Describe the differences between addition and condensation polymerisation

Organic Chemistry – Polymers

What is Polymerisation?

Polymerisation is the process of linking many small molecules (monomers) together to form a large chain called a polymer. Think of it like building a long train from many carriages – each carriage is a monomer, and the whole train is the polymer.

Addition Polymerisation

In addition polymerisation, monomers with double bonds (like \$C=C\$) add together without losing any atoms. The reaction is usually a chain‑growth process and no small molecules are released.

  • 🔗 Key Feature: Chain‑growth mechanism.

  • 🧪 Typical Monomers: Ethylene (\$C2H4\$), Styrene (\$C8H8\$).

  • ⚙️ Example Polymers: Polyethylene (\$(-CH2-CH2-)n\$), Polystyrene (\$(-CH2-CH(Ph)-)_n\$).

  • 💧 By‑product: None.

🧩 Analogy: Imagine Lego bricks snapping together – each brick (monomer) attaches to the next without any glue or extra pieces.

Condensation Polymerisation

Condensation polymerisation (also called step‑growth) involves monomers with two different functional groups. When they react, a small molecule such as water or methanol is released.

  • 🔗 Key Feature: Step‑growth mechanism.

  • 🧪 Typical Monomers: Diamines & diacids (e.g., hexamethylenediamine & adipic acid).

  • ⚙️ Example Polymers: Nylon‑6,6 (\$(-NH-(CH2)6-NH-CO-(CH2)4-CO-)n\$), PET (\$(-O-CH2-CH2-O-CO-Ph-CO-)n\$).

  • 💧 By‑product: Water, methanol.

🧪 Analogy: Think of building a chain with glue – each link (monomer) is joined by glue (reaction), and a drop of water is left behind.

Comparison Table

FeatureAddition PolymerisationCondensation Polymerisation
MechanismChain‑growthStep‑growth
Monomer typeUnsaturated (e.g., \$C=C\$)Two functional groups (e.g., –NH₂ & –COOH)
By‑productNoneSmall molecule (H₂O, MeOH)
Typical PolymersPolyethylene, PolystyreneNylon‑6,6, PET

Exam Tips

Key Points to Remember:

  1. Define addition polymerisation and condensation polymerisation clearly.

  2. List the main differences: mechanism, monomer type, by‑product, and examples.

  3. Use the comparison table to structure your answer.

  4. Show a simple reaction scheme for each type (you can draw it with text).

  5. Remember that condensation always releases a small molecule – a good hint for identification.

Good luck! 🚀