the nonbonded steric interactions operating on different parts of the catalytic
species and on the polymerization active participants, the aromatic ligand, the
polymer chain, and the coordinating monomer in the following order. The organic
ligand consists of a large substituted cyclopentadienyl group, the fluorenyl group,
and a smaller, sterically less challenging, unsubstituted cyclopentadienyl group tied
together with an isopropylidene bridge. The steric interaction between the flat and
spatially extended fluorenyl ligand forces the growing polymer chain, during the
polymerization, to adopt the conformation that permits its orientation towards the
free space left (or right) of the unsubstituted cyclopentadienyl moiety of the ligand.
The incoming monomer in turn (to avoid an excessive repulsive steric exposure)
orients itself in a manner such that its methyl group is “trans” positioned with
respect to the growing polymer chain (alkyl group).
The system thus reaches its minimum energy during the formation of the
metalacyclobutane transition state. In this coordination mode, the coordinated
monomer points with its methyl group head-down into the empty space in the
central region of the fluorenyl ligand with little or no direct interaction with the
ligand. The importance of the “head-down” orientation of the monomer with
respect to the “upward” orientation of the chain during the π-complex formation
and transition state was recognized and proposed early on after extensive molecular
mechanics and force field calculations performed by Corradini and coworkers
[64, 69, 71, 73–75]. The model underwent later additional refinement and took its
current form after experiments conducted by several research groups supported the
idea of the formation of an α-hydride agostic Zr bond assisting and stabilizing the
chain conformation orientation in the metalacyclobutane transition state geometry,
before the actual propylene insertion [98–106]. It implies that the insertion transition state relies on the formation of an α-agostic bond between either one of the two
available hydrides on the last carbon (α-C) of the growing polymer chain and the
transition metal center. This agnostic interaction provokes the rotation of the highly
directional sp
3 orbital of the α carbon towards the π orbital of the 1,2-coordinated
propylene monomer, allowing for larger orbital overlapping and contributing to the
stabilization of the insertion transition state and thus affecting profoundly the
stereochemistry of insertion; selection of one or the other α-hydride for agostic
interaction could bring the growing polymer chain either into the congested quadrant of the ancillary ligand or its free quadrant left or right of the cyclopentadienyl
group [99].
After insertion of the first monomer, the alkyl group (polymer chain), now
enlarged by one monomer, is moved to the coordination side that has become vacant
after the monomer insertion and is replaced itself by a new incoming propylene
monomer presenting a different face. Thus, another cycle begins with another
monomer face at the other enantiotopic coordination site (see Fig. 7, right). The
systematic and repetitious cycles would then lead to the formation of polypropylene
chains with alternatively inverted stereogenic centers. The systematic transformation
of the two S and R enantiomorphic antipodes into one another, after each olefin
insertion, ensures that the relative positions of the σ-and π-bonded ligands in the
equatorial plane of the transition metal are exchanged and that monomers with
56
A. Razavi
species and on the polymerization active participants, the aromatic ligand, the
polymer chain, and the coordinating monomer in the following order. The organic
ligand consists of a large substituted cyclopentadienyl group, the fluorenyl group,
and a smaller, sterically less challenging, unsubstituted cyclopentadienyl group tied
together with an isopropylidene bridge. The steric interaction between the flat and
spatially extended fluorenyl ligand forces the growing polymer chain, during the
polymerization, to adopt the conformation that permits its orientation towards the
free space left (or right) of the unsubstituted cyclopentadienyl moiety of the ligand.
The incoming monomer in turn (to avoid an excessive repulsive steric exposure)
orients itself in a manner such that its methyl group is “trans” positioned with
respect to the growing polymer chain (alkyl group).
The system thus reaches its minimum energy during the formation of the
metalacyclobutane transition state. In this coordination mode, the coordinated
monomer points with its methyl group head-down into the empty space in the
central region of the fluorenyl ligand with little or no direct interaction with the
ligand. The importance of the “head-down” orientation of the monomer with
respect to the “upward” orientation of the chain during the π-complex formation
and transition state was recognized and proposed early on after extensive molecular
mechanics and force field calculations performed by Corradini and coworkers
[64, 69, 71, 73–75]. The model underwent later additional refinement and took its
current form after experiments conducted by several research groups supported the
idea of the formation of an α-hydride agostic Zr bond assisting and stabilizing the
chain conformation orientation in the metalacyclobutane transition state geometry,
before the actual propylene insertion [98–106]. It implies that the insertion transition state relies on the formation of an α-agostic bond between either one of the two
available hydrides on the last carbon (α-C) of the growing polymer chain and the
transition metal center. This agnostic interaction provokes the rotation of the highly
directional sp
3 orbital of the α carbon towards the π orbital of the 1,2-coordinated
propylene monomer, allowing for larger orbital overlapping and contributing to the
stabilization of the insertion transition state and thus affecting profoundly the
stereochemistry of insertion; selection of one or the other α-hydride for agostic
interaction could bring the growing polymer chain either into the congested quadrant of the ancillary ligand or its free quadrant left or right of the cyclopentadienyl
group [99].
After insertion of the first monomer, the alkyl group (polymer chain), now
enlarged by one monomer, is moved to the coordination side that has become vacant
after the monomer insertion and is replaced itself by a new incoming propylene
monomer presenting a different face. Thus, another cycle begins with another
monomer face at the other enantiotopic coordination site (see Fig. 7, right). The
systematic and repetitious cycles would then lead to the formation of polypropylene
chains with alternatively inverted stereogenic centers. The systematic transformation
of the two S and R enantiomorphic antipodes into one another, after each olefin
insertion, ensures that the relative positions of the σ-and π-bonded ligands in the
equatorial plane of the transition metal are exchanged and that monomers with
56
A. Razavi