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.

Azo Compounds

“…These free radicals are reactive intermediates (half-lives (t 1/2 ) of less than 10 − 3 s) and have been used to initiate a variety of commercial processes, such as vinyl monomer polymerizations and copolymerizations; curing resins; grafting vinyl monomers onto polymer backbones; autoxidation of hydrocarbons; anti-Markovnikov additions to terminal olefins; halogenations; and telomerizations (1,3,9,(18)(19)(20)(21)(22)(23)(24)(25)(26). Although the initiating species in these processes is the free radical, the term free-radical initiator is used synonymously with the precursor azo compound.…”

“…CH 3 1-(tert-Butylazo)formamide (16) has been prepared by oxidizing 1-tertbutylsemicarbazide (28) with potassium permanganate (42,50):…”

“…UVLS R N N R 1 + n CH 2 CHX heat UVLS R ( CH 2 CHX ) n Azonitriles are used as initiators for free-radical additions to olefins, autoxidations, telomerizations, and halogenations (1,3,6,9,(18)(19)(20)(21)(22)(23)(24)(25)(26). Monomers are grafted onto polyols with the aid of azonitrile initiators.…”

“…Simple symmetrical azo compounds have been named by adding the prefix azo-to the name of the parent molecule, eg, azomethane for CH 3 N N CH 3 . Azo compounds with simple substituents were named by prefixing the sub'stituent and locant, eg, 1-methylazoethane for C 2 H 5 N N CH(CH 3 ) 2 .…”

“…However, starting with Chemical Abstracts Volume 76 (1972), azo compounds are indexed as derivatives of diazene, H N N H, eg, diazene, dimethyl-. Previously, "diimide" had been used instead of diazene in naming certain other azo compounds, eg, diimide, diacetyl for CH 3 …”