Appreciate that knowledge and understanding of the production and…

The idea

Polymer chemistry shows how understanding grows and changes what can be made. Poly(ethene) was first produced by accident in the 1930s at very high pressure with a trace of oxygen; the chains branched, packed badly, and gave a soft, low density material. In the 1950s catalysts were developed that let ethene polymerise at low pressure and gave straight chains, which pack closely, attract each other over more of their length and give the rigid, higher density poly(ethene) used for pipes and crates. Later work added plasticisers, co-polymers and, more recently, biodegradable and more easily recycled polymers. The pattern to learn is that controlling the structure of the chain, usually through the catalyst, controls the properties of the material.

What earns the marks

• Describing a change in production, such as the move from high-pressure to catalysed low-pressure polymerisation.
• Linking that change to a change in chain structure, such as branched against straight chains.
• Linking the chain structure to a property such as density, rigidity or melting point.
• Naming a more recent development such as biodegradable polymers or improved recycling.

Common mistakes and how to put them right

• Writing only that polymers have improved over time, with no chemistry to support it.

• Saying high density poly(ethene) is denser because its molecules are heavier, rather than more closely packed.

• Offering a condensation polymer such as a polyester as an example of an addition polymer development.

• Describing the catalyst as changing the product's formula rather than the arrangement of its chains.

Exam tips

• Tie every historical point to a structural reason; the marks are for chemistry, not for dates.
• One well-developed example scores better than a list of unexplained names.
• Use branching and packing as your explanatory vocabulary for density and strength.