Chemistry Reference
In-Depth Information
+
(8-45)
ROO
+ H + Co
2+
Co
3+
+ ROOH
The radical destroyed in reaction (8-44) is not available for initiating polymer-
ization. There is thus an optimum low level of transition metal reducing agent for
efficient generation of radicals.
The general redox reaction
-
+
(8-46)
A
+ B + X
A — B + X
could proceed with any molecule AB, provided the reducing agent X is
strong enough to split
the AB bond. Sodium would be effective with ethyl
a
chloride,
in which the C
Cl
link has a bond dissociation energy of about
290 kJ/mol:
-
+
(8-47)
CH
3
CH
2
+ Cl + Na
CH
3
— CH
2
— Cl + Na
Such redox couples are not attractive because the reducing agent is readily
oxidized in air and may attack some solvents in preference to the intended sub-
strate. For practical purposes the A—B bond in redox systems must be relatively
weak and this limits such materials to the peroxide compounds described in the
preceding section.
Many redox initiators are water soluble and are widely used in emulsion
polymerizations (Chapter 10) in which the radicals are generated in the aque-
ous phase. When metal ions are used as reducing agents in organic media
they are commonly in the form of salts of carboxylic acids.
Redox initiation can be arranged to proceed quickly under mild reaction con-
ditions and is particularly useful for low and ambient temperature radical poly-
merizations. The two components of the redox couple are stable when handled
separately whereas a conventional thermal decomposition initiator with the same
activity would be difficult to store and transport.
8.5.3
Photochemical Initiation
Radicals can be produced by ultraviolet irradiation of a monomer like styrene
which absorbs sufficiently strongly in this wavelength region. Photochemical ini-
tiation may also be provided by thermal initiators or by compounds like benzoin
ethers:
H
O
OCH
3
h
v
C
C
(8-48)
C
C
+
O
OCH
3
H
Current practice in the ultraviolet curing of paints, adhesives, and inks relies
heavily on the use of photoinitiators of this type. An alternative procedure
