Predicting the type of polymerisation

Welcome to the World of Polymers!

Ever wondered how a gaseous substance like ethene becomes a solid plastic bottle or how your non-stick frying pan works? The answer lies in polymerisation. In this chapter, we are going to learn how to predict when addition polymerisation happens, how to draw the results, and why these materials are both a blessing and a curse for our planet. Don't worry if it sounds like a lot—we will break it down into simple, bite-sized pieces!

1. What is Addition Polymerisation?

To understand polymerisation, think of a single monomer as a single LEGO brick. When you snap hundreds or thousands of these bricks together to form a long chain, you have created a polymer.

In addition polymerisation, the monomers simply add onto one another. No atoms are lost, and no "extra" molecules are produced. It is 100% efficient in terms of atoms!

How to spot a monomer that can do this:

For addition polymerisation to happen at the AS Level, your monomer must have a C=C double bond. This means it is an alkene (or a derivative of an alkene).

Memory Aid: Think Addition = Alkene. If you see that double bond, it’s an addition reaction waiting to happen!

Prerequisite Concept: The "Magic" of the Double Bond

The double bond consists of a strong \(\sigma\) (sigma) bond and a weaker \(\pi\) (pi) bond. During polymerisation, the \(\pi\) bond breaks. This "opens up" the carbon atoms, allowing them to reach out and grab the carbon atoms of the next monomer.

Key Takeaway: Addition polymers are formed from monomers containing a C=C double bond. The polymer is the only product formed.

2. Predicting the Polymer from the Monomer

If you are given a monomer, how do you draw the polymer? Let's use ethene as our first example.

Step-by-Step: Ethene to Poly(ethene)

1. Draw your monomer (ethene) in a "H" shape, so the double bond is in the center and the other groups are pointing up and down: \(CH_{2}=CH_{2}\).
2. Break the double bond to make it a single bond: \(–CH_{2}–CH_{2}–\).
3. Extend "trailing bonds" out of the sides of the carbons. These represent the chain continuing.
4. Put the whole thing in square brackets and add a small 'n' at the bottom right. This 'n' stands for a very large number of repeating units.

Example: Chloroethene to Poly(chloroethene) (PVC)

Chloroethene monomer: \(CH_{2}=CHCl\)
Polymer Repeat Unit: \([–CH_{2}–CHCl–]_{n}\)
Did you know? PVC is the material used for most modern water pipes and even some "vegan leather" clothing!

Quick Review: To draw a polymer, change the \(C=C\) to a \(C-C\), draw bonds coming out the sides, and wrap it in brackets with an 'n'.

3. Identifying the Monomer from a Polymer Chain

Sometimes, the exam will give you a long section of a polymer and ask: "What was the original monomer?" This is like looking at a finished train and trying to figure out what a single carriage looks like.

The "Two-Carbon Rule":

Since addition polymers come from alkenes, the repeat unit will always involve two carbon atoms in the main chain.

1. Look at the long chain and find the pattern that repeats every two carbons.
2. Isolate those two carbons and whatever is attached to them (the "repeat unit").
3. To get the monomer, simply remove the trailing bonds and put the C=C double bond back in.

Common Mistake to Avoid: Students often try to include more than two carbons in the repeat unit. Unless the monomer was very unusual, always look for the pattern every two carbons along the "backbone."

Key Takeaway: Monomer = Repeat unit minus the trailing bonds + a double bond.

4. Real-World Examples in the Syllabus

You specifically need to know these two:

• Poly(ethene): Used for plastic bags and bottles. It is flexible and cheap.
• Poly(chloroethene) (PVC): Used for window frames and piping. It is much tougher and more rigid than poly(ethene) because of the large chlorine atoms.

5. The Environmental Cost: Disposal of Polymers

While polymers are incredibly useful, they have two major "villain traits" that make them bad for the environment:

1. They are Non-Biodegradable

Because addition polymers are made of strong C–C and C–H bonds, they are very chemically inert (unreactive). Bacteria in the soil don't have the "tools" (enzymes) to break these bonds down.
Analogy: Putting a plastic bottle in a landfill is like putting a stone there—it will just sit there for hundreds of years!

2. Harmful Combustion Products

If we try to get rid of them by burning them (combustion), we run into trouble:
• General risk: All carbon-based polymers can produce Carbon Monoxide (CO) if there isn't enough oxygen (incomplete combustion).
• PVC risk: Because PVC contains chlorine, burning it releases Hydrogen Chloride (HCl) gas, which is highly acidic and toxic. It can cause acid rain and is dangerous to breathe.

Encouraging Note: Don't worry if the environmental part seems like "common sense"—in Chemistry 9701, you get marks for using specific terms like non-biodegradable, inert, and naming specific gases like HCl!

Final Summary Checklist

• Can I identify a monomer by looking for the C=C?
• Can I draw a repeat unit with square brackets and an 'n'?
• Can I "reverse-engineer" a polymer chain to find the monomer?
• Do I know that addition polymers are unreactive (inert) and why that makes them hard to dispose of?
• Do I know that burning PVC releases toxic HCl gas?