Linear, high molecular weight polyethylene from a discrete, mononuclear phosphinoarenesulfonate complex of nickel(ii)

Article abstract of DOI:10.1039/c1cc00095k

A well-defined, homogeneous catalyst, [(Ph)(2-(2′,6′-(OMe) ₂-C₆H₃)-C₆H₄)P(2-SO ₃-C₆H₄)]Ni(Ph)PPh₃, in which a single, bulky ortho-biphenyl substituent on the chelating phosphine blocks one axial position, is very active for formation of linear polyethylene (M _n = 403000 g mol^-1, M_w/M_n = 1.87).

Full text of DOI:10.1039/c1cc00095k

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COMMUNICATION
Linear, high molecular weight polyethylene from a discrete, mononuclear
phosphinoarenesulfonate complex of nickel(II)w
Philippe Perrotin,a Jenny S. J. McCahill,a Guang Wub and Susannah L. Scott*abc
Received 6th January 2011, Accepted 8th April 2011
DOI: 10.1039/c1cc00095k
A well-defined, homogeneous catalyst, [(Ph)(2-(20,60-(OMe)2-
C6H3)-C6H4)P(2-SO3-C6H4)]Ni(Ph)PPh3, in which a single,
bulky ortho-biphenyl substituent on the chelating phosphine
blocks one axial position, is very active for formation of linear
polyethylene (Mn = 403 000 g molꢀ1, Mw/Mn = 1.87).
Mecking et al. found that a heptane-insoluble complex,
[PO]NiMe(pyridine), produces strictly linear, high molecular
weight polyethylene (Mn = 184 000 g molꢀ1) in that reaction
medium.18 The finding was tentatively attributed to the
aggregated nature of the active sites of the heterogeneous
catalyst, making it difficult to develop a rational strategy for
new ligand design. Recently, Shen and Jordan reported poly-
Discrete palladium(II) complexes bearing an ortho-phosphino-
arenesulfonate ligand ([PO] = [Ar2P(2-SO3–C6H4)]) display
the remarkable ability to produce linear polyethylene as
well as linear copolymers of ethylene with polar vinyl
monomers.1–10 However, the polymers are generally formed
only in modest yields and with low molecular weights
(typically, Mn o 104).11–13 Nickel-based analogues of these
catalysts are attractive targets because of their lower metal
cost and, often, significantly higher activities.14–16 For
example, Rieger and co-workers describe discrete nickel(II)
complexes [PO]Ni(Ph)(PPh3) that catalyze the homopolymer-
ization of ethylene, although they obtained only low molecular
weight materials (Mn r 1000 g molꢀ1).17 Much higher
polymerization activities are observed in the presence of a
PPh3 scavenger such as Ni(COD)2 or B(C6F5)3. Replacing
PPh3 by a weaker s-donor ligand such as DMSO or an amine
also improves the catalytic activity, leading to turnover
frequencies as high as 106 hꢀ1 without the need for a
scavenger.18–20 However, there is no improvement in the
molecular weight of the polyethylene. Base-free [PO]Ni(II)
complexes with Z3-benzyl or Z3-allyl ligands were reported
to produce polyethylene with very slightly increased molecular
weights (1300 o Mn o 2,100 g molꢀ1), at the expense of lower
activities.21,22
ethylene with very high molecular weight (Mw = 915 000 g molꢀ1
)
as well as polydispersity (Mw/Mn = 29) from a self-assembled,
tetranuclear {[PO]Pd(II)}4 cluster.23 The authors considered the
possibility of cooperative effects between the Pd(II) centers, but
attributed the formation of high molecular weight polymer
primarily to blocking of one of the axial faces of each Pd(II) site
due to their aggregation.
In the well-known a-diimine family of Pd(II) and Ni(II)
polymerization catalysts, steric bulk in the pseudo-axial positions
causes the molecular weight of the resulting polyethylene to
increase.15 By applying this strategy, the production of relatively
high molecular weight polyethylene (Mn = 189 000 g molꢀ1
)
was recently achieved using a mononuclear [PO]Pd(II) complex
bearing two bulky, ortho-biphenyl substituents on the chelating
In a significant advance towards the production of high
molecular weight polyethylene using this family of catalysts,
a Mitsubishi Chemical Center for Advanced Materials,
University of California, Santa Barbara, CA 93106-5121, USA
b Department of Chemistry & Biochemistry, University of California,
Santa Barbara, CA 93106-9510, USA
c Department of Chemical Engineering, University of California,
Santa Barbara, CA 93106-5080, USA.
E-mail: sscott@engineering.ucsb.edu; Fax: +1 805-893-4731;
Tel: +1 805-893-5606
w Electronic supplementary information (ESI) available: Synthesis
and characterization details, polymerization procedures, and crystal
data for 2a and 2b.CCDC 806742 and 806743. For ESI and crystallo-
graphic data in CIF or other electronic format, see DOI: 10.1039/
c1cc00095k
Scheme 1
c
6948 Chem. Commun., 2011, 47, 6948–6950
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Fig. 1 Molecular structure of 2a (50% probability). Hydrogen atoms
and co-crystallized benzene molecules have been omitted for clarity.
Fig. 2 Molecular structure of 2b (50% probability). Hydrogen atoms
and co-crystallized benzene molecules have been omitted for clarity.
The central carbon of the –CH2–CHQCH2 group is disordered
(2 positions).
phosphine.5 Here we report that a simple modification of this
approach suffices to allow the production of linear, high
molecular weight polyethylene from a discrete, monomeric
[PO]Ni(II) complex.
one biphenyl group has been replaced by a less sterically-
demanding phenyl substituent. Since only one ortho-aryl
group interacts with Ni(II) in 2a, we predicted that this ligand
modification would allow the complex to retain its axial
aryl–Ni(II) interaction.26 This presumably disfavors the
formation of bis-olefin hydride species that lead to chain
transfer, while affording more design versatility for the
Ni(II) complex. The Zwitterionic ligand 1b was prepared
according to the one-pot procedure in Scheme 1, and
was characterized by multinuclear NMR, elemental analysis
and ESI-MS (see ESIw). 1b was subsequently deprotonated
with Na2CO3 and combined with either [Ni(Br)allyl]2 or
trans-[Ni(PPh3)2(Ph)Cl] to generate 2b and 3b, respectively.
Our initial attempts to prepare a PPh3-stabilized phenyl-
Ni(II) complex using the [PO] ligand 1a, bearing two (20,60-
dimethoxy[1,10-biphenyl]-2-yl) substituents, failed, presumably
because of the competing steric demands of the two
phosphines. However, the base-free allyl complex 2az was
obtained in moderate yield after deprotonation of the ligand
by Na2CO3 and transmetallation with 0.5 equiv. [NiBr(allyl)]2,24
Scheme 1. 2a was characterized by 1H, 13C and 31P NMR,
elemental analysis and electrospray ionization mass spectro-
metry (ESI-MS, see ESIw). Its structure was confirmed by
single-crystal X-ray diffraction, Fig. 1. One of the two ortho-
aryl groups sits above an axial coordination site, and may
show a very weak interaction between Ni and Cipso (Ni–C(16),
3.10 A).25
2
In the 31P{1H} NMR spectrum, the large JPP value (280 Hz)
for 3b is consistent with
a trans-arrangement of the
phosphines. These complexes were also characterized by 1H
and 13C NMR and ESI-MS (see ESIw). The molecular
structure of 2b appears by its geometry to display the expected
apical Ni–Ar interaction, Fig. 2, although the distance
between Ni and Cipso (Ni–C(16), 3.15 A) is slightly longer
than in 2a.
Ethylene polymerization by 2a at 90 1C in toluene
gave polyethylene with moderate molecular weight
(Mn = 66000 g molꢀ1), Table 1 (entry 1). The low productivity
of this catalyst presumably arises from the low reactivity of the
Z3-allyl ligand. In addition, steric crowding of the Ni(II) center
may limit access to an incoming monomer, precluding opti-
mization using related [PO]Ni(R)L systems (R, alkyl, aryl; L,
labile ligand).
Ethylene polymerizations with 2b and 3b were conducted in
toluene, in which both complexes are readily soluble.27 The
results are summarized in Table 1. While 2b was unreactive at
room temperature and afforded only a trace of polymer at
90 1C, 3b produced high molecular weight polyethylene
We sought to address the issue of steric bulk with the design
of the novel o-phosphinoarenesulfonate ligand 1b, in which
Table 1 Ethylene polymerization resultsa
(Mn = 403 000 g molꢀ1) with good activity (1.8 ꢁ 104 TO hꢀ1
)
Entry Cat T/1C Yield/g 10ꢀ3 TOFf 10ꢀ3 Mn PDI Tmh/1C
g
at 25 1C. The high melting point of the polymer (136.1 1C) is
consistent with highly linear polyethylene.
1b
2b
3b
4b
5c
6b
7c
2a
2a
2b
3b
3b
90
25
90
25
90
0.14
0
0
5.31
24.2
10.1
8.64
0.26
—
—
18.4
251
36.1
95.6
66
—
2.10 133.6
When the reactor containing 3b was pressurized with
ethylene at an initial temperature of 60 1C, a large exotherm
(DT = +55 1C) was observed. When the reaction was
conducted at 90 1C, the temperature was stable (ꢂ3 1C), but
resulted in the formation of a large amount of low molecular
weight polyethylene (10 000 g molꢀ1, entry 5). While partly
due to the decreased solubility of ethylene at elevated
temperatures, this experiment implies a dramatic increase in
the rate of chain transfer with polymerization temperature,
which may be due in part to thermal disruption of the
aryl–Ni interaction. Similar decreases in molecular weight
—
—
—
—
—
403
10
440
6
1.87 136.1
2.25 118.1
1.89 136.9
6.74 127.1
3bd 25
3be 25
a
300 mL reactor, 20 mmol Ni (entries 1–3) or 10 mmol Ni (entries 4–7),
80 mL toluene, 400 psi ethylene added on demand. Polymerization
b
c
d
time 1 h. Polymerization time 20 min. With 1 equiv. Ni(COD)2.
e
f
equiv. B(C6F5)3. Turnover frequency
With
1
=
mol C2H4
polymerized (mol Ni)ꢀ1 hꢀ1
h
.
In g molꢀ1, determined by GPC.
g
Measured by DSC.
c
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Chem. Commun., 2011, 47, 6948–6950 6949

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with increased temperature have been reported for both
(a-diimine)nickel(II) complexes28 and Ni(II) catalysts with
chelating P,O ligands.29
1 E. Drent, R. van Dijk, R. van Ginkel, B. van Oort and R. I. Pugh,
Chem. Commun., 2002, 744–745.
2 S. Liu, S. Borkar, D. Newsham, H. Yennawar and A. Sen,
Organometallics, 2007, 26, 210–216.
The polymerization activity of 3b in the absence of a PPh3
scavenger is remarkable, compared to other [PO]NiPh(PPh3)
systems17,21 or SHOP-type Ni(II) catalysts,30 which show much
lower activities without a scavenger (o4000 TO hꢀ1). It
appears that the bulky ligand 1b promotes dissociation of
the Lewis base.
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Nevertheless, addition of one equiv. Ni(COD)2 to 3b
increased the productivity of the catalyst by a factor of two,
without affecting either Mn or the polydispersity index (PDI)
significantly. The latter remains unchanged at ca. 2 (compare
entries 4 and 6), and the high Tm value is consistent with highly
linear PE. The effect of the strong Lewis acid B(C6F5)3 on the
activity of 3b was even more pronounced, but resulted in low
molecular weight polyethylene with a high polydispersity index
(entry 7), suggesting the presence of multiple catalytically active
sites. A small-scale experiment in a NMR tube with 3b and a
stoichiometric amount of B(C6F5)3 revealed the formation
of a complex mixture of species, although the adduct
Ph3P–B(C6F5)3 was not detected.31
In sum, we report a single-component nickel(II) catalyst that
produces linear, high molecular weight polyethylene. Blocking
only one of the transition metal faces is sufficient to inhibit
associative displacement of the growing polymer chain by an
incoming monomer. Also, by analogy to recent work with
dialkyl(biaryl)phosphine ligands in Pd-catalyzed coupling
catalysts where metal–arene interactions are observed,32,33 it
is possible that a weak interaction between Ni and the aryl
17 R. J. Nowack, A. K. Hearley and B. Rieger, Z. Anorg. Allg. Chem.,
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903–908.
C
ipso (e.g., C16 in Fig. 2) contributes to the formation of high
molecular weight polyethylene. Most important, this work
points to new opportunities regarding [PO] ligand design.
We are currently exploring the reactivity of these complexes
in the copolymerization of ethylene with polar monomers.
The authors thank Dr Jerry Hu and Dr James Pavlovich
for their assistance with NMR and mass spectrometry, respec-
tively, and Obum Kwon and Prof. G. Bazan for their help with
GPC. This work was supported partially by the National
Science Foundation under Award No. CBET08-54425.
21 X. Zhou, S. Bontemps and R. F. Jordan, Organometallics, 2008,
27, 4821–4824.
22 S. Noda, T. Kochi and K. Nozaki, Organometallics, 2009, 28,
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25
(Ni and C van der Waals radii) = 3.33 A. Source http://www.
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27 20 mmol 2a or 3b dissolve readily in 5 mL toluene at 20 1C.
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Notes and references
z Crystal data for complex 2a + 2C6H6. C49H47NiO7PS, M = 869.6,
29 U. Klabunde and S. D. Ittel, J. Mol. Catal. A: Chem., 1987, 41,
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ꢀ
triclinic, space group P1, a = 11.165(7), b = 11.531(7), c = 17.445(11) A,
a = 72.330 (9)1, b = 90.065 (10)1, g = 83.002 (10)1, V = 2122(2) A3,
T = 150 K, Z = 2, F(000) = 912, reflections measured: 15 376, unique
reflections: 7172 [R(int) = 0.0414]. The final wR2 was 0.1936. Complex
2b + C6H6. C35H33NiO5PS, M = 655.35, monoclinic, space group
P2(1)/c, a = 12.848(4), b = 11.461(4), c = 22.104(7) A, b =
106.795(5)1, V = 3116.1(17) A3, T = 150 K, Z = 4, F(000) = 1368,
reflections measured: 24 608, unique reflections: 6423 [R(int) = 0.0827].
The final wR2 was 0.1313. CCDC 806742 and 806743.
30 P. Kuhn, D. Se
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c
6950 Chem. Commun., 2011, 47, 6948–6950
This journal is The Royal Society of Chemistry 2011