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.
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.