Welcome to the World of Polymers!
In this chapter, we are going to explore how tiny molecules can join together to create "giant" molecules. You interact with polymers every single day—from the plastic bottle on your desk to the clothes you are wearing. Understanding polymerisation is like learning how to build a massive bridge using nothing but millions of identical Lego bricks!
Prerequisite Check: Before we dive in, remember that this chapter focuses on alkenes. Alkenes are hydrocarbons that contain a C=C double bond. This double bond is the "secret key" that allows polymerisation to happen.
1. What is Addition Polymerisation?
Imagine you are in a room full of people. Each person is standing with their arms folded (this is like the double bond). To form a long human chain, everyone must "unfold" their arms and reach out to hold the hands of the people next to them. This is exactly what happens in addition polymerisation!
Key Definitions
• Monomer: A small, single molecule (usually an alkene) that can be joined together with others. Think of this as a single "link" in a chain.
• Polymer: A long-chain molecule made up of many repeating units joined together. This is the finished "chain."
• Addition Polymerisation: A process where many monomers containing C=C bonds join together to form a polymer, and nothing else is produced. The "addition" means \(1 + 1 + 1 = 3\); no atoms are lost!
How it works (Step-by-Step)
1. The \( \pi \) bond (the second part of the double bond) in the alkene monomer breaks.
2. Each carbon atom now has an "extra" electron it can use to form a new single bond.
3. These carbon atoms bond to the carbon atoms of the neighboring monomers.
4. This process repeats thousands of times to create a very long saturated chain (only single bonds remain).
Quick Review: In addition polymerisation, the double bond in the monomer becomes a single bond in the polymer.
2. Essential Examples: Poly(ethene) and PVC
The Cambridge syllabus requires you to know two specific examples of addition polymers. Let's look at them closely.
A. Poly(ethene)
• Monomer: Ethene \( (CH_2=CH_2) \)
• Polymer: Poly(ethene)
• Use: Plastic bags, cling film, and plastic bottles.
B. Poly(chloroethene) - also known as PVC
• Monomer: Chloroethene \( (CH_2=CHCl) \)
• Polymer: Poly(chloroethene) or PVC
• Use: Water pipes, insulation for electrical wires, and window frames.
Did you know? PVC is much stronger and more rigid than poly(ethene) because the large Chlorine atoms change how the chains sit next to each other!
Key Takeaway: To name the polymer, just put the word "poly" in front of the monomer name in brackets. Example: propene becomes poly(propene).
3. Deduce and Identify: The Repeat Unit
A repeat unit is the simplest part of the polymer chain that, if you copied and pasted it over and over, would give you the whole chain. Don't worry if this seems tricky at first; there is a simple trick to getting this right every time!
How to find the Repeat Unit from a Monomer:
1. Draw the monomer in a "H-shape" (put the two carbons of the C=C bond in the middle).
2. Change the C=C double bond to a C–C single bond.
3. Draw two "continuation bonds" sticking out from the sides of the carbons.
4. Place square brackets around the unit and a small 'n' at the bottom right.
Memory Aid: The repeat unit never has a double bond! If you draw a double bond in a repeat unit, it's like trying to hold hands while your arms are still folded—it doesn't work!
How to find the Monomer from a Polymer section:
1. Look at the polymer chain and identify the simplest repeating pattern (usually a 2-carbon section).
2. Isolate that 2-carbon section.
3. Remove the "continuation bonds" on the sides.
4. Put the C=C double bond back in between the two carbons.
Common Mistake to Avoid: When drawing the polymer, students often forget to draw the side groups (like \( -H \), \( -CH_3 \), or \( -Cl \)) exactly where they were on the monomer. Make sure they stay attached to the same carbon atom!
4. The Problem with Plastics: Disposal
While polymers are incredibly useful, they come with a significant environmental "price tag." The same properties that make them useful (they are strong and don't react easily) make them hard to get rid of.
1. Non-biodegradability
Most poly(alkenes) are chemically inert. This is because they consist of very strong C–C and C–H bonds. Bacteria in nature don't have the "tools" (enzymes) to break these bonds down. Consequently, these plastics can sit in a landfill for hundreds of years without rotting.
2. Harmful Combustion Products
If we try to burn (incinerate) plastic waste to save space, we run into another problem:
• Carbon Monoxide \( (CO) \): Produced if there isn't enough oxygen (incomplete combustion). This gas is toxic.
• Hydrogen Chloride \( (HCl) \): When PVC is burned, it releases \( HCl \) gas. This is highly acidic and creates acid rain, which can damage buildings and kill aquatic life.
• Toxic Dioxins: Burning certain plastics can release complex organic chemicals that are very poisonous.
Quick Review Box:
• Problem: Plastics don't break down (non-biodegradable).
• Reason: Strong, non-polar C–C bonds.
• Danger: Burning them creates toxic gases like \( HCl \) and \( CO \).
Summary Checklist
Before you move on, make sure you can:
• [ ] Define monomer, polymer, and addition polymerisation.
• [ ] Draw the polymer and repeat unit for ethene and chloroethene.
• [ ] Identify a monomer if you are shown a picture of a polymer chain.
• [ ] Explain why poly(alkenes) are difficult to dispose of safely.
You've got this! Polymerisation is just a big game of molecular "connect the dots." Keep practicing the drawings, and they will become second nature!