In polymer chemistry, polymerisation inhibitors (US: polymerization inhibitors) are chemical compounds added to monomers to prevent their self-polymerisation. Unsaturated monomers such as acrylates, vinyl chloride, butadiene and styrene require inhibitors for both processing and safe transport and storage. Many monomers are purified industrially by distillation, which can lead to thermally-initiated polymerisation. Styrene, for example, is distilled at temperatures above 100 °C whereupon it undergoes thermal polymerisation at a rate of ~2% per hour.[1] This polymerisation is undesirable, as it can foul the fractionating tower; it is also typically exothermic, which can lead to a runaway reaction and potential explosion if left unchecked. Once initiated, polymerisation is typically radical in mechanism and as such many polymerisation inhibitors act as radical scavengers.