Remote Perfluoroalkyl Substituents are Key to Living Aqueous Ethylene Polymerization

Article abstract of DOI:10.1002/anie.201913117

In various nickel(II) salicylaldiminato ethylene polymerization catalysts, which are a versatile mechanistic probe for substituent effects, longer perfluoroalkyl groups exert a strong effect on catalytic activities and polymer microstructures compared to the trifluoromethyl group. This effect is accounted for by a reduced electron density on the active sites, and is also supported by electrochemical studies. Thus, β-hydride elimination, the key step of chain transfer and branching pathways, is disfavored while chain-growth rates are enhanced. This enhancement occurs to an extent that enables living polymerizations in aqueous systems to afford ultra-high-molecular-weight polyethylene for various chelating salicylaldimine motifs. These findings are mechanistically instructive as well as practically useful for illustrating the potential of perfluoroalkyl groups in catalyst design.

Full text of DOI:10.1002/anie.201913117

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Accepted Article
Title: Remote Perfluoroalkyl Substituents are Key to Living Aqueous
Ethylene Polymerization
Authors: Manuel Schnitte, Janine Sophie Scholliers, Kai Riedmiller,
and Stefan Mecking
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201913117
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10.1002/anie.201913117
Angewandte Chemie International Edition
RESEARCH ARTICLE
Remote Perfluoroalkyl Substituents are Key to Living Aqueous
Ethylene Polymerization
Manuel Schnitte[a], Janine S. Scholliers[a], Kai Riedmiller[a], and Stefan Mecking*[a]
Abstract: Perfluoroalkyl groups are well established substituents to
enhance the solubility of molecular catalysts in apolar and fluorous
media. However, they have been little studied to control the catalytic
properties of active sites. In various nickel(II) salicylaldiminato
ethylene polymerization catalysts, which are a versatile mechanistic
probe for substituent effects, longer perfluoroalkyl groups exert a
strong effect on catalytic activities and polymer microstructures
compared to trifluoromethyl groups as a reference. This can be
accounted for by a reduced electron density on the active sites, also
supported by electrochemical studies. Hereby β-hydride elimination
as the key step of chain transfer and branching pathways is disfavored
while chain growth rates are enhanced. This occurs to an extent that
enables living polymerizations in aqueous systems to afford ultra high-
molecular-weight polyethylene for various chelating salicylaldimine
motifs. These findings on a mechanistically instructive as well as
practically useful probe illustrates the potential of perfluoroalkyl
groups for catalyst design beyond solubility, which benefits from the
synthetic accessibility and stability of these electron-withdrawing
groups.
catalytic reactivity are rare.[8] Although their synthetic chemistry is
well established their potential is largely unexplored in this regard.
We now report a case study showing how perfluoroalkyl
substituents enhance catalyst performance. As a catalyst system,
which is both mechanistically instructive to unravel the role of the
perfluoroalkyl substituents as well as practically useful neutral
Ni(II) polymerization catalysts were studied.[9]
Results and Discussion
Perfluoroalkyl (e.g. n-C6F13) substituted N-terphenyl
salicylaldiminato Ni(II) catalysts (Figure 1) were recently
demonstrated to be uniquely capable of
a
truly living
polymerization of ethylene under aqueous conditions.[10] In this
process narrow distributed ultra high-molecular-weight strictly
linear polyethylene (UHMWPE) is generated, in the unusual form
of monodisperse uniform-shape nanocrystals.
Introduction
The properties of molecular catalysts are often controlled by
substituents chosen to the purpose. Perfluorinated alkyls –
(CF2)nCF3 – are well-established groups to adjust the solubility of
homogeneous catalysts.[1] They can enhance catalysts’
lipophilicity and allow for reactions under very apolar conditions
or with very apolar substrates. This enables e.g. catalysis in
supercritical carbon dioxide as an alternative and an
environmentally friendly solvent[2], applications in biphasic
mixtures for efficient catalyst recovery[3] or selective reagent
separation[4], and catalyst heterogenization and new delivery
methods.[5] Strategies to make a catalyst ‘fluorous’ are versatile
and perfluorinated groups can also be placed in the structure of a
catalyst precursor to enhance its activation and reactivity via its
miscibility in different phases.[6] Fluorous catalysts in general
show a versatile solubility behavior and are not limited to
applications in equally fluorous solvents, that are often not desired
due to their costs and environmental persistence.[7]
Figure 1. Catalyst precursor with C6F13-substituents in remote positions capable
of truly living ethylene polymerization in aqueous media (left).[10] Interaction
promoting chain transfer and branch formation in the case of electron-donating
substituents, exemplified by methyl substituents (right).[11]
The strong influence of substituents in these remote positions
of the chelating ligand, distant from the active center, can be
related to a weak interaction between the distal aryl rings and the
metal atom (Figure 1, right), that promotes decoordination of
ethylene and favors β-hydride elimination (BHE).[11] BHE is the
key step in chain transfer and branch formation. This weak
interaction is promoted by electron-donating groups, which afford
hyperbranched ethylene oligomers. In contrast, with electron-
withdrawing trifluoromethyl groups linear polyethylene is formed
under otherwise identical conditions.[12,13] Note that a H-F
interaction with the growing chain, what has been suggested for
early[14] and late transition metal catalysts[15], is clearly not
operative.[16]
In contrast to their extensive utilization for solubility, examples
where perfluorinated alkyl groups control the active metal center’s
[a]
Manuel Schnitte, Janine S. Scholliers, Kai Riedmiller, Prof. Dr.
Stefan Mecking
Chair of Chemical Materials Science, Department of Chemistry
University of Konstanz
D-78457 Konstanz, Germany
E-mail: stefan.mecking@uni-konstanz.de
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.201913117
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RESEARCH ARTICLE
Figure 2. Catalyst precursors studied in this work. Compared to the reference systems (RF = CF3), the novel catalysts are substituted with linear long perfluoroalkyl
groups (RF = C6F13) in remote positions. The coordinated labile ligand L differs for lipophilic precursors (L = Pyr) and hydrophilic precursors (L = PEG), suitable for
aqueous polymerizations.
To understand their role in catalysis, we investigated the influence
of long perfluoroalkyl substituents (linear C6F13 groups) in different
length on catalytic performance and polymer properties was
observed. Compared to their CF3 analogues, the C6F13-substitued
catalysts 1-C6F13/Pyr and 2-C6F13/Pyr showed higher activities
(15 x 103 TO h-1 vs. 35 x 103 TO h-1, entries 1 and 2, Table 1) and
produced narrower distributed polyethylene of higher molecular
weights (167 x 103 g mol-1 vs. 504 g mol-1, entries 1 and 2, Table
1) with less branches (2.3 vs. < 1.0 branches per 1000 carbon
atoms, entries 1 and 2, Table 1) at 30 °C reaction temperature.
Chain transfer (which limits molecular weight and broadens
molecular weight distribution) and branch formation proceed
established salicylaldiminato Ni(II) motifs under
a set of
conditions. We targeted the N-(quar)terphenyl based types 1/RF-
L and 2/RF-L (cf. Figure 2), known to be active in aqueous and
non-aqueous ethylene polymerization towards linear high-
molecular-weight polyethylene.[13,17–19] We also modified N-
naphthyl type catalyst 3/RF-L (cf. Figure 2), capable of a
controlled/living ethylene polymerization in a variety of solvents,
and introduced C6F13-substituents in selected positions.[20] Anthryl
moieties were placed in close proximity to the nickel center to
shield the axial site and impede chain transfer reactions.[13,21]
The synthesis of the respective salicylaldimines was
straightforward according to known procedures, slightly modified
to handle the highly lipophilic intermediate products (cf. the
Supporting Information, SI, for details of synthesis and
characterization of all catalyst precursors). As a key step, a
copper-promoted Ullmann coupling of 1,3-diiodobenzene and
perflurorohexyl iodide provided access to the 1,3-
(diperfluorohexyl)phenyl structure motif incorporated in the
aforementioned catalyst structures. Lipophilic precatalysts were
obtained by reaction of salicylaldimines with [(tmeda)NiMe2] in the
presence of pyridine as labile ligand. Hydrophilic analogs, suitable
for aqueous polymerizations, were obtained in the presence of α-
methoxy-ω-amino poly(ethylene glycol) (H2N-PEG-OMe, cf.
Figure 2).
through BHE. The above data indicates
a very effective
suppression of these pathways by the C6F13-substitution. Similar
trends of activity and branching were observed at a higher
reaction temperature of 50 °C comparing CF3- and C6F13-
substitued catalyst types 1-RF/Pyr and 2-RF/Pyr.
In case of catalyst type 3-RF/Pyr, the C6F13-substituted
derivative again produced polyethylene with lower branches.
Other than this, no significant differences were observed. Both
catalysts perform a highly controlled polymerization as indicated
by chains per nickel ratios close to unity and narrow molecular
weight distributions of Mw/Mn = 1.2.
The observed increase in polymerization rate and molecular
weights vs. the CF3 reference, together with a decrease in
branching indicate a significant influence of the perfluoroalkyl
substituents in remote positions of the catalyst structures on the
active nickel center. To further illuminate the origin of this effect,
cyclic voltammetry experiments were performed on all pyridine
precatalysts. The observed oxidation and reduction transitions for
the Ni(II)/Ni(III) pair showed that, compared to the reference
trifluoromethyl catalysts, the electron density at the metal center
is significantly lower in the C6F13-substituted complexes as
Ethylene polymerization studies in heptane were carried out
at different temperatures (30-50 °C), with C6F13-substituted
catalyst precursors and the analogous CF3 complexes to identify
effects of perfluoroalkyl substituent under lipophilic conditions
(Table 1). All catalysts studied were active in ethylene
polymerization and a significant effect of perfluoroalkyl chain
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10.1002/anie.201913117
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RESEARCH ARTICLE
Table 1. Ethylene polymerization experiments in heptane.
[a]
[c]
E1
T
[°C]
yield PE
[g]
Mn
chains/
[Ni]
Tm[d] [°C]
(Cryst. [%])
Branches
/1000 C[e]
[c]
entry
precatalyst
TOF[b]
Mw / Mn
[mV]
[103 g mol-1]
1
2
3
4
5
6
7
8
9
10
1-CF3/Pyr
1-C6F13/Pyr
1-CF3/Pyr
1-C6F13/Pyr
2-CF3/Pyr
2-C6F13/Pyr
2-CF3/Pyr
2-C6F13/Pyr
3-CF3/Pyr
3-C6F13/Pyr
254
560
254
560
329
445
329
445
357
395
30
30
50
50
30
30
50
50
50
50
0.68
1.66
2.28
5.68
0.99
1.33
2.68
5.23
1.40
1.24
14.6
35.3
48.8
121.4
21.2
28.4
57.3
111.8
50.0
44.2
167
504
80
1.9
1.6
3.3
2.9
2.4
1.6
4.6
2.5
1.2
1.2
0.8
0.7
5.7
13.8
2.2
0.9
6.6
13.6
0.8
0.6
134 (53)
133 (49)
124 (47)
124 (50)
136 (57)
133 (55)
125 (51)
123 (49)
135 (47)
135 (46)
2.3
< 1.0
12.4
8.4
82
91
4.7
294
82
2.9
17.8
13.4
3.7
77
618
683
< 1.0
cond.: 5 µmol catalyst loading, 20 minutes reaction time, 40 bar ethylene pressure, in 100 mL heptane. [a] determined via cyclic voltammetry. [b] given in 103
x mol [C2H4] x mol-1 [Ni] x h-1. [c] determined via GPC at 160 °C. [d] determined via DSC (heating rate: 10 K/min), 2nd heating cycle reported. [e] determined via
selective detection of methyl and methylene IR-bands during GPC measurements (versus standards with known degree of branching).
evidenced by an increase in measured forward peak potentials
(column E1, Table 1) found for all catalyst types studied. This
remarkable decrease in electron density (e.g. 211 mV [RF = CF3]
vs. 541 mV [RF = C6F13], catalyst 1-RF/Pyr, Table 1) correlates
qualitatively with the observed catalytic and polymer properties in
case of catalyst types 1-RF/Pyr and 2-RF/Pyr (cf. Figure 3). Note,
that this effect on polymer microstructure may be enhanced
further by the higher steric demand[22] of perfluoroalkyl vs.
trifluoromethyl substituents.
In case of catalyst types 3-RF/Pyr, only a small difference in
forward peak potentials was observed (357 mV [RF = CF3] vs.
395 mV [RF = C6F13], entries 9 and 10, Table 1). This lower effect
of the substitution pattern compared to the other two types of
catalysts is expected, given that only one of the aryl substituents
in o-position of the N-phenyl is altered, and the phenyl ring closest
to the nickel center (8-position of the naphthylamine moiety, cf.
Figure 2) was not modified. This electrochemical data agrees well
with the similar catalytic properties observed for 3-C6F13/Pyr and
3-CF3/Pyr (Table 1), the C6F13-substitution lead to a slight
decrease in branching only while catalytic activity and molecular
weight are similar to the CF3 analog.
To probe for any pronounced solvent effects, e.g. through
interactions with the active site or via the solvent quality for the
polymer product formed, we performed polymerization
experiments in toluene as a somewhat more polar and aromatic
solvent (cf. the SI for comprehensive experimental and analytical
data of polymerization experiments in toluene with all catalyst
precursors at different temperatures). In summary, we observed
the same trends for catalysts 1-RF/Pyr and 2-RF/Pyr as outlined
Figure 3. Comparison of experimental polymerization data with cylic
voltammetry data for different catalyst precursors (1-RF/Pyr, top and 2-RF/Pyr,
bottom). Catalytic activity (TOF, red line), forward peak potential determined via
cyclic voltammetry (E1, green line), molecular weight (blue line) and branching
values (black line) of polyethylenes formed versus different catalyst precursors
given. Experimental data from experiments at 30 °C in heptane (entries 1, 2, 5,
and 6, Table 1).
above with a superior role of catalysts with long perfluoroalkyl
substituents. Namely, compared to the CF3 reference (1)
increased molecular weights (240 x 103 g mol-1 vs. 623 x 103 g
mol-1, entries 1 and 2, Table S1); (2) slightly increased catalytic
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Table 2. Ethylene polymerization experiments in aqueous surfactant solution.
[b]
yield PE
[g]
Mn
chains/
[Ni]
Tm[c] [°C]
(Cryst. [%])
Branches
/1000 C[d]
dh (Vol.)[e]
[nm]
[b]
entry
precatalyst
1-CF3/PEG
1-C6F13/PEG
2-CF3/PEG
2-C6F13/PEG
3-CF3/PEG
3-C6F13/PEG
TON[a]
27.9
45.7
22.7
50.6
7.8
Mw / Mn
[103 g mol-1]
142 (55)
134
/ 135 (36)
1
2
3
4
5
6
5.87
9.61
4.78
10.65
1.64
9.39
484
866
408
885
486
1609
2.0
1.6
1.5
1.6
1.6
0.5
0.8
1.6
22 (0.05)
25 (0.08)
23 (0.07)
31 (0.12)
16 (0.14)
21 (0.17)
143 (73) / 135 (48)
133
1.3
< 1.0
< 1.0
< 1.0
2.6
140 (66) / 135 (49)
134
1.8
140 (64) / 133 (41)
133
1.4
138 (63) / 135 (51)
134
1.6
142 (65) / 136 (41)
137
44.6
1.2
< 1.0
cond.: 7.5 µmol catalyst loading, 40 bar ethylene pressure, 4 hours reaction time, 15 °C reaction temperature, 6.0 g sodium dodecyl sulfate, 1.5 g cesium
hydroxide, 0.75 mL mesitylene, in 150 mL degassed water, ultrasound applied prior to ethylene pressurization. [a] given in 103 x mol [C2H4] x mol-1 [Ni]. [b] determined
via GPC at 160 °C. [c] determined via DSC, reported as [1st heating cycle (crystallinity) \ 2nd heating cycle (crystallinity)] with 10 K/min heating rate, second line: 1st
heating cycle with 1 K/min heating rate. [d] determined via selective detection of methyl and methylene IR-bands during GPC measurements (calibrated versus
samples with known degree of branching). [e] determined via DLS, volume mean and PDI reported.
activities (61 x 103 TO h-1 vs. 73 x 103 TO h-1, entries 1 and 2,
Table S1); and (3) decreased degrees of branching (1.7 vs. 1.1
branches per 1000 carbon atoms, entries 1 and 2, Table S1) were
found. In line with the above polymerizations in heptane and cyclic
voltammetry experiments, no significant impact of the longer
perfluoroalkyl substituents in catalysts 3-RF/Pyr was observed.
3-C6F13/Pyr produce polyethylene with slightly lower activity and,
accordingly, with slightly lower molecular weights.
The high functional group tolerance of late transition metal
catalysts allows their use even in polar protic environments.[23]
During polymerization in aqueous surfactant solution with Ni(II)
salicylaldiminato catalysts, the polyethylene is formed in the very
unusual form of nanoscale single crystals.[17] They are
characterized by a high degree of order, that arises from the
unique particle growth mechanism, where the active center’s
growing chain is directly deposited on the crystal growth front,
leaving no opportunity for any disorder.[18,24] This allows for
effective generation of anisotropic polymer nanoparticles, that are
otherwise difficult to access.[25]
All hydrophilic catalyst precursors of types 1-RF, 2-RF and
3-RF (with H2N-PEG-OMe as labile ligand) were active for several
hours under established reaction conditions in water (Table 2).
Remarkably, all C6F13-substituted catalysts studied are clearly
superior in aqueous polymerization compared to their CF3
analogues, in terms of catalytic activity and properties of the
produced polyethylenes. In case of catalysts 1-C6F13/PEG and
2-C6F13/PEG, the perfluoroalkyl substitution lead to a comparable
substantial increase in catalytic activities (e.g. 23 x 103 TO vs.
51 x 103 TO, entries 3 and 4, Table 2), molecular weights (408 x
103 g mol-1 vs. 885 x 103 g mol-1, entries 3 and 4, Table 2) and
decreased polydispersities (1.8 vs. 1.4, entries 3 and 4, Table 2)
and branch contents (1.6 vs. < 1.0 branches per 1000 carbon
atoms, entries 1 and 2, Table 2) vs. the trifluoromethyl reference.
Under optimized reaction conditions (10 °C reaction temperature,
high surfactant loading), both catalysts 1-C6F13/PEG and 2-
C6F13/PEG are capable of a truely living ethylene polymerization
in water, as evidenced by (1) linear relationships between yields
and molecular weights; (2) narrow molecular weight distributions
of Mw/Mn < 1.3; and (3) chain per nickel ratios close to unity (cf.
the SI, Table S2 and S3 for comprehensive experimental and
analytical data, and Figure 4). Even linear and narrow distributed
UHWMPE with Mn = 2.0 x 106 g mol-1 is accessible at extended
reaction times, as one nickel center grows one polymer chain over
the entire polymerization experiment.
Likewise, catalyst 3-C6F13/PEG stands out as the first reported
N-naphthyl type nickel(II) salicylaldiminato catalyst with adequate
activities in water and accessible molecular weights of Mn = 1.6 x
106 g mol-1 with very narrow distributions of Mw/Mn =1.1, that is
formed in a strictly living polymerization (cf. the SI, Table S4 for
comprehensive experimental and analytical data, and Figure 4).
Different from the non-aqueous polymerization with catalyst motif
3-RF, in the aqueous polymerization the CF3 analog is much less
active (45 x 103 TO vs. 8 x 103 TO, entries 5 and 6, Table 2) and
less controlled compared to the perfluoroalkyl complex. As
polymerizations under aprotic conditions and insights from cyclic
voltammetry experiments did not suggest a significant electronic
influence of the long perfluoroalkyl groups on the active center for
this catalyst type 3-RF, we tentatively address this different
behavior in the aqueous system to the highly hydrophobic groups
on the salicylaldiminato ligand. Possibly, the four C6F13-
substituents create a highly apolar environment around the metal
center, that promotes rapid dissociation of the hydrophilic polar
labile H2N-PEG-OMe ligand into the aqueous solution and allows
for an effective catalyst activation. 1H-NMR spectra of catalysts 3-
RF/PEG point to a very strong binding of the labile ligand to the
nickel center, as indicated by a hindered rotation (diastereotopic
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10.1002/anie.201913117
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RESEARCH ARTICLE
character) of the amino functionality and the protons in α- and β-
positions (cf. the SI, Figure S10, S12 and S15), that might prohibit
an effective activation in case of the catalyst with exclusively
trifluoromethyl substituents.
(Figure 5). Even these highly fluorinated catalysts (e.g. 36
perfluorinated carbon atoms in chemical structure of 2-
C6F13/PEG) are able to form small uniform particles under
aqueous conditions, which are composed of completely
disentangled polyethylene as evidenced by high melting points in
the first heating cycle, that are not observed for slow heating
rates.[27]
Figure 5. DLS traces of polyethylene dispersions obtained from aqueous
polymerization with different catalyst precursors (entries 2,4 and 6, Table 2).
Conclusion
In summary, we present a clear case demonstrating the utility
of perfluoroalkyl substituents to control catalytic properties. This
demonstrates their potential for catalysis beyond the established
use for achieving solubility in very apolar and fluorous media. The
C6F13 groups in the catalyst motifs studied substantially increased
catalyst activity and molecular weight of the polymer product, and
reduced branching. This establishes the mechanistic picture of a
reduced electron density on the active metal sites – also
supported by cyclic voltammetry on the catalyst precursors –
when compared to the more commonly used trifluoromethyl
groups. For the catalytic chain growth polymerization studied
here, this favors chain growth and disfavors ß-hydride elimination
which is the key step of chain transfer and branching pathways.
This occurs to an extent that enables truly living polymerizations
even in aqueous systems. Perfluoroalkyl groups are similarly or
slightly more electron withdrawing compared to CF3 groups.[28]
This accounts for their beneficial impact observed here. Further,
longer substituents in the remote positions of the o-terphenyl motif
also reduce BHE[22], possibly by hindering conformations suitable
for an aryl-Ni interaction.
Figure 4. Molecular weights and polydispersity indices of formed polyethylenes
versus yields from different aqueous polymerizations with different catalyst
precursors (cf. the SI, Tables S2, S3 and S4 for comprehensive experimental
and analytical data).
Considering the particle formation process, the generation of
very small (<100 nm) particles requires a high degree of
dispersion of the catalyst precursor[26], and under ideal conditions
an entire particle is grown by a single active site.[10] With the
catalysts studied here, stable dispersions with a single well-
defined particle population were obtained as observed by DLS
Experimental Section
Experimental Details are described in detail in the Supporting Information.
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[15] M. P. Weberski, C. Chen, M. Delferro, C. Zuccaccia, A. Macchioni, T. J.
Acknowledgements
Marks, Organometallics 2012, 31, 3773–3789.
Financial support by the DFG (SFB 1214) is gratefully
acknowledged. We thank Anke Friemel for NMR measurements
and Sonja Stadler for a sample of 2-CF3/Pyr. We thank Larissa
Casper from Rainer Winters group for support with cyclic
voltammetry experiments.
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Keywords: aqueous ethylene insertion polymerization •
fluorinated ligands • homogeneous catalysis • living
polymerization • UHMWPE
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10.1002/anie.201913117
Angewandte Chemie International Edition
RESEARCH ARTICLE
Entry for the Table of Contents
Layout 1:
RESEARCH ARTICLE
Long perfluoroalkyl substituents in
remote positions can influence the
active center of nickel(II)
Manuel Schnitte, Janine S. Scholliers,
Kai Riedmiller, S. Mecking*
Page No. – Page No.
salicylaldiminato catalysts to form
linear polyethylene of higher
molecular weights and in higher
activities.
Remote Perfluoroalkyl Substituents
are Key to Living Aqueous Ethylene
Polymerization
This article is protected by copyright. All rights reserved.