7.7.2 Formation of Polyamides

Amide link

• Polyamides are also formed using condensation polymerisation

An amide link - also known as a peptide link - is the key functional group in a 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 than dicarboxylic acid. However, a more expensive alternative

The monomers for making polyamides

Formation of polyamides

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

• Nylon 6,6 is a synthetic polyamide
• Its monomers are 1,6-diaminohexane and hexane-1,6-dioic acid
  • The ‘6,6’ part of its name arises from the 6 carbon atoms in each of Nylon 6,6 monomers

Nylon 6,6 is a synthetic polyamide made using diamine and dicarboxylic acid monomers

Kevlar

• Kevlar is another example of a polymer formed through condensation polymerisation
• The polymer chains are neatly arranged with many hydrogen bonds between them
• This results in a strong and flexible polymer material with fire resistance properties
• These properties also lend Kevlar to a vital application in bullet-proof vests
• The monomers used to make Kevlar
  • 1,4-diaminobenzene
  • Benzene-1,4-dicarboxylic acid

• As seen with Nylon, a dioyl chloride can be used instead of the acid as well (benzene-1,4-dioyl chloride)

Kevlar is made using a diamine and dicarboxylic acid monomers

Aminocarboxylic acids

• So far, condensation polymerisation has covered the use of monomers that contain 2 of the same functional group (eg. diamine, Diol etc.)
• It is possible to carry out a condensation polymerisation where one monomer provides both of the function groups necessary for an amide/peptide link
• For example 6-aminohexanoic acid has an amino group and a carboxylic acid group on the same molecule
• Molecules like this are called amino carboxylic acids
• They are able to polymerise to form a structure similar to Nylon 6,6

6-aminohexanoic acid can be polymerised to make the synthetic polymer Nylon 6,6

Making 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

Protein hydrolysis

• Proteins (polypeptides) can be broken down into its constituent amino acids
• This process occurs through a hydrolysis reaction

Proteins are broken down by hydrolysis