Chemistry Reference
In-Depth Information
C
4
H
6
O
C
2
H
5
n
Ti
C
2
H
5
n
The insertions of the monomers are believed to occur in two steps [
268
,
269
]. In the first one,
the incoming monomer coordinates with the transition metal. This results in formation of a short-lived
s
-allylic species. In second one, the metal-carbon bond is transferred to the coordinated monomer with
formation of a
-butenyl bond. Coordination of the diene can take place through both double bonds,
depending upon the transition metal [
270
] and the structure of the diene. When the monomer coordinates
as amonodentate ligand, then a
syn
complex forms. If however, it coordinates as a bidentate ligand, then an
anti
complex results [
271
]. In the
syn
complex, carbons one and four have the same chirality while in the
anti
complex they have opposite chiralities [
268
]. Due to lower thermodynamic stability the
p
complex
isomerizes to a
syn
complex [
268
]. If the allylic system does not have a substituent at the second carbon,
then the isomerization of
anti
to
anti
usually occurs spontaneously even at room temperature [
268
].
Transition metal alkyls probably cannot be classified as typical Ziegler-Natta catalysts. Some of them,
however, exhibit strong catalytic activity and were, therefore, investigated [
272
-
275
]. This also led to the
conclusion that
syn
-allylic complexes as
intermediates. It is similar to the mechanism visualized for the Ziegler-Natta catalysis [
276
-
283
]. The
initial formation of the
the polymerization mechanisms involve formations of
p
-allylic ligands and the solvents used in catalyst preparations strongly influence the
catalytic activity and stereospecificity of the product [
280
]. NMR studies of polymerizations of conjugated
dienes with
p
-crotyl-nickel iodide [
281
] showed that the monomers are incorporated at the metal-carbon
bonds with formations of
p
syn
-
p
-crotyl ligands. The
syn
-ligands transform to
trans
-1,4-segments next to the
crotyl group and the
-1,4-units in the polymers [
282
]. In summary, the
general mechanisms of
cis
and
trans
placement by coordinated catalysts were pictured as follows [
280
]:
trans
-1,4-segments become
trans
M
T
+
M
T
X
X
R
R
R
L
M
T
M
T
X
X
cis-1,2 placement
R
L
M
T
L
R
R
M
T
X
X
