6.6.1 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

 

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

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

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)

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 

 

Hydrolysis of polyethylene terephthalate (PET), a polyester