CIE A Level Chemistry (9701) exams from 2022

Revision Notes

7.7.5 Degradabiity of Polymers

Poly(alkenes) & Biodegradability

  • Many of the polymers in use have been produced through addition polymerisation of alkenes
  • The (poly)alkene chains are non-polar and saturated
  • This makes them chemically inert and therefore non-biodegradable
  • (poly)alkenes can be melted and recycled into new uses
    • However, even in the new applications, the (poly)alkenes are not biodegradable
  • Recycling plants can burn used plastic materials
    • The energy released from burning can be used to generate electricity
    • Burning plastics in oxygen releases carbon dioxide and water (complete combustion) which can contribute to global warming

Photodegradation of Polymers

  • Polyesters and polyamides are biodegradable polymers for a number of reasons
  • One such reason is their ability to breakdown with the use of light
  • Carbonyl groups (C=O) along polymer chains are able to absorb energy from the Electromagnetic Spectrum
    • In particular Ultraviolet (UV) light
  • Absorbing UV light weakens the carbonyl areas of polymers and breaks them down into smaller molecules

Disadvantages of photo degradability

  • Despite this ability being a great advantage of polyesters and polyamides, it may pose a problems when the polymers are repurposed
  • When applied to a new use, the biodegradability could give a weaker polymer
  • Breaking down polymers also poses another challenge
    • Once used, polymeric materials are taken to landfill sites where many other materials are piled on top of each other
    • This could mean that photodegradable polyesters or polyamides do not have access to UV light in order to break down naturally

Biodegrading Polyesters & Polyamides

Biodegradable polymers

  • Both polyesters and polyamides can be broken down using hydrolysis reactions
  • This is a major advantage over the polymers produced using alkene monomers (polyalkenes)
  • When polyesters and polyamides are taken to landfill sites, they can be broken down easily and their products used for other applications

Hydrolysis of polyamides

  • Hydrolysis is a breaking up of a molecules using water
  • In acidic hydrolysis, acid (such as hydrochloric acid) acts as the catalyst
    • Polyamides are heated with dilute acid
    • This reaction breaks the polyamide into carboxylic acid molecules and ammonium chloride ions
  • Alkaline hydrolysis
    • The polyamide is heated with a species containing hydroxide ions (eg. sodium hydroxide)
    • This breaks the polymer into the sodium salts of its monomers (dicarboxylic acids and diamines)
    • If the poly amide link used an aminocarboxylic acid as the monomer, then a sodium salt of the original amino acid is reformed

Polymerisation - Amide Hydrolysis, downloadable AS & A Level Chemistry revision notes

When polyamides are degraded by hydrolysis, carboxylic acids and amines are formed

Hydrolysis of polyesters

  • Ester linkages can also be degraded through hydrolysis reactions
  • Acid hydrolysis forms the alcohols and carboxylic acids that were used to form the polyesters

 

Polymerisation - Ester Hydrolysis, downloadable AS & A Level Chemistry revision notes

When polyesters are degraded by hydrolysis, carboxylic acids and alcohols are formed

Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.
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