Welcome to the World of Polymers!
Hi there! Today we are diving into one of the most useful areas of chemistry: Polymers. You might think of "plastic" when you hear the word polymer, and you'd be right! But polymers are also in the food we eat and the clothes we wear. In this chapter of the Developing Fuels (DF) section, we are going to look at how small molecules join together to make giant ones.
Don’t worry if this seems a bit overwhelming at first. We’ll break it down step-by-step until you're an expert!
1. What exactly is a Polymer?
To understand a polymer, imagine a long freight train. The whole train is the polymer. Each individual carriage on that train is a monomer.
Key Definitions:
• Monomer: A small, single molecule that can be joined together with others to form a long chain.
• Polymer: A very large molecule (macromolecule) made up of many repeating units (monomers) joined together.
Analogy: Think of Lego bricks. One single brick is the monomer. When you snap 50 of them together in a line, you’ve built a polymer!
Quick Review: The Vocabulary
• Poly- means "many"
• Mono- means "one"
• -mer means "part"
2. Addition Polymerisation
In the Developing Fuels storyline, we focus on Addition Polymerisation. This specifically happens with alkenes (hydrocarbons with a double bond).
How does it work?
In an addition reaction, the C=C double bond in the monomer "opens up." This allows the carbon atoms to bond to the monomers next to them, creating a long, continuous chain of C-C single bonds.
Did you know?
In addition polymerisation, nothing else is made. 100% of the atoms in the monomers end up in the polymer. It’s a very "green" and efficient way to make materials!
The Conditions
For these monomers to hold hands and form a chain, they usually need:
1. High pressure
2. A catalyst
Key Takeaway: Addition polymerisation only happens if the monomer is unsaturated (has a double bond). The polymer produced is saturated (has only single bonds).
3. Drawing the Relationship: Monomer to Polymer
The exam will often ask you to draw the repeat unit of a polymer from a given monomer. Here is a foolproof step-by-step guide.
Step-by-Step: Drawing a Polymer from a Monomer
Let’s use ethene \( (CH_2=CH_2) \) as our example to make poly(ethene).
1. Draw the monomer but arrange the groups so the C=C is in the center and the other atoms are sticking up or down (like a "capital H" shape).
2. Change the double bond \( (C=C) \) into a single bond \( (C-C) \).
3. Draw "extension bonds" out the sides of the carbons. These represent the chain continuing.
4. Put square brackets around the unit, ensuring the extension bonds pass through the brackets.
5. Add a small 'n' at the bottom right corner. This \( n \) stands for a very large number of repeating units.
Example:
Monomer: \( CH_2=CH_2 \)
Polymer Repeat Unit: \( -[CH_2-CH_2]_n- \)
Common Mistake to Avoid:
Students often forget to change the double bond to a single bond in the polymer. Remember: The double bond must break to allow the chain to grow!
4. Working Backwards: Polymer to Monomer
Sometimes the exam gives you a long chain and asks, "What was the monomer?"
How to "Spot the Monomer":
1. Identify the repeat unit. Look for the pattern that keeps showing up every two carbons.
2. Draw that section by itself.
3. Remove the side extension bonds.
4. Put the double bond back in between the two central carbons.
Memory Trick:
• Monomer to Polymer: Break the bond, add brackets.
• Polymer to Monomer: Fix the bond, remove brackets.
5. Real-World Examples in DF
You should be familiar with these common addition polymers:
• Poly(ethene): Made from ethene. Used for plastic bags and bottles.
• Poly(propene): Made from propene. Used for ropes and crates.
• Poly(chloroethene): Also known as PVC. Made from chloroethene. Used for water pipes and window frames.
Naming Rule:
To name the polymer, simply put "poly" in front of the monomer name in brackets.
Example: Chloroethene becomes poly(chloroethene).
Summary Checklist
Quick Review Box:
• Do you know the difference between a monomer and a polymer?
• Can you explain why alkenes are used in addition polymerisation? (Hint: The \( C=C \) bond).
• Can you draw a repeat unit with brackets and an \( n \)?
• Can you identify a monomer from a polymer chain by adding the double bond back?
Keep going! You’re doing great. Polymers are just giant puzzles made of tiny pieces. Once you see the pattern, you’ve got it! 🌟