Addition polymerisation

Overview

Addition polymerisation is a cornerstone of modern materials science and a frequently examined topic in Edexcel GCSE Chemistry (2.14). It is the process by which thousands of small, unsaturated molecules, known as monomers, join together to form a single, very large molecule called a polymer. The term 'addition' is key – in this reaction, no other product is formed; the monomers simply add to one another. This process is responsible for creating many of the plastics we use every day, from carrier bags to window frames. Understanding this topic not only secures vital marks in your exam but also provides insight into the properties and environmental impact of these ubiquitous materials. Examiners typically assess this through questions requiring you to draw and interpret polymer and monomer structures, and to evaluate the challenges of polymer disposal. This topic links directly to your understanding of alkenes (Topic 2.13) and has synoptic links to concepts of bonding (Topic 1) and the impact of materials on the environment (Topic 10).

Key Concepts

Concept 1: The Monomer and the Double Bond

The starting point for any addition polymer is the monomer. For addition polymerisation, the monomer must be an alkene – a hydrocarbon containing at least one carbon-carbon double bond (C=C). This double bond is the site of the reaction. It is a region of high electron density, making it reactive. During polymerisation, one of the two bonds in the C=C double bond breaks open. This allows the carbon atoms to form new single bonds with adjacent monomer molecules, initiating a chain reaction.

Example: The simplest alkene monomer is ethene (C₂H₄). Its structure contains a C=C double bond. When many ethene molecules react together under conditions of high temperature and pressure with a catalyst, the double bonds break, and they link together to form poly(ethene).

Concept 2: Forming the Polymer Chain

Polymerisation is a chain reaction. Once the double bond of a monomer 'opens up', it can link to another opened-up monomer, which in turn links to another, and so on. This creates a very long, saturated chain called a polymer. The original double bonds are all converted into single bonds within the polymer backbone. Because the polymer is made of only carbon-carbon single bonds, it is now a saturated molecule. This is why addition polymers are very unreactive and non-biodegradable.

Concept 3: Representing Polymers - The Repeating Unit

Since a polymer chain can be thousands of atoms long, it's impractical to draw the entire molecule. Instead, we draw the repeating unit. This is the shortest section of the chain that, if repeated, would build the entire polymer. To draw it correctly:

1. Identify the monomer.
2. Change the C=C double bond to a C-C single bond.
3. Draw single bonds extending from these carbons outwards.
4. Enclose this structure in square brackets.
5. Draw the extension bonds passing through the brackets to show the chain continues.
6. Write a subscript 'n' outside the brackets to signify a large number of repeats.

This notation is crucial and frequently tested. A common mistake is to draw the extension bonds stopping at the brackets, which is incorrect and will lose marks.

Mathematical/Scientific Relationships

There are no complex mathematical formulas in this topic at GCSE level. The key relationship is structural:

n (Monomer) → [Repeating Unit]ₙWhere 'n' represents a very large number. This equation shows that 'n' individual monomer molecules react to form a polymer chain made of 'n' repeating units. The relative formula mass of the polymer is therefore approximately 'n' times the relative formula mass of the monomer.

Practical Applications

Addition polymers are everywhere in modern life. The properties of the polymer depend on the monomer used and the conditions of polymerisation.

Environmental Impact and Disposal

The very property that makes polymers so useful – their inertness due to strong C-C bonds – also makes them an environmental problem. They are non-biodegradable, meaning they persist in landfill sites for hundreds of years. This has led to a focus on more sustainable disposal methods.