Biaryl Group 4 Metal Complexes as Non-Metallocene Catalysts for Polyethylene with Long Chain Branching

Article abstract of DOI:10.1002/ejic.202000734

A series of biaryl Group 4 complexes with a bidentate and a tridentate pincer ligand have been synthesized and characterized. The complexes have been applied as metallocene analogues for the controlled polymerization of ethylene and the copolymerization of ethylene and 1-hexene, with a particular focus on the control of the degree of long chain branching in these polymers.

Full text of DOI:10.1002/ejic.202000734

Communication
doi.org/10.1002/ejic.202000734
EurJIC
European Journal of Inorganic Chemistry
Branched Polyethylene
Biaryl Group 4 Metal Complexes as Non-Metallocene Catalysts
for Polyethylene with Long Chain Branching
[
a]
[a]
[b]
[b]
Maria M. Gragert, Atanas K. Tomov, Serge Bettonville, Gaëlle Pannier,
[
a]
[a]
Andrew J. P. White, and George J. P. Britovsek*
Abstract: A series of biaryl Group 4 complexes with a bident- and the copolymerization of ethylene and 1-hexene, with a par-
ate and a tridentate pincer ligand have been synthesized and ticular focus on the control of the degree of long chain branch-
characterized. The complexes have been applied as metallo- ing in these polymers.
cene analogues for the controlled polymerization of ethylene
Introduction
Polyethylene (PE) is currently the most widely used and least
expensive synthetic polymer on the market, accounting for ca.
32 % of global plastics demand in 2015, which translates to
approximately 86 million tonnes. Metal catalysed polyethylene
production has revolutionised the PE film industry and Group
4
metallocene complexes of type I have been at the forefront
[
1]
of these catalyst developments (Figure 1). The success of
metallocenes in olefin polymerisation is in part due to the cata-
lyst stability offered by the cyclopentadienyl (Cp) ligand scaffold
and the tuneability in terms of polymer properties such as
molecular weight and chain branching. Chain branching is clas-
sified as either short chain branching (SCB) or long chain
branching (LCB). By convention, SCB implies branches of 6 or
less carbon atoms, while LCB implies branches of approximately
Figure 1. Group 4 metallocene (I), bidentate (II) and tridentate (III) biaryl
complexes.
of type III could serve as metallocene alternatives with im-
proved branching control. The orthogonal planar steric require-
1
5
ments of η -phenyl donors compared to η -Cp ligands allow
easier access to the metal centre, a feature that could poten-
tially affect SCB or LCB formation.
[
2]
1
40 or more carbons. Even a small degree of LCB (e.g. a few
6
Group IV complexes with σ-bound aryl ligand(s) have been
previously used as catalysts for ethylene polymerization. An
early example by van Koten used the 2,6-bis[(dimethyl-
branches per 10 carbon atoms) can result in large improve-
ments in the rheological behaviour compared to purely linear
PE, which dramatically impacts processing and handling prop-
erties.
and several mechanisms based on macromers, C–H-activation
[
5]
[
2,3]
amino)methyl]phenyl pincer ligand in combination with TiCl
,
4
The origin of LCB formation is not well understood
which were active in both ethylene homo-polymerisation and
[
6]
[
2–4]
co-polymerisation with 1-hexene. Another important class of
aryl Group IV catalysts for olefin polymerization, identified by
high throughput screening methods, are the tridentate aryl
or chain walking have been proposed.
In search of alternative non-metallocene catalysts that en-
able control over branching levels, we have investigated
whether bi-aryl complexes of type II and pincer-type complexes
[
7,8]
pyridine amide pincer complexes reported in 2003.
These
complexes received considerable attention and have shown ac-
tivity in both ethylene homo-polymerisation and co-polymeri-
[
[
a] Dr. M. M. Gragert, Dr. A. K. Tomov, Dr. A. J. P. White, Prof. G. J. P. Britovsek
Department of Chemistry, MSRH, Imperial College
White City Campus, 80 Wood Lane, London W12 0BZ, United Kingdom
b] Dr. S. Bettonville, Dr. G. Pannier
[
9–12]
sation with alkenes, as well as 1-hexene polymerisation.
Around the same time, tridentate aryl pyridine aryloxide ligands
were reported by Chan and co-workers following the reaction
of 2-(2′-phenol)-6-arylpyridine substrates with tetrabenzyl-
INEOS, NOH Technology Centre,
Rue de Ransbeek 310, Brussels, 1120, Belgium
Supporting information and ORCID(s) from the author(s) for this article are
available on the WWW under https://doi.org/10.1002/ejic.202000734.
[
13]
titanium and -zirconium precursors.
Synthetic variations on
tridentate pyridine-2-phenolate-6-(σ-aryl) templates have been
©
2020 The Authors. European Journal of Inorganic Chemistry published by
subsequently investigated with titanium, zirconium and haf-
Wiley-VCH GmbH · This is an open access article under the terms of the
Creative Commons Attribution License, which permits use, distribution and
reproduction in any medium, provided the original work is properly cited.
This manuscript is part of the Special Collection Pincer Chemistry and Catal-
ysis.
[14–16]
nium.
More recent examples of mono-aryl complexes are
[
17]
Phebox-based complexes
plexes reported by Veige and Bercaw.
and the aryl bis(aryloxide) com-
[
18,19]
Only a few biaryl
[
20]
Group 4 metal complexes of type II have been reported,
in-
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[
21,22]
variably either with X being cyclopentadienyl,
tris(biaryl) complexes.
or as bis- or
[
23,24]
We report here the first mono(biaryl) complexes of type II
and tridentate pincer complexes of type III with ancillary X li-
gands that can be activated with co-catalysts for ethylene po-
lymerisation catalysis. M–C(aryl) bonds are generally stronger
than aliphatic M–C bonds which should ensure that ethylene
insertion occurs preferentially in M–C(alkyl) bonds, rather than
[
25,26]
the M–C (phenyl) bonds.
alkene insertion into the M–C(aryl) bond can be an essential
However, it has been shown that
[
9,12]
catalyst activation process.
Results and Discussion
Figure 3. Molecular structure of 1-Ti(NMe
angles (°): Ti(1)–N(20) 1.852(2), Ti(1)–C(1) 2.1061(13), N(20)–Ti(1)–N(20A)
11.25(18), C(1A)–Ti(1)–C(1) 88.39(7), N(20)–Ti(1)–C(1) 111.81(10).
2 2
) . Selected bond lengths (Å) and
1
Synthesis of the ligand precursor 3,3′,5,5′-tetra(tert-butyl)-bi-
phenyl (1) was achieved via Suzuki-coupling of 3,5-di(tert-
butyl)phenyl bromide and 3,5-di(tert-butyl)phenyl boronic and the molecular structure is shown in Figure 3. All complexes
[
27]
[28]
acid, followed by bromination to give 1-Br in 86 % yield.
are highly air-sensitive and traces of moisture result in the for-
2
Lithiation of 1-Br in pentane resulted in a suspension of 1-Li2 mation of biphenyl 1.
2
(
see Figure 2). Initial attempts to prepare Group 4 metal com-
In order to improve catalyst stability, a tridentate dianionic
plexes were carried out using the tin precursor 1-SnMe , which diphenylpyridine ligand 2 with an additional donor was investi-
2
was prepared from 1-Li and Me SnCl , but none of the trans- gated. This C–N–C pincer type ligand has been extensively used
2
2
2
[
29–38]
metalation attempts using 1-SnMe were successful. Addition for late transition metal complexes,
but there are no re-
of 1-Li2 to [Ti(NMe ) Cl ] did give the yellow complex 1- ports of early transition metal complexes with such ligands.
2
2 2
2
Ti(NMe2)2 in moderate yield. Purification was achieved by 2,6-Bis(2-bromophenyl)pyridine 2-Br was prepared as reported
2
washing with tetramethylsilane (TMS) to remove any residual previously,[
39]
with improved yields obtained using the
biphenyl (1). Attempts to convert 1-Ti(NMe ) to 1-TiCl with 2,6-diiodopyridine precursor instead of 2,6-dibromopyridine.
2
2
2
Me SiCl were unsuccessful.
Lithiation of 2,6-bis(2-bromophenyl)pyridine 2-Br in diethyl
ether at –30 °C resulted in the dilithium salt 2-Li ·OEt , which
2 2
3
2
[
40]
Other Group 4 complexes 1-Zr(NMe ) , 1-ZrCl and 1-HfCl
2
2
2
2
were prepared in a similar manner, using [Zr(NMe ) Cl ] or was treated with [Ti(NMe ) Cl ] to give 2-Ti(NMe ) as a yellow-
2
2
2
2 2
2
2 2
freshly sublimed ZrCl and HfCl respectively (see Supporting brown complex in 44 % yield (see Figure 4). Attempts to con-
4
4
Information). Crystals suitable for X-ray analysis of 1-Ti(NMe2)2 vert 2-Ti(NMe2)2 into the dichloro complex by reaction with
were obtained from a concentrated solution in hexane at –30 °C Me SiCl were unsuccessful.
3
Figure 2. Synthesis of Group 4 complexes with bidentate biaryl ligand 1.
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obtained with the titanium complex 1-Ti(NMe ) at 80 °C and at
2
2
5
0 °C for the zirconium analogue 1-Zr(NMe ) . Polymer molec-
2 2
ular weights decrease at higher polymerization temperatures. In
the case of 1-Zr(NMe ) , rather broad molecular weight distribu-
2
2
tions are observed, the origins of which are unclear at this stage
but could indicate the formation of multiple active sites upon
activation of these pre-catalysts. Complexes 2-Ti(NMe ) and 1-
2
–1
–1
–1
HfCl gave low activities (< 5 g mmol
h
bar ) and the poly-
mer products were not analysed in this case.
The best performing catalyst 1-Ti(NMe ) was also evaluated
2
Figure 4. Synthesis of a titanium complex with tridentate biaryl ligand 2.
2
2
for ethylene/1-hexene co-polymerisation under industrially
more relevant conditions. From Table 2 it can be seen that activ-
ities have significantly improved under these conditions and
excellent incorporation levels of 1-hexene up to 18 wt.-% was
achieved at 70 °C. The polymer melting points for these co-
polymers are relatively high for this level of 1-hexene incorpora-
tion, but they are considerably lower than for the PE homopoly-
mer obtained under similar reaction conditions (run 10), whose
molecular weight was too high to be determined by GPC. The
reason for the higher melting points are not fully understood
at this stage, but could be due to the very high molecular
weights (Mw ≈ 1,000 kDa) obtained for these co-polymers.
Crystals were obtained from a concentrated solution of com-
plex 2-Ti(NMe2)2 in diethyl ether at –30 °C. (see Supporting
Information). XRD analysis shows a distorted trigonal bipyrami-
dal geometry with the N-atoms in the equatorial plane and
C-atoms in axial position (Figure 5). The C–Ti–C angle of
1
45.4 °C and the Ti–N and Ti–C bond lengths are within the
[
40,41]
typical range.
2 2
Table 2. Ethylene/1-hexene polymerisation with 1-Ti(NMe ) .
A[c]
M
kDa
M
kDa
PDI
C
[d]
SCB
Mp.[e]
[°]C
#
T
PE
g
n
w
6
[
°]C
wt.-% /1000C
8[
9
1
a]
a)
70
90
22
31
7
182
256
350
57
34
–
985
925
–
17
28
–
18
11
–
30
19
–
129
127
136
(
0[b] 70
[f]
[
(
%
1
a] Conditions: 1-Ti(NMe
MAO on silica): 200 mg, 10 bar, 60 min, 1-hexene: 30 g, H
. [b] Conditions: 1-Ti(NMe (2 μmol), 1.8 L isobutane, MMAO (20 mmol),
0 bar, 60 min, H /C = 0.15 mol-%. [c] A: activity in g mmol
d] Determined by IR. [e] Determined by DSC. [f] Polymer was too viscous for
2
)
2
(12.1 μmol), 1.8 L isobutane. Activating support
2
2 4
/C H = 0.15 mol-
2 2
)
–
1
–1
–1
2
2
H
4
h
bar .
[
GPC analysis.
Figure 5. Molecular structure of complex 2-Ti(NMe
Å) and angles (°): Ti(1A)–N(19A) 1.8760(16), Ti(1A)–C(8A) 2.176(2), Ti(1A)–
N(1A) 2.2290(15), N(19A)–Ti(1A)–N(22A) 109.68(7), N(19A)–Ti(1A)–C(8A)
8.19(7), C(8A)–Ti(1A)–C(14A) 145.38(8), N(19A)–Ti(1A)–N(1A) 121.97(7).
2 2
) . Selected bond lengths
(
9
Polymer Analysis
Polymer analysis was carried out using GPC, IR and melt rheol-
ogy measurements. In general, all polymers in Table 1 and
Table 2 show high molecular weight and broad polydispersity.
Ethylene Polymerisation Results
The response of M and M values to the addition of H (1 mol-
Ethylene polymerisation reactions were carried out in toluene at
n
w
2
%
) was investigated, but the addition of H offered little control
4
bar pressure, using MAO as the co-catalyst, either with or with-
2
over the molecular weight with these catalysts.
The presence of long chain branching in the polyethylene
materials was investigated using flow activation energies (Ea),
out H as chain transfer agent (Table 1). The best activities were
2
Table 1. Ethylene polymerisation with Ti and Zr catalyst of 1.
which are listed in Table 1. E values are a measure of the flow
a
[d]
#
Complex[a]
μmol]
T PE
[°C] [g]
A[b]
M
n
M
w
PDI
E
a
activation energy required for polymer chains to overcome in-
ternal resistance. They are determined from rheology measure-
ments by measuring the storage modulus G′, the loss modulus
G′′ and the complex viscosity η* vs. the oscillating frequency ω.
Polyethylene samples are measured at four different tempera-
tures between 160 and 190 °C, using Time Temperature Super-
position (TTS), whereby the data at 190 °C are taken as the
[
[kDa] [kDa]
1
2
3
1-Ti(NMe
1-Ti(NMe
1-Ti(NMe
2
2
2
)
2
)
2
)
2
(25)[c] 25
2.8
22
46
24
30
664
301
282
14
13
10
46±2
52±7
44±2
(24)
(25)
80
80
11.0 92
11.7 94
[
c]
4
5
6
7
1-Zr(NMe
1-Zr(NMe
1-Zr(NMe
1-Zr(NMe
2
)
2
2
2
2
(14)
(14)
(21)
(14)
25
25
50
80
3.1
2.7
11
44
39
8.9
10.7 480
418
47
45
43
53
43±2
51±1
38±1
23±5
[
c]
2
2
2
)
)
)
104 5.3
87 2.0
226
110
[
42,43]
[
c]
reference curve.
Figure 6 shows a typical analysis obtained
6.1
from frequency sweep measurements at various temperatures.
Shifting the graphs to the 190 °C-reference curve and plotting
[
a] Conditions: 300 mL of toluene, 500 equiv. MAO, 4 bar, 75 min. [b] A:
–1
–1
–1
activity in g mmol
h
2
bar . [c] 1 mol-% H added. [d] Flow activation en-
ergy in kJ mol–1, determined by rheology.
the displacement a against 1/T allows the flow activation en-
T
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European Journal of Inorganic Chemistry
ergy E to be determined from the slope (see Supporting Infor- metallocene catalysts with the co-catalyst MAO results in active
a
[
44]
mation for details).
ethylene polymerisation catalysts that show good incorporation
of 1-hexene. Analyses of the polyethylene homopolymers by
rheological measurements have established the presence of
long chain branches, presumably formed via macromer inser-
tion.
Deposition Numbers 1552482–1552483 contain the supplementary
crystallographic data for this paper. These data are provided free of
charge by the joint Cambridge Crystallographic Data Centre and
Fachinformationszentrum Karlsruhe Access Structures service
www.ccdc.cam.ac.uk/structures.
Conflict of Interest
There are no conflicts to declare.
Acknowledgments
We are grateful to INEOS Technologies for funding.
Figure 6. Rheological determination of storage (G′) and loss modulus (G′′), as
well as complex viscosity η* vs. oscillating frequency ω from 160–200 °C for
PE samples from run 3 in Table 1.
Keywords: Ethylene · Polymerisation · Titanium · Pincer ·
Branching
Flow activation energies of up to 28 kJ mol–1 are typically
reported for linear high density polyethylene, while higher val-
ues indicate the presence of long chain branches by virtue of
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European Journal of Inorganic Chemistry
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Received: August 1, 2020
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© 2020 The Authors. European Journal of Inorganic Chemistry
published by Wiley-VCH GmbH

Communication
doi.org/10.1002/ejic.202000734
EurJIC
European Journal of Inorganic Chemistry
Branched Polyethylene
A series of non-metallocene Group 4
metal complexes containing bidentate
and tridentate pincer ligands with
M. M. Gragert, A. K. Tomov,
S. Bettonville, G. Pannier,
A. J. P. White, G. J. P. Britovsek* .... 1–6
1
η -aryl donors has been investigated
as catalysts for polyethylene forma-
tion, with a particular focus on long
chain branching as determined by
rheology measurements.
Biaryl Group 4 Metal Complexes as
Non-Metallocene Catalysts for Poly-
ethylene with Long Chain Branching
doi.org/10.1002/ejic.202000734
Eur. J. Inorg. Chem. 0000, 0–0
www.eurjic.org
6
© 2020 The Authors. European Journal of Inorganic Chemistry
published by Wiley-VCH GmbH