GCSE Chemistry revision

Condensation polymerisation

Synthetic and naturally occurring polymers (chemistry only)

AQA 4.7.3.2 Higher Tier only

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Condensation polymers are polymers formed from many individual monomers joining together whilst releasing a small molecule, usually water.
The process is called condensation polymerisation because water is released, similar to how gaseous water condenses.
Unlike addition polymers, condensation polymers typically use two different types of monomer that react together.

1. Introduction AQA

Condensation polymers are polymers formed from many individual monomers joining together whilst releasing a small molecule, usually water.
The process is called condensation polymerisation because water is released, similar to how gaseous water condenses.
Unlike addition polymers, condensation polymers typically use two different types of monomer that react together.

A dicarboxylic acid monomer contains two carboxylic acid (–COOH) functional groups, one at each end of the molecule.
A diol monomer contains two alcohol (–OH) functional groups, one at each end of the molecule.
Having two functional groups on each monomer is essential, as it allows the chain to extend in both directions to form a long polymer.

2. The Two Monomers Used to Make Polyesters AQA

A dicarboxylic acid monomer contains two carboxylic acid (–COOH) functional groups, one at each end of the molecule.
A diol monomer contains two alcohol (–OH) functional groups, one at each end of the molecule.
Having two functional groups on each monomer is essential, as it allows the chain to extend in both directions to form a long polymer.

During the reaction, the dicarboxylic acid loses its –OH group and the diol loses a hydrogen atom from its –OH group.
These three atoms (O, H, and H) combine to form a water molecule (H₂O), which is released as a by-product.
The carbon from the dicarboxylic acid then bonds directly to the oxygen from the diol, forming the ester link (–COO–).
The ester link is the characteristic bond in polyesters and gives this class of polymer its name.

3. How the Ester Link Forms AQA

During the reaction, the dicarboxylic acid loses its –OH group and the diol loses a hydrogen atom from its –OH group.
These three atoms (O, H, and H) combine to form a water molecule (H₂O), which is released as a by-product.
The carbon from the dicarboxylic acid then bonds directly to the oxygen from the diol, forming the ester link (–COO–).
The ester link is the characteristic bond in polyesters and gives this class of polymer its name.

A dimer is formed when just two monomers combine, but to show the repeating unit, the – OH and –H at the ends are removed, releasing another water molecule.
Large brackets are drawn around the repeating unit, cutting through the bonds at each end to show that the unit continues in both directions.
The letter n is placed in front of each reactant monomer and in the bottom right corner of the repeating unit to represent the large number of monomers involved.
Because two water molecules are produced per repeating unit, a coefficient of 2n is placed in front of H₂O in the equation.

4. Writing the Repeating Unit AQA

A dimer is formed when just two monomers combine, but to show the repeating unit, the – OH and –H at the ends are removed, releasing another water molecule.
Large brackets are drawn around the repeating unit, cutting through the bonds at each end to show that the unit continues in both directions.
The letter n is placed in front of each reactant monomer and in the bottom right corner of the repeating unit to represent the large number of monomers involved.
Because two water molecules are produced per repeating unit, a coefficient of 2n is placed in front of H₂O in the equation.

Each monomer must have at least two functional groups so that bonds can form on both sides, allowing the chain to grow.
At least two different types of functional group must be present overall (e.g. carboxyl and hydroxyl groups) so that the monomers can react with each other.
A small molecule, typically water, must be eliminated during each bond-forming step.
A Real-Life Example: Poly(ethyl ethanoate) Ethanedioic acid (a dicarboxylic acid) reacts with ethanediol (a diol) to form the polyester poly(ethyl ethanoate) and water.
This real example follows the same general pattern: ester links form between the monomers and water molecules are released as by-products.

5. Conditions Required for Condensation Polymerisation AQA

Each monomer must have at least two functional groups so that bonds can form on both sides, allowing the chain to grow.
At least two different types of functional group must be present overall (e.g. carboxyl and hydroxyl groups) so that the monomers can react with each other.
A small molecule, typically water, must be eliminated during each bond-forming step.
A Real-Life Example: Poly(ethyl ethanoate) Ethanedioic acid (a dicarboxylic acid) reacts with ethanediol (a diol) to form the polyester poly(ethyl ethanoate) and water.
This real example follows the same general pattern: ester links form between the monomers and water molecules are released as by-products.

Polyesters are generally biodegradable, meaning bacteria and other microorganisms can break down the ester links naturally.
This is a significant advantage over addition polymers such as plastics, which are generally not biodegradable and persist in the environment for a very long time.

6. Biodegradability of Polyesters AQA

Polyesters are generally biodegradable, meaning bacteria and other microorganisms can break down the ester links naturally.
This is a significant advantage over addition polymers such as plastics, which are generally not biodegradable and persist in the environment for a very long time.