is twofold. First, Marks’ anion interacts more strongly with the solvent than the
other anions because its charge is the most localized. Therefore, solvent destabilization of the transition state with respect to the reactant is highest for Marks’ anion.
Second, the steric bulk makes it more difficult for the fluoroaluminate to get close to
the zirconocenium cation. The MAO-Me
À model also has a more concentrated
charge than MeB(C 6 F 5 ) 3
À , which in solution has an effect similar to that found for
Marks’ anion but it is smaller and therefore the steric effects do not contribute as
much to the barrier. The two effects combined are capable of explaining the
experimental results.
It seems that the site epimerization mechanism, complex as it is, can be related to
three main factors. Optimization of each would contribute to the lowering of its
frequency of occurrence and to improvement in the stereoregularity and physical
properties of the produced s-PP. Whereas a direct link could be establish between
site epimerization and the ligand substituents’ bulkiness and positions on one hand,
and the anion’s size and charge distribution on the other hand, the connection
between structural flexibility and site epimerization, though proven empirically,
is not very clear.
However, although optimization of the stereorigidity and substitutional factors
are, more or less, controllable and within practical grasp, the anion–cation interaction, although very well understood, is in practice very difficult to implement in a
large-scale catalyst manufacturing plant for commercial purposes. Understanding
the effect of anion size, charge, and charge distribution on the site epimerization
transition energy barrier in particular and controlling site epimerization in general,
is of utmost importance not just for academic reasons but also for the commercialization of s-PP using large-scale supported catalysts.
6 Metallocene Molecular Symmetry and the Catalyst’s
Syndiotactic Specificity
In the previous sections, various factors affecting the stereospecificity of the
syndiotactic-specific catalysts systems were discussed. A final topic that should
be covered to complete these discussions is the relevance of the symmetry of the
metallocene molecules and its role in the tactic behavior of the final catalyst.
As shown in previous sections, for many different reasons the perfect bilateral
symmetry of the original metallocene structure is most likely not maintained in the
solution phase. The enantioselectivity calculations with model catalysts have also
revealed that the catalyst system will behave in a syndioselective way as long as the
conditions for the proper arrangement of ligand, polymer chain and minimum
energy monomer coordination mode is provided at each coordination position.
Thus, the legitimate question to be answered is whether the C s or bilateral
symmetry of a metallocene molecule is a good indicator for the syndiospecificity
of the final catalyst.
Syndiotactic Polypropylene: Discovery, Development, and Industrialization. . .
89
other anions because its charge is the most localized. Therefore, solvent destabilization of the transition state with respect to the reactant is highest for Marks’ anion.
Second, the steric bulk makes it more difficult for the fluoroaluminate to get close to
the zirconocenium cation. The MAO-Me
À model also has a more concentrated
charge than MeB(C 6 F 5 ) 3
À , which in solution has an effect similar to that found for
Marks’ anion but it is smaller and therefore the steric effects do not contribute as
much to the barrier. The two effects combined are capable of explaining the
experimental results.
It seems that the site epimerization mechanism, complex as it is, can be related to
three main factors. Optimization of each would contribute to the lowering of its
frequency of occurrence and to improvement in the stereoregularity and physical
properties of the produced s-PP. Whereas a direct link could be establish between
site epimerization and the ligand substituents’ bulkiness and positions on one hand,
and the anion’s size and charge distribution on the other hand, the connection
between structural flexibility and site epimerization, though proven empirically,
is not very clear.
However, although optimization of the stereorigidity and substitutional factors
are, more or less, controllable and within practical grasp, the anion–cation interaction, although very well understood, is in practice very difficult to implement in a
large-scale catalyst manufacturing plant for commercial purposes. Understanding
the effect of anion size, charge, and charge distribution on the site epimerization
transition energy barrier in particular and controlling site epimerization in general,
is of utmost importance not just for academic reasons but also for the commercialization of s-PP using large-scale supported catalysts.
6 Metallocene Molecular Symmetry and the Catalyst’s
Syndiotactic Specificity
In the previous sections, various factors affecting the stereospecificity of the
syndiotactic-specific catalysts systems were discussed. A final topic that should
be covered to complete these discussions is the relevance of the symmetry of the
metallocene molecules and its role in the tactic behavior of the final catalyst.
As shown in previous sections, for many different reasons the perfect bilateral
symmetry of the original metallocene structure is most likely not maintained in the
solution phase. The enantioselectivity calculations with model catalysts have also
revealed that the catalyst system will behave in a syndioselective way as long as the
conditions for the proper arrangement of ligand, polymer chain and minimum
energy monomer coordination mode is provided at each coordination position.
Thus, the legitimate question to be answered is whether the C s or bilateral
symmetry of a metallocene molecule is a good indicator for the syndiospecificity
of the final catalyst.
Syndiotactic Polypropylene: Discovery, Development, and Industrialization. . .
89