Polymerisation is a chemical reaction where monomer molecules join together to form polymer chains or three-dimensional networks. The ‘drying’ of oil is a complex process, but involves the double bonds of the unsaturated fatty acids in the triglyceride molecules (the monomer units) reacting with oxygen in the air.
These triglyceride molecules can then link together, forming a three-dimension network. In paint, where an oil binder has been mixed with pigments, these cross-linking reactions can sometimes be catalysed by certain pigments. This might affect the speed and type of polymerisation reactions which take place and therefore the physical and chemical characteristics of the paint film that is produced.