A ruthenium catalyst yielding crosslinked polyethylene

Article abstract of DOI:10.1039/c1cc11677k

A ruthenium phosphane aryl sulfonate was found to be an efficient catalyst for the polymerization of ethene. Surprisingly, the resulting polyethylene is crosslinked.

Full text of DOI:10.1039/c1cc11677k

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COMMUNICATION
A ruthenium catalyst yielding crosslinked polyethylenew
Laurence Piche, Jean-Christophe Daigle and Jerome P. Claverie*
Received 23rd March 2011, Accepted 18th May 2011
DOI: 10.1039/c1cc11677k
A ruthenium phosphane aryl sulfonate was found to be an
efficient catalyst for the polymerization of ethene. Surprisingly,
the resulting polyethylene is crosslinked.
More than 50 years after the discovery of Ziegler and Natta
Scheme 1 Ruthenium phosphane aryl sulfonate complexes.
on olefin polymerization, the quest for insertion-coordination
catalysts capable of providing extended control over the polymer
structure is still ongoing.1 Olefin polymerization catalyzed by
late transition metals has attracted much interest and opened
the way to a myriad of new catalysts.2 These catalysts are
not only more robust towards impurities, but also allow the
copolymerization of ethene with polar monomers. The majority
of those catalysts are based on Ni and Pd, but a major advance
was reported by Brookhart et al.3 and Gibson et al.4
who simultaneously discovered very active bis(imino)pyridine
Fe and Co precatalysts which can polymerize ethene in the
presence of methylaluminoxane (MAO) with activities comparable
to the Ziegler–Natta system. By contrast to Ni, Pd, Fe and Co,
the reactivity of Ru complexes in insertion polymerization has
hardly been scrutinized. Nomura et al. have reported that Ru
complexes bearing bis(oxazoline)pyridine (pybox) ligands
show very low activity for the polymerization of ethene upon
activation with MAO (TON = 75 mol ethene per mol Ru).5
Dias et al. have demonstrated that bis(imino)pyridine Ru
cationic alkyl complexes show no activity towards ethene
insertion.6
Fig. 1 MALDI TOF mass spectrum of
1 (anthracene matrix,
dichloromethane). Insets show isotope patterns for the molecular ion
(top: simulated, bottom: observed).
catalytic regioselective allylation reactions.10 We report herein
a Ru phosphane aryl sulfonate complex that is active for the
preparation of high-density polyethylene.
Aryl sulfonate phosphane complex 1 was prepared in 87% yield
by reaction of two equivalents of the aryl sulfonate phosphane
ligand with RuCl2(PPh3)3 in dichloromethane (Scheme 1). The
31P NMR of the reaction mixture showed the disappearance of the
signals corresponding to the starting materials and appearance of a
new singlet at 27 ppm implying a symmetrical structure. Analogous
reactions with RuCl3, [RuCl2(COD)]n, or RuCl2(DMSO)4 as
starting material generated again complex 1 but in lower yields.
The elemental analysis of this orange compound agrees with the
formula Ru(–PPh2–C6H4–SO3À)2. Thus, 1 is a dimeric or polymeric
complex whereby bridging occurs via sulfonate units, as already
reported in the case of Pd.11 Further evidence for the structure was
suggested from a matrix-assisted laser desorption/ionization time of
flight MALDI-TOF MS study of this compound in dichloro-
methane (Fig. 1).12 In this case, only the monomeric form of 1 is
detected.
Since ruthenium possesses excellent functional group tolerance
in olefin metathesis and has low moisture and air-sensitivities,7
while being approximately one-third the price of palladium per
mole, it would be very advantageous to develop Ru polymeri-
zation catalysts containing phosphane aryl sulfonate ligands
with selectivities identical to those encountered with Pd. Indeed,
phosphane aryl sulfonate Pd complexes exhibit unprecedented
versatility, permitting the copolymerization of ethene with a
wide variety of polar olefins.8,9 To our knowledge, phosphane
aryl sulfonate Ru polymerization catalysts have never been
reported, but recently it has been shown that Ru(IV) complexes
bearing phosphane aryl sulfonate ligands can be used for
Complex 1 reacts in the presence of water to form the well-
defined mononuclear species 2 which can be crystallized from
a layered acetone–water (1 : 1) solution (Fig. 2). The structure
can be described as a distorted octahedron with the PRuP
angle being significantly larger than 901 and O7–Ru–O8 angle
being smaller than 901, probably as a consequence of the bulk
of the aryl phosphane groups. Accordingly, bulkier o-sulfonated
phosphanes Ar2PC6H4SO3H where Ar = Ph(o-OMe) or
Ar = Ph[o-C6H3(2,6-OMe)2] only reacted in less than 10%
Quebec Center for Functional Materials-NanoQAM,
Department of Chemistry, UQAM, Succ Centre Ville – CP 8888,
Montreal, Qc, H3C3P8, Canada. E-mail: claverie.jerome@uqam.ca;
Fax: +1 514 987 4054
w Electronic supplementary information (ESI) available: Experimental
and crystallographic details. CCDC 809322. For ESI and crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/
c1cc11677k
c
7836 Chem. Commun., 2011, 47, 7836–7838
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65 1C (run 1) at higher ethene pressure (comparing runs 2–4
to runs 5–8). Above 95 1C, catalyst activity decreases
(runs 10–12), due to catalyst decomposition. Overall, the
polymerizations were sluggish. After 30 minutes (run 9) only
a minute amount of polymer was isolated. Consequently,
polymerizations were conducted overnight. Control experiments
involving Ru complex 1 without MAO (run 13), 1 with an
aluminium alkyl (run 14) and ligand + MAO without any Ru
source (run 15) only yielded trace amounts of solid material.
Interestingly, polymerizations performed with precatalysts
1 or 2 gave similar results, both in terms of activity and
polymer microstructure, and these two compounds were used
interchangeably. These precatalysts are also very stable, and
can even be stored in air at room temperature.
Fig. 2 Thermal ellipsoids diagram (50%) of 2. All hydrogen atoms
and solvent are omitted for clarity. Selected bond distances (A) and
angles (1): Ru–P1 2.272(5), Ru–P2 2.281(5), Ru–O1 2.126(2),
Ru–O4 2.176(1); O(1)–Ru–O(8) 168.91(6), P(1)–Ru–P(2) 98.57 (2),
P(1)–Ru–O(4) 171.07(4), O(1)–Ru–P(1) 92.36(4), O(8)–Ru–O(7)
84.18(6), O(1)–Ru–P(2) 93.27(4).
Most surprisingly, the resulting polymer does not appear to
be fully soluble, even when gently stirred at 160 1C in
trichlorobenzene, TCB, for 24 hours, conditions under which
even ultra high molecular weight polyethylene is solubilized.14
Evidence that the solid collected is indeed polyethylene stems
from solid-state 13C NMR using magic-angle spinning (MAS)
cross-polarization (CP) and direct polarization (DP) sequences
(Fig. 3). The main resonance corresponds to crystalline poly-
ethylene, whereas the small one convoluted in the main peak
corresponds to amorphous polyethylene.15 Importantly, no
other peak could be detected by solid-state NMR, indicating
that the polymer is ‘linear’ polyethylene. This is also confirmed
by the presence of a high melting point (136 1C) as measured
by differential scanning calorimetry (DSC).
yield with RuCl2(PPh3)3 to form analogous Ru complexes.
This is consistent with the important steric strain generated by
the two chelating phosphanes in an octahedral environment.
The six membered cycles Ru(1)–P(1)–C(1)QC(2)–S(1)–O(1)–
and Ru(1)–P(2)–C(19)QC(20)–S(2)–O(4)– adopt a half-boat
conformation, with C(25) C(13) O(3) and O(5) in pseudo axial
positions and C(31) C(7) O(2) and O(6) in pseudo equatorial
positions. This half boat conformation has been reported for
the majority of aryl sulfonate catalysts.9c,e,h,13 It is surprising
that the phosphane moieties are cis to each other as these
ligands with a strong trans influence are expected to prefer trans
positions with respect to each other.
Since the polymer is not totally soluble, we have decided
to measure its gel content (portion of polymer insoluble in
the solvent) and its degree of swelling (which is the volume
fraction of the swollen gel divided by the volume fraction of
the dried gel). Analysis by high-temperature GPC has indicated
that less than 20% of each polymer sample is soluble at 160 1C in
TCB (based on integration by differential refractometer). Thus,
gel content is higher than 80% for each sample. The degree of
swelling in xylene at 125 1C after 48 hours was 15, 12 and 18,
respectively, for polymers from runs 1, 4 and 6, corresponding to
a molecular weight of 104 to 4 Â 104 g molÀ1 between two
crosslinks. Therefore, the polymer is only slightly crosslinked.
Only for runs 1 and 6, the soluble fraction of the polymer
was in sufficient amount to be analyzed by GPC. By comparing
The results of ethene polymerization are consigned in Table 1.
The optimal conditions for polymerization were found to be
Table 1 Ethene polymerization by Ru catalystsa
TON/
molE
per molRu
Run
no.
Complex/
mmol
Cocatalystb
[Al/Ru]
Ethene/
atm
T/1C
1
2
3
4
5
6
7
8
1 (5)
2 (5)
2 (5)
1 (5)
1 (6)
1 (6)
2 (5)
2 (6)
1 (10)
1 (10)
1 (10)
1 (10)
2 (5)
2 (5)
(5)f
1600
300
300
300
150
300
900
1300
800
800
800
800
0
65c
50
80
95
65
65
65
65
80
80
110
125
65
65
65
80
65
20
20
20
20
7.5
7.5
7.5
7.5
20
20
20
20
3.4
14
3.4
20
20
2270
560
696
505
316
392
255
239
32d
403e
122
54
—
—
—
1110
340
9
10
11
12
13
14b
15
16g
17h
300
300
300
150
2 (5)
2 (5)
a
b
Reaction conditions: solvent = toluene (50 mL) overnight. Molar
ratio of Al/Ru, cocatalyst is MAO except for run 14, for which it
c
is AlEt3. An important exotherm was observed. Reaction time =
d
e
f
30 minutes. Reaction time = 180 minutes. 5 mmol of ligand was
used instead of complex 1. 2.5 mL of hexene was added. 2.5 g of
norbornene was added.
g
h
Fig. 3 13C CP/MAS (above) and 13C DP/MAS (below) Solid State
NMR spectra of polyethylene (run 1).
c
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the number average degree of polymerization (Xn = 21 800 for
run 1, 12 400 for run 6) to the ethene to catalyst ratio (2270 for
run 1, 316 for run 6), it is clear that only a small fraction of the
Ru is active. Interestingly, the soluble portion of run 1,
analyzed by triple detection GPC has a high molecular weight
(Mn = 610 000 g molÀ1, Mw/Mn = 1.3). The Mark-Houwink
plot is linear, with a slope of 0.75 which corresponds to the
plot of a polyethylene devoid of short branches. The absence
of branches on the soluble portion of the polymer is consistent
with the fact that the polymer is only slightly crosslinked.
that Ru is suitable for ROMP only (even RuCl3ÁxH2O is an
efficient catalyst for the ROMP of norbornene!18) but our
finding offers a contrasted view of the reactivity of Ru
complexes.
This work was supported by NSERC (Discovery). LP
and JCD thank NSERC and CSMO-caoutchouc-AEQ for
financial support (individual PhD fellowships). We thank
Prof. D. Zargarian for insightful discussions.
Notes and references
When a preformed linear polyethylene sample (Mn
=
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system (1 and 200 eq MAO, 80 1C overnight) in the absence of
ethene, its molecular weight distribution remains unchanged,
indicating that crosslinking occurs concomitantly to the forma-
tion of the polymer. Attempts to isolate an uncrosslinked
polymer in the early stages of the reaction failed. When a polymeri-
zation was stopped after 30 minutes (run 9), the amount of
polymer was insufficient for solubility testing and GPC analysis,
but after 3 hour reaction time (run 10), the polymer was already
found to be crosslinked.
One possibility to account for crosslinking is that the
initiation step yields a growing chain metallated at both ends.
After successive insertions and b-H elimination, an a–o olefin
is obtained. Reinsertion of this difunctional macromonomer
could then be responsible for the presence of crosslinks. Such a
mechanism would entail that a-olefins are incorporated by the
Ru catalysts. Copolymerization of ethene and hexene catalyzed
by 1 generates again an insoluble copolymer with a melting
point of 88 1C, as measured by DSC, indicating that this
catalyst is capable of inserting hexene, however, we were not
able to homopolymerize hexene with these catalysts. Although
the suggested crosslinking mechanism is probable (but difficult
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To conclude, we found the first example of a significantly
active homogeneous ruthenium catalyst for the polymerization
of ethene. Based on the analysis of the soluble fraction of the
polymer, the catalyst generates high molecular weight poly-
ethylene devoid of short branches. However, this polyethylene
is crosslinked. This surprising finding has interesting repercussions
for the preparation of novel polyolefins with unusual macro-
molecular architectures17 and more importantly it addresses
new mechanistic avenues concerning the reactivity of alkyl Ru
complexes. In polymerization catalysis, it is often considered
14 A commercial sample of Gur 4022 from Ticona (Mn E 5 Â 106 g molÀ1
)
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c
7838 Chem. Commun., 2011, 47, 7836–7838
This journal is The Royal Society of Chemistry 2011