Addition polymerisation

AS Level Chemistry · Polymerisation. 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

In addition polymerisation many small alkene molecules (monomers) join to make one long chain. The π bond of each C=C breaks and the electrons are used to form single bonds to the next monomers. The polymer is the only product, and it is saturated.

The repeat unit is the smallest part of the chain that repeats. For a monomer with one C=C bond it has only two carbon atoms in the chain. Any other groups hang from the chain as side groups.

Poly(alkene)s are hard to dispose of. Their strong, non-polar C–C and C–H bonds are not attacked by microorganisms, so they are non-biodegradable. Burning them can give harmful products.

Rules to remember

Common mistake

Students put every carbon atom of the monomer into the chain, for example drawing poly(propene) as –CH2–CH2–CH2–. Only the two carbons of the C=C bond are in the chain; the CH3 is a side group, giving –CH2–CH(CH3)–.

Worked example

A section of an addition polymer is –CH2–CCl2–CH2–CCl2–CH2–CCl2–. Identify the monomer.

  1. The pattern repeats every two chain carbons, so the repeat unit is –CH2–CCl2–.
  2. Put a double bond back between these two carbons: CH2=CCl2.
  3. Both chlorine atoms are on the same carbon, carbon 1.

Answer: 1,1-dichloroethene, CH2=CCl2.

Practice questions

Try each one, then open the answer.

1. What is the repeat unit of poly(propene)?

  1. A
    –CH2–CH2–CH2–
  2. B
    –CH2–CH(CH3)–
  3. C
    –CH(CH3)=CH–
  4. D
    –CH2–CH2–CH(CH3)–
Show answer

Answer: B. In addition polymerisation the C=C of the monomer opens to a C–C single bond and every atom is kept. Propene, CH2=CHCH3, gives –CH2–CH(CH3)–; the methyl group hangs off the backbone rather than joining it.

2. What is the repeat unit of poly(tetrafluoroethene), made from CF2=CF2?

  1. A
    –CF2=CF2–
  2. B
    –CF2–
  3. C
    –CF2–CF2–
  4. D
    –CF=CF–
Show answer

Answer: C. The C=C double bond opens to form single bonds to neighbouring units, so the repeat unit is the two-carbon unit –CF2–CF2– with no double bond.

3. Which statement describes addition polymerisation?

  1. A
    many monomers containing a C=C bond join to form one saturated product, with no other product
  2. B
    monomers join together with the loss of a small molecule such as water
  3. C
    monomers containing –OH and –COOH groups join to form ester links
  4. D
    small alkanes join together when heated with a catalyst
Show answer

Answer: A. In addition polymerisation the π bonds of the alkene monomers open and link the monomers into a long saturated chain; the polymer is the only product. Loss of a small molecule describes condensation.

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