Poly(ethene)

Poly(ethene), or polythene, is the polymer the 0620 syllabus uses to teach addition polymerisation. The examinable skill is moving between the monomer and the repeating unit: given ethene, draw the polymer; given the polymer, work back to ethene.

Key facts

Property	Detail
Repeating unit	-(CH2-CH2)-
Monomer	Ethene, CH2=CH2
Type	Addition polymer
Structure	Long-chain saturated hydrocarbon molecules
Key properties	Flexible, unreactive, electrical insulator, non-biodegradable
Main uses	Bags, film, bottles, insulation

Where this compound appears in 0620

• Polymers (Topic 12): addition polymerisation, and drawing the repeating unit from the monomer (and the reverse).
• Alkenes (Topic 12): poly(ethene) links back to the reactivity of the ethene double bond.
• Air quality and climate change (Topic 11): the disposal problems of non-biodegradable plastics.

Formation

n(CH2=CH2) -> -(CH2-CH2)-n

The C=C double bond of each ethene molecule opens and the monomers join end to end. Only one product forms — the polymer — because no small molecule is lost. That is what makes it addition rather than condensation polymerisation. Compare the two in polymers: addition vs condensation.

Properties and uses

Poly(ethene) is flexible, lightweight, waterproof, chemically unreactive and an electrical insulator, which is why it is chosen for the jobs below. It is also thermoplastic: it softens when heated and can be remoulded, which is what makes it possible to recycle by melting.

Use	Property exploited
Plastic bags and film	Flexible, lightweight, waterproof
Bottles and containers	Strong, mouldable, chemically resistant
Electrical insulation	Does not conduct electricity

Environmental issues

• Non-biodegradable: microorganisms cannot break the strong C-C backbone, so it persists for hundreds of years.
• Landfill and litter: waste accumulates and harms wildlife, especially in the oceans.
• Burning: poly(ethene) contains only carbon and hydrogen, so incomplete combustion produces toxic carbon monoxide and soot (not acidic HCl — that comes from PVC).

Disposal is managed by recycling (sorting, melting and remoulding), reducing use, developing biodegradable plant-based plastics, and incineration with energy recovery.

Worked exam question

The repeating unit of poly(ethene) is -(CH2-CH2)-.

(a) Name the monomer used to make poly(ethene) and draw its structure. (2 marks)

(b) Name the type of polymerisation and explain why no other product is formed. (2 marks)

(c) Explain why poly(ethene) is non-biodegradable and give one environmental problem this causes. (2 marks)

Mark scheme

• (a) The monomer is ethene [1]; drawn as CH2=CH2 with the C=C double bond clearly shown [1]
• (b) Addition polymerisation [1]; the monomers join when the C=C double bond opens, so the polymer is the only product and no small molecule is lost [1]
• (c) Microorganisms cannot break down the strong C-C bonds in the chain [1]; so it accumulates in landfill / litters the environment / harms marine life for a very long time [1]

Examiner note: in (a) the double bond must be drawn in the monomer — an ethene shown with a single bond scores zero. In (b), the “only product” mark depends on stating that no small molecule is lost, which is what separates addition from condensation.

Common exam mistakes

• Keeping the double bond in the repeating unit. In poly(ethene) all the bonds are single; the C=C double bond belongs only to the monomer.
• Omitting the continuation bonds and the n. The repeating unit needs a bond passing out through each side of the brackets and a subscript n outside.
• Calling it condensation polymerisation. Poly(ethene) forms by addition — no water or other small molecule is released.
• Saying burning it gives acidic HCl fumes. With only C and H present, incomplete combustion gives carbon monoxide and soot; HCl is produced by PVC, a different polymer.