reveals that, under similar polymerization conditions, the catalyst systems 6/MAO
(and 7/MAO) produce s-PP polymers with much higher molecular weights compared
to polymers produced with 1/MAO (and 2/MAO) catalyst systems. On the other
hand, for both systems the polymers’ stereoregularities and microtacticities, as
measured by the size of the related pentad (rmmr and rrmr) intensities, within the
expected experimental errors, were very close.
The general model and mechanism that was discussed in Sect. 2.4 and proposed
for the mechanism of the syndiospecific polymerization of propylene with the C S
symmetric metallocene catalyst system 1/MAO vindicate the similar microtacticity
and stereoregularity of the syndiotactic polymers perfectly, but does not in any way
account for or rationalize, the polymerization behavior of catalyst system diphenylmethylidene-μ-(cyclopentadienyl-fluorenyl)zirconium dichloride 6/MAO (7/MAO)
with respect to the dramatic increases in the molecular weights of the resulting s-PP
polymers.
The molecular structures of 1 and 6, depicted in Figs. 2 and 13, expose their
extraordinary overall resemblance and the reason for the production of s-PP chains
with very similar microstructures. They do not, however, provide any clue as to
why the molecular weights of their corresponding s-PP polymers are so different.
A review of the solid state interatomic bond distances obtained from crystal
structure data for complexes 1 and 6 [19, 20] also does not give, at the first sight,
any indications that might justify the different catalytic performance in this respect.
It seems that upon introduction of the phenyl groups in the bridge, the important
Zr–C bond distances in the modified complex 6 have not undergone palpable
changes. Both aromatic ring systems in complex 6 are η
5 -bonded to the zirconium
center and the observed slight variations in Zr–C and C–C bond distance are, in
general, well within the expected experimental uncertainties.
However, closer inspection of the bond angles reveals some minor changes. The
centroid–Zr–centroid (and Cl–Zr–Cl) bond angle in complex 6 is smaller than the
corresponding bond angles for complex 1.The angle has decreased from a value of
118.60
(and 98.20
) to a value of 117.60
(and 96.60
), i.e., by 1.00
(and 1.60
).
These angular changes imply that in complex 6, as a result of the repulsive
interaction between the two aromatic phenyl substituents in the bridge, the external
tetrahedral angle, i.e. the angle including the bridging carbon and the bonds
connected to the two phenyl groups, is increased and as a direct consequence the
internal tetrahedral angle, including the bridging carbon and its bonds to cyclopentadienyl and fluorenyl bridge-head carbon atoms, is decreased.
1 The smaller
internal angle forces the transition metal Zr (Hf) to move slightly outward in
order to fit better inside the ligand and to have a more efficient d orbitals
overlapping with their aromatic ring π-systems.
2
1 (a) Tetrahedral formed by the bridging carbon and the four bonds connecting it to two phenyl
groups, a cyclopentadienyl, and a fluorenyl group. (b) Interpretation of the X-ray data in [20] was
given by Professor Jerry Atwood (private communication) with respect to Zr displacement.
2 See footnote 1, part b.
Syndiotactic Polypropylene: Discovery, Development, and Industrialization. . .
67
(and 7/MAO) produce s-PP polymers with much higher molecular weights compared
to polymers produced with 1/MAO (and 2/MAO) catalyst systems. On the other
hand, for both systems the polymers’ stereoregularities and microtacticities, as
measured by the size of the related pentad (rmmr and rrmr) intensities, within the
expected experimental errors, were very close.
The general model and mechanism that was discussed in Sect. 2.4 and proposed
for the mechanism of the syndiospecific polymerization of propylene with the C S
symmetric metallocene catalyst system 1/MAO vindicate the similar microtacticity
and stereoregularity of the syndiotactic polymers perfectly, but does not in any way
account for or rationalize, the polymerization behavior of catalyst system diphenylmethylidene-μ-(cyclopentadienyl-fluorenyl)zirconium dichloride 6/MAO (7/MAO)
with respect to the dramatic increases in the molecular weights of the resulting s-PP
polymers.
The molecular structures of 1 and 6, depicted in Figs. 2 and 13, expose their
extraordinary overall resemblance and the reason for the production of s-PP chains
with very similar microstructures. They do not, however, provide any clue as to
why the molecular weights of their corresponding s-PP polymers are so different.
A review of the solid state interatomic bond distances obtained from crystal
structure data for complexes 1 and 6 [19, 20] also does not give, at the first sight,
any indications that might justify the different catalytic performance in this respect.
It seems that upon introduction of the phenyl groups in the bridge, the important
Zr–C bond distances in the modified complex 6 have not undergone palpable
changes. Both aromatic ring systems in complex 6 are η
5 -bonded to the zirconium
center and the observed slight variations in Zr–C and C–C bond distance are, in
general, well within the expected experimental uncertainties.
However, closer inspection of the bond angles reveals some minor changes. The
centroid–Zr–centroid (and Cl–Zr–Cl) bond angle in complex 6 is smaller than the
corresponding bond angles for complex 1.The angle has decreased from a value of
118.60
(and 98.20
) to a value of 117.60
(and 96.60
), i.e., by 1.00
(and 1.60
).
These angular changes imply that in complex 6, as a result of the repulsive
interaction between the two aromatic phenyl substituents in the bridge, the external
tetrahedral angle, i.e. the angle including the bridging carbon and the bonds
connected to the two phenyl groups, is increased and as a direct consequence the
internal tetrahedral angle, including the bridging carbon and its bonds to cyclopentadienyl and fluorenyl bridge-head carbon atoms, is decreased.
1 The smaller
internal angle forces the transition metal Zr (Hf) to move slightly outward in
order to fit better inside the ligand and to have a more efficient d orbitals
overlapping with their aromatic ring π-systems.
2
1 (a) Tetrahedral formed by the bridging carbon and the four bonds connecting it to two phenyl
groups, a cyclopentadienyl, and a fluorenyl group. (b) Interpretation of the X-ray data in [20] was
given by Professor Jerry Atwood (private communication) with respect to Zr displacement.
2 See footnote 1, part b.
Syndiotactic Polypropylene: Discovery, Development, and Industrialization. . .
67