Coordination polymerization of ethylene in water by Pd(II) and Ni(II) catalysts

Article abstract of DOI:10.1039/a908633a

Ethylene is polymerized in water as a reaction medium by Pd(II) and Ni(II) complexes to afford branched or linear homopolymer.

Full text of DOI:10.1039/a908633a

Coordination polymerization of ethylene in water by Pd(II) and Ni(II) catalysts
Anke Held, Florian M. Bauers and Stefan Mecking*
Institut für Makromolekulare Chemie und Freiburger Materialforschungszentrum der Albert-Ludwigs-Universität
Freiburg, Stefan-Meier-Str. 31, D-79104 Freiburg, Germany. E-mail: mecking@ruf.uni-freiburg.de
Received (in Cambridge, UK) 29th October 1999, Accepted 5th January 2000
Ethylene is polymerized in water as a reaction medium by
Pd(II) and Ni(II) complexes to afford branched or linear
homopolymer.
overall chain transfer rate will strongly influence product
molecular weight.
A very slow (ca. 1 turnover per day) coordination polymeri-
zation of ethylene in water catalyzed by a Rh complex has
previously been investigated.9 We now report on the homo-
polymerization of ethylene in water by neutral Ni(II) and
cationic Pd(II) complexes.
Water possesses unique properties as a reaction medium. It is
highly polar and immiscible with most organic compounds, has
a high heat capacity and also features a strong propensity for
micelle formation. In addition, water is an ideal medium from
an environmental and safety perspective. Thus, emulsion and
suspension polymerization of olefinic monomers is employed
on a vast scale, e.g. for the direct production of water-based
lattices, used for coatings and paints. In contrast to these free
radical polymerizations, transition metal catalyzed coordination
polymerization reactions in water have received less attention,
as the early transition metal catalysts1 used predominately are
extremely sensitive to moisture.
Late transition metal complexes are generally less sensitive to
polar media due to their less oxophilic nature. Due to the
propensity of late transition metal alkyl complexes for b-
hydride elimination, dimers or oligomers are usually obtained in
C–C linkage of ethylene.2 Only a limited number of catalysts for
polymerization to high molecular weight products are known.
Most of them are based either on neutral Ni(II) complexes3,14a of
formally monoanionic bidentate ligands or on cationic Fe, Co,
Ni or Pd complexes4 of neutral multidentate ligands with bulky
substituted N donor atoms.5 The aforementioned stability of late
transition metal complexes is demonstrated by the tolerance of
some of these polymerization catalysts towards polar function-
alized comonomers3d,4b,c and polar organic solvents.3c,d,4c,6
In the context of possible side reactions in transition metal
catalysis in aqueous media7 (such as hydrolysis of metal alkyl
species, attack of water on coordinated substrates or coordina-
tion of water to the metal center as a ligand), regarding
polymerization reactions a conceivable effect of water on chain
transfer8 is of specific interest as small (absolute) changes in the
Exposure of an aqueous suspension of Pd complex 14c to
ethylene at elevated pressure results in formation of high
molecular weight, highly branched polyethylene (Table 1). The
reaction is effective under mild conditions (7 bar, room
temperature), an increase of the ethylene pressure to 40 bar
resulting in a doubling of productivity (entry 1 vs. 3).
Comparison of 3 and 16 h experiments reveals no significant
deactivation of the cationic catalyst in the presence of water
(entry 2 vs. 3). In comparison to polymerization employing a
homogeneous solution of 1 in CH2Cl2 as a non-aqueous solvent
(entry 3 vs. 4), activities are similar. However, branching is
reduced in comparison to reaction in CH2Cl2 (69 to 83 branches
per 1000 C atoms in runs 1 to 3 vs. 109 branches per 1000 C in
1
run 4; determined by H NMR). Also, GPC analysis reveals a
much higher molecular weight of the polymers obtained in
water (Table 1). These findings correspond to the physical
appearance of the polymers: whereas the material obtained in
CH2Cl2 is a highly viscous oil, the polymers obtained in the
aqueous reaction medium are rubbery solids. In preliminary
Table 1 Polymerization results
Reaction conditions
Results
Productivity/ Average activity/
Entry
No.
n(cat.)/
Catalyst mmol
Ethylene
pressure/bar Reaction medium
Reaction Polymer mol(ethylene) mol(ethylene)
Mwa/g
Mna/g mol21
(Mw/Mn)
time/h
yield/g
mol(cat.)21
mol(cat.)21 h21 mol21
1
2
3
4
5
6
7
8
9
10
11
12
13
1
1
1
1
2ab
2ab
2ab
2ab
2bb
2bb
2ab
2ab
2bb
81
61
61
7
40
40
50
50
50
50
50
50
50
50
50
50
H2O
H2O
H2O
CH2Cl2
acetone+H2O 50+50
acetone+H2O 50+50
acetone+H2O 5+95
toluene+H2O 5+95
toluene+H2O 5+95
toluene+H2O 5+95
toluene
23
3
16
14
1.5
3
2
2
1.5
3
2
7.4
2.3
10.9
14.4
2.5
3.2
2.2
5.9
1.0
1.6
9.0
22.2
5.2
3 260
1 340
6 380
8 430
680
940
710
2 360
310
550
140
450
400
600
450
310
360
1 180
210
180
160 000
181 100
179 400
70 600 (2.3)
63 500 (2.8)
77 700 (2.3)
61
32 300
14 500 (2.2)
c
c
130
121
108
89
116
104
12
c
c
2 230
970 (2.3)
3 030
960 (3.1)
c
c
c
c
26 680
30 440d
20 600
13 340
15 220d
11 780
580 000
94 000
28 800
13 900 (42)
3 770 (25)
5 440 (5.3)
26
9
acetone
toluene
2
1.75
Reaction temperature: room temp. (entries 1 to 4), 70 °C (entries 5 to 13). Total volume of water and/or organic solvent: 100 mL.
a Determined vs. polystyrene (entry 1 to 4) resp. linear polyethylene (entry 5 to 13) standards. b Phosphine scavenger [Rh(CH2NCH2)2(acac)] (Ni+Rh
2+1). c Not determined. d Probably mass transfer limited.
Chem. Commun., 2000, 301–302
This journal is © The Royal Society of Chemistry 2000
301

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experiments, the water ligand of the cationic complex [(ArNNC-
polyethylenes were carried out by D. Lilge (BASF) and 31P
NMR analyses were provided by D. Hunkler (Freiburg).
(Me)C(Me)NNAr)PdMe(OH2)]+SbF6 (Ar = 2,6-iPr2C6H3)10
2
was found to be displaced completely upon addition of ethylene
in low temperature NMR experiments. This result implies that
in catalysis in aqueous media with cationic complexes of this
type no severe blocking of coordination sites by water should be
expected, in accordance with the above polymerization ex-
periments.
Notes and references
1 Ziegler Catalysts, ed. G. Fink, R. Mülhaupt and H. H. Brintzinger,
Springer, Berlin, 1995.
2 G. Wilke, Angew. Chem., Int. Ed. Engl, 1988, 27, 185; Angew. Chem.,
1988, 100, 189. M. Peuckert and W. Keim, Organometallics, 1983, 2,
594.
In ethylene polymerisation by a neutral Ni(II) complex with a
P,O ligand, introduction of a sulfonate substituent has been
reported to enhance formation of higher molecular weight
products (toluene as reaction medium).3c,d At the same time, the
sulfonate group affects water solubility. In order to produce
higher molecular weight polymer the absence of strongly
coordinating phosphine ligands is required.3d,11 For this reason
3 (a) W. Keim, R. Appel, A. Storeck, C. Krueger and R. Goddard, Angew.
Chem., Int. Ed. Engl., 1981, 20, 116; Angew. Chem., 1981, 93, 91. (b)
W. Keim, F. H. Kowaldt, R. Goddard and C. Krueger, Angew. Chem.,
Int. Ed. Engl., 1978, 17, 466; Angew. Chem., 1978, 90, 493. (c) K. A.
Ostoja-Starzewski and J. Witte, Angew. Chem., Int. Ed. Engl., 1987, 26,
63; Angew. Chem., 1987, 99, 76. (d) U. Klabunde and S. D. Ittel, J. Mol.
Catal., 1987, 41, 123. (e) C. Wang, S. Friedrich, T. R. Younkin, R. T.
Li, R. H. Grubbs, D. A. Bansleben and M. W. Day, Organometallics,
1998, 17, 3149. (f) L. K. Johnson, A. M. A. Bennett, S. D. Ittel, L. Wang,
A. Parthasarathy, E. Hauptman, R. D. Simpson, J. Feldman and E. B.
Coughlin (DuPont), WO98/30609, 1998.
4 Ni, Pd: (a) L. K. Johnson,, C. M. Killian and M. Brookhart, J. Am.
Chem. Soc., 1995, 117, 6414. (b) L. K. Johnson, S. Mecking and M.
Brookhart, J. Am. Chem. Soc., 1996, 118, 267. (c) S. Mecking, L. K.
Johnson, L. Wang and M. Brookhart, J. Am. Chem. Soc., 1998, 120, 888.
(d) L. K. Johnson, C. M. Killian, S. D. Arthur, J. Feldman, E. McCord,
S. J. McLain, K. A. Kreutzer, M. A. Bennett, E. B. Coughlin, S. D. Ittel,
A. Parthasarathy, D. Tempel and M. Brookhart (UNC-Chapel Hill/
DuPont) WO 96/23010, 1996. Co, Fe: (e) B. L. Small, M. Brookhart and
A. M. A. Bennett, J. Am. Chem. Soc., 1998, 120, 4049. (f) G. J. P.
Britovsek, V. Gibson, B. S. Kimberley, P. J. Maddox, S. J. McTavish,
G. A. Solan, A. J. P. White and D. J. Williams, Chem. Commun., 1998,
849.
5 A much larger variety of ligands is applicable for the particular case of
palladium-catalyzed alternating olefin–carbon monoxide copolymeriza-
tion as the involvement of CO can reduce the propensity for chain
transfer: E. Drent and P. H. M. Budzelaar, Chem. Rev., 1996, 96, 663;
A. Sen, Acc. Chem. Res., 1993, 26, 303. Alternating olefin–CO
copolymerization in aqueous media: Z. Jiang and A. Sen, Macromole-
cules, 1994, 27, 7215; G. Verspui, G. Papadogianakis and R. A.
Sheldon, Chem. Commun., 1998, 401; C. Bianchini, H. Man Lee, A.
Meli, S. Moneti, V. Patinec, G. Petrucci and F. Vizza, Macromolecules,
1999, 32, 3859.
6 Tolerance of the palladium diimine complexes towards moisture has
previously been noted, ref. 4b,c.
7 Aqueous-Phase Organometallic Chemistry, ed. B. Cornils and W. A.
Herrmann, Wiley-VCH, Weinheim, 1998.
8 E.g. the presence of additional ligands can promote chain transfer
reactions in ethylene oligomerization: W. Keim and F. H. Kowaldt,
Erdoel, Erdgas, Kohle/Compend.-Dtsch. Ges. Mineraloelwiss. Kohle-
chem., 1978, 78–79, 453.
9 L. Wang, R. S. Lu, R. Bau and T. C. Flood, J. Am. Chem. Soc., 1993,
115, 6999.
10 1H NMR (CD2Cl2, 300 MHz, 250 °C): 7.4–7.0 (m, 6H, Haryl), 4.6 (s,
2H, OH2), 2.90 and 2.86 (septet, J 7 Hz, 2 H, CHMe2 and CAHMe2), 2.18
[s, 6H, NNC(Me)C(Me)NN], 1.32, 1.26, 1.15 and 1.10 (d, J 7 Hz, 6H;
CHMeMeA and CAHMeMeA), 0.26 (s, 3H, PdMe).
11 For formation of polyethylene employing a PPh3 complex in hexane
suspension cf. ref. 3b.
we
have
employed
complex
2a,12,13
utilizing
[Rh(H2CNCH2)2(acac)] as a phosphine scavenger. For compar-
ison to water-soluble 2a with respect to aqueous polymerization
in the presence of a water-immiscible solvent (vide infra) the
novel complex 2b was prepared by oxidative addition of
2
+
4-MeC6H4C(O)C(SO3 H33C16NMe3 )NPPh3 to [Ni(cod)2] in
the presence of PPh3, or by addition to [Ni(PPh3)4].13
Introduction of the large H33C16NMe3+ cation results in a strong
increase in lipophilicity: whereas 2a dissolves in the aqueous
phase upon addition of water to a toluene solution of the
complex, 2b remains in the organic phase.
The stability of C–C linkage catalysts based on Ni complexes
with anionic bidentate ligands towards protic media, including
water, had been noted early on by the original inventors.14
However, successful polymerization to afford higher molecular
weight products in water has not been reported, to the best of our
knowledge. The complex [{k2P,O-Ph2PC(Ph)NC(OEt)O}NiPh-
(PEt3)] (removal of PEt3 by phosphine scavengers) was
reported to be completely inactive for ethylene polymerization
in organic media in the presence of 1000 eq. water.3d,15 For this
reason, we were somewhat surprised to observe formation of
linear polyethylenes employing catalysts 2a and 2b in an
aqueous environment, utilizing only a small amount of water-
miscible (acetone) or -immiscible (toluene) organic solvent to
enable injection of the phosphine scavenger or the scavenger
and 2b, respectively (Table 1, entries 7 to 10). By comparison to
polymerization in neat toluene or acetone16 (entries 11 to 13),
polymer molecular weight is significantly reduced and pro-
ductivity is lowered in aqueous media. From the data presented,
no dramatic effect of water on chain transfer reactions is
evident, the lowering of polymer molecular weight also being
attributable to a slower chain growth (as reflected by the lower
productivity) caused by the lower solubility of ethylene in
water. Considering catalyst stability, comparison of entries 5 vs.
6 and 9 vs. 10 shows that the catalysts are still active for
polymerization after several hours in water. A preliminary
comparison of the hydrophilic 2a and the lipophilic 2b in the
multiphase system water/toluene/insoluble polymer (entries 8
and 9) reveals that phosphine abstraction from 2a is not
significantly hampered by the different solubilities of 2a and
[Rh(H2CNCH2)2(acac)].
12 D. L. Beach and J. J. Harrison (Gulf), Eur. Pat. A 52929, 1982.
13 Characteristic NMR data (13C{1H}, 75 MHz; 31P{1H}, 202 MHz),
ligands 4-MeC6H4C(O)C(SO32M+)NPPh3. M+ = H33C16NMe3+: NMR
(CDCl3), 13C: d 188.5 [d, 2J(C,P) 6 Hz, CNO], 81.0 [d, 1J(C,P) 104 Hz,
CNP], 52.2 (H33C16NMe3+). 31P: d 16.8. M+ = Na+: NMR (CDCl3), 13C:
Interestingly, when performing the Ni(II)-catalyzed polymer-
ization in the presence of ionic or non-ionic surfactants (SDS,
Triton X-100), stable polyethylene emulsions are obtained.
Typically, emulsions with particle sizes in the range of f 80 to
300 nm are obtained at a catalyst productivity of, e.g. 1300
mol(ethylene) per mol(Ni) (conditions of run 7, surfactant
added).
In conclusion, high molecular weight polymers can result
from the coordination polymerizaton of ethylene in water at
high catalyst activities. Branched or linear polymers are
accessible in water as a reaction medium.
The authors thank R. Mülhaupt for his interest in our work.
Financial support by BASF AG is gratefully acknowledged, and
we thank B. Manders and M. O. Kristen for valuable
discussions. A.H. thanks the Deutsche Forschungsgemeinschaft
for a Graduiertenkolleg stipend. A generous loan of PdCl2 was
provided by Degussa-Huels AG. GPC analyses of linear
d 191.5 (CNO), 81.3 [d, 1J(C,P) 113 Hz, CNP]. 31P: d 16.7. 2b NMR 13
C
(toluene-d8): d 187.1 [dd, 2J(C,P) 23 Hz, 3J(C,P) 8 Hz, C–O], 104.7 [d,
1J(C,P) 44 Hz; NC(SO32)P], 52.7 (H33C16NMe3+). 31P (C6D6): d 38.3
[d, 2J(P,P) 272 Hz, NC(SO32)P], 21.0 [d, 2J(P,P) = 272 Hz, PPh3]. 2a
NMR (C6D6), 13C: d 187.9 (br, C–O), 101.2 [d, 1J(C,P) 42 Hz,
NC(SO32)P]. 31P: d 35.4 [d, 2J(P,P) 276 Hz, NC(SO32)P], 21.0 [d,
2J(P,P) 276 Hz, PPh3].
14 (a) R. Bauer, H. Chung, G. Cannell, W. Keim and H. van Zwet (Shell),
US Pat. 3637636, 1972. (b) R. Bauer, H. Chung, K. W. Barnett, P. W.
Glockner and W. Keim (Shell), US Pat. 3686159, 1972.
15 Polymerization in the presence of ‘up to 100 eq.’ of water/Ni for a
neutral nickel complex with a chelating N,O-ligand has recently been
claimed: D. A. Bansleben, S. Friedrich, T. R. Younkin, R. H. Grubbs, C.
Wang and R. T. Li (W. R. Grace), WO 98/42664, 1998.
16 The broad molecular weight distributions observed in entries 11 and 12
are in agreement with previous studies: ref. 3d.
Communication a908633a
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Chem. Commun., 2000, 301–302