Condensation Polymers (OCR A Level Chemistry)

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Condensation Polymers

  • Addition polymerisation has been covered in reactions of alkenes
    • They are made using monomers that have C=C double bonds joined together to form polymers such as polyethene

  • Condensation polymerisation is another type of reaction whereby a polymer is produced by repeated condensation reactions between monomers
  • Natural condensation polymers are all formed by elimination of water
    • Although the process of condensation polymerisation involves the elimination of a small molecule

  • Condensation polymers can be identified because the monomers are linked by ester or amide bonds
  • Condensation polymers can be formed by:
    • dicarboxylic acids and diols
    • dicarboxylic acids and diamines
    • amino acids

Polyester

  • Is formed by the reaction between dicarboxylic acid monomers and diol monomers
  • Polyester is produced by linking these monomers with ester bonds / links

This polymer structure shows an ester functional group linking monomers together

Formation of polyesters

  • A diol and a dicarboxylic acid are required to form a polyester
    • A diol contains 2 -OH groups
    • A dicarboxylic acid contains 2 -COOH groups

The position of the functional groups on both of these molecules allows condensation polymerisation to take place effectively

  • When the polyester is formed, one of the -OH groups on the diol and the hydrogen atom of the -COOH are expelled as a water molecule (H2O)
  • The resulting polymer is a polyester
    • In this example, the polyester is poly(ethylene terephthalate) or PET, which is sometimes known by its brand names of Terylene or Dacron

 

7-7-polymerisation-making-teryle

Expulsion of a water molecule in this condensation polymerisation forms the polyester called (ethylene terephthalate) (PET)

Formation of polyesters - hydroxycarboxylic acids

  • So far the examples of making polyesters have focused on using 2 separate monomers for the polymerisation
  • There is another route to making polyesters
  • A single monomer containing both of the key functional groups can also be used
  • These monomers are called hydroxycarboxylic acids
    • They contain an alcohol group (-OH) at one end of the molecule while the other end is capped by a carboxylic acid group (-COOH)

Both functional groups that are needed to make the polyester come from the same monomer

Polyamides

  • Polyamides are polymers where repeating units are bonded together by amide links
  • The formula of an amide group is -CONH

    An amide link - also known as a peptide link - is the key functional group in a polyamide

Polyamide monomers

  • A diamine and a dicarboxylic acid are required to form a polyamide
    • A diamine contains 2 -NH2 groups
    • A dicarboxylic acid contains 2 -COOH groups

  • Dioyl dichlorides can also used to react with the diamine instead of the acid
    • A dioyl chloride contains 2 -COCl groups
    • This is a more reactive monomer but more expensive than dicarboxylic acid

example-dioyl-chloride-monomer

The monomers for making polyamides

Formation of polyamides

This shows the expulsion of a small molecule as the amide link forms

Amino acids - formation of proteins

  • Proteins are vital biological molecules with varying functions within the body
  • They are essentially polymers made up of amino acid monomers
  • Amino acids have an aminocarboxylic acid structure
  • Their properties are governed by a branching side group - the R group

Amino acids contain an amine group, an acid group and a unique R group

 

  • Different amino acids are identified by their unique R group
  • The names of each amino acid is given using 3 letters
  • For example Glutamine is known as ‘Gln’
  • Dipeptides can be produced by polymerising 2 amino acids together
    • The amine group (-NH2) and acid group (-COOH) of each amino acid is used to polymerise with another amino acid

  • Polypeptides are made through polymerising more than 2 amino acids together

Dipeptides and polypeptides are formed by polymerising amino acid molecules together

Acid & Base Hydrolysis

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 the breakdown of molecules using water
  • In acidic hydrolysis, an acid (such as hydrochloric acid) acts as the catalyst
    • Polyamides such as Kevlar are heated with dilute acid
    • This reaction breaks the polyamide into a dicarboxylic acid and ammonium 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 acid salt and diamines)

polyamide-hydrolysis

Hydrolysis of Kevlar, a polyamide

Hydrolysis of polyesters

  • Ester linkages can also be degraded through hydrolysis reactions
    • The acidic and alkaline hydrolysis of polyethylene terephthalate (PET) is shown below
  • Acid hydrolysis forms the diol and dicarboxylic acid that were used to form the polyesters
  • Alkaline hydrolysis forms the diol and dicarboxylic acid salt 

 

polyester-hydrolysis

Hydrolysis of polyethylene terephthalate (PET), a polyester

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Philippa

Author: Philippa

Expertise: Chemistry

Philippa has worked as a GCSE and A level chemistry teacher and tutor for over thirteen years. She studied chemistry and sport science at Loughborough University graduating in 2007 having also completed her PGCE in science. Throughout her time as a teacher she was incharge of a boarding house for five years and coached many teams in a variety of sports. When not producing resources with the chemistry team, Philippa enjoys being active outside with her young family and is a very keen gardener.