Polymers

What are Polymers?

Polymers are very large molecules made up of many smaller repeating units called monomers. These monomers join together through a process called polymerisation, forming long chains.

There are two main types of polymers:

• Synthetic polymers – man-made, such as plastics like poly(ethene) and poly(propene).
• Natural polymers – found in nature, such as proteins, starch, and DNA.

Polymers have different properties depending on the monomers used and how the chains are arranged.

Addition Polymerisation

Addition polymerisation involves unsaturated monomers that contain double bonds (C=C). During polymerisation, these double bonds open up and link together to form long chains without producing any other molecules.

This process is common for making many everyday plastics.

Examples include:

• Poly(ethene) – made from ethene monomers (CH₂=CH₂).
• Poly(propene) – made from propene monomers (CH₂=CHCH₃).

The repeating unit in the polymer chain is the monomer without the double bond.

For instance, ethene monomers join by opening their double bonds to form a long chain of single bonds:

Ethene monomer: CH₂=CH₂

Poly(ethene) repeating unit: 6CH₂ 6CH₂ 6

The polymer chain is simply many of these units joined together.

For example, if 1000 ethene molecules join, the polymer chain will have 1000 repeating 6CH₂ 6CH₂ 6 units.

Example: Calculate the number of carbon atoms in a poly(ethene) chain made from 5000 ethene monomers.

Each monomer has 2 carbon atoms, so total carbon atoms = $5000 \times 2 = 10,000$.

Condensation Polymerisation

Condensation polymerisation involves monomers with two different functional groups. When these monomers join, a small molecule such as water is released.

This type of polymerisation forms polymers like polyesters and polyamides.

For example, in making a polyester, a dicarboxylic acid ( 6COOH 6) reacts with a diol ( 6OH 6) to form ester links ( 6COO 6) and water.

The polymer chain grows as these monomers repeatedly join, releasing water each time.

Example: In polyester formation, each bond formed between monomers releases one water molecule.

Properties of Polymers

Polymers have strong covalent bonds within their chains, making the chains themselves very stable.

However, the forces between polymer chains (intermolecular forces) are much weaker and affect the polymer 2s flexibility and melting point.

The strength of these intermolecular forces depends on the polymer 2s structure:

• Weak forces 6 chains slide past each other easily 6 flexible polymers.
• Strong forces 6 chains held tightly 6 rigid or hard polymers.

There are two main types of polymers based on how they respond to heat:

• Thermoplastics 6 soften when heated and can be reshaped. Their polymer chains are not cross-linked, so they slide over each other when heated.
• Thermosetting plastics 6 do not soften when heated. They have strong cross-links between chains, forming a rigid 3D network.

Examples:

• Poly(ethene) is a thermoplastic.
• Epoxy resin is a thermosetting plastic.

Example: Poly(ethene) is flexible because its chains are not cross-linked and have weak intermolecular forces, allowing them to slide past each other easily.