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Fig. 2.1
Bipyridine-coordinated iron alkyl
complex (
A
) for the polymerization of acetoni-
trile (Yamamoto et al.). (
B
): P
O complex for
ethene oligomerization (Keim et al.).
relevant source materials for polymerization catalysis. The commercially significant
Shell Higher Olefin Process (SHOP) takes advantage of the competing
-H elimina-
tion and the facile-associative olefin exchange reactions which occur during the
polymerization process and often lead to dimers or oligomeric products [9]. For that
reason, SHOP is the first choice in the design of linear
-olefins (C
4
-C
20
) which are
commonly used for detergents, plasticizers, lubricants and a variety of fine
chemicals [10]. This capability spurred scientists to expend fundamental research
in the design of a variety of new late transition metal catalysts. The development,
therein, was started in the late 1960s to early 1970s by Keim et al. (Fig. 2.1, B)
[11] who finally found the first defined complex providing the formation of
-olefins and polyethene without the use of a co-catalyst by conversion of
bis(cyclooctadienyl)nickel, phosphorane and triphenylphosphine [10]. Treatment of
a toluene solution of the Ni-P
O complexes with ethene afforded activities up to
6000 mol ethene per mol of the catalyst and formed linear products (99%) with
an
-olefins content of 98%. Replacement of toluene with
n
-hexane formed high mo-
lecular weight linear polyethene. The significance of these results is manifested in
the high scientific interest which was increasingly centered on detailed mechanistic
investigations [12, 13]. Keim et al. identified that a nickel hydride compound oper-
ates as the active catalyst species in nickel-catalyzed oligo- and polymerizations reac-
tions (Fig. 2.2). An ethene molecule is inserted in the Ni-H bond, resulting in the
formation of a nickel ethyl unit. Further insertions of monomer in the nickel-alkyl
bond and subsequent
-olefins with high selectivity.
The chemical modification of SHOP-catalysts was the center of interest of Kla-
bunde and Ittel who intensively focused on the synthesis of high molecular
weight products and on the copolymerization of ethene with
-H elimination produce linear
-olefins. Therein,
the combination of ethene with polar monomers and carbon monoxide depicts
the major concern [14]. They realized the importance of the nature of donor li-
gands coordinating to the nickel center as the primary factor limiting the polymer
molecular weight (Fig. 2.3, C).
A first approach aimed at removing the defective, strong donor ligands (for ex-
ample PPh
3
) from the coordination sphere was the addition of catalyst promoters
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