Mechanism for Olefin Polymerization

Mechanism for 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] α-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 α-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.

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Chain Termination: β-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

β-Hydrogen Elimination - β-hydrogen transferred to the metal. ibid.
H [M] C H2 H [M] C H2 H [M] P [M] H

H P

H P

β-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