Chemistry Reference
In-Depth Information
The observed
k
p
in these simplified reactions will be composed of contribu-
tions from the ion pairs and free ions:
k
p
obs
5
α
k
p
1
ð
2
αÞ
k
p
1
(11-38)
where
is the degree of dissociation of ion pairs into free solvated ions and the
k
p
values are defined in (11-37). In solvents like CH
2
Cl
2
(dielectric constant
α
B
10
12),
10
5
liter/mol sec. These rate con-
stants are similar to the
k
p
values for anionic polymerizations with free ions
[
Section 11.2.3.2
]. The anionic reactions are carried out in solvents with lower
dielectric constants, however, so the comparison is not strictly valid. In media of low
polarity, like bulk monomer, the few
k
p
values that have been measured for cationic
olefin polymerizations are of the order of 10
6
values for olefins are of the order of 10
3
the
k
p
10
9
liter/mol sec. It is not surprising
that carbenium ions are more reactive than carbonions under more or less equivalent
conditions. A carbenium ion has vacant bonding orbitals while those in the propagat-
ing carbanion and monomer are already filled, and the formation of an anionic transi-
tion state requires use of the antibonding orbitals of the monomer.
In general, both
k
p
and
k
p
will decrease with increasing solvent polarity. The
transition-state theory of chemical reactions suggests that this is because the ini-
tial state (monomer plus ion or ion pair) is more polar than the activated complex
in which the monomer is associated with the cation, and the charge is dispersed
over a larger volume. More polar solvents will tend, then, to stabilize the initial
state at the expense of the transition complex and reduce
k
p
and
k
p
.
Note, however, that more polar solvents enhance the rate of polymerization
even though they reduce the specific rate constants for propagation. This apparent
conflict reflects the greater degree of dissociation of ion pairs into free ions in
higher dielectric constant media.
The
k
p
values are generally at least 100 times as great as the corresponding
k
p
figures for olefin monomers. In many cationic polymerizations in media with
dielectric constants greater than about 10, it is likely that the concentration of free
ions is such that the contribution of ion pairs to the overall rate of polymerization
is negligible.
11.4.4
Termination and Transfer Processes
True termination reactions interrupt the growth of a macrocation without generat-
ing a new active center which can add more monomer. This can occur through
the formation of a stable cation:
CH
3
CH
3
CH
3
CH
3
CH
3
H
CH
3
2
CH
2
C
CH
2
C
AlCl
4
CH
2
C
CH
2
C
H+
CH
2
C
C
C
CH
3
CH
3
CH
3
CH
3
CH
3
CH
3
AlCl
4
(11-39)
