Nickel(II) α-diimine catalysts for the atom transfer radical polymerization of styrene

Article abstract of DOI:10.1016/j.ica.2006.06.026

The synthesis and evaluation of ^Cy[N,N]NiX₂ complexes (where ^Cy[N,N] = C₆H₁₁{single bond}N{double bond, long}CH{single bond}CH{double bond, long}N{single bond}C₆H₁₁; X = Cl, Br)

Full text of DOI:10.1016/j.ica.2006.06.026

Inorganica Chimica Acta 359 (2006) 4417–4420
www.elsevier.com/locate/ica
Note
Nickel(II) a-diimine catalysts for the atom transfer
radical polymerization of styrene
*
Rachel K. O’Reilly, Michael P. Shaver, Vernon C. Gibson
Department of Chemistry, Imperial College, Exhibition Road, South Kensington, London SW7 2AZ, United Kingdom
Received 31 May 2006; accepted 11 June 2006
Available online 22 June 2006
Abstract
The synthesis and evaluation of Cy[N,N]NiX2 complexes (where Cy[N,N] = C6H11AN@CHACH@NAC6H11; X = Cl, Br) as catalysts
for atom transfer radical polymerization are reported. Cy[N,N]NiCl2 offers poor control over the polymerization of MMA and styrene
due to catalyst insolubility. The more soluble bromo catalyst Cy[N,N]NiBr2, promotes rapid styrene polymerization, but with inefficient
initiation, affording higher than expected molecular weights based on [M]o/[I]o ratios. Utilizing 1-PEBr results in efficient initiation to
give low polydispersities (Mw/Mn ꢀ 1.2) and polystyrene molecular weights that correlate with monomer:initiator ratios.
ꢀ 2006 Elsevier B.V. All rights reserved.
Keywords: a-Diimines; Polymerization; Styrene; Nickel; ATRP
1. Introduction
MMA via an ATRP mechanism [7]. Soon after, Sawamoto
reported that commercially available NiBr2(PPh3)2 (2) could
be used to induce the living radical polymerization of MMA
in conjunction with CCl3Br initiator and a Lewis acid
activator [8]. In bulk, or at high monomer concentrations,
it was subsequently shown that a Lewis acid activator was
not required [9]. This reaction was improved again by using
the more thermally robust and soluble NiBr2-
(PnBu3)2 (3) for the ATRP of MMA and MA [10]. Most
recently, a nickel catalyst supported on a cross-linked triphe-
nylphosphine-functionalized polystyrene resin has been
used to facilitate catalyst removal from PMMA [11].
The controlled polymerization of styrene by nickel cata-
lysts has proved more challenging. While a combination of
NiCl2, tetraethylthiuram disulfide and PPh3 was shown to
generate polystyrene via a reverse ATRP protocol [12],
only moderate control over the polymerization was
observed, with molecular weights of the resultant polysty-
rene higher than expected from [M]o/[I]o ratios; polydisper-
sities ranged from 1.34 to 1.60. To date, there has been no
report of the ATRP of styrene using nickel catalysts. Here,
we describe our studies on nickel complexes containing
a simple a-diimine ligand which show selectivity for the
controlled polymerization of styrene.
The field of atom transfer radical polymerization
(ATRP) [1] continues to grow since the initial reports by
Matyjaszewski [2] and Sawamoto [3]. An important quest
has been to understand the role of the metal and ligands
in facilitating efficient halogen atom transfer processes, as
well as exploring new metal–ligand combinations. Our
work in this field has focussed on iron catalysts supported
by a-diimine ligands, where we have shown that iron(II)
complexes of N-alkyl diimines are good ATRP catalysts
for the polymerization of styrene [4] and methyl methacry-
late (MMA) [5], while catalysts bearing aryl substituents at
nitrogen or attached to the 2,3 positions of the ligand back-
bone switch the polymerization mechanism to catalytic
chain transfer [6].
Many metals with accessible one-electron redox couples
have been exploited as catalysts for ATRP. In 1996, Teyssie
and co-workers reported that the nickel(II) complex, [Ni{o,
o0-(Me2NCH2)2C6H3}Br] (1) can be used to polymerize
´
*
Corresponding author. Tel.: +44 20 7594 5830; fax: +44 20 7594 5810.
E-mail address: v.gibson@imperial.ac.uk (V.C. Gibson).
0020-1693/$ - see front matter ꢀ 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2006.06.026

4418
R.K. O’Reilly et al. / Inorganica Chimica Acta 359 (2006) 4417–4420
4 (X = Cl) as a fine green solid (91%) and 5 (X = Br) as
a yellow micro-crystalline solid (81%). C14H24N2Cl2Ni (4)
requires C, 48.0; H, 6.9; N, 8.0. Found: C, 47.8; H, 6.7;
N, 8.0%. IR (cmꢁ1) (C@N) 1633 m; MS(FAB+)(m/z) 313
[M ꢁ Cl]+, leff = 3.2 BM. C14H24N2Br2Ni (5) requires C,
38.2; H, 5.4; N, 6.3. Found: C, 38.0; H, 4.9; N, 6.0%. IR
(cmꢁ1) (C@N) 1624 m; MS(FAB+)(m/z) 357 [M ꢁ Br]+;
leff = 3.1 BM.
2.3. Polymerization of styrene
2. Experimental
All polymerizations were set up and performed under an
atmosphere of oxygen-free, dry nitrogen using standard
Schlenk-line and glove-box techniques. To an ampoule
equipped with a magnetic stirrer bar was added a solution
of monomer (styrene; 200 eq.), initiator (1-phenylethyl
chloride, 1-phenylethyl bromide; 1 eq.), and catalyst
2.1. General
Complexes 4 and 5 were handled by means of standard
high vacuum Schlenk and cannula techniques. Air sensitive
compounds were transferred to a nitrogen filled glove-box
and, unless stated otherwise, stored at room temperature.
Infra-red spectra were obtained with thin sample films on
NaCl plates on a Perkin–Elmer 1710X FT-IR spectrome-
ter. Mass spectra were recorded on either a VG Autospec
or a VG Platform II spectrometer. Elemental analyses were
performed by the microanalytical services of the Depart-
ment of Chemistry at London Metropolitan University.
Magnetic susceptibilities were determined by the Evans
NMR method [13]. For solubility purposes, measurements
were recorded in 90:10 d7-DMF/cyclohexane for 4 and
90:10 d8-toluene/cyclohexane for 5. Gel permeation chro-
matography (GPC) was performed using Viscotek Trisec
software connected to a Knauer differential refractometer.
Samples were injected onto two linear 10 lm columns using
chloroform as eluent at a flow rate of 1.0 cm3minꢁ1 at
room temperature. Cyclic voltammetry (CV) was per-
formed using a MacLab potentiostat operated by EChem
1.3.2 software. The working electrode and reference elec-
trode were purchased from Bioanalytical (ref: MF-2013
and RE-5B). Reagents were obtained from Aldrich Chem-
ical Co. and purified by standard procedures unless other-
wise stated.
(
Cy[N,N]NiX2; 1 eq.) and then sealed. The ampoules were
placed in a preheated sand bath at 120 ꢁC for an allotted
period of time. Aliquots (0.3 mL) were removed at appro-
priate intervals. Conversion was determined by integration
of the monomer versus polymer backbone resonances in the
1H NMR spectrum of the crude mixture in CDCl3. Polymer
was dissolved in THF and precipitated into acidified meth-
anol, filtered and washed with methanol. The precipitate
was collected and dried for 24 h under vacuum. Samples
were then analyzed by GPC versus styrene standards (Poly-
mer Laboratories EasiCal PS1). Catalysts, monomers and
initiators were dried and purified prior to use.
3. Results and discussion
The nickel(II) complexes Cy[N,N]NiX2(Cy[N,N] = C6H11
AN@CHACH@NAC6H11) (X = Cl, 4; Br, 5) were pre-
pared by treatment of NiX2(dme) with Cy[N,N] according
to the procedure outlined in Scheme 1. Complex 4 was iso-
lated as a green solid in ca. 90% yield; magnetic susceptibil-
ity measurements showed it to be paramagnetic, indicative
of a tetrahedral geometry (leff = 3.2, d8-high spin, S = 1).
The low solubility of 4 in traditional solvents necessitated
the use of dimethylformamide as a solvent for Evans’
NMR measurements. In an analogous procedure, 5 was
isolated as a yellow solid in ca. 80% yield, magnetic suscep-
tibility measurements (leff = 3.1) again being supportive of
a tetrahedral geometry. The bromo derivative, 5, is found
to be substantially more soluble than 4 in hydrocarbon sol-
vents; toluene was employed for magnetic susceptibility
studies.
2.2. Synthesis of 4 and 5
Dichloromethane (ca. 40 mL) was added to NiX2(DME)
(0.006 mol) and Cy[N,N], Cy-N@CHACH@N-Cy [14]
(1.55 g, 0.007 mol) in a Schlenk vessel. After stirring at
40 ꢁC for 12–24 h, a precipitate formed which was isolated
by filtration. The solid was washed with portions of Et2O
(3 · 20 mL) at 0 ꢁC to remove excess ligand and afforded
Scheme 1. Synthesis of complexes 4 and 5.

R.K. O’Reilly et al. / Inorganica Chimica Acta 359 (2006) 4417–4420
4419
Fig. 2. Plot of Mn vs. conversion (Mw/Mn in parentheses) for styrene
polymerization using 5/1-PECl.
Fig. 1. CV traces for 5 at different scan rates.
Since ATRP depends on a single electron redox couple,
electrochemical techniques provide useful insight into
which complexes should be suitable catalysts for ATRP.
The CV data for 5 (Fig. 1) indicate a reversible and acces-
sible redox couple (E1/2 ꢁ500 and DEp 130 mV) that is sur-
prisingly easier to oxidize than its iron(II) analogue (E1/2
ꢁ120 and DEp 130 mV) [4]. The electrochemical viability
of these catalysts for ATRP thus became apparent.
Initially, 4 was tested for the polymerization of styrene
(200 eq.) using 1-PECl as an initiator at 120 ꢁC. However,
the insolubility of 4 in styrene led to uncontrolled polymer-
izations, with broad polydispersities (>2.5) and molecular
weights much higher than predicted on the basis of mono-
mer:initiator ratio. The polymerization was rapid, and
PDIs and molecular weights were consistent with a thermal
polymerization. Addition of 5% DMF in an attempt to
improve solubility led to catalyst decomposition. Complex
4 was also incompatible or insoluble in both isopropanol
and dichlorobenzene, solvents traditionally used as polar
additives to improve solubility and activity [15,16]. Con-
trastingly, the polymerization of styrene using 5 under
identical conditions proceeded in a much more controlled
fashion with a kobs of 0.31 hꢁ1 and Mn of 21900
(Mn,cal = 18900), Mw/Mn = 1.38. This is markedly faster
than that found for the analogous, and previously reported
[4], iron(II) complex (kobs = 0.25 hꢁ1) and is consistent
with the more favourable oxidation potential of the nickel
catalyst. Halide end-groups were observed at ca. d 4.5 in
the 1H NMR spectrum of the resultant polystyrene
strongly indicating an ATRP mechanism.
Fig. 3. Plot of Mn vs. conversion for 5 initiated with 1-PEBr. Selected
values of Mw/Mn are given in parenthesis.
was used, but molecular weights were close to theoretical
values (Fig. 3) and polydispersities were lowered signifi-
cantly. It is interesting to note that polymerization rates
are slowed by an order of magnitude, as bromo initiators
are expected to accelerate reaction kinetics [17]. It is sus-
pected that the activation and deactivation reactions are
rapid, but the atom transfer equilibrium now favors the
dormant species.
Somewhat surprisingly, 5 was ineffective at controlling
the polymerization of MMA, with uncontrolled polymeri-
zation being observed over a range of temperatures and
with a selection of different initiators. Atom transfer poly-
merization catalysts must match redox behaviour to the C–
X bond strength of the growing polymer chain. As the high
redox potential of these nickel catalysts is effective for sty-
rene, the more reactive C–X bond of MMA may result in
loss of control.
Although 5/1-PECl afforded linear kinetics, the molecu-
lar weight of the resultant polymer samples isolated during
the polymerization were significantly higher than calcu-
lated values and molecular weight distributions were some-
what broader than expected for
a well-controlled
4. Conclusions
polymerization (Fig. 2). These observations are consistent
with poor initiation by 1-PECl.
While Cy[N,N]NiCl2 (4) offers poor control over the
polymerization of MMA and styrene due to catalyst insol-
ubility, the corresponding bromo complex Cy[N,N]NiBr2
(5) affords rapid styrene polymerization with moderate
control when initiated by 1-PECl. Inefficient initiation
To improve initiation, 5 was tested using the bromo
initiator, 1-PEBr, where the weaker C–Br bond would
be expected to facilitate the initiation step. The polymer-
ization was significantly slower (76% conversion in 54 h
and kobs = 0.02 hꢁ1) than when the chloride initiator

4420
R.K. O’Reilly et al. / Inorganica Chimica Acta 359 (2006) 4417–4420
generates polymer with molecular weights higher than
expected from [M]o/[I]o ratios and with somewhat broad-
ened molecular weight distributions. Utilizing 1-PEBr
results in much more efficient initiation with molecular
weights of the polystyrene increasing linearly with conver-
sion and polydispersities as low as 1.15. These observations
highlight the need to match catalyst, initiator and mono-
mer reactivity when developing suitable catalyst systems
for ATRP. There now exists much scope for optimization
of this a-diimine nickel catalyst system through fine-tuning
of the steric and electronic environment around the metal
centre, not only to improve catalyst performance, but also
to extend their application to a greater range of radically
polymerizable monomers.
References
[1] K. Matyjaszewski, J. Xia, Chem. Rev. 101 (2001) 2921.
[2] J.S. Wang, K. Matyjaszewski, J. Am. Chem. Soc. 117 (1995) 5614.
[3] M. Kato, M. Kamigaito, M. Sawamoto, T. Higashimura, Macro-
molecules 28 (1995) 1721.
[4] V.C. Gibson, R.K. O’Reilly, W. Reed, D.F. Wass, A.J.P. White,
D.J. Williams, Chem. Commun. (2002) 1850.
[5] V.C. Gibson, R.K. O’Reilly, D.F. Wass, A.J.P. White, D.J. Williams,
Macromolecules 36 (2003) 2591.
[6] M.P. Shaver, L.E.N. Allan, H.S. Rzepa, V.C. Gibson, Angew.
Chem., Int. Ed. 45 (2006) 1241.
[7] C. Granel, P. Dubois, R. Jerome, P. Teyssie, Macromolecules 29
(1996) 8576.
[8] H. Uegaki, Y. Kotani, M. Kamigaito, M. Sawamoto, Macromole-
cules 30 (1997) 2249.
[9] G. Moineau, M. Minet, P. Dubois, P. Teyssie, T. Senninger, R.
Jerome, Macromolecules 32 (1999) 27.
Acknowledgements
[10] H. Uegaki, Y. Kotani, M. Kamigaito, M. Sawamoto, Macromole-
cules 31 (1998) 6756.
[11] E. Duquesne, J. Habimana, P. Degee, P. Dubois, Macromolecules 38
(2005) 9999.
[12] P. Li, K.Y. Qiu, Polymer 43 (2002) 5873.
BP Chemicals (R.K.O’R.) and the Natural Sciences and
Engineering Research Council of Canada (M.P.S.) are
thanked for financial support.
[13] D.F. Evans, J. Chem. Soc. (1959) 2003.
[14] A.T.T. Hsieh, B.O. West, J. Organomet. Chem. 112 (1976) 285.
[15] V.V. Saraev, P.B. Kraikivskii, S.N. Zelinskiy, D.A. Matveev, A.I.
Vilms, A.V. Rohin, K. Lammertsma, J. Mol. Cat. A., Chem. 236
(2005) 125.
[16] F. Dawans, Tetrahedron Lett. (1971) 1943.
[17] M.B. Gillies, K. Matyjaszewski, P.-O. Norrby, T. Pintauer, R. Poli,
P. Richard, Macromolecules 36 (2003) 8551.
Appendix A. Supplementary data
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.ica.2006.06.
026.