Chemistry Reference
In-Depth Information
capped polymers and new Lewis acids. Thus, even though the initiator is ostensibly
diethylaluminum chloride there may be major contributions to the polymerization
from ethyl aluminum dichloride or aluminum chloride.
11.4.2.3
Stable Organic Cation Salts
Some organic cation salts can be isolated as crystalline solids. Stability of carbe-
nium ions is enhanced if the electron-deficient carbon is conjugated with olefin or
aromatic groups or with atoms with unshared electron pairs (O, N, S). The posi-
tive charge is diffused over a larger region as a consequence. Examples of such
initiators
SbCl
6
are
hexachloroantimonate
ð
Þ
salts
of
triphenyl methyl
[(C
6
H
5
)
3
C
"
] and cycloheptrienyl
ð
C
7
H
7
Þ
carbenium ions.
These initiators are primarily of academic interest because their initiation pro-
cesses are fairly straightforward compared to the complicated equilibria that can
exist in other systems (e.g.,
Fig. 11.2
). This simplifies the study of the kinetics of
the propagation and other processes involved in cationic polymerization.
However, since these cations are stable, their use is limited to the initiation of
more reactive monomers like
N
-vinyl carbazole and alkyl vinyl ethers.
11.4.3
Propagation Reactions
Determination of propagation rate constants in cationic (and in anionic) systems
is complicated by the simultaneous occurrence of different types of propagating
sites. In olefin polymerizations, some portion of the active centers may exist as
free ions and others as ion pairs of varying degrees of solvation. In the solvents in
which cationic polymerizations are normally carried out, the polymerization is
mainly due to free ions. In low dielectric constant media like benzene or hydro-
carbon monomers, however, ion pairs will dominate the reaction.
The initiation and propagation processes are influenced by equilibria between
various degrees of association of the active center and its counterion. At a mini-
mum, it is necessary to conceive of the existence of contact (associated) ion pairs,
solvent-separated ion pairs, and free solvated ions. A simplified reaction scheme
[3]
is presented in reaction (11-37).
K
i
±
K
p
±
M
A
ZX
n
B
+
M
AM
ZX
n
B
M
ZX
n
B
Polymer
solvent
separated
ion pairs
M
K
p
±
M
K
i
±
M
solvated
free ions
A
ZX
n
B
M
AM
+
ZX
n
B
M
+
ZX
n
B
+
+
Polymer
(11-37)
The existence of contact pair ions (as in Eq. 9-1) is neglected in this represen-
tation because the dielectric constants of the solvents usually used for cationic
polymerizations are high enough to render concentrations of intimate ion pairs
negligible compared to those of solvated ion pairs.
