Polymers
Master the chemistry of large molecules built from smaller units — and understand why they matter in the real world.
The Big Idea: Polymers are massive molecules made by joining many identical or different smaller molecules (monomers) together with covalent bonds. There are two main ways to build them — addition (simple joining) and condensation (joining with water loss) — each with completely different properties and environmental consequences.
Polymers: The Basics
Before we talk about how to make polymers, we need to understand what they are.
What's a Polymer?
A polymer is a very large molecule made from many smaller molecules called monomers joined together by strong covalent bonds. Think of it like a long chain where each link is a monomer.
Definition
Polymer: A large molecule made from many monomers joined by covalent bonds.
Monomer: A small molecule that can join with others to form a polymer.
The key word is many — we're not talking about two or three units. Polymers can contain hundreds or thousands of monomers. This is why they're so different from regular molecules: they're much larger, often stronger, and have completely different chemical properties.
Common Examples
- Poly(ethene) — made from ethene monomers; used for plastic bags and bottles
- Poly(chloroethene) (PVC) — made from chloroethene monomers; used for pipes, vinyl records
- Nylon — made from two different monomers; used for fibres, ropes
- PET (polyethylene terephthalate) — used for plastic bottles, clothing fibres
Why Size Matters
Polymer molecules are extraordinarily large compared to regular molecules. If you tried to write out the full chemical formula showing every atom, it would be kilometres long. This is why we use something called repeat units — we write out just one repeating section and put an n subscript to mean "this pattern repeats many times."
💡 Quick Check: A polymer is not a mixture of monomers — it's a single large molecule where all the monomers are covalently bonded into one chain.
Addition Polymerisation
This is the simpler of the two polymer-making routes. It's called "addition" because monomers simply add on to the growing chain — nothing is removed.
How It Works
Addition polymerisation only happens with monomers that contain a carbon-carbon double bond (C=C), also called alkenes. Here's the process:
The Addition Polymerisation Mechanism
1. The C=C double bond breaks in each monomer
2. Monomers link together by forming new single bonds
3. The polymer chain grows longer and longer
4. Only one product is made — the polymer itself
Why Only One Product?
This is the crucial distinction: in addition polymerisation, nothing leaves the system. All the atoms from all the monomers end up in the final polymer. No water is lost, no small molecules are produced as byproducts — just a very long polymer chain.
The Classic Example: Ethene → Poly(ethene)
Let's walk through the most important example you need to know.
Ethene monomer: Has the structure C₂H₄ with a C=C double bond.
H H
| |
C = C
| |
H H
(ETHENE MONOMER)
When many ethene monomers undergo addition polymerisation, the double bonds break and the monomers link into a long chain:
Condensation Polymerisation
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