Welcome to the World of Polymers!
In this chapter, we are going to explore polymers—the giant molecules that make up almost everything around us. From the plastic bottle on your desk to the DNA in your cells and the clothes you are wearing, polymers are the "bricks and mortar" of the modern world.
Don't worry if organic chemistry feels a bit like a puzzle right now. We are going to break it down step-by-step into two main types: addition polymers and condensation polymers. By the end of these notes, you’ll be able to draw them, name them, and even explain why some are better for the environment than others.
1. Addition Polymers: The "Hand-Holders"
You might remember these from your first year, but they are vital to understand before we move on. Addition polymers are made from monomers (the small building blocks) that contain a carbon-carbon double bond (C=C), like alkenes.
How they form
Imagine a room full of people (monomers). Each person has their arms crossed (the double bond). To form a polymer, everyone opens their arms and grabs the hand of the person next to them. Now you have a long, continuous chain!
Key point: In addition polymerization, the double bond "opens up" to form single bonds with neighboring molecules. No atoms are lost during this process.
Properties and Examples
- Intermolecular Forces: These chains are held together by van der Waals forces. The longer the chain, the stronger the forces, and the more solid the plastic.
- Poly(chloroethene) (PVC): This is a famous example. On its own, it is hard and brittle (think of water pipes). However, we can add plasticisers.
Analogy for Plasticisers: Imagine a deck of cards. If the cards are stuck together, you can't bend them. A plasticiser acts like a thin layer of oil between the cards, letting them slide past each other. This makes the polymer flexible (think of cling film or "fake leather" jackets).
Quick Review: Common Mistakes to Avoid
When drawing the repeating unit of an addition polymer, students often forget to:
1. Change the double bond to a single bond.
2. Extend the bonds out through the brackets (these are called "trailing bonds").
3. Use the letter \( n \) to show there are many units.
Key Takeaway: Addition polymers are made from alkenes, have a non-polar C-C backbone, and are generally very unreactive (inert).
2. Condensation Polymers: The "Water-Losers"
This is the core of the A-level content. Unlike addition polymers, condensation polymers form when two different monomers react together and lose a small molecule (usually water or \( HCl \)) in the process.
A. Polyesters
These are formed when a dicarboxylic acid reacts with a diol. They are held together by ester links (\( -COO- \)).
Example: Terylene (PET)
Made from benzene-1,4-dicarboxylic acid and ethane-1,2-diol.
Real-world use: Plastic bottles and clothing fibers.
B. Polyamides
These form when a dicarboxylic acid reacts with a diamine. They are held together by amide links (\( -CONH- \)).
- Nylon 6,6: Made from 1,6-diaminohexane and hexanedioic acid. (The "6,6" comes from the 6 carbons in each monomer!)
- Kevlar: Made from benzene-1,4-diamine and benzene-1,4-dicarboxylic acid. It is incredibly strong because of its flat structure and hydrogen bonding.
Did you know? Kevlar is used in bulletproof vests because the polymer chains are so tightly "glued" together by hydrogen bonds that they can stop a moving projectile!
Comparison of Intermolecular Forces
This is a favorite exam question! Why are polyamides stronger than polyesters?
1. Polyesters have permanent dipole-dipole forces between the polar \( C=O \) groups.
2. Polyamides have hydrogen bonding between the \( N-H \) of one chain and the \( C=O \) of another. Hydrogen bonds are much stronger!
Drawing Hint: The "Box Method"
Don't worry if the monomer structures look scary. Use "boxes" to represent the middle of the molecule:
\( HOOC-[BOX]-COOH \) + \( HO-[BOX]-OH \) \( \rightarrow \) \( -[BOX]-COO-[BOX]-O- \) + \( H_2O \)
Simply remove the \( -OH \) from the acid and the \( -H \) from the alcohol/amine to create the link!
Key Takeaway: Condensation polymers lose a small molecule, contain polar links (ester or amide), and often have stronger intermolecular forces than addition polymers.
3. Biodegradability and Disposal
Why can we recycle some plastics but others stay in landfills for 1,000 years? It all comes down to the bonds.
Addition Polymers vs. Condensation Polymers
- Addition Polymers (e.g., Polyethene): They have a backbone made of non-polar C-C bonds. These bonds are very strong and cannot be attacked by water or bacteria. Therefore, they are non-biodegradable.
- Condensation Polymers (e.g., Polyesters): They have polar bonds (like \( C=O \) and \( C-N \)). These are susceptible to hydrolysis (breaking down using water). Because they can be broken down, they are biodegradable.
Methods of Disposal
1. Landfill: Cheap and easy, but uses up land and non-biodegradable plastics stay there forever.
2. Incineration (Burning): Provides energy, but releases \( CO_2 \) (greenhouse gas). If you burn PVC, it also releases toxic \( HCl \) gas!
3. Recycling: Saves raw materials (crude oil), but it is difficult and expensive to sort different types of plastic.
Common Mistake: Students often say all polymers are non-biodegradable. Remember: Polyesters and polyamides can be hydrolyzed because of their polar linkages!
Key Takeaway: The polar bonds in condensation polymers allow them to be broken down by hydrolysis, making them more environmentally friendly than addition polymers.
Summary Checklist
Before you sit your exam, make sure you can:
- Identify if a polymer is addition or condensation.
- Draw the repeating unit from the monomers.
- Draw the monomers from a polymer chain.
- Explain why Kevlar is strong (Hydrogen bonding!).
- Explain why addition polymers are not biodegradable (non-polar C-C bonds).
- Discuss the pros and cons of recycling vs. incineration.
You've got this! Polymers might seem like a lot of drawing, but once you spot the patterns in the functional groups, it becomes much easier.