CIE A Level Chemistry复习笔记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

7.7-Polymerisation-Amide-Hydrolysis

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

7.7-Polymerisation-Ester-Hydrolysis

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

 

 

 

 

 

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