Condensation Polymers (HL) (DP IB Chemistry): Revision Note

Condensation polymers

What is condensation polymerisation?

• In condensation polymerisation, monomers join together and release a small molecule as a by-product

  • The most common small molecules are water or hydrogen chloride

• Example of condensation polymers include:

  • Nylon-6,6 is a synthetic polymer used in textiles and engineering plastics

  • Proteins are natural polymers formed from amino acids

  • Polyesters are synthetic polymers

• Condensation polymers often contains either:

  • Ester links (-COO-)

    • Formed between a carboxylic acid and an alcohol group

    • Commonly found in polyesters

  • Amide links (-CONH-)

    • Formed between a carboxylic acid and an amine group

    • Commonly found in polyamides, such as proteins and nylon

• Properties of condensation polymers include:

  • Strong – like addition polymers, they are held together by strong covalent bonds

  • Biodegradable – some condensation polymers can break down over time, especially those with ester or amide bonds

  • Versatile – can be tailored for flexibility, toughness, or water resistance depending on the monomers used

  These properties make condensation polymers useful in clothing, packaging, biomedical materials, and biodegradable plastics.

What is a polyester?

• A polyester is a condensation polymer formed by linking monomers with ester bonds / links (-COO-)

• Polyesters can be formed from:

  • Diols and dicarboxylic acids

    • Diols contains two alcohol –OH groups

    • Dicarboxylic acids contains two carboxylic acid –COOH groups

  

  • Hydroxycarboxylic acids

    • Hydroxycarboxylic acids have –COOH and –OH groups on the same molecule

Visualising polyester formation

• The formation of a polyester requires the reaction of:

  • A hydrogen from the diol –OH groups

  • An -OH group from the dicarboxylic acids

• This results in:

  • Water molecules being eliminated

  • Ester links in the resulting polyester

• For example, the polyester poly(ethylene terephthalate) or PET

  • PET has brand names of Terylene or Dacron

• Hydroxycarboxylic acids can also form polyesters, by the reaction of:

  • A hydrogen from the –OH group

  • The -OH group from the carboxylic acid

• For example, the polyester poly(2-hydroxybutanoate) or PHB

  • PHB is also known as poly(2-hydroxybutanoic acid) and polyhydroxybutyrate

What are polyamides?

• A polyamide is a condensation polymer formed by linking monomers with amide / peptide links (-CONH-)

• Polyamides can be formed from:

  • Diamines and dicarboxylic acids

    • Diamines contains two amine –NH₂ groups

    • Dicarboxylic acids contains two carboxylic acid –COOH groups

  

  • Diamines and dioyl dichlorides

    • Dioyl chloride contains 2 –COCl groups

    • This monomer is more reactive and more expensive than dicarboxylic acid

  

  • Amino acids

    • Amino acids have -COOH and -NH₂ groups on the same molecule

  

Visualising polyamide formation

• The formation of a polyamide requires the reaction of:

  • A hydrogen from the diamine –NH₂ groups

  • An -OH group from the dicarboxylic acids

• This results in:

  • Water molecules being eliminated

  • Amide links in the resulting polyamide

  

• Amino acids can also form polyamides, by the reaction of:

  • A hydrogen from the –NH₂ group

  • The -OH group from the carboxylic acid

• The amide link (–CONH–) is called a peptide bond when it is formed between amino acids

Biodegradable polymers

• Polyesters and polyamides can be broken by hydrolysis 

  • This is possible because of the ester / amide bonds, which are susceptible to water attack

• This is a major advantage over addition polymers

  • Addition polymers like poly(ethene) cannot be hydrolysed and persist in the environment

• Hydrolysis occurs during

  • Digestion

  • Decomposition

• When polyesters and polyamides are taken to landfill sites, they can be broken down easily and their products used for other applications

• Both condensation and hydrolysis reactions are controlled by enzymes

Comparing addition and condensation polymerisation

• Addition polymerisation

  • Uses monomers with carbon–carbon double bonds

  • Forms no by-products

  • Produces polymers like poly(ethene) that have saturated carbon backbones.

• Condensation polymerisation

  • Uses monomers with two different functional groups (e.g. –COOH and –NH₂)

  • Releases a small molecule like water as a by-product

  • Produces polymers like nylon and proteins that contain amide or ester links between the monomers.