A polymer is a large molecule containing many repeating structural units. In addition polymerisation, alkene monomers join as C=C bonds become C–C bonds in the chain. No small molecule is eliminated. Ethene forms poly(ethene), with repeat unit –CH2–CH2–; propene forms –CH2–CH(CH3)–. Show brackets around one repeat unit, a bond passing through each bracket and n outside to indicate repetition.
Polymer properties depend on chain length, branching, attractions and cross-links. A thermoplastic can soften as chains gain mobility without normally breaking their covalent backbones. Extensive covalent cross-links restrict movement and can prevent melting before decomposition. A polymer name or recycling code alone does not prove biodegradability. Use the specified repeat structure to count atoms and identify whether its formation is addition rather than condensation.
Example 1
Give the repeat unit formed from chloroethene, CH2=CHCl.
The double bond opens to give –CH2–CHCl–. Chlorine remains attached to the same carbon in each repeat unit.
Example 2
A simplified poly(ethene) segment contains 100C2H4 repeat units. Ignoring end groups, how many carbon atoms are present?
Each repeat unit contributes two carbons, so 100×2=200 carbon atoms.
Watch out:Addition polymerisation joins unsaturated monomers without eliminating a small molecule.
Addition polymers and repeat units practice | Year 11 Chemistry | Ascent Arcade