Chemistry Reference
In-Depth Information
Table 3.4
Decomposition of benzoyl peroxide in various
solvents at 79.8
C[
15
]
Approximate%
decomposition in 4 h
Solvent
Anisole
43.0
Benzene
50.0
Carbon tetrachloride
40.0
Chlorobenzene
49.0
Chloroform
44.0
Cyclohexane
84.0
Cyclohexene
40.0
Ethyl acetate
85.0
Ethylbenzene
46.0
Methyl benzoate
41.0
Methylene chloride
62.0
Nitrobenzene
49.0
Tetrachloroethylene
35.0
Toluene
50.0
In the gaseous phase, the cleavage is usually homolytic because it requires the least amount of
energy [
12
]. In solution, however, the dissociation may be either one of the two, depending upon the
nature of the R groups. Heterolytic cleavage may be favored, in some cases, if the two groups, R and
R
0
, differ in electron attraction.
The same is true if the reaction solvent has a high dielectric constant. Solvation of the ions that
would form due to heterolytic cleavage is also a promoting influence for such a cleavage:
:
+BS
A:B + 2S
!
SA
where, S represents the solvent.
In sum total, the types and the amounts of side reactions that can take place are a function of the
structures of the peroxides, the stability of the formed radicals, the solvent, and the monomer that is
being polymerized. The stability of the radicals that form can also affect the amount of radicals being
captured by the monomers. Also, it was reported that while generally the character of free radicals is
neutral, some of them are electrophilic (such as chloro) and others are nucleophilic (such as
-butyl).
This tendency, however, is relatively slight when compared with positive and negative ions [
15
].
There is much information in the literature on the rates and manner of decomposition of many
peroxides in various media. Beyond that, diagnostic tests exist that can aid in determining the
decomposition rates of a particular peroxide in a particular media [
13
]. Table
3.4
is presented to
show how different solvents affect the rate of decomposition of benzoyl peroxide into radicals.
Some initiators can function as both, thermal and photoinitiators. Such an initiator, for instance, is
2,2
0
-azobisisobutyronitrile. Also, Engel and coworkers [
16
] reported synthesis of an initiator that can
function both as a thermal free radical initiator and a photoinitiator (see Sect.
3.2.4
). It can be
illustrated as follows:
t
N
O
N
O
O
The claimed advantage of this initiator is that it can be used to form block copolymers.
