According to the decomposition mechanism, it can be divided into thermal decomposition type, photodecomposition type and redox type. The specific types are shown in Figure 1.1 below:

Figure 1.1 Classification of free radical initiators

   According to the chemical structure, it can be divided into:
• Peroxides: including organic peroxides, such as dibenzoyl peroxide (BPO) and inorganic peroxides, such as potassium persulfate (KPS) and ammonium persulfate (APS);
• Azo compounds: such as azobisisobutyronitrile (AIBN);
• Others: including some special photoinitiators, radiation sources, etc.
   The following is a brief introduction to the characteristics of different types of initiators:
1. Peroxygen initiator
   Peroxygen initiators are divided into organic peroxygen and inorganic peroxygen. Commonly used initiators include dibenzoyl peroxide (BPO), lauroyl peroxide (LPO), tert-butyl peroxybenzoate (TBPB), etc. Inorganic peroxides are represented by persulfates, including potassium persulfate (KPS) and ammonium persulfate (APS).
   The molecular structure of peroxygen initiators contains peroxy groups, which homolyze to generate free radicals when exposed to heat or light. The decomposition formula is shown in the figure below:

Figure 1.2 Decomposition reaction of diacyl peroxide

2. Azo initiators
   The use temperature of azo initiators is in the range of 45 to 80°C. The decomposition reactions are almost all first-order reactions. Only one kind of free radical can be formed. During decomposition, nitrogen gas escapes and there is no induced decomposition. In addition, azo initiators are relatively stable and can be safely stored alone.
   Among them, the most commonly used are azobisisobutyronitrile (AIBN) and azobisisoheptanitrile (ABVN).

Figure 1.3 Decomposition reaction of azobisisobutyronitrile

3. Redox s ystem
   Inorganic peroxides, high-valent transition metal ions, high-valent oxidation atoms and a small amount of reducing agents are combined to generate primary free radicals through oxidation-reduction reactions to initiate polymerization reactions. This type of initiating system is called a water-soluble oxidation-reduction initiating system.
   Most organic peroxides are insoluble in water, but soluble in common solvents and most olefinic monomers; when preparing a redox system, an oil-soluble reducing agent must be used. Organic peroxide-tertiary amine systems are commonly used, among which dibenzoyl peroxide and N,N'-dimethylaniline are the most typical systems. This system decomposes much faster than dibenzoyl peroxide alone.
   Compared with the previous peroxides and azo compounds, the decomposition activation energy of the oxidation-reduction initiating system is lower, so it can initiate polymerization at a lower temperature (room temperature or below room temperature).
   It should be noted that if metal ions remain in the polymer, some properties of the polymer will usually be deteriorated.

Ⅲ. How to choose the initiator?
   From a thermodynamic point of view, most olefin monomers have a tendency to polymerize, but whether the actual polymerization reaction can occur depends on kinetic conditions, that is, a sufficient polymerization rate is required. Therefore, the reasonable selection of initiators is particularly critical.
   Initiator selection should follow the following principles:
• Select the type according to the polymerization method: commonly used oil-soluble initiators (such as peroxides, azo compounds) for bulk and suspension polymerization; water-soluble initiators (such as persulfate) for emulsion and aqueous solution polymerization.
• Avoid reactions with system components: If the system is reducing, peroxygen initiators should be avoided to prevent side reactions.
• Match half-life to polymerization temperature: The initiator decomposition rate should match the polymerization temperature to control the reaction rate.
   In addition, in industry, initiators with different half-lives are often used in combination to maintain a uniform and stable rate during the polymerization process.
   Table 1-1 below lists the types of initiators that can be selected at different temperatures:

   Finally, try to choose an initiator whose decomposition products are less toxic and safe to store. Factors that need to be considered when selecting an initiator include whether it is easy to color and whether it is toxic. Ease of use and economic benefits are also factors to consider.
A summary of common thermal initiator and photoinitiator types is shown in the following table:

Table 1-3 Summary of common photoinitiators

Table 1-4 Summary of common redox systems

Ⅳ. New initiators and development trends
   As environmental protection regulations become increasingly strict and application fields continue to expand, initiators are developing in the direction of high efficiency, safety, and environmental protection:
• Macromolecule photoinitiator: low migration, low odor, suitable for food packaging materials;
• Visible light initiator: low energy consumption, strong penetration, suitable for thick film curing and biomedical applications;
• Aqueous redox system: environmentally friendly, suitable for low-temperature emulsion polymerization;
• Enzymatic/bio-based initiator: a truly green initiator system.
Ⅴ. Practical application cases
• Polyvinyl chloride (PVC) production: AIBN or percarbonate initiators are commonly used to perform suspension polymerization at 45 - 65°C to balance activity and safety.
• Acrylic emulsion: KPS or APS are often used as water-soluble initiators, and the redox system is used to achieve low-temperature polymerization and improve the uniformity of molecular weight distribution.
• UV curable coating: Using α-hydroxyketone photoinitiator, curing is completed within a few seconds under UV lamp irradiation, which is highly efficient and energy-saving.
Ⅵ. Conclusion
   Although free radical initiators do not directly participate in the final product, they are like the "engine" of the polymerization reaction, directly determining the efficiency, controllability and product performance of the reaction.
   Selecting an initiator is a science as well as an art - it is necessary to find a delicate balance between activity and stability, efficiency and safety, cost and environmental protection. As the demand for green chemistry and precise synthesis increases, the development of a new generation of initiators that are low-temperature, efficient, biocompatible, and can respond intelligently is becoming an important direction at the forefront of the field.

References
[1] Pan Zuren, Sun Jingwu, "Polymer Chemistry", Chemical Industry Press, 1980.
[2] Pan Zuren, Ding Zaizhang "Free Radical Polymerization", Chemical Industry Press, 1983.
[3] Wang Jian, Zhang Qian, Zhao Jinyuan, et al. Research progress on free radical polymerization initiators [J]. Plastic Additives, 2022, (04): 29-34+40.
[4] Xu Cheng, Tang Huadong. Research progress on free radical polymerization initiators [J]. Zhejiang Chemical Industry, 2015, 46(06): 34-37.
[5] Li Xuechun, Sun Fang. Introduction and research progress of free radical photoinitiators [J]. University Chemistry, 2021, 36(06): 5-14.