Chemistry Reference
In-Depth Information
with both the propagating chain end and the incoming monomer. Coordination holds the monomer in
place during the process of addition to the chain. This coordination is broken simultaneously with
formation of a new coordination with the new monomer [
258
]:
HH
M
T
H
H
H
M
T
H
H
H
H
H
H
H
H
H
H
H
H
M
T
H
where, M
T
means metal.
This capability of the initiator to control the placement of the monomer overrides the common
tendency for some syndiotactic placement. While syndiotactic polypropylene has been prepared with
heterogeneous catalysts, the yield of syndiotactic placement is low. Soluble Ziegler-Natta catalysts
on the other hand can yield high amounts of syndiotactic placement. This is discussed in the next
section. When 1,2-disubstituted olefins are polymerized with Ziegler-Natta catalysts, the di tacticity
of the products depends on the mode of addition. It also depends on the structure of the monomer,
whether it is
cis
or
trans
. A threodiisotactic structure results from a
syn
addition of a
trans
monomer.
A
syn
addition of a
cis
monomer results in the formation of an erythrodiisotactic polymer. For
instance,
-propylenes give
erythro
and
threo
diisotactic polymers, respectively
[
255
]. To avoid 1,2-interactions in the fully eclipsed conformation, the carbon bonds in the monomer
units rotate after the addition of the monomer to the polymeric chain [
240
]. Also, a systematic
investigation of chain propagation by ethylene insertion into a metal-C
6
H
5
bond showed that the
backside insertion barriers depend little on the identity of the metal, because backside insertion
requires little deformation of the metal-ligand framework [
256
]. For systems that are sterically
unencumbered the insertion reaction proceeds through frontside and backside channels in equal
parts. This is due to the fact that both transition states are close in energy. The influence of the ligand
upon insertion is such that good
cis
and
trans
-1-
d
donor ligands lower the front-side insertion barrier, because they
favor trigonal planar geometry over trigonal pyramidal coordination. The activity of various metal
centers can be strongly influenced, however, by sterically bulky ligands. Insertion barriers are
generally lowered by steric bulk, because compression of the active site favors the transition state
geometry over the
p
p
-complex geometry [
256
].
4.5.2 Homogeneous Ziegler-Natta Catalysts
The catalysts that fit into this category are quite diverse chemically. They can be, for instance,
reaction products of vanadium tetrachloride with dialkyl aluminum chloride or reaction products of
bis-cyclopentadienyl titanium dichloride with aluminum alkyls. Organic titanates also form soluble
catalysts upon reactions with certain aluminum alkyls.
The homogeneous catalysts can range in efficiency from very high to very low. Usually, a much
narrower molecular weight distribution is obtained in polymers formed with these catalysts than with
the heterogeneous ones. In addition, syndiotactic poly(
78
C).
a
-olefins) form at low temperatures (
