4 Fluorenyl Substituents and Catalyst Enantioselectivity
As in the case of the bridge substituent modifications, this section describes the
results of a systematic investigation involving fluorenyl substitution patterns. The
investigations have shown that any modification of cyclopentadienyl substitution in
complex 1 is at best ineffective [proximal position(s)] or completely disruptive
[distal position(s)] to syndiospecificity of the final catalyst. As far as the fluorenyl
substitution pattern is concerned, the combined substitutions at position 4 and 5
proved to be generally disruptive to enantioselectivity and had some adverse effects
on polymer stereoregularity due to blockage of the central space in the frontal
positions of the fluorenyl that accommodate the propylene methyl group
[131–136]. By contrast, substitution at the 2 and 7 positions of the fluorenyl does
not bring about any improvement in enantioselectivity of the parent complex 1. On
the other hand, double substitution involving positions 3 and 6 of the fluorenyl
moiety of the ligand proved to bring about the desired improvements in catalyst
stereoselectivity and in the resulting polymer’s stereoregularity, melting point, and
crystallinity. This will be discussed below as a successful case study. The study also
shows that, as well as the position of the substituents, the size of the substituent is an
important factor such that the larger the substituents, the more dramatic the effect
they exert.
4.1 Stereoregularity Improvement and Frontal Substituents
The zirconocene complex (η
5 -C 5 H 4 -μ-CPh 2 -η
5 -3,6-dit
Bu-C 13 H 6 )ZrCl 2 , 9, can be
synthesized according to a similar synthetic procedure that describes the synthesis
of 6 (see Sect. 8 and [20]) with only difference being that, instead of the
unsubstituted fluorene used for preparation of 6, the 3,6-di-tert-butyl-substituted
fluorene is employed for the synthesis of the ligand of complex 9. The procedures
for the aromatization of the ligand, its reaction with ZrCl 4 , and isolation and
identification of complex 9 all follow similar reaction procedures and work-up
steps as for the preparation of 1 and 6 (see Sect. 8). Complex 9 is identified by its
1 H
NMR spectrum and its single-crystal X-ray structure. The molecular structure of
complex 9 is depicted in Fig. 15. As can be seen from molecular views, the
metallocene molecule 9 projects the overall symmetry and bonding characteristics
that were described for 6 except for the presence of two protruding tert-butyl
substituents at positions 3 and 6 in its frontal section. When activated with MAO,
complex 9 polymerizes propylene to highly syndiotactic polypropylene very
efficiently. Catalyst prepared with the complex 9/MAO is more active than the
corresponding catalysts prepared with complexes 1 and 6 under the same polymerization conditions. Tables 7 and 8 present the polymerization conditions, results, and
polymer analyses of the syndiotactic polymers produced with the 9/MAO catalyst
system at different polymerization temperatures [28–30]. A quick comparative
70
A. Razavi
As in the case of the bridge substituent modifications, this section describes the
results of a systematic investigation involving fluorenyl substitution patterns. The
investigations have shown that any modification of cyclopentadienyl substitution in
complex 1 is at best ineffective [proximal position(s)] or completely disruptive
[distal position(s)] to syndiospecificity of the final catalyst. As far as the fluorenyl
substitution pattern is concerned, the combined substitutions at position 4 and 5
proved to be generally disruptive to enantioselectivity and had some adverse effects
on polymer stereoregularity due to blockage of the central space in the frontal
positions of the fluorenyl that accommodate the propylene methyl group
[131–136]. By contrast, substitution at the 2 and 7 positions of the fluorenyl does
not bring about any improvement in enantioselectivity of the parent complex 1. On
the other hand, double substitution involving positions 3 and 6 of the fluorenyl
moiety of the ligand proved to bring about the desired improvements in catalyst
stereoselectivity and in the resulting polymer’s stereoregularity, melting point, and
crystallinity. This will be discussed below as a successful case study. The study also
shows that, as well as the position of the substituents, the size of the substituent is an
important factor such that the larger the substituents, the more dramatic the effect
they exert.
4.1 Stereoregularity Improvement and Frontal Substituents
The zirconocene complex (η
5 -C 5 H 4 -μ-CPh 2 -η
5 -3,6-dit
Bu-C 13 H 6 )ZrCl 2 , 9, can be
synthesized according to a similar synthetic procedure that describes the synthesis
of 6 (see Sect. 8 and [20]) with only difference being that, instead of the
unsubstituted fluorene used for preparation of 6, the 3,6-di-tert-butyl-substituted
fluorene is employed for the synthesis of the ligand of complex 9. The procedures
for the aromatization of the ligand, its reaction with ZrCl 4 , and isolation and
identification of complex 9 all follow similar reaction procedures and work-up
steps as for the preparation of 1 and 6 (see Sect. 8). Complex 9 is identified by its
1 H
NMR spectrum and its single-crystal X-ray structure. The molecular structure of
complex 9 is depicted in Fig. 15. As can be seen from molecular views, the
metallocene molecule 9 projects the overall symmetry and bonding characteristics
that were described for 6 except for the presence of two protruding tert-butyl
substituents at positions 3 and 6 in its frontal section. When activated with MAO,
complex 9 polymerizes propylene to highly syndiotactic polypropylene very
efficiently. Catalyst prepared with the complex 9/MAO is more active than the
corresponding catalysts prepared with complexes 1 and 6 under the same polymerization conditions. Tables 7 and 8 present the polymerization conditions, results, and
polymer analyses of the syndiotactic polymers produced with the 9/MAO catalyst
system at different polymerization temperatures [28–30]. A quick comparative
70
A. Razavi