Chemistry Reference
In-Depth Information
11.4.5
Kinetics of Cationic Polymerization of Olefins
Cationic polymerizations differ from free-radical and homogeneous anionic syn-
theses of high polymers in that the cationic systems have not so far been fitted
into a generally useful kinetic framework involving fundamental reactions like
initiation, propagation, and so on. To explain the reasons for the peculiar prob-
lems with cationic polymerizations we will, however, postulate a conventional
polymerization reaction scheme and show where its inherent assumptions are
questionable in cationic systems.
An “ideal” reaction scheme is shown in
Fig. 11.3
. Note that the Lewis acid
ZX
n
is referred to as a catalyst. This is the common terminology for such poly-
merizations but it will be realized that ZX
n
is really an initiator and the compound
BA, which is often called a cocatalyst, is really a coinitiator.
Referring to
Fig. 11.3
, we see that the reaction sequence is greatly oversimpli-
fied to begin with, because we have ignored the existence and equilibria between
free solvated ions and ion pairs of various degrees of intimacy. Thus, all the rate
constants that are listed are actually composite values (cf.
Eq. 11-38
) which can
vary with the nature of the medium and counterion. The observed
k
p
values can
even vary with the total concentration of reactive species, because organic ion
pairs tend to cluster in the more nonpolar environment, and the reactivity of
aggregated and individual ion pairs is not generally equal.
We can detect a further complication from observations that some cationic
polymerizations exhibit bimodal (two peaks) molecular weight distributions. This
can happen if different active species (say, free ions and some form of ion pairs)
engage in propagation or transfer reactions faster than they can come to equilib-
rium with each other. Under these circumstances there can be two effectively
K
-
+
Pre-initiation
equilibrium
Z X
n
+ B A
Z X
n
B
A
Catalyst cocatalyst
-
-
k
i
+
+
Initiation
A
Z X
n
B
+ M
AM
Z X
n
B
monomer
-
k
p
-
+
+
Propagation
M + AM
n
Z X
n
B
AM
Z X
n
B
n +1
-
-
k
t
+
+
M
n +1
Termination
Z X
n
B
+A
Z X
n
B
AM
n + 1
-
-
+
k
tr
+
Z X
n
B
M
n +1
+A
Z X
n
B
Transfer
AM
n + 1
+ M
FIGURE 11.3
Simplified reaction scheme for cationic polymerizations.
