3.3 Bridge Substituents and Syndiotactic Polypropylene
Molecular Weight
An increase in the size of the bridge, exemplified by replacement of the singlecarbon isopropylidene inter-annular bridge with a two-carbon ethano-bridge, in
syndiospecific catalyst system 5/MAO provides the desired increase in the molecular
weight of the s-PP polymer, but is unfortunately accompanied by some undesired
side effects such as a decrease in the catalytic activity and stereospecificity and a
lowering of the stereoregularity of the s-PP polymer. Other investigations related to
the increase in the size of the bridge have also shown, for example, that the
replacement of the carbon bridge with a silicon bridge has a similar deleterious
effect on the enantioselectivity of the final catalysts [123, 124]. These findings are
suggestive of the fact that any bridge modification with the purpose of improving
the overall final catalyst properties must maintain the single-carbon arrangement of
the bridge in the basic skeleton of complex 1. This leaves only one remaining option
for further consideration: modification of the bridge substituents.
The effect of the bridge substituents on the molecular weight of s-PP polymer was
accidentally discovered during a systematic study of structural modifications for
syndiospecific catalyst systems. The aim of the study was initially to perform
substitutional modifications on the ligand without destroying the bilateral symmetry
in the original metallocene molecule 1. In the course of the study, by us and other
research groups, the two methyl substituents of the bridge were exchanged individually or simultaneously for one or two phenyl groups, one or two hydride groups, and a
series of other potential aliphatic or aromatic groups [125–127]. Of the many bridge
substituent modifications, the effect of replacing the two methyl substituents of the
bridge in 1 with two phenyl groups causes the most spectacular results with respect to
increasing the molecular weight of s-PP and is discussed below in some detail.
3.4 Polymerization Behavior of Diphenylmethylidene
(cyclopentadienyl-fluorenyl)MCl 2 /MAO: Methyl Versus
Phenyl Substituent in the Bridge
The replacement of the two methyl groups in the isopropylidene bridge of the
complex 1 (and 2) by two phenyl groups leads to the formation of two new
complexes, diphenylmethylidene(cyclopentadienyl-fluorenyl)MCl 2 or (η
5
-C 5 H 4 -
CPh 2 -η
5
-C 13 H 8 )MCl 2 ; M ¼ Zr, (6) or Hf (7). Complexes 6 and 7 can be prepared
according to a method very similar to that used for the preparation of 1 and 2
(see Sect. 8).
Two different views of the molecular structure of 6 (and 7) are portrayed in
Fig. 13. From both a molecular symmetry point of view and from solid structural
characteristics (bond distances and bond angles) point of view, complexes 6 and 7
appear almost indistinguishable from 1 and 2 [20]. It was, therefore, expected that
Syndiotactic Polypropylene: Discovery, Development, and Industrialization. . .
65
Molecular Weight
An increase in the size of the bridge, exemplified by replacement of the singlecarbon isopropylidene inter-annular bridge with a two-carbon ethano-bridge, in
syndiospecific catalyst system 5/MAO provides the desired increase in the molecular
weight of the s-PP polymer, but is unfortunately accompanied by some undesired
side effects such as a decrease in the catalytic activity and stereospecificity and a
lowering of the stereoregularity of the s-PP polymer. Other investigations related to
the increase in the size of the bridge have also shown, for example, that the
replacement of the carbon bridge with a silicon bridge has a similar deleterious
effect on the enantioselectivity of the final catalysts [123, 124]. These findings are
suggestive of the fact that any bridge modification with the purpose of improving
the overall final catalyst properties must maintain the single-carbon arrangement of
the bridge in the basic skeleton of complex 1. This leaves only one remaining option
for further consideration: modification of the bridge substituents.
The effect of the bridge substituents on the molecular weight of s-PP polymer was
accidentally discovered during a systematic study of structural modifications for
syndiospecific catalyst systems. The aim of the study was initially to perform
substitutional modifications on the ligand without destroying the bilateral symmetry
in the original metallocene molecule 1. In the course of the study, by us and other
research groups, the two methyl substituents of the bridge were exchanged individually or simultaneously for one or two phenyl groups, one or two hydride groups, and a
series of other potential aliphatic or aromatic groups [125–127]. Of the many bridge
substituent modifications, the effect of replacing the two methyl substituents of the
bridge in 1 with two phenyl groups causes the most spectacular results with respect to
increasing the molecular weight of s-PP and is discussed below in some detail.
3.4 Polymerization Behavior of Diphenylmethylidene
(cyclopentadienyl-fluorenyl)MCl 2 /MAO: Methyl Versus
Phenyl Substituent in the Bridge
The replacement of the two methyl groups in the isopropylidene bridge of the
complex 1 (and 2) by two phenyl groups leads to the formation of two new
complexes, diphenylmethylidene(cyclopentadienyl-fluorenyl)MCl 2 or (η
5
-C 5 H 4 -
CPh 2 -η
5
-C 13 H 8 )MCl 2 ; M ¼ Zr, (6) or Hf (7). Complexes 6 and 7 can be prepared
according to a method very similar to that used for the preparation of 1 and 2
(see Sect. 8).
Two different views of the molecular structure of 6 (and 7) are portrayed in
Fig. 13. From both a molecular symmetry point of view and from solid structural
characteristics (bond distances and bond angles) point of view, complexes 6 and 7
appear almost indistinguishable from 1 and 2 [20]. It was, therefore, expected that
Syndiotactic Polypropylene: Discovery, Development, and Industrialization. . .
65