How It Works: Why Monomers Join Together

⚙️ How It Works: Why Monomers Join Together

Addition polymerisation works because alkene monomers contain a carbon-carbon double bond (C=C). A double bond consists of two separate bonds between the same pair of carbon atoms — one sigma (σ) bond and one pi (π) bond. At GCSE level, the key point is that double bonds can "open" under the right conditions.

When heat, pressure, and a catalyst are applied, the pi (second) bond in the C=C double bond breaks. This leaves each carbon with a "free" bonding capacity — an unpaired electron that actively wants to bond with something. When this activated monomer meets another monomer, the free bonds link together, and the chain grows.

The process is called "addition" because all atoms from all monomers end up in the polymer — nothing is removed or produced as a by-product. This contrasts with condensation polymerisation (A-level concept) where water molecules are released as monomers join.

Why polymer properties vary: The length of the polymer chain, the side groups attached to the chain, and how the chains are arranged (tangled randomly or aligned in parallel) all affect the final properties. Longer chains with more intermolecular contact produce stiffer, stronger materials. Short, branched chains produce softer, more flexible materials like low-density polythene.