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The process of turning monomers into a polymer is called polymerisation. Synthesis processes are classified using two different systems: the first according to the way in which the polymers grow, and the second according to the mechanism by which the chemical reactions occur.
In the first system, synthesis processes are either chain growth or step growth:
- In chain growth polymerisation, an initiator molecule starts the reaction. Monomers are then joined onto an initiated chain. This is a fast process that produces long chains soon after the reaction begins. A chain is terminated when no more monomers are available or when the chain reacts with another chain.
- In step growth polymerisation, any monomer may react with any other, so no initiator is required. Monomers first join to form short chains (dimers, trimers…), which start to combine into longer chains once the supply of monomers begins to run out. Step growth is slower than chain growth; the process is terminated when all available monomers are used up.
In the second system, synthesis processes are either addition reactions or condensation reactions:
- The polymer is the only product of addition reactions. They often involve free radicals – chemical species having an un-paired electron.
- Condensation reactions involve the loss of a small molecule such as H2O. Because of this the polymer is not the only product of the reaction. They can result in polymers whose repeat unit does not at first sight resemble the monomer.
Chain growth polymerisation usually occurs by an addition reaction, and step growth polymerisation usually occurs by a condensation reaction. However there are several exceptions, which is why the two different systems of classification are needed.
|Polymer growth mechanism||Chain growth||Step growth|
|Chemical reaction mechanism||Addition reactions|
The following application describes the mechanisms of the chain and step growth polymerisation.
Effect of number of functional groups
The functional groups on a monomer are those that react during synthesis to form bonds with other monomers. The number of these influences the structure of the synthesis product.
|Number of functional groups on monomer||Resulting molecule|
|One||Dimer (a molecule formed from two monomers)|
|Two||Straight chain polymer|
|Three or more||Branched or network polymer|