Inhibition of Free Radical Polymerization: A Review

Polymerization reactions have caused several severe accidents in the past since they are prone to runaways due to their highly exothermic and auto-accelerating nature. The heat generated during these uncontrolled runaway reactions surpasses the heat removal capacity of the cooling system leading to the auto-acceleration of the reactions. If proper measures are not taken to attenuate this auto-accelerative nature, dangerous consequences ensue, such as rampant boiling of the reaction system fluids or vapor production from secondary reactions. Both these consequences may eventually lead to over-pressurization followed by a thermal explosion. Thus, to eliminate the associated risk, polymerization reactions in industries are carried out in the presence of inhibitors which are injected into the reaction system before the initiation of polymerization. In this review, I have summarized various accidents that have happened in the past due to runaway polymerization implicating that there is an urgent necessity to do further research in this relatively less explored field of polymerization inhibition. To this end, I have completed an exhaustive survey of the various types of inhibitors used in industries and their inhibition mechanisms focusing mainly on the auto-initiated polymerization of styrene, methyl methacrylate, and acrylic acid monomer. Lastly, the synergism in the inhibition performance of a mixture of two types of inhibitors was also compared and discussed in detail.