Chemistry Reference
In-Depth Information
Chain transfer always results in a lower polymer molecular weight than would
occur in its absence. The effect on the rate of propagation varies, however, and
depends on the relative rates of the transfer and reinitiation steps
(Eq. 8-69)
com-
pared to that of the normal propagation reaction. Several cases are conventionally
distinguished. These are all instances of chain transfer, but they are usually given
different names, as shown below, depending on the net effects on molecular
weight and polymerization rate.
In effective (normal) chain transfer, the new radical formed by the transfer
reaction is reactive enough to initiate growth of a new macroradical within about
the same time period as that required for addition of a monomer to a monomer-
ended radical in normal polymerization. In this case, then,
k
r
.
k
p
and the rate of
polymerization
R
p
is not altered. If
k
p
c
k
tr
, the polymer molecular weight is
reduced but the product is still macromolecular. Normal chain transfer may be a
characteristic of the particular polymerization or it may be deliberately contrived
to control the polymer molecular weight.
If
k
p
{
k
tr
and
k
r
.
k
p
, there will be a large number of transfer reactions com-
pared to monomer additions, and only low-molecular-weight products will be
made. This process, which is called
telomerization
, is illustrated by the radical
reaction of ethylene and CCl
4
which yields waxy products of the general structure
Cl
3
C(CH
2
CH
2
)
n
Cl with
n
#
12. The chain transfer reaction and reinitiation pro-
cess in this case are
k
tr
(8-70)
CH
2
CCl
3
CH
2
+
CCl
4
CH
2
CH
2
Cl
+
k
r
(8-71)
CCl
3
CH
2
+CH
2
=
CH
2
Cl
3
C
CH
2
If the reactivity of the radical formed by chain transfer is lower than that of
propagating macroradicals, the rate of polymerization will be reduced along with
the polymer molecular weight. Such cases are examples of retardation. If the
reduction in polymerization rate and molecular weight are so severe as to make
both effectively nil on the scale of polymer measurements, the process is labeled
inhibition
. Retardation and inhibition are reviewed separately in
Section 8.9
,
because some retarders and inhibitors operate through reactions other than trans-
fer. Degradative chain transfer, which results when the monomer itself is a true
chain transfer agent that reinitiates poorly, is also more clearly explained by post-
poning its consideration to
Section 8.8.5
.
The ideal free-radical kinetics without chain transfer culminate in
Eqs. (8-64)
and (8-65)
in which termination of the growth of polymeric radicals is accounted
for only by mutual reaction of two such radicals. Chain transfer can also end the
physical growth of macroradicals, and the polymerization model will now be
amended to include the latter process. This can be easily done by changing
Eq. (8-62)
to include transfer reactions in the rate of polymer production,
d
[polymer]
/dt
. Combining
Eqs. (8-62) and (8-68)
,
