Condensation Polymers (AQA A Level Chemistry)

• 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, the OH group from the COOH on the acid and an H atom from the alcohol are expelled as a water molecule

• 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

• Diacyl (or dioyl) dichlorides can also used to react with the diamine instead of the acid
  • An acid chloride or acyl chloride is a carboxylic acid where the -OH has been replaced by a chlorine  
    • A dioyl dichloride or diacyl dichloride 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

Answer:

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

• The reaction between the amine group and the carboxylic acid is slow
• Consequently, the dicarboxylic acid is usually first converted to a diacyl dichloride which reacts with the diamine much faster

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-benzenediamine
  • 1,4-benzenedicarboxylic acid

• As seen with Nylon, a diacyl dichloride can be used instead of the acid

Kevlar is made using a diamine and dicarboxylic acid monomers

Intermolecular Forces

• Condensation polymers such as Nylon-6,6, Kevlar® and Nomex® are long chain molecules which can be drawn out to form fibres
• During this process the linear molecules align and become increasingly linked by hydrogen bonds between adjacent chains
• The causes the strength of the fibre to increase during the drawing process
• Between Kevlar® and Nomex® two aramid polymers, Kevlar® has a higher melting point as the chains are straighter and lie closer together, meaning more effective hydrogen bonds are formed

Hydrogen bonding present between Kevlar® polymer chains