Condensation Polymers (HL) (HL IB Chemistry)

What is condensation polymerisation?

• 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 links
• Condensation polymers can be formed by:
  • Dicarboxylic acids and diols
  • Hydroxycarboxylic acids
  • Dicarboxylic acids and diamines
  • Amino acids

What is polyester?

• Polyester is a polymer, most commonly formed by the condensation polymerisation of dicarboxylic acid monomers and diol monomers
• A polyester is produced by linking these monomers with ester bonds / links

Ester Link in a Polyester

This polymer structure shows an ester functional group linking monomers together

Formation of polyesters using diols and dicarboxylic acids

• A diol and a dicarboxylic acid are required to form a polyester
  • A diol contains two alcohol –OH groups
  • A dicarboxylic acid contains two carboxylic acid –COOH groups

Diol and dicarboxylic acid monomer structures 

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

• When the polyester is formed, one of the diol –OH groups and a hydrogen atom from one of the carboxylic acid –COOH  groups are eliminated as a water molecule (H₂O)
• The resulting polymer is a polyester
  • For example, the polyester poly(ethylene terephthalate) or PET, which is sometimes known by its brand names of Terylene or Dacron
    
    

  Formation of Poly(ethylene terephthalate)

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

Formation of polyesters using 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 –OH group at one end of the molecule and a carboxylic acid –COOH group at the other end of the molecule

Formation of polyesters from hydroxycarboxylic acids

Both functional groups that are needed to form the ester link of the polyester come from the same monomer

What are polyamides?

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

Amide links in Polyamides

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

Polyamide monomers

The monomers for making polyamides

Formation of polyamides

Formation of Polyamides from Monomers

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

Biodegradable polymers

• Both polyesters and polyamides can be broken down using hydrolysis reactions
• This is a major advantage over the addition 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 occurs during
  • Digestion
  • Decomposition
• Both condensation and hydrolysis reactions are controlled by enzymes

Diagram to show hydrolysis of condensation polymers

Water is added which causes the polymer to break down into the original monomers