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Addition polymerization
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A polymer is a large molecule formed by the joining together of many smaller molecules, called monomers. Polymerization is the chemical process of forming a polymer. In addition polymerization the reaction which occurs is an addition reaction - that is the monomers join together with no other product being formed. This contrasts with condensation polymerization where the monomers join together with the loss of many smaller molecules, usually water or hydrogen chloride gas. You will see this type of reaction when you make nylon. In order to have addition polymerization the monomer molecule must have at least one double bond. This double bond opens up during polymerization, and allows the monomer molecule to be attached to two neighbouring monomer molecules, which in turn join to further monomers to form a long chain. |
It is difficult to represent the whole process on paper, as the polymer molecule may contain many thousands of monomer units. As a result we tend to show only a small section of the polymer chain. We use the letter n to represent a large, but variable, number of monomer molecules joined together in the polymer chain. The following equation shows the formation of poly(ethene) from ethene:
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The polymerization of ethene is carried out at high temperature and pressure and in the presence of a nickel catalyst.
Remember that it is the double bonds that join together. Students often make the mistake, with more complicated monomers, of joining together other parts of the molecules. If you are asked to draw out this type of equation, start by writing down two C=C double bonds right next to each other, so that no other groups can be placed between them. Draw in the attached atoms, and then remove the double bonds and join them together - this is your section of polymer. The equation for the formation of poly(propene) may make this a little clearer:
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This type of reaction is often started using a free radical initiator.
