association) the structure will again adopt the tetrahedral geometry (Fig. 19,
bottom). This counter-ion-assisted change in geometry operates on all metallocenic
catalyst systems, but is probably more crucial for the syndiotactic-specific catalyst
systems and syndiospecific polymerization where dynamic processes such as chain
migration and, particularly, site epimerization are vital or fatal for its existence.
In this aspect, the syndiospecific polymerization process resembles a poly-insertion
reaction following at each step the mechanism of a SN2-type reaction. For the
isospecific system, the reagent propylene “always” attacks the tri-coordinated
intermediate from the same side with the same face because of its homotopicity.
For the syndiospecific case, the stereochemistry is determined by the reagent,
propylene, attacking the tri-coordinated intermediate form on the opposite side
with different faces because of the enantiotopic nature of the active site.
5.1 Catalysts Stereorigidity and Site Epimerization
In the preceding sections, the term “site epimerization” was frequently used as the
phenomenon responsible for the formation of the meso dyad (m) stereodefects and
the lower stereoregularity and crystallinity of the s-PP polymers. To increase the
stereoregularity and melting point of these polymers, obviously, the concentration
of the rrmr pentads must be suppressed in the polymer chain’s backbone by
devising means that enable circumventing their formation during syndiospecific
polymerization of propylene.
The occurrence of site epimerization is generally explained [165–172] in the
following manner: The R and S configured active sites that are formed during the
initial stages of the activation are equi-energetic and can interconvert (epimerize)
during the polymerization, particularly in the absence of a coordinating monomer or
a stabilizing solvent molecule, particularly at higher polymerization temperatures.
The interconversion occurs when occasionally the polymer chain swings back
(“back-skip”) to its initial coordination position after an insertion before the coordination of the next monomer (Fig. 6). Under these conditions, two (and more than two
in the case of Hf-based catalyst 2) consecutive insertions will take place at the same
enantiomorphic coordination position, resulting in enchainment of two monomer
units with the same prochiral face and insertion of two units with same stereogenic
center. This leads to the formation of meso dyad, m-type, stereodefects in the s-PP
polymer chains (and/or short isotactic blocks for the Hf analogue).
Little was known initially about the driving forces underlying the phenomenon of
site epimerization. Empirically, it has been observed that (in addition to low monomer concentration and higher polymerization temperature dependency) the catalysts
formed with metallocene structures with inherently lower stereorigidity or higher
flexibility (such as structures 5, 10, and 11) tend to undergo a more frequent site
epimerization than those known to be less flexible, as can be seen from data presented
in Table 13. Additionally, it was shown that the frontal substituents in more
stereorigid catalysts systems impact the site epimerization rate (see 9/MAO rrmr in
Syndiotactic Polypropylene: Discovery, Development, and Industrialization. . .
83
bottom). This counter-ion-assisted change in geometry operates on all metallocenic
catalyst systems, but is probably more crucial for the syndiotactic-specific catalyst
systems and syndiospecific polymerization where dynamic processes such as chain
migration and, particularly, site epimerization are vital or fatal for its existence.
In this aspect, the syndiospecific polymerization process resembles a poly-insertion
reaction following at each step the mechanism of a SN2-type reaction. For the
isospecific system, the reagent propylene “always” attacks the tri-coordinated
intermediate from the same side with the same face because of its homotopicity.
For the syndiospecific case, the stereochemistry is determined by the reagent,
propylene, attacking the tri-coordinated intermediate form on the opposite side
with different faces because of the enantiotopic nature of the active site.
5.1 Catalysts Stereorigidity and Site Epimerization
In the preceding sections, the term “site epimerization” was frequently used as the
phenomenon responsible for the formation of the meso dyad (m) stereodefects and
the lower stereoregularity and crystallinity of the s-PP polymers. To increase the
stereoregularity and melting point of these polymers, obviously, the concentration
of the rrmr pentads must be suppressed in the polymer chain’s backbone by
devising means that enable circumventing their formation during syndiospecific
polymerization of propylene.
The occurrence of site epimerization is generally explained [165–172] in the
following manner: The R and S configured active sites that are formed during the
initial stages of the activation are equi-energetic and can interconvert (epimerize)
during the polymerization, particularly in the absence of a coordinating monomer or
a stabilizing solvent molecule, particularly at higher polymerization temperatures.
The interconversion occurs when occasionally the polymer chain swings back
(“back-skip”) to its initial coordination position after an insertion before the coordination of the next monomer (Fig. 6). Under these conditions, two (and more than two
in the case of Hf-based catalyst 2) consecutive insertions will take place at the same
enantiomorphic coordination position, resulting in enchainment of two monomer
units with the same prochiral face and insertion of two units with same stereogenic
center. This leads to the formation of meso dyad, m-type, stereodefects in the s-PP
polymer chains (and/or short isotactic blocks for the Hf analogue).
Little was known initially about the driving forces underlying the phenomenon of
site epimerization. Empirically, it has been observed that (in addition to low monomer concentration and higher polymerization temperature dependency) the catalysts
formed with metallocene structures with inherently lower stereorigidity or higher
flexibility (such as structures 5, 10, and 11) tend to undergo a more frequent site
epimerization than those known to be less flexible, as can be seen from data presented
in Table 13. Additionally, it was shown that the frontal substituents in more
stereorigid catalysts systems impact the site epimerization rate (see 9/MAO rrmr in
Syndiotactic Polypropylene: Discovery, Development, and Industrialization. . .
83