Describe the formation of poly(ethene) as an example of addition polymerisation using ethene monomers. 🧪
Poly(ethene), also known as polyethylene, is a long chain polymer made from the monomer ethene (C₂H₄). It is the most common plastic used in everyday items like bags, bottles and toys. 📚
In addition polymerisation, the double bond of each monomer is opened and the monomers link together to form a chain. No atoms are lost from the monomers. The general reaction is:
\$\ce{CH2=CH2} \xrightarrow{\text{radical initiator}} \ce{[-CH2-CH2-]_{n}}\$
| Step | What Happens | Illustration (LaTeX) |
|---|---|---|
| 1. Initiation | Peroxide breaks to give two radicals. | \$\ce{RO-OR} \rightarrow 2\ce{RO.}\$ |
| 2. Propagation | Radical adds to ethene, opening the double bond. | \$\ce{RO. + CH2=CH2 \rightarrow RO-CH2-CH2.}\$ |
| 3. Termination | Two radicals combine or a radical abstracts H. | \$\ce{RO-CH2-CH2. + .CH2-CH2-OR \rightarrow RO-CH2-CH2-CH2-CH2-OR}\$ |
Think of each ethene monomer as a LEGO block with a double‑bond “connector”. When a radical initiator (the “magnet”) pulls the connector open, the blocks snap together one after another, forming a long chain. The chain grows until the magnet stops pulling or two chains snap together. This is like building a long train of cars that keeps adding cars until the engine stops.
Example: Making a plastic bag. Thousands of ethene molecules link together to give a flexible, strong material that can hold liquids.