Monomers Molecules can polymerise if saturated molecule bonds are broken up. In this process, the molecule becomes a radical. Radicals are highly reactive, i.e. they look to find a bonding partner as fast possible which saturates the open bond. If there is a large number of other radicals in the vicinity of the radical, then there is a high probability that a radical will find another one with which it will then bond to form a heavier molecule that is then no longer a monomer. Heavy molecules can also exist as radicals and react with other heavy radicals. Often, the free bond of a radical is a carbon bond. Two free carbon bonds can very easily attach and saturate each other by means of a single or a double bond. If the larger molecules formed in this way again form radicals to which other carbon bonds are attached, polymers with any number of carbon bonds grow in this way.
In customised polymerization reactions, the mean chain length can be limited by adding radical interceptors to the reaction mixture. These are molecules whose radicals also attach very easily to free carbon bonds and form stable bonds. This process is referred to as ‘termination reaction’. The cleavage of molecular bonds for formation of radicals can be triggered in particular by high-energy radiation. In a plasma, high-energy UV radiation is generated. It makes sure that not only ions and molecules but also radicals are formed in the plasma-gas mix. For this reason, plasma is a very good tool for initiating polymerisation reactions. This is be foundation for the principle of plasma polymerisation.