in both cases according to the same basic mechanism and they both produce similar
s-PP chains, it could be reasonably assumed that the even the isotactic stereodefects
(mmmm) in the backbone of polymer chains formed with (1 and 2)/MAO catalyst
systems are also generated following the same basic mechanism, operating only
with slightly less efficiency in the case of 2/MAO. As for the substantial differences
related to the activities of the two catalysts and molecular weights of their
polymers, different kinetic behavior for 1/MAO and 2/MAO catalyst systems is
suspected.
2.4 Mechanism of Syndiospecific Polymerization
In order to describe correctly the mechanism of the syndiospecific polymerization
with (1 and 2)/MAO catalyst systems, as mentioned before, access to a realistic
model representing the active site is of utmost importance. The information extracted
from the molecular structures of complexes 1 and 4 provides all necessary elements
required to construct such a “realistic” active site model. From the discussion in
Sects. 2.1 and 2.2, it can be reasonably assumed that the structure of the actual active
centers, at least during the π-complex formation, are very similar and close to the
structures of complex 4 depicted in Fig. 3. All is needed is to imagine the PMe 3
molecule as removed and replaced by a propylene molecule. In this form, it can be
used to describe the different stereochemical events, step by step, during different
stages of polymerization, activation, monomer selection/coordination, monomer
insertion, chain propagation and chain termination responsible for the formation of
s-PP with the microstructure . . .rrrrrrmmrrrrrmrrr. . . with complex 1/MAO:
1. The stereorigid bridged metallocene dichloride 1, as depicted in Fig. 2, is a
prochiral molecule and possesses bilateral symmetry. The activation of
metallocene dichloride 1 leads to the formation of enantiomeric metallocenium–
monoalkyl cationic species that are both electron-deficient and coordinatively
unsaturated and structurally very similar to the enantiomeric molecule depicted
in Fig. 3. These enantiomeric metallocenium–monoalkyl species have the necessary vacant coordination position and energetically low lying, accessible empty
fragment orbitals to be used for coordination and activation of incoming propylene
molecules via interaction with their available π orbitals.
2. The cationic metallocenium–monoalkyl species [21] are composed of equal
numbers of R and S mirror-image related enantiomers and have monomer re/si
π-face selective properties (Fig. 5).
3. The re or si face-selectivity is induced by the unique steric arrangement of the
chelating ancillary ligand engulfing the resident chiral transition metal center via
a delicately balanced, cooperative and nonbonded steric interactions between
different parts of the “active” catalytic species, ligand, polymer chain, and
coordinating monomer. The nonbonded steric interactions govern the whole
scenery of the syndiospecific polymerization process during all its individual steps.
Syndiotactic Polypropylene: Discovery, Development, and Industrialization. . .
53
s-PP chains, it could be reasonably assumed that the even the isotactic stereodefects
(mmmm) in the backbone of polymer chains formed with (1 and 2)/MAO catalyst
systems are also generated following the same basic mechanism, operating only
with slightly less efficiency in the case of 2/MAO. As for the substantial differences
related to the activities of the two catalysts and molecular weights of their
polymers, different kinetic behavior for 1/MAO and 2/MAO catalyst systems is
suspected.
2.4 Mechanism of Syndiospecific Polymerization
In order to describe correctly the mechanism of the syndiospecific polymerization
with (1 and 2)/MAO catalyst systems, as mentioned before, access to a realistic
model representing the active site is of utmost importance. The information extracted
from the molecular structures of complexes 1 and 4 provides all necessary elements
required to construct such a “realistic” active site model. From the discussion in
Sects. 2.1 and 2.2, it can be reasonably assumed that the structure of the actual active
centers, at least during the π-complex formation, are very similar and close to the
structures of complex 4 depicted in Fig. 3. All is needed is to imagine the PMe 3
molecule as removed and replaced by a propylene molecule. In this form, it can be
used to describe the different stereochemical events, step by step, during different
stages of polymerization, activation, monomer selection/coordination, monomer
insertion, chain propagation and chain termination responsible for the formation of
s-PP with the microstructure . . .rrrrrrmmrrrrrmrrr. . . with complex 1/MAO:
1. The stereorigid bridged metallocene dichloride 1, as depicted in Fig. 2, is a
prochiral molecule and possesses bilateral symmetry. The activation of
metallocene dichloride 1 leads to the formation of enantiomeric metallocenium–
monoalkyl cationic species that are both electron-deficient and coordinatively
unsaturated and structurally very similar to the enantiomeric molecule depicted
in Fig. 3. These enantiomeric metallocenium–monoalkyl species have the necessary vacant coordination position and energetically low lying, accessible empty
fragment orbitals to be used for coordination and activation of incoming propylene
molecules via interaction with their available π orbitals.
2. The cationic metallocenium–monoalkyl species [21] are composed of equal
numbers of R and S mirror-image related enantiomers and have monomer re/si
π-face selective properties (Fig. 5).
3. The re or si face-selectivity is induced by the unique steric arrangement of the
chelating ancillary ligand engulfing the resident chiral transition metal center via
a delicately balanced, cooperative and nonbonded steric interactions between
different parts of the “active” catalytic species, ligand, polymer chain, and
coordinating monomer. The nonbonded steric interactions govern the whole
scenery of the syndiospecific polymerization process during all its individual steps.
Syndiotactic Polypropylene: Discovery, Development, and Industrialization. . .
53