Welcome to the World of Polymers!
Ever wondered why your plastic water bottle is so different from a plastic PVC pipe, even though they are both "plastic"? In this chapter, we are going to explore addition polymerisation. This is the chemical process that allows us to take tiny molecules and link them together into giant, incredibly useful chains. If you've ever played with LEGO or linked paperclips together, you already understand the basic idea behind polymerisation!
1. What is Polymerisation?
At its simplest, polymerisation is the process of joining a large number of small molecules, called monomers, together to form a very long chain called a polymer.
Analogy Time: Imagine a single LEGO brick. That is your monomer. When you snap 1,000 of those identical bricks together to make one long line, that long line is your polymer.
Key Terms to Know:
Monomer: The small, repeating unit that serves as the building block of a polymer. (In AS Level, these are usually alkenes).
Polymer: A long-chain molecule made up of many repeating units.
Addition Polymerisation: A reaction where many monomers containing C=C double bonds join together to form a polymer, and nothing else is produced. The double bond "opens up" to link to the next molecule.
Quick Review: Remember that alkenes have a pi (\(\pi\)) bond which is relatively weak. In addition polymerisation, this \(\pi\) bond breaks, allowing the carbon atoms to form new sigma (\(\sigma\)) bonds with neighboring monomer molecules.
Takeaway: Addition polymerisation turns unsaturated monomers (alkenes) into saturated polymer chains.
2. Key Examples: Poly(ethene) and PVC
The Cambridge 9701 syllabus focuses on two main examples that you need to be able to describe and draw.
A. Poly(ethene)
This is the most common plastic in the world, used for carrier bags and cling film. It is made from ethene monomers.
The Reaction:
\(n(CH_2=CH_2) \rightarrow -(CH_2-CH_2)_n-\)
Don't worry if the symbols look scary! The "\(n\)" just represents a very large number (thousands of molecules). The brackets show the repeat unit.
B. Poly(chloroethene) - also known as PVC
You probably know PVC from construction pipes or vegan leather. The monomer is chloroethene (also called vinyl chloride).
The Reaction:
\(n(CH_2=CHCl) \rightarrow -(CH_2-CHCl)_n-\)
Did you know? PVC is very hard and brittle on its own. To make it flexible for things like fake leather or electrical cable insulation, chemicals called plasticisers are added to sit between the polymer chains and let them slide over each other.
3. How to Draw Polymers and Repeat Units
This is a favorite exam topic! Students often lose marks for small drawing errors. Follow these steps to get it right every time.
Step-by-Step: Drawing a Polymer from a Monomer
- Draw the monomer: Draw the \(C=C\) bond in the center and arrange the four groups around it in a "H" or "X" shape.
- Open the bond: Change the double bond (\(C=C\)) to a single bond (\(C-C\)).
- Add "extension" bonds: Draw bonds sticking out horizontally from the two carbons. These must pass through the brackets.
- Add brackets and 'n': Put square brackets around the unit and a small subscript "\(n\)" at the bottom right.
Common Mistake to Avoid: Never leave the double bond inside the brackets of a polymer! The polymer chain only has single bonds between the carbons.
The Repeat Unit
A repeat unit is the specific part of the polymer that, if you copied and pasted it, would make the whole chain. It looks exactly like the polymer drawing but without the "\(n\)".
Takeaway: The monomer has a double bond; the repeat unit and polymer have single bonds with trailing "connector" lines.
4. Identifying the Monomer from a Polymer Chain
Sometimes the exam will give you a long section of a polymer chain and ask: "What was the monomer?"
The Trick:
1. Look for the pattern. Find the group of atoms that repeats every two carbons.
2. Isolate that two-carbon section.
3. Remove the side "extension" bonds and put the double bond back in between the two carbons.
Memory Aid: "Poly" means "many." To get the monomer name, just take the polymer name and remove "poly."
Example: Poly(propene) comes from the monomer propene.
5. Polymers and the Environment
While plastics are useful, they cause big problems for our planet. You need to know why they are hard to get rid of.
A. Non-biodegradability
Addition polymers like poly(ethene) are non-biodegradable. This means bacteria and fungi cannot break them down. Why?
- They consist of very strong C-C and C-H bonds.
- The chains are non-polar, so they aren't easily attacked by chemical reagents or biological enzymes.
B. Disposal Problems
Since they don't rot, we have three main options, each with a downside:
- Landfill: They take up space for hundreds of years.
- Recycling: Difficult and expensive because different types of plastic must be sorted perfectly.
- Combustion (Burning): This provides energy, but it can be dangerous.
Danger Alert! When PVC (poly(chloroethene)) is burned, it releases Hydrogen Chloride (HCl) gas. This gas is highly acidic and toxic, contributing to acid rain and respiratory problems.
Takeaway: Polymers are chemically "lazy" (inert), which makes them stick around in the environment forever. Burning PVC is extra dangerous because of toxic HCl gas.
Quick Review Box
Addition Polymerisation Summary:• Monomer: Must have a \(C=C\) (Unsaturated).
• Polymer: Only has \(C-C\) (Saturated).
• Identify Monomer: Find two carbons in the chain, cut them out, and add a double bond.
• Environment: Inert/non-polar = won't rot. Burning PVC = toxic \(HCl\).
Final Encouragement: Polymerisation is all about patterns. Once you can see the two-carbon repeat unit, you've mastered the hardest part! Keep practicing drawing those brackets and you'll do great.