![]() Once that benzylic radical forms, it can continue to propagate, enchaining more styrene molecules. It is stabilized by delocalization because of the benzene ring. There are two possible radicals that could result, but only one actually forms. Note that this step takes place with regioselectivity. This is an example of a propagation step, because old radicals have disappeared and new radicals have appeared. The radical formed via initiation induces cleavage of the pi bond in styrene, resulting in a benzylic radical. Accordingly, tert-butyl peroxides and other such peroxides would not be used to initiate the polymerization of styrene. It's much more common to see them abstract hydrogen atoms. Oxygen-centered radicals do not typically add to double bonds. ![]() Simple peroxides are not very good at initiating radical polymerization, despite the fact that they also contain O-O bonds that are readily cleaved. Not just any radical will peform that task. Phenyl radicals and isobutyronitrile radicals are both able to initiate the polymerization of styrene. The benzoate radical that forms from that homolysis further decomposes via decarboxylation, rapidly giving rise to a phenyl radical. In this case, the initiation reaction is sparked by the weak O-O single bond. Benzoyl peroxide (BPO) is one common example. Other initiators can also be used in radical polymerizations. This radical is both tertiary and delocalized. When AIBN is used as an initiator, the compound decomposes partly because of the strong N-N triple bond that is formed, and partly because of the relatively stable radical that results. ![]() Radical reactions start with an initiation reaction, in which radicals first come into being. A typical example is the formation of polystyrene under the influence of the radical initiator, azoisobutyronitrile (AIBN). There are a number of very reliable methods of carrying out radical polymerization, leading to high molecular weight materials. Radical polymerization is a very common approach to making polymers. ![]()
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