Chemistry Reference
In-Depth Information
occurs at a transition metal ion on the surface layer of the metal trichloride (or perhaps dichloride)
lattice. Here the halide is replaced by an alkyl group (R). The adjacent chloride site is vacant and
accommodates the incoming monomer molecule. Using titanium chloride as an illustration:
R
R
R
R
Cl
Cl
Cl
Cl
CH
2
Cl
Ti
+
Cl
Ti
Cl
Ti
Cl
Ti
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
four center intermediate
where, represents a vacant site in the d-orbital. The newly formed transition metal-alkyl
bond becomes the active center and a new vacant site forms in place of the previous transition
metal-alkyl bond.
The driving force for the reactions [
236
,
237
] depends on
p
-type olefin complexes. In these
p
complexes, the
-electrons of the olefins overlap with the vacant d-orbitals of the transition metals.
This results in the
-bonds being transitory. Also, the d-orbitals of the metals can simultaneously
overlap with the vacant anti-bonding orbitals of the olefins. This decreases the distances between
the highest filled bonding orbitals and the empty (or nearly empty) d-orbitals. In such situations,
the carbon-metal bonds of the transition metals weaken and the alkyl groups migrate to one end of the
incoming olefin [
238
]. The insertion process results in a
p
cis
-opening of the olefinic double bond [
237
,
239
,
240
].
The above scheme of propagation might also be pictured for bimetallic active centers.
Complexations precede monomer insertions at the vacant octahedral sites and are followed
by insertion reactions at the metal-carbon bonds [
237
]. When the transition metals are
immobilized in crystal lattices, the active centers and the ligands are expected to interchange
at each propagation step.
The above model for monometallic mechanism, though now widely accepted, is still occasionally
questioned. Some evidence, for instance, has been presented over the years to support a bimetallic
mechanism [
242
]. It was shown that elimination of the organometallic portion of the complex catalyst
during polymerization of propylene results in deactivation of the catalyst. By contrast, replacement of
the initial organometallic compound with another one results in a change in the polymerization rate,
but not in deactivation of the catalyst.
In addition, some monometallic mechanisms based on a different mode of monomer insertion were
also proposed. An example is a reaction mechanism that was proposed by Ivin et al. [
241
]. This
mechanism is based on an insertion mechanism involving an
a
-hydrogen reversible shift, carbene,
and a metallocyclobutane intermediate:
M
T
M
T
H
M
T
M
T
H
H
H
M
T
where, M
T
means metal. The stereospecificity is dependent upon the relative configuration of the
substituted carbons of the metallocyclobutane ring. Hydrogen transfer from the metal to the more
substituted carbon exclude branching [
241
]. The following evidence supports the above mechanism.
