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5.5.6Unit 5. Transition Metals and Organic Nitrogen Chemistry

Polyamides and Poly(ethenol) — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions

5 min read 5 parts Lesson 36 of 43 Unit quiz

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Part 1 of 5Forming Polyamides

5.5.6 Polyamides and Poly(ethenol)

A polyamide is a condensation polymer with amide links.

Amide link
  • Reaction of an amine group –NH₂ with a carboxylic acid group –COOH or acyl chloride gives the amide link –CONH–.
Nylon-6,6 monomers
  • Hexane-1,6-diamine, H₂N(CH₂)₆NH₂, reacts with hexanedioic acid, HOOC(CH₂)₄COOH, or its diacyl chloride.
Diacyl chloride route
  • Using hexanedioyl dichloride is faster at room temperature and loses hydrogen chloride instead of water.
Name meaning
  • The numbers 6,6 show that both monomers contain six carbon atoms.
LinkGroups joinedSmall molecule lost
Ester–COOH + –OHH₂O
Amide–COOH + –NH₂H₂O
Amide from acyl chloride–COCl + –NH₂HCl

Part 2 of 5Amino Acid Polyamides

1
A single monomer can carry both functional groups, such as 6-aminohexanoic acid, H₂N(CH₂)₅COOH.
2
The –NH₂ end of one molecule reacts with the –COOH end of another, forming an amide link and releasing water.
3
The chain keeps growing from both ends because every new unit leaves an –NH₂ and a –COOH group free.
4
The polymer formed is nylon-6, with repeat unit –NH(CH₂)₅CO–.
5
Proteins form in the same way from 2-amino acids, which is why they are described as natural polyamides.

Part 3 of 5Hydrolysis and Biodegradability

Hydrolysis and Biodegradability
Acid hydrolysis
  • Boiling with aqueous acid breaks the amide links and gives the dicarboxylic acid and the ammonium salt of the diamine.
Alkaline hydrolysis
  • Boiling with aqueous sodium hydroxide gives the sodium carboxylate salt and the free diamine.
Reason for breakdown
  • The C=O carbon of the amide is partly positive, so it is attacked by water or hydroxide ions.
Use
  • Polyamides can be broken down slowly and are therefore more biodegradable than polyalkenes, though still slow.

Part 4 of 5Poly(ethenol)

Why not from ethenol
  • Ethenol, CH₂=CHOH, is an unstable enol that rearranges to ethanal, so it cannot be polymerised.
Starting polymer
  • Poly(ethenyl ethanoate), made by addition polymerisation of ethenyl ethanoate, is the usual starting material.
Hydrolysis step
  • Hydrolysing the ester side groups with aqueous alkali converts –OCOCH₃ groups into –OH groups.
Properties
  • The many –OH groups hydrogen bond with water, so poly(ethenol) dissolves in water and is used in dissolvable packaging.

Part 5 of 5Exam Checklist & Tips

5.5.6 – Key Points for the Exam

  • Amide link The –CONH– group joining monomer units in a polyamide.
  • Nylon-6,6 A polyamide made from a six-carbon diamine and a six-carbon diacid or diacyl chloride.
  • Hydrolysis Breaking a bond by reaction with water, often catalysed by acid or alkali.
  • Poly(ethenol) A water-soluble polymer with an –OH on every second carbon of the chain.

Common Mistakes

  • Believing poly(ethenol) is made directly from ethenol; the enol is unstable and forms ethanal.
  • Writing the repeat unit of nylon-6,6 with only one monomer's carbons instead of both.
  • Saying acyl chloride routes release water; they release hydrogen chloride.
  • Stating all condensation polymers are biodegradable; they only break down more easily than addition polymers.

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