catalysts by changing the cocatalyst from [(
i
Bu) 2 Al] 2 O to MAO [139]. Comprehensive theoretical molecular modeling focusing on such interesting polymerization/
oligomerization transformation mechanisms is still in progress.
In order to shed some light on the nature of active sites relating to the transformation between ethylene polymerization and nonselective oligomerization over the
two model catalyst systems, several spectroscopic methods including NMR, ESR,
and MALDI-TOF MS were applied. For the case of the model 3f/MAO system, a
very characteristic isotropic, hyperfine structure multiplet assigned to a cationic [Cr
(η
6 -arene) 2 ]
+ sandwich complex was observed from the ESR spectrum, indicating
that part of the complex was reduced to a Cr(I)
+ species, which was coordinated by
two molecules of toluene (or arenes from the dissociated Ph 3 SiO groups) to yield
the cationic (η
6 -arene) 2 complex. Similar ESR results for the same cationic [Cr(η
6 -
arene) 2 ]
+ sandwich species were also obtained in the model 9f/MAO catalyst
system at different Al/Cr molar ratios.
In the
29 Si NMR spectrum of the model 9f catalyst activated by MAO, aluminum
species containing the Ph 3 SiO group were observed. Thus, the Cr–C bond was likely
to be produced by transferring the methyl group from MAO to the chromium center
during the activation. Correspondingly, the Ph 3 SiO group was transferred from the
Cr center to the aluminum of MAO to produce the aluminum species containing a
Ph 3 SiO group, accompanied by the formation of a cationic [Cr(η
6 -arene) 2 ]
+
sandwich complex.
The alkyl radical is known to be an important intermediate during the activation
reaction between transition metal-based polyolefin catalysts and metal alkyl
cocatalysts. However, it is difficult to characterize by spectroscopic methods due
to its high reactivity and short lifetime. An investigation to confirm the generation
of alkyl radicals during the activation of model 3f with TiBA by fullerene radical
trapping combined with ESR as well as MALDI-TOF MS was performed. A new
ESR signal (Fig. 21b) of the multiple addition paramagnetic adducts of butyl
radicals to fullerene was successfully observed compared with the ESR signal
without fullerene, and the addition of butyl radicals to fullerene was confirmed by
MALDI-TOF MS analysis (Fig. 21a). The butyl radical intermediate could be
considered to be generated during the reduction and alkylation of BC with TiBA.
Similar ethyl and methyl radical formation has been previously reported during the
activation reaction in other olefin polymerization catalyst systems [140, 141].
In order to understand the identity (active or inactive) of the cationic [Cr(η
6 -
arene) 2 ]
+ species in model 3f/TiBA catalyst system, a temperature-dependent ESR
experiment (220–350 K) was performed to monitor ethylene polymerization (in the
NMR tube). In Fig. 22, it can be seen that the multiplet (g ¼ 1.995) of the cationic
[Cr(η
6 -arene) 2 ]
+ species remained unchanged, indicating that this kind of species
was not active at these reaction conditions. No other ESR signals were observed
during the temperature raising process (220–350 K), although the solid PE had been
observed at 290 K. This result indicated that the active species for ethylene
polymerization cannot be observed by ESR spectroscopy. Therefore, the valence
state of the active species in model 3f/TiBA catalyst system might be Cr(II), which
was always ESR silent. However, Cr(III) cannot be excluded on the basis of the
Phillips Cr/Silica Catalyst for Ethylene Polymerization
175
i
Bu) 2 Al] 2 O to MAO [139]. Comprehensive theoretical molecular modeling focusing on such interesting polymerization/
oligomerization transformation mechanisms is still in progress.
In order to shed some light on the nature of active sites relating to the transformation between ethylene polymerization and nonselective oligomerization over the
two model catalyst systems, several spectroscopic methods including NMR, ESR,
and MALDI-TOF MS were applied. For the case of the model 3f/MAO system, a
very characteristic isotropic, hyperfine structure multiplet assigned to a cationic [Cr
(η
6 -arene) 2 ]
+ sandwich complex was observed from the ESR spectrum, indicating
that part of the complex was reduced to a Cr(I)
+ species, which was coordinated by
two molecules of toluene (or arenes from the dissociated Ph 3 SiO groups) to yield
the cationic (η
6 -arene) 2 complex. Similar ESR results for the same cationic [Cr(η
6 -
arene) 2 ]
+ sandwich species were also obtained in the model 9f/MAO catalyst
system at different Al/Cr molar ratios.
In the
29 Si NMR spectrum of the model 9f catalyst activated by MAO, aluminum
species containing the Ph 3 SiO group were observed. Thus, the Cr–C bond was likely
to be produced by transferring the methyl group from MAO to the chromium center
during the activation. Correspondingly, the Ph 3 SiO group was transferred from the
Cr center to the aluminum of MAO to produce the aluminum species containing a
Ph 3 SiO group, accompanied by the formation of a cationic [Cr(η
6 -arene) 2 ]
+
sandwich complex.
The alkyl radical is known to be an important intermediate during the activation
reaction between transition metal-based polyolefin catalysts and metal alkyl
cocatalysts. However, it is difficult to characterize by spectroscopic methods due
to its high reactivity and short lifetime. An investigation to confirm the generation
of alkyl radicals during the activation of model 3f with TiBA by fullerene radical
trapping combined with ESR as well as MALDI-TOF MS was performed. A new
ESR signal (Fig. 21b) of the multiple addition paramagnetic adducts of butyl
radicals to fullerene was successfully observed compared with the ESR signal
without fullerene, and the addition of butyl radicals to fullerene was confirmed by
MALDI-TOF MS analysis (Fig. 21a). The butyl radical intermediate could be
considered to be generated during the reduction and alkylation of BC with TiBA.
Similar ethyl and methyl radical formation has been previously reported during the
activation reaction in other olefin polymerization catalyst systems [140, 141].
In order to understand the identity (active or inactive) of the cationic [Cr(η
6 -
arene) 2 ]
+ species in model 3f/TiBA catalyst system, a temperature-dependent ESR
experiment (220–350 K) was performed to monitor ethylene polymerization (in the
NMR tube). In Fig. 22, it can be seen that the multiplet (g ¼ 1.995) of the cationic
[Cr(η
6 -arene) 2 ]
+ species remained unchanged, indicating that this kind of species
was not active at these reaction conditions. No other ESR signals were observed
during the temperature raising process (220–350 K), although the solid PE had been
observed at 290 K. This result indicated that the active species for ethylene
polymerization cannot be observed by ESR spectroscopy. Therefore, the valence
state of the active species in model 3f/TiBA catalyst system might be Cr(II), which
was always ESR silent. However, Cr(III) cannot be excluded on the basis of the
Phillips Cr/Silica Catalyst for Ethylene Polymerization
175