Chemistry Reference
In-Depth Information
8.3.2.2
Atom Transfer Reactions
Radicals can undergo other reactions as well as monomer addition. Atom
abstraction reactions usually involve transfer of a hydrogen or halogen atom. An
example from micromolecular chemistry involves the chlorination of hydrocar-
bons at about 200
C or during irradiation with light of wavelength less than
4875
10
2
10
m:
3
'
C
'
2
-
2C
(8-14a)
'
1
'
1
C
CH
4
-
HC
CH
3
(8-14b)
CH
3
1
'
C
'
2
-
CH
3
C
'
1
C
(8-14c)
Reaction (8-14a) is the initiation step, while reactions (8-14b) and (8-14c) are
atom abstraction propagation reactions. Atom abstraction reactions in free-radical
polymerizations are called
chain transfer reactions
. They are discussed in some
detail in
Section 8.8
.
8.3.2.3
Other Propagation Reactions
Radicals can undergo other reactions such as rearrangements and fragmentations.
These are also propagation reactions by definition but they do not concern us
here because free-radical polymerizations are not usually performed under condi-
tions in which these processes are significant.
8.3.3
Termination
The sequence of monomer additions is terminated by the mutual annihilation of
two radicals. Such termination reactions can occur if the radicals combine to form
a paired electron bond as in
H
H
H
H
C
C
C
C
CH
2
+
CH
2
CH
2
CH
2
(8-15)
This process is called
termination by combination
and would be written in general
terms as in
k
td
M
ðn
1
mÞ
M
n
1
M
m
!
(8-16)
with
k
tc
as the corresponding rate constant.
Alternatively the two radicals can form two new molecules by a disproportion-
ation reaction in which a hydrogen atom is transferred:
