<Tr> <Td> H 2 O 2 + Fe 2 + ⟶ Fe 3 + + HO − + HO ⋅ (\ displaystyle (\ ce ((H2O2) + Fe ^ (2 +) -> (Fe ^ (3 +)) + (HO ^ (-)) + HO ^ (.)))) Figure 3: Redox reaction of hydrogen peroxide and iron . </Td> </Tr> <Dl> <Dt> Persulfates </Dt> <Dd> The dissociation of a persulfate in the aqueous phase (Figure 4). This method is useful in emulsion polymerizations, in which the radical diffuses into a hydrophobic monomer - containing droplet . Figure 4: Thermal degradation of a persulfate </Dd> <Dt> Ionizing radiation </Dt> <Dd> α -, β -, γ -, or x-rays cause ejection of an electron from the initiating species, followed by dissociation and electron capture to produce a radical (Figure 5). Figure 5: The three steps involved in ionizing radiation: ejection, dissociation, and electron - capture </Dd> <Dt> Electrochemical </Dt> <Dd> Electrolysis of a solution containing both monomer and electrolyte . A monomer molecule will receive an electron at the cathode to become a radical anion, and a monomer molecule will give up an electron at the anode to form a radical cation (Figure 6). The radical ions then initiate free radical (and / or ionic) polymerization . This type of initiation of especially useful for coating metal surfaces with polymer films . Figure 6: (Top) Formation of radical anion at the cathode; (bottom) formation of radical cation at the anode </Dd> <Dt> Plasma </Dt> <Dd> A gaseous monomer is placed in an electric discharge at low pressures under conditions where a plasma (ionized gaseous molecules) is created . In some cases, the system is heated and / or placed in a radiofrequency field to assist in creating the plasma . </Dd> <Dt> Sonication </Dt> <Dd> High - intensity ultrasound at frequencies beyond the range of human hearing (16 kHz) can be applied to a monomer . Initiation results from the effects of cavitation (the formation and collapse of cavities in the liquid). The collapse of the cavities generates very high local temperatures and pressures . This results in the formation of excited electronic states, which in turn lead to bond breakage and radical formation . </Dd> <Dt> Ternary Initiators </Dt> <Dd> A ternary initiator is the combination of several types of initiators into one initiating system . The types of initiators are chosen based on the properties they are known to induce in the polymers they produce . For example, poly (methyl methacrylate) has been synthesized by the ternary system benzoyl peroxide - 3, 6 - bis (o - carboxybenzoyl) - N - isopropylcarbazole - di - η - indenylzicronium dichloride (Figure 7). Figure 7: benzoyl peroxide - 3, 6 - bis (o - carboxybenzoyl) - N - isopropylcarbazole - di - η - indenylzicronium dichloride This type of initiating system contains a metallocene, an initiator, and a heteroaromatic diketo carboxylic acid . Metallocenes in combination with initiators accelerate polymerization of poly (methyl methacrylate) and produce a polymer with a narrower molecular weight distribution . The example shown here consists of indenylzirconium (a metallocene) and benzoyl peroxide (an initiator). Also, initiating systems containing heteroaromatic diketo carboxylic acids, such as 3, 6 - bis (o - carboxybenzoyl) - N - isopropylcarbazole in this example, are known to catalyze the decomposition of benzoyl peroxide . Initiating systems with this particular heteroaromatic diket carboxylic acid are also known to have effects on the microstructure of the polymer . The combination of all of these components--a metallocene, an initiator, and a heteroaromatic diketo carboxylic acid--yields a ternary initiating system that was shown to accelerate the polymerization and produce polymers with enhanced heat resistance and regular microstructure . </Dd> </Dl> <Dd> The dissociation of a persulfate in the aqueous phase (Figure 4). This method is useful in emulsion polymerizations, in which the radical diffuses into a hydrophobic monomer - containing droplet . Figure 4: Thermal degradation of a persulfate </Dd> <Dd> α -, β -, γ -, or x-rays cause ejection of an electron from the initiating species, followed by dissociation and electron capture to produce a radical (Figure 5). Figure 5: The three steps involved in ionizing radiation: ejection, dissociation, and electron - capture </Dd>

Explain the role of benzoyl peroxide in chain polymerization reaction