Polymerization initiator refers to the compounds that are easily decomposed into free radicals (i.e. primary radicals) by heat. It can be used to initiate free radical polymerization and copolymerization of olefinic and diene monomers, as well as crosslinking curing of unsaturated polyesters and polymer crosslinking reactions. The polymerization initiator controls the chain initiation reaction of the polymerization reaction and directly affects the conversion rate of the monomer and the properties of the polymer.
- The medical field: Redox initiators composed of peroxides and amines can be widely used in the field of medical polymers. Because the redox initiator can reduce the decomposition activation energy, the polymerization can be carried out at a lower temperature, which saves energy and improves the properties of the polymer. For example, organic redox initiators composed of organic peroxides and aromatic tertiary amines (represented by dibenzoyl peroxide (BPO)-N,N-dimethylaniline (DMA) and BPO-N,N-dimethyl-p-toluidine (DMT)) are widely used to prepare dental self-gelling resin and medical polymer bone cement.
- The chemical field: The peroxide initiator contains peroxy group (-O-O-), after heating, the -O-O- bond breaks and splits into two corresponding free radicals, thus initiating monomer polymerization, so it is suitable for aqueous solution polymerization and emulsion polymerization of polymer synthesis. For example, using ammonium persulfate as initiator and N-methylene bisacrylamide as cross-linking agent, acrylic acid (AA) and hydroxyethyl methacrylate (HEMA) are copolymerized in aqueous solution to prepare AA-HEMA copolymers with good water resistance. In addition, because the photoinitiator has the advantages of initiating monomer polymerization at room temperature and below, and less chain transfer side reactions, it can synthesize syndiotactic polymers. For example, photoinitiated polymerization of vinyl monomers can be used to produce a variety of coating materials, adhesives, printing inks, and photoresist. Initiators such as thioxanthone thioacetate have an one-component structure and can be used as triple photosensitizers and hydrogen donors.
Figure 1. Benzoyl peroxide polymerization initiator.
There are various classification methods for polymerization initiators. (1) According to the molecular structure, it can be divided into azo, peroxy, and redox. (2) According to the solubility, it can be divided into water-soluble initiators (such as inorganic persulfates, hydrogen peroxide, and water-soluble azo initiators) and oil-soluble (soluble in monomers or organic solvents) organic initiators. (3) According to the decomposition methods, the initiator can be divided into two types, which are thermal decomposition type and redox decomposition type. (4) According to the temperature range, it can be divided into high temperature (above 100℃), such as alkyl peroxide, alkyl hydroperoxide, and peroxy ester, medium temperature (40～100℃), such as azobisisobutyronitrile, diacyl peroxide, and persulfate, and lower temperature (0～40℃), such as redox initiation system. Therefore, the initiator should be selected according to the temperature requirements of the polymerization reaction. If the high-temperature initiator is used for polymerization in the middle temperature range, the decomposition rate is too low and the polymerization time is extended. If the medium-temperature initiator is used for polymerization in the high-temperature range, the decomposition rate is too fast, the initiator is consumed prematurely, and the reaction is stopped in the low polymerization conversion stage.
- Lu Jinju, Han Shouxin.(2008) “Types and applications of azo initiators " Papers from the National Symposium on Oilfield Chemicals and Water-soluble Polymers. 125-129.
- Zou Shengou.(2001) “Polymerization initiator organic peroxide”. Guangdong Chemical Industry. 1:5-9.