Chemistry Reference
In-Depth Information
The configuration of the polymer about the double bond can be controlled by vary-
ing polymerization conditions. A structure with about 85%
trans
configurations has
received the most attention. This product retains its elastomeric character down to about
2
95
C. It forms easily melted crystallites on stretching and is readily processed in
conventional rubber equipment. Norbornene, 11-27, also yields a useful polymer:
RuCl
n
CH
CH
n
(11-71)
(BuOH
)
11-27
Metathesis catalysis has also been used to prepare polyacetylene, which is of
interest because of its electrical conductivity.
Metathesis catalysts vary widely. They always contain a transition metal com-
pound which is usually employed in combination with one or more cocatalysts
like AlEt
3
. The most important catalysts for cyclopentene polymerization are
derived from W or Mo. Low-molecular-weight alcohols are used in low concen-
trations to reduce the formation of oligomeric products.
Catalysts comprising a metal
carbon double bond (metallocarbenes, or metal-
locenes) are efficient. With these initiators, the polymerization mechanism appears
to involve coordination of the C
C double bond in the cyclo- or dicycloalkene at
a vacant
d
orbital on the metal. The metallocyclobutane intermediate which is
formed decomposes to produce a new metal carbene and a new C
Q
C bond.
Propagation consists of repeated insertions of cycloalkenes at the metal carbene.
Q
H
H
+
W
C
W
C
W
C
H
R
R
R
(11-72)
H
H
WC
CH
2
CH
2
CH
2
CH
WC
H
C
R
R
PROBLEMS
11-1
Isobutene is polymerized commercially by a cationic mechanism initiated
by strong acids like AlCl
3
. It is not polymerized by free-radicals or anionic
initiators. Acrylonitrile is polymerized commercially by free-radical
means. It can also be polymerized by anionic initiators like potassium
amide but does not
respond to cationic initiators. Account
for
the
