Control of molecular weight in Ni(II)-catalyzed polymerization via the reaction medium

Article abstract of DOI:10.1039/b811888d

The reaction medium controls polymerization with highly active (κ²-P,O)-phosphinesulfonato nickel methyl complexes to afford polyethylenes ranging from low molecular weight (M_n) branched material to high molecular weight (M_n

Full text of DOI:10.1039/b811888d

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Control of molecular weight in Ni(II)-catalyzed polymerization
via the reaction mediumw
Damien Guironnet, Thomas Runzi, Inigo Gottker-Schnetmann and Stefan Mecking*
¨
¨
Received (in Cambridge, UK) 11th July 2008, Accepted 14th August 2008
First published as an Advance Article on the web 16th September 2008
DOI: 10.1039/b811888d
The reaction medium controls polymerization with highly active
(j2-P,O)-phosphinesulfonato nickel methyl complexes to afford
polyethylenes ranging from low molecular weight (Mn) branched
material to high molecular weight (Mn) strictly linear polymer.
Reaction of the phosphinesulfonic acid 2 with [(tmeda)-
NiMe2] (tmeda = N,N,N0,N0-tetramethylethylene diamine) at
ꢁ30 1C in THF resulted in expulsion of one Ni–Me group as
methane, and precipitation of 3 in quantitative yield (Scheme
1
1). H NMR spectra of 3 in dmso-d6 indicate a 2 : 1 ratio of
Ni-bound methyl groups to tmeda thus confirming the
dinuclear structure of 3 depicted.
Olefin polymerization by cationic complexes of d8 metals (late
transition metals) have been studied intensely in the last
decade.1 Due to their functional group tolerance, ethylene
and 1-olefins can be copolymerized with polar monomers such
as acrylates.2 Substantial and unique branching patterns can
be introduced in ethylene homopolymerization.3 These studies
prompted renewed interest in neutral nickel(II) ethylene poly-
merization catalysts.4,5 For example, by comparison to their
cationic Ni(II) counterparts, they are more tolerant towards
protic reagents such as water.6
The reaction of 2 with [(tmeda)NiMe2] in the presence of an
excess of pyridine afforded the mononuclear pyridine complex
4 in high yield (Scheme 1).z Crystals suitable for X-ray
diffraction analysis were grown from dichloromethane solu-
tion. The solid state structure of 4 exhibits a square planar
coordination geometry around the nickel center, the methyl
ligand being cis to the phosphine (Fig. 1).z This isomer
appears to prevail also in solution exclusively. A single
resonance is observed for the Ni–CH3 moiety in 1H NMR
In general, late transition metal alkyl complexes are prone
to b-hydride elimination. This promotes chain transfer in
catalytic olefin oligomerization and polymerization. In the
development of late transition metal polymerization catalysts,
substantial effort has been devoted to designing catalysts such
that chain transfer is prevailed by chain growth in order to
form high molecular weight polymer rather than oligomers or
dimers, e.g. by appropriate steric shielding of the metal
center.4 We report on a remarkable effect on polymer mole-
cular weight and microstructure for a given catalyst simply by
choice of the reaction medium (for a brief notation that with
Ni(II) phosphinoenolato complexes higher molecular weight
product is obtained in hexane vs. ethylene oligomerization in
toluene as a solvent cf. ref. 5a).
3
spectra (doublet, JPH = 7.6 Hz).
The polymerization of ethylene with 3 and 4 was studied
(Table 1). The complexes are highly active single site catalysts
for the polymerization of ethylene. In toluene they convert
ethylene to low molecular weight moderately branched poly-
ethylene (with 10–18 methyl branches per 1000 carbon atoms).
Activities exceed those reported for 1/phosphine scavenger by
an order of magnitude, in the absence of any scavenger (entries
1-1 and 1-13). The observed (1.5–4.5) ꢀ 105 TO hꢁ1 are among
the highest activities reported for neutral late transition metal
catalysts.4c,i,n The catalyst derived from 3 or 4 is compatible
with potentially coordinating solvents, e.g. THF and super-
critical CO2. Like in toluene, low molecular weight moderately
branched materials were obtained.
Neutral P^O-chelated Pd(II) phosphinosulfonato complexes
are versatile functional group-tolerant olefin polymerization
catalysts.7,8 Analogous Ni(II) complexes 1 [(P^O)NiPh(PPh3)]
(P^O = Ar2P–C6H4–SO3) were found to polymerize ethylene to
low molecular weight material (Mw = 103 to 4 ꢀ 103 g molꢁ1
)
with an activity up to 5 ꢀ 104 TO hꢁ1 upon activation with a
scavenger for PPh3.9 Our interest in neutral Ni(II) phosphino-
sulfonate complexes evolved from studies of catalytic polymeri-
zation in dense CO2.10
Chair of Chemical Materials Science, Dept. of Chemistry, University
of Konstanz, 78464 Konstanz, Germany.
E-mail: stefan.mecking@uni-konstanz.de; Fax: +49 7531 88-5152;
Tel: +49 7531 88-5151
w Electronic supplementary information (ESI) available: Complex
synthesis, polymerization procedures and crystal data of 4. CCDC
693181. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/b811888d
Scheme 1
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by following the ethylene uptake over time with a mass flow
meter). Unfortunately, GPC analysis of a number of samples
was hampered by a strong pressure increase on the columns,
indicative of very high molecular weight fractions. However,
NMR analysis confirms a high Mn of these samples, and no
endgroups are observable (Table 1).
Coordinating solvents can promote chain transfer.11 For
THF, toluene and scCO2 p- or s-donor coordination to the
Ni(II) center may be possible. However, polymerization in methyl
cyclohexane which is devoid of any donor functionality proceeds
with similar activities as in toluene and likewise results in low
molecular weight polyethylene (entry 1-13). Also, polymerization
in the potentially p-donating 1-octene affords high molecular
weight polyethylene with activities comparable to those in
heptane (entry 1-9). Remarkably, not even traces of 1-octene
are incorporated within the experimental accuracy of 13C NMR
analysis. A comparison of the polymerization experiments in the
aforementioned hydrocarbons also reveals no correlation of
polymer molecular weights observed with the Flory–Huggins
parameters of the polymer and respective solvent.12
Fig. 1 Molecular structure of 4. Ellipsoids are shown with 50%
probability. Hydrogen atoms are omitted for clarity.
Surprisingly, polymerization in heptane as a reaction
medium afforded strictly linear high molecular weight
polyethylene. Number average molecular weights exceed
105 g molꢁ1 (Table 1), even at high polymerization tempera-
tures which usually favor b-hydride transfer (entry 1-11 and
Fig. 2). At 90 1C in heptane, the catalyst is stable for hours
during polymerization (entries 1-10 and 1-11, also confirmed
Phenomenologically, the polymerization behaviour appears
to correlate with the solubility of the catalyst precursor in the
reaction medium, at the concentrations employed for poly-
merization studies. Insolubility (heptane, 1-octene) correlates
with formation of linear high molecular weight polymer. This
working hypothesis is underlined by polymerization with 5, an
analogue of 4 which however is soluble in heptane due to the
nonyl substituent on the pyridine ligand. Low molecular
weight branched polymer is formed with very high activities
(entry 1-17). Polymerization results in heptane–toluene solvent
mixtures strongly depend on the ratio of the solvents (entries
1-15 and 1-16). This also underlines that the polymer mole-
cular weight and microstructure formed rather correlates with
solvent properties than the chemical nature or mere presence
or absence of a solvent. A tentative explanation for these
findings is a multinuclear nature of the catalyst formed from
Fig. 2 GPC trace of polyethylene (entry 1-6).
Table 1 Polymerization resultsa
10ꢁ3Mn
(GPC)/
10ꢁ3Mn
(NMR)/ Tm/d
b
c
Av. TOF/
mol (C2H4)
Ni/
Entry Cat. mmol Solvent
P/
T/ t/
Polymer
Cryst.d Branchese/
(mol%) x1000 C
bar 1C min mol (Ni)ꢁ1 hꢁ1 yield/g
g molꢁ1 Mw/Mn g molꢁ1
1C
b
1-1
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
10
10
4
9
5
5
5
Toluene 40
H3CC6H11 40
scCO2
60
60
50
30
30
30
3.5 ꢀ 105
2.0 ꢀ 105
3.7 ꢀ 103
1.3 ꢀ 104
2.2 ꢀ 103
7.9 ꢀ 103
1.5 ꢀ 105
1.3 ꢀ 105
5.9 ꢀ 103
2.6 ꢀ 104
1.9 ꢀ 104
7.4 ꢀ 104
4.5 ꢀ 105
1.1 ꢀ 105
1.9 ꢀ 104
1.7 ꢀ 104
b
24.8
14.3
0.52
7.15
0.25
1.99
(0.5)
(0.8)
(1.8)
(1.7)
n.d.h
n.d.h
n.d.h
3.2
(1.9)
(1.7)
11.3
n.d.h
4.0
0.8
0.8
1.8
15
16
1-2
1-3f
1-4
n.d.h
127
136
135
135
56
59
68
52
7
o1
o1
o1
14
f
1-Octene
Heptane
Heptane
Toluene
Toluene
1-Octene
Heptane
Heptane
THF
40
40
40
40
40
40
40
40
40
50 135
n.d.h
n.d.h
101
415
415
415
0.8
0.8
415
1-5
1-6
1-7
1-8
90
70
70
90
90
90
60
60
60
30
30
30
21.4
(0.8)
(0.5)
71
9.06
0.41
1.83
14
1-9
o1
o1
n.d.
11
15
15
1-10
1-11
1-12
1-13
1-15
1-16
1-17
a
9
8
8
n.d.h
184
415
nd
90 160
12.0
137
95
90
90
56
—
—
—
56
90
90
90
90
90
30
10
12
60
60
8.27
9.41
4.78
2.61
3.83
(0.6)
(0.5)
(0.6)
135
(1.7)
(1.8)
(1.6)
2.9
0.8
0.8
0.9
4.5 H3CC6H11 40
8
5
8
H/Tg 70/30 40
H/Tg 80/20 40
415
0.7
138
o1
Heptane
40
(0.4)
(2.1)
18
Polymerization performed in a 250 mL steel reactor. Determined by GPC, referenced to linear PE. Determined by 1H NMR at
c
130 1C. From DSC. From 13C NMR at 130 1C, methyl branches observed exclusively. 5 g of ethylene added to 45 mL of CO2 at 30 MPa;
d
e
f
g
h
then compressed to 65 MPa. Heptane–toluene mixture (v/v). Analysis hampered by a strong pressure increase of the GPC columns, indicative
of very high molecular weight fractions.
ꢂc
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methyl complexes 3–5 are single component precursors to
highly active polymerization catalysts. Polymer molecular
weight and microstructure depend strongly on the reaction
medium employed. By appropriate choice very high molecular
weight polyethylene can be obtained, with catalysts which
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¨
That is, the large majority of polymer chains were rather short,
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We thank Lars Bolk for GPC, DSC and viscosimetry
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