Polymers
Section: 11. Organic Chemistry | Syllabus: Cambridge IGCSE Chemistry (0620)
Polymers and Monomers
A polymer is a large molecule built up from many smaller molecules called monomers, joined together in a long chain. Plastics are materials made from polymers.
Poly(ethene): an Example of Addition Polymerisation
Poly(ethene) is formed by addition polymerisation, using ethene monomers. The C=C double bond in each ethene molecule opens up, and thousands of monomers join end-to-end to form one long polymer chain - no other product is formed.
n(CH₂=CH₂) → -(CH₂-CH₂)n-
(many ethene monomers → poly(ethene))
Worked Example
- Question (based on a real exam question): four statements are given about poly(ethene): (1) it is a plastic; (2) it is formed by addition polymerisation; (3) it is formed from small molecules called polymers; (4) it burns to produce carbon dioxide and hydrogen. Which statements are correct?
- Answer: 1 and 2. Statement 3 is wrong - the small molecules are called monomers, not polymers (the polymer is the large product). Statement 4 is wrong - combustion of poly(ethene), a hydrocarbon, produces carbon dioxide and water, not hydrogen.
- Question (based on a real exam question): draw the displayed formula of the monomer used to make poly(ethene).
- Answer: ethene, H₂C=CH₂ - two carbon atoms joined by a double bond, each bonded to two hydrogen atoms.
Common Mistakes
- ❌ Calling the small starting molecules "polymers" instead of "monomers"
- ✓ the small molecules that join together are monomers; the single large molecule they form is the polymer - it is easy to get these two terms swapped round
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