How to choose an initiator in the synthesis of acrylic resins

Emulsion polymerization proceeds almost entirely via a free-radical polymerization mechanism. The main function of the initiator is to generate free radicals, thereby enabling the growth of polymer chains and an increase in molecular weight. The active center and the site of polymer growth exist independently from each other within micelles in the aqueous phase. The type and amount of initiator are directly related to the polymerization rate, emulsion stability, as well as the yield and quality of the product. There are two main categories of initiators commonly used in emulsion polymerization. One category is thermal decomposition initiators, in which the initiator molecule itself undergoes thermal decomposition to produce free radicals ； Another category consists of redox initiators, which are composed of an oxidizing agent and a reducing agent, and generate free radicals through an electron transfer mechanism. 1. Thermal decomposition initiators: If sufficient vibrational energy is provided to the atoms, it can cause chemical bonds to break, but an appropriate amount of energy is required for this to occur in order for it to function as an initiator. If the required energy is very low, the chemicals will be unstable during storage and prone to decomposition ； If the required energy is very high, its decomposition rate is too slow. Compounds containing O-O and N=N bonds have a bond dissociation energy of 30–35 kcal/mol; they decompose very slowly at room temperature and can be considered stable. Their decomposition temperature lies between 60–80°C, making them suitable for use as initiators. The temperature for the emulsion polymerization of acrylic acid is usually between 50 and 90°C, with the initiator decomposing to produce radicals that enable the polymerization reaction to proceed. The main types of thermal decomposition initiators include: persulfates: potassium persulfate, sodium persulfate, ammonium persulfate; hydrogen peroxide; organic peroxides; diacyl peroxides; peracidic compounds; peracid esters; azo compounds. In the emulsion polymerization of acrylates, persulfates are commonly used as initiators. Persulfates are soluble in water but generally insoluble in monomers. The reaction is carried out at temperatures of 50–90°C, with a dosage of 0.5% or less of the monomer amount. Acidic media promote the decomposition of persulfates; the decomposition process is as follows: the sulfate ion radicals and hydroxyl radicals generated by the decomposition of the initiator remain at the ends of the polymer molecules after polymerization, endowing the polymer with hydrophilicity. Hydrogen ions are generated in the aforementioned reaction, causing the pH value of the system to decrease. As the pH value drops, the decomposition rate of persulfate increases. The decomposition rate of most initiators is related to the pH value. Many emulsion systems are sensitive to pH levels; changes in pH can affect the stability of the emulsion. To maintain stability, pH regulators, or pH buffers, such as sodium bicarbonate, ammonium bicarbonate, and mono-diphosphates, are typically added. It is very important to control the pH value of the emulsion system, but buffers are dielectrics that can affect the size of the latex ions and the stability of the emulsion. The typical addition amount is about 0.5% of the monomer amount. 2. Redox initiators: At lower reaction temperatures, free radicals can be generated through redox reactions; for example, the most commonly used persulfate and sulfite redox system, where an electron from the reducing agent sulfite is transferred to persulfate, resulting in sulfate and sulfite ions that initiate polymerization. The ends of the polymer chain segments are equipped with sulfate groups or sulfonic acid groups. Peroxysulfate and thiosulfate systems are also commonly used. There is also persulfate and sodium formate hyposulfite (di-white block): peroxide–sodium formate hyposulfite. Redox systems that use metal ions such as iron, chromium, and copper as reducing agents, for example: hydrogen peroxide–ferrous ion system; persulfate–ferrous ion system. Organic redox systems, for example: isopropylbenzene hydrogen peroxide and benzenesulfinic acid. The two components of a redox system are inert at room temperature, but they react rapidly when in contact with each other. The use of a redox initiation system allows for polymerization to be initiated at near room temperature or even at low temperatures of 5°C. Redox systems have many advantages; firstly, the activation energy required for chain scission is relatively low, at around 10 kcal/mol, whereas the activation energy for thermal decomposition initiators is between 30 and 35 kcal/mol. The persulfate-sodium bisulfite and hydrogen peroxide-ferrous sulfate combinations listed above are both commonly used. Lower polymerization temperatures yield higher polymer molecular weights, as higher reaction temperatures promote the transfer of free radicals to the solvent, monomers, and polymer chains, as well as cause chain termination reactions that reduce the average molecular weight of the polymer. Secondly, it is also easy to control the polymerization reaction rate, saving energy consumption. Oil-soluble initiators such as benzoyl peroxide, azobisisobutyronitrile, and their derivatives are commonly used in suspension polymerization and solvent polymerization, and have also been reported to be used in emulsion polymerization. Radiation-induced generation of free radicals has been applied in the polymerization of acrylates, and radiation grafting can modify certain properties of polymers, which is also highly practical. The amount of initiator affects the reaction rate of emulsion polymerization and the molecular weight of the polymer. As the initiator concentration increases, the rate of radical generation rises, as does the chain termination rate, resulting in a decrease in the average molecular weight of the polymer. M. Nomura and others’ study on oil-soluble monomers indicated that Mn∝-0.6.

BPO, KPS, etc. are all acceptable; generally, it is 0.2~5% of the monomer amount

I’m not sure if anyone has tried using water-soluble azo-based initiators such as V50 and VA044; those who are interested can take a look at www.rxgdchem.com

Water-soluble azo catalysts are quite expensive. Hehe, I’ve used them in radical polymerization before, but to be honest, I couldn’t see any particular advantages in their use. One thing that seemed okay to me was that the heat release during the reaction was more uniform, so the catalytic effect was likely stable; this might help maintain consistency in molecular weight. As for other benefits, I didn’t notice any. Acrylic compounds are usually polymerized using emulsion methods, so it’s recommended to use KPS and persulfates – it depends on how high your requirements are

In the past, we usually used persulfates, and the properties of the resulting emulsions remained quite stable. Follow-up tests conducted after more than a year showed almost no changes at all.

It depends on whether the polymerization is carried out by emulsion polymerization or solution polymerization – these are the two most basic approaches to polymerization. If it is emulsion polymerization, water-based initiators such as potassium persulfate and sodium persulfate are used; whereas for solution polymerization, oil-based initiators like benzoyl peroxide are used.