Polymers and polymerisation

Introduction to Big Molecules

Have you noticed that a single thread of cotton is weak, but when many are joined together, they form a strong shirt? In Chemistry, we have small molecules that join like a long chain to form very big ones. These big molecules are called polymers.

A polymer is a giant molecule formed by the joining together of many small repeating units called monomers.

Simply put, if a monomer is like a single block of LEGO, the polymer is the long wall you build with many blocks.

What is Polymerisation

When you use a needle and thread to join pieces of cloth together, you are doing something like polymerisation. It is the process that turns small units into one big structure.

It is just the chemical reaction that glues small molecules together to make a giant one.

Addition Polymerisation

This type happens when monomers that have double bonds (unsaturated) just add to themselves without losing anything. It is like a group of people holding hands to form a long line; nobody leaves the group. Common examples include Polythene and PVC.

1. The double bond in the monomer (like ethene) breaks.

2. The monomers link up head-to-tail.

3. A long straight chain is formed.

Condensation Polymerisation

This one is different. When the monomers join, a small molecule like water (H_2O) or ammonia (NH₃) is pushed out. It is like two people hugging and their water bottles falling out of their pockets in the process. Examples include Nylon and Terylene.

Classification of Polymers

Polymers can be natural or synthetic. Natural ones are found in nature, like the cellulose in the wood we use for furniture or the starch in the garri we eat. Synthetic ones are man-made in factories, like the plastic buckets and 'nylon' bags we use at home.

Worked Example for Degree of Polymerisation

Question: If a sample of Polythene has a molar mass of 28,000 g mol^-1, how many ethene units are in the chain? (C = 12, H = 1).

1. First, find the molar mass of one monomer (Ethene, C_2H4): (12 x 2) + (1 x 4) = 28 g mol^-1.

2. Use the formula: Number of units = Total Molar Mass / Monomer Molar Mass.

3. Calculation: 28,000 / 28 = 1,000 units. This means 1,000 ethene molecules joined together.