Controlled Polymerization in Polar Solvents to Ultrahigh Molecular Weight Polyethylene

Article abstract of DOI:10.1021/jacs.8b03223

Novel complexes based on 2,4,8-triarylnaphthylamine [[(2,4-{3,5-(CF₃)₂C₆H₃}₂-8-(3,5-R₂C₆H₃)-C₁₀H₄) -Rfnet=C(H) - {3-(9-anthryl)-2-O-C₆H₃}-κ²-N,O]Ni(CH₃)(pyridine)] (4-Py, R = Me; 5-Py, R = CF₃) were synthesized from 8-arylnaphthylamines which can be generated via selective lithiation, borylation, and coupling in a one-pot procedure. Due to their living characteristics in ethylene polymerization and the particularly low propensity of 5-Py for β-H elimination as reflected by a high linearity even at elevated polymerization temperatures of 60 °C, ultrahigh molecular weight polyethylene (UHMWPE) is formed in polar organic solvents (THF, diethyl ether) as a reaction medium in a controlled polymerization (M_w/M_n = 1.2 at M_n = 1.1 × 10⁶ g mol^-1).

Full text of DOI:10.1021/jacs.8b03223

Article
pubs.acs.org/JACS
Cite This: J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
Controlled Polymerization in Polar Solvents to Ultrahigh Molecular
Weight Polyethylene
Philip Kenyon, Melissa Worner, and Stefan Mecking*
̈
Chemical Materials Science, Department of Chemistry, University of Konstanz, Universitatsstrasse 10, 78457 Konstanz, Germany
̈
S
* Supporting Information
ABSTRACT: Novel complexes based on 2,4,8-triarylnaphthylamine
[[(2,4-{3,5-(CF3)2C6H3}2-8-(3,5-R2C6H3)-C10H4)NC(H){3-(9-
anthryl)-2-O-C6H3}-κ2-N,O]Ni(CH3)(pyridine)] (4-Py, R = Me; 5-Py,
R = CF3) were synthesized from 8-arylnaphthylamines which can be
generated via selective lithiation, borylation, and coupling in a one-pot
procedure. Due to their living characteristics in ethylene polymerization
and the particularly low propensity of 5-Py for β-H elimination as
reflected by a high linearity even at elevated polymerization temper-
atures of 60 °C, ultrahigh molecular weight polyethylene (UHMWPE)
is formed in polar organic solvents (THF, diethyl ether) as a reaction medium in a controlled polymerization (Mw/Mn = 1.2 at
Mn = 1.1 × 106 g mol−1).
INTRODUCTION
■
The reaction medium or solvent is of paramount importance
for any chemical reaction. This is particularly pronounced for
polymerization reactions. Here, not only do reactions and
interactions of the solvent with reactive species like catalysts or
initiators occur, but the reaction medium also affects the
product morphology.1 Organic reaction media and solvents for
catalytic polymerization are restricted to hydrocarbons (or
halogenated solvents) to date, however. This is related to the
incompatibility of traditional polymerization catalysts with
Figure 1. Terphenyl-based catalysts Me1-Py and CF31-Py and 2-Py, a
terphenyl derivative for high molecular weights.
compounds containing other heteroatoms.2
Less electrophilic late transition metals are much more
tolerant in this regard. This has been well recognized for the
copolymerization of ethylene with polar vinyl monomers.3−16
From these studies, a conclusive picture has evolved on how
different functional groups interact with the polymerization
active metal sites.17
Due to the propensity of late transition metal alkyl
complexes for β-H elimination, chain transfer often limits
molecular weights and much effort has been devoted to the
design of catalysts that nonetheless can provide high molecular
weight polymers.10−12,15,16,18−25 Although no comprehensive
picture exists, it appears that high concentrations of
coordinating compounds tend to reduce polymer molecular
weights, likely by hindering chain growth or even by actively
promoting chain transfer.
A particular case regarding reaction media for insertion
polymerization are aqueous polymerizations,26 enabled by the
water-tolerance of neutral Ni(II) salicylaldiminato com-
plexes.27,28 Starting from water-soluble complexes like CF31-L
or 2-L (cf. Figure 1, L = water-soluble ligand instead of
pyridine), surfactant-stabilized single lamella polyethylene
nanoscale crystals are formed. The particular aqueous particle
formation process suppresses the formation of entanglements.26
This is of interest among others to improve the processing
properties of ultrahigh molecular weight polyethylene
(UHMWPE).29 Nascent nonentangled morphologies have
been much sought to this end, traditionally limited to
hydrocarbon reaction media.30,31
A further significant improvement of the N-terphenyl motif
of 1 or 2 was achieved with Brookhart’s and Daugulis’
diarylnaphthylamine-based catalysts like 3-Py. These polymer-
ize ethylene to moderately branched UHMWPE with molecular
weights up to 1.4 × 106 g mol−1 and narrow molecular weight
distributions, even at elevated temperatures of 60 °C.
For the terphenyl system, distal substituents have an extreme
effect on the outcome of polymerization. While CF31-Py forms
linear polyethylene, Me1-Py yields hyperbranched oligomers.
This has recently been recognized to result from a weak Ni···
arene interaction that promotes chain transfer and branching in
the latter case.32
Received: March 23, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/jacs.8b03223
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Article
We now report on living/controlled polymerizations using
THF and diethyl ether as a reaction medium where, with
enhanced catalysts, UHMWPE can be obtained. The insights
on the role of distal substituents32 prompted us to explore the
effect of electron-withdrawing substituted bis(trifluoromethyl)-
phenyl groups in the 4- and particularly the 8-position of the
naphthylamine moiety (4-Py and 5-Py, Figure 2).
The polymerization properties of the novel catalyst
precursors and 2-Py and 3-Py for comparison were studied
over a range of temperatures (40−60 °C; Table 1). It is
immediately clear that the naphthylamine-based 3-Py, 4-Py,
and 5-Py produce fewer branches than 2-Py under identical
reaction conditions (Figure 3). In addition to an apparently
reduced propensity for β-H elimination and chain transfer, the
lower branching can also be related to a higher steric
congestion making insertion of ethylene into secondary alkyls
formed via chain walking less favorable. A similar increase in
sterics leading to a reduction in branching was observed for
diimine palladium catalysts.35
The introduction of an additional electron-withdrawing
substituted aryl that points away from the metal center (in
the 4-position) does not have a notable impact on branching.
At a given temperature, 3-Py and 4-Py yield essentially identical
degrees of branching. Significant differences are observed for 5-
Py, however. At all temperatures, 5-Py produces a more linear
polymer than 4-Py. Even at 60 °C, only one methyl branch per
1000 C atoms is observed. While this reduction in branches via
changing the remote substituent is less dramatic than the
change observed for Me1-Py and CF31-Py (vide supra), it
suggests that an analogous effect influences the rate of β-H
elimination in naphthylamine based catalysts.
Figure 2. Catalyst based on diarylnaphthylamine introduced by
Brookhart and Daugulis (3-Py) and novel catalysts based on
triarylnaphthylamine (4-Py and 5-Py).
Remarkably, polymerizations with 3-Py, 4-Py, and 5-Py
exhibit a living character (Table 1, entries 1−3, 5, 6, 8, and 9) as
concluded from narrow molecular weight distributions and a
consideration of the number of chains formed per metal center
present. The molecular weight is therefore limited by the rate of
polymerization, with the more active catalysts (3-Py and 4-Py)
producing higher molecular weight polymers. At 60 °C, it can
be seen that chain transfer does occur (from the number of
chains produced, Table 1, entries 4, 7, and 10), but it remains a
rare event and the increase in activity compensates for chain
transfer. Notably, 5-Py retains a very narrow molecular weight
distribution of Mw/Mn = 1.2. Attempts were made to carry out
polymerizations at 70 °C; however, a constant temperature
could not be maintained over the course of the reaction with
the setup employed due to the high activity of these catalysts.
By comparison, in polymerizations with 2-Py, molecular
weights are determined by β-H elimination and the reaction
is in the regime of chain transfer, and as expected for this case,
molecular weights decrease with increasing polymerization
temperature (Table 1, entries 11−13).
RESULTS AND DISCUSSION
■
Rather than the previously performed multistep reaction33
based on C−H activation,34 naphthylamine derivative NA-CF3
was synthesized in a one-pot reaction beginning with a
selective, quantitative lithiation reaction, followed by introduc-
ing a boronic acid group. Suzuki coupling with 3,5-bis-
(trifluoromethyl)bromobenzene gave NA-CF3 in an 80%
yield (Scheme 1).
Scheme 1. One-Pot Synthesis of NA-CF3
Encouraged by the highly controlled nature of these
polymerizations and the ability to form highly linear poly-
ethylene at elevated temperature at the same time, polymer-
izations in THF and diethyl ether as reaction media were
studied. These polar, aprotic solvents can coordinate to the
metal center, hindering the coordination of ethylene and
suppressing polymerization. Indeed, a lower productivity of the
catalyst27 and limited molecular weight was observed at the
lower end of the temperature range studied (Table 2, entries 2
and 3).
While higher polymerization temperatures can help to
overcome the effects of solvent coordination, the reduced
chain propagation and relative increase of β-H elimination lead
to high rates of chain transfer and low molecular weight
polymers as observed for 2-Py (Table 2, entry 6). Naphthyl-
amine derived catalysts with low rates of chain transfer are
therefore unique in allowing polymerization to high molecular
weights in THF (or other coordinating solvents). Using these
2,4,8-Triarylnaphthylamine derivatives were then synthesized
in a manner analogous to the previously reported 2,8-
diarylnaphthylamine derivatives. Selective 2,4-dibromination
was followed by Suzuki coupling, and the desired salicylaldi-
mines (4 and 5) were obtained by condensation with the
appropriate aldehyde (Scheme 2). Precatalysts (4-Py and 5-Py)
were then obtained by reaction with [NiMe2(TMEDA)] in the
presence of excess pyridine (Scheme 3).
B
DOI: 10.1021/jacs.8b03223
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Journal of the American Chemical Society
Article
Scheme 2. Synthesis of 2,4,8-Triarylnaphthylamines and Subsequent Condensation to Salicylaldimines 4 and 5
Scheme 3. Complexation to Give Precatalysts 4-Py and 5-Py
Based on 2,4,8-Triarylnaphthylamine Derivatives
catalysts, UHMWPE can be obtained in THF (Table 2, entries
1, 4, and 5).
Compared to polymerizations in toluene, there is a slight but
noted decrease in the number-average molecular weight (Mn)
for polymerizations in THF. As this decrease is associated with
a broadening in the molecular weight distribution for 3-Py and
4-Py, it could be argued that this is due to increasing
precipitation disturbing the controlled polymerization.21
Figure 3. Branches per 1000 carbon atoms (determined from 13C
NMR spectra) for complexes 2-Py, 3-Py, 4-Py, and 5-Py at various
polymerization temperatures in toluene.
Table 1. Ethylene Polymerization Results with Naphthylamine Derivative Complexes 3-Py, 4-Py, and 5-Py and Representative
a
Terphenyl Derivative Complex 2-Py as Precatalysts in Toluene
b
c
c
d
d
e
entry precatalyst T (°C) yield (g)
TOF
Mn (103 g/mol) Mw/Mn
Tm (°C) crystallinity (%) branches/1000 C
chains per nickel
1
2
3
4
5
6
7
8
9
3-Py
3-Py
3-Py
3-Py
4-Py
4-Py
4-Py
5-Py
5-Py
5-Py
2-Py
2-Py
2-Py
40
40
50
60
40
50
60
40
50
60
40
50
60
3.6
6.4
5.09
3.06
7.75
818
1232
1021
1437
796
1.2
1.3
1.3
1.3
1.3
1.3
1.3
1.1
1.2
1.2
2.3
2.5
2.1
135
132
130
125
134
131
126
135
132
131
128
118
115
39
47
45
43
46
47
44
53
48
51
58
55
57
1.0
1.2
1.9
4.5
1.1
1.8
4.3
0.7
1.0
1.2
4.2
10.6
13.0
0.9
1.0
1.1
2.3
1.0
1.3
1.8
0.8
1.2
1.9
6.9
43
f
5.8
16.5
3.8
23.6
5.39
11.0
21.1
2.99
7.74
16.4
7.7
1186
1451
527
12.6
2.1
5.4
885
10
11
12
13
11.5
10.0
11.8
16.8
1206
287
g
g
g
10.7
12.6
18.0
32
h
17
106
a
b
c
Polymerization conditions: 5 μmol of precatalyst, 100 mL of toluene, 40 bar of C2H4, 30 min. 104 × mol [C2H4] × mol−1 [Ni] × h−1. Determined
d
e
f
g
by GPC at 160 °C. Determined by DSC. Determined by 13C NMR spectroscopy. Polymerization carried out for 90 min. Polymerization carried
out for 40 min. Includes 0.6 ethyl and 0.4 n-propyl branches.
h
C
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Table 2. Ethylene Polymerization Results with Naphthylamine Derivative Complexes 3-Py, 4-Py, and 5-Py and Representative
a
Terphenyl Derivative Complex 2-Py as Precatalysts in THF
b
c
c
d
d
e
entry precatalyst T (°C) yield (g)
TOF
19.1
Mn (103 g/mol) Mw/Mn
Tm (°C) crystallinity (%) branches/1000 C
chains per nickel
1
2
3
4
5
6
3-Py
4-Py
4-Py
4-Py
5-Py
2-Py
60
35
35
60
60
60
13.4
5.9
1097
1279
519
1.4
1.2
1.4
1.7
1.2
2.1
120
135
135
120
128
99
44
49
50
46
52
52
5.7
0.9
2.4
0.7
0.9
1.9
1.8
f
2.81
0.88
g
1.8
1.2
9.6
13.7
993
7.0
10.1
11.2
14.4
12.0
1111
3.5
h
5.6
26.1
400
a
b
c
Polymerization conditions: 5 μmol of precatalyst, 100 mL of THF, 40 bar of C2H4, 30 min. 104 × mol [C2H4] × mol−1 [Ni] × h−1. Determined
d
e
f
by GPC at 160 °C. Determined by DSC. Determined by 13C NMR spectroscopy. Polymerization carried out for 90 min in toluene.
g
h
Polymerization carried out for 90 min in diethyl ether. Includes 1.7 ethyl and 1.0 n-propyl branches.
However, the UHMWPE produced by 5-Py retains a narrow
molecular weight distribution, suggesting precipitation is not
the source of this broadening.
The probable source of molecular weight broadening is an
increase in the number of dormant states produced by
polymerization in THF (Figure 4). Besides the pyridine
with the formed polymer will decisively influence parameters
like crystallization rates, and we are currently exploring its
impact on morphology. The overcoming of the restriction to
hydrocarbon solvents opens up broad opportunities for control
of supramolecular structure formation during polymerization.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/jacs.8b03223.
Synthetic procedures for the preparation of new ligands
and complexes; H, 13C, and 19F NMR spectra for new
1
Figure 4. Proposed dormant states in THF for naphthylamine derived
catalysts. Ar = 2,8-diarylnaphthylamine (3-Py) or 2,4,8-triarylnaphthyl-
amine (4-Py, 5-Py).
compounds; procedures for the polymerization of
1
ethylene; examples of H and 13C NMR spectra for
polymers; DSC traces of polymers produced in THF;
GPC traces of UHMWPE produced in THF (PDF)
coordinated resting state (formed by the recombination of
the equivalent of pyridine introduced with the catalyst)
observed by NMR previously,21 a species with coordinated
THF is present and due to the increase in β-H elimination
chain walking species become more common.
AUTHOR INFORMATION
Corresponding Author
*Stefan.Mecking@uni-konstanz.de
■
As 5-Py shows similar productivity in THF and toluene, the
formation of THF coordinated species seems to be less
significant than that for 3-Py and 4-Py. The lowest rates of β-H
elimination are also observed for 5-Py, suggesting that chain
walking is also minimized. These remote substituents are
therefore the main factor in allowing for controlled polymer-
ization to UHMWPE in THF.
ORCID
Philip Kenyon: 0000-0003-0244-1347
Stefan Mecking: 0000-0002-6618-6659
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
CONCLUSION
We thank Nic Friederichs and Dieter Bilda for fruitful
discussions. We thank Lars Bolk for DSC and GPC analyses
and Anke Friemel and Ulrich Haunz for support with NMR
measurements. Financial support by SABIC is gratefully
acknowledged.
■
Salicylaldiminato catalysts based on arylnaphthylamine deriva-
tives show low rates of β-H elimination and chain transfer
which allow for polymerization to lightly branched UHMWPE
at high temperatures. The rate of β-H elimination can be
reduced by introducing electron-withdrawing (CF3) substitu-
ents on the aryl ring in the 8-position of the naphthylamine
backbone. While rare chain transfer events do occur at higher
temperatures (i.e., one per active site even at 60 °C),
polymerization remains controlled, leading to UHMWPE
with narrow molecular weight distributions (Mw/Mn < 1.3).
At the same time, linear polyethylene with only one branch per
1000 carbon atoms is obtained.
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