Classification of Polymers – A First Look
You already know what a polymer is: a long chain made of many small repeating units (monomers). But not all polymers are the same. Think about the difference between a rubber band, a plastic water bottle, and a cotton T-shirt. They feel different, stretch differently, and are made from different starting materials. That's why we classify polymers — to understand their behaviour and how they're made.
The classification is done along four main axes: source, structure, mode of polymerization, and intermolecular forces. Each gives you a different lens to look at a polymer.
1. Classification by Source
This is the simplest: where does the polymer come from?
- Natural polymers – found in nature. Examples: cellulose (cotton, wood), proteins (wool, silk), natural rubber, starch, DNA.
- Synthetic polymers – man-made in a lab or factory. Examples: nylon, polyester, polythene, PVC, Teflon.
- Semi-synthetic polymers – natural polymers chemically modified. Examples: cellulose acetate (rayon), vulcanized rubber.
Semi-synthetic polymers are a middle ground. Rayon is made from natural cellulose but treated with chemicals — it's not fully natural, not fully synthetic.
2. Classification by Structure
This is about how the polymer chains are arranged. Imagine a pile of spaghetti versus a ladder versus a tree branch.
- Linear polymers – long, straight chains. They pack closely, so they have high density, high melting points, and are strong. Example: high-density polythene (HDPE).
- Branched polymers – chains with side branches. They pack less tightly, so they are less dense and melt at lower temperatures. Example: low-density polythene (LDPE).
- Cross-linked polymers – chains connected by covalent bonds in a 3D network. They are rigid, hard, and do not melt on heating (thermosetting). Example: bakelite, melamine.
Don't confuse "branched" with "cross-linked". Branched chains are still separate; cross-linked chains are chemically bonded together into one giant molecule.
3. Classification by Mode of Polymerization
This is about the chemical reaction that builds the polymer. There are two fundamentally different mechanisms.
Addition polymerization – monomers add to each other without losing any small molecule. The monomer usually has a double bond that opens up. The polymer has the same empirical formula as the monomer.
Example: ethene (CH2=CH2) → polythene (−CH2−CH2−)n
Condensation polymerization – monomers join by eliminating a small molecule like water, ammonia, or HCl. The polymer has a different formula from the monomer (because something is lost). These polymers usually have functional groups like ester, amide, or ether links.
Example: hexamethylenediamine + adipic acid → nylon-6,6 + water
| Feature | Addition | Condensation |
|---------|----------|--------------|
| By-product | None | Small molecule (H₂O, NH₃, etc.) |
| Monomer | Contains C=C double bond | Two different functional groups |
| Example | Polythene, PVC, Teflon | Nylon, polyester, bakelite |
4. Classification by Intermolecular Forces
This is the most practical classification for everyday use. The forces between polymer chains determine whether the material is stretchy, stiff, or fibre-like.
The strength of intermolecular forces increases in this order:
Elastomers < Plastics (Thermoplastics < Thermosetting) < Fibres
Let's unpack that.
Elastomers – weak van der Waals forces between chains, but the chains are lightly cross-linked. They can stretch a lot and snap back. Example: natural rubber, neoprene.
Fibres – strong intermolecular forces (hydrogen bonds, dipole-dipole) plus linear, closely packed chains. They are strong, tough, and have high tensile strength. Example: nylon, polyester, silk.
Plastics – intermediate forces. They are further divided into:
- Thermoplastics – linear or branched chains with moderate forces. They soften on heating and harden on cooling (reversible). Example: polythene, PVC, polystyrene. …