Condensation Polymers (Higher Tier)

Definition of Condensation Polymers

Condensation polymers are large molecules formed when monomers join together through condensation reactions. In these reactions, each time a bond forms between monomers, a small molecule such as water is lost. This contrasts with addition polymers, where monomers simply add together without losing any atoms or molecules.

In condensation polymerisation, the monomers have two functional groups that react to form a polymer chain with repeating units linked by ester or amide bonds.

Monomers for Condensation Polymers

Condensation polymers are made from either:

• Two different monomers, each with two reactive functional groups (e.g. a dicarboxylic acid and a diol)
• A single monomer containing two different functional groups (e.g. a molecule with both an alcohol and a carboxylic acid group)

These functional groups react to form strong covalent bonds, releasing small molecules like water as a by-product.

Formation and Structure

The polymer chains consist of many repeating units joined by either ester or amide linkages:

• Polyesters contain ester linkages formed between carboxylic acid and alcohol groups.
• Polyamides contain amide linkages formed between carboxylic acid and amine groups.

Every time a bond forms between monomers, a molecule of water is released. This is why the process is called condensation polymerisation.

The resulting polymers are long chains with repeating units, which can be represented in a simplified form showing the repeating section in brackets.

For instance, when a dicarboxylic acid reacts with a diol, the -OH group from the acid and the -H from the alcohol combine to form water, and an ester bond forms between the monomers:

-COOH + HO- 12; -COO- + H₂O

This process repeats many times to form a long polyester chain.

Example: When ethane-1,2-diol (a diol) reacts with benzene-1,4-dicarboxylic acid (a dicarboxylic acid), they form the polyester Terylene, with ester linkages and water molecules released at each bond.

Examples of Condensation Polymers

Polyesters are formed from dicarboxylic acids and diols. Terylene is a common polyester used in textiles and plastic bottles.

Polyamides are formed from dicarboxylic acids and diamines. Nylon is a well-known polyamide used in fabrics, ropes, and fishing lines.

Natural condensation polymers include:

• Proteins 12; formed from amino acids linked by amide bonds (peptide bonds). (See the Amino Acids and Proteins topic for full details.)
• Cellulose 12; a natural polymer made from glucose units linked by glycosidic bonds (not condensation polymerisation but a natural polymer formed by a different type of reaction).

Properties and Uses

Condensation polymers have strong intermolecular forces due to the polar ester or amide linkages, which give them:

• High melting points compared to many addition polymers
• Good strength and durability
• Resistance to chemicals and wear

These properties make them useful in:

• Textiles and clothing (e.g. nylon fibres)
• Plastic bottles and packaging (e.g. polyesters like Terylene)
• Engineering plastics and ropes

Learning Example: Formation of a Polyester

Ethane-1,2-diol (HO0302CH₂0202CH₂02OH) reacts with benzene-1,4-dicarboxylic acid (HOOC02C₆H₄02COOH) to form Terylene.

Each reaction between an 02COOH group of the acid and an 02OH group of the diol forms an ester linkage and releases one water molecule:

02COOH + HO02 12; 02COO02 + H₂O

The polymer chain grows by repeating this reaction many times, producing a long polyester chain with repeating ester linkages.