Chemistry Reference
In-Depth Information
8.8.1
Transfer to Initiator
Many peroxides are quite active chain transfer agents, but they are norm
ally
pres-
ent in such low quantities that they do not have a significant effect on DP
n
. The
usual values of [I] are 10
2
4
10
2
2
M
and thus the [I]
/
[M] term in
Eq. (8-79)
is
2
of the order of 10
2
5
2
10
2
3
.The
C
I
values vary from zero for azo compounds to
figures that approach one for hydroperoxides.
Several methods can be used to determine
C
I
. If transfer to all species except
initiator is negligible,
Eq. (8-77)
can be rearranged to
"
#
1
DP
n
2
k
tc
R
p
k
p
½
2
k
td
R
p
k
p
½
C
I
½
I
C
M
(8-78)
2
2
2
2
5
½
M
M
M
R
p
and DP
n
are measured at various values of [I] and [M], and a plot of the
left-hand side of the above equation against [I]
/
[M] yields
C
I
if the ratio
k
td
/k
tc
is
known from other measurements.
A complication can arise at the higher initiator concentrations involved in
such plots because radicals from the initiators can terminate polymer radicals.
This process, called
primary radical
termination
, reduces polymer molecular
weight and reduces initiator efficiency.
8.8.2
Transfer to Monomer
Transfer of a hydrogen can occur either from the monomer to a propagating
macroradical [reaction (8-79)] or in the reverse direction [reaction (8-80)]. In
either case, the active site is transferred to the monomer, and the growth of the
polymer radical is terminated.
H
H
C
(8-79)
CH
2
+
CH
2
C
CH
2
CH
2
X
+
CH
2
CX
X
X
H
H
H
C
C
(8-80)
CH
2
+
CH
2
C
CH
2
CHX
+
CH
3
X
X
X
H
H
H
C
C
CH
2
+
CH
2
C
CH
2
C
CH
2
CH
2
C
CH
2
C
X
X
X
X
X
X
(8-81)
Reinitiation from the monomer radical in reaction (8-81) can produce a
branched polymer because the vinyl end of the new polymer radical can react
with another growing radical to produce a structure that can grow by adding
monomer from the new radical in the interior of the macromolecule:
