Question
Differentiate between addition polymerisation and condensation polymerisation with suitable examples. Give the mechanism and the type of monomers involved in each.
Solution — Step by Step
Addition polymerisation is a reaction in which monomers containing C=C double bonds (alkenes) join together repeatedly to form a polymer. No small molecule is eliminated — the monomers simply add to each other.
Type of monomers: Unsaturated compounds (with double or triple bonds) — alkenes, dienes
Mechanism: Chain growth via free radical, cationic, or anionic intermediates
General pattern:
Each monomer loses its double bond as it joins. The double bond electrons form new single bonds between monomers.
Condensation polymerisation is a reaction where monomers with two functional groups react repeatedly, and a small molecule (typically water, HCl, or methanol) is eliminated at each step.
Type of monomers: Bifunctional monomers — each must have two reactive groups (e.g., -NH₂ and -COOH, or -OH and -COOH, or two -OH groups + diisocyanate)
Mechanism: Step-growth — any two monomers or oligomers can react at any point
General pattern:
| Feature | Addition Polymerisation | Condensation Polymerisation |
|---|---|---|
| Monomer type | Unsaturated (C=C or C≡C) | Bifunctional (two reactive groups) |
| Byproduct | None | Small molecule (H₂O, HCl, etc.) |
| Mechanism | Chain growth | Step growth |
| Bond formed | C-C single bonds | Ester, amide, or other bonds |
| Examples | Polythene, PVC, Teflon, Polystyrene | Nylon-6,6; Dacron (Terylene); Bakelite |
| Polymer composition | Same as monomer (just addition) | Differs from monomer (atoms lost) |
Addition Polymer Examples:
- Polythene (Polyethylene): Monomer = ethylene (CH₂=CH₂). Forms: -[-CH₂-CH₂-]ₙ-
- PVC (Polyvinyl Chloride): Monomer = vinyl chloride (CH₂=CHCl). Used in pipes, flooring.
- Teflon (PTFE): Monomer = tetrafluoroethylene (CF₂=CF₂). Non-stick coating.
- Natural rubber: Monomer = isoprene (2-methylbuta-1,3-diene). A natural addition polymer.
Condensation Polymer Examples:
- Nylon-6,6: Monomers = hexamethylene diamine + adipic acid. Amide bonds formed (-CO-NH-). Water eliminated. Used in fibres, ropes.
- Dacron (Terylene/PET): Monomers = ethylene glycol + terephthalic acid. Ester bonds formed (-CO-O-). Used in polyester fabric.
- Bakelite: Monomers = phenol + formaldehyde. Condensation gives a cross-linked thermoset.
Use this quick test:
- Does the monomer have a C=C or C≡C double bond, and only that (no two separate functional groups)? → Addition polymerisation
- Does the monomer have two separate functional groups (like -NH₂ at one end and -COOH at the other)? → Condensation polymerisation
- Is a small molecule (water, HCl) also produced in the reaction? → Condensation polymerisation (confirmed)
Why This Works
The mechanism difference is fundamental:
In addition polymerisation, a reactive intermediate (radical or ion) opens the double bond of one monomer and adds to the next. The chain grows rapidly by consuming monomers one at a time.
In condensation polymerisation, any two molecules with complementary functional groups can react. A dimer can react with another monomer, or with another dimer, or with a trimer — the chain grows gradually in many places simultaneously (step growth). This is why condensation polymers typically have lower molecular masses unless driven to completion.
Alternative Method — Classify by Bond Type in Polymer
Another way to remember:
- Polymer backbone is purely C-C bonds → addition polymer
- Polymer backbone has heteroatoms (O, N) in the chain → condensation polymer (ester = -CO-O-; amide = -CO-NH-)
Check: Nylon has -CO-NH- in backbone → condensation. Polythene has only C-C → addition.
JEE Main and NEET both test polymer identification. The question usually gives a monomer structure and asks for the polymer type and name. For Nylon-6 (from caprolactam — a cyclic amide): this is also a condensation polymer (ring-opening). Don’t confuse with Nylon-6,6 which uses two different monomers.
Common Mistake
Students often say “Nylon is an addition polymer because nylon fibres are made by adding heat.” The word “addition” in everyday language is different from “addition polymerisation” in chemistry. Nylon is a condensation polymer (amide bonds, water eliminated). Judge by the chemistry — monomer type and whether a byproduct forms — not by the everyday meaning of the word “addition.”