Chemistry Reference
In-Depth Information
the molecular structures of the products. It is possible to vary the length and stereoregularity of the
blocks. This is accomplished by feeding alternately different monomers into the reactor. When it is
necessary that the blocks consist of pure homopolymers, then after each addition the reaction is
allowed to subside. If any residual monomer remains, it is removed [
96
]. This requires a long lifetime
for the growing chains and an insignificant amount of termination. The stability of the anion depends
upon the catalyst system. One technique for catalyst preparation is to form TiCl
3
by reducing TiCl
4
with diethyl aluminum chloride followed by careful washing of the product of reduction to remove
the by-product Al(C
2
H
5
)Cl. Other reports describe using the
a
-form of TiCl
3
or heat treating it to
b
g
form the
-forms that yields more stereospecific products.
The transition metal oxide catalysts on support, such as the CrO
3
/silica-alumina (Phillips) and
MoO
3
/Al
2
O
3
(Standard Oil), are used to copolymerize minor quantities of
or
-olefins with ethylene.
Such copolymerizations introduce short pendant groups into polymer backbones.
Ethylene and other olefins can also be copolymerized with carbon monoxide to form polymers of
aliphatic ketones, using transition metal catalysts, like palladium(II) coupled with non-coordinating
anions. There are numerous reports of such catalysts in the literature. One example is a compound
composed of bidentate diarylphosphinopropane ligand and two acetonitrile molecules coordinating
Pd
2+
coupled with BF
3
counterions. This compound, bis(acetonitrile)palladium(II)-1,3-bis(diphenyl-
phosphino)propane-(tetrafluoborate), can be illustrated as follows [
97
]:
a
N
++
(BF
4
)
2
R
3
Pd
N
Ph
Ph
P
R =
P
Ph
Ph
When the tetrafluoroborate is replaced with a perchlorate,
the compound is a very active
catalyst [
97
].
One copolymer of ethylene and carbon monoxide are available commercially. The material
offered under the trade name of Carilon is actually a terpolymer, because it contains a small quantity
of propylene. It is reported [
98
] that use of a palladium catalyst permits formation of perfectly
alternating interpolymer.
08
The product is reported to be a tough, chemical resistant material.
Hustad et al. [
99
] developed a technique to make polydisperse polyethylene diblock copolymers
with 1-octene with a distribution of block lengths. When melted and compressed into films, the
distinct polymeric segments self-assemble into layered patterns of semi-crystalline and hard and
amorphous phases. Because each phase has a different refractive index, the block copolymer, shown
below, can function as a photonic crystal and scatter visible light.
x
1-x
1-x
x
C
6
H
13
C
6
H
13
