Condensation Polymers (Oxford AQA International A Level Chemistry)

Condensation Polymers

• Condensation polymerisation is type of reaction whereby a polymer is produced by repeated condensation reactions between monomers

• Natural condensation polymers are all formed by elimination of water

• 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

Polyesters

• Polyesters are formed by the reaction between dicarboxylic acid monomers and diol monomers

  • An ester bond / link is formed

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

• 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

  • An example is Terylene (also known as polyethylene terephthalate or 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 group (-OH) at one end of the molecule while the other end is capped by a carboxylic acid group (-COOH)

Polyamides

• Polyamides are polymers where repeating units are bonded together by amide links

• The formula of an amide group is -CONH

  

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  • An acid chloride or acyl chloride is a carboxylic acid where the -OH has been replaced by a chlorine  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

Monomers required to form polyamides

Formation of polyamides

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

• Dipeptides can be produced by polymerising two 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

General structure of an amino acid

Forming a polypeptide from amino acids

Intermolecular forces

• There are intermolecular forces between the polymer chains

• The properties vary depending on the type of intermolecular force

• Permanent dipole-dipole forces can form between polyesters

• Hydrogen bonds can form between polyamides such as nylon

  • This means the polymer chains can align and attract one another forming strong fibres

Intermolecular forces between polyester

Intermolecular forces between polyamide chains

Repeating Units in Polyesters & Polyamides

Terylene

• Terylene is a polyester

• Its monomers are:

  • Benzene-1,4-dicarboxylic acid

  • Ethane-1,2-diol

The structure of Terylene

Nylon 6,6

• Nylon 6,6 is a synthetic polyamide

• Its monomers are

  • 1,6-diaminohexane

  • 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

The structure of Nylon 6,6

Kevlar®

• Kevlar® is another example of a polymer formed through condensation polymerisation

• The polymer chains are neatly arranged with many hydrogen bonds between them

• 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

The structure of Kevlar®

Uses of Condensation Polymers

• Polyesters such as Terylene, also known as polyethylene terephthalate (or PET) is a thermoplastic which can be repeatedly heated to soften and melt it and cooled to solidify it

  • Terylene can be extruded to form fine fibres for use in artificial fabrics or moulded into fizzy drinks bottles and containers

• The best known example of an artificial polyamide is nylon

  • Nylon-6,6 contains a diamine and dicarboxylic acid, each of which contains six carbon atoms

  • Nylon-6,6 proved to be a cheap substitute for silk which is used to make ropes, twines, Velcro® and is often added to natural fibres in clothing and carpets to make them last longer

• Aromatic polyamides include Nomex® and Kevlar®

  • They are very tough and lightweight and used to make bulletproof vests (Kevlar®) and fireproof suits (Nomex®)