O at the other, so every monomer molecule must carry both an NH₂ group and a COOH group, and one kind of molecule is enough
  3. B1 the monomer is 6-aminohexanoic acid, H₂N(CH₂)₅COOH, and a molecule of water is lost as each amide link forms, which makes the polymerisation a condensation

• 2 marks · Explain · CPD-Q8

  Explain why nylon-6,6 has a much higher melting point than poly(ethene).

  Mark scheme

  1. B1 nylon chains are held by hydrogen bonds between N–H and C=O groups on neighbouring chains
  2. B1 poly(ethene) chains are held only by weaker van der Waals forces, so less energy is needed to separate them

• 2 marks · Suggest · CPD-Q13

  Suggest why waste PVC is a particular problem for incineration.

  Mark scheme

  1. B1 PVC contains chlorine, so burning it produces hydrogen chloride
  2. B1 HCl is toxic and acidic, so it has to be removed from the waste gases, for example by a base, adding cost

• 2 marks · Suggest · CPD-Q15

  Suggest why a biodegradable polyester cup may still be found intact after many years in a landfill site.

  Mark scheme

  1. B1 landfill waste is compacted, dry and short of oxygen
  2. B1 so few microorganisms are active to catalyse the hydrolysis of the ester links

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