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could be isolated. Oligomeric products were not detected by GC. DSC measure-
ments showed a melting point of 137.0
H
f
=213.52 J g
-1
) for the material
(heating rate 10 K min
-1
). In the IR spectrum bands at 2919 and 2849 cm
-1
C(
(C-
H-stretching, CH
2
), 1469 cm
-1
(C-H-deformation, CH
2
) und 718 cm
-1
(CH
2
-rock-
ing) were observed.
1
H-NMR measurements in
para
-xylene-d
10
at 90
C provided a
main resonance at 1.52 ppm (CH
2
, main chain) and a weak signal at 1.09 ppm
(CH
3
, side chain). The
13
C-NMR spectrum revealed only one resonance at
29.97 ppm (CH
2
, main chain). GPC measurements in 1,2,4-trichlorobenzene at
145
10
5
g mol
-1
and a polydispersity of 3.6.
On the basis of the analytical results, the white solid material was identified as a
predominantly linear, semi crystalline polyethene with a relatively high molecular
weight belonging to the class of High Density Polyethenes (HDPEs) [59].
The polymerization results clearly demonstrate that also in the case of
bis(phosphine)s the steric demand of the ligand sphere is a prerequisite for the
formation of higher olefins from ethene (
15b
,
c
). At the same time, there occurs a
moderate, but observable selectivity for linear products. It was revealed that only
the highly sterically crowded 2,4,6-triisopropylphenyl substitution efficiently acts
in blocking the axial positions of the tested compounds. Here, the associative dis-
placement of
C gave a molecular weight
M
W
of 1
-H elimination products by free monomer is retarded in a way that
allows the formation of polyethenes with high molecular weights. The polymeriza-
tion behavior of
15b-d
reflects the observations in NMR spectroscopy concerning
the conformational rigidity of the ligand spheres. Only the highly rigid ligand ar-
chitecture of
14d
efficiently fixes the substituents (isopropyl units) above and be-
low the coordination plane and enables the complex to work as a polymerization
catalyst. The predominantly linear character of the polymers produced using
15d
disagrees with reports on highly branched amorphous polyethenes formed by di-
imine-Pd systems. An explanation for this difference may again be derived from
quantum mechanical considerations performed on Ni(II), Pd(II)-diimine com-
plexes [33]. There it is suggested that an isomerization of the polymer chain at-
tached to the metal center - which can result in the formation of a side group -
mechanistically requires a rotation of the polymer chain. The axis for this rotation
would be the olefin-to-metal bond of the olefinic end group (vinyl end group)
formed by a
-H elimination process. A methyl side group would then be formed
by reinsertion of this vinyl end group in a 2,1-fashion, whereby a new monomer
unit is incorporated. According to these calculations, such a mechanism will not
take place step by step but rather in a simultaneous way. Under these circum-
stances, an increase in the steric demand of the ligand sphere may suppress
chain rotation which is necessary in obtaining branched polymer microstructures.
This will - if the reinsertion of the vinyl end group alternatively takes place in a
1,2-orientation - finally lead to the formation of less branched, more linear poly-
mers. This argument was used to explain the linearity of the polyethenes result-
ing from Fe(II)- and Co(II)-bisimino(pyridine) complexes and might also be ac-
cepted as true for bis(phosphine) catalysts such as
15b-d
[34].
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