Initiators, Free‐Radical

Abstract

Most commercial free‐radical applications employ initiators such as peroxides and azo compounds. Lesser amounts of carbon–carbon initiators and photoinitiators, and high energy ionizing radiation are also used commercially to generate free radicals. The chemical initiators are substances possessing labile oxygen–oxygen, carbon–nitrogen, or carbon–carbon covalent bonds that under certain thermal, chemical, or photochemical conditions undergo homolytic scission of the labile bond to produce free radicals. The free radicals produced are useful in many industrial chain reactions, eg, halogenations, additions to carbon–carbon double bonds, polymerizations of vinyl monomers, grafting reactions, curing of rubbers and unsaturated polymers, cross‐linking of polyolefins, modification of polyolefins, and reactive processing. In this article free‐radical initiator decomposition pathways are reviewed. Initiator reactivity is related to chemical structure and to first‐order decomposition parameters such as temperature, frequency factor, and activation energy. Reactivity of derived free radicals is also related to structure and to the occurrence of secondary reactions such as β‐scission and induced decomposition. The energy and the chemical properties of the free radicals determine commercial utility, thus the interrelation of initiators, their kinetics, and the radicals produced provide guidelines for initiator selection.

Decomposition mechanism of α-alkoxyalkyl-hydroperoxides in the liquid phase: temperature dependent kinetics and theoretical calculations

“…It has been reported that tertiary hydroperoxides can produce corresponding tertiary alcohols under acidic conditions, due to the stabilization of the corresponding carbenium ion. 37 In that case, the observed 1 st order decays and corresponding k values were attributed to -AHs possessing a tertiary -OOH (B in Scheme 1). This interpretation is supported by the theoretical calculations as shown below.…”

Free‐radical bulk polymerization of styrene with a new trifunctional cyclic peroxide initiator

“…Extensive research has been carried out7, 8 for improving the rate/molecular weight balance. Difunctional initiators and their advantages over monofunctional ones are also well documented in the literature, especially for polystyrene (PS) production, as discussed in Sanchez and Myers,4, 5 Kamath,7 Kamath and Harpell,9 Kim and Choi,10 Choi et al,11 Villalobos et al,12, 13 Dhib et al14 and Cavin et al15, 16 Studies involving difunctional perketals in ethylene polymerization17 have shown increased levels of high‐molecular‐weight components in the products. In rubber‐modified PS, difunctional peroxides not only increase the weight‐average molecular weight ( M w ) but also broaden the molecular weight distribution 18, 19.…”

“…Over the years, improvements in reactor technology and heat removal have resulted in better rate performance for polymer production. Monofunctional initiators (azo compounds or peroxides) are widely used for polymerizations of different monomers as described, for instance, in Gao and Penlidis,2, 3 Sanchez and Myers,4, 5 or any industrial initiator catalogue 6. The bulk or less solvent free‐radical polymerization of STY at higher temperatures with the aid of appropriate initiators is of significant industrial importance because they bring about increased polymerization rates and narrower polydispersity.…”

The development of theoretical relationships between some handling parameters (setting time and setting temperature), composition (relative amounts of initiator and activator) and ambient temperature for acrylic bone cement

“…When the two cement components are mixed, BPO and DMpT react to form the benzoyldimethyl-4-methylphenylammonium benzoate ion pair ( Figure 2). 7 This rapidly decomposes to liberate benzoyl and N,N-dimethyl-ptoluidine radicals. Although it is known, and regularly reported, that the benzoyl radical initiates the polymerization of methyl methacrylate, there is some evidence that the radical from DMpT also participates in this process.…”