All syndiotactic-specific catalysts that have been discussed so far had one
principal element in common: their precatalyst’s metallocene molecules in solid
phase all presented bilateral molecular symmetry or a vertical plane of symmetry
(σ v ) in the solid state. However, from the discussions in Sect. 4.3 it is clear that the
whole notion of a “perfect” molecular symmetry, bilateral or otherwise, in the
solution phase should be handled with caution and not t taken very literally. We saw
that even with initially “perfect” bilateral symmetric metallocene molecules like 1
(2) and 6 (7), the perfect symmetry may be lost, at least in part, in solution and
during different stages of the polymerization due to incessant η
5 , η
3 , and possibly η
1
reversible hapticity change or occasional ring slippage [149–160, 161, and
references therein; 162, 163]. Thus, the question to be answered is whether or not
the bilateral symmetry embodies the necessary and/or sufficient condition(s) for
syndiotactic specificity of a catalyst system. It is apparent that a catalyst’s
syndiospecificity, as defined in preceding sections, is the result of the combined
actions of several factors, the absence of any one of which could lead to the
malfunctioning of the catalyst and destruction of the whole syndiospecificity
process. The delicate steric balance between the three main participants (ligand,
polymer chain, and monomer) as well as the interaction of cation with the counter-ion
and proper functioning of the dynamic processes are all essential for its existence.
Therefore, it should not be surprising if, occasionally, catalysts prepared with
structurally “acceptable” metallocenes and with perfect bilateral symmetry do not
function accordingly.
6.1 Syndio- and Nonsyndiospecific Catalyst Systems with C s
Symmetric Metallocene Structures
A disappointing example of this kind is the performance of the bilaterally symmetric
complex dimethysilyl-(cyclopentadienyl-tetramethyl-cyclopentadienyl)ZrCl 2 . After
its activation with MAO and exposure to propylene, the resulting enantiotopic
catalyst system does not function in a syndiotactic-specific manner and the polypropylene polymers that it produces are completely atactic. Here, obviously the presence
of constantly rotating, relatively voluminous methyl groups, particularly those two
placed in the distal positions (with respect to the bridge-head carbon of the substituted
cyclopentadienyl group), blocks the central free space in front of the active site and
interferes vigorously with the proper “head-down” coordination mode of propylene
and disrupts completely the enantio-face selective process of the system.
Another example demonstrating the determining role of the molecular bilateral
symmetry versus the (delicate and fragile) steric balance between the chain, monomer, and ligand substituents for the syndiospecificity process is the polymerization
behavior of the metallocene complex diphenylmethylidene-(cyclopentadienyloctahydrofluorenyl)ZrCl 2 , with the chemical formula (η
5
-C 5 H 4 -μ-CPh 2 -η
5
-C 13 H 16 )
ZrCl 2 , 8.
90
A. Razavi
principal element in common: their precatalyst’s metallocene molecules in solid
phase all presented bilateral molecular symmetry or a vertical plane of symmetry
(σ v ) in the solid state. However, from the discussions in Sect. 4.3 it is clear that the
whole notion of a “perfect” molecular symmetry, bilateral or otherwise, in the
solution phase should be handled with caution and not t taken very literally. We saw
that even with initially “perfect” bilateral symmetric metallocene molecules like 1
(2) and 6 (7), the perfect symmetry may be lost, at least in part, in solution and
during different stages of the polymerization due to incessant η
5 , η
3 , and possibly η
1
reversible hapticity change or occasional ring slippage [149–160, 161, and
references therein; 162, 163]. Thus, the question to be answered is whether or not
the bilateral symmetry embodies the necessary and/or sufficient condition(s) for
syndiotactic specificity of a catalyst system. It is apparent that a catalyst’s
syndiospecificity, as defined in preceding sections, is the result of the combined
actions of several factors, the absence of any one of which could lead to the
malfunctioning of the catalyst and destruction of the whole syndiospecificity
process. The delicate steric balance between the three main participants (ligand,
polymer chain, and monomer) as well as the interaction of cation with the counter-ion
and proper functioning of the dynamic processes are all essential for its existence.
Therefore, it should not be surprising if, occasionally, catalysts prepared with
structurally “acceptable” metallocenes and with perfect bilateral symmetry do not
function accordingly.
6.1 Syndio- and Nonsyndiospecific Catalyst Systems with C s
Symmetric Metallocene Structures
A disappointing example of this kind is the performance of the bilaterally symmetric
complex dimethysilyl-(cyclopentadienyl-tetramethyl-cyclopentadienyl)ZrCl 2 . After
its activation with MAO and exposure to propylene, the resulting enantiotopic
catalyst system does not function in a syndiotactic-specific manner and the polypropylene polymers that it produces are completely atactic. Here, obviously the presence
of constantly rotating, relatively voluminous methyl groups, particularly those two
placed in the distal positions (with respect to the bridge-head carbon of the substituted
cyclopentadienyl group), blocks the central free space in front of the active site and
interferes vigorously with the proper “head-down” coordination mode of propylene
and disrupts completely the enantio-face selective process of the system.
Another example demonstrating the determining role of the molecular bilateral
symmetry versus the (delicate and fragile) steric balance between the chain, monomer, and ligand substituents for the syndiospecificity process is the polymerization
behavior of the metallocene complex diphenylmethylidene-(cyclopentadienyloctahydrofluorenyl)ZrCl 2 , with the chemical formula (η
5
-C 5 H 4 -μ-CPh 2 -η
5
-C 13 H 16 )
ZrCl 2 , 8.
90
A. Razavi