Introduction to Addition Polymerisation
Hi there! Welcome to one of the most practical chapters in Chemistry. Have you ever looked at a plastic shopping bag or a PVC water pipe and wondered how they were made? Today, we are going to learn about addition polymerisation. This is the chemical process that turns small, simple molecules into the massive, useful materials we use every day. Don't worry if it sounds a bit technical—by the end of these notes, you'll be able to spot a polymer and its starting ingredients with ease!
Quick Review: Before we start, remember that alkenes (like ethene) have a C=C double bond. This double bond is the "secret ingredient" that makes addition polymerisation possible.
1. What is Addition Polymerisation?
At its simplest, addition polymerisation is the process of joining a very large number of small molecules together to form one very long chain. This happens without losing any atoms—everything in the small molecules ends up in the big chain.
Key Terms:
- Monomer: The small, individual molecule that reacts to form the polymer. (Think of a single Lego brick).
- Polymer: The long-chain molecule made up of many repeating units. (Think of a long tower built from those Lego bricks).
The Analogy: Imagine a room full of people standing with their arms crossed. Each person is a monomer. If everyone reaches out their arms to hold hands with the person next to them, they form a long human chain. That chain is the polymer!
Did you know? The "poly" in polymer comes from the Greek word for "many," and "mer" means "part." So, a polymer is just "many parts" joined together.
Key Takeaway:
In addition polymerisation, unsaturated monomers (containing double bonds) join together to form a saturated polymer (containing only single bonds).
2. The Chemistry of the Process
In an addition reaction, the \(\pi\) bond (the second bond in the C=C) breaks. Each carbon atom then uses that "freed-up" electron to form a new single covalent bond with a neighbouring molecule.
Example 1: Poly(ethene)
When thousands of ethene molecules \( (H_2C=CH_2) \) react under high pressure and temperature with a catalyst, they form poly(ethene).
The equation looks like this: \( n(CH_2=CH_2) \rightarrow -(CH_2-CH_2)_n- \)
The \( n \) represents a very large number of molecules.
Example 2: Poly(chloroethene) or PVC
If we use chloroethene \( (CH_2=CHCl) \) as our monomer, we get poly(chloroethene), commonly known as PVC. This is the stuff used for window frames and drainpipes because it is very strong and durable.
Step-by-Step: How to draw a Polymer from a Monomer
- Draw the monomer but change the C=C double bond to a C-C single bond.
- Draw two "continuation bonds" sticking out horizontally from the carbon atoms.
- Put large square brackets around the unit.
- Write a small \( n \) at the bottom right-hand corner.
Quick Review Box:
Monomer: Has a double bond \( (C=C) \).
Repeat Unit: Has single bonds \( (C-C) \) and brackets.
3. Deducing Repeat Units and Monomers
One common exam task is to "work backwards" or "identify the repeat unit." Don't let this trick you! It’s like finding the pattern in a wallpaper design.
How to find the Repeat Unit from a Polymer chain:
Look at the long chain and find the smallest section that repeats itself over and over. Usually, this involves two carbon atoms in the main backbone.
How to identify the Monomer from a Polymer section:
- Identify the repeat unit.
- Remove the continuation bonds and the brackets.
- Change the single bond between the two carbons back into a double bond.
Memory Aid: To go from Polymer to Monomer, just "Add the double bond back!"
Common Mistake to Avoid:
When drawing a polymer, make sure the continuation bonds actually pass through the square brackets. This shows that the chain continues beyond what you've drawn.
4. Environmental Impact and Disposal
While polymers are incredibly useful, they come with a "green" cost. Because polymers like poly(ethene) and PVC are alkanes (long chains of C-C and C-H bonds), they are very unreactive. This leads to several problems:
1. Non-biodegradability
Most addition polymers are non-biodegradable. This means bacteria in the soil cannot break them down because the C-C bonds are very strong and non-polar. As a result, they sit in landfills for hundreds of years.
2. Harmful Combustion
We could burn them for energy, but this is also risky:
- Burning any plastic can release toxic gases like carbon monoxide \( (CO) \).
- Burning PVC \( (poly(chloroethene)) \) is particularly dangerous because it releases hydrogen chloride \( (HCl) \) gas, which is highly acidic and corrosive.
3. Disposal Challenges
Because they don't rot, they clutter the environment and harm wildlife (like turtles eating plastic bags). Recycling is the best option, but it requires sorting different types of plastic, which can be expensive and difficult.
Key Takeaway:
Polymers are chemically inert (unreactive). This makes them great for storage but terrible for the environment after we throw them away.
Summary Checklist
Before you finish, make sure you can:
- Define addition polymerisation correctly.
- Draw the repeat unit for poly(ethene) and poly(chloroethene).
- Identify a monomer if you are shown a section of a polymer.
- Explain why plastics are difficult to dispose of (non-biodegradable and toxic fumes).
Great job! Addition polymerisation is all about patterns. Once you see the "double bond to single bond" pattern, you've mastered it!