Chemistry Reference
In-Depth Information
Polyethylene is most commonly polymerized with cyclopentadiene metallocenes
of Ti, Zr, and Hf, with zirconenes preferred for their stability. Polymerization rates
are very high and are a function of the Al/transition metal ratio. Polymer molecular
weight decreases with increased reaction temperature, because higher temperatures
accelerate
-hydride transfer.
The metallocenes generally used for ethylene polymerization are achiral, and
yield atactic polypropylene. Stereoregular polypropylene requires the use of chiral
metallocene catalysts, such as [Et(Indenyl)
2
ZrMe]
%
[B(C
6
F
5
)]
.
Supported metallocene catalyst systems are preferred to soluble versions in
conventional polyolefin plants, which were designed to use
β
supported
Ziegler
Natta or Cr
2
O
3
-based catalysts. Metallocenes can be supported on a
number of substrates, such as SiO
2
, MgCl
2
,orAl
2
O
3
. Supported catalysts also
provide polypropylene with fewer stereochemical defects.
Production problems with current metallocene-aluminoxane catalysts reflect
the high aluminoxane/metal ratio needed for good catalyst productivity and stere-
ochemical control. Aluminoxane production is slow and relatively inefficient and
the large amounts used make post-polymerization catalyst removal processes nec-
essary. These deficiencies are expected to be remedied by different cocatalysts,
which are being disclosed in the patent literature.
11.6
Olefin Metathesis Catalysts
Metathesis
means “transposition of parts.” It is a disproportionation reaction,
known in synthetic organic chemistry as a route to substituted olefins. The reac-
tion leaves the number of C
Q
C bonds unchanged but shuffles their substitutents.
R
1
CH
2
CH
R
1
R
1
CH
CH
R
1
(11-69)
+
+
R
2
R
2
R
2
R
2
CH
CH
CH
CH
It is applied to macromolecules in the ring-opening polymerization of
cycloalkenes and bicycloalkenes. Polypentenamer is obtained by the polymeriza-
tion of cyclopentene catalyzed by transition metal compounds.
n
CH
CH
CH
2
CH
2
CH
2
(11-70)
It is part of a homologous series of linear unsaturated polymers that are termed
polyalkenamers
. Polypentenamer is an interesting member of this series because
cyclopentene is economically available from petrochemical byproducts and
because the polymer is a readily vulcanizable elastomer (since it contains residual
C
a
C double bonds; Section 1.3.3
