Chemistry Reference
In-Depth Information
pressure less branching occurs. Brookhart et al. illustrate the mechanism of polymerization as
follows [
18
]:
Ar
Ar
Ar
X
R
R
M
M
M
ethylene
activator
X
Ar
Ar
Ar
beta-hydride
elimination
Ar
ethylene
R
M
Ar
Ar
R
R
M
M
2
migraatory
insertion
polymer
Ar
Ar
Ar
Ar
migratory
insertion
ethylene
R
M
R
M
chain migration
via insertion
Ar
Ar
Ar
additional
chain insertion and migation
R
M
branched polymer
Ar
where X
2,6-dialkylphenyl.
A patent for the polymerization process of olefins (especially ethylene,
¼
CH
3
, Br; M
¼
Pd, Ni; R
¼
(CH
2
CH
2
)
n
CH
3
;Ar
¼
-olefins, cyclopentene, and
some fluorinated olefins) describes the above catalytic systems [
20
]. The hindered diimines stabilize alkyl
Ni(II) or Pd(II) with cationic complexes. After preparation, the complexes are reduced with methylalu-
minoxane and then activated with Lewis acids capable of forming non-coordinating counterions [
20
].
In addition, preparation of catalysts based on iron and cobalt [
21
] was also reported. These are
complexes of bulky pyridine bis-imine ligands with iron or cobalt that are also activated by
methylaluminoxane:
a
N
N
N
M
Cl
Cl
The iron-based catalysts are reported to be considerably more active than the cobalt analogs [
21
].
The yield of linear, narrow molecular weight distribution polyethylene per gram is reported to be very
