Ultrarigid Indenyl-based Hafnocene Complexes for the Highly Isoselective Polymerization of Propene: Tunable Polymerization Performance Adopting Various Sterically Demanding 4-Aryl Substituents

Article abstract of DOI:10.1021/acs.organomet.6b00814

Two novel silyl-bridged C₂-symmetric (2-methyl-4-aryl-7-methoxy) substituted bisindenyl based ansa-hafnocene complexes of varied steric demand (I, 4-phenyl; II, 4-[(3′,5′-methyl)-phenyl]) were synthesized and examined in the coordinative polymerization of propene. Both complexes enable a comparative study with the state of the art homogeneous metallocene catalyst (III, 4-[(3′,5′-tert-butyl)-phenyl]) for high melting ultrahigh molecular weight isotactic polypropylene. All three activated complexes exhibit extremely concise stereoregularity along with high molecular weights and high melting transitions at low to moderate polymerization temperatures. Increased sterical encumbrance of the 4-aryl substituent prevents the process of chain release reactions more effectively, especially due to enhanced reduction of β-methyl elimination. Accordingly, end group analysis disclosed the highest selectivity toward allylic chain ends as a result of β-methyl elimination with the less sterically encumbered complex I. Examination of the catalytic activity of I-III disclosed considerable impact of the varied 4-aryl substituents on the maximum productivity with respect to the applied polymerization conditions considering the combined influence of activation, monomer diffusion rate, catalyst deactivation, and rate of chain growth.

Full text of DOI:10.1021/acs.organomet.6b00814

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Article
pubs.acs.org/Organometallics
Ultrarigid Indenyl-based Hafnocene Complexes for the Highly
Isoselective Polymerization of Propene: Tunable Polymerization
Performance Adopting Various Sterically Demanding 4‑Aryl
Substituents
Martin R. Machat,† Dominik Lanzinger,‡ Alexander Pothig,§ and Bernhard Rieger*,†
̈
†Wacker-Lehrstuhl fur Makromolekulare Chemie, Technische Universitat Munchen, Lichtenbergstraße 4, 85748 Garching bei
̈
̈
̈
Munchen, Germany
̈
‡Advanced Materials & Systems Research, BASF SE, GCP/PM-B001, 67056 Ludwigshafen am Rhein, Germany
§Catalysis Research Center, Technische Universitat Munchen, Ernst-Otto-Fischer Straße 1, 85748 Garching bei Munchen, Germany
̈
̈
̈
S
* Supporting Information
ABSTRACT: Two novel silyl-bridged C2-symmetric (2-methyl-4-aryl-7-
methoxy) substituted bisindenyl based ansa-hafnocene complexes of varied
steric demand (I, 4-phenyl; II, 4-[(3′,5′-methyl)-phenyl]) were synthesized
and examined in the coordinative polymerization of propene. Both
complexes enable a comparative study with the state of the art
homogeneous metallocene catalyst (III, 4-[(3′,5′-tert-butyl)-phenyl]) for
high melting ultrahigh molecular weight isotactic polypropylene. All three
activated complexes exhibit extremely concise stereoregularity along with
high molecular weights and high melting transitions at low to moderate
polymerization temperatures. Increased sterical encumbrance of the 4-aryl
substituent prevents the process of chain release reactions more effectively,
especially due to enhanced reduction of β-methyl elimination. Accordingly,
end group analysis disclosed the highest selectivity toward allylic chain ends
as a result of β-methyl elimination with the less sterically encumbered
complex I. Examination of the catalytic activity of I−III disclosed
considerable impact of the varied 4-aryl substituents on the maximum
productivity with respect to the applied polymerization conditions
considering the combined influence of activation, monomer diffusion
rate, catalyst deactivation, and rate of chain growth.
exceptional potential in the precise single-site catalysis of high
INTRODUCTION
■
12−16
i
molecular weight PP.
The remarkable high molecular
In 1985 the first indenyl based ansa-zirconocene-methyl-
aluminoxane (MAO) catalytic system for the narrow molecular
weight distributed isospecific polymerization of propene was
discovered by the combined work of Kaminsky and
Brintzinger.1−3 Since then vast interest aroused to optimize
the polymerization performance of metallocene complexes in
the single-site catalysis to polypropylene (PP).4 Ewen’s
symmetry rules describing the relationship between complex
symmetry and polymer microstructure were crucial for further
development of complex design with respect to the desired PP
properties.5 The synthesis of polypropylenes with variable
microstructures, which were not accessible using heterogenic
Ziegler−Natta systems,6,7 evoked huge scientific effort
concerning polypropylene with tailored tacticities particularly
comprising elastic behavior.8−11 In 1994 Spaleck and co-
workers reported the catalysis to highly isotactic polypropylene
using − SiMe2− bridged (2-methyl-4-phenyl) substituted rac-
bisindenyl zirconocene complexes (SBI type) which revealed
weights obtained with these systems activated with MAO are
attributed to the effective shielding of the ligand preventing
exchange reactions with AlMe3 setting free the polymer chain.17
Further strategies toward highly isoselective PP catalysis were
developed using C1-symmetric ansa-cyclopentadienyl(cp)-fluo-
renyl coordinated zirconocene complexes, titanium salen
complexes and bis(phenolate) ether complexes (Zr, Hf)
respectively.18−23 While initially low activities were observed
applying hafnocene complexes for the polymerization of olefins
alternative activation reagents to MAO enabled the utilization
of hafnocene complexes as highly productive metallocene
catalysts.24−27 In 2012 further adjustment of the primary
Spaleck-type ligand structure by Schobel et al. lead to ultrarigid
̈
metallocene complexes producing polypropylenes with ex-
Received: October 26, 2016
© XXXX American Chemical Society
A
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tremely high molecular weights and extraordinary precision
with respect to the stereo- and regioselective polymerization
behavior.28 To date the resulting polymer exhibits the highest
melting transition for untreated iPP (ex reactor) and was further
investigated regarding the exceptional thermal characteristics of
the almost perfect PP polymer. Just recently, the high
potential in terms of catalytic activity of SBI-type metallocence
complexes was reported as a result of weak electrophilic
attraction of the metal center toward the respective counter-
ion.30,31 Calculations on related zirconocene complexes
supported the additional effect of 3′,5′-substitution on the 4-
phenyl substituent with respect to the stereo- and regiose-
lectivity.16,32 In order to gain deeper insight into the
outstanding catalyst performance of these ultrarigid 2,4,7-
substituted bisindenyl hafnocene complexes the focus is drawn
to different 4-aryl substituents possessing a varied 3′5′-
substitution pattern of decreased steric encumbrance.
temperatures isolating the desired rac-hafnocene complexes I−
III.
Single crystals suitable for X-ray diffraction analysis were
obtained of I and II by diffusion of pentane into a saturated
complex solution in toluene or benzene. Crystals of complex I
possess an additional incorporation of 1.0 eq. of benzene. The
crystal structure of complex III is already literature known
(CCDC: 841806).28 ORTEP style representations of I and II
are given in Figure 1, important bond lengths and angles of all
three complexes are depicted in Table 1.
29
i
RESULTS AND DISCUSSION
■
Syntheses. Varied ligand structures with different 4-aryl
substituents were obtained starting with the literature known
indene derivative 1 followed by a Suzuki cross-coupling
reaction in order to implement different aryl groups in position
4 of the indenyl fragment.28,33 Whereas 3a and b react with
SiMe2Cl2 in a regioselective manner after the deprotonation
n
with BuLi, a mixture of regioisomers regarding position 1 and
3 of the indenyl fragment was obtained with the phenyl
substituted fragment. Therefore, the alternative reaction
pathway B (Scheme 1) for the synthesis of ligand 4 was
Scheme 1. Synthesis Route to 2,4,7-Substituted Hafnocene
Complexes with Varied 4-Aryl Substituents
Figure 1. ORTEP style representation of I (top) and II (bottom) with
ellipsoids drawn at 50% probability level. Hydrogen atoms are omitted
for clarity.
In the solid state, the largest dihedral angle is observed for
complex II possessing the largest bite angle. The shortest bond
length between the central metal and the cp-center as well as
the lowest D-value indicating that the metal center is less
exposed to the surrounded environment, were measured in the
solid state of complex III. Unfortunately, a consistent trend of
Table 1. Characteristic Angles and Distances of the Solid
State Structures (I−III)
a
a
a
bite angle
(deg)
dihedral angle
(deg)
Hf−Cpcentroid
D
developed. The lithiated substrate 1 turned out to react with
SiMe2Cl2 regioselective at position 1 of the indenyl fragment
followed by a subsequent introduction of the phenyl groups.
Deprotonation of ligands 4, 5a-b with tBuLi in toluene and the
successive addition of HfCl4 led to a mixture of the
corresponding racemic and meso complex. Separation was
conducted with different pentane/toluene mixtures at adjusted
(Å)
(Å)
0.949 4
I
58.9
59.6
57.8
41.1/45.2
48.6
2.224
2.231
2.218
2
II
III28
0.949
0.926
42.6
a
According to refs 34 and 35. Complex I differs from accurate C2
symmetry.
B
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bond lengths and angles with respect to an increased steric
demand of the 4-aryl substituents is not observed in the solid
state. The incorporation of solvent molecules in the case of I as
well as lattice effects leading to enhanced compression
tional and motional processes the steric interaction of the 3′,5′-
substituents of both indenyl fragments tends to widen the bite
angle in connection with increased steric encumbrance. This
suggestion is supported by the space filling representations of
Figure 2 and the schematic representation of Figure 3. The
result of combined compressing and widening forces (green
arrows, Figure 3) is expected to preserve the rigid complex
framework in a broad temperature range.
Productivity. All three complexes were tested regarding
their catalytic behavior in the polymerization of propene under
identical conditions (Table 2). The activated complexes I-III
are capable of producing high molecular weight and highly
isotactic polypropylenes possessing high productivities. To gain
detailed information on the particular catalytic activities Figure
4 visualizes essential factors determining the overall productiv-
ity.
t
especially for the Bu-substituted complex during crystal
packaging disables the opportunity to draw further conclusion
regarding the chemical behavior in solution. Nevertheless, the
impact of increased sterically encumbered 4-aryl substituents on
the coordination gap aperture36 is conveniently illustrated by
the solid state structures (Figure 2).
Figure 2. Space filling representation of I−III illustrating the effect of
different 4-aryl substituents on the coordination gap aperture (cga).
Evaluation of geometric characteristics of the complex
structures is presented based on the graphic illustration of
Figure 3. All three complexes carry a methoxy substituent in
Figure 4. Main influencing factors affecting the productivity in the
coordinative polymerization of olefins.
Activation of all three complexes proceeds in two steps
starting with an alkylation of the bishalogenated complex using
200 eq. of triisobutylaluminum (TIBA) at 60 °C for 1 h. The
cationic, catalytically active species is generated in situ by the
addition of 5 eq. of [Ph3C][B(C6F5)4]. Monitoring the reaction
of I−III with 200 eq. of TIBA at 60 °C via UV VIS
spectroscopy indicates a fast substitution reaction due to a shift
of the absorbance maxima to lower wavelength (see Supporting
Information). Although the substitution reaction is much
slower for complex III compared to I and II a distinct shift of
the absorbance maxima is observed after 1 h. The reaction of
[Ph3C][B(C6F5)4] with alkylated metallocenes is stated to
proceed quite rapid leading to the assumption that the overall
activation process should be comparable for all three
complexes.24,25,38,39
The polymers from complexes I−III produced at temper-
atures up to 30−50 °C almost exclusively precipitate during the
polymerization therefore limiting the productivity since
accessibility of further monomer units to the catalytically
activated complexes is significantly reduced. In addition, during
precipitation of the polymer also active catalyst may be
removed from the solution. At higher polymerization temper-
atures the polymer remains in solution enabling higher yields in
the batch process as the produced polymer is acting as
additional viscous solvent during the polymerization reaction.
Hence, the averaged chain length as well as the stereo- and
regioregularity of the polymer are crucial for the viscosity of the
reaction mixture in addition to the total amount of produced
polymer. In general, enhanced viscosity reduces the productiv-
ity since diffusion of monomer to the metal center is rather
limited. Due to the direct relation of viscosity and polymer
formation the diffusion rate is particularly lowered at longer
reaction times. Thus, the amount of catalyst is adjusted
generating comparable reaction mixtures keeping limitation by
Figure 3. Graphic illustration of I−III pointing out the steric
interactions leading to the rigid complex framework.
position 7 of the indenyl fragment, which is known to cause
stereorigidity in the complex structure by repulsive interaction
with the − SiMe2− bridge. As a consequence the bite angle is
lowered.28,37 In addition, two opposing interactions determin-
ing the conformation of the 4-aryl substituents related to the
indenyl fragment are essential for the structure of these
complexes. This phenomena, being partially described by the
dihedral angle, is a result of reaching the energetic minimum in
a coplanar structure by maximized delocalization, interfering
with the reduction of steric interactions by rotation of the 4-aryl
substituent. On a molecular level including rotational, vibra-
C
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a
Table 2. Conditions and Results for the Polymerization of Propene with Complexes I−III in Toluene
b
c
d
e
f
g
h
i
entry
complex
n
p
Tp
Tm
mmmm
Mw
Đ
P
1
I
2
3
0
165
162
161
149
128
108
164
163
162
154
143
122
171
170
165
160
152
128
155
156
>99
>99
98
1600
1400
680
200
29
2.0
1.6
1.8
2.2
2.4
2.1
1.5
1.5
1.7
1.7
2.1
1.7
1.2
1.5
1.6
1.5
2.4
2.1
2.3
1.9
4000
16000
33000
40000
9500
2
I
1
1
2
5
5
2
2
2
2
5
5
2
2
2
2
5
5
2
2
4
4
4
4
4
3
4
4
4
4
4
3
4
4
4
4
4
3
3
30
50
70
90
110
0
j
j
j
j
3
I
4
I
88
5
I
80
6
I
67
6
3600
7
II
II
II
II
II
II
III
III
III
III
III
III
I
>99
>99
>99
93
2200
1500
710
200
31
3200
8
30
50
70
90
110
0
15000
16000
47000
11000
7500
9
j
j
j
10
11
12
13
14
15
16
17
18
19
20
89
76
6
l
l
l
l
l
l
l
l
l
l
l
l
l
99.9
5800
1700
1100
410
97
1000
l
30
50
70
90
110
50
50
99.5
13000
14000
46000
50000
21000
30000
l
99.3
j
l
98.4
k
j
94
84
93
97
12
j
290
380
II
19000
a
b
c
d
e
tp = 30 min; Vtoluene = 300 mL; scavanger (TIBA) = 2.2 mmol. In micromoles. p = pAr + ppropene in bar; pAr = 1.3 bar. Tp in °C 2 °C. In °C.
f
g
h
i
j
Determined via 13C NMR spectroscopy assuming the enantiomorphic site model. In kg·mol−1. Đ = Mw/Mn. in kgPP·[molM·(mol/L)·h]−1 5 °C.
k
l
8 °C. According to ref 28.
mass-transport to a minimum or at least comparable for all
three catalyst systems.
complex cation−anion interaction potentially affecting the
coordination strength, the anion mobility, the monomer
assisted dissociative displacement or the prevalent formation
of solvated/outer-sphere ion pairs may depict a substantial
reason for the different catalytic behavior of complexes I-
III.45,46 These interactions may directly affect the rate constant
(kP), itself representing a very complex aggregate accounting all
factors of influence related to chain propagation. Most
essentially, the influence of varied steric demand of the 4-aryl
substituents in complexes I-III on the chain propagation step
should be expressed by this constant. However, experimental
determination of kP appears to be rather difficult and is only
feasible monitoring the molecular weight with respect to the
reaction time. Since the reaction time to build up one polymer
chain is usually well below one second only few examples of
quantitative kinetic investigation are known using quenched
flow techniques.43,47−49 Consequentially, a more qualitative
evaluation of the impact of varied ligand structure on the
catalytic activity is in the scope of this work. The temperature
dependent productivities of complexes I-III adjusted to the
present complex and monomer concentration are depicted in
Figure 5.
Olefin polymerization basically proceeds via π-coordination
of an olefin unit followed by the migratory insertion via four-
center transition state. Variation of the ligand structure of
complexes I-III may affect the monomer association/
dissociation equilibrium but since the rate-determining step is
generally assigned to the process of olefin insertion, the focus is
directed to the influence of varied ligand structure on the rate
of chain propagation (v).40−42 The latter takes place first order
with respect to the amount of added catalyst (cat) and
monomer (C3H6), the rate constant of monomer insertion (kP)
and the fraction of catalytically active complex (χ) (eq 1).43
ν = kP·χ·[cat]·[C3H6]
(1)
To compare the catalytic activity of complexes I-III as a
preferably unadulterated function of the chain propagation rate,
the productivities are normalized to catalyst and monomer
concentration (the latter determined by the equation of
Busico44). A crucial point for the catalytically active fraction
(χ) is the initial complex activation procedure being already
discussed in a previous section, but also catalyst deactivation
due to decomposition at high temperatures becomes
progressively important thereby irreversibly lowering χ. This
process is indicated by a significant drop of activity over the
polymerization time excluding a prevalent impact of mass-
transport limitation due to enhanced viscosity. The possible
formation of resting states during the polymerization is an
important as well as hardly predictable issue. Beside the
interaction with aluminum alkyls being always present in the
reaction mixture the role of the counterion is also essential in
this connection. Whereas the applied [B(C6F5)4]− anion
possesses a low coordination strength to the catalytically active
cationic complex in the case of [MeB(C6F5)3]− the tight
cation−anion interaction results in an equilibrium of active and
dormant species.45 The impact of varied ligand structure on the
It is illustrated, that the maximum productivity of all three
complexes I-III is shifted with respect to the applied
polymerization temperature and the absolute value of the
maximized productivity increases. Complex I comprising the
smallest steric demand at the 4-aryl substituent, reveals its
highest productivity at about 60−65 °C (∼40 000 kgPP·[molM·
(mol/L)·h]−1). The maximum productivity is displaced up to
about 70 °C (∼45 000 kgPP·[molM·(mol/L)·h]−1) for complex
II and to 80−85 °C (∼50 000 kgPP·[molM·(mol/L)·h]−1) for
t
the Bu-substituted complex III, respectively. At low polymer-
ization temperatures the less sterically demanding complex I
exhibits the highest productivities leading to the assumption
that chain propagation is rather limited in the case of bulkier
substituted systems. Interaction of the skipping polymer chain
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Figure 5. Spline representation of the productivity for complexes I−
III in the polymerization of propene (see Table 2 for details). Error
bars are depicted according to possible deviations during the
preparation process.
Figure 6. Consumption of propene with respect to the polymerization
time at 70 °C (entries 4, 10, and 16) standardized on 2.0 μmol of I−
III.
due to the migratory insertion mechanism with the 4-aryl
substituents may exhibit a crucial role considering low rotation
rates. Switching to polymerization temperatures of 30 °C
results in a productivity increase of all three complexes I-III.
Further temperature increase to 50 °C rises the productivity of
I distinctly, whereas the more sterically encumbered complexes
II-III need polymerization temperatures of TP ≥ 70 °C to
become highly active. Higher steric demand from I to III in the
complex framework seems to limit the catalytic activity at low
to moderate temperatures. The temperature shifted progressive
increase of the productivity of I vs II and III may be at least
partially ascribed to different solubility of the produced
polymer. Without going into detail on the polymer properties
at this point, a more precise stereoselective behavior as well as
higher molecular weights are obtained with a bulkier 4-aryl
substituent. As a result, polypropylene produced at 50 °C with I
is partly soluble, whereas that of II and especially III still
precipitates thereby removing active catalyst at an early stage of
the polymerization process. Consequently, II and III display
high catalytic activities not until predominant solubility of the
produced polymer is provided. Regarding high polymerization
temperatures (TP ≥ 70 °C) enhanced sterical encumbrance in
the complex structure accounts for higher catalytic activities.
Considering the volume uptake of propene during the adopted
polymerization time this observation seems to be a reason for
different thermal stability of the catalytically activated
complexes (Figure 6, 7).
At 70 °C the productivity, representing the averaged catalytic
performance over the polymerization time, is in a comparable
range for all three activated complexes I-III. Nevertheless, the
volume uptake of propene indicates a distinct different catalytic
behavior. Although complex I and II show very high initial
activities (Figure 6, 70 °C, large slope of the curves), the
sterically most encumbered complex III reveals the highest
activities at longer polymerization times resulting in a similar
overall volume uptake of all three catalytic systems. At 90 °C
the initial activities are almost identical for all three complexes,
but III is the only complex which is active during the whole
polymerization time. By contrast, the catalytically active species
of complex I is completely deactivated after about 4 min and II
after about 10 min.
Figure 7. Consumption of propene with respect to the polymerization
time at 90 °C (entries 5, 11, and 17) standardized on 2.0 μmol of I−
III.
three complexes I-III produce polymers with ultrahigh
molecular weights particularly at low polymerization temper-
atures. Switching to higher temperatures and lower monomer
concentrations endothermal elimination processes become
more favored (Figure 8).
An increased steric demand of the 4-aryl substituent results
in higher molecular weights of the produced polymers
suggesting progressive prevention of elimination reactions.
This effect is especially pronounced for III at lower
polymerization temperatures although the lowest propagation
rates of all complexes are observed under these conditions.
Crucial for the molecular weight of the polymer chains are
the chain release reactions. Investigation of the olefinic end
group composition facilitates the determination of chain release
reactions using the catalytically activated complexes I-III. Due
to low concentration of olefinic end groups in high molecular
weight polymer samples the end group composition can only
be identified in polymer samples produced at higher polymer-
ization temperatures and lower monomer concentrations via 1H
NMR spectroscopy (Figure 9). In the case of complex I and II
β-methyl elimination is observed as most favored chain release
reaction pathway leading to the formation of allylic end groups
(Figure 9, green). Vinylidene end groups as a result of β-
hydride elimination can be detected as second preferred chain
Polymer properties. The molecular weights are deter-
mined by the rate ratio of propagation and chain release. All
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Figure 10. Determination of allylic and vinylidene chain ends of
polypropylene produced at 90 and 110 °C (entries 5, 6, 11, 12, 17, and
18).
Figure 8. Weight-average molecular weight (Mw) of polypropylene
dependent on the polymerization temperatures with I−III (see Table
2 for details).
suggest that especially β-methyl elimination is accelerated rising
the temperature. This is in line with a higher estimated
activation barrier for the process of β-methyl compared to β-
hydride elimination considering the notable energy difference
for C−C vs C−H bond cleavage.56 An increased steric demand
of the 4-aryl substituents prevents both elimination pathways,
whereupon the process of β-methyl elimination is obviously
reduced more distinctly.
i
For PP produced by complex III very low amounts of
regiodeffects are reported.28 The results of I and II confirm the
concise regioselectivity for these type of complexes since
neither 2,1-erythro nor 3,1-isomerization regio deffects are
detected using standard 13C NMR spectroscopy (s. Exper-
imental). The remarkable selectivities especially at elevated
polymerization temperatures may be attributed to the high
rigidity in all three catalytic systems. The rigidity preserves the
accurate molecular structure being indispensible for a presice
regio control mechanism in conditions usually stimulating
potentially undesired rotational, vibrational and motional
processes.
All three complexes exhibit almost perfect stereoselective
behavior in the polymerization of propene at low temperatures
following the mechanism of enantiomorphic site control.57−60
Switching to higher polymerization temperatures as well as
lower monomer concentrations isolated stereoerrors become
detectable. The tacticity is determined by the ratio of the
mmmm-pentad to the sum of all pentads (Figure 11).
1
Figure 9. Determination of olefinic end group composition via H
NMR spectroscopy (at 140 °C, C6D5Br) of polypropylene (entries 5,
11, and 17; Tp = 90 °C) with I (black), II (red), and III (blue).
release reaction (Figure 9, orange). Regarding complex III the
probability toward vinylidene end groups is higher compared to
allylic end groups. Internal end groups as a result of allylic C−H
bond activation are also monitored for all three complexes
(Figure 9, purple).50,51 The impact of the polymerization
temperature on the selectivity of β-hydride and β-methyl
elimination is depicted in Figure 10 considering the polymer-
ization results at 110 °C.
At 90 °C as well as 110 °C the catalytically active species of
complex I bearing the lowest steric demand at the 4-aryl
substituent possesses the highest selectivity toward allylic chain
ends. This fact contradicts with previous studies revealing much
higher selectivities toward β-methyl transfer in the case of the
bulkier substituted Cp*2HfCl2/MAO system (M: Zr, Hf)
compared to the unsubstituted Cp2HfCl2/MAO system (M: Zr,
Hf).52−55 Rising the polymerization temperature favors the
pathway of β-methyl elimination in all three cases. Varying the
temperature from 90 to 110 °C particularly affects the end
group selectivity of complex III resulting in the predominant
formation of allylic chain ends at 110 °C in contrast to the
observed end group composition at 90 °C.
In accordance to Figure 8 the average molecular weight
decreases with elevated temperatures since all chain release
reactions more preferentially occur. The results of Figure 10
Figure 11. Tacticities (% mmmm) of the polymers produced with
complexes I−III determined via 13C NMR spectroscopy (see Table 2
for details).
F
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Assignment of the isotacticity (% mmmm) via 13C NMR
spectroscopy indicates at least 99% mmmm regarding polymer-
ization temperatures up to 30 °C for I and II with respect to
the signal-to-noise ratio, although no other pentads are
observable. All tacticities of polymers produced with complex
III (except entry 17 + 18) were determined with an accuracy of
0.1%.28 Investigation of the pentad distribution at polymer-
ization temperatures above 50 °C reveals the presence of
additional pentads (mmmr, mmrr, mrrm) attributed to isolated
stereo errors.
High melting temperatures above 160 °C are observed for
polymerization temperatures up to 50 °C in all cases. An overall
decrease of the melting transitions is noticed for the polymers
of complexes I-III applying higher polymerization temper-
atures, at which the effect is less pronounced with increased
steric demand of the ligand. The results are in line with the
determined tacticities (Figure 11) as well as molecular weights
(Figure 8).
Regarding industrial application, the focus is directed to
highly productive catalyst systems, most substantially, compris-
ing long-term stability. A precise stereo- and regio control
mechanism in combination with high molecular weight
polymer chains are indispensable to ensure the quality of the
produced high performance polyolefins. Considering the
production process, elevated temperatures are always preferred
to improve heat transmission (larger ΔT) as well as the
processability, since better solubility of the produced polymer is
assured. In this context, the most promising results are obtained
with the catalytic system III revealing the highest selectivities
and productivities combined with the most distinctive temper-
ature stability. The high rigidity in the complex framework,
provided by the combined steric effects of the 7-OMe and the
sterically encumbered 4-[(3′,5′-tert-butyl)-phenyl] substituents,
is suggested to be key for this essential contribution. The
introduced rigidity-concept as a result of the unique
substitution pattern displays a new, alternative approach for
further catalyst design and development.
Taking into consideration that the polymerizations between
70−110 °C were performed at isobar pressure, lower monomer
concentrations at higher temperatures evoke predominant
impact on the stereo control mechanism beside the general
loss of selectivity caused by rising the temperature itself.
Particularly stereoerror formation via chain end epimerization61
is favored at low monomer concentration and is attributed to
be the main reason turning down the isotacticity employing
complex I-III under these conditions.44,62−65 This assumption
is supported by polymers produced with I and II at 50 °C and 3
bar, possessing significantly lower tacticities (I: 93% mmmm, II:
97% mmmm) than the ones produced at 4 bar. Consequently,
complex III comprising the highest steric demand at the 4-aryl
substituent is able to preserve the formation of isotactic
sequences at elevated temperatures best, whereas I produces
polymers with the lowest amount of mmmm-pentads. These
results indicate that an increased steric demand more
sufficiently prevents the cascade process of chain end
epimerization comparing complex I-III. Disregarding the
predominant stereoerror formation via chain end epimerization
in the presence of low monomer concentration, we believe, that
the enantiofacial selectivity of our systems is still accurate at
elevated temperatures due to the high rigidity of the complexes.
One example demonstrating the precise catalytic control at
higher temperatures is the polymerization with III at 70 °C
increasing the monomer concentration from 0.88 M (Entry 16)
to 1.53 M. The result is a highly isoselective polymer with an
mmmm-pentad distribution of 99.0%.28
CONCLUSION
■
In this report two novel C2-symmetric indenyl based ansa-
hafnocene complexes with 4-aryl substituents of varied steric
demand (I-II) were synthesized enabling a comparative study
with the literature known complex (III). The latter is stated as
benchmark for the catalysis of high melting ultrahigh molecular
i
weight PP comprising extraordinary precise stereo- and
regioregularity. Complete characterization including X-ray
diffraction analysis was conducted for both new complexes
illustrating the crucial impact of the 4-aryl substituent on the
coordination gap aperture. The activated complexes I-III were
examined regarding their polymerization behavior applying
identical conditions. The varied steric demand of the 4-aryl
substituent revealed considerable impact on the maximum
productivity with respect to the polymerization temperature.
Therefore, the direct relationship between complex structure
and catalytic activity is extensively discussed with respect to the
combined impact of activation, monomer diffusion rate, catalyst
deactivation and rate of chain growth. The highest temper-
atures to reach the maximized productivity in combination with
the largest overall catalytic activity were necessary for the most
sterically encumbered complex III. GPC analysis determined
remarkable high molecular weights for the polymers of all three
complexes at low polymerization temperatures. A decrease of
the average molecular weight is observed applying higher
polymerization temperatures as well as lower monomer
concentrations. Increased steric encumbrance of the ligand
leads to higher molecular weights due to enhanced prevention
of chain release reactions. Olefinic end group analysis discloses
increased selectivity toward allylic vs vinylidene end groups
regarding elevated polymerization temperatures, since β-methyl
elimination is rather accelerated. Furthermore, the process of
the β-methyl elimination is particularly prevented in the case of
increased steric demand at the 4-aryl substituent. At low to
moderate polymerization temperatures going along with higher
The melting behavior of the polymers is mainly influenced by
the tacticity of the polypropylenes. In the case of short chain
lengths the average molecular weight possesses a more
distinguished impact on the melting transition.66 The melting
transition of all generated polymers is depicted in Figure 12.
Figure 12. Melting transition of polypropylene samples produced at
different polymerization temperatures with I−III (see Table 2 for
details).
G
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Organometallics
Article
2H, H−Ar), 6.60 (m, 2H, −CH), 6.53 (d, 3J = 8.5 Hz, 2H, H−Ar),
6H, −CH3), −0.44 (s, 6H, Si−CH3). 13C NMR (126 MHz, CDCl3
298 K): δ (ppm) = 153.51, 150.19, 145.96, 134.30, 129.05, 125.54,
106.65, 105.62, 54.99, 48.18, 17.70, −3.71.
monomer concentrations the catalytic performance in terms of
stereoregularity is very concise for all three complexes I-III.
Consequentially, the mmmm-pentad is exclusively detected
using 13C NMR spectroscopy. The formation of isolated
stereoerrors, predominant a result of chain end epimerization,
rather occurs at lower monomer concentrations. The sterically
most encumbered complex III is capable of still preserving the
highest isotacticities impeding the cascade reaction of chain end
epimerization better than I and II. Accurate regioselecitivties
for all polymers produced with I-III were detected in a broad
temperature range. High rigidity in the complex framework is
key for a precise stereo and regio control up to elevated
temperatures. DSC analysis provides a high melting behavior
for the polymer of all three complexes dependent on the varied
polymerization conditions in analogy to the determined
tacticities and molecular weights.
3
4.24 (s, 2H, −CH−Si), 3.85 (s, 6H, −OCH3), 2.24 (d, J = 1.4 Hz,
7-(3′,5′-Dimethylphenyl)-4-methoxy-2-methylindene, 3a. All sol-
vents were degassed prior to use. A solution of 4.63 g (30.8 mmol,
1.10 equiv) of (3,5-dimethylphenyl)boronic acid in ethanol (40 mL)
and a solution of 3.88 g (3.36 mmol, 0.12 equiv) Pd(PPh3)4 in toluene
(200 mL) were added to 6.70 g (28.0 mmol, 1.00 equiv) of 4-bromo-
7-methoxy-2-methylindene. After the subsequent addition of 56.0 mL
(56.0 mmol, 2.00 equiv) of 1 M NaOH solution, the mixture was
refluxed for 3 days. Water (250 mL) and toluene (250 mL) were
added, phases were separated, and the aqueous phase was extracted
two times with toluene (2 × 100 mL). The combined organic layers
were dried over Na2SO4, and the solvent was removed in vacuo.
Compound 3a was isolated via column chromatography (toluene, Rf =
0.75) as a white solid (6.10 g, 82%). Anal. Calcd for C19H20O: C,
86.32; H, 7.63. Found: C, 86.69; H, 7.60. 1H NMR (300 MHz, CDCl3,
298 K): δ (ppm) = 7.11 (s, 2H, H−Ar′), 7.07 (d, 3J = 8.3 Hz, 1H, H−
EXPERIMENTAL SECTION
■
3
Ar), 6.97 (s, 1H, H−Ar′), 6.84 (d, J = 8.3 Hz, 1H, H−Ar), 6.64 (s,
General. All reactions containing air- and moisture-sensitive
compounds were performed under argon atmosphere using standard
Schlenk or glovebox techniques. All chemicals, unless otherwise stated,
were purchased from Aldrich, Acros, or VWR and used as received.
Dry toluene and n-pentane were obtained from an MBraun MB-SPS-
800 solvent purification system. Deuterated dichloromethane was
refluxed over CaH2 and distilled prior to use. Propene (99.5% by
Westfalen AG) was purified by passage through two columns filled
with BASF catalyst (R3-11) and molecular sieves 3−4 Å. Elemental
analysis was conducted with a EURO EA (HEKA tech) instrument
equipped with a CHNS combustion analyzer.
1H, −CH), 3.88 (s, 3H, −OCH3), 3.39 (s, 2H, −CH2−), 2.35 (s,
6H, CH3−Ar′), 2.13 (s, 3H, −CH3). 13C NMR (75 MHz, CDCl3 298
K): δ (ppm) = 152.05, 145.28, 143.04, 141.52, 138.40, 134.95, 131.42,
128.72, 126.66, 125.76, 123.46, 109.66, 55.97, 43.96, 21.68, 16.93.
Bis[4-phenyl-7-methoxy-2-methylindenyl)]dimethyl Silane, 4. All
solvents were degassed prior to use. A solution of 6.10 g (50.0 mmol,
4.00 equiv) of phenylboronic acid in ethanol (70 mL) as well as a
solution of 3.46 g (2.99 mmol, 0.24 equiv) of Pd(PPh3)4 in toluene
(400 mL) were added to 6.66 g (12.5 mmol, 1.00 equiv) of bis(4-
bromo-7-methoxy-2-methylindenyl)dimethyl silane (2). After the
subsequent addition of 50.0 mL (50.0 mmol, 4.00 equiv) of 1 M
NaOH solution, the mixture was refluxed for 3 days. Water (200 mL)
and toluene (200 mL) were added, phases were separated, and the
aqueous phase was extracted two times with toluene (2 × 100 mL).
The combined organic layers were dried over Na2SO4, and the solvent
was removed in vacuo. A 1:1 mixture of rac-/meso-isomers (4) was
obtained via column chromatography (pentane/EtOAc 20/1, Rf =
0.36). Recrystallization in DCM/MeOH led to the desired product as
colorless needles (5.20 g, 79%). Anal. Calcd for C36H36O2Si × 1/3
CH2Cl2: C, 78.34; H, 6.63. Found: C, 78.26; H, 6.60 (presence of
1H and 13C NMR measurements were recorded on a Bruker ARX-
300, AV-500C, AV400, or AV500 spectrometer at ambient temper-
ature. Chemical shifts δ are reported in ppm relative to
tetramethylsilane and calibrated to the residual H or 13C signal of
1
the deuterated solvent. Polymer spectra were measured with an ARX-
300 spectrometer at 140 °C in bromobenzene-d5 with 50−60 mg/mL.
Gel permeation chromatography (GPC) was performed with a PL-
GPC 220 instrument equipped with 2× Olexis 300 mm × 7.5 mm
columns and triple detection via differential refractive index detector,
PL-BV 400 HT viscometer, and light scattering (Precision Detectors
model 2040; 15°, 90°). Measurements were performed at 160 °C
using 1,2,4-trichlorobenzene (TCB; 30 mg BHT/L) with a constant
flow rate of 1 mL/min and a calibration set with narrow MWD
polystyrene (PS) and polyethylene (PE) standards. Samples were
prepared dissolving 0.9−1.1 mg of polymer in 1.0 mL of stabilized
TCB for 10−15 min at 160 °C immediately before each measurement.
Differential scanning calorimetry (DSC) analysis was conducted on
a DSC Q2000 instrument. Polymer (3−8 mg) was sealed into a DSC
aluminum pan and heated from 20 to 200 °C at 10 °C/min. After
holding the temperature for 2 min, the sample was cooled down to 20
°C at 10 °C/min and heated up again in the same manner. The
reported values are those determined in the second heating cycle.
Synthesis. All compounds that are not listed below were
synthesized according to literature procedures.28,67,68
1
additional CH2Cl2 was confirmed via NMR spectroscopy). H NMR
(300 MHz, CDCl3, 298 K): δ (ppm) = 7.54 (m, 4H, H−Ar′), 7.45 (m,
4H, H−Ar′), 7.33 (m, 2H, H−Ar′), 7.24 (dd, 3J = 8.2, 1.7 Hz, 2H, H−
Ar), 6.74 (m, 4H, H−Ar, −CH), 4.23, 4.06 (s, 2H, −CH−Si), 3.92,
3.85 (s, 6H, −OCH3), 2.30, 2.17 (s, 6H, −CH3), −0.20, −0.22, −0.34
(s, 6H, Si−CH3). 13C NMR (75 MHz, CDCl3 298 K): δ (ppm) =
153.70, 153.66, 149.82, 149.63, 144.35, 144.26, 141.42, 133.48, 128.97,
128.95, 128.43, 127.75, 127.68, 126.78, 126.77, 126.34, 125.19, 125.08,
105.41, 105.35, 54.78, 54.67, 47.06, 46.62, 17.81, 17.72, −2.65, −3.83,
−3.85.
Bis[4-(3′,5′-dimethylphenyl)-7-methoxy-2-methylindenyl)]-di-
methyl Silane, 5a. Compound 3a (5.97 g, 22.6 mmol, 2.00 equiv) was
diluted in 100 mL of dry toluene/dioxane (1/1) in a pressurizable
n
Schlenk flask. At −10 °C, 6.42 mL (22.6 mmol 2.00 equiv) of BuLi,
2.4 M in hexane, was added dropwise to the solution. After stirring at
room temperature for 2 h, 1.38 mL (11.3 mmol, 1.00 equiv) of
dichlorodimethylsilane was added at −10 °C. After stirring at 60 °C
for additional 24 h, the reaction mixture was poured into 25 mL of
water. Diethyl ether (100 mL) was added, phases were separated, and
the organic layer was washed with water (70 mL) and brine (70 mL).
The organic phase was dried with Na2SO4, the solvent was evaporated,
and the crude product was recrystallized in a DCM/MeOH mixture
leading to 3.11 g (47%) of a 1:1 rac-/meso-isomer mixture as white
solid. Anal. Calcd for C40H44O2Si: C, 82.14; H, 7.58. Found: C, 81.79;
Bis(4-bromo-7-methoxy-2-methylindenyl)dimethyl Silane, 2. 4-
Bromo-7-methoxy-2-methylindene (10.3 g, 43.1 mmol, 2.00 equiv)
was diluted in 200 mL of dry toluene/dioxane (1/1) in a pressurizable
n
Schlenk flask. At −10 °C, 17.2 mL (43.1 mmol 2.00 equiv) of BuLi,
2.5 M in hexane, was added dropwise to the solution. After stirring at
room temperature for 2 h, 2.62 mL (21.5 mmol, 1.00 equiv) of
dichlorodimethylsilane was added at −10 °C. After stirring at 60 °C
for additional 24 h, the reaction mixture was poured into 50 mL of
water. Diethyl ether (150 mL) was added, phases were separated, and
the organic layer was washed with water (150 mL) and brine (150
mL). The organic phase was dried with Na2SO4, the solvent is
evaporated, and the crude product was recrystallized in a DCM/
MeOH mixture leading to 8.25 g (72%) of colorless needles (2). Anal.
Calcd for C24H26Br2O2Si: C, 53.95; H, 4.90. Found: C, 53.99; H, 4.78.
1H NMR (500 MHz, CDCl3, 298 K): δ (ppm) = 7.28 (d, 3J = 8.5 Hz,
1
H, 7.53. H NMR (300 MHz, CDCl3, 298 K): δ (ppm) = 7.19, 7.18
3
(d, J = 8.3 Hz, 2H, H−Ar), 7.12, 7.10 (s, 4H, H−Ar′), 6.96 (s, 2H,
H−Ar′), 6.71 (m, 4H, H−Ar, −CH), 4.22, 4.02 (s, 2H, −CH−Si),
3.90, 3.84 (s, 6H, −OCH3), 2.39, 2.36 (s, 12H, CH3−Ar′), 2.28, 2.15
(s, 6H, −CH3), −0.23, −0.25, −0.39 (s, 6H, Si−CH3). 13C NMR (75
H
DOI: 10.1021/acs.organomet.6b00814
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Organometallics
Article
MHz, CDCl3 298 K): δ (ppm) = 153.62, 153.59, 149.57, 149.39,
144.38, 144.29, 141.43, 137.87, 137.86, 133.51, 133.43, 128.05, 127.99,
127.92, 126.87, 126.85, 126.78, 126.74, 125.34, 125.20, 105.35, 105.28,
54.78, 54.65, 47.12, 46.61, 21.60, 17.81, 17.74, −2.57, −3.76, −3.79.
rac-Dimethylsilanediylbis(4-phenyl-7-methoxy-2-methylindenyl)-
hafnium Dichloride, I. Compound 4 (750 mg, 1.42 mmol, 1.00 equiv)
was dissolved in 50 mL of dry toluene and cooled down to −78 °C,
mixture was poured into 1.0 L of acidified methanol. Precipitated
polymer was removed from the autoclave, and all combined polymer
was washed exhaustively and dried at 70 °C in vacuo overnight.
ASSOCIATED CONTENT
■
S
* Supporting Information
t
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.6b00814.
and 1.67 mL (2.84 mmol, 2.00 equiv) of 1.7 M BuLi solution in
pentane was added dropwise. After maintaining the temperature for 1
h, the reaction mixture was stirred for additional 3 h at room
temperature. The yellow suspension was cooled down to 0 °C and
subsequently transferred via cannula to a suspension of 454 mg (1.42
mmol, 1.00 equiv) of HfCl4 in 25 mL of dry toluene at −78 °C. The
reaction mixture was allowed to thaw overnight resulting in an orange
suspension. The suspension was filtered, and the residue was extracted
with dry toluene (2 × 100 mL). The extract was concentrated to 10
mL and cooled down to −20 °C overnight. Pure rac-isomer (110 mg,
10%) was obtained as an orange crystalline solid after removal of the
overlaying solution. Anal. Calcd for C36H34Cl2HfO2Si: C, 55.71; H,
4.42. Found: C, 55.52; H, 4.32. 1H NMR (400 MHz, C2D2Cl2, 298 K):
δ (ppm) = 7.60 (m, 4H, H−Ar′), 7.41 (m, 4H, H−Ar′, H−Ar), 7.31
(m, 4H, H−Ar′), 6.86 (s, 2H, −CH), 6.43 (d, 3J = 7.8 Hz, 2H, H−
Ar), 3.92 (s, 6H, −OCH3), 2.24 (s, 6H, −CH3), 1.21 (s, 6H, Si−CH3).
13C NMR (100 MHz, C2D2Cl2, 298 K): δ (ppm) = 156.11, 140.40,
1H NMR and 13C NMR spectra of the new key
1
compounds I and II, UV VIS spectra, H NMR spectra
for end group analysis, 13C NMR spectra for tacticity
determination, charts providing propene consumption
during the polymerization time, SC-XRD data (PDF)
Crystallographic information for compounds I and II
(CIF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: rieger@tum.de. Tel: +49-89-289-13570. Fax: +49-89-
289-13562.
133.16, 132.93, 130.63, 129.07, 129.01, 128.26, 127.60, 121.71, 120.33,
103.26, 85.70, 17.67, 6.17.
ORCID
rac-Dimethylsilanediylbis[4-(3′,5′-dimethylphenyl)-7-methoxy-2-
methylindenyl)]hafnium Dichloride, II. Compound 5a (1.33 g, 2.27
mmol, 1.00 equiv) was dissolved in 70 mL of dry toluene and cooled
down to −78 °C, and 2.68 mL (4.55 mmol, 2.00 equiv) of 1.7 M tBuLi
solution in pentane was added dropwise. After maintaining the
temperature for 1 h, the reaction mixture was stirred for additional 3 h
at room temperature. The yellow suspension was cooled down to 0 °C
and subsequently transferred via cannula to a suspension of 727 mg
(2.27 mmol, 1.00 equiv) of HfCl4 in 40 mL of dry toluene at −78 °C.
The reaction mixture was allowed to thaw overnight resulting in a dark
yellow suspension. After filtration, the solvent of the filtrate was
distilled off, and the residue was washed with dry pentane (2 × 100
mL) and a 1:2 toluene/pentane-mixture (240 mL). After recrystalliza-
tion in a toluene/pentane mixture, 360 mg (19%) of yellow powder
was obtained containing the pure rac-isomer. Anal. Calcd for
Bernhard Rieger: 0000-0002-0023-884X
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors thank Philipp Pahl and Daniel Wendel for
proofreading the manuscript and valuable discussions. The
second author D. L. substantially contributed to this work
during his time as a co-worker of the Wacker-Lehrstuhl fur
Makromolekulare Chemie.
̈
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■
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DOI: 10.1021/acs.organomet.6b00814
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