Chemistry Reference
In-Depth Information
4.4.4 Thermodynamics of Anionic Polymerization
The change of the equilibrium constant for the dissociation of ion pairs into free ions can be expressed
as:
ln
K ¼DH=RT þ DS=R
where
is negative. The rate constant is a composite
of the rate constant for ion pairs and the rate constant for solvent separated ion pairs. It can be
expressed as:
K
increases with decreasing temperatures and
DH
K
p
¼ðK
C
þ K
S
K
CS
Þ=ð
1
þ K
CS
Þ
where
K
S
is the constant for solvent separated ion pair, and
is the equilibrium constant for the interconversions between unsolvated and solvated ion pairs. The
relationship of the three rate constants to temperature can be expressed as follows:
K
C
is the rate constant for contact ion pair,
K
C
¼
E
C
K
C
=RT
ln
ln
ln
K
S
¼
ln
E
S
K
S
=RT
K
CS
¼
E
CS
K
CS
=RT
ln
ln
where
E
C
,
E
S
, and
E
CS
are the activation energies. Sawada [
406
] writes the apparent activation
energy,
DE
app
as follows:
0
CS
DE
app
¼ DE
ðÞ
S
þ DH
=ð
þ K
CS
Þ
1
DE
(
)S
is the activation energy of propagation for solvent-separated ion pair, and
DH
CS
is the
where
enthalpy change of formation of solvent-separated ion pair from contact ion pair.
4.5 Coordination Polymerization of Olefins
The catalysts for these polymerizations can be separated into two groups. To the first one belong the
so-called Ziegler-Natta catalysts. To the second one, transition metal oxides on special supports, like
carbon black or silica-alumina, etc. Besides the two, there are related catalysts, like transition metal
alkyls or metal halides that also catalyze some coordinated anionic polymerization. This group also
includes transition metal-
-allylic compounds and transition metal hydrides. The mechanism of
polymerization is generally coordinated anionic, based on all the evidence to date.
The Ziegler-Natta catalysts received their initial attention when Ziegler showed that some
transition metal halides, upon reaction with aluminum alkyls, can initiate polymerizations of ethylene
[
224
]. Polymers, which form, are linear and high in molecular weight. The reactions require much
lower pressures than do free-radical polymerizations of ethylene. Simultaneously, Natta
demonstrated [
225
] that similar catalysts can polymerize various other olefins like propylene,
butylene, and higher
p
-olefins. High molecular weight linear polymers form, as well, and, what is
more important, highly stereospecific ones.
The two disclosures stimulated intensive research into the mechanism of catalysis. Much knowl-
edge was gained to date. Some uncertainties about the exact mechanisms of the reactions still persist.
a
