Anthony KERMAGORET - THESES ET MEMOIRES DE L'UDS

1. Introduction
Chapitre VI
The commercial interest for high-density polyethylene (HDPE), linear low-density
polyethylene (LLDPE) and linear α-olefins triggers research in ethylene polymerization and
oligomerization. Since the works of Ziegler and Natta, alkylation of the metal centre with
aluminum alkyls became the key for the formation of active catalysts in polymerization and
oligomerization. 1-3 This activation process applies well to nickel precatalysts 4-6 and Brookhart
et al. have described α-diimines nickel complexes 1 which are highly active in ethylene
polymerization in the presence of MAO (methylaluminoxane) as cocatalyst. 7-10
R' R'
R R
N N
R
Ni
R
Br Br
1
R = i-Pr
R' = H, Me, or 1,8-naphth-diyl
The nature of the chelating ligand is crucial for the catalyst performances and the use of α-
diimine ligands with less bulky substituents turned their nickel complexes into ethylene
oligomerization rather than polymerization catalysts. In this case, they showed a high
selectivity for linear α-olefins with a Schulz-Flory mass distribution of the oligomers. 11,12
These results have inspired the synthesis of many nickel precatalysts coordinated by N,N-, 10-12
N,O-, 13-16 or P,N- 17,18 type ligands and activated by various alkylaluminum derivatives such as
MAO, MMAO (modified MAO), AlEtCl2, AlEt3, AlMe3, etc. With the N,O chelating ligands
salicylaldimines, 13,19-22 anilinotropones 23-25 or anilinoperinaphthenones, 26 it was observed that
very bulky groups retard chain transfer and lead to high molecular weight polyethylenes. 27-30
Nickel catalysts for ethylene oligomerization are applied in the Shell Higher Olefin
Process (SHOP) without any cocatalyst, 31 and lead to linear α-olefins with remarkable
selectivity. The known SHOP-type catalyst 2 is synthesized by oxidative-addition of the β-
keto phosphorus ylid 3 on a zerovalent nickel compound, such as [Ni(COD)2] (COD = 1,5-
cyclooctadiene), in the presence of a 2 electron donor ligand, in general a phosphine such as
PPh3, to stabilize the resulting neutral Ni(II) complex [eqn (1)] 32,33 (throughout this paper, the
drawings will indicate a localized C=C bond for the enolate moiety but electronic
delocalization involves the C-O bond). 17,18
3