Inhibitors are chemical substances that are used in small amounts to suppress the polymerization reaction of a monomer. An inhibitor has to be completely consumed before a polymerization reaction can proceed at normal rates. The time required to completely consume the inhibitor is often referred to as an ”induction” time. Inhibitors react with polymerization initiation radicals to produce products that cannot induce further reaction. Inhibitors are different from reaction ”retarders”. A retarder does not suppress the reaction but merely slows it down, i.e. the reaction continues to increase at a slower rate until the retarder is consumed. Some impurities in monomers can act as retarders.
Small amounts of inhibitors can substantially prolong the shelf life of a reactive monomer. Common polymerization inhibitors, typically antioxidants, include MEHQ (monomethyl ether hydroquinone), TBC (4-t-butylcatechol), HQ (hydroquinone), PTZ (phenothiazine), etc. The effectiveness of most commonly used inhibitors depends on the presence of dissolved oxygen to convert free radicals to peroxy radicals that in turn react with the inhibitor to stabilize the monomer. Both inhibitors and oxygen deplete over time. Understanding inhibitor requirements is essential for polymerization reactions safety.
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