Addition Polymers: Building Big from Small
Welcome to your study notes on Polymers! This topic is a key part of the Developing Fuels (DF) section of your Salters Chemistry course. You might wonder why "Polymers" are in a section about fuels. It’s because the same crude oil we use for petrol also provides the alkenes used to make plastics.
In this chapter, we are going to look at how tiny molecules (monomers) join together to create giant chains (polymers). Don't worry if this seems like a lot of drawing at first—once you see the pattern, it becomes much easier!
1. What is Addition Polymerisation?
Addition polymerisation is a process where many small, unsaturated molecules (called monomers) join together to form a very long chain (the polymer).
Prerequisite Check: Remember that "unsaturated" means the molecule contains at least one C=C double bond. In the "Developing Fuels" section, we focus on alkenes as our starting materials.
The "Paperclip" Analogy: Imagine you have a box of 1,000 individual paperclips. Each paperclip is a monomer. If you hook them all together to make one long chain, that chain is the polymer. In chemistry, we "hook" them by opening up the C=C double bond.
Did you know? The word "Polymer" comes from the Greek words poly (many) and meros (parts). So, it literally means "many parts"!
2. How the Process Works (Step-by-Step)
When addition polymerisation happens, the \(\pi\)-bond (the second part of the double bond) breaks. Each carbon atom from that double bond then uses its newly "free" electron to form a single \(\sigma\)-bond with a neighboring molecule.
Step-by-Step Guide to Drawing a Polymer:
1. Start with your monomer (e.g., ethene). Draw it in a "H-shape" with the C=C bond in the middle.
2. Change the C=C double bond into a C-C single bond.
3. Draw square brackets around the unit.
4. Extend the side bonds through the square brackets (this shows the chain continues).
5. Add a small letter \(n\) at the bottom right corner to show there are "many" of these units.
Common Mistake to Avoid: Never keep the double bond in your polymer drawing! The double bond must "open up" to allow the molecules to join together. If you draw a double bond inside the brackets, the chain can't grow.
Quick Review: Key Terms
• Monomer: The small starting molecule (e.g., ethene).
• Polymer: The long-chain molecule made of many monomers (e.g., poly(ethene)).
• Repeating Unit: The specific arrangement of atoms in the polymer that is repeated over and over.
3. Relating Monomers to Polymers (and Vice Versa)
The OCR syllabus requires you to be able to move between the structure of a monomer and the structure of the polymer. This is like a puzzle—you just need to find the repeating pattern.
A. From Monomer to Polymer
If you are given propene \(CH_2=CHCH_3\):
1. Arrange it so the C=C is central: \(C=C\) with two H's on the left carbon, and one H and one \(CH_3\) group on the right carbon.
2. Open the bond: \( -[CH_2-CH(CH_3)]_n- \).
3. The name simply becomes poly(propene). Always put the monomer name in brackets after "poly".
B. From Polymer to Monomer
If you are given a long chain, look for the repeating unit (the smallest section that repeats).
1. Identify the two carbon atoms that make up the backbone of the repeat unit.
2. Draw those two carbons by themselves.
3. Put the C=C double bond back between them.
4. Add the groups that were attached to them in the polymer.
Memory Aid: "Polymer to Monomer? Double the bond, stop the chain!" (Remove the brackets and trailing bonds, then add the double bond back in).
4. Real-World Examples in "Developing Fuels"
Since we are in the Developing Fuels section, we focus on synthetic polymers derived from the petrochemical industry:
1. Poly(ethene): Used for plastic bags and bottles. It is flexible and cheap.
2. Poly(propene): Used for ropes and crates because it is stronger than poly(ethene).
3. Poly(chloroethene) (PVC): The monomer is chloroethene. PVC is used for water pipes and window frames because it is stiff and chemically resistant.
4. Poly(tetrafluoroethene) (PTFE): Also known as Teflon. Used for non-stick pans because the C-F bonds are incredibly strong and slippery.
Key Takeaway: The properties of the polymer (like strength or flexibility) depend on the monomer used and the intermolecular forces between the long polymer chains.
5. Summary Checklist
Before you move on, make sure you can:
• Identify a monomer from a polymer structure.
• Draw the repeating unit for any given alkene monomer.
• Explain that addition polymerisation involves "opening" the C=C double bond.
• Use the correct notation: square brackets, bonds passing through the brackets, and the \(n\) symbol.
Don't worry if drawing the 3D-style monomers feels tricky at first. Just remember to keep the carbons of the double bond in a straight line and hang everything else (H, \(CH_3\), Cl) off the top and bottom! You've got this!