parent syndiospecific catalyst system 1/MAO, the energy barrier for the site
epimerization is even higher and reaches above 13 kcal/mol [146].
The polymer chain’s passage between the two available coordination sites
involves many nonbonded repulsive steric interactions with different parts of the
ligand and the transformation of the β-agostic resting state into a symmetrical
β
0 -agostic state, via a symmetric transition state in which the alkyl chain rests
approximately in the plane formed by the metal and the ligands’ centroids, half
way between the two coordination positions. During the transformation, on either
side of the reaction energy profile, other intermediates with α- and α
0 -agostic bonds
are also formed [170]. Overall, the site epimerization mechanism that does not take
into account the role of the counter-anion in the process has a very large activation
energy compared to the propagation activation energy. Such a mechanism does not
agree with the experimental results, which show that the pentad distributions are in
reality influenced by a rather frequent site epimerization.
5.4 Counter-Ion-Assisted Site Epimerization
In the regular and real olefin polymerization processes with metallocene catalysts,
the anion is an ever-present participant and an integral part of the functioning
catalyst system. It is systematically displaced and moves (in and) out of the
coordination sphere in a concerted action, while the monomer units approach for
coordination to be paired again with the cation after the insertion process is
complete. Assuming that the monomer approach proceeds along the most favorable
path (i.e., via a cis approach with respect to the anion), immediately after the
monomer insertion the anion is positioned at the outer coordination sphere, on the
side of the polymer chain (Fig. 20a).
The resulting metalloocenium–polymeryl cation has an empty coordination site
available for the re-formation of a close-contact ion pair with the anion. To avoid
unnecessary steric interaction with the alky group (polymer chain), the anion can
first undergo a spatial reorganization in the outer coordination sphere and then
approach the free coordination site unhindered (Fig. 20b) [168, 169]. Alternatively,
it can approach the cation directly, forcing in the process the polymer chain to move
to the empty coordination site. The first process is what normally occurs during a
syndiospecific propagation step (Fig. 20c), while the second process leads to the site
epimerization since the immediate ion pairing and the simultaneous displacement
of the polymer chain, results in a complex in which the polymer chain occupies the
same coordination site as in the previous catalytic cycle (Fig. 20d).
Intuitively, it can be reasonably assumed that the anion reorganization in the outer
coordination sphere is spontaneous and occurs freely; its rearrangement is governed
purely by diffusion. Indeed, our gas-phase calculations [146] show that for the cation/
anion model system, Me 2 C(CpFlu)ZrMe-Me-B(C 6 F 5 ) 3 , the energy barrier is about
0.5 kcal/mol, and well within the error associated with the computational method. On
the other hand, the formation of the close-contact ion pair is also an exothermic and
Syndiotactic Polypropylene: Discovery, Development, and Industrialization. . .
85
epimerization is even higher and reaches above 13 kcal/mol [146].
The polymer chain’s passage between the two available coordination sites
involves many nonbonded repulsive steric interactions with different parts of the
ligand and the transformation of the β-agostic resting state into a symmetrical
β
0 -agostic state, via a symmetric transition state in which the alkyl chain rests
approximately in the plane formed by the metal and the ligands’ centroids, half
way between the two coordination positions. During the transformation, on either
side of the reaction energy profile, other intermediates with α- and α
0 -agostic bonds
are also formed [170]. Overall, the site epimerization mechanism that does not take
into account the role of the counter-anion in the process has a very large activation
energy compared to the propagation activation energy. Such a mechanism does not
agree with the experimental results, which show that the pentad distributions are in
reality influenced by a rather frequent site epimerization.
5.4 Counter-Ion-Assisted Site Epimerization
In the regular and real olefin polymerization processes with metallocene catalysts,
the anion is an ever-present participant and an integral part of the functioning
catalyst system. It is systematically displaced and moves (in and) out of the
coordination sphere in a concerted action, while the monomer units approach for
coordination to be paired again with the cation after the insertion process is
complete. Assuming that the monomer approach proceeds along the most favorable
path (i.e., via a cis approach with respect to the anion), immediately after the
monomer insertion the anion is positioned at the outer coordination sphere, on the
side of the polymer chain (Fig. 20a).
The resulting metalloocenium–polymeryl cation has an empty coordination site
available for the re-formation of a close-contact ion pair with the anion. To avoid
unnecessary steric interaction with the alky group (polymer chain), the anion can
first undergo a spatial reorganization in the outer coordination sphere and then
approach the free coordination site unhindered (Fig. 20b) [168, 169]. Alternatively,
it can approach the cation directly, forcing in the process the polymer chain to move
to the empty coordination site. The first process is what normally occurs during a
syndiospecific propagation step (Fig. 20c), while the second process leads to the site
epimerization since the immediate ion pairing and the simultaneous displacement
of the polymer chain, results in a complex in which the polymer chain occupies the
same coordination site as in the previous catalytic cycle (Fig. 20d).
Intuitively, it can be reasonably assumed that the anion reorganization in the outer
coordination sphere is spontaneous and occurs freely; its rearrangement is governed
purely by diffusion. Indeed, our gas-phase calculations [146] show that for the cation/
anion model system, Me 2 C(CpFlu)ZrMe-Me-B(C 6 F 5 ) 3 , the energy barrier is about
0.5 kcal/mol, and well within the error associated with the computational method. On
the other hand, the formation of the close-contact ion pair is also an exothermic and
Syndiotactic Polypropylene: Discovery, Development, and Industrialization. . .
85