Formation of Polyamides (CIE A Level Chemistry)

Amide link

• Polyamides are also formed using condensation polymerisation

Section of a polyamide highlighting the amide links 

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

Examples of the monomers required to form polyamides

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

Formation of polyamides

Forming an amide link

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

Forming nylon 6,6

Nylon 6,6 is a synthetic polyamide made using specific 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 resistant 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)

Forming Kevlar

Kevlar is made using specific 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

Forming nylon 6,6 using a single monomer

6-aminohexanoic acid polymerises 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

The functionality of an amino acid

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 name 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 are used to polymerise with another amino acid
• Polypeptides are made by polymerising more than 2 amino acids together

Forming dipeptides and polypeptides

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

Hydrolysing proteins

Hydrolysis of proteins produces the component amino acids