high enough to generate measurable crystallinity and melting points. The
syndiotactic polymers that were produced at higher polymerization temperatures
have much lower stereoregularity and exhibit no crystallinity and/or melting points.
Here again, like in the case of s-PP polymers produced with the 10/MAO catalyst
system, the main reason for lower stereoregularity is the rapidly increasing concentration of the site epimerization related (rrmr pentad) stereodefects. They double in
size by increasing the polymerization temperature from 40
C to 60
C and reach
values of over 10% at a polymerization temperature of 80
C and above for the very
same reasons explained for syndiotactic polymers produced with the 10/MAO
catalyst system. The enantiomorphic site control related stereo-errors (rmmr
pentads) are high but remain almost unchanged (as for 10/MAO-produced
polymers) over the entire measured temperature range.
The large enantioselectivity difference between the two catalyst systems involving
complexes 10 and 11 (with rmmr pentad concentration of over 3% for 11/MAO and
about 1.7% for 10/MAO) demonstrate the crucial role of the positions 3,6 for
introducing the substitutions in the fluorenyl moiety of the ligand.
The syndiospecific catalysts systems (10 and 11)/MAO in which one of the
ligand’s main ingredients, an η
5
-bonded cyclopentadienyl moiety, is exchanged for
an η
1
-bonded amido group with respect to the parent syndiospecific system made
with complex 1, are convincing examples for the validity of the fact that the proper
steric balance between the three participants (ligand, polymer chain, and propylene)
in the active transition state structure and the preference for one of the two monomer
enantio-faces at each of the two enantiotopic coordination positions rather than for
any particular geometry or symmetry, are the origin of syndiospecificity.
The polymerization behavior of (10 and 11)/MAO catalyst systems also
demonstrates nicely that no matter what the chemical makeup of the metallocene
precursor, composition of the ligand, or nature of the transition metal, the resulting
catalysts (once their structure fulfills the prerequisites discussed based on the model
given in Sect. 3.2) will act in a syndiotactic-specific manner and produce crystalline
s-PP. The overall stereoselectivity is, however, determined by the frequency and
rate of the site epimerization process. The lower stereoregularity of the new
syndiotactic chains is probably the related to higher flexibility of the structures of
complexes 10/11 in solution due to the absence of one of the η
5
-bonded aromatic
ring systems and the dynamic environment of a reversibly inverting amido group
(dynamic umbrella-type reversible inversion at the amido-nitrogen center see:
[148]) imparting a high degree of flexibility to the structure and/or facilitating
fluorenyl ring slippage (see Sect. 4), leading occasionally to the dissymmetry of
the site and the formation of lop-sided tight contact ion-pairing and short meso dyad
units/sequences.
It is worth mentioning that the zirconium analogue of 10 produces only
oligomeric materials.
80
A. Razavi
syndiotactic polymers that were produced at higher polymerization temperatures
have much lower stereoregularity and exhibit no crystallinity and/or melting points.
Here again, like in the case of s-PP polymers produced with the 10/MAO catalyst
system, the main reason for lower stereoregularity is the rapidly increasing concentration of the site epimerization related (rrmr pentad) stereodefects. They double in
size by increasing the polymerization temperature from 40
C to 60
C and reach
values of over 10% at a polymerization temperature of 80
C and above for the very
same reasons explained for syndiotactic polymers produced with the 10/MAO
catalyst system. The enantiomorphic site control related stereo-errors (rmmr
pentads) are high but remain almost unchanged (as for 10/MAO-produced
polymers) over the entire measured temperature range.
The large enantioselectivity difference between the two catalyst systems involving
complexes 10 and 11 (with rmmr pentad concentration of over 3% for 11/MAO and
about 1.7% for 10/MAO) demonstrate the crucial role of the positions 3,6 for
introducing the substitutions in the fluorenyl moiety of the ligand.
The syndiospecific catalysts systems (10 and 11)/MAO in which one of the
ligand’s main ingredients, an η
5
-bonded cyclopentadienyl moiety, is exchanged for
an η
1
-bonded amido group with respect to the parent syndiospecific system made
with complex 1, are convincing examples for the validity of the fact that the proper
steric balance between the three participants (ligand, polymer chain, and propylene)
in the active transition state structure and the preference for one of the two monomer
enantio-faces at each of the two enantiotopic coordination positions rather than for
any particular geometry or symmetry, are the origin of syndiospecificity.
The polymerization behavior of (10 and 11)/MAO catalyst systems also
demonstrates nicely that no matter what the chemical makeup of the metallocene
precursor, composition of the ligand, or nature of the transition metal, the resulting
catalysts (once their structure fulfills the prerequisites discussed based on the model
given in Sect. 3.2) will act in a syndiotactic-specific manner and produce crystalline
s-PP. The overall stereoselectivity is, however, determined by the frequency and
rate of the site epimerization process. The lower stereoregularity of the new
syndiotactic chains is probably the related to higher flexibility of the structures of
complexes 10/11 in solution due to the absence of one of the η
5
-bonded aromatic
ring systems and the dynamic environment of a reversibly inverting amido group
(dynamic umbrella-type reversible inversion at the amido-nitrogen center see:
[148]) imparting a high degree of flexibility to the structure and/or facilitating
fluorenyl ring slippage (see Sect. 4), leading occasionally to the dissymmetry of
the site and the formation of lop-sided tight contact ion-pairing and short meso dyad
units/sequences.
It is worth mentioning that the zirconium analogue of 10 produces only
oligomeric materials.
80
A. Razavi