Hey there! Absolutely, alkene polymerisation is a super interesting and important topic in GCSE Chemistry, and I'd be happy to break it down for you. Think of it like building a very long LEGO chain from lots of individual LEGO bricks! 🧪
What are Alkenes and Polymerisation?
First off, let's quickly recap. Alkenes are a family of hydrocarbons containing at least one carbon-carbon double bond ($C=C$). This double bond is the key to polymerisation!
Polymerisation is simply a chemical reaction where many small molecules, called monomers, join together to form a very large molecule called a polymer. Imagine linking hundreds or thousands of identical paper clips together – the paper clips are your monomers, and the long chain is your polymer.
The Magic of Addition Polymerisation ✨
For alkenes, the process is called addition polymerisation. The 'addition' part means that all the atoms from the monomers are incorporated into the polymer – no small molecules (like water) are lost during the reaction. This is different from other types of polymerisation you might learn later.
How it Works (Using Ethene as an Example):
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The Monomer: Ethene
The simplest alkene is ethene, with the formula $CH_2=CH_2$. Each ethene molecule has that crucial $C=C$ double bond.
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Opening the Double Bond
Under specific conditions (like high temperature, pressure, and a catalyst), the carbon-carbon double bond ($C=C$) in ethene molecules 'opens up'. One of the two bonds in the double bond breaks, leaving each carbon atom with a 'free' bond ready to attach to something else.
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Joining the Chains
As one ethene molecule's double bond breaks, it allows it to connect to another ethene molecule whose double bond has also broken. This process repeats over and over again, linking thousands of ethene molecules together in a long, continuous chain.
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The Polymer: Poly(ethene)
The resulting super long chain is called poly(ethene), or more commonly, polythene! It looks like this: $-[CH_2-CH_2]_n-$. This is where you see the original ethene unit, but now as a repeating single-bonded section of a much larger molecule.
The General Equation for Addition Polymerisation
You can represent this process with a general equation. For ethene polymerisation, it looks like this:
$n \, CH_2=CH_2 \rightarrow -[CH_2-CH_2]_n-$
- The '$n$' on the left side represents a very large number of individual ethene monomers.
- The square brackets on the right, $-[CH_2-CH_2]_n-$, show the repeat unit of the polymer. This is the section that comes from the original monomer, and it's repeated '$n$' times to form the long polymer chain.
Other Examples
This process isn't just for ethene! Other alkenes can polymerise too:
- Propene ($CH_2=CH-CH_3$) polymerises to form poly(propene) (often called polypropylene), used for ropes, carpets, and storage containers.
- Chloroethene (also known as vinyl chloride, $CH_2=CHCl$) polymerises to form poly(chloroethene) (PVC), used for drainpipes, window frames, and electrical insulation.
So, the key takeaway is that the carbon-carbon double bond in alkenes is what allows them to 'open up' and link together to form these useful, long-chain polymers! Hope this clears things up for your GCSEs! Good luck! 🍀