Possible mediators of the "living" radical polymerization

Article abstract of DOI:10.1016/j.saa.2005.10.007

The stable radicals derived from different compounds were detected in process of styrene autopolymerization. The nitroxide radicals are produced from nitrosocompound, hindered hydroxylamine, nitrophenols and nitroanisoles. The phenoxyl radicals are formed from quinine methides, and naphtoxyl radicals are generated from 2-nitro-1-naphtol. The radicals are identified, the kinetics of their formation and follow-up evolution are studied. These radicals can participate in process of living radical polymerization as the mediators and can effect significantly on kinetics of polymerization and structure of the resulting polymer.

Full text of DOI:10.1016/j.saa.2005.10.007

Spectrochimica Acta Part A 63 (2006) 802–815
Possible mediators of the “living” radical polymerization
M.V. Motyakina, A.M. Wassermana,∗, P.E. Stottb, G.E. Zaikovc
a Institute of Chemical Physics, Russian Academy of Science, ul. Kosygina 4, Moscow 119991, Russia
b Crompton Corporation, 199 Benson Road Middlebury, CT 06749, USA
c Institute of Biochemical Physics, Russian Academy of Science, ul. Kosygina 4, Moscow 119991, Russia
Received 1 July 2005; received in revised form 23 September 2005; accepted 1 October 2005
Abstract
The stable radicals derived from different compounds were detected in process of styrene autopolymerization. The nitroxide radicals are produced
from nitrosocompound, hindered hydroxylamine, nitrophenols and nitroanisoles. The phenoxyl radicals are formed from quinine methides, and
naphtoxyl radicals are generated from 2-nitro-1-naphtol. The radicals are identified, the kinetics of their formation and follow-up evolution are
studied. These radicals can participate in process of living radical polymerization as the mediators and can effect significantly on kinetics of
polymerization and structure of the resulting polymer.
© 2006 Published by Elsevier B.V.
Keywords: Styrene autopolymerization; Mediators; Nitroxide radicals; ESR
1. Introduction
phenols, nitroanisols, nitronaphtol, hindered hydroxylamine,
andquinonemethidesintheprocessofstyrenethermalautopoly-
merization. We have identified the radicals and studied kinetics
of their formation and transformation.
“Living” free radical polymerization controlled by a stable
radical ranks among the techniques for producing polymers with
well-defined microstructure (see, for example, [1–11]). Besides,
the “living” free radical polymerization does not require high
purity monomers and solvents that make this technique very
attractiveforindustrialapplication. Thebasicconceptconsistsof
the reversible trapping of the propagating polymeric chain with
a counter radical. The trapping reaction (reversible inhibition)
makes the classical termination almost negligible, thus lead-
ing to narrow molecular weight distribution of polymer. Either
stable nitroxide radicals or species generating nitroxide radi-
cals in situ are normally employed for the reversible trapping
of the growing polymeric chains. Nitrons, nitrosocompounds,
and alkoxyamines are the examples of such species [4,5,10,11].
There are few examples of employing of those chemical com-
pounds which are neither stable nitroxide radicals nor the species
which generate nitroxide radicals in situ. Therefore, a searching
for new compounds capable of effective participation in “living”
radical polymerization is one of the topical problems.
2. Experimental
Styrene was purified by distillation under vacuum (20 Torr) at
44–45 ◦C (n2D0 = 1.5462). m-Xylene was purified by distillation
with following selection of fraction boiling at 139–139.5 ◦C.
m-Dichlorobenzene was purified by distillation under vacuum
(14 Torr) at 67 ◦C.
Nitrosocompound, N,N-diethylhydroxylamine, quinone
methides have been obtained from Crompton Corporation. 2,4-
Dinitrophenol, 2,4-dinitroanysole and 2-nitroanysole have
been obtained from Acros. These compounds were used
without additional purification. 4-Nitrophenol (Chemapol)
and 3-nitrophenol (Acros) were recrystallized from benzene,
2-nitrophenol (Acros) was recrystallized from ethanol with
following sublimation in vacuum. Purity of the used compounds
was controlled by thin-layer chromatography.
In this connection we performed the EPR investigation of sta-
ble radicals derived from molecules of nitrosocompound, nitro-
The compounds were weighted and brought to vial with inert
solvent or styrene solution. The solution was stirred. After dis-
solving of compound ∼0.2 ml of the solution was placed in
a glass tube of 3 mm diameter. The solutions were studied in
the presence of oxygen and in the absence of oxygen. The
∗
Corresponding author.
E-mail address: spinchem@chph.ras.ru (A.M. Wasserman).
1386-1425/$ – see front matter © 2006 Published by Elsevier B.V.
doi:10.1016/j.saa.2005.10.007

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803
deoxygenated solutions were prepared by bubbling argon
through the solution or by repeating freeze-thaw cycles. The
experimental results did not depend on the method of deoxy-
genating. The glass tubes were sealed.
ESR spectra were recorded on the RADIOPAN ESR-
spectrometer equipped with
a flowing air thermostat-
temperature controller. For determination of the stable radical
concentration several nitroxyl radical concentration standards
were prepared by dissolving known amounts of nitroxyl radical
(4-hydroxy-2,2,6,6,-tetramethyl-pyperine-N-oxyl) in m-xylene.
The ESR spectra were doubly integrated, and the value of the
integral was plotted as a function of concentration. The radical
concentration was read from the calibration curve. The glass
tubes for all solutions were similar.
The Bruker programs were used for the theoretical analyze
of ESR spectra and determination of hyperfine constants.
3. Results and discussion
3.1. N-(1,4-Dimethylpentyl)-4-nitroso-aniline
Nitroso-compounds are widely used for the trapping of
carbon-centered radicals. The trapping reaction results in for-
mation of nitroxyl radical [12]. This nitroxyl radical can take
part in the process of living radical polymerization [4,5].
ESR spectrum of radicals in deoxygenated and oxygen-
containing styrene solutions of N-(1,4-dimethylpentyl)-4-
nitroso-aniline (structureisgiven in Scheme1) hasbeendetected
at 110 and 130◦ C. The ESR spectrum (Fig. 1a) consists of three
basicspectralcomponentsandadditionalsplitting. Threelinesof
the spectrum are due to interaction of uncoupling electron with
nitrogen nuclei (aN = 11.5 G), additional splitting is assigned to
interaction of uncoupling electron with protons of aromatic ring
(aH = 2.5 G). The spectrum is very well known and is typical for
aromatic nitroxyl radical [13–15].
Aromatic nitroxyl radicals are derived from reaction of N-
(1,4-dimethylpentyl)-4-nitroso-aniline with active radicals R•,
involved in polymerization. Scheme 1 shows the mechanism of
nitroxyl radical formation.
Kinetics of the nitroxyl radical formation generated dur-
ing styrene polymerization in the presence of N-(1,4-
dimethylpentyl)-4-nitroso-aniline is shown in Fig. 1b. It is
important to notice that nitroxyl radicals appear already during
the first minutes of thermal treatment. The radical concentra-
Fig. 1. (a) The ESR spectrum of nitroxyl radical (I) in styrene solution at
110 ◦C. The same spectrum is observed at 130 ◦C. (b) Kinetics of nitroxyl
radical (I) formation in styrene solution of N-(1,4-dimethylpentyl)-4-nitroso-
aniline: (᭹) in the presence of oxygen at 110 ◦C, nirosocompound concentration
is 4.5 × 10−3 M; (ꢀ) in the presence of oxygen at 130 ◦C, nirosocompound
concentration is 3.19 × 10−2 M; (ꢁ) in the presence of oxygen at 110 ◦C, niroso-
compound concentration is 3.19 × 10−2 M; (ꢂ) in the absence of oxygen at
130 ◦C, nirosocompound concentration is 2.42 × 10−2 M.
tion at temperature of 130 ◦C is higher than at temperature
of 110 ◦C. Maximum nitroxyl radical concentration at 130 ◦C
is approximately 102 times less than the initial concentration
of N-(1,4-dimethylpentyl)-4-nitroso-aniline in the styrene solu-
tion. At the same time concentration of the nitroxyl radicals in
oxygen-containing samples is approximately two times less than
concentration of the radicals in deoxygenated samples. It seems
that nitroxyl radicals are terminated by the side reactions in the
presence of oxygen.
Nitroxyl radical concentration increases during the first min-
utes of observation and then varies only slightly. The station-
ary radical concentration can be explained by the reaction of
reversible inhibition (Scheme 2).
Scheme 1.
Scheme 2.

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Scheme 3.
Scheme 4.
oxygen according to Scheme 3. Second reaction is an interaction
of DEHA with alkyl styrene radicals (Scheme 4).
Kinetics of formation of radicals (CH3CH2)NO• in DEHA
solutions in m-xylene and in styrene at 130 ◦C are presented in
Fig. 2b.
Within first minutes of heating radicals are detected in
DEHA solution in m-xylene. Concentration of the radicals grad-
ually decreases during heating. Radical formation is apparently
induced by oxidation of DEHA (Scheme 3). Radical termina-
tion is apparently due to reaction of radicals with molecules of
solvent or due to disproportional reaction leading to formation
of diamagnetic products.
Concentration of radical in DEHA solution in styrene
increases dramatically for first minutes. An increase of radi-
cal concentration is apparently induced by reaction of DEHA
with alkyl radicals of styrene (Scheme 4). With time the radi-
cal concentration decreases. A decrease of radical concentration
is due to the reactons much as in DEHA solution in m-xylene.
In addition, termination of nitroxyl radicals in styrene can be
caused by reaction with alkyl radicals of styrene (Scheme 5).
The product (CH3CH2)NOR is unstable and can participate
in process of living radical polymerization.
Fig. 2. (a) The ESR spectrum of radical (CH3CH2)2NO• in DEHA solu-
tion in styrene (1.7 × 10−1M) at 130 ◦C. (b) Variation of nitroxyl radical
(CH3CH2)2NO• concentration in DEHA solutions (1.7 × 10−1 M) in styrene
(ꢀ) and in m-xylene (ꢁ) at 130 ◦C.
We emphasize that a shape of the ESR signal does not change
during thermal treatment. It means that only one type of radicals
having the structure (I) has been detected.
3.3. Nitrocompounds
ItshouldliketonotethatreactiongivenatScheme2iscapable
to provide the “living chain” styrene polymerization.
Aromatic nitrocompounds are widely used for inhibition and
regulation of radical polymerization. Depending on structure
nitrocompounds can be both strong and weak inhibitors [17].
Besides, stable radicals formed in process of radical polymeriza-
tion in the presence of aromatic nitrocompounds can be involved
in living free radical polymerization [4,5,18]. Reactions of nitro
compound with active radicals involved in polymerization pro-
cess can be schematically written as in Scheme 6 [19–22].
According to Scheme 6, nitroxyl and aminyl radicals are
formed. Nitroxyl radicals are more stable than aminyl radicals.
Therefore, nitroxyl radicals accumulate in reaction system and
can participate in free radical polymerization. To effect on poly-
merization it is necessary to understand mechanism of reaction
in the presence of nitrocompounds and to clarify the structure
3.2. N,N-Diethylhydroxylamine (DEHA)
ESR spectrum of radicals formed in DEHA solution in
styrene at 130 ◦C is presented in Fig. 2a. The similar spec-
trum was detected in DEHA solution in m-xylene at 130 ◦C.
The spectrum can be positively assigned to nitroxyl radical
(CH3CH2)NO•. Analyze of spectrum was carried out by “Sim-
phonia” program (Bruker). Theoretical spectra were calcu-
lated. Good agreement was observed with experimental spectra.
Parameters used in simulation were following: hyperfine con-
stant of interaction with nitrogen aN = 15.2 G, hyperfine constant
of interaction with four protons of CH2 group aH = 10.3 G, line
width ꢀH = 1.4 G.
Consider possible mechanisms of nitroxyl radical formation
in DEHA solutions.
Nitroxyl radicals in DEHA solution in m-xylene can be
formed as a result of oxidation of DEHA by oxygen remained
in the solution after deoxygenating. Hindered hydroxylamines
react readily with oxygen [16]. Result of the reaction is forma-
tion of nitroxyl radical (Scheme 3).
Scheme 5.
Scheme 6.
Nitroxyl radicals in DEHA solution in styrene can be formed
by two reactions. First reaction is an oxidation of DEHA by

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805
Scheme 7.
ber of nitrogroups in molecule of aromatic compound [24,25].
According to ref. [24], one of the possible ways of radical forma-
tion in the nitrophenol-containing systems is the disproportional
reaction (Scheme 7).
Using Schemes 6 and 7, the possible mechanism of nitroxide
radical formation in 2,4-dinitrophenol solution of m-xylene can
be written as in Scheme 8.
The spectrum presented in Fig. 3a is likely to be assigned to
radical II. The value of aN = 9.6 G is caused by interaction of
uncoupling electron with nitrogen atom, the value of aN = 3.2 G
is caused by interaction with aromatic ring protons. Unfortu-
nately, at present time it is impossible to make positive con-
clusion about mechanism of formation and structure of nitroxyl
radicals formed in 2,4-dinitrophenol solution of m-xylene stored
for a long time.
3.3.1.1.2. 2,4-Dinitrophenol in styrene. ESR spectrum
(Fig. 3b) of stable radicals was detected in 2,4-dinitrophenol
solution in styrene kept at room temperature in the presence
of oxygen for 24 h. The spectrum can be assigned to nitroxide
radicals with hyperfine constants of aN = 10.0 G and aH = 3.2 G.
Notice that spectra in Fig. 3a and b are distinctively different. It
is reasonable to assume that formation of radicals, whose spec-
trum is presented in Fig. 3b, is related to two processes: firstly,
to reactions presented in Scheme 8, and, secondly, to the styrene
oxidation by 2,4-dinitrophenol leading to formation of active
radicals that finally transform to nitroxide radicals according to
Scheme 6. An extended analysis of formation and of structure of
active radicals formed in 2,4-dinitrophenol solutions of styrene
at room temperature needs some additional investigation.
Fig. 3. (a) The ESR spectrum of radical observed in 2,4-dinitrophenol solution in
m-xylene (1.36 × 10−1 M) after heating (180 min) at 130 ◦C. The spectrum was
recorded at room temperature after 24 days. (b) The ESR spectrum of radicals
observed in 2,4-dinitrophenol solution (9.1 × 10−2 M) in styrene in the presence
of oxygen at room temperature. The spectrum was recorded after storage of
sample for 1 day.
of stable radicals formed in the result of nitrocompound trans-
formation.
3.3.1. Nitrophenols
3.3.1.1. 2,4-Dinitrophenol.
3.3.1.1.1. 2,4-Dinitrophenol in m-xylene. First of all it was
necessary to clarify the possibility of stable radical formation
during the heating of 2,4-dinitrophenol in inert solvent. The m-
xylene was used as inert solvent. Solutions of 2,4-dinitrophenol
(1.36 × 10−1 M) in m-xylene with oxygen and without oxygen
were heated at 130 ◦C for 3 h. No radicals were detected by
ESR spectroscopy during the heating. After the heating, the
solutions were stored at room temperature. The solutions grad-
ually changed in color (yellow–green color appeared). Besides,
ESR signal appeared in the deoxygenated solution stored for
24 days (Fig. 3a). The signal is weak (concentration is about
8 × 10−7 M). The signal can be presented as triplet of triplets
and can be assigned to stable nitroxide radical with isotropic
coupling constants of aN = 9.6 G and aH = 3.2 G. It is important
to notice that aromatic nitrocompounds are the true oxidants
[23]. ESR spectra of stable nitrogen-containing radicals are
usually detected under heating of nitroaromatic compounds.
Concentration of the radicals increases with increasing num-
Scheme 8.

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Fig. 5. Kinetics ofnitroxyl radicalformationinstyrene solutionat 130 ◦C:(᭹) in
thepresenceofoxygen, 2,4-dinitrophenolconcentrationis1.38 × 10−1 M;(ꢀ)in
the presence of oxygen, 2,4-dinitrophenol concentration is 4.12 × 10−2 M; (ꢃ)
in the absence of oxygen, 2,4-dinitrophenol concentration is 4.34 × 10−2 M; (ꢂ)
in the absence of oxygen, 2,4-dinitrophenol concentration is 4.57 × 10−2 M.
radical with hyperfine constants of aN = 11.3 G and aH = 2.8 G
after 27 min of heating whereas after heating we observe nitrox-
ideradicalwithhyperfineconstantsof aN = 9.4 Gand aH = 3.8 G.
Kinetics of the nitroxyl formation at 130 ◦C is presented
in Fig. 5. Notice that nitroxyl radical concentration in styrene
solution increases with the increasing of 2,4-dinitrophenol con-
centration in the solution. However, the maximum nitroxyl
radical concentration is almost 104 times less than initial 2,4-
dinitrophenol concentration in the solution. Nitroxyl radical
concentration in deoxygenated solutions differs slightly from
the concentration in oxygen-containing solutions. Nitroxyl rad-
ical concentration increases during the first minutes of thermal
treatment and then decreases slowly. It is possible to assume that
nitroxyl radicals derived from nitrocompounds can take part in
the reversible inhibition.
Fig. 4. The ESR spectra of radicals formed in 2,4-dinitrophenol solution
(9.1 × 10−2 M) in styrene with oxygen at 130 ◦C.
The transformation of ESR spectra of radicals formed in
styrene solution of 2,4-dinitrophenol at 130 ◦C in the presence
of oxygen is in Fig. 4.
3.3.1.2. 4-Nitrophenol in styrene. ESR spectra of radicals
formed during heating of 4-nitrophenol solution in styrene in
the presence and in the absence of oxygen are presented on the
Fig. 6A. Fig. 6B presents kinetics of radical formation. All spec-
tra are assigned to nitroxide radicals formed in accordance with
Scheme 6.
ESR spectra of the sample heated for a short time differ dis-
tinctively from ESR spectra of the sample heated for a long time.
The general length of spectrum and distances between picks
change during the heating. It is not inconceivable that the spec-
trum observed at initial stage of heating can be a superposition
of spectra of nitroxide radicals III and IIIa, as well as, of alkoxy
nitroxide radical IV and aminyl radical V formed in accordance
with Scheme 6.
It should be paid attention on three moments.
Firstly, The ESR spectra in the presence of oxygen do not differ
from ESR spectra in the absence of oxygen. Nitroxide radi-
cals formed during polymerization have hyperfine constants of
aN = 11.7 G and aH = 3.0 G.
Secondly, there are no changes in ESR spectrum during heat-
ing.
Finally, the concentration of radicals generated in styrene solu-
tion of 4-nitrophenol is lower than in styrene solution of 2,4-
dinirophenol (compare results presented on Figs. 5 and 6). The
difference may be caused by different reactivity of nitro groups
in the two compounds.
Radicals IV and V are much less stable than radicals III and
IIIa, and are terminated during the heating and after heating
[20,26]. Radicals III and IIIa accumulate at different stages of
styrene polymerization reaction. That is why we detect nitroxide
3.3.1.3. 2-Nitrophenol in styrene. The ESR spectra of radicals
formed in styrene solution of 2-nitrophenol at 130 ◦C on the air
and in argon atmosphere are shown on Figs. 7 and 8. The kinetics
of radical formation is shown in Fig. 9.

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807
Secondly, concentration of the radicals formed in fresh made
samples is less than in samples stored for 1–2 days at room
temperature or at fridge temperature. Besides, kinetics of radical
formation in fresh made samples differs from kinetics of radical
formation in stored samples.
3.3.2. Nitroanisoles
3.3.2.1. 2,4-Dinitroanisole. ESR spectra of radicals formed in
styrene solutions of 2,4-dinitroanisole are given in Fig. 11a. The
ESR spectra registered in the presence of oxygen do not differ
from ESR spectra registered in the absence of oxygen. The ESR
spectra obtained are assigned to nitroxide radicals with hyper-
fine constants of aN = 10.8 G and aH = 3.0 G. It is important to
notice that the signal of ESR is detected not only in the sam-
ples heated at 130 ◦C but in samples kept at room temperature
as well.
Kinetics of radical formation and termination are shown in
Fig. 11b. Oxygen does not affect on the radical concentration.
Comparison with 2,4-dinitrophenol results (Fig. 5) shows
that 2,4-dinitrophenol produces more radicals than 2,4-
dinitroanisole with the same sample concentration.
It is necessary to pay special attention on the results of inves-
tigation of solution (1.07 × 10−1 M) stored for 6 days at room
temperature. It was found a large (5 × 10−5 M) concentration of
nitroxide radicals after storage (before heating). Besides, con-
Fig. 6. (A) The ESR spectra of radicals observed in solutions of 4-nitrophenol in
styrene (1.73 × 10−1 M; a, with argon; b, with oxygen) at 130 ◦C. (B) Kinetics
of radical formation in solutions of 4-nitrophenol in styrene at 130 ◦C. Concen-
trations of 4-nitrophenol are 1.73 × 10−1 M (with oxygen (᭹) and with argon
(ꢁ)) and 1.83 × 10−1 M (with oxygen (ꢀ) and with argon (ꢃ)).
There are no practically difference between spectra and kinet-
ics in the presence and the absence of oxygen. The radical
concentration is about the same as in 4-nitrophenol solutions
and is not changed after first 10 min of heating. But there is
transformation of ESR spectra. The general length of spectrum
and distances between picks change during the heating. It seems
that the spectra observed are a superposition of spectra of var-
ious types of stable radicals. Nitroxide radical spectrum with
hyperfine constants of aN = 10.1 G and aH = 3.5 G is obtained
after heating.
3.3.1.4. 3-Nitrophenol in styrene. ESR spectra of radicals
formed during heating of styrene solution of 3-nitrophenol in
the presence of oxygen are presented on the Fig. 10a.
The ESR spectra registered in the presence of oxygen do not
differ from ESR spectra registered in the presence of argon. The
ESR spectra are assigned to nitroxide radicals with hyperfine
constants of aN = 11.3 G and aH = 3.0 G. There is no change in
spectra of samples during heating at 130 ◦C.
The kinetics of radical formation is shown in Fig. 10b.
It should be paid attention on two peculiarities of radical
kinetics.
Firstly, the radical concentration in solution with oxygen is
higher than the radical concentration in solution without oxy-
gen. Perhaps, in this case oxygen initiates process of radical
formation.
Fig. 7. The ESR spectra of radicals observed in 2-nitrophenol solution
(2.44 × 10−1 M) in styrene with argon at 130 ◦C.

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Fig. 8. The ESR spectra of radicals observed in 2-nitrophenol solution
(2.44 × 10−1 M) in styrene with oxygen at 130 ◦C.
centration of the radicals increases dramatically during heating
of the sample.
3.3.2.2. 2-Nitroanisole in styrene. The ESR spectra of radicals
formed in 2-nitroanisole solution in styrene at 130 ◦C are shown
in Fig. 12a. The presence of oxygen does not affect on the ESR
spectrum. The spectrum is poor-resolved triplet. The ESR spec-
Fig. 10. (a) The ESR spectra of radicals observed in 3-nitrophenol solution
(1.32 × 10−1 M) in styrene at 130 ◦C in the presence of oxygen. (b) Kinetics of
radical formation in 3-nitrophenol solution in styrene at 130 ◦C. (᭹) With oxy-
gen, 1.81 × 10−1 M, stored in fridge for 2 days; (ꢃ) with argon, 1.73 × 10−1 M;
(ꢂ) with oxygen, 1.73 × 10−1 M; (ꢀ) with oxygen, 1.32 × 10−1 M, stored at
room temperature for 1 day.
tra are assigned to nitroxide radicals with hyperfine constants of
aN = 12.7 G and aH = 2.6 G. There is no change of spectra dur-
ing heating at 130 ◦C. Kinetics of radical formation is given in
Fig. 12b. The presence of oxygen does not affect significantly
on the concentration of radicals. In addition concentration of
radicals in 2-nitroanisole solutions differs slightly from the con-
centration of radicals in 2-nitrophenol solutions (Fig. 9). It is
important to pay attention on the following peculiarity: solution
stored for 1 day at room temperature gives more radicals during
the heating than fresh made solution.
Thus, the formation of stable nitroxide radicals in 2,4-
dinitrophenol solution in xylene in the absence of oxygen, and
in 2,4-dinitrphenol solution in styrene and 2,4-dinitroanisole
Fig. 9. Kinetics of radical formation in 2-nitrophenol solution (2.44 × 10−1 M)
in styrene at 130 ◦C. (᭹) With oxygen and (ꢃ) with argon.

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809
Scheme 9.
solution in styrene in the presence of oxygen during their stor-
age is observed. Nitroxide radical formation is observed during
the heating of 2,4-dinitrophenol, 2-nitrophenol, 3-nitrophenol,
4-nitrophenol, 2,4-dinitroanisole, 2-nitroanisole solutions in
styrene at 130 ◦C either in the presence or in the absence of oxy-
gen. Scheme 6 shows reactions of radical formation. In the cases
of 2-nitrophenol and 2,4-dinitrophenol transformation of ESR
spectra was observed during the heating. This transformation
may be caused by a superposition of spectra of different stable
radicals formed during the reaction. Concentration of radicals
formed in 2-nitrophenol solution differs slightly from concentra-
tion of radicals formed in 2-nitroanisole solution. But fresh made
2,4-dinitrophenol solution in styrene produces more radicals
than 2,4-dinitroanisole solution at the same conditions. There-
fore, substitution OH group on OCH3 affects on nitroxide radical
concentration differently depending on phenol structure. Con-
centration of radicals formed during styrene polymerization in
the presence of 3-nitrophenol, 2-nitroanisole, 2,4-dinitroanisole
increases after preliminary storage of solutions at room or fridge
temperature. Oxygen affects on concentration of radicals in
3-nitrophenol and 2,4-dinitroanisole solutions in styrene. The
stable nitroxyl radicals from nitrophenols and nitroanisols can
take part in living radical polymerization.
Fig. 11. (a) The ESR spectra of radicals observed in 2,4-dinitroanisole solution
(1.07 × 10−1 M) in styrene in the presence of oxygen. (b) Kinetics of radical
formation in 2,4-dinitroanisol solutions in styrene at 130 ◦C. (ꢃ) 1.07 × 10−1 M,
with oxygen, stored at room temperature for 6 days; 1.32 × 10−1 M, with oxygen
(ꢂ) and with argon (ꢁ).
3.3.3. 2-Nitro-1-naphtol
The ESR spectrum (triplet with hyperfine constant of
aN = 27.1 G) is observed in solution of 2-nitro-1-naphtol
(Scheme 9, compound VI) in m-xylene at 130 ◦C in the pres-
ence of argon (Fig. 13a). The ESR signal can be assigned to
iminoxyl radical (Scheme 9, compound VII).
In solution nitronaphtols can be in tautomeric equilibrium
with quinone shown in Scheme 10.
Iminoxyl radical VII can be formed as a result of breaking of
weak N OH bond in compound VIa initiated by some impurity.
It is important to note that concentration of iminoxyl radical is
small and slightly decreases in time (Fig. 13b).
The ESR spectrum of radicals formed in solution of 2-nitro-1-
naphtol in styrene at 130 ◦C is shown in Fig. 14. The spectrum
can be assigned to naphtoxyl radical (VIII), in which interac-
tion of uncoupling electron with five protons of aromatic rings
(aH = 4.4 G) takes place. Formation of the naphtoxyl radical, for
example, can be determined by hydrogen of OH-group abstrac-
tion reaction of 2-nitro-1-naphtol with active styrene radical. We
note that the naphtoxyl radical is stable and the radical concen-
tration increases all time (Fig. 13b).
Fig. 12. (a) The ESR spectra of radicals observed in 2-nitroanisole solutions
(1.83 × 10−1 M) in styrene at 130 ◦C. a, with oxygen; b, with argon. (b) Kinetics
of radical formation in 2-nitroanisole solutions in styrene (1.83 × 10−1 M) at
130 ◦C. (᭹) With argon, (ꢀ) with oxygen; (ꢁ) with oxygen, stored at room
temperature for 1 day; 2.74 × 10−1 M, (ꢃ) with argon, (ꢂ) with oxygen.
So, naphtoxyl radicals form in solution of 2-nitro-1-naphtol
in styrene at 130 ◦C. It should like also to note the difference
between 2-nitrophenol and 2-nitro-1-naphtol. Nitroxyl radicals

810
M.V. Motyakin et al. / Spectrochimica Acta Part A 63 (2006) 802–815
Fig. 14. ESRspectraofradicalsobservedinsolution2-nitro-1-naphtolinstyrene
(2.11 × 10−1 M) with argon at 130 ◦C and at room temperature after heating.
Fig. 13. (a) ESR spectrum of radicals observed in solution of 2-nitro-1-naphtol
in m-xylene (2.11 × 10−1 M) with argon at 130 ◦C for 8 min of heating. (b)
Kinetics of radical formation in solutions of 2-nitro-1-naphtol (2.11 × 10−1 M)
in styrene with argon (᭹) and in m-xylene with argon (ꢃ) at 130 ◦C.
were derived from 2-nitrophenol while naphtoxyl radicals were
derived from 2-nitro-1-naphtol.
Scheme 11.
3.4. Quinone methides
Quinone methides substituted at methylene group are more
stable than compound IX. They form in processes of oxidation
of hydrocarbons and polymers inhibited by hindered phenols
[35,36]. Hindered quinone methides react with alkyl radicals
and inhibit radical polymerization of styrene [37]. They inhibit
liquid-phase oxidation of hydrocarbons [37] and oxidation of
polymers [38]. Reactions of quinone methides with alkyl (R•)
Quinone methides belong to a class of unsaturated cyclic
ketones [27,28]. Their high reactivity attracts great interest.
The simplest quinone methides—unstable compounds. In the
moment of formation they polymerize themselves or interact
with the medium. High reactivity of quinone methides is deter-
mined mainly by low-lying excited level of the molecules [29].
Stability of quinone methides increases sufficiently by shield-
ing of oxygen atom [29,30]. For instance, compound IX was
obtained as individual compound. However, even at room tem-
perature this compound undergoes transformations with for-
mation of characteristic products of radical reactions [31,32].
ESR spectrum (triplet of triplets) was observed in solutions of
compound IX in inert solvents. The spectrum was assigned to
biradical X or phenoxyl radicals XI or XII [32–34] (Scheme 11).
and peroxide (RO• ) radicals can be presented by following
2
schemes [37,38] (Scheme 12).
Scheme 10.
Scheme 12.

M.V. Motyakin et al. / Spectrochimica Acta Part A 63 (2006) 802–815
811
Scheme 13.
stage of polymerization process (up to 80 min) at 130 ◦C ESR
spectrum of phenoxyl radical (XIII) radicals dominates. This
ESR spectrum corresponds to the reaction of phenoxyl radical
(XIII) formation is presented in Scheme 13.
Note that R• is active radical involved in process of styrene
polymerization.
Hyperfine structure constants determined from ESR spec-
trum (Fig. 15aA) are following: aH(1) = 8.5 G, aH(2) = 1.8 G. The
constant aH(1) is due to interaction of uncoupling electron with
hydrogen atom H(1). The constant aH(2) is determined by inter-
action of uncoupling electron with hydrogen atoms of phenyl
ring. The ESR spectra of phenoxyl radicals were studied exten-
sively [13,14,39]. Values of the hyperfine structure constants
allow us to determine the radical structure (XIII) uniquely.
Initial ESR spectrum transforms during the thermal treat-
ment at 130 ◦C. Signal intensity of radical (XIII) decreases, and
at the same time signal intensity of secondary stable radical
increases (Fig. 15aB). It means that radical (XIII) concentra-
tion decreases, and at the same time concentration of secondary
phenoxyl radical increases. The ESR spectrum of this radical
is triplet determined by interaction of uncoupling electron with
two hydrogen atoms of phenyl ring (aH = 1.8 G). Secondary phe-
noxyl radical arises due to reaction of disproportionation [13]
and reaction of active radical R• with quinine XIV (Scheme 14).
It is important to notice that the concentration of secondary
phenoxyl stable radicals is high at later stage (300 min) of the
process.
Fig. 15. (a) ESR spectra of radicals derived from quinine methide Q in styrene
solution (4.95 × 10−2 M) at 130 ◦C for 20 min (A) and 150 min (B). (b) Kinetics
of radical formation in styrene solution of quinine methide Q at 130 ◦C: (᭹)
4.95 × 10−2 M, in the presence of oxygen; (ꢀ) 4.09 × 10−2 M, in the absence
of oxygen.
In this work, formation of stable radicals during styrene poly-
merization at 130 ◦C in the presence of quinone methides Q (4-
benzylidene-2,6-di-tert-butyl-cyclohexa-2,5-dienone), QE (2,6-
di-tert-butyl-ethylidenecyclohexa-2,5-dien-1-one) and TBSQ
(3,5,3ꢀ,5ꢀ-tetra-butylstilbenequinone, structures are given
below) was studied.
Kinetics of the radical formation at 130 ◦C is presented in
Fig. 15b. Notice that the stable radical concentration is approxi-
mately 103 times less than the concentration of quinone methide
Q in styrene solution. The concentration of radicals in the pres-
ence of oxygen is significantly less than the radical concentration
in the absence of oxygen. It can be explained by the fact that
phenoxyl radicals are usually unstable in the presence of oxy-
gen [13]. After 140 min of thermal treatment at 130 ◦C radical
concentration varies insignificantly. It seems that reversible inhi-
bition takes place (Scheme 15).
3.4.1. Quinone methide “Q”
(4-benzylidene-2,6-di-tert-butyl-cyclohexa-2,5-dienone)
ESR spectrum of radicals in deoxygenated and oxygen-
containing styrene solutions of quinone methide “Q” has been
detected at 130 ◦C (Fig. 15a). The ESR spectrum obtained is a
combination of ESR spectra of different radicals. During the first
3.4.2. Quinone methides QE
(2,6-di-tert-butyl-ethylidenecyclohexa-2,5-dien-1-one) and
TBSQ (3,5,3ꢀ,5ꢀ-tetra-butylstilbenequinone)
3.4.2.1. Quinone methides QE and TBSQ in inert solvents.
No ESR signal in solutions of QE in inert solvents (m-

812
M.V. Motyakin et al. / Spectrochimica Acta Part A 63 (2006) 802–815
Scheme 14.
dichlorobenzene, m-xylene) was detected at room temperature.
ESR spectra of radicals formed in these solutions with argon
at 130 ◦C are presented in Fig. 16a. ESR spectrum (triplet of
triplets, aH(1) = 9.1 G, aH(2) = 1.8 G) was observed for the first
minutes of heating. The spectrum can be assigned positively to
phenoxyl radical XVa. Hyperfine constant, aH(1), is due to inter-
action of uncoupling electron with two protons of CH2-group
Fig. 16. (a) The ESR spectra of radicals formed in QE solution in m-xylene
(7.85 × 10−2 M) with argon at 130 ◦C. (b) The ESR spectra of radicals formed
in TBSQ solution in m-xylene with argon (9.2 × 10−3 M) at 130 ◦C.
[39]. The ESR spectrum changes during heating. It is due to
formation and accumulation of stable radicals that are products
of transformation of radical XVa (Scheme 16).
in para-position of phenoxyl radical. Hyperfine constant, aH(2)
,
is determined by interaction of uncoupling electron with two
meta-protons of aromatic ring. Similar spectra of radicals XVa
are very well known and are described in literature in details
ESR spectra of radicals formed in TBSQ solution in m-xylene
with argon at 130 ◦C are presented in Fig. 16b. The spectrum
observed for first minutes of heating is similar the spectrum
Scheme 15.

M.V. Motyakin et al. / Spectrochimica Acta Part A 63 (2006) 802–815
813
Scheme 16.
observed in solutions of QE. The spectrum can be assigned to
radical XVb (aH(1) = 9.1 G, aH(2) = 1.8 G). During heating the
shape of signal does not changes practically while the signal
intensity decreases significantly. It seems likely that the radical
termination proceeds without formation of new radical products.
Formation of stable phenoxyl radical in solutions of quinone
methides QE and TBSQ in inert solvent can be understood
in terms of involving of exited level of quinine methods [29].
Remind that molecule of quinone methide has low-lying exited
level. The first step of reaction can be described by the formation
of biradical XVI (Scheme 17).
Phenoxyl radical XV can be formed as the result of hydrogen
abstraction reaction of carbon center of biradical XVI with sol-
vent molecule (RH) or with other molecule of quinone methide
(Scheme 17). ESR spectrum of the phenohyl radical XV is
detected for first minutes of heating at 130 ◦C. With time this
radical either changes slightly (solutions of QE) or terminates
(solutions of TBSQ).
Fig. 17. The ESR spectra of radicals formed in QE solution in styrene
(8.4 × 10−2 M) with argon at 130 ◦C.
aH(1) = 9.1 G, aH(2) = 1.8 G) observed within first minutes can be
positively assigned to phenoxyl radical XVa. Mechanism of rad-
ical formation was discussed above (Scheme 17). With time the
ESR spectrum changes. It seems that in addition to the reactions
leading to formation of radical XVa reaction of QE with active
alkyl radical (R•) of styrene (Scheme 18) proceeds. This reaction
leads to formation of phenoxyl radical XVII. Besides, isomer-
ization and disproportional reactions of radicals XVa and XVII
can take place also. As a result, the ESR spectrum observed is
a superposition of spectra of different phenoxyl radicals formed
in solution of QE in styrene at 130 ◦C.
3.4.2.2. Quinone methides QE and TBSQ in styrene. No EPR
signal in solutions of QE in styrene was detected at room tem-
perature. ESR spectra of radicals formed in styrene at 130 ◦C
are presented in Fig. 17. The ESR spectrum (triplet of triplets,
ESR spectra of radicals formed in TBSQ solution in styrene
at 130 ◦C are shown in Fig. 18. During first minutes of heating
radicals XVb (triplet of triplets, aH(1) = 9.1 G, aH(2) = 1.8 G)
form. Mechanism of radical formation was discussed above
Scheme 17.
Scheme 18.

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M.V. Motyakin et al. / Spectrochimica Acta Part A 63 (2006) 802–815
Fig. 19. (a) Kinetics of stable radical formation in QE solutions in styrene
(8.4 × 10−2 M) with argon (ꢀ) and in m-xylene (7.85 × 10−2 M) with argon
(ꢁ) at 130 ◦C. (b) Kinetics of stable radical formation in TBSQ solutions in
styrene (9.9 × 10−3 M) with argon (ꢀ) and in m-xylene (9.2 × 10−3M) with
argon (ꢁ) at 130 ◦C.
superposition of ESR spectra of different phenoxyl radicals is
observed.
Kinetics of formation of stable radicals in solutions of QE in
m-xylene and in styrene at 130 ◦C are shown in Fig. 19a.
The concentration of phenoxyl radicals in solution of QE
in m-xylene increases during first 25 min and then changes
insignificantly (at least during 250 min). Overall concentration
of stable radicals in solution of QE in styrene increases during
first 100 min and, then changes insignificantly. It is important to
emphasize that stationary concentration of stable phenoxyl radi-
cals in styrene solution remains invariable during a long time (at
least during 300 min). Besides, the stationary concentration of
stable radicals in QE solution in styrene is significantly higher
than in QE solution in m-xylene.
Fig. 18. The ESR spectra of radicals formed in TBSQ solution in styrene
(9.9 × 10−3 M) with argon at 130 ◦C.
(Scheme 17). With time concentration of radicals XVb
decreases, and new radicals appear in solution. It seems, that
in addition to the reactions, presented in Scheme 17, reaction
of TBSQ with active alkyl radical (R•) of styrene (Scheme 19)
proceeds. Besides, further transformations of radicals XVb
and XVIII leading to new phenoxyl radicals (for instance, in
accordance with Scheme 14) are also possible. As a result,
Kinetics of formation of stable radicals in TBSQ solutions
in m-xylene and in styrene at 130 ◦C are shown in Fig. 19b.
Phenoxyl radicals XVb form during first minutes in solution
of TBSQ in m-xylene. The radicals disappear rapidly, and after
13 min the radical concentration is negligible. Phenoxyl radi-
cals form during first minutes in the solution of TBSQ styrene.
The concentration of the phenoxyl radicals in solution remains
sufficiently high during the long time (300 min) and does not
decrease.
So, stable phenoxyl radicals form in solutions of quinone
methides QE and TBSQ in styrene at 130 ◦C. The phenoxyl
radicals can participate in reversible inhibition and in mediation
of living radical polymerization.
Scheme 19.

M.V. Motyakin et al. / Spectrochimica Acta Part A 63 (2006) 802–815
815
4. Conclusion
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Result presented in this study show that stable radicals are
derived from inhibitor molecules in process of styrene polymer-
ization. The nitroxide radicals are produced from nitrosocom-
pound, hindered hydroxylamine, nitrophenols and nitroanisoles.
The phenoxyl radicals are produced from quinine methides and
naphoxyl radicals are generated from 2-nitro-1-naphtol. These
radicals can participate in process of living radical polymeriza-
tion as the mediators and can effect significantly on kinetics of
polymerization and structure the resulting polymer.
Toperformfunctionofreversibleinhibitorsthestableradicals
must fill several requirements [4].
First, they are bound to be stable at temperatures of the living
polymerization carrying out.
Second, the concentration of stable radicals should fall in
the range of 10−5 to 10−3 M at normal concentrations of the
growing polymeric radicals. At lower concentrations, the liv-
ing feature of polymerization is affected by strong contribution
of quadratic termination of the growing polymeric radicals. At
higher concentrations, the rate of polymerization is too slow.
Third, the concentration of the stable radicals-inhibitors may
not vary essentially during the polymerization except for initial
time interval when the process attains the steady state regime.
Essential reduction of the stable radicals concentration during
polymerization causes a rise in fraction of “dead” chains. Gen-
erally, this happens due to side chemical reactions.
As is evident from our work, the majority of examined com-
pounds fit satisfactorily the requirements formulated above.
These compounds generate stable radicals in the necessary
concentrations, which generally vary only slightly during poly-
merization. It must be emphasized that the listed requirements
are necessary but not sufficient for realization of living radical
polymerization. Nevertheless, there is reason to hope that the
compounds represented in this paper offer the new contribu-
tors of stable radicals capable by means of reversible inhibition
to control the growth of the polymeric chain in reactions of
polymerization. Effectiveness of these compounds will be char-
acterized in future researches.
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Acknowledgements
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The authors thank Prof. V.B. Golubev and Dr. M.Yu. Zarem-
ski for helpful discussion.
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