Chemistry Reference
In-Depth Information
used is seldom explicitly specified, however, although it can usually be inferred
from the particular context.
8.5
Methods of Producing Radicals
A large number of methods are available to bring about free-radical polymeriza-
tions. The more widely used initiation techniques are reviewed in this section.
8.5.1
Thermal Decomposition of Initiators
The thermal scission of a compound to yield two radicals was used for illustration
in reaction (8-6), because this is the most common means of generating radicals.
Thermal decomposition is ideally a unimolecular reaction with a first-order con-
stant,
k
d
, which is related to the half-life of the initiator,
t
1
/
2
,by
Eq. (8-33)
.
It is convenient in academic studies to select an initiator whose concentration
will not change significantly during the course of an experiment. From experience
it seems that a
t
1
/
2
of about 10 h at the particular reaction temperature is a good
rule-of-thumb in this regard. This corresponds to a
k
d
of 2
10
2
5
sec
2
1
from
Eq. (8-33)
. More generally, initiators used in polymer production have
k
d
's under
usage conditions which are as low as about 10
2
4
sec
2
1
. This corresponds to a 2-h
half-life. Even faster decompositions are needed in the high-pressure polymeriza-
tion of ethylene where reactor residence times are very short at temperatures
between about 130 and 280
C.
In general, initiation should be as fast as is practical to produce as much poly-
mer as possible per unit of reaction time. The reaction cannot be allowed to
proceed more quickly than the rate at which the exothermic heat of polymeriza-
tion can be removed from the system, however. The decomposition rates of free-
radical initiators are very temperature sensitive (the
t
1
/
2
of benzoyl peroxide drops
from 13 h at 70
C to 0.4 h at 100
C), and a runaway reaction can result from
overheating if the rate of initiation is not limited appropriately.
Many polymerizations are carried out at temperatures between 0 and 100
C.
Initiation at the required rates under these conditions is confined to compounds
with activation energies for thermal homolysis in the range 100
3
165 kJ/mol. If
the decomposition process is endothermic, the activation energy can be consid-
ered to be approximately equal to the dissociation energy of the bond which is
being split. It can be expected, then, that useful initiators will contain a relatively
weak bond. (The normal C
C sigma bond dissociation energy is of the order of
350 kJ/mol, and alkanes must be heated to 300
a
500
C to yield radicals at the
rates required in free-radical polymerizations.)
The major class of compounds with bond dissociation energies in the
100
O peroxide linkage. There are numerous
varieties of compounds of this type, and some are listed in
Table 8.1
. Thermal
165 kJ/mol range contain the O
a
