Chemistry Reference
In-Depth Information
been pictured by others [
259
,
260
]. In the above model, the chain ends control monomer placement.
This differs significantly from the mechanism of isotactic placement by catalytic site control with the
heterogeneous catalysts. The active sites of homogeneous catalyst do not discriminate between
the faces of the incoming monomers in the step of coordination. Instead, steric hindrance between
the substituents of the terminal units of the propagating chains and the ligands attached to the
transition metals prevent rotations about the metal-carbon bonds. In addition, steric repulsion forces
the monomers to coordinate at opposite faces with each successive step of propagation, resulting in
syndiotactic placement.
4.5.3 Steric Control in Polymerization of Conjugated Dienes
The subject has also received considerable attention. Nevertheless, the mechanism is still not fully
understood. It is reasonable to assume that the form and structure of the catalyst and the valence of the
transition metals must play a role. The conformation of the monomer (s-
cis
or s-
trans
) is probably
also important.
It is known that CoCl
2
/Al(C
2
H
5
)Cl can polymerize
isomer
[
263
]. This suggests that a two-point coordination is required. Several reaction schemes that provide
for an attack at either C
1
or at C
4
positions were proposed over the years [
263
]. One mechanism for
polymerization of butadiene suggests that complexes of the catalysts in solvents of low dielectric
constant will either act as ion pairs or as independent solvated entities. Also, the growing chain may
be bound by either a
trans
-1,3-pentadiene but not the
cis
p
or a
s
linkage, and it is suspected that a continuous
s ! p
isomerism is
possible [
264
]:
R
C
4
H
6
C
4
H
6
M
T
M
T
X
X
M
T
X
R
polymer
M
T
X
where, M
T
means metal.
Soluble catalysts from transition metal acetoneacetonates (from nickel to titanium) combined with
triethylaluminum or triethylaluminum chloride yield
placement
decrease when bases are added [
265
]. This decrease is proportional to the base strength. The addition
is believed to decrease electron densities of the orbitals of the transition metals [
266
]. This suggests
that electrostatic interactions between the nearly non-bonding electrons of transition metals with the
dienes or with growing chain ends must play an important role. Such interactions must affect
placements of the incoming monomers. Hirai et al. studied ESR signals obtained during
polymerizations of butadiene, 1,3-pentadiene, and isoprene [
267
] with catalyst based on
n
cis
polymers of the dienes. The
cis
-butyltitanate/triethylaluminum. They concluded that the catalyst must possess two substituted
p
-allyl groups and one alkoxy group during the chain growing process [
267
]:
