chloride groups, occupying the prospective coordination sites designated for future
monomer coordination and polymer chain growth. By interacting with their
substituents, they regulate and direct the orientation of the growing polymer
chain, determine the re or si selective coordination of propylene, and govern the
whole scenery of the stereospecific polymerization process.
For future discussions, it is also important to know that the fluorenyl–Zr–cyclopentadienyl bond angle in the zirconium complex 1 is 118.6
(119.4
for hafnium
complex 2) and deviates by about 10
from the ideal tetrahedral angle of 109.5
(tetrahedral formed around the bridging carbon and the bonds with its four
substituents, two methyl groups plus fluorenyl and cyclopentadienyl groups) and
a Cl–Zr–Cl bond angle of 98.2
(Cl–Hf–Cl ¼ 97.5
).
2.2 Crystal Structure of the Metallocenium–Monoalkyl
Cation and the Structure of the Putative Active Site
Before the discovery of the metallocene-based syndiospecific catalyst system there
existed, thanks to intensive investigations by several research groups worldwide,
sufficient evidence to assume that the active species involved in metallocene-based
olefin polymerization catalysis are cationic in nature [77–96]. It is now widely
accepted that the active site species are cationic metallocenium–monoalkyl
complexes. They are formed generally during the activation step(s) when the
catalyst precursor, the metallocene dichloride, is treated with the alkyl-aluminum
co-catalysts/activators in a “two-step” alkylation and alkyl abstraction reaction
(in reality the alkylation process comprises a rather complex multi-step halide/
alkyl exchange with reversible and nonreversible reactions and side reactions
leading to the final cationic species and some unspecified nonreactive or dormant
species. See also [97] and the quoted references). Therefore it is reasonable to
assume that similar cationic species are being formed during the activation stages
of 1 (and 2)/MAO catalyst systems with MAO or other alkylating, ionizing agents.
It was recognized that for mechanistic elucidation of the elementary steps involved
in stereospecific propylene polymerization catalysis with 1 (and 2)/MAO, access
to the molecular structure of the corresponding zirconocenium–methyl cation of
complex 1 would be very useful. Its molecular structure and X-ray determined
interatomic parameters could help to design a practical and realistic model for the
hypothetical active site at which the actual syndiospecific polymerization of propylene
is actually taking place. The geometrical data for the cationic complex allow better
visualization and insight into the immediate stereo-electronic environment of the
hypothetical active site, and elaboration of its functioning.
The metallocenium complex 3, [(η
5
-C 5 H 4 -μ-CMe 2 -η
5
-C 13 H 8 )Zr
+
Me][B(C 6 F 5 ) 4 ]
À ,
is prepared via the double methylation of complex 1 and the subsequent reaction of the
di-methylated reaction product with an equivalent of the reagent trityl-tetrakispentaflurophenylborate, (C 6 H 5 ) 3 C-B(C 6 F 5 ) 4 , in toluene at room temperature.
Syndiotactic Polypropylene: Discovery, Development, and Industrialization. . .
49
monomer coordination and polymer chain growth. By interacting with their
substituents, they regulate and direct the orientation of the growing polymer
chain, determine the re or si selective coordination of propylene, and govern the
whole scenery of the stereospecific polymerization process.
For future discussions, it is also important to know that the fluorenyl–Zr–cyclopentadienyl bond angle in the zirconium complex 1 is 118.6
(119.4
for hafnium
complex 2) and deviates by about 10
from the ideal tetrahedral angle of 109.5
(tetrahedral formed around the bridging carbon and the bonds with its four
substituents, two methyl groups plus fluorenyl and cyclopentadienyl groups) and
a Cl–Zr–Cl bond angle of 98.2
(Cl–Hf–Cl ¼ 97.5
).
2.2 Crystal Structure of the Metallocenium–Monoalkyl
Cation and the Structure of the Putative Active Site
Before the discovery of the metallocene-based syndiospecific catalyst system there
existed, thanks to intensive investigations by several research groups worldwide,
sufficient evidence to assume that the active species involved in metallocene-based
olefin polymerization catalysis are cationic in nature [77–96]. It is now widely
accepted that the active site species are cationic metallocenium–monoalkyl
complexes. They are formed generally during the activation step(s) when the
catalyst precursor, the metallocene dichloride, is treated with the alkyl-aluminum
co-catalysts/activators in a “two-step” alkylation and alkyl abstraction reaction
(in reality the alkylation process comprises a rather complex multi-step halide/
alkyl exchange with reversible and nonreversible reactions and side reactions
leading to the final cationic species and some unspecified nonreactive or dormant
species. See also [97] and the quoted references). Therefore it is reasonable to
assume that similar cationic species are being formed during the activation stages
of 1 (and 2)/MAO catalyst systems with MAO or other alkylating, ionizing agents.
It was recognized that for mechanistic elucidation of the elementary steps involved
in stereospecific propylene polymerization catalysis with 1 (and 2)/MAO, access
to the molecular structure of the corresponding zirconocenium–methyl cation of
complex 1 would be very useful. Its molecular structure and X-ray determined
interatomic parameters could help to design a practical and realistic model for the
hypothetical active site at which the actual syndiospecific polymerization of propylene
is actually taking place. The geometrical data for the cationic complex allow better
visualization and insight into the immediate stereo-electronic environment of the
hypothetical active site, and elaboration of its functioning.
The metallocenium complex 3, [(η
5
-C 5 H 4 -μ-CMe 2 -η
5
-C 13 H 8 )Zr
+
Me][B(C 6 F 5 ) 4 ]
À ,
is prepared via the double methylation of complex 1 and the subsequent reaction of the
di-methylated reaction product with an equivalent of the reagent trityl-tetrakispentaflurophenylborate, (C 6 H 5 ) 3 C-B(C 6 F 5 ) 4 , in toluene at room temperature.
Syndiotactic Polypropylene: Discovery, Development, and Industrialization. . .
49