Cationic complexes of monovalent nickel as catalysts for styrene polymerization

Article abstract of DOI:10.1007/s10975-005-0126-4

The interaction of the [Ni(PPh₃)₃]BF₄ complex with styrene and the products of styrene conversion in the polymerization reaction were studied by EPR and ¹³C NMR spectroscopy. The structure of the σ-carbocationic complex of Ni(I) formed by the interaction of styrene with the [Ni(PPh₃)₃]BF₄ cationic phosphine complex of Ni(I) was characterized in detail. It was found that the reaction of styrene polymerization occurred with the participation of the coordination center of the σ-carbocationic complex (coordination catalysis), whereas the reaction of telomerization occurred with the participation of the cationic center of this complex (ionic catalysis). The resulting polymer contained active terminal double bonds; it is a promising macromonomer for the synthesis of grafted copolymers. The discovered capacity of alcohols to undergo nucleophilic addition to a growing polymer chain offers strong possibilities for preparing functional polymers and block copolymers.

Full text of DOI:10.1007/s10975-005-0126-4

Kinetics and Catalysis, Vol. 46, No. 5, 2005, pp. 712–718. Translated from Kinetika i Kataliz, Vol. 46, No. 5, 2005, pp. 757–763.
Original Russian Text Copyright © 2005 by Saraev, Kraikivskii, Annenkov, Vil’ms, Matveev, Danilovtseva, Ermakova, Kuznetsova, Lammertsma.
Cationic Complexes of Monovalent Nickel
as Catalysts for Styrene Polymerization
V. V. Saraev*, P. B. Kraikivskii*, V. V. Annenkov**, A. I. Vil’ms*, D. A. Matveev*,
E. N. Danilovtseva**, T. G. Ermakova***, N. P. Kuznetsova***, and K. Lammertsma****
* Irkutsk State University, Irkutsk, 664033 Russia
** Institute of Limnology, Siberian Division, Russian Academy of Sciences, Irkutsk, 664033 Russia
*** Favorskii Institute of Chemistry, Siberian Division, Russian Academy of Sciences, Irkutsk, 664033 Russia
**** Vrije Universiteit, Amsterdam, the Netherlands
Received October 1, 2004
Abstract—The interaction of the [Ni(PPh3)3]BF4 complex with styrene and the products of styrene conversion
in the polymerization reaction were studied by EPR and 13C NMR spectroscopy. The structure of the σ-car-
bocationic complex of Ni(I) formed by the interaction of styrene with the [Ni(PPh3)3]BF4 cationic phosphine
complex of Ni(I) was characterized in detail. It was found that the reaction of styrene polymerization occurred
with the participation of the coordination center of the σ-carbocationic complex (coordination catalysis),
whereas the reaction of telomerization occurred with the participation of the cationic center of this complex
(ionic catalysis). The resulting polymer contained active terminal double bonds; it is a promising macromono-
mer for the synthesis of grafted copolymers. The discovered capacity of alcohols to undergo nucleophilic addi-
tion to a growing polymer chain offers strong possibilities for preparing functional polymers and block copol-
ymers.
INTRODUCTION
intense band at 30000 cm–1 and a weak band at
21000 cm–1 appeared in the UV spectrum. These
absorption bands are characteristic of a carbocation
containing a benzene ring conjugated with the carbon
atom bearing a positive charge [26]. Based on these
data, it was hypothesized that the resulting Ni(I) com-
plex with styrene, in which a positive charge is local-
ized on styrene, is a σ-cationic complex of Ni(I), which
occurs in solution in the dimeric state
Catalytic systems based on nickel, which are prima-
rily known as catalysts for the low-molecular-weight
oligomerization of olefins [1–4], are of considerable
current interest because it was found that a number of
nickel complexes can efficiently catalyze the polymer-
ization of unsaturated hydrocarbons [5–21].
Despite the considerable number of publications
devoted to studies of polymerization on nickel complex
catalysts, the nature of the active center and the mecha-
nism of polymerization are not clearly understood. This
is due to the fact that the majority of studies have been
oriented to reaction products without considering the
structure of the active center and its transformation in
the presence of a monomer, as well as the intricacy of
test systems containing various potential centers of
polymerization. In this context, the [Ni(PPh3)3]BF4 cat-
ionic phosphine complex of Ni(I) is of interest. As
found previously [22, 23], this complex is an active cat-
alyst for styrene polymerization.
Ph
2BF4–
Ni(PPh3)2
(Ph3P)2Ni
Ph
In this work, we studied the interaction of the
[Ni(PPh3)3]BF4 complex with styrene and the products
of styrene conversion in the polymerization reaction
using EPR and 13C NMR spectroscopy in order to
obtain detailed information on the structure of the car-
bocationic complex of Ni(I) and on the process mecha-
nism of styrene ligand activation in the coordination
sphere of Ni(I).
The [Ni(PPh3)3]BF4 tricoordinated phosphine com-
plex of monovalent nickel has a doubly degenerated
ground state. As a consequence of the Jahn–Teller
effect, it was detected as a broadened asymmetric sig-
nal in the EPR spectrum at T = 77 K (Fig. 1) (gz = 2.385,
gy = 2.12, gx = 2.07, Ax(1P) = 8.2 mT; Ay(1P) = 6.4 mT)
[24, 25]. Previously [22], it was found that, upon the
interaction of the [Ni(PPh3)3]BF4 cationic complex
with styrene at room temperature (a 2 : 1 molar ratio of
EXPERIMENTAL
All of the operations were performed in an atmo-
styrene to Ni), the EPR signal disappeared and an sphere of purified argon using Schlenk techniques.
0023-1584/05/4605-0712 © 2005 MAIK “Nauka /Interperiodica”

CATIONIC COMPLEXES OF MONOVALENT NICKEL AS CATALYSTS
713
Glass Schlenk filters were used for filtration. All of the
prepared and synthesized reagents were stored in
sealed ampules in an argon atmosphere.
DPPH
Toluene, benzene, and heptane (high-purity grade)
were distilled from sodium metal in the presence of
benzophenone before use. Boron trifluoride–diethyl
ether adduct (Merck) was distilled from LiH in an
argon atmosphere before use. Absolute ethanol was
prepared by distillation from sodium metal (10 g/l).
1
Diethyl ether was purified to remove the peroxides,
distilled from sodium, and stored over a sodium mirror
[27].
Styrene was purified by shaking it with a 5% alkali
solution until the alkali became colorless [27]; thereaf-
ter, it was washed with distilled water, dried with anhy-
drous calcium chloride, and distilled in vacuum (boil-
ing temperature, 32°ë/10 Torr).
2
The [Ni(PPh3)3]BF4 complex was synthesized in
accordance with a procedure based on the oxidation of
the Ni(PPh3)4 complex by the boron trifluoride–diethyl
ether adduct [28]: 0.23 ml of BF3 · OEt2 was added to a
stirred solution of 0.5 g of Ni(PPh3)4 in 10 ml of toluene
(B/Ni = 4 : 1) for 10 min at room temperature in an
argon atmosphere with the use of a syringe. Next, the
mixture was cooled to –10°C, and the precipitate
formed was filtered off. The [(PPh3)3Ni]BF4 complex
was isolated as a brown solid from the filtrate by precip-
itating with heptane. The product yield was 75% of the
theoretical value. For C54H45P3NiBF4 anal. calc. (%):
Ni, 6.3; C, 69.5; H, 4.80; P, 9.99. Found (%): Ni, 6.6;
C, 69.3; H, 4.86; P, 9.78.
8.0 mT
Fig. 1. EPR spectra of the monovalent nickel complexes (1)
[Ni(PPh ) ]BF and (2) [(PPh ) Ni(P(OEt) ) ]BF in a tol-
3 3
4
3 2
3 2
4
uene solution. T = 77 K.
meter at a wavelength of 870 nm. The results were
treated in accordance with a published procedure [30].
The polymerization of styrene was performed as fol-
lows: Toluene, a weighed portion of the nickel complex,
and other reagents were successively placed in a ther-
mostated reactor in an argon atmosphere. The concen-
tration of the nickel complex was equal to 1 × 10–4 mol/l.
Next, styrene was added with intense stirring. The
amount of reacted styrene was determined by GLC on a
GALS gas chromatograph with a flame-ionization detec-
tor (using packed column 5 mm in diameter and 150 cm
in length with Apiezon). After completion of the reac-
tion, the polymer was precipitated with an excess of
ethanol, reprecipitated from toluene, and vacuum dried
to constant weight.
The EPR spectra were measured on a PS-100X
spectrometer operating at 9.6 GHz at the boiling tem-
perature of liquid nitrogen. As reference substances,
Mn(II) in MgO and DPPH were used.
NMR spectra were obtained on a Varian VXR-500S
spectrometer at 25°C, and IR spectra were measured as
KBr pellets on a Bruker IFS 25 spectrophotometer.
RESULTS AND DISCUSSION
Interaction of styrene with the cationic complex
of Ni(I). Upon the addition of styrene to a toluene solu-
tion of the [Ni(PPh3)3]BF4 complex (styrene/Ni molar
ratio of 2 : 1), EPR signal 1 (Fig. 1) from the initial
complex of Ni(I) disappeared and a group of signals
appeared in the 13C NMR spectrum at 156, 140, 136,
135, and 129 ppm (Fig. 2). These signals are character-
istic of a carbocation containing a benzene ring conju-
gated with the carbon atom bearing a positive charge
[26]. The signal at 156 ppm was a superposition of two
signals at 156.83 and 155.86 ppm. This was likely due
to the occurrence of two conformers of the carboca-
tionic complex of Ni(I) with a dynamic equilibrium
between these conformers. A broadening of these sig-
nals is indicative of short relaxation times of the
dynamic processes. Upon the subsequent introduction
of a strong ligand, triethyl phosphite, into the system
(P(OEt)3/Ni molar ratio of 2 : 1), signal 2 appeared in
the EPR spectrum (Fig. 1) (gz = 2.014; gy = 2.185; gx =
The molecular weights of polymers were deter- 2.201; Az(P) = 17.5 mT; Az(P') = 8.1 mT; Ay(P) =
mined by the isopiestic method in an acetone solution 21.0 mT; Ax(P)=13.5 mT), which fully coincided with
using azobenzene as a standard compound [29]. For the
spectrum
of
the
cationic
complex
turbidimetric titration, 0.05% polymer solutions in [(PPh3)2(P(OEt)3)2Ni]BF4 [31]. In this case, a group of
dioxane were used; precipitation was performed with signals at 156, 140, 136, 135, and 129 ppm disappeared
water; absorbance was measured on a KFK-3 photo- from the 13C NMR spectrum. The disappearance of the
KINETICS AND CATALYSIS Vol. 46 No. 5 2005

714
SARAEV et al.
NMR signals of a carbocation and the appearance of
The addition of new portions of the fresh monomer
EPR signal 2 suggest the replacement of styrene by tri- to the reaction mass up to a styrene/Ni ratio of 700 : 1
ethyl phosphite ligands and the degradation of diamag- resulted in the activation of the reaction at almost the
netic dimeric carbocation structures.
same rate. The high viscosity of the system hindered a
further addition of the monomer. The product isolated
at this step was a powder whose molecular weight
(3600) was higher than the molecular weight (2500) of
a polymer obtained at a styrene/Ni ratio of 200 : 1. The
turbidimetric titration curves (Fig. 4) are consistent
with the observed increase in the degree of styrene
polymerization; they are indicative of a similarity
between the molecular-weight distributions of the sam-
ples. It is likely that, on the addition of new portions of
the fresh monomer, previously formed macromolecules
containing residual terminal double bonds also partici-
pated in the renewed polymerization.
The reaction did not occur with methyl methacrylate
(MMA) and methyl acrylate as monomers: we failed to
detect the formation of polymer products by GLC anal-
ysis. The fact that the monomers with an acceptor sub-
stituent at the double bond are nonreactive under the
conditions examined suggests the cationic character of
the active nickel complex.
The experimental data on the structure of the car-
bocationic complex of Ni(I) allowed us to propose
Scheme 1, which is the most probable reaction scheme
for styrene activation on [Ni(PPh3)3]BF4.
The BF–4 counterions are not shown in Scheme 1.
In accordance with the proposed reaction scheme, in
the interaction of styrene with cationic complex 1, the
substitution of styrene for a phosphine ligand occurs
with the formation of π complex 2. Complex 2 is con-
verted into σ-carbocationic complex 3 as a conse-
quence of the rapid π–σ rearrangement and positive
charge transfer from the nickel ion to coordinated sty-
rene. It is likely that the π–σ rearrangement is an equi-
librium process, and equilibrium is shifted toward the
formation of the σ structure. Because of its coordina-
tive unsaturation, σ-carbocationic complex 3 forms
diamagnetic dimer 4 in a toluene solution.
Styrene polymerization. The introduction of
0.5 mol % of the [Ni(PPh3)3]BF4 cationic complex into
a styrene solution in toluene resulted in the rapid poly-
merization of styrene at 10–40°ë. The reaction
occurred to the complete consumption of the monomer
(Fig. 3); however, the polymer yield was only 67%.
This suggests that a low-molecular-weight oligomer
fraction soluble in ethanol was also formed, which is
characteristic of processes on nickel-containing cata-
lysts [21]. The resulting product was a paraffin-like
mass. The IR spectrum of this product exhibited an
absorption band at 1640 cm–1, which is characteristic of
the stretching vibrations of double bonds. The 13C
NMR spectra exhibited a signal in the region 137–
138 ppm, which corresponds to the terminal unsatur-
The addition of MMA to styrene did not result in the
formation of copolymers: the NMR spectra of the
resulting products were analogous to the spectra of
polystyrene. Only a weak absorption band at 1720 cm–1,
which belongs to the stretching vibrations of the carbo-
nyl group, appeared in the IR spectrum. The yield of
polystyrene decreased with MMA concentration
(Fig. 5), although MMA did not inhibit styrene poly-
merization even in a considerable excess in the reaction
mixture. The polystyrene specimens obtained in the
system with MMA were powdered solids. According to
turbidimetric titration data (Fig. 6, curves 3, 4), they
had a higher molecular weight and a narrower molecu-
lar-weight distribution, as compared with the polymer-
ization products of pure styrene (Fig. 6, curve 1). The
introduction of MMA after the consumption of styrene
(Fig. 6, curve 2) did not affect the molecular-weight
distribution, although the molecular weight somewhat
ated groups
, whose concentration was
CH2=C
Ph
1−2% of the total amount of styrene units.
Ph
Ph
(Ph3P)2Ni
+ (Ph3P)3Ni
+ PPh3
1
2
Ph
Ph
(Ph3P)2Ni
Ph
Ni(PPh3)2
(Ph3P)2Ni
4
3
Scheme 1.
KINETICS AND CATALYSIS Vol. 46 No. 5 2005

CATIONIC COMPLEXES OF MONOVALENT NICKEL AS CATALYSTS
715
160
150
140
130
120
170
150
130
110
90
δ, ppm
70
50
30
10
13
Fig. 2. C NMR spectrum of the [Ni(PPh ) ]BF + 2C H system. Solvent: deuterotoluene.
3 3
4
8 8
increased. This effect of MMA on the polymerization explained by the participation of this amine in coordina-
tion to Ni(I) ions as a competitive reaction. A stronger
catalyst poison, for example, triethyl phosphite at a
P(OEt)3/Ni molar ratio of 2 : 1, completely inhibited the
polymerization reaction as a result of the replacement of
styrene in the coordination sphere of Ni(I) by triethyl
phosphite and the formation of the coordinatively satu-
rated paramagnetic complex [(PPh3)2Ni(P(OEt)3)2]BF4.
of styrene suggests the participation of MMA at the
steps of initiation or termination of polymer chains,
although the nature of the observed phenomenon
invites further investigation.
Polymerization reaction scheme. The occurrence
of two reaction centers in the σ-carbocationic complex
of Ni(I) (α-carbon bound to the transition metal and
β-carbon, at which a positive charge is localized)
allows activated styrene to react by both coordination
and ionic mechanisms. To determine the true mecha-
nism of styrene polymerization, we used a typical test
for ionic polymerization with triethylamine. The intro-
duction of the tertiary amine into the reaction mixture
decreased the rate of polymerization (by 30% at an
NEt3/Ni molar ratio of 5 : 1); however, it did not com-
pletely inhibit the polymerization even with a 100-fold
excess of the base. Consequently, the polymerization of
styrene on the cationic complex of Ni(I) mainly
occurred by a coordination mechanism. The effect of
The carbocationic center in activated styrene mani-
fests itself in the reactions of nucleophilic addition of
alcohols. These reactions occur on the cationic com-
plex of Ni(I) under mild conditions. As found previ-
ously [22], the reaction products (telomers) contain the
following fragments:
OR
C
Ph
Me
The results of this study and previously published
the tertiary amine on the rate of polymerization can be data [22, 23] allowed us to propose Scheme 2 for the
KINETICS AND CATALYSIS Vol. 46 No. 5 2005

716
SARAEV et al.
Styrene concentration, mol %
Absorbance, arb. units
100
2
2.0
1
80
60
40
20
1.5
1.0
0.5
0
0
20
40
60
Reaction time, min
0.20
0.25
0.30
0.35
Fig. 3. Styrene consumption (as a percentage on the basis of
the initially added amount) in polymerization. Reaction
temperature: 10°C.
Precipitant volume fraction
Fig. 4. Integral (solid lines) and differential (dashed lines)
curves of turbidimetric titration of the product of styrene
polymerization (1) at an initial styrene/Ni ratio of 200 : 1
and (2) after the addition of three styrene portions of 200,
200, and 100 parts. Reaction temperature: 20°C.
reactions of styrene on the [Ni(PPh3)3]BF4 cationic
complex.
For simplicity, Ni(I) complexes in Scheme 2 are rep-
resented as fragments: the second metal–carbon bonds
was formed. The insertion of styrene into a metal–car-
bon bond resulted in an elongation of the carbon chain
that links the nickel atoms together (6). At the next step,
the carbocation in complex 7 arranged itself into a more
stable form as a result of hydride transfer between adja-
cent carbon atoms and the localization of a positive
charge on the β-carbon atom. After the migration of the
metal–carbon coordination bond from γ- to α-carbon
in Ni(I) dimers, BF–4 counterions, and PPh3 phosphine
ligands are not shown. Moreover, the steps of formation
of carbocationic complex dimer 4, which are described
in detail in Scheme 1, are not shown.
In accordance with Scheme 2, in an excess of sty-
rene in the reaction mixture, styrene underwent coordi-
nation to carbocationic complex dimer 4 (evidently, by
the replacement of a phosphine ligand), and π complex 5 atom (8), the cycle of carbon chain growth by one unit
Ph Ph
Ph Ph
Ph
Ph
Ni
Ph
Ph
Ni
Ni
Ni Ni
Ni Ni
Ni
4
5
6
7
Ph
Ph
Ph
Ph
Ph
Ph
Ni
Ph
Ni
Ph
Ph
Ph
Ph
+
Ni
Ni
Ni
2
9
8
HOR
Ph
Ph
Ph
+
Ni
n
(n = 25–35)
OR
1
Scheme 2.
KINETICS AND CATALYSIS Vol. 46 No. 5 2005

CATIONIC COMPLEXES OF MONOVALENT NICKEL AS CATALYSTS
717
Polystyrene yield, %
was complete. Repeated cycles resulted in the forma-
tion of σ-cationic complex 9 with an oligomer (poly-
mer) chain. The oligomer (polymer) in complex 9,
which occurred in an equilibrium π form, was replaced
by the parent monomer (2), and the process turned back
to the beginning of a new cycle. Complex 9 in an equi-
librium σ form can also enter into nucleophilic addition
reactions at the carbocationic center with the formation
of telomers and the regeneration of starting cationic
complex 1.
80
60
Thus, we have characterized in detail the structure
of the σ-carbocationic complex of Ni(I) formed in the
reaction of styrene with the [Ni(PPh3)3]BF4 cationic
phosphine complex of Ni(I). The reaction of styrene
polymerization occurred with the participation of the
coordination center of the σ-carbocationic complex of
Ni(I) (coordination catalysis), whereas the telomeriza-
tion reaction occurred with the participation of the cat-
ionic center of this complex (ionic catalysis). The
resulting polymer contained active terminal double
bonds; it is a promising macromonomer for the synthe-
sis of grafted copolymers. The discovered capacity of
alcohols to undergo nucleophilic addition to a growing
polymer chain offers strong possibilities for preparing
functional polymers and block copolymers.
40
20
0
20
40
60
80
100
Styrene concentration, mol %
Fig. 5. Dependence of the limiting yield of polystyrene on
the styrene content of the starting mixture with MMA. Total
monomers/Ni = 200 : 1. Reaction temperature: 20°C.
ACKNOWLEDGMENTS
This work was supported by the Russian Foundation
for Basic Research and NWO (the Netherlands) (interna-
tional project no. 03-03-89007) and the Ministry of Edu-
cation of the Russian Federation (grant nos. 03.06.047
(Chemical Technologies) and 05.01.003 (Universities
of Russia)).
Absorbance, arb. units
3
4
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