"Initiators, Free-Radical". In: Encyclopedia of Polymer Science and ...

Vol. 6 INITIATORS, FREE-RADICAL 563
INITIATORS, FREE-RADICAL
Introduction
Free-radical initiators are chemical substances that, under certain conditions,
initiate chemical reactions by producing free radicals:
Initiators contain one or more labile bonds that cleave homolytically when
sufficient energy is supplied to the molecule. The energy must be greater than the
bond dissociation energy (BDE) of the labile bond. Radicals are reactive chemical
species possessing a free (unbonded or unpaired) electron. Radicals may also be
positively or negatively charged species carrying a free electron (ion radicals).
Initiator-derived radicals are very reactive chemical intermediates and generally
have short lifetimes, ie, half-life times less than 10 − 3 (1).
The principal commercial initiators used to generate radicals are peroxides
and azo compounds. Lesser amounts of carbon–carbon initiators and photoinitiators,
and high energy ionizing radiation are also employed commercially to generate
radicals. Emerging technologies use N-alkoxyamines as free-radical initiators
or employ atom or group transfer facilitated by transition metals.
Free-Radical Formation and Use
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There are three general processes for supplying the energy necessary to generate
radicals from initiators: thermal processes, microwave or ultraviolet (uv) radiation
Encyclopedia of Polymer Science and Technology. Copyright John Wiley & Sons, Inc. All rights reserved.
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