At higher polymerization temperatures, however, the energy for overcoming the
interconversion barrier is apparently provided even for cationic systems and the
polydispersity of the polymers begins to decrease with the gradually increasing
polymerization temperature, as the difference between the two populations of
active species vanishes due to very fast δ–λ interconversion. The two polymer
chain populations coalesce around 80
C into one single population. At 90
C, the
catalyst produces an amorphous polypropylene polymer that contains a large
portion of atactic pentads but a narrow molecular weight distribution.
At higher temperatures, at the same time as the polydispersity decreases because
of the rapid reversible δ–λ interconversion, the rigid structural integrity is also more
and more compromised. The concentration of enantiomorphic site control related
stereo-errors (rmmr) increases, but the rate of site epimerization and concentration
of rrmr pentads increase much more dramatically, parallel to the increase in the
conformational interconversion rate and the resulting molecular flexibility.
The explanation for the formation of the longer polymer chains with the 5/MAO
catalyst system requires a different approach and necessitates a comparative
evaluation of the interatomic parameters for structures 5 and 1, including some
important bond angles and bond distances. It is apparent from structural data [113]
that the substitution of one carbon atom (in the bridge of 1) with two carbon atoms
(in the bridge of 5) provokes the increase in centroid–Zr–centroid angle (a smaller
bite angle!) by several degrees (127.28
/127.08
versus 118.6
for 1). The
cyclopentadienyl and fluorenyl ring systems are forced to move closer and adopt
a more parallel position. This new ring arrangement causes the increase in the
average cyclopentadienyl Zr–centroid bond distance (Zr–C cp ¼ 2.268 A ˚ /2.285 A ˚
for 5, 5
0 versus 2.170 A ˚ for 1) and the decrease in the average fluorenyl Zr–centroid
bond distance (Zr–C flu ¼ 2.196 A ˚ /2.196 A ˚ for 5, 5
0 versus 2.240 A ˚ for 1). The net
effect is that under these conditions slightly more ligand coverage is provided for
the transition metal’s coordination sites. The increase in the centroid–Zr–centroid
angle at the same time is accompanied by an increase in the frontier orbital energies
and a change in the degree of their hybridization [107, 108, 122]. Furthermore, the
increased steric congestion around the coordination sites increases the nonbonded
steric contacts between the ligand and the growing chain substituents, preventing it
from getting close enough to the Zr center for an adequate orbital overlapping
between the metal fragment orbitals and hydrides for a meaningful α- or β-agostic
exchange. The lower probability of the β-agostic approach favors the formation of a
longer chain and, on the other hand, the lower probability of α-agostic interaction is
favorable for enhanced conformational orientation of the chain and faster insertion/
propagation steps, resulting in a lower degree of enantioselectivity and lower
activity due to lower insertion rate (and also smaller bite angle!). Thus both from
a steric and a valence orbital energetic point of view, the approach and subsequent
orbital overlapping between chain-end hydride orbitals and transition metal
fragment orbitals for a proper agostic interaction (of any kind) will be less
favored for the new catalyst formed with 5 (similar results have been reported by
Kim et al. [114]).
64
A. Razavi
interconversion barrier is apparently provided even for cationic systems and the
polydispersity of the polymers begins to decrease with the gradually increasing
polymerization temperature, as the difference between the two populations of
active species vanishes due to very fast δ–λ interconversion. The two polymer
chain populations coalesce around 80
C into one single population. At 90
C, the
catalyst produces an amorphous polypropylene polymer that contains a large
portion of atactic pentads but a narrow molecular weight distribution.
At higher temperatures, at the same time as the polydispersity decreases because
of the rapid reversible δ–λ interconversion, the rigid structural integrity is also more
and more compromised. The concentration of enantiomorphic site control related
stereo-errors (rmmr) increases, but the rate of site epimerization and concentration
of rrmr pentads increase much more dramatically, parallel to the increase in the
conformational interconversion rate and the resulting molecular flexibility.
The explanation for the formation of the longer polymer chains with the 5/MAO
catalyst system requires a different approach and necessitates a comparative
evaluation of the interatomic parameters for structures 5 and 1, including some
important bond angles and bond distances. It is apparent from structural data [113]
that the substitution of one carbon atom (in the bridge of 1) with two carbon atoms
(in the bridge of 5) provokes the increase in centroid–Zr–centroid angle (a smaller
bite angle!) by several degrees (127.28
/127.08
versus 118.6
for 1). The
cyclopentadienyl and fluorenyl ring systems are forced to move closer and adopt
a more parallel position. This new ring arrangement causes the increase in the
average cyclopentadienyl Zr–centroid bond distance (Zr–C cp ¼ 2.268 A ˚ /2.285 A ˚
for 5, 5
0 versus 2.170 A ˚ for 1) and the decrease in the average fluorenyl Zr–centroid
bond distance (Zr–C flu ¼ 2.196 A ˚ /2.196 A ˚ for 5, 5
0 versus 2.240 A ˚ for 1). The net
effect is that under these conditions slightly more ligand coverage is provided for
the transition metal’s coordination sites. The increase in the centroid–Zr–centroid
angle at the same time is accompanied by an increase in the frontier orbital energies
and a change in the degree of their hybridization [107, 108, 122]. Furthermore, the
increased steric congestion around the coordination sites increases the nonbonded
steric contacts between the ligand and the growing chain substituents, preventing it
from getting close enough to the Zr center for an adequate orbital overlapping
between the metal fragment orbitals and hydrides for a meaningful α- or β-agostic
exchange. The lower probability of the β-agostic approach favors the formation of a
longer chain and, on the other hand, the lower probability of α-agostic interaction is
favorable for enhanced conformational orientation of the chain and faster insertion/
propagation steps, resulting in a lower degree of enantioselectivity and lower
activity due to lower insertion rate (and also smaller bite angle!). Thus both from
a steric and a valence orbital energetic point of view, the approach and subsequent
orbital overlapping between chain-end hydride orbitals and transition metal
fragment orbitals for a proper agostic interaction (of any kind) will be less
favored for the new catalyst formed with 5 (similar results have been reported by
Kim et al. [114]).
64
A. Razavi