Nitroxyl radicals of the imidazoline series as agents of pseudoliving polymerization of styrene

Article abstract of DOI:10.1007/s11172-010-0278-x

Radical polymerization of styrene in the presence of 2,2,4,5,5-pentamethyl- 2,5-dihydro- Imidazol-1-oxyl, 2,2-diethyl-4,5,5-trimethyl-2,5-dihydroimidazol-1- oxyl, 2,2,5,5-tetramethyl- 4-phenyl-2,5-dihydroimidazol-1-oxyl, 2,2,5-trimethyl-4,5-diphenyl-2,5-dihydroimidazol- 1-oxyl, and 2-methyl-2,3-diphenyl-1,4-diazaspiro[4.5]deca-3-en-1-oxyl was studied. Effect of substituents in the nitroxyl radical and the nature of initiator on the features of "pseudoliving" polymerization and the molecular-weight characteristics of polystyrene synthesized were considered. Nitroxyl radicals of imidazoline series, like TEMPO and its derivatives, allow one to regulate polymerization of styrene and obtain polymers with relatively low values of polydispersity index.

Full text of DOI:10.1007/s11172-010-0278-x

Russian Chemical Bulletin, International Edition, Vol. 59, No. 8, pp. 1556—1564, August, 2010
1556
Nitroxyl radicals of the imidazoline series as agents
of pseudoliving polymerization of styrene
E. V. Kolyakina,a M. A. Lazarev,a M. V. Pavlovskaya,a I. A. Kirilyuk,b
I. F. Zhurko,b I. A. Grigor´ev,b and D. F. Grishina
aScientific Research Institute of Chemistry
N. I. Lobachevsky Nizhnii Novgorod State University,
23, korp. 5 prosp. Gagarina, 603950 Nizhnii Novgorod, Russian Federation.
Fax: +7 (831) 465 8162. Eꢀmail: grishin@ichem.unn.ru
bN. N. Vorozhtsov Novosibirsk Institute of Organic Chemistry,
Siberian Branch of the Russian Academy of Sciences,
9 prosp. Akad. Lavrent’eva, 630090 Novosibirsk, Russian Federation.
Fax: +7 (838 3) 330 8850
Radical polymerization of styrene in the presence of 2,2,4,5,5ꢀpentamethylꢀ2,5ꢀdihydroꢀ
imidazolꢀ1ꢀoxyl, 2,2ꢀdiethylꢀ4,5,5ꢀtrimethylꢀ2,5ꢀdihydroimidazolꢀ1ꢀoxyl, 2,2,5,5ꢀtetramethylꢀ
4ꢀphenylꢀ2,5ꢀdihydroimidazolꢀ1ꢀoxyl, 2,2,5ꢀtrimethylꢀ4,5ꢀdiphenylꢀ2,5ꢀdihydroimidazolꢀ
1ꢀoxyl, and 2ꢀmethylꢀ2,3ꢀdiphenylꢀ1,4ꢀdiazaspiro[4.5]decaꢀ3ꢀenꢀ1ꢀoxyl was studied. Effect
of substituents in the nitroxyl radical and the nature of initiator on the features of "pseudoliving"
polymerization and the molecularꢀweight characteristics of polystyrene synthesized were conꢀ
sidered. Nitroxyl radicals of imidazoline series, like TEMPO and its derivatives, allow one to
regulate polymerization of styrene and obtain polymers with relatively low values of polydisperꢀ
sity index.
Key words: controlled radical polymerization, imidazoline radicals, nitroxyl radicals, styrene.
Radical polymerization in the regime of "living" chains
is one of the efficient methods for the synthesis of polyꢀ
mers with certain molecular weight and low polydisperꢀ
sity. Nitroxide mediated polymerization (NMP) is the
most widespread version used for its performing.1—6 The
NMP mechanism is based on the reaction of propagation
Under certain temperature conditions, the inhibition
process (see Scheme 1, pathway a) becomes reversible,
leading to regeneration of propagation radicals. The equiꢀ
librium of the reversible inhibition reaction is determined
by the ratio of the rate constants of radical recombination
(kr) and alkoxyamine dissociation (kd) reactions. In turn,
the constant ratio indicated considerably depends on
the structure of a nitroxyl radical. It is known1—6 that
2,2,6,6ꢀtetramethylpiperidinꢀ1ꢀoxyl (TEMPO) was one
of the first nitroxyl radicals suggested for accomplishing
"pseudoliving" radical polymerization at the temperatures
of 120—140 °C. In the last years, a clear tendency is
observed in the development of this concept consisting in
the search for new sterically hindered radicals, which in
contrast to the TEMPO derivatives allow one to carry out
the "pseudoliving" polymerization under milder temperaꢀ
ture conditions (<100 °C).7—16 An approach consisting in
the introduction of bulky substituents (for example, spiro
structures7—10 or silyloxyalkyl groups11,12) at αꢀposition to
the >N—O• group in piperidine nitroxyls has proved very
efficient, as well as in the synthesis of new sterically
hindered nitroxyls, such as SG1,13,14 1,1ꢀdiadamantylꢀ
nitroxyl,15 and bisnitroxyl.16 In addition, a number of
research groups are developing an original method for
•
radicals (~Pn ) with stable nitroxyl radicals (>N—O•) to
form alkoxyamines (~Pn—O—N<, Scheme 1, pathway
a), that results in suppression of bimolecular termination
of growing chains (see Scheme 1, pathway b).
Scheme 1
kp
∼Pn• is a polymer radical, kr, kd, kp, and kt are the rate
constants of recombination, dissociation, propagation,
and termination of a chain, respectively, M is a monomer
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1519—1527, August, 2010.
1066ꢀ5285/10/5908ꢀ1556 © 2010 Springer Science+Business Media, Inc.

Nitroxide imidazole radicals in polymerization
Russ.Chem.Bull., Int.Ed., Vol. 59, No. 8, August, 2010
1557
carrying out polymerization in the regime of "living"
chains, consisting in the generation of sterically hindered
nitroxyl radicals directly during polymerization (in situ)
by entrapment of bulky radicals of propagation or initiaꢀ
tor with spinꢀtraps.17—23
phenylꢀ2,5ꢀdihydroimidazolꢀ1ꢀoxyl (4), and 2ꢀmethylꢀ
2,3ꢀdiphenylꢀ1,4ꢀdiazaspiro[4.5]decaꢀ3ꢀenꢀ1ꢀoxyl (5).
Results and Discussion
In a number of recent works,24—26 imidazoline nitroxyls
were suggested for the use in carrying out "pseudoliving"
polymerization of styrene (ST). It was found that these
compounds allow one to obtain polystyrene (PS) with
relatively low values of polydispersity index (1.2—1.3) and
required molecular weight (MW) in the temperature range
80—130 °C.
In order to further study specific features of controlled
synthesis of macromolecules and search for new efficient
regulators based on nitroxyl radicals, we studied polymerꢀ
ization of ST in the presence of nitroxyls of imidazoline
series containing various sterically hindered groups at poꢀ
sitions 2, 4, and 5: 2,2,4,5,5ꢀpentamethylꢀ2,5ꢀdihydroꢀ
imidazolꢀ1ꢀoxyl (1), 2,2ꢀdiethylꢀ4,5,5ꢀtrimethylꢀ2,5ꢀdiꢀ
hydroimidazolꢀ1ꢀoxyl (2), 2,2,5,5ꢀtetramethylꢀ4ꢀphenylꢀ
2,5ꢀdihydroimidazolꢀ1ꢀoxyl (3), 2,2,5ꢀtrimethylꢀ4,5ꢀdiꢀ
It was found experimentally that the ratio of concenꢀ
trations of an initiator, a nitroxyl, and a monomer plays a
key role for the synthesis of polymers with a narrow moꢀ
lecularꢀweight distribution (MWD). For instance, when
concentrations of an initiator and a nitroxyl are low with
respect to a monomer (1 : 1.5 : 1000 and 1 : 1.5 : 500), PS
with high polydispersity indexes, viz., 1.6—3.0, are formed
independent on the temperature conditions (Table 1). If
the concentrations of an initiator and a nitroxyl is inꢀ
creased with respect to ST by the order of magnitude
(1 : 1.5 : 100, see Table 1), PS with low enough polydisꢀ
persity indexes (Mw/Mn) can be synthesized. However, a
controlled process of polymerization of ST with such a ratio
of reagents mainly occurred at temperatures ≥ 100 °C. In
the lowꢀtemperature range, polymerization is completely
Table 1. Effect of synthetic conditions on molecularꢀweight characteristics of PS ([ST]0 = 8.71 mol L–1
)
Nitroxyl
T/°С
[ST]0 : [AIBN]0 : [Nitroxyl]0
t/h
Conversion (%)
Mn•10–3
Mw/Mn
1
100
1000 : 1 : 1.5
500 : 1 : 1.5
1000 : 1 : 1.5
500 : 1 : 1.5
100 : 1 : 1.5
100 : 1 : 1.5
100 : 1 : 1.5
100 : 1 : 1.5
1000 : 1 : 1.5
100 : 1 : 1.5
1000 : 1 : 1.5
1000 : 1 : 1.5
500 : 1 : 1.5
100 : 1 : 1.5
1000 : 1 : 1.5
500 : 1 : 1.5
1000 : 1 : 1.5
500 : 1 : 1.5
100 : 1 : 1.5
100 : 1 : 0.5
100 : 1 : 1.0
100 : 1 : 1.5
100 : 1 : 2.0
4
10
4
7
10.0
7.9
8.0
7.1
3.7
4.0
3.0
4.5
48.0
—
10.0
21.0
20.0
2.3
4.0
8.9
8.1
9.7
2.0
10.4
5.5
5.0
3.1
2.41
2.16
1.99
1.74
1.43
1.60
1.38
1.38
1.81
120
3
10
10
75
38
31
4
30
4
3
10
10
4
10
3
10
10
10
39
81
97
10
25
10
27
44
71
9
0
4
35
89
19
6
2
3
70
90
120
80
—
100
120
2.41
1.82
1.61
1.28
3.19
3.68
2.27
1.63
1.37
2.00
1.50
1.23
1.25
4
5
100
120
8
21
61
14
75
79
82
79
120

1558
Russ.Chem.Bull., Int.Ed., Vol. 59, No. 8, August, 2010
Kolyakina et al.
inhibited. Further increase in the concentration of
nitroxyl (see Table 1) do not considerably improve the
Mw/Mn, rather it retards the rate of polymerization. To
sum up, from analysis of the experimental data on polyꢀ
merization of ST in the presence of imidazoline nitroxyls
it follows that the temperature of 120 °C and the high
concentration of a nitroxyl with respect to a monomer
(1.5 : 100) are the optimum conditions for the "pseudoꢀ
living" process.
It should be noted that polymerization of ST in the
presence of these nitroxyls and peracyl or azonitrile iniꢀ
tiators (benzoyl peroxide (BP) and azoisobutyric acid
dinitrile (AIBN)) has some specific features. In all the
cases, periods of induction were observed in the initial
step (Fig. 1). When the peroxide initiator was used, the
induction period ranged from ~4 to 10 h depending on
the structure of the nitroxyl radical 1—5. In the case of
AIBN, induction periods were somewhat shorter and
changed in the range from ~40 min to 6 h. Since the halfꢀ
life periods for AIBN and BP at 120 °C are less than 1 and
3 min, respectively,27,28 it can be stated that the initiators
under conditions selected are completely consumed
already within 30 min (≥ 10 halfꢀlife periods) after the
process began. Thus, formation of PS after the time indiꢀ
cated (ti) points to the direct involvement of alkoxyamines
into its synthesis, which are formed in the polymerization
system (in situ) due to recombination of nitroxyls with
radicals of initiator and macroradicals emerging in the
process of polymerization of ST selfꢀinitiation.
The polymerization processes studied can be described
by the first order kinetic equation in monomer (see Fig. 1).
The dependences of ln([M]0/[M]) versus time given have
a linear character to the high degrees of conversion
(~70%), that points to the constant concentration of growꢀ
ing radicals and is an indirect evidence of "pseudoliving"
character of polymerization of ST in the presence of
nitroxyls of imidazoline series. When specific features
of polymerization of ST in the presence of identical nitroxyl
radicals and various initiators are compared, it is seen that
the process of polymerization of ST in the presence of BP
and nitroxyls 2 and 3 occurs at retarded rates and higher
absolute values of induction period as compared to polyꢀ
merization in the presence of AIBN. Polymerization of ST
in the presence of 5 for both BP and AIBN proceeds virtuꢀ
ally within the same period of time at comparable rates.
The experimental data obtained are an evidence that
upon initiation with BP, the rate of polymerization of ST
is weakly dependent on the structure of substituents in the
imidazoline nitroxyls, though it somewhat decreases in
the following order: 4 > 5 > 3 ≈ 2 ≈ 1. In this case, comꢀ
parison of the slopes corresponding to the lines of nitroxyls
1 and 3 (see Fig. 1, a) indicates that the introduction of a
phenyl ring at position 4 has no effect on the rate of
polymerization of ST. Thus, as in the case of TEMPO
derivatives, "remote" sterical effect is not observed.29,30
Polymerization of ST has the highest rate in the presence
of nitroxyl radicals with bulky substituents at positions 2
and/or 5 of the imidazole ring (αꢀposition with respect to
the nitroxyl fragment).
ln([M]0/[M])
a
2.0
1.6
1.2
0.8
0.4
1
2
3
4
5
20
ln([M]0/[M])
40
60
t/h
b
1.6
1.2
0.8
0.4
2
3
5
For high concentrations of a nitroxyl radical, converꢀ
sion of a monomer after ti is described by the equation5,31
ln[M]0/[M] = (kpKeq2f[I]0/[>NO•]t )t,
(1)
i
where [M]0 and [M] are the initial and current concentraꢀ
tions of a monomer, [I]0 is the initial concentration of an
initiator (AIBN or BP), f is the efficiency of initiation
(0.5—0.6), Keq = kd/kr, [>NO•]ti is the concentration of
a nitroxyl radical.
20
40
60
t/h
Fig. 1. Dependence of ln([M]0/[M]) from time on polymerizaꢀ
tion of styrene initiated with BP (a) and AIBN (b) at 120 °C in
the presence of nitroxyls 1—5 (a) and 2, 3, 5 (b). The ratio of
A certain acceleration of polymerization when nitroxyls
4 and 5 are used can be due to the increase in the dissociaꢀ
concentrations of polymerization system components: [ST]0
=
= 8.71 mol L–1, [ST]0 : [Initiator]0 : [Nitroxyl]0 = 100 : 1 : 1.5.

Nitroxide imidazole radicals in polymerization
Russ.Chem.Bull., Int.Ed., Vol. 59, No. 8, August, 2010
1559
tion rate constants of alkoxyamines formed in situ (kd, see
Eq. (1)), as well as due to the decrease in the recombinaꢀ
tion rate constants of nitroxyls and propagation radicals
(kr, see Eq. (1)) because of the steric hindrance created by
bulky groups. In fact, a number of works using alkoxyꢀ
amines of various structure as an example demonstrate
that an increase in the bulkiness of the groups at αꢀposiꢀ
tion with respect to the nitroxyl fragment leads to a conꢀ
siderable increase in the dissociation rate constants (kd)
of such alkoxyamines12—14,24,32 and decrease in the reꢀ
combination rate constant (kr) of stable nitroxyl with a
propagation macroradical.11,12,32
Retardation of polymerization rate in the presence of
sterically hindered nitroxyl 5 as compared to 4 is apparꢀ
ently due to a number of reasons. In particular, this cirꢀ
cumstance can be related to the soꢀcalled "levelled" steric
effect.33 This effect consists in the fact that the dissociaꢀ
tion constants of alkoxyamines change nonmonotonously
with the increase in the size of substituents of the nitroxyl
fragment. For the substituents of a very large size, kd do
not virtually increase or can decrease. Another explanaꢀ
tion for the effect observed can consist in the proceeding of
side disintegration reactions of alkoxyamines, for example,
intramolecular and intermolecular hydrogen transfer.34
Initiation with AIBN shows no clear dependence of
the rate of polymerization and induction period versus
structure of a nitroxyl radical. For instance, from Fig. 1, b
one can see that the rate of polymerization of ST deꢀ
creases in the order 2 > 3 > 5. An increase in steric hinꢀ
drance created by substituents at Cα atoms of the nitroxyl
group changes in another sequence. Considerable inducꢀ
tion period and low rate of polymerization of ST in the
presence of nitroxyl 5 containing bulky substituents are
due to a number of factors. As it follows from Eq. (1), a
degree of conversion will depend not only on the equilibꢀ
rium constant, but also on the ratio of concentrations
of initiating radicals and a nitroxyl. For AIBN, a "cage
effect" becomes a considerable factor at 120 °C, which
determines efficiency of initiator and, therefore, ability of
radicals formed to react with nitroxyl radicals and further
initiation of polymerization. In the case of the in situ
synthesis of alkoxyamines from nitroxyl radicals 2, 3, and
5 and radical initiator, the current concentrations of the
alkoxyamine formed and residual nitroxyl will depend on
kr and efficiency of the outcome of initial radicals from
the "cage". It should be noted that the introduction of
sterically hindered groups in the nitroxyl fragment of
alkoxyamine to a greater extent affects not the dissociaꢀ
tion rate constants of the corresponding alkoxyamines,
but the decrease in the recombination rate constant of
stable nitroxyl with propagation macroradical.11,12 When
nitroxyl 5 is used, for which the kr value is lower virtually
by the order of magnitude32 than in the case of nitroxyls 2
and 3, recombination of initial radicals of initiator in the
"cage" with the formation of nonradical products is more
likely. It is logically to suggest that polymerization of ST
in the presence of alkoxyamines synthesized in situ from
nitroxyl 5 proceeds under conditions of decreased conꢀ
centrations of alkoxyamine and an excess of nitroxyl radiꢀ
cals. This leads to an increase in the induction period and
Table 2. Molecularꢀweight characteristics of PS synthesized at 120 °C in the presence of various initiators*
Initiator Nitroxyl Conversion
(%)
Mn•10–3
Mw/Mn
Initiator Nitroxyl
Conversion Mn•10–3
Mw/Mn
(%)
BP
1
2
3
4
18
27
42
59
82
17
40
53
73
80
19
28
39
61
82
26
43
49
64
80
2.2
2.8
4.1
5.2
7.4
1.5
3.7
6.0
8.0
8.4
2.0
2.3
3.2
4.6
5.9
4.4
5.2
5.8
6.5
7.9
1.27
1.32
1.39
1.35
1.40
1.18
1.25
1.28
1.27
1.27
1.23
1.24
1.25
1.26
1.24
1.65
1.61
1.63
1.63
1.60
BP
5
2
3
5
28
39
53
63
83
8
28
51
66
71
19
27
48
66
75
22
31
61
75
82
3.3
4.6
5.6
6.3
7.8
1.3
1.7
2.5
4.1
4.5
2.2
2.3
2.8
3.5
3.7
3.0
3.3
4.4
4.7
5.0
1.38
1.42
1.43
1.43
1.40
1.40
1.39
1.38
1.38
1.38
1.27
1.24
1.28
1.25
1.25
1.27
1.23
1.30
1.23
1.23
AIBN
* [ST]0 = 8.71 mol L–1, the ratio [ST]0 : [Initiator]0 : [Nitroxyl]0 = 100 : 1 : 1.5.

1560
Russ.Chem.Bull., Int.Ed., Vol. 59, No. 8, August, 2010
Kolyakina et al.
retardation of the process. Analogous decrease in the rate
was observed in the works,35,36 in which, together with
alkoxyamines, related nitroxyl radicals were involved into
polymerization. Such a change in kinetics of polymerizaꢀ
tion is due to the "persistent radical effect".5
In the case of synthesis of PS with involvement of
nitroxyls 2, 3, and 5 and AIBN, experimental Mn values
do not correlate with those calculated using formula (2)
(see Fig. 2, b). Two regions can be selected on the graph:
theoretical values of number average MW of 2000 g mol–1
and higher. In the diagram region of low MW, the Mn
values found experimentally are higher as compared to
theoretical. In the case of nitroxyl 3, in the region to 2000
g mol–1 the MW of PS decreases with conversion of the
monomer. In the region of MW higher 2000 g mol–1 for
all the nitroxyls studied, dependences of Mn from converꢀ
sion are approximated by straight lines. However, the MW
values found experimentally are lower than those preꢀ
dicted theoretically (Fig. 2, b), that is reflected in the
lower slopes of functional curves. The effect observed is
due to the higher concentration of growing radicals durꢀ
ing polymerization of ST initiated by AIBN with involveꢀ
ment of dihydroimidazole nitroxyls, due to a considerable
contribution of the thermal component. This is also
reflected in higher rate of polymerization and has been
discussed above.
The number average molecular weights of the polyꢀ
mers synthesized in the presence of either BP, or AIBN
with the concentration of nitroxyls being 1.5 mol.% linꢀ
early increase with conversion of the monomer (Table 2).
The polydispersity indexes of PS are low (see Table 2), with
the correlation between polydispersity indexes of polyꢀ
styrene and the rate of the synthesis of the latter being
observed. Polymers formed at low rate are characterized
by the lowest values of polydispersity indexes (1.2—1.3).
An increase in the rate of polymerization leads to the
increase in polydispersity indexes. This regularity is
the most pronounced in the synthesis of PS in the presꢀ
ence of BP and sterically hindered nitroxyls 4 and 5 (see
Table 2).
In addition, it should be noted another distinguishing
feature of polymerization of ST in the presence of difꢀ
ferent initiators. The number average values of polystyꢀ
rene MW synthesized in the presence of BP and nitroxyls
1—5 (Fig. 2, a) are in good agreement with the theoretiꢀ
cally calculated values of Mn:
The fact found experimentally is explained by the low
rate of alkoxyamines decomposition formed in situ in the
presence of AIBN in the first moment of time. This effect
has been predicted theoretically and substantiated earꢀ
lier.5,37 To make allowance for the rate of decomposition
of initial adducts, a number of correction factors were
suggested5,37 to introduce into the denominator of Eq. (2):
Mncalc = MST([M]0 — [M])/(2f[I])0,
(2)
where MST is the MW of a monomer unit of styrene.
Mncalc = MST([M]0 — [M])/{[I]0[1 – exp(–kdt)]},
(3)
In all the cases, the dependences given are extrapoꢀ
lated to zero, that points that polymerization of ST in the
presence of selected nitroxyls has "pseudoliving" characꢀ
ter and good enough correlation with the "ideal" condiꢀ
tions for controlled process.
Mncalc = MST([M]0 — [M])/{[I]0[1 –
– exp(–kdt)] + Rit},
(4)
Mn•10–3
Mn•10–3
10
10
a
b
8
8
6
4
2
1
2
3
4
5
6
4
2
2
3
5
10 Mn •10–3
calc
2
4
6
8
10 Mn •10–3
calc
2
4
6
8
Fig. 2. Correlation of experimental Mn values of PS with theoretically calculated Mncalc values at different degrees of conversion of the
PS monomer synthesized in the presence of BP (a) and AIBN (b) at 120 °C and nitroxyls 1—5 (a) and 2, 3, 5 (b). The ratio of
concentrations of polymerization system components: [ST]0 = 8.71 mol L–1, [ST]0 : [Initiator]0 : [Nitroxyl]0 = 100 : 1 : 1.5.

Nitroxide imidazole radicals in polymerization
Russ.Chem.Bull., Int.Ed., Vol. 59, No. 8, August, 2010
1561
where kd is the rate constant of dissociation of alkoxyꢀ
amines, Ri is the rate of thermal selfꢀinitiated polymerizaꢀ
tion of styrene, t is the time.
mation of alkoxyamines showed40 that in the case of orꢀ
ganic peroxides and azonitriles, alkoxyamines of different
structures can be formed. When alkoxyamines containing
several monomeric units are derived from BP, the MM of
polymers and the rates of their synthesis are completely
predictable. A clear correlation is present between charꢀ
acteristics and structure of a nitroxyl radical. The presence
of "anomalies" indicated previously in the case of AIBN is
explained by the difference in the rates of alkoxyamine
decomposition and a possibility to proceed for the side
nonradical decomposition reactions of initial adducts of
cyanoisopropyl and nitroxyl radicals, for example, intraꢀ
molecular hydrogen transfer.34
A possibility to carry out postpolymerization is an
irrefutable evidence of polymerization in controlled reꢀ
gime using the nitroxyls selected. The process was carried
out at 120 °C in the presence of macroinitiators obtained
using nitroxyls 1—5. Postpolymerization resulted in
the formation of a polymer with the MW higher than the
starting polymer (Table 3). In all the cases (when peracyl
and azonitrile initiators were used), the curves of the
MWD of postpolymers are unimodal, and their mode is
displayed to the region of higher MW as compared to the
starting samples (Fig. 4). In this case, the MWD curves of
initiating polymers and postpolymers have nonoverlapped
regions, i.e., the lowꢀmolecularꢀweight fractions of initiꢀ
ating polymers are absent in postpolymers. Polydispersity
indexes of postpolymers obtained in the presence of
nitroxyls 1—3 are low enough (1.2—1.4) and comparable
with the macroinitiator polydispersity indexes (see Table 3).
This is an evidence that the length of all the chains
included in the initiating polymers is increased due to the
addition of new monomeric units. When postpolymerizaꢀ
For the theoretical calculations of MW by Eqs (3) and
(4), the dissociation rate constant value (kd)32 for nitroxyl 3
of 4.4•10–5 s–1 and Ri (see Ref. 38) of 6•10–7 mol L–1 s–1
were used. To determine [M], known values of kp (see
Ref. 38) and kt,37 equal to 2026 and 108 L mol–1 s–1, reꢀ
spectively, were used, as well as a recombination rate
constant for nitroxyl 3 kr equal to 1.6•108 L mol–1 s–1
(see Ref. 32). It is seen from the data given in Fig. 3 that
the dependence of Mn from the monomer conversion calꢀ
culated by Eq. (2) is linear within all the range of converꢀ
sions (curve 1). If a correction coefficient, making allowꢀ
ance for the alkoxyamines dissociation rate constant (see
Eq. (3)), is introduced, the Mn values calculated theoretiꢀ
cally decrease with the increase in conversion to 20% (see
Fig. 3, curve 2). Introduction into the denominator of a
term responsible for the thermal selfꢀinitiated polymerꢀ
ization of styrene (Ri), whose value is constant only for the
initial degrees of the monomer conversion (see Eq. (4)),
leads to yet greater decrease in the calculated values in the
region of initial conversions (see Fig. 3, curve 3). The Mn
values obtained experimentally (curve 4) for the low
conversions better correlate with values calculated using
Eqs (3) and (4). Analogous regularities have been noted
earlier.39
A reason for the differences in both the polymerization
rates and molecularꢀweight characteristics of polymers
obtained in the presence of nitroxyls 1—5 and different
initiators (BP or AIBN) consists in different reactivity of
the initial radicals formed upon decomposition of initiaꢀ
tor. For instance, kinetic analysis performed for the forꢀ
Table 3. Molecularꢀweight characteristics of PSꢀmacroinitiators
(indicated with an asterisk) and postpolymers synthesized at
120 °C
Mn•10–3
Initiator Nitroxyl t/h
Conversion Mn•10–3 Mw/Mn
8
6
4
(%)
BP
1
2
3
4
5
2
3
5
50*
14
69*
45
73*
47
43*
62
6.3*
10.4
7.9*
11.0
4.1*
7.5
1.35*
1.43
1.26*
1.30
1.27*
1.31
50*
14
30*
14
2
4
2
3
30*
14
30*
14
69*
41
63*
43
6.9*
11.7
6.3*
8.1
1.63*
1.72
1.43*
1.42
1
40
80
C (%)
Fig. 3. Dependence of experimental and theoretically calculated
Mn of PS from conversion (C) of the monomer: 1, 2, 3 are the
Mn values calculated using Eqs (2), (3), and (4), respectively;
4 is the experimental Mn of PS synthesized in the presence of
nitroxyl 3, the ratio concentrations of polymerization system comꢀ
ponents: [ST]0 = 8.71 mol L–1, [ST]0 : [Initiator]0 : [Nitroxyl]0 =
= 100 : 1 : 1.5.
AIBN
17*
16
51*
67
2.5*
7.4
1.38*
1.23
20*
50
70*
38
48*
98
2.8*
6.7
1.28*
1.43
73*
81
4.8*
5.2
1.25*
1.75

1562
Russ.Chem.Bull., Int.Ed., Vol. 59, No. 8, August, 2010
1
Kolyakina et al.
able effect on the kinetic regularities of polymerization
of ST and molecularꢀweight characteristics of polymers
synthesized. These specific features are seen the most
clear when a peroxide initiator was used.
a
2
3
Experimental
Initiators BP and AIBN were recrystallized from hexane and
ethanol, respectively. Solvents were purified according to the
standard procedures,41,42 ST was purified off from hydroquinone
by repeated washing with 5% aqueous alkali until the monomer
became colorless, then was washed off from residual alkali with
DI water until the aqueous layer was neutral, and dried with
calcium dichloride. The dried monomer was distilled under
reduced pressure. Physicoꢀchemical constants agree with the
literature data.43
4
Synthesis of nitroxyl radicals. All the nitroxyl radicals
synthesized were characterized by IR spectroscopy on a Bruker
Vector 22 FTIR spectrometer (in KBr pellets with the conꢀ
centration 0.6%, the pellet was 1 mm thick), UV spectroscopy
on a HP Agilent 8453 spectrometer (EtOH with concentration
10–4 mol L–1), NMR spectroscopy on a Bruker AV300 spectroꢀ
meter (1H, 300.1 MHz; 13C, 75.5 MHz) or Bruker AMꢀ400
spectrometer (1H, 400.136 MHz; 13C, 100.6 MHz) in CDCl3,
and elemental (C, H) analysis.
3.0
3.5
4.0
4.5
5.0 logM
b
2
1
Nitroxyl radicals 1—3 were synthesized according to the
procedures published earlier.44—46 Nitroxyl radicals 4 and 5 were
obtained according to the general Scheme 2 from 2,2ꢀdimethylꢀ
4ꢀphenylꢀ2ꢀhydroimidazole 1ꢀoxide (6a) and 3ꢀphenylꢀ
1,4ꢀdiazaspiro[4.5]decaꢀ1,3ꢀdiene 1ꢀoxide (6b), whose synthesis
is described in the work.47
3
4
Scheme 2
3.0
3.5
4.0
4.5
5.0logM
Fig. 4. The MWD curves of PS postpolymer samples (1, 2)
synthesized at 120 °C. Macroinitiators (curves 3, 4) were obꢀ
tained in the presence of BP (a) and AIBN (b) at the following
ratio of component concentrations: [ST]0 = 8.71 mol L–1
,
[ST]0 : [Initiator]0 : [Nitroxyl]0 = 100 : 1 : 1.5; 1, 3 — in the presꢀ
ence of nitroxyl 2, 2, 4 — in the presence of nitroxyl 3. Converꢀ
sions (%): a — 73 (3), 47 (1), 43 (4), 62 (2); b — 51 (3), 67 (1),
50 (4), 98 (2). The curves are normalized on conversion.
tion was performed with involvement of macroinitiators
based on compounds 4 and 5, the samples with somewhat
higher values of Mw/Mn (~1.7) than for the starting polyꢀ
mers were obtained (see Table 3). It is possible, that this
is due to the "dead" polymer formed in the starting PS
during the synthesis because of termination of alkoxyꢀ
amines by monoꢀ and bimolecular reactions.34
On the whole, the data obtained are an unambiguous
evidence that polymerization of ST in the presence of
nitroxyls of imidazoline series follows the "pseudoliving"
mechanism. It was showed that spatial groups at αꢀposiꢀ
tion with respect to the nitroxyl fragment have considerꢀ
6, 7: R = Me (a); R + R = (CH ) (b)
2 5
Synthesis of 5ꢀmethylꢀ4ꢀphenylꢀ2Hꢀimidazoleꢀ1ꢀoxides 7a,b.
A solution of methylmagnesium iodide in anhydrous diethyl
ether, obtained from magnesium turnings (0.72 g, 30 mmol) and

Nitroxide imidazole radicals in polymerization
Russ.Chem.Bull., Int.Ed., Vol. 59, No. 8, August, 2010
1563
methyl iodide (1.25 mL, 20 mmol) in anhydrous diethyl ether
(30—40 mL), was added dropwise to a solution of nitrone 6a,b
(10 mmol) in minimum dry benzene so that the ether simmered.
The reaction mixture was stirred for 1 h until the starting
compound disappeared according to the TLC data (Silufol
UV 254, chloroform). Then, water was added thoroughly
dropwise until a viscous inorganic phase was formed. The organic
solution was decanted. The inorganic mass was washed with
diethyl ether ((2—3)Ѕ10 mL). A combined organic solution
was dried with magnesium sulfate, then, lead dioxide (15 g) was
added to the solution, which was vigorously stirred for 2—3 h.
The oxidant was filtered off and 2—3 times washed with a mixture
of chloroform with ethanol (4 : 1), the solvents were evaporated
under reduced pressure. The residue was recrystallized from
hexane. Compounds 7a,b obtained correspond to the authentic
samples.48
2,2,5ꢀTrimethylꢀ4ꢀphenylꢀ2Hꢀimidazoleꢀ1ꢀoxide (7a). The
yield was 1.9 g (96%), colorless crystals, m.p. 87—89 °C. 1H NMR
(400 MHz, CDCl3), δ: 1.52 (s, 6 H, 2 Me); 2.25 (s, 3 H, Me);
7.43 (m, 3 H, mꢀH, pꢀH, Ph); 7.70 (m, 2 H, oꢀH, Ph). 13C NMR
(100 MHz, CDCl3), δ: 10.68 (Me, C(5)); 24.06, 24.15 (2 Me,
C(2)); 99.03 (C(2)); 127.59 (oꢀC, Ph); 128.70 (mꢀC, Ph); 130.89
(pꢀC, Ph); 132.18 (ipsoꢀC, Ph); 135.56 (C(5)); 166.38 (C(4)).
2ꢀMethylꢀ3ꢀphenylꢀ1,4ꢀdiazaspiro[4.5]decaꢀ1,3ꢀdiene
1ꢀoxide (7b). The yield was 2.3 g (93%), colorless crystals,
m.p. 75—77 °C. 1H NMR (300 MHz; CDCl3), δ: 1.35 (m, 3 H,
CH2, (CH2)5); 1.85 (m, 5 H, CH2, (CH2)5); 2.12 (m, 2 H, CH2,
(CH2)5); 2.28 (s, 3 H, Me); 7.43 (m, 3 H, pꢀH, Ph); 7.75 (m,
2 H, oꢀH, Ph). 13C NMR (75 MHz, CDCl3), δ: 10.46 (Me);
22.94 (2 CH2, (CH2)5); 24.51 (CH2, (CH2)5); 34.63 (2 CH2,
(CH2)5); 101.56 (C(5)); 127.54 (oꢀC, Ph); 128.53 (mꢀC,
Ph); 130.53 (pꢀC, Ph); 132.36 (ipsoꢀC, Ph); 135.71 (C(2));
166.20 (C(3)).
(H—C=, Ph); 2997, 2977, 2938, 2923, 2857 (CH3, CH2); 1597,
1570 (C=N, C=C). UV, λmax/nm: 249 (logε = 4.61).
Polymerization. Initiators and nitroxyl radicals were accuꢀ
rately weighted and dissolved in the monomer. The solutions
were dozed into glass tubes and degassed under reduced pressure
by triple refreezing in the liquid nitrogen. The tubes were kept in
a thermostat at a certain temperature (Т 1 °C) for a given time,
followed by isolation of a polymer. Polystyrene was purified
off from organic impurities by double reprecipitation from a
chloroform solution into isopropanol and dried in vacuo at 50 °C
until the mass was constant. The polymer yield (conversion
of the monomer) was determined gravimetrically.
To perform postpolymerization, the reprecipitated polymer
obtained in the presence of imidazoline nitroxyls was dissolved
in the monomer in concentration 50 wt.%. Preparation of tubes,
polymerization, isolation of postpolymers, and calculation of
conversions were carried out similarly to the procedures described
previously.
Analysis of polymers. Analysis of molecularꢀmass characꢀ
teristics of polymers49 was carried out on a KNAUER liquid
chromatograph equipped with a cascade of Phenomenex columns
(300 mm×7.8 mm, phenogel, 10 μm) with pores of 105 and
103 Å in diameter and two detectors (refractometric and UV).
Tetrahydrofuran was an eluent, the column temperature was
25.0 0.1 °C. Calibration of the column was carried out using
polystyrene standards with molecular mass from 2.9•103 to
1.2•106 Da.
This work was financially supported by the Russian
Foundation for Basic Research (Project No. 08ꢀ03ꢀ00100).
References
Synthesis of nitroxyl radicals 4 and 5. A solution of phenylꢀ
magnesium bromide, obtained from magnesium (1.5 g, 63 mmol)
and bromobenzene (4.0 mL, 38 mmol) in anhydrous diethyl
ether (30 mL) was added dropwise to a solution of methylnitrone
7a,b (9.5 mmol) in minimum dry benzene. The reaction mixture
was stirred for another 1 h, then, water was added thoroughly
dropwise until a viscous inorganic phase was formed. The organic
solution was decanted. The inorganic mass was washed with
diethyl ether (2×10 mL). A combined organic solution was dried
with magnesium sulfate, then, manganese dioxide (15 g) was
added to the solution, which was vigorously stirred for 2—3 h.
The oxidant was filtered off and 2—3 times washed with a mixꢀ
ture of chloroform with ethanol (4 : 1), the solvents were
evaporated under reduced pressure. The residue was subjected
to column chromatography on silica gel, using the chloroform—
hexane (1 : 1) solvent mixture as an eluent. Product 4 was
recrystallized from hexane, whereas 5 from the mixture ethyl
acetate—hexane (1 : 8).
1. G. V. Korolev, A. P. Marchenko, Usp. Khim., 2000, 69, 447
[Russ. Chem. Rev. (Engl. Transl.), 2000, 69].
2. M. Yu. Zaremskii, V. B. Golubev, Vysokomolekulyar. Soedin.,
Ser. C, 2001, 43, 1689 [Polym. Sci., Ser. C (Engl. Transl.),
2000, 43].
3. D. F. Grishin, L. L. Semenycheva, Usp. Khim., 2001, 70,
486 [Russ. Chem. Rev. (Engl. Transl.), 2001, 70].
4. K. Matyjaszewski, T. P. Davis, Handbook of Radical Polyꢀ
merization, WileyꢀInterscience, Hoboken, 2002, 920 pp.
5. H. Fischer, Chem. Rev., 2001, 101, 3581.
6. W. A. Braunecker, K. Matyjaszewski, Prog. Polym. Sci., 2007,
32, 93.
7. Y. Miura, N. Nakamura, I. Taniguchi, Macromolecules, 2001,
34, 447.
8. Y. Miura, N. Nakamura, I. Taniguchi, A. Ichikawa, Polyꢀ
mer, 2003, 44, 3461.
9. Y. Miura, A. Ichikawa, I. Taniguchi, Polymer, 2003, 44,
5187.
10. Md. A. Mannan, A. Ichikawa, Y. Miura, Polymer, 2007,
48, 743.
2,2,5ꢀTrimethylꢀ4,5ꢀdiphenylꢀ2,5ꢀdihydroimidazolꢀ1ꢀoxyl
(4). The yield was 1.1 g (42%), yellow crystals, m.p. 128—132 °C.
•
Found (%): C, 77.72; H, 6.82; N, 10.12. C18H19N2O . Calculꢀ
ated (%): C, 77.39; H, 6.86; N, 10.03. IR, ν/cm–1: 3065, 3032
11. C. A. Knoop, A. Studer, J. Am. Chem. Soc., 2003, 125,
16327.
(H—C=, Ph); 2982, 2932 (Me); 1599, 1572 (C=N, C=C). UV,
12. K. O. Siegenthaler, A. Studer, Macromolecules, 2006, 39,
1347.
λ
max/nm: 248 (logε = 4.20).
2ꢀMethylꢀ2,3ꢀdiphenylꢀ1,4ꢀdiazaspiro[4.5]decaꢀ3ꢀenꢀ1ꢀoxyl
13. S. Grimaldi, J. P. Finet, F. le Moigne, A. Zeghdaoui,
P. Tordo, D. Benoit, M. Fontanille, Y. Gnanou, Macroꢀ
molecules, 2000, 33, 1141.
(5). The yield was 1.1 g (37%), yellow crystals, m.p. 161—163 °C.
Found (%): C, 78.80; H, 7.29; N, 8.76. C21H23N2O . Calculꢀ
ated (%): C, 78.96; H, 7.26; N, 8.77. IR, ν/cm–1: 3067, 3031
•

1564
Russ.Chem.Bull., Int.Ed., Vol. 59, No. 8, August, 2010
Kolyakina et al.
14. D. Benoit, S. Grimaldi, S. Robin, J. P. Finet, P. Tordo,
Y. Gnanou, J. Am. Chem. Soc., 2000, 122, 5929.
15. A. Debuigne, D. ChangꢀSeng, L. Lichun, G. K. Hamer,
M. K. Georges, Macromolecules, 2007, 40, 6224.
16. J. Ruehl, N. L. Hill, E. D. Walter, G. Millhauser, R. Braslau,
Macromolecules, 2008, 41, 1972.
34. G. S. Ananchenko, H. Fischer, J. Polym. Sci., Part A: Polym.
Chem., 2001, 39, 3604.
35. D. Benoit, V. Chaplinski, R. Braslau, C. J. Hawker, J. Am.
Chem. Soc., 1999, 121, 3904.
36. A. Studer, K. Harms, Ch. Knoop, Ch. Muller, T. Schulte,
Macromolecules, 2004, 37, 27.
17. D. F. Grishin, L. L. Semenycheva, E. V. Kolyakina, Dokl.
Akad. Nauk, 1998, 362, 634 [Dokl. Chem. (Engl. Transl.),
1998, 362].
18. D. F. Grishin, L. L. Semyonycheva, E. V. Kolyakina,
Mendeleev Commun., 1999, 250.
37. M. Souaille, H. Fischer, Macromolecules, 2002, 35, 248.
38. M. Buback, R. G. Gilbert, R. A. Hutchinson, B. Klumperman,
F.ꢀD. Kuchta, B. G. Manders, K. F. O´Driscoll, G. T. Russell,
J. Schweer, Macromol. Chem. Phys., 1995, 169, 3267.
39. F. Chauvin, P.ꢀE. Dufils, D. Gigmes, Y. Guillaneuf,
S. R. A. Marque, P. Tordo, D. Bertin, Macromolecules,
2006, 39, 5238.
19. E. V. Kolyakina, D. F. Grishin, Usp. Khim., 2009, 78, 579
[Russ. Chem. Rev. (Engl. Transl.), 2009, 78].
20. M. Yu. Zaremskii, A. P. Orlova, E. S. Garina, A. V. Olenin,
M. B. Lachinov, V. B. Golubev, Vysokomolekulyar. Soedin.,
Ser. A, 2003, 45, 871 [Polym. Sci., Ser. C (Engl. Transl.),
2003, 45].
21. C. Detrembleur, V. Sciannamea, C. Koulic, M. Claes,
M. Hoebeke, R. Jerome, Macromolecules, 2002, 35, 7214.
22. V. Sciannamea, R. Jérôme, C. Detrembleur, Chem. Rev.,
2008, 108, 1104.
23. E. H. H. Wong, M. H. Stenzel, T. Junkers, C. Barnerꢀ
Kowollik, J. Polym. Sci., Part A: Polym. Chem., 2009, 47,
1098.
24. D. Zubenko, I. Kirilyuk, G. Roshchupkina, I. Zhurko,
V. Reznikov, S. Marque, E. G. Bagryanskaya, Helv. Chim.
Acta, 2006, 89, 2341.
25. M. A. Lazarev, I. A. Kirilyuk, I. A. Grigor´ev, D. F. Grishin,
Vysokomolekulyar. Soedin., Ser. B, 2007, 49, 1735 [Polym.
Sci., Ser. B (Engl. Transl.), 2000, 49].
26. M. V. Pavlovskaya, I. A. Kirilyuk, A. A. Shchepalov, I. A.
Grigor´ev, D. F. Grishin, Vysokomolekulyar. Soedin., Ser. B,
2008, 50, 2181 [Polym. Sci., Ser. B (Engl. Transl.), 2000, 50].
27. J. P. van Hook, A. V. J. Tobolsky, J. Am. Chem. Soc., 1958,
80, 779.
28. B. Barnett, W. E. Vaughan, J. Phys. Chem., 1947, 51, 926.
29. F. J. Hua, Y. L. Yang, Polymer, 2001, 42, 1361.
30. K. Matyjaszewski, T. Shigemoto, J. M. J. Frechet, M. Leduc,
Macromolecules, 1996, 29, 4167.
31. A. Goto, T. Fukuda, Prog. Polym. Sci., 2004, 29, 329.
32. D. P. Zubenko, Ph.D. Thesis (Chem.), International
Tomographic Center SB RAS, Novosibirsk, 2008, 119 pp.
(in Russian).
40. G. V. Korolev, G. M. Bakova, M. P. Berezin, Vysokomoleꢀ
kulyar. Soedin., Ser. A, 2001, 43, 405 [Polym. Sci., Ser. A
(Engl. Transl.), 2000, 43].
41. A. Weissberger, E. Proskauer, J. Riddick, E. Toops, Organic
Solvents, Interscience Publishers, INC, New York, 1955.
42. Organic Syntheses, Third Printing, Interscience Publishers,
New York—London, 1946—1952, 26—32.
43. Entsiklopediya polimerov [Encyclopedia of Polymers], Sovetskaya
Entsiklopediya, Moscow, 1972, 1, 1224 pp. (in Russian).
44. L. B. Volodarskii, V. A. Reznikov, V. S. Kobrin, Zh. Org.
Khim., 1979, 13, 415 [J. Org. Chem. USSR (Engl. Transl.),
1979, 13].
45. D. Zubenko, Y. Tsentalovich, N. Lebedeva, I. Kirilyuk,
G. Roshchupkina, I. Zhurko, V. Reznikov, S. R. A. Marque,
E. Bagryanskaya, J. Org. Chem., 2006, 71, 6044.
46. T. K. Sevast´yanova, L. B. Volodarskii, Izv. Akad. Nauk
SSSR, Ser. Khim., 1972, 2339 [Bull. Acad. Sci. USSR, Div.
Chem. Sci. (Engl. Transl.), 1972, 31, 2276].
47. I. A. Kirilyuk, I. A. Grigor´ev, L. B. Volodarskii, Izv. Akad.
Nauk SSSR, Ser. Khim., 1991, 2113 [Bull. Acad. Sci. USSR,
Div. Chem. Sci. (Engl. Transl.), 1991, 40, 1871].
48. L. B. Volodarskii, A. N. Lysak, V. A. Koptyug, Khim.
Geterotsikl. Soedin., 1968, 334 [Chem. Heterocycl. Compd.
(Engl. Transl.), 1968].
49. B. G. Belen´kii, L. Z. Vilenchik, Khromatografiya polimerov
[Chromatography of Polymers], Moscow, Khimiya, 1978,
344 pp. (in Russian).
33. H. Fischer, A. Kramer, S. R. A. Marque, P. Nesvadba,
Macromolecules, 2005, 38, 9974.
Received October 22, 2009;
in revised form May 13, 2010