Mechanistic studies of catalytic polyethylene chain growth in the presence of water

Article abstract of DOI:10.1002/anie.200601387

Position blocked? Quantitative studies of polyethylene chain growth in the presence of water by variable-temperature (VT) ¹H NMR spectroscopy show, unexpectedly, that water can compete with monomer binding at the vacant coordination site in cat

Full text of DOI:10.1002/anie.200601387

Communications
Polymerization
DOI: 10.1002/anie.200601387
Mechanistic Studies of Catalytic Polyethylene
Chain Growth in the Presence of Water**
Andreas Berkefeld and Stefan Mecking*
Emulsion polymerization of unsaturated monomers is among
the most important polymerization processes. With this
method aqueous dispersions of surfactant-stabilized polymer
particles, with sizes of 50 nm to 1000 nm, are obtained. These
dispersions are employed broadly, for example, for environ-
mentally benign coatings. In comparison to traditional free-
radical techniques which are employed commercially,[1] the
synthesis of polymer dispersions by transition-metal-cata-
lyzed insertion polymerization is complimentary in terms of
the polymers accessible, and provides a much broader scope
of control of polymer microstructures.[2–5,8] Synthesis of
polyethylene dispersions of unusually small particle sizes
and defined microstructures has been studied recently.[2g–i]
These versatile transition-metal-catalyzed polymeri-
zations in aqueous systems raise the question of fundamental
underlying reaction steps, namely 1) the competition of
monomer and water for binding sites at the metal center
(that is, reversible blocking of active sites), 2) chain growth in
the presence of water, and 3) the reactivity of higher metal
À
alkyl species M R (R > Me) formed during polymerization
towards water in general. Recent theoretical studies indicate
that ethylene binding is preferred over water coordination in
various catalyst systems, namely neutral nickel salicylaldimi-
nato and cationic palladium or nickel diimine complexes.[6]
As the model system for experimental studies we chose
the cationic a-diimine palladium system studied by Brookhart
et al., because the palladium alkyl olefin resting states can be
detected conveniently by low-temperature NMR spectrosco-
py.[7,9,10] To enable quantitative studies in homogeneous
solution, [D8]THF was employed as a water-miscible solvent
with a low freezing point.
Addition of water to a solution of the diethyl ether
_
_
BAr4F = B(3,5-(CF3)2C6H3)4, NN = ArNC(Me)(Me)CNAr,
F
adduct[7] [(NN)PdMe(OEt2)][BAr4 ] ((1-OEt2), where
with Ar= 2,6-iPr2C6H3) in [D8]THF at À608C cleanly
afforded the water complex 1-OH2. The relative binding
constant of water versus [D8]THF was estimated to be
[*] Dipl.-Chem. A. Berkefeld, Prof. Dr. S. Mecking
Lehrstuhl für Chemische Materialwissenschaft
Fachbereich Chemie
Universität Konstanz
Universitätsstrasse 10, 78457 Konstanz (Germany)
Fax: (+49)7531-88-5152
E-mail: stefan.mecking@uni-konstanz.de
[**] Financial support by the DFG (Me1388/3-2) is gratefully acknowl-
edged. We thank U. Haunz for technical support, and H. Möller for
discussions on NMR spectroscopy.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
6044
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Chemie
K = 4 ” 104 (THF versus diethyl ether, K = 4). Key signals in
can be prepared cleanly in the NMR tube (Figure 2). Vice
versa, starting from 1-OEt2, the addition of water after the
1
the H NMR spectrum (12 equiv water,[13] À608C) are d =
_
6.32 (br, s, Pd(CH3)(OH2)), 2.31 and 2.27 (s, (H3C)CN, NN),
0.34 ppm (s, Pd(CH3)(OH2)) (for complete NMR data see the
Supporting Information).[11]
The resonance signal for excess free water occurs at d =
3.30 ppm. As the singlet signal of coordinated water is not
conclusive, the assignment was confirmed unambiguously by
an EXCY NMR exchange experiment on the signals of free
and coordinated water (see Supporting Information). This
observation of separate signals for free and coordinated water
shows that the exchange is slow on the NMR time scale. The
corresponding coalescence temperature, TC(OH2), was deter-
mined to be between 0 and 58C. This corresponds to a first-
order exchange rate constant kC(PdMe,OH2) on the order of
103 sÀ1.[12]
Exposure of 1-OH2 in [D8]THF to ethylene at À608C
resulted in its partial conversion into the known[7] olefin
complex 1-C2H4 (Scheme 1 and Figure 1). At temperatures
above À308C ethylene insertion occurs. Thus, the insertion of
ethylene in the presence of water can be detected and higher
_
alkyl water complexes [(NN)Pd(C2n+1H4n+3)(OH2)]+ (2-OH2)
Figure 2. Higher palladium alkyl water species, 2-OH2, detected by
1H NMR spectroscopy after consumption of 24 equiv of ethylene
(3.6 equiv water, À208C, [D8]THF).
consumption of a defined amount of ethylene also resulted in
the clean formation of complexes 2-OH2. Key 1H NMR
signals: (3.6 equiv water, À208C): d = 5.98 (br, s,
_
Pd(C2n+1H4n+3)(OH2)), 2.31 and 2.25 ppm (s, H3C)CN, NN).
Note that the signal of bound water (and also the diimine
backbone) in 2-OH2 are different from the corresponding
signals of the methyl complex 1-OH2 (Figure 1).
The water- versus olefin-binding equilibrium constants
(1-OH2+C2H4 Q 1-C2H4 + OH2, K1 and 2-OH2 + C2H4 Q
2-C2H4 + OH2, K2) were determined at various temperatures
(accompanied by olefin insertion at T> À308C) from integral
intensities of the four species involved (Scheme 1, Table 1).
The determination of the equilibrium constants K over a
range of ethylene and water concentrations demonstrated
that K is concentration independent (see Supporting Infor-
mation). K ranges from 1 to 4 depending on the temperature.
That is, on an order of magnitude scale ethylene and water
have a similar binding capability in the system studied;
ethylene binding is slightly preferred. This finding is surpris-
ing as theoretical studies[6] had pointed to a strong preference
for ethylene binding.[14] By comparison to other s-donor
ligands, such as diethyl ether or THF, water competes much
Scheme 1. Exchange of water versus ethylene monomer, andchain
growth in the presence of water.
Table 1: Relative binding constants (K of water versus ethylene
exchange) andrate constants of ethylene insertion ( ki).
T [8C]
K[a]
ki [”10À3 sÀ1
]
absence
of water
presence
(R=Me)
(R>Me)
of water[b]
À60
À20
À10
4.7Æ0.2
1.9Æ0.1
1.3Æ0.3
4.4Æ0.1
1.4Æ0.2
1.9Æ0.1
n.i.[c]
n.i.[c]
2.7Æ0.1
2.2Æ0.1
10.3Æ0.4
10.2Æ0.1
Figure 1. Palladium water and olefin species detected by 1H NMR
spectroscopy during chain growth (12 equiv ethylene, 3.6 equiv water,
À208C, [D8]THF).
[a] Binding equilibrium constants, see Supporting Information. [b] 4–
12 equiv water. [c] no insertion occurs.
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more effectively for coordination at the cationic palladium
[1] a) D. Distler, Wäßrige Polymerdispersionen, Wiley-VCH, Wein-
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Introduction, Academic Press, San Diego, 1997.
center. A reasonable explanation might be the low steric
hindrance of the water molecule. Notably, relative binding in
higher alkyl species (2-OH2 + C2H4 Q 2-C2H4 + OH2, K2)
does not differ significantly from that for the methyl complex.
The temperature dependence of the equilibria is low, as
expected; K decreases by a factor of 2 to 3 over a temperature
range of 50 K.
It is conceivable that the presence of water has an
influence on the migratory-insertion step, for example, as the
fifth ligand in five-coordinate intermediates (for example,
five-coordinate intermediates are assumed to be the key
species in chain transfer in the palladium diimine system[7,10]).
[2] Ethylene polymerization: a) A. Held, F. M. Bauers, S. Mecking,
Chem. Commun. 2000, 301 – 302; b) A. Tomov, J. P. Broyer, R.
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Mecking, Angew. Chem. 2001, 113, 3112 – 3115; Angew. Chem.
Int. Ed. 2001, 40, 3020 – 3022; d) R. Soula, C. Novat, A. Tomov,
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Chowdhry, S. Mecking, Macromolecules 2003, 36, 6711 – 6715;
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S. Mecking, J. Am. Chem. Soc. 2006, 128, 7708 – 7709.
À
Insertion into the Pd Me bond was monitored in the presence
À
and absence of water by the decrease of the Pd Me species
(see Supporting Information). First-order insertion rate
constants determined in the presence (3.6–12 equiv) or
absence of water, corrected for the water versus monomer
equilibrium, are identical within experimental error (Table 1).
This result confirms that water does not affect the crucial
migratory insertion step; further, that establishing the H2O
versus C2H4 binding equilibrium is fast compared to insertion
and thus not rate determining. A free enthalpy of DG° =
74 kJmolÀ1 was determined which coincides with a free
enthalpy of 73 kJmolÀ1 in CD2Cl2 solution reported previ-
[3] Butadiene polymerization: a) H. Ono, T. Kato, J. Polym. Sci.
Part A 2000, 38, 1083 – 1089; b) V. Monteil, A. Bastero, S.
Mecking, Macromolecules 2005, 38, 5393 – 5399.
[4] ROMP: a) S.-Y. Lu, P. Quayle, P. C. Booth, S. G. Yeates, J. C.
Padget, Polym. Int. 1993, 32, 1 – 4; b) I. Kühn, B. Mohr, Y.
Durant, R. Schwab, R. Leyrer (BASF), DE 19859191, 2000;
c) J. P. Claverie, S. Viala, V. Maurel, C. Novat, Macromolecules
2001, 34, 382 – 388; d) D. Quenemer, A. Chemtob, V. Heroguez,
Y. Gnanou, Polymer 2005, 46, 1067 – 1075; e) see also: D. M.
Lynn, S. Kanaoka, R. H. Grubbs, J. Am. Chem. Soc. 1996, 118,
784 – 790.
[5] Review: A. Held, F. M. Bauers, S. Mecking, Angew. Chem. 2002,
114, 564 – 582; Angew. Chem. Int. Ed. 2002, 41, 544 – 561.
[6] I. H. Hristov, R. L. DeKock, G. D. W. Anderson, I. Göttker-
Schnetmann, S. Mecking, T. Ziegler, Inorg. Chem. 2005, 44,
7806 – 7818.
ously.[7]
_
Alkyl complexes [(NN)Pd(C2n+1H4n+3)(OH2)]+ (2-OH2),
are stable at low temperatures (< 08C; Figure 2). At room
temperature, decomposition with the formation of a black
solid, presumably Pd-black, was observed. Remarkably, the
decomposition was accelerated in the simultaneous presence
of water and ethylene, even at low temperature. As a
conceivable pathway, b-hydride elimination seems unlikely,
as no olefin signals are observed in the NMR spectra. Possible
explanations are 1) hydrolysis of higher (secondary) alkyl
species,[6] 2) reductive coupling of the alkyl fragment to the
ligand, or 3) binuclear reductive alkyl-chain coupling.[15] In
contrast, the methyl complex, 1-OH2, was found to be stable
in aqueous [D8]THF solution for days at room temperature
[7] L. K. Johnson, C. M. Killian, M. Brookhart, J. Am. Chem. Soc.
1995, 117, 6414 – 6415.
[8] For an early work on ethylene polymerization with the cationic
__
__
complex[(N NN)RhMe(OH2)2]2+ (NNN = triazacyclononane)
in an aqueous system to afford a suspension of low-molecular-
weight polymer, see L. Wang, R. S. Lu, R. Bau, T. C. Flood, J.
Am. Chem. Soc. 1993, 115, 6999 – 7000.
[9] D. T. Tempel, L. K. Johnson, R. L. Huff, P. S. White, M.
Brookhart, J. Am. Chem. Soc. 2000, 122, 6686 – 6700.
[10] L. H. Shultz, D. J. Tempel, M. Brookhart, J. Am. Chem. Soc.
2001, 123, 11539 – 11555.
[11] NMR data in other solvents: A. Held, S. Mecking, Chem. Eur. J.
2000, 6, 4623 – 4629 and ref. [2a].
[12] H. Friebolin, Ein- und zweidimensionale NMR - Spektroskopie:
eine Einfꢀhrung, 3rd. ed., Wiley-VCH, Weinheim, 1999.
[13] Addition of less than 2 equiv of water gave rise to two singlet
signals for coordinated water in a 30:70 ratio (d = 6.055/
6.019 ppm, À608C; overall integral intensity of 2H relative to
À
(10 equiv excess water). No hydrolysis of the Pd Me bond
occurred.
In summary, we have reported the first direct observation
of ethylene migratory-insertion chain growth in the presence
_
of water. Higher alkyl species [(NN)PdR(OH2)]+ (R > Me)
were prepared cleanly. Surprisingly, water can compete with
ethylene for the vacant coordination site in the system
À
À
À
the Pd Me signal) but only one Pd Me signal was observed.
This finding might result from hydrogen bridges of coordinated
water (Pd(Me)(OH2)) to solvent (minor signal) and to water
molecules in solution (major signal). Increasing the amount of
excess water results in the stepwise disappearance of the signal
from the minor species.
studied. This situation is true for cationic Pd Me species
and also for higher palladium alkyl species, the latter being
the relevant species in chain growth. Insertion rate constants
are not affected by the presence of water.
Experimental data can be found in the Supporting
Information.
[14] The theoretical studies[6] were based on toluene as the solvent,
whereas the present studies were carried out in THF as the
solvent, which can form hydrogen bridges. This feature may
affect the coordination behavior of water.
Received: April 7, 2006
Published online: August 9, 2006
[15] A. C. Gottfried, M. Brookhart, Macromolecules 2003, 36, 3085 –
3100.
Keywords: chain growth · homogeneous catalysis · palladium ·
.
polymerization · water
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