Chemistry Reference
In-Depth Information
1. There are no unambiguous examples where a characterized metaloalkyl-olefin compound may be
induced to react.
2. There is a close identity between the catalysts that cause the Ziegler-Natta type polymerizations
This mechanism was argued against, however, as an over simplification, because it ignores the
experimentally observed regiospecificity of may propylene polymerizations [
243
]. On the other hand,
it is argued that, correct regiospecificity of the monomer is accounted for by isotactic or syndiotactic
propagation. Different energies of steric control can be qualitatively explained by the Ivin mecha-
nism. This can be done through simple considerations of different distances between the substituted
carbons of a four-membered ring [
243
].
In addition, Cavallo et al. [
244
] reported a study of ethylene polymerization using a model for the
heterogeneous Ziegler-Natta catalyst (Mg
2
CI
6
Ti). Propagation as well as the termination reactions
were considered. From this study they concluded that in the absence of a coordinated olefin, the Ti-C
(chain)
-bond does not occupy an octahedral coordination position but is oriented along an axis that
is intermediate between the two octahedral coordination positions that are available. The propagation
reactions occur in stepwise fashions, and the most favored mechanism requires rearrangements of the
growing chains out of the four-center transition-state planes. The insertion reactions are facilitated by
a
s
-agostic interactions. The most favored termination reactions are
b
-hydrogen transfers to the
monomers. This type of reaction is favored, relative to the C-H
s
-bond activation of a coordinated
monomer, as well as to the
b
-hydrogen transfer to the metal [
244
].
4.5.1.1 Steric Control with Heterogeneous Catalysts
Different independent approaches were used to investigate the mechanism of polymerization of
a
-olefins with heterogeneous catalysts. As a result, it was shown that isotactic polymerization of
racemic mixtures of
-olefins are stereoselective [
245
,
246
]. Also, optically active polymers form
with optically active catalysts [
247
]. Stereoelectivity and stereoselectivity are due to the intrinsic
asymmetry of the catalytic centers [
248
]. This conclusion comes in part from knowledge that
propylene coordinates asymmetrically in platinum complexes [
248
]. A study of chemical and
configurational sequences supports this [
249
]. In addition, polymerization of a racemic mixture of
(
a
)-3-methyl-1-pentene with ordinary Ziegler-Natta catalysts (e.g., Al(C
2
H
5
)
3
+ TiCl
4
or Al
(C
2
H
5
)
2
Cl + TiCl
3
) yields a racemic mixture of isotactic polymers. The mixture can be separated
by column chromatography into pure optically active components. These polymeric chains are
exclusively either poly[(
R
)(
S
)-3-methyl-1-pentene] [
248
,
250
,
251
].
This means that typical Ziegler-Natta catalysts have essentially two types of active sites that differ
only in a chiral sense. Such sites polymerize the monomers stereoselectively, i.e., either (
S
)-3-methyl-1-pentene] or poly[(
R
R
)or(
S
)
with the exclusion of the other enantiomeric form:
(S)
R
TiCl
4
/AlR
3
+
+
(S)
(S)
(S)
R
R
R
R
(R)
R
(R)
R
(R)
R
(R)
where R =
C
2
H
5
H
