Benzoyl peroxide (BPO) can be used as a catalyst for the polymerization of styrene and other molecules to form polystyrene. This kind of polystyrene has a very wide range of uses, and goes deep into our daily lives. It can be used to make plastic products from cups to various packaging materials.
The polymerization process of polystyrene is a free radical chain reaction, but unless extreme conditions are used, it is difficult to form initial free radicals from stable styrene molecules. This is where initiators such as benzoyl peroxide need to be added. Benzoyl peroxide (BPO) can break the weak O-O bond (because two oxygen atoms have the same electronegativity, they will fission) and generate two benzoyl radicals.
The "dot" next to the atom indicates that there are unpaired electrons, which is the reason why free radicals are so reactive. They try to find another paired free radical to generate covalent bonds. These benzoyl groups then eliminate the CO 2 that forms the phenyl group. Because stable molecules will become reactive free radicals, this is called the initiation step, which will trigger the formation of free radicals.
Although benzoyl peroxide (BPO) only produces a small amount of phenyl groups in the reaction process, once formed, they can interact with styrene monomer in the propagation step. One free radical disappears, but is replaced by another, so the free radical "spreads" in the chain reaction.