Drawing Addition Polymerisation Reactions Revision Notes | GCSE AQA Higher Chemistry

1. What Happens When Small Molecules Join Up?

Imagine thousands of identical paper clips linking together to form one massive chain. That's essentially what happens in addition polymerisation - except instead of paper clips, we're using molecules.

The key insight is this: when alkenes polymerise, nothing gets lost. Every atom from the original molecules ends up in the final polymer. This is what makes addition polymerisation different from other types of reactions you might have seen.

2. The Building Blocks - Understanding Monomers

Think of monomers as LEGO bricks. Each brick (monomer) has the same basic structure but can connect to other identical bricks. The key scientific idea happens at the C=C double bond - this is where the connection occurs.

The most common example is ethene ($C_2H_4$). It has a double bond between two carbon atoms, and this double bond is the key to polymerisation. When thousands of ethene molecules join up, they form polyethene (polythene) - the plastic used in shopping bags.

3. Drawing the Polymerisation Process

Here's where students often get confused, but there's a simple pattern to follow. When drawing addition polymerisation, you need to show three things: the monomer, the arrow, and the polymer structure.

Worked example: Drawing Addition Polymerisation of Ethene

Draw the addition polymerisation of ethene to form polyethene.

1. $H_2C=CH_2$ — Start with ethene showing the C=C double bond clearly
2. $nH_2C=CH_2$ — The 'n' means many molecules - typically thousands
3. $nH_2C=CH_2 \rightarrow$ — Conditions: high pressure, catalyst (usually not specified at GCSE)
4. $-(CH_2-CH_2)_n-$ — The double bond opens up to form single bonds in the chain
5. Repeating unit: $-CH_2-CH_2-$ — This is the smallest section that repeats along the chain

Answer: $nH_2C=CH_2 \rightarrow -(CH_2-CH_2)_n-$

Notice something crucial: count the atoms in the monomer ($C_2H_4$) and in the repeating unit ($C_2H_4$). They're identical! This is the hallmark of addition polymerisation - no atoms are lost or gained.

4. Working with Different Monomers

The beauty of addition polymerisation is that it works with any alkene. Let's look at propene ($CH_3CH=CH_2$) as another example. The process is identical - only the side groups change.

Worked example: Drawing Addition Polymerisation of Propene

Draw the addition polymerisation of propene ($CH_3CH=CH_2$) to form polypropene.

1. $CH_3CH=CH_2$ — Propene has a methyl group ($CH_3$) attached to the double bond
2. $nCH_3CH=CH_2 \rightarrow$ — Many propene molecules will join together
3. $-(CH_2-CH(CH_3))_n-$ — The double bond opens, methyl groups alternate along the chain
4. $-CH_2-CH(CH_3)-$ — This unit contains the same atoms as the original monomer

Answer: $nCH_3CH=CH_2 \rightarrow -(CH_2-CH(CH_3))_n-$

Again, check the atom count: propene monomer has $C_3H_6$, and so does the repeating unit. The pattern holds true regardless of which alkene you start with.

5. Common Drawing Mistakes and How to Avoid Them

Students often make predictable errors when drawing polymerisation reactions. Here are the most common traps and how to avoid them.

The biggest mistake? Forgetting that addition polymerisation produces no waste products. If you find yourself with extra atoms or molecules left over, you've made an error. Everything from the monomers ends up in the polymer chain.

6. Exam Technique - Getting Full Marks

Exam questions on addition polymerisation follow predictable patterns. You'll either be asked to draw the polymerisation reaction or identify the monomer from a given polymer structure.

For drawing questions, examiners award marks for: showing the correct monomer structure [1 mark], using 'n' to represent many molecules [1 mark], drawing the correct polymer structure with repeating unit clearly shown [2 marks], and including continuation bonds [1 mark].

Remember, addition polymerisation is fundamentally simple: alkenes join together by opening their double bonds. Master this concept, and you'll find these questions straightforward. The key is careful drawing and systematic checking of your work.

Exam precision note: In Drawing Addition Polymerisation Reactions - Addition polymerisation - Synthetic and naturally occurring polymers (chemistry only), always identify the key chemical idea first, then link it to evidence, data, or an equation. A high-scoring GCSE answer names the particles involved, states what changes, and justifies the conclusion using correct scientific language. Where calculations are used, show each step with units and keep significant figures sensible. Finally, check that terminology matches AQA wording so your answer is unambiguous and mark-scheme friendly.

Addition polymerisation happens when many alkene molecules join together. The double bond in each monomer opens so that new single bonds can form along the polymer chain. Students should focus on identifying the alkene monomer and then drawing the repeating unit correctly.