Students should be able to: explain how low density and high density…

The idea

Ethene molecules can be joined by addition polymerisation to form poly(ethene), but the conditions used change the polymer's structure and properties. Low density poly(ethene), LDPE, is made at high pressure with a catalyst that produces branched chains, so the chains pack loosely and the polymer is flexible and less dense. High density poly(ethene), HDPE, is made at lower pressure with a different catalyst that produces straighter, less branched chains, so the chains pack closely, giving a denser, stronger, more rigid polymer. The same structural idea explains the difference between thermosoftening and thermosetting polymers: thermosoftening polymers have separate chains held by weak intermolecular forces and can slide when heated, whereas thermosetting polymers have covalent cross-links forming a rigid network that cannot slide, so they do not melt.

What earns the marks

• Ethene undergoes addition polymerisation, in which many ethene molecules join to form a long poly(ethene) chain.
• Low density poly(ethene) is produced using high pressure and a catalyst that gives branched polymer chains.
• The branching in low density poly(ethene) prevents close packing, so the chains are held by weaker forces overall and the polymer is flexible and less dense.
• High density poly(ethene) is produced using lower pressure and a different catalyst that gives straighter, less branched chains.
• Straighter chains pack closely together in high density poly(ethene), giving stronger intermolecular forces, a denser and more rigid polymer.
• Thermosoftening polymers consist of separate chains with weak intermolecular forces, so the chains can slide when heated and the polymer softens.
• Thermosetting polymers have covalent cross-links between chains forming a rigid three-dimensional network, so the chains cannot slide and the polymer does not melt.

Common mistakes and how to put them right

• Thinking that low density and high density poly(ethene) are made from different monomers.

  Put it right: Both are made from ethene, and only the polymerisation conditions and chain branching differ.

• Stating that high density poly(ethene) has stronger covalent bonds between its chains.

  Put it right: The difference lies in how closely the chains pack and the resulting intermolecular forces.

• Describing thermosetting polymers as having no forces between chains.

  Put it right: They have covalent cross-links that hold the network rigid.

Exam tips

• For each polymer, link production conditions to chain structure and then to a property, for example high pressure gives branched chains, which pack loosely and make LDPE flexible.
• When comparing thermosoftening and thermosetting polymers, always refer to the forces or bonds between chains, not within them.
• Use clear comparative language such as 'branched versus straighter chains' and 'weak intermolecular forces versus covalent cross-links' to structure your answer.