Polymers

O Level / IGCSE Chemistry · Organic chemistry. A short explanation of the idea, the rules to remember, the mistake to avoid, a worked example and practice questions with answers.

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The idea

A polymer is a large molecule built up from many smaller molecules called monomers. Plastics are made from polymers.

In addition polymerisation the monomers are alkenes. The C=C bonds open and the molecules join in a chain, so the polymer is the only product. In condensation polymerisation the monomers have a reactive group at each end. Each time two join, a small molecule such as water is also formed. A dicarboxylic acid and a diamine give a polyamide such as nylon. A dicarboxylic acid and a diol give a polyester such as PET. Proteins are natural polyamides made from amino acids.

Most plastics are unreactive and do not break down naturally, which makes them hard to dispose of.

Rules to remember

Common mistake

Students leave the double bond in the repeat unit of an addition polymer. The C=C becomes a single C–C bond, and each of the two carbon atoms has a bond going out to the next unit.

Worked example

Part of an addition polymer chain is –CH2–CHF–CH2–CHF–CH2–CHF–. What is the monomer?

  1. Find the part that repeats along the chain. It is two carbon atoms long: –CH2–CHF–.
  2. Remove the two bonds that go out to the neighbouring units.
  3. Put a double bond between the two carbon atoms: CH2=CHF.
  4. Check: the monomer and the repeat unit both contain 2 C, 3 H and 1 F.

Answer: CH2=CHF

Practice questions

Try each one, then open the answer.

1. Which is a difference between addition and condensation polymerisation?

  1. A
    addition polymerisation also forms water as a second product
  2. B
    addition polymerisation always needs two different monomers
  3. C
    condensation polymerisation also makes a small molecule such as water
  4. D
    condensation polymerisation always needs monomers that contain C=C bonds
Show answer

Answer: C. In addition polymerisation alkene monomers join through their C=C bonds. In condensation polymerisation a small molecule is released each time two monomers link.

2. Why are many plastics a problem for the environment?

  1. A
    they do not rot away in landfill, and burning them can release toxic gases
  2. B
    they react with oxygen in the air, which lowers the oxygen level
  3. C
    they are made from plants, so they use up land needed for farming
  4. D
    they dissolve slowly in water and make the sea more alkaline
Show answer

Answer: A. Addition polymers are not broken down by microorganisms, so they persist in landfill and the sea; burning some, such as PVC, releases harmful gases.

3. Which pair of monomers can react to form a polyester?

  1. A
    a dicarboxylic acid and a diamine
  2. B
    an alkane and chlorine
  3. C
    ethene and propene
  4. D
    a dicarboxylic acid and a diol
Show answer

Answer: D. A diol has an –OH at each end, which forms ester links with the –COOH groups. A diamine would form amide links, giving a polyamide.

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