for Mechanism Olefin Polymerization Chain Propagation:
Cossee-Arlman Mechanism = good basic mechanism.
Cossee et al., J. Catal., 1964, 3, 80 & 99.
1,2-insertion
alkene coordination
R [M]
R [M] [M]
R CH2 C H2 [M] R
Green-Rooney Mechanism involving metathesis-like step = totally wrong !
CHP CH2P [M] -elimination [M] H CHP [M] H PHC H [M] R reductive elimination PH2C [M] R
alkene coordination
R
metathesislike process
This Mechanism DOES NOT Occur
Proposed by Green, Rooney et al., J. Chem. Soc., Chem. Commun., 1978, 604. Refuted convincingly by Grubbs et al., J. Am. Chem. Soc., 1985, 3377.
Brookhart-Green Mechanism = an improvement on the Cossee-Arlman mechanism it includes an -agostic interaction which helps to facilitate 1,2-insertion.
Brookhart et al., J. Organomet. Chem., 1983, 250, 395.
H P [M] H H P [M] H H P
H
CH2P [M]
H P
H [M]
H P
H [M]
[M]
Supporting calculations: Ziegler et al., Organometallics, 2004, 104. Supporting experiments: Brintzinger et al., Angew. Chem., Int. Ed., 1990, 1412 (Zr), Piers and Bercaw, J. Am. Chem. Soc., 1990, 9406.
1
Chain Termination:
-Hydrogen Transfer - H- transferred from the growing polymer chain to an incoming olefin. This is the dominant chain termination mechanism under the usual experimental conditions.
Ziegler et al., J. Am. Chem. Soc., 1999, 154.
H [M] C H2
H2 C H P C H2 [M] H H P C H2 [M] H H P P [M] H [M] H
H H
-Hydrogen Elimination - -hydrogen transferred to the metal. ibid.
H [M] C H2 H [M] C H2 H [M] P [M] H
H P
H P
-Methyl Elimination - only occurs in special cases.
Bercaw et al., Organometallics, 1994, 1147 (Sc), Jordan et al., Organometallics, 1994, 1424 (Hf), Resconi et al., Organometallics, 1996, 5046 (Zr), Resconi et al., Organometallics, 1992, 1025 (Zr, Hf).
H [M] C H2
P Me
Me [M] C H2
H P
Me [M] C H2
H P
Me [M] P [M] Me
Chain transfer to aluminium - MAO usually contains leftover AlR3 - chain transfer to Al is more common at lower alkene monomer concentration.
Resconi et al., Macromolecules, 1990, 4489, Naga et al., Polymer, 1998, 5059.
H [M] C H2 R Me P + R Al R [M] CH2 CHMeP R Al R R R [M] R + Al C H2 CHMeP
R
2
Atactic Polymerization of Propene
C2v
Zr
CH2R CH3 H 3C
R Zr H
H
or
H 3C
H Zr R
H
Background: 1) Assigning the symmetry of olefin polymerization catalysts Assign the symmetry just by considering the metal and the cyclopentadienyl ligand (C2v, C2, Cs, C1 are the most common see below). Don't worry about the other groups since we are only concerned with the symmetry that the complex presents to the alkyl and alkene group, and these groups swap sides after each insertion.
M
M
M
M
M
C2v
Cs
Cs
C2
C1
2) 1,2-Insertion versus 2,1-Insertion With early transition metal complexes, insertion generally occurs in a 1,2-fashion. Catalysts that produce regular poly(-alkenes) are designed in order to maximize 1,2-insertion relative to 2,1-insertion (generally reaches approximately 100%):
R [M]
1 2
1,2-Insertion Me
Me [M]
2 1
R results in a less bulky group on the metal
R [M]
2 1
2,1-Insertion
[M]
1 2
R
Me Me
3
Atactic Polymerization of Propene cont.
C2v
Zr
CH2R CH3 H 3C
R Zr H
H
or
H 3C
H Zr R
H
agostic interaction locks the growing polymer chain in position. however, the polymer chain (R) doesn't care if it is up or down, and more importantly, neither does the methyl group of propene: it binds via either the re- or si-face.
si-face Me 2 1 H re-face 3 1 Me 2 H 3
The only real preference is for the growing chain and the methyl group to be as far away from each other as possible (they prefer to occupy opposite quadrants). This indifference of the alkene to bind via either the re- or the si-face (irrespective of which side of the complex the alkene is bonded) leads to atactic polypropylene.
Zr
Me Me
B(C6F5)3
Zr
Me
Zr
Me CH3
Zr C H2 CH Me CH3
atactic
Zr
CH2
CH Me
CH2
CH Me
CH3
Me Zr C H2 CH Me CH3
4
Isotactic Polymerization of Propene
C2
Zr
CH2R CH3 H 3C metal coordinated to the re-face
R Zr H
H
Agostic interaction locks the growing polymer chain in position (growing chain oriented in order to minimize unfavourable steric interactions with the ligand). In the drawing above, the polymer chain wants to occupy the top right quadrant, and the methyl group of propene wants to occupy the bottom left quadrant. In this arrangement, the growing polymer chain and the methyl group of propene are also as far apart as possible. Very important only decently isotactic once R H. Irrespective of which side of the metallocene propene is bound, it binds via the re-face. This results in isotactic polypropylene.
m m m
R Zr H H3C
H Me H H2C
H Zr H R' Zr
CH3 n isotactic
metal coordinated to the re-face
CH2R
epimerization
metal coordinated to the re-face
Rate ???
has no effect on the stereochemistry RH2C Zr C H2 Me H
m = meso inicates a meso-relationship between adjacent stereogenic centres (meso = both R groups on the same side of the polymer chain)
For the other C2-symmetric enantiomer, the alkene will bind only via the si-face, also resulting in isotactic polypropylene. Therefore, the two enantiomers of the C2-symmetric catalyst do not have to be resolved.
other C2 symmetric enantiomer
metal coordinated to the si-face
H
R Zr H CH3
5
Syndiotactic Polymerization of Propene
H H
Cs
CH3 Zr R
H3C
H Zr R
H H H3C Zr R
H
In the drawing above, the polymer chain occupies the bottom right quadrant. The methyl group of propene occupies top left quadrant two competing influences: (1) desire to avoid the growing polymer chain, (2) desire to avoid the more bulky fluorenyl group. The first influenece is more powerful.
r H3C H Zr R metal coordinated to the si-face H Me H
Zr
r
r
H
H Zr R'
CH3
syndiotactic
H2C
n
CH2R
slow epimerization
metal coordinated to the re-face
leads to errors (meso)
r = racemic inicates a rac-relationship between adjacent stereogenic centres (rac = R groups on the opposite sides of the polymer chain)
Zr
CH2
Me RH2C H
In the preparation of atactic or isotactic polypropylene, chain epimerization (the chain alkyl group moving from one side of the complex to the other without insertion occurring) does not result in errors. For the atactic case, the polymer is random anyway. For isotactic polypropylene formation, the alkene binds via the same face irrespective of which side of the complex it is bonded to. By contrast, in the formation of syndiotactic PP, chain epimerization results in stereoerrors. Instead of strict alternation between binding to the re- and si-face, the alkene bonds to the same face twice, resulting in a meso placement, rather than rac.
6
Hemiisotactic Polymerization of Propene
H H CH3 Zr R H3C Zr
H
H or
H Zr H3C R
H
isospecific
R alkene binds exclusively through the si-face
Me
C1
alkene binds through the si-face
H Zr R H Me CH3 H R Zr CH3 or H
R Zr H CH3 aspecific
H H R Zr
CH3 or H
R Zr H
CH3
alkene binds through the re-face
For the C1-symmetric catalyst drawn above, both sides of the complex are different: When bonded to the right hand side of the complex, the polymer chain occupies the bottom right quadrant. In order to minimize unfavourable steric interactions with the growing polymer chain, the alkene bonds to the left hand side of the complex by its si-face isospecific monomer placement. When bonded to the left hand side of the complex, the polymer chain can't decide whe...