Unsaturated monomers such as acrylates, vinyl chloride, butadiene and styrene require inhibitors for both processing and safe transport and storage.
[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.
[2] In an industrial setting compounds from both classes will usually be used together, with the true inhibitor providing optimal plant performance and the retarder acting as a failsafe.
Certain compounds marketed as true inhibitors, such as p-phenylenediamines, phenothiazine and hydroxylamines like HPHA and DEHA, are also thought to react through the intermediary of aminoxyl radicals.
For styrene, nitrophenol compounds such as dinitro-ortho-cresol and di-nitro-sec-butylphenol (DNBP) have long been the important,[7] however they are coming under regulatory pressure due to their high toxicity.