Ruthenium carborane complexes in the controlled radical polymerization of methyl methacrylate

Article abstract of DOI:10.1007/s11172-006-0219-x

New initiators based on closo- and exo-nido-ruthenacarboranes with phosphine and diphosphine ligands were proposed as chain growth regulators. They allow conducting the controlled synthesis of poly(methyl methacrylate) under radical initiation conditions.

Full text of DOI:10.1007/s11172-006-0219-x

Russian Chemical Bulletin, International Edition, Vol. 55, No. 1, pp. 89—93, January, 2006
89
Ruthenium carborane complexes in the controlled
radical polymerization of methyl methacrylate
E. V. Kolyakina,a I. D. Grishin,a D. N. Cheredilin,b F. M. Dolgushin,b I. T. Chizhevsky,bꢀ and D. F. Grishina
ꢀ
aResearch Institute of Chemistry of the
N. I. Lobachevsky Nizhnii Novgorod State University,
23, building 5, prosp. Gagarina, 603950 Nizhnii Novgorod, Russian Federation.
Fax: +7 (831) 265 8162. Eꢀmail: grishin@ichem.unn.runnet.ru
bA. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences,
28 ul. Vavilova, 119991 Moscow, Russian Federation.
Fax: +7(495) 135 6549. Eꢀmail: chizbor@ineos.ac.ru
New initiators based on closoꢀ and exoꢀnidoꢀruthenacarboranes with phosphine and
diphosphine ligands were proposed as chain growth regulators. They allow conducting the
controlled synthesis of poly(methyl methacrylate) under radical initiation conditions.
Key words: ruthenium carborane complexes, methyl methacrylate, controlled radical polyꢀ
merization.
Scheme 1
The controlled synthesis of macromolecules opens up
broad prospects for controlling the chain growth under
radical initiation conditions.1,2 A substantial drawback of
radical polymerization is the high reactivity of the radical
species, which actively participate not only in the chain
growth, but also in side reactions, for example, disproporꢀ
tionation, dimerization, and chain transfer, which finally
results in high polydispersity of the polymers and makes
the process uncontrolled.3 The soꢀcalled Atom Transfer
Radical Polymerization (ATRP), first accomplished
in 1995 in the synthesis of polyolefins, is now the most
efficient method for the targeted control of the elemenꢀ
tary steps of radical polymerization.4,5 This approach is
actually based on the Kharasch reaction, discovered
in 1945,6 in which alkyl halides act as sources of carbonꢀ
centered radicals in the presence of transition metal comꢀ
plexes where the oxidation state of the metal can easily
and reversibly change (Scheme 1).
The possibility of fast and reversible redox transformaꢀ
tions of the metal and the presence of a halogen atom
bound to the metal atom are necessary conditions for the
high activity of metal complexes in this type of processes.7
Currently, ATRP is carried out using various transition
metal complexes, including copper,8 nickel,9 iron,10
rhodium,11 molybdenum,12 palladium,13 and rutheꢀ
nium.14—16 Examples of using metallacarborane catalysts
with chargeꢀcompensating phosphorusꢀ17 or sulfurꢀconꢀ
taining18,19 substituents at the carborane cage ligand in
this process and in the Kharasch reaction are known.
As shown already in the first experiments, RuII carboꢀ
rane complexes exhibit the highest activity among all
Bimolecular termination
Mn is a transition metal in the oxidation state n;
Х, Y = Cl or Br; L is the organic ligand
known ruthenium catalysts used in the reaction of styrene
and methyl methacrylate (MMA) with CCl4 (Kharasch
reaction) (with 3ꢀHꢀ3,3ꢀ(PPh3)2ꢀ8ꢀS(Et)Phꢀ3,1,2ꢀ
RuC2B9H10 used as the catalyst, the turnover number is
1800 and 1500 per hour, respectively, at 40 °C).18
This communication describes the use of classical
closoꢀ and exoꢀnidoꢀruthenacarboranes 1—5, containing
readily available nonꢀsubstituted or dimethylꢀsubstituted
carborane ligands as regulators of the radical polymerizaꢀ
tion of MMA. The general principles of the ATRP cataꢀ
lysts20 are fulfilled, in particular, for complexes 1—5 where
the ruthenium atom can easily change its oxidation state;
in addition, each complex contains a halogen atom bound
to the Ru atom.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 85—89, January, 2006.
1066ꢀ5285/06/5501ꢀ0089 © 2006 Springer Science+Business Media, Inc.

90
Russ.Chem.Bull., Int.Ed., Vol. 55, No. 1, January, 2006
Kolyakina et al.
Scheme 2
Results and Discussion
18ꢀElectron complexes 1—4 were synthesized accordꢀ
ing to published methods.21—23 exoꢀnidoꢀRuthenacarboraꢀ
nes 1 and 2 were obtained from the К+ salts of nidoꢀdiꢀ
carbaundecaborates [nidoꢀ7,8ꢀR2ꢀ7,8ꢀC2B9H10]–K+
(R = H, Me) and Ru(PPh3)3Cl2 by a known but slightly
modified procedure,21 in which the THF—ether binary
mixture used previously as the solvent was replaced by
benzene. Complex 3 was synthesized by the thermal
exoꢀnido→closoꢀrearrangement of ruthenacarborane 1 in
benzene,22 and complex 4 was obtained by the reaction of
ruthenacarborane 1 with 1,4ꢀbis(diphenylphosphino)buꢀ
tane (dppb) in benzene at room temperature.23
the Ru atom in complex 5 is coordinated to the doubly
charged (2–) {C2B9} ligand, neutral diphosphine, and
singly charged (1–) chloride ligand. Thus, the sixꢀcoordiꢀ
nate Ru atom has a formal oxidation state +3 and a defiꢀ
cient 17ꢀelectron configuration. The paramagnetic naꢀ
ture of complex 5 is also confirmed by the ESR spectrum
(see Experimental). An interesting feature of compound 5
is the presence of oꢀphenylcycloboronated diphosphine
ligand. Note that in the series of closoꢀmetallacarborane
complexes of transition metals, this is only the second25
structurally characterized complex of this type. The presꢀ
ence of an oꢀcycloboronated fragment in the complex
accounts for the observed differences in the Ru—P(1)
(2.306(4) Å) and Ru—P(2) (2.369(4) Å) bond
lengths. Attention is also attracted by the substantially
shortened Ru—Cl bond length (2.375(1) Å). In 18ꢀelecꢀ
tron closoꢀruthenacarboranes having a similar structure,
the Ru—Cl distance is much longer, for example, in
neutral complex 3 (Ru—Cl, 2.429 Å)22 or in the anꢀ
Table 1. Selected geometric parameters of molecule 5
Bond
d/Å
Angle
ω/deg
Ru(3)—P(1)
Ru(3)—P(2)
2.306(4)
2.369(4)
B(8)—Ru(3)—P(1)
P(1)—Ru(3)—P(2)
80.8(3)
85.66(13)
Ru(3)—Cl(1) 2.375(3)
P(1)—Ru(3)—Cl(1) 91.35(12)
P(2)—Ru(3)—Cl(1) 86.06(12)
Ru(3)—C(1)
Ru(3)—C(2)
Ru(3)—B(4)
Ru(3)—B(7)
Ru(3)—B(8)
P(1)—C(15)
P(1)—C(18)
P(1)—C(24)
P(2)—C(17)
P(2)—C(30)
P(2)—C(36)
C(1)—C(2)
2.319(1)
2.316(1)
2.245(1)
2.203(1)
2.223(1)
1.827(1)
1.829(1)
1.812(1)
1.829(1)
1.836(1)
1.833(1)
1.635(1)
The paramagnetic oꢀcycloborated closoꢀruthenaꢀ
carborane 5 was prepared on heating of ruthenacarꢀ
borane 2 with a 10% excess of 1,3ꢀbis(diphenylphosꢀ
phino)propane (dppp) in toluene (Scheme 2). Comꢀ
plex 5, formed together with the known cationic comꢀ
plex [RuCl(Ph2P(CH2)3PPh2)2]+[nidoꢀ7,8ꢀMe2ꢀ7,8ꢀ
C2B9H10]– (6),24 was isolated from the reaction mixture
in 64% yield by column chromatography on silica gel (see
Experimental).
C(15)—P(1)—Ru(3)
C(18)—P(1)—Ru(3)
C(17)—P(2)—Ru(3)
C(16)—C(15)—P(1)
120.5(3)
105.8(4)
113.5(4)
117.1(7)
C(15)—C(16)—C(17) 112.0(8)
C(16)—C(17)—P(2)
C(19)—C(18)—P(1)
C(18)—C(19)—B(8)
112.4(7)
113.0(8)
119.3(8)
The structure of 5 was established using Xꢀray diffracꢀ
tion and ESR data, and its composition was confirmed by
elemental analysis. The molecular structure of 5 is shown
in Fig. 1, while selected bond lengths and angles are sumꢀ
marized in Table 1. According to Xꢀray diffraction data,
C(15)—C(16) 1.498(1)
C(16)—C(17) 1.537(1)
C(18)—C(19) 1.419(1)

Rutheniumꢀcatalysed MMA polymerization
Russ.Chem.Bull., Int.Ed., Vol. 55, No. 1, January, 2006
91
C(16)
C(17)
C(15)
C(29)
C(37)
C(38)
Cl(1)
C(35)
C(28)
C(34)
P(2)
C(39)
C(27)
C(36)
C(33)
C(23)
C(30)
C(24)
P(1)
C(40)
C(41)
C(26)
Ru(3)
C(18)
C(31)
C(25)
C(32)
C(22)
C(14)
C(19)
C(13)
C(21)
B(7)
B(8)
B(9)
C(2)
B(4)
C(1)
C(20)
B(11)
B(12)
B(6)
B(5)
B(10)
Fig. 1. Molecular structure of complex 5.
ionic complexes [3,3ꢀL2ꢀ3ꢀClꢀ3,1,2ꢀRuC2B9H11]–[Cat]+
(L = PPh3, Cat = Et4N, Ru—Cl, 2.515 Å;26 L = CO,
Cat = Au(PPh3)2, Ru—Cl, 2.452 Å).27 The distances from
the ruthenium atom to the C2B3 pentagonal plane of the
carborane ligand are similar in all of these compounds.
We found that ruthenium carborane complexes 1—5
actively initiate polymerization of methyl methacrylate in
the presence of CCl4; in some cases, the degree of monoꢀ
mer conversion is close to 100% (Table 2).
As follows from the obtained data (see Table 2),
18ꢀelectron ruthenium(II) exoꢀnidoꢀcomplexes (comꢀ
pounds 1 and 2) are the best choice as regards the degree
of conversion and initiation efficiency; with these comꢀ
pounds, the monomer conversion exceeds 90%. The reꢀ
sulting polymers have rather high molecular weights
(Mn > 50 thousand) and a polydispersity of <2.
When focusing on the synthesis of narrowꢀpolydisperꢀ
sity macromolecules, of most interest are ruthenacarboꢀ
rane complexes 3—5 of the closoꢀstructure. If they are used
in the synthesis of poly(methyl methacrylate) (PMMA),
the polydispersity of the sample is in a range of 1.25—1.62
(see Table 2), which corresponds to the basic paramꢀ
eters of living radical polymerization.1—3,20 However,
MMA polymerization involving the ruthenium closoꢀcomꢀ
plexes proceeds at a much lower rate than that with
exoꢀnidoꢀcomplexes, reaches a much lower limiting conꢀ
version, and gives a polymer with a lower molecular weight
(see Table 2). All the proposed diamagnetic closoꢀruthenaꢀ
carboranes exceed in activity related ruthenium cycloꢀ
pentadienyl derivatives, for example, RuCl(Cp*)(PPh3)2,
which can also control the molecularꢀmass characterisꢀ
tics of the polymers. These compounds are mainly used
for the synthesis of PMMA together with coꢀcatalysts, in
particular, aluminum alkoxides, because the rate of polyꢀ
merization in the presence of these compounds without
additives is low and a high degree of conversion (~90%) is
attained only after 300—600 h (see Refs 7, 28, and 29).
Comparison of the polydispersity of PMMA syntheꢀ
sized in the presence of complex 3 and compounds 4, 5
indicates that binding of phosphine ligands by methylene
fragments creates favorable steric conditions for control
of the polymer chain growth by the ruthenacarborane
Table 2. Polymerization of MMA catalyzed by ruthenacarboranes
Comꢀ z
q
tp Converꢀ
Mw
Mn
Mw/Mn
plex
/h sion (%)
1
2
3
4
5
II
II
18e
18e
18e
18e
45.4
80.3
45.4
110
94.0
98.5 100200
87.8
82.7
70.0
83500
53500
51800
36200
26800
41900
1.56
1.93
1.62
1.25
1.37
IV
IV
III 17e
58900
33600
57400
114
Note. Conditions: ruthenacarborane concentration, 0.125 mol.%;
temperature, 80 °C, initiator, CCl4 (0.25 mol.%). z is the oxidaꢀ
tion number, q is the electron environment, tp is the polymerizaꢀ
tion time, Mw is the weight average molecular weight, Mn is the
number average molecular weight, Mw/Mn is the polydispersity
coefficient.

92
Russ.Chem.Bull., Int.Ed., Vol. 55, No. 1, January, 2006
Kolyakina et al.
of complex 6. The data of 1H, 31P{1H}, and 11B NMR spectra of
complex 6 fully coincided with the published data,24 which indiꢀ
cates that these compounds are identical. Complex 5. Found (%):
C, 48.73; H, 4.79; B, 12.07. C31H30P2B9ClRu•CH2Cl2. Calꢀ
culated (%): C, 48.50; H, 5.18; B, 12.28. ESR (CH2Cl2,
gstand = 2.0023, T = 77 K): gav = 2.1096.
XꢀRay diffraction study of 5. The crystals of 5•3C6H6 are
triclinic, C49H57B9ClP2Ru, M = 940.77, space group P–1, at
173 K a = 10.131(9), b = 10.232(11), c = 26.14(3) Å,
α = 80.90(3), β = 84.16(4), γ = 61.17(3)°, V = 2343(4) Å3, Z = 2,
dcalc = 1.335 g cm–3, µ(MoꢀKα) = 4.95 cm–1 (no absorption
correction was applied). The intensities of 8478 reflections
(7996 independent reflections, Rint = 0.071) were measured
on a Syntex P21 diffractometer (graphite monochromator,
molecule, which finally results in a narrowꢀpolydispersity
polymer with Mw/Mn ≤ 1.25 in the case of complex 4. The
samples of PMMA obtained in the presence of diꢀ
phosphineꢀsubstituted complexes, diamagnetic 4 and
paramagnetic 5, show only slight differences in polydisꢀ
persity coefficients (see Table 2).
The very similar polydispersities of the polymer
samples obtained with complexes 1 and 3 having different
structures are due, in our opinion, to a rearrangement of
the exoꢀnido complex 1 into closo complex 3, which readily
occurs in benzene at 80 °C.22 This rearrangement does
not take place in the case of sterically more hindered
dimethylꢀsubstituted exoꢀnidoꢀcomplex 2 (see Ref. 22),
in whose presence a polymer with a greater polydispersity
coefficient is formed (Mw/Mn ≤ 1.93). This fact indicates
that the chain growth step and the formation of macroꢀ
molecular characteristics of PMMA samples are affected
by both steric and structural features of the catalysts.
Complexes 1 and 3 are also active in the polymerizaꢀ
tion of MMA according to the ATRP mechanism in
the presence of other radical initiators, for example,
ethyl 2ꢀbromoisobutyrate (MMA : initiator : catalyst =
1 : 0.25 : 0.125). The relatively low (<1.85) polydispersity
values for PMMA are retained.*
λ(MoꢀKα) = 0.71073 Å, θ/2θ scan mode, 2θ
= 50°). The
max
structure was solved by the direct method and refined by the
fullꢀmatrix leastꢀsquares method on F 2hkl with anisotropic therꢀ
mal parameters for all nonhydrogen atoms. The structure conꢀ
tains three solvating benzene molecules, one of which is disorꢀ
dered. The hydrogen atoms of the carborane ligand were located
from the difference synthesis and included in the final refineꢀ
ment with fixed coordinates. The other hydrogen atoms were
placed into geometrically calculated positions and included in
the refinement in the riding model. The final Rꢀfactors R1 = 0.099
(refinement on Fhkl for 5492 reflections with I > 2σ(I )),
wR2 = 0.226 and S = 1.019 (refinement on F2hkl for all indepenꢀ
dent reflections). All calculations were carried out on a personal
computer using the SHELXTL software.30 The full tables of
atom coordinates, bond lengths, bond angles, and anisotropic
thermal parameters were deposited with the Cambridge Strucꢀ
tural Database.
Thus, this study has shown that ruthenium comꢀ
plexes with carborane ligands are efficient catalysts for
controlled ATRP synthesis of PMMA. The complexes
with closoꢀstructure are more efficient regulators of the
molecular mass characteristics of the polymers than
exoꢀnidoꢀruthenacarboranes.
Polymerization of MMA. The physicochemical parameters
of the solvents correspond to published data.31 Methyl methꢀ
acrylate was purified by vacuum distillation to collect the fracꢀ
tion with b.p. 38 °C (15 Torr).32
Experimental
Carbon tetrachloride was used in the ATRP as the initiator.
The samples were prepared as follows: the monomer and exact
portions of the initiator and the catalyst (ruthenacarboranes)
were placed in glass tubes, and degassed three times. The tubes
were frozen in liquid nitrogen and polymerization was carried
out at a residual pressure of 1.3 Pa. The polymerization kinetics
was monitored under isothermal conditions by the weighing
method. The tube was place in a thermostat for a strictly speciꢀ
fied period of time. After that, the tube was taken out and frozen
in liquid nitrogen to terminate polymerization. The resulting
polymer was precipitated by petroleum ether, reprecipitated from
a chloroform solution, and dried in vacuo to a constant weight,
and the degree of conversion was calculated.
Analysis of poly(methyl methacrylate). The molecular weight
and the molecular weight distribution of PMMA were deterꢀ
mined by GPC on a Knauer setup with a linear column
(Phenomenex, USA). A RI Detektor Kꢀ2301 differential refracꢀ
tometer was used ad the detector. Chloroform was used as the
eluent. Narrowꢀpolydispersity polystyrene samples were used
for calibration. The average molecular weights were converted
from calibration against polystyrene samples to the polymers
under study using standard formulas for PMMA.33
Complexes 1—4,21—23 5, and 6 were synthesized under arꢀ
gon using anhydrous solvents prepared by standard procedures.
The products were isolated and purified by column chromatoꢀ
graphy using silica gel Merck (230—400 mesh). Commercial
diphosphines dppp and dppb (Strem Chemicals) were used. The
ESR spectrum of complex 5 was recorded on a Varian Eꢀ12A
radiospectrometer. Elemental analysis of complex 5 was perꢀ
formed at the laboratory of microanalysis of the A. N.
Nesmeyanov Institute of Organoelement Compounds.
Synthesis of 3ꢀchloroꢀ1,2ꢀdimethylꢀ3,3ꢀ{[1´,1´ꢀdiphenylꢀ5´ꢀ
phenylꢀ5´ꢀ(5´,8ꢀµꢀ(oꢀphenylene)]ꢀ1´,5´ꢀdiphosphapentane}ꢀ
closoꢀ3,1,2ꢀdicarbollylruthenium (5) and 1´,3´ꢀbis(diphenylꢀ
phosphinopropane)chlororuthenium(II)ꢀ7,8ꢀdimethylꢀ7,8ꢀdicarbaꢀ
nidoꢀundecaborate (6). Complex 2 (0.36 g, 0.44 mmol) was added
to a solution of dppp (0.2 g, 0.48 mmol) in toluene (20 mL) and
the reaction mixture was refluxed for 2.5 h with stirring using
a magnetic stirrer. The solvent was evaporated in vacuo, the
residue was chromatographed on a column with silica gel,
the major reaction product (complex 5) being eluted with a
nꢀC6H12—CH2Cl2 mixture (1 : 1). The yield of 5 was 64%. The
second reaction product was eluted from the column by more
polar CH2Cl2, which was removed in vacuo to give 0.05 g (11%)
The authors are grateful to S. P. Solodovnikov (A. N.
Nesmeyanov Institute of Organoelement Compounds) for
recording the ESR spectrum of complex 5.
* S. Delfosse, A. Demonceau, I. T. Chizhevsky, and A. F. Noels,
unpublished results.

Rutheniumꢀcatalysed MMA polymerization
Russ.Chem.Bull., Int.Ed., Vol. 55, No. 1, January, 2006
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This work was supported by the Russian Foundation for
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Received January 31, 2005;
in revised form February 21, 2006