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

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Vol. 6 INITIATORS, FREE-RADICAL 589
in industrial polymer applications owing to cost or efficiency considerations. However,
some systems may be well-suited for initiating specific radical reactions or
polymerizations, eg, grafting of monomers to cellulose using ceric ion (225).
Initiation through Radiation and Photoinitiators
High energy ionizing radiation sources (eg, x-rays, γ -rays, α-particles, β-particles,
fast neutrons, and accelerator-generated electrons) can generate radical sites on
organic substrates (226). If the substrate is a vinyl monomer, radical polymerization
can occur (227). If the substrate consists of a polymer and a vinyl monomer,
then polymer cross-linking, degradation, grafting of the monomer to the polymer,
and polymerization of the monomer can all occur (228). Radical polymerizations of
vinyl monomers with ionized plasma gases have been reviewed (229). Ultrasonic
polymerization of vinyl monomers using special initiators (eg, dodecanethiol) has
been described (230).
Initiation of radical reactions with uv radiation is widely used in industrial
processes (231). In contrast to high energy radiation processes where the energy
of the radiation alone is sufficient to initiate reactions, initiation by uv irradiation
usually requires the presence of a photoinitiator, ie, a chemical compound or compounds
that generate initiating radicals when subjected to uv radiation. There
are two types of photoinitiator systems: those that produce initiator radicals by
intermolecular hydrogen abstraction and those that produce initiator radicals by
photocleavage (232–239).
In the case of intermolecular hydrogen abstraction, a hydrogen (H) atom
donor is required. Typical donors have an active H atom positioned alpha to an
oxygen or nitrogen, eg, alcohols (R2CHOH), ethers (R2CHOR), and tert-amines
(R2CHNR2), or an active H atom directly attached to sulfur, eg, thiols (RSH).
Some of the commercial photoinitiators that undergo intermolecular H abstraction
from the H atom donor upon excitation by uv radiation are listed in Table 10. A
reaction illustrating this photoinitiation process is given below for benzophenone
(photoinitiator) and an alcohol (H atom donor):
Upon exposure to uv light, ground-state benzophenone is excited to the triplet
state (a diradical), which abstracts an alpha H atom from the alcohol, resulting
in the formation of two separate initiating radicals. With amine H atom donors,
an electron transfer may precede the H transfer, as in triplet exciplex formation
between benzophenone and amine (eq. 46):
(44)