Synthesis of a new stable β-sulfinyl nitroxide and the corresponding alkoxyamine for living/controlled radical polymerization of styrene: Kinetic and ESR studies

Article abstract of DOI:10.1021/ma011063k

The syntheses Of a new nitroxide bearing a sulfoxide group at the β-position (with respect to nitrogen) and the corresponding N-β-sulfinyl alkoxyamine are reported. Styrene was polymerized in bulk in the presence of this new alkoxyamine. The polymerization satisfies the usual criteria of a living/ controlled radical polymerization, with a linear increase of molecular weight vs yield and a constant transient radical concentration with time. However, the polymerization rates were independent of alkoxyamine concentration but much higher than the thermal polymerization ones: R_p/R_th = 2.6 at 90 °C, 3.7 at 100 °C, and 3.5 at 110 °C. Kinetic and ESR studies showed that both transient and persistent radical concentrations do not follow the corresponding theoretical evolutions with time but reach stationary states. The different rate constants (k_d and k_c) and corresponding activation energies were estimated, showing that the sulfoxide group has a large effect mainly on the combination reaction, the value of which is unusually low for such system (k_c ≈ 10⁵ L mol^-1 s^-1).

Full text of DOI:10.1021/ma011063k

Macromolecules 2002, 35, 2461-2466
2461
Synthesis of a New Stable â-Sulfinyl Nitroxide and the Corresponding
Alkoxyamine for Living/Controlled Radical Polymerization of Styrene:
Kinetic and ESR Studies
Er ic Dr ock en m u ller a n d J ea n -Ma r ie Ca ta la *
Institut Charles Sadron, CNRS-ULP, 6 rue Boussingault, 67083 Strasbourg Cedex, France
Received J une 21, 2001
ABSTRACT: The syntheses of a new nitroxide bearing a sulfoxide group at the â-position (with respect
to nitrogen) and the corresponding N-â-sulfinyl alkoxyamine are reported. Styrene was polymerized in
bulk in the presence of this new alkoxyamine. The polymerization satisfies the usual criteria of a living/
controlled radical polymerization, with a linear increase of molecular weight vs yield and a constant
transient radical concentration with time. However, the polymerization rates were independent of
alkoxyamine concentration but much higher than the thermal polymerization ones: Rp/Rth ) 2.6 at 90
°C, 3.7 at 100 °C, and 3.5 at 110 °C. Kinetic and ESR studies showed that both transient and persistent
radical concentrations do not follow the corresponding theoretical evolutions with time but reach stationary
states. The different rate constants (kd and kc) and corresponding activation energies were estimated,
showing that the sulfoxide group has a large effect mainly on the combination reaction, the value of
which is unusually low for such system (kc ≈ 105 L mol-1 s-1).
Sch em e 1
In tr od u ction
Since original report of Rizzardo and Solomon1 on the
ability of nitroxides to control the radical polymerization
of styrene, living/controlled radical polymerizations
mediated by nitroxides have been of special interest as
an alternative method to synthesize well-defined archi-
tectures. During the past 15 years, the mechanism has
been studied,2,3 and this new synthetic route has been
extended to a wide range of monomers.4 This process is
mainly based on the equilibrium between dormant and
active species (Scheme 1).
The dormant species (P-X), in the form of an
alkoxyamine, carry a thermoreversible bond which
produces, upon heating, a nitroxyl radical (X•) and the
corresponding alkyl radical (P•). These transient alkyl
radicals undergo several monomer additions before
combining with free nitroxide to regenerate the dormant
species. If the equilibrium constant and the product kdkc
are high enough, cycles between the active and the
dormant forms proceed sufficiently fast to ensure a
narrow chain distribution.
lower the temperature of polymerization and thus to
minimize thermal polymerization. One nitroxide that
achieves this goal is the N-tert-butyl-N-[1-diethylphospho-
no-(2,2-dimethylpropyl)) nitroxide (DEPN)8,9 synthe-
sized by Tordo et al. with a phosphonylated group at
the R-position to the nitrogen atom. This compound
leads to a controlled polymerization of styrene at 120
°C and a polymerization rate that depends on the
alkoxyamine concentration and remains higher than the
rate of thermal polymerization. The basic features of
this process and its persistent radical effect were
described in detail by Fischer2 and Fukuda.3 A power-
law analysis predicts the evolution of persistent radical
concentration and monomer conversion according to eqs
2 and 3.
Because of the instability of nitroxides bearing an
R-hydrogen atom, the nitroxides have been restricted
to tert-alkylated nitroxides such as di-tert-butyl nitrox-
ide, 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO), or
TEMPO derivatives.5 These compounds allowed the
synthesis of well-defined polymers and copolymers of
styrene derivatives but were unable to control the
polymerization of other vinylic monomers. Moreover,
polymerization rate is generally independent of the
alkoxyamine concentration6 due to the low dissociation
rate compared to the rate of thermal radicals genera-
tion.7 Under these conditions, nitroxide and alkyl radical
concentrations soon reach a stationary state and are
related by relation 1, with [P-X]0 the initial alkoxyamine
concentration.
[X•] ) (3ktK2[P-X]02)1/3t1/3
ln([M]0/[M]) ) (3kp/2)(K[P-X]0/3kt)1/3 2/3
(2)
(3)
t
The persistent radical concentration should increase
following the t1/3 dependence, whereas the monomer
consumption should have a t2/3 dependence and a
1/3
[P-X]0 dependence. This kinetic behavior should be
observed if the process is dominated by dissociation of
the alkoxyamine. To enhance the homolysis of C-O
bond, we synthesized an alkoxyamine bearing a strongly
polar sulfoxide group. We used this alkoxyamine in the
polymerization of styrene at different temperatures; the
results of kinetic and ESR studies are presented in this
paper.
K ) kd/kc ) [P•]stat[X•]stat/[P-X]0
(1)
Exp er im en ta l Section
Ch a r a cter iza tion s. 1H NMR (CDCl3, 200 MHz) and 13C
NMR (CDCl3, 50 MHz) measurements were performed on a
Therefore, new persistent radicals are sought that
lead to a high rate constant of dissociation in order to
10.1021/ma011063k CCC: $22.00 © 2002 American Chemical Society
Published on Web 02/22/2002

2462 Drockenmuller and Catala
Macromolecules, Vol. 35, No. 7, 2002
Bruker AC 200 spectrometer at room temperature. ESR
spectra were recorded on a Bruker ESP-300 X-band ESR
spectrometer equipped with a HP 53150A frequency meter and
a Boonton microwatt meter. The nitroxide concentrations were
determined by integration of the ESR spectra and calibration
with a TEMPO solution in benzene. Size exclusion chroma-
tography (SEC) was carried out at room temperature on a
Shimadzu apparatus equipped with a refractometer (Shi-
madzu) and five columns PL GEL (10 µm particles) (three
mixed B, 103 Å, 105 Å) using THF as eluant (flow rate: 1 mL
min-1). A calibration curve obtained from polystyrene stan-
dards was used to determine the molecular weights.
663 mg of pure alkoxyamine was obtained (28%) as a colorless
oil. 1H NMR: δ 0.92, 1.16 (2s, (CH3)3CCH, 9H), 1.21, 1.27 (2s,
(CH3)3CN, 9H), 1.31-1.58 (m, CH3CH2SO, CH3CHSO, CH3-
CHO, 9H), 2.30-3.05 (m, CH3CH2SO, 2H), 3.76-3.97 (m,
CH3CHSO, (CH3)3CCHN, 2H), 5.00 (dq, CH3CHO, 1H), 7.15-
7.40 (m, aromatics, 5H). 13C NMR: δ 8.6 (s, CH3CH2SO, 1C),
13.0, 13.9 (2s, CH3CHSO, 1C), 26.0, 27.9 (2s, (CH3)3CCH, 1C),
29.2, 29.8 (2s, (CH3)3CCH, 3C), 30.5, 31.1 (2s, (CH3)3CN, 3C),
32.0, 84.9 (2s, CH3CHO, 1C), 36.4, 36.8 (2s,(CH3)3CCH, 1C),
46.0 (s, CH3CHO, 1C), 57.8 (s, (CH3)3CN, 1C), 62.3 (s, CH3CH2-
SO, 1C), 62.9 (s, CH3CHSO, 1C), 127.2 (s, m-aromatics, 2C),
127.6, 128.1 (2s, p-aromatic, 1C), 128.8, 129.1 (2s, o-aromatics,
Ma ter ia ls. Diethyl sulfide (Aldrich, 98%) was oxidized to
diethyl sulfoxide using 3-chloroperoxybenzoic acid (Aldrich,
75%) in methylene chloride and then distilled under reduced
pressure.10 n-BuLi (1.6 M in hexanes) was purchased from
Aldrich. Di-tert-butyl peroxalate11 and tert-butylhydroxy-
lamine12 were synthesized as described previously. Tetra-
hydrofuran (THF) was distilled from sodium benzophenone
dianion. Ethylbenzene (Aldrich, 99%) was distilled twice from
calcium hydroxide. Styrene (Aldrich, 99%) was distilled twice
from calcium hydride prior to use. All other materials were
used as received.
2C), 144.7, 147.4 (2s, CCHO, 1C). Anal. Calcd for C21H37
NO2S: C, 68.61; H, 10.15; N, 3.81. Found: C, 68.35; H, 9.95;
N, 3.72. HRMS (EI, 70 eV): exact mass calculated for C21H37
NO2S: [M]+ 367.60, found: 367.75.
Typ ica l P olym er iza t ion E xp er im en t . A solution of
alkoxyamine 4 (0.06 g, 0.16 mmol) in neat styrene (17.66 g,
170 mmol) was distributed among 10 glass tubes. The contents
were degassed by freeze-pump-thaw cycles, and the tubes
were sealed off under vacuum. One tube was heated to 100
°C for 12 h, and after cooling, the polymerization medium was
dissolved in THF. After evaporation of excess monomer and
solvent, the polymer was freeze-dried in benzene. Conversion
was evaluated gravimetrically; molecular weight and polydis-
-
-
N-ter t-Bu tyl-r-ter t-bu tyl Nitr on e (1). A solution of tert-
butylhydroxylamine (9.98 g, 0.112 mol), trimethylacetaldehyde
(13.4 mL, 0.123 mol), and a catalytic amount of p-toluene-
sulfonic acid (PTS) (0.01 g, 0.5 mmol) in ether (100 mL) was
stirred for 48 h at room temperature. After evaporation of the
solvent, the residue was placed under dynamic vacuum
overnight in order to remove excess hydroxylamine by subli-
mation. After crystallization in pentane 9.1 g of pure nitrone
was obtained (52%) as white crystals. 1H NMR: δ 1.27 (s,
persity index were determined by SEC (yield ) 61.8%, Mn
)
73100 g mol-1, PDI ) 1.23).
Deter m in a tion of th e Ra te Con sta n t of Dissocia tion
(kd ) by ESR Sp ectr oscop y. Benzene solutions containing
alkoxyamine 4 (0.18 × 10-3 and 0.38 × 10-3 mol L-1) and
excess galvinoxyl (1 × 10-3 mol L-1) were degassed and then
heated at temperatures between 70 and 100 °C. The release
of persistent radicals was monitored by ESR spectroscopy, and
their concentrations were estimated from the integration of
the first peak of nitroxide 3 spectrum centered at 3360 G.
(CH3)3CCH, 9H), 1.48 (s, (CH3)3CN, 9H), 6.57 (s, CH, 1H). 13
C
NMR: δ 26.1 (s, (CH3)3CCH, 3C), 28.2 (s, (CH3)3CN, 3C), 32.4
(s, (CH3)3CN, 1C), 69.6 (s, (CH3)3CCH, 1C), 139.7 (s, CH, 1C).
Anal. Calcd for C9H19NO: C, 68.74; H, 12.18; N, 8.91. Found:
C, 68.67; H, 11.92; N, 8.73.
Resu lts a n d Discu ssion
N-ter t-Bu tyl-N-(1-ter t-bu tyl-2-eth ylsu lfin yl)p r op ylh y-
d r oxyla m in e (2). A solution of N-tert-butyl-R-tert-butyl ni-
trone (1.51 g, 9.6 mmol) in THF (45 mL) was added dropwise
to a cooled (-78 °C), stirred solution of R-lithiated sulfoxide
prepared from n-BuLi (6 mL, 9.6 mmol) and diethyl sulfoxide
(1.1 mL, 9.6 mmol) in THF (45 mL) at -20 °C. The mixture
was stirred for 4 h at -78 °C and quenched with a saturated
solution of ammonium chloride (20 mL). The mixture was then
allowed to warm to room temperature and extracted twice with
dichloromethane (2 × 40 mL). The combined organic layers
were washed with water, dried over magnesium sulfate,
filtered, and evaporated to give 1.67 g of crude hydroxylamine
(66%) which was used without further purification in the
Syn th esis of â-Su lfin yl Nitr oxid e (3) a n d Cor r e-
sp on d in g N-â-Su lfin yla lk oxya m in e (4). The new
alkoxyamine bearing a sulfoxide function was prepared
as outlined in Scheme 2.
The first step involves the synthesis of N-tert-butyl-
R-tert-butyl nitrone (1) by acid-catalyzed condensation
of trimethylacetaldehyde and tert-butylhydroxylamine
in ether. The nitrone was then submitted to nucleophilic
addition of R-lithiated diethyl sulfoxide to yield â-sulfi-
nylhydroxylamine (2). Such compounds have been widely
studied since they are easily accessible intermediates
for the synthesis of enantiomerically pure amines after
reduction of the sulfinyl function.13 In the third step,
the hydroxylamine function was oxidized to obtain the
corresponding â-sulfinyl nitroxide 3.
1
oxidation step. H NMR: δ 1.10 (s, (CH3)3CCH, 9H), 1.19 (s,
(CH3)3CN, 9H), 1.37 (t, J ) 7.5 Hz, CH3CH2SO, 3H), 1.43 (d,
J ) 7.5 Hz, CH3CHSO, 3H), 2.53 (m, CH3CHHSO, 1H), 2.88
(m, CH3CHHSO, 1H), 3.53 (s, (CH3)3CCHN, 1H), 3.60 (q, J )
7.5 Hz, CH3CHSO, 1H), 4.82 (s, NOH, 1H).
A triplet appeared in the ESR spectrum shown in
N-ter t-Bu tyl-N-(1-ter t-bu tyl-2-eth ylsu lfin yl)p r op yl Ni-
tr oxid e (3). A solution of hydroxylamine 2 (1.67 g, 6.4 mmol)
and copper(II) acetate monohydrate (0.02 g, 0.1 mmol) in
methanol (50 mL) was stirred at room temperature under O2
(optional) for 2 h. The solvent was evaporated without heating,
and the pure nitroxide was separated by column chromatog-
raphy on silica gel eluting with a 1:9 cyclohexane/ethyl acetate
mixture (Rf ) 0.2). After evaporation of the solvents, 0.82 g of
pure nitroxide was obtained (49%) as a red liquid and stored
at -20 °C. ESR (benzene) triplet, aN ) 14.7 G, g ) 2.0064.
Anal. Calcd for C13H28NO2S: C, 59.50; H, 10.75; N, 5.34.
Found: C, 59.75; H, 10.86; N, 5.18.
2-P h en yl-2-(N-ter t-bu tyl-N-(1-ter t-bu tyl-2-eth ylsu lfin yl)-
p r op yl n itr oxid e)eth a n e (4). A solution of nitroxide 3 (1.60
g, 6.09 mmol) and di-tert-butyl peroxalate (2.86 g, 12.2 mmol,
explosive when scratching crystals!) in ethylbenzene (15 mL)
was stirred at 35 °C under argon for 15 h. Excess ethylbenzene
was removed under vacuum, and the residue was chromato-
graphed on silica gel eluting with a 7:3 cyclohexane/ethyl
acetate mixture (Rf ) 0.4). After evaporation of the solvents,
Figure 1, revealing the usual nitrogen coupling (aN
)
14.7 G), whereas â-hydrogen coupling aH was not
resolved. The absence of hydrogen coupling can be
attributed to the steric hindrance introduced by the tert-
alkyl substituents; the R-hydrogen atom is eclipsed by
a bulky tert-butyl group. For a better understanding of
polymerization kinetics, the corresponding alkoxyamine
4 was synthesized. The low-temperature initiator di-
tert-butyl peroxalate was used to make the spin trap
reaction mild.14 Di-tert-butyl peroxalate decomposes
cleanly at 35 °C to give two carbon dioxide molecules
and two tert-butoxy radicals. tert-Butoxy radicals ab-
stract a benzylic hydrogen from ethylbenzene to form
tert-butyl alcohol and 2-phenylethyl radicals. These
radicals can either couple or react with nitroxides to
form the alkoxyamine. The final mixture was composed
of the alkoxyamine and a nonnegligible amount of side
reaction products. The pure alkoxyamine was obtained

Macromolecules, Vol. 35, No. 7, 2002
â-Sulfinyl Nitroxide 2463
F igu r e 2. Evolution of average molecular weights as a
function of conversion for the bulk polymerization of styrene
in the presence of alkoxyamine 4: (lines) theoretical values;
(2) 110 °C; (b) 100 °C; (9, 0) 90 °C; (2, b, 9) [4] ) 8.2 × 10-3
F igu r e 1. ESR spectrum of nitroxide 3 in benzene.
Sch em e 2
mol L-1; (0) [4] ) 4.0 × 10-3 mol L-1
.
F igu r e 3. Evolution of polydispersity indexes (PDI ) Mw/Mn)
as a function of conversion for the bulk polymerization of
styrene in the presence of alkoxyamine 4: (2) 110 °C; (b) 100
°C; (9, 0) 90 °C; (2, b, 9) [4] ) 8.2 × 10-3 mol L-1; (0) [4] )
4.0 × 10-3 mol L-1
.
alkoxyamine ([4] ) 4.0 × 10-3 mol L-1); the fraction of
dead chains is therefore higher and leads to higher
polydispersity indexes which increase from 1.2 to 1.35
with increasing conversion (Figure 3).
The linear increases of ln([M]0/[M]) vs time in Figure
4 shows that the concentration of the growing radicals
remains constant during all of the experiments. At each
temperature, the polymerization rate (Rp) is clearly
higher than the thermal polymerization rate (Rth) of
pure styrene, Rp/Rth ) 2.6 at 90 °C, 3.7 at 100 °C, and
3.5 at 110 °C. This has never been reported for styrene
at temperatures below 120 °C. These results suggest
that the polymerization rate is not dominated by
thermal radicals generated in the medium but mainly
by radicals from the alkoxyamine end group of polymer
chains. Therefore, the conversion should follow the
theoretical relation (3).
The double-logarithmic plot of ln([M]0/[M]) vs time
(Figure 5) confirms that monomer conversion follows a
first-order time dependence; the slopes are equal to 1
instead of 2/3 as suggested by relation 3. This first-order
time dependence of monomer conversion associated with
high values of the polymerization rates (compared to
by purification by column chromatography and stored
then at -20 °C.
Kin etic a n d ESR Stu d ies. Bulk polymerization of
styrene mediated by alkoxyamine 4 was studied at three
temperatures: 90, 100, and 110 °C. To check whether
the polymerization satisfies the criteria of a living/
controlled radical polymerization, the molecular weight,
yield, and conversion were determined as a function of
time. Figure 2 shows that the experimental values of
Mn obtained for [4] ) 8.2 × 10-3 mol L-1 fall on the same
theoretical straight line passing through the origin.
There is a small deviation at high conversions due to
unavoidable termination reactions. This phenomenon
is more pronounced at the lower concentration of

2464 Drockenmuller and Catala
Macromolecules, Vol. 35, No. 7, 2002
F igu r e 6. Variation of nitroxide 3 concentration as a function
of time for the bulk polymerization of styrene in the presence
of alkoxyamine 4: (2) 110 °C; (b) 100 °C; (9, 0) 90 °C; (2, b,
F igu r e 4. Monomer conversion vs time for the bulk polym-
erization of styrene in the presence of alkoxyamine 4: (+)
thermal polymerizations of neat styrene; (2) 110 °C; (b) 100
°C; (9, 0) 90 °C; (2, b, 9) [4] ) 8.2 × 10-3 mol L-1; (0) [4] )
9) [4] ) 8.2 × 10-3 mol L-1; (0) [4] ) 4.0 × 10-3 mol L-1
.
4.0 × 10-3 mol L-1
.
F igu r e 7. Temperature dependence of the equilibrium con-
stant (K) for the bulk polymerization of styrene in the presence
of alkoxyamine 4: (9) [4] ) 8.2 × 10-3 mol L-1; (O) [4] ) 4.0
F igu r e 5. Double-logarithmic plot of conversion as a function
of time for the bulk polymerization of styrene in the presence
of alkoxyamine 4: (2) 110 °C; (b) 100 °C; (9, 0) 90 °C; (2, b,
× 10-3 mol L-1
.
9) [4] ) 8.2 × 10-3 mol L-1; (0) [4] ) 4.0 × 10-3 mol L-1
.
Ta ble 1. Kin etic P a r a m eter s for th e Bu lk P olym er iza tion
of Styr en e in th e P r esen ce of Alk oxya m in e 4 (kp Va lu es
fr om Ref 16)
the corresponding thermal ones) are generally observed
when an additional radical initiator is added in the
medium.15 It implies theoretically that the nitroxide
concentration does not depend on the alkoxyamine
concentration. To check this hypothesis, we carried out
experiments with two different alkoxyamine concentra-
tions (8.2 × 10-3 and 4.0 × 10-3 mol L-1) at 90 °C.
Figure 4 shows that the monomer conversion follows the
same straight line in both cases; i.e., the polymerization
rate is not related to the alkoxyamine concentration.
However, no additional initiator was added in our
experiments and to better understand this kinetic
phenomenon, we followed the persistent radical con-
centrations by ESR experiments at the three tempera-
tures (Figure 6).
T
kp (L
[4] (mol
L-1
K (mol
L-1
kc (L
(°C) mol-1 s-1
)
)
)
kd (s-1
)
mol-1 s-1
)
110
100
90
1563
1188
890
8.2 × 10-3 3.3 × 10-9 2.6 × 10-3 7.9 × 105
8.2 × 10-3 1.5 × 10-9 9.9 × 10-4 6.6 × 105
8.2 × 10-3 5.2 × 10-10 3.6 × 10-4 6.9 × 105
4.0 × 10-3 5.1 × 10-10 3.6 × 10-4 7.1 × 105
90
890
this suggests that transient radical and nitroxide reach
stationary states in the process, both concentrations
being correlated by relation 1. Therefore, the equilibri-
um constant (K) was evaluated at each temperatures
from the transient radical concentrations, calculated
from the slopes of the plot of ln([M]0/[M]) vs time and
kp values16 (Table 1) and the nitroxide concentrations
taken at the maximum of the pseudo plateau. From the
van’t Hoff plot (Figure 7) of the equilibrium constant,
the following Arrhenius equation was established:
Surprisingly, the evolution of nitroxide concentration
1
does not follow the /3-order time dependence variation
described by eq 2. The nitroxide concentration increases
rapidly with time and reaches a maximum or a pseudo
plateau that depends on temperature; its length varies
from 3 h at 100 °C to 10 h or more at 90 °C. Moreover,
increasing the alkoxyamine concentration from 4.0 ×
10-3 to 8.2 × 10-3 mol L-1 doubles the nitroxide
concentration. Coupled with the previous kinetic result,
K/mol L-1 ) 1.3 × 106 exp(-107/RT)
(4)
According to eq 4, K extrapolates to 8.0 × 10-9 mol L-1
at 120 °C. This value is slightly higher than the one

Macromolecules, Vol. 35, No. 7, 2002
â-Sulfinyl Nitroxide 2465
F igu r e 8. First-order plot of nitroxide 3 concentration vs time
in benzene: (b, O) 100 °C; (2, 4) 90 °C; ([, ]) 80 °C; (9, 0) 70
°C; (b, 2, [, 9) [4] ) 0.18 × 10-3 mol L-1; (O, 4, ], 0) [4] )
F igu r e 10. Temperature dependence of therate constant of
combination (kc) for the bulk polymerization of styrene in the
presence of alkoxyamine 4: (9) [4] ) 8.2 × 10-3 mol L-1; (O)
0.38 × 10-3 mol L-1; [galvinoxyl] ) 1 × 10-3 mol L-1
.
[4] ) 4.0 × 10-3 mol L-1
.
where [NO•] denotes the nitroxide 3 concentration and
[ALK]0 the initial alkoxyamine 4 concentration.
ln(([ALK]0 - [NO•])/[ALK]0) ) -kdt
(5)
As expected, the alkoxyamine concentration has no
effect on the dissociation rate constant values. From the
van’t Hoff plot represented in Figure 9, the variation of
the rate constant of dissociation with temperature was
determined (eq 6).
kd/s-1 ) 1.6 × 1013 exp(-116/RT)
(6)
The low value of the frequency factor is in the range
found in the literature for various alkoxyamines (A ≈
1013-1015 s-1).19 The activation energy (Ea ) 116 kJ
mol-1) is also within the range 110-140 kJ mol-1 for
alkoxyamines that release 2-phenylethyl radicals.20
From K and kd values (Table 1), the combination rate
constant was calculated using relation 1. The van’t Hoff
plot of kc, represented in Figure 10, leads to the
Arrhenius equation
F igu r e 9. Temperature dependence of alkoxyamine 4 rate
constant of dissociation (kd) in benzene: (9) [4] ) 0.18 × 10-3
mol L-1; (O) [4] ) 0.38 × 10-3 mol L-1; [galvinoxyl] ) 1 × 10-3
mol L-1
.
determined for styrene polymerization mediated by
DEPN9 (K120 ) 6 × 10-9 mol L-1) and is 40 times higher
than that estimated for styrene-TEMPO17 (K120 ) 2 ×
10-11 mol L-1). Since the equilibrium constant reflects
the competition between the dissociation and combina-
tion reactions (K ) kd/kc), it was interesting to determine
both rate constants in order to evaluate their respective
role in the process.
Deter m in a tion of Dissocia tion (kd ) a n d Com bi-
n a tion (kc) Ra te Con sta n ts. The rate constant of
dissociation (kd) was evaluated at different temperatures
from the Fischer et al.18 method by determining the
nitroxide concentration formed during the thermolysis
of an alkoxyamine in the presence of excess galvinoxyl.
The galvinoxyl radical traps 2-phenylethyl radicals,
thereby suppressing the backward reaction that leads
to initial alkoxyamine. We carried out the experiments
in benzene solutions of galvinoxyl (1 × 10-3 mol L-1),
at temperatures between 70 and 100 °C for two
alkoxyamine concentrations (0.18 × 10-3 and 0.38 ×
10-3 mol L-1).
kc/L mol-1 s-1 ) 4.0 × 106 exp(-6/RT)
(7)
The low activation energy is not surprising for this
reaction, although the value of kc (7 × 105 L mol-1 s-1
at 90 °C) is low in comparison to the one obtained for
sterically encumbered nitroxides like di-tert-butyl ni-
troxide (kc ) 1.2 × 107 L mol-1 s-1).21 For such
90
nitroxides, it is known that rate constants for the
homolysis and the reversible coupling reactions show a
corresponding dependence on nitroxide structure and
mainly in steric factors.22 As the di-tert-butyl alkoxyamine
90
has a kd slightly higher (2-phenylethyl radical, kd
)
6.4 × 10-4 s-1 20
)
than the kd of alkoxyamine 4, it was
expected that the kc values should be of the same order
of magnitude. This not observed, and it seems reason-
able to relate the strong difference between the kc values
to a better stabilization of the canonical form (2) of the
nitroxide 3, by the sulfoxide group (Scheme 3).
Nevertheless, these rate constant values cannot ex-
plain why the polymerization rate is independent of the
alkoxyamine concentration and higher than the thermal
polymerization rate. The theoretical analysis relates this
The dissociation rate constant was determined from
the slope of the first-order plot of nitroxide concentration
evolution vs time (Figure 8) according to relation 5,

2466 Drockenmuller and Catala
Macromolecules, Vol. 35, No. 7, 2002
Refer en ces a n d Notes
Sch em e 3
(1) Solomon, D. H.; Rizzardo, E.; Cacioli, P. U.S. Patent 4,581,429,
1986.
(2) Fischer, H. J . Polym. Sci., Polym. Chem. 1999, 37, 1885 and
references therein.
(3) Fukuda, T.; Goto, A.; Ohno, K. Macromol. Rapid Commun.
2000, 21, 151 and references therein.
(4) Hawker, C. J .; Bosman, A. W.; Harth, E. Chem. Rev. 2001,
101, 3661 and references therein.
(5) (a) Veregin, R. P. N.; Georges, M. K.; Kazmaier, P. M.; Hamer,
G. K. Macromolecules 1993, 26, 5316. (b) Chong, Y. K.; Ercole,
F.; Moad, G.; Rizzardo, E.; Thang, S. H. Macromolecules 1999,
32, 6895.
(6) Catala, J .-M.; Bubel, F.; Hammouch, S. O. Macromolecules
1995, 28, 8441.
(7) Fukuda, T.; Terauchi, T. Chem. Lett. 1993, 293.
(8) (a) Grimaldi, S.; Lemoigne, F.; Finet, J .-P.; Tordo, P.; Nicol,
P.; Plechot, M. Patent, WO96/24620, 1996. (b) Le Mercier,
C.; Bernard-Henriet, C.; De Sainte Claire, V.; Le Moigne, F.;
Tordo, P. Polym. Prepr. 1999, 40 (2), 403.
(9) Benoit, D.; Grimaldi, S.; Robin, S.; Finet, J .-P.; Tordo, P.;
Gnanou, Y. J . Am. Chem. Soc. 2000, 122, 5929.
(10) Dyer, J . C.; Harris, D. L.; Evans, S. A. J . Org. Chem. 1982,
47, 3660.
(11) Bartlett, P. D.; Benzing, E. P.; Pincock, R. E. J . Am. Chem.
Soc. 1960, 82, 1762.
(12) Greene, F. D.; Pazos, J . F. J . Org. Chem. 1969, 34, 2269.
(13) Chang, Z. Y.; Coates, R. M. J . Org. Chem. 1990, 55, 3464.
(14) Miura, Y.; Hirota, K.; Moto, H.; Yamada, B. Macromolecules
1999, 32, 8356.
(15) Hammouch, S. O.; Catala, J .-M. Macromol. Rapid Commun.
1996, 17, 683.
behavior to radicals generated by a source other than
the alkoxyamine; experimentally, this was observed by
addition of an initiator in the medium.15 However, in
our experiment no initiator is added; the formation of
this “excess” of active species can be related to the
mechanism of thermal radicals formation. It was shown,
from a study of thermal polymerization of styrene,23 that
the number of primary radicals formed in the medium
was higher than the number of radicals engaged in
styrene initiation. Our hypothesis is based on the
trapping of these intermediate radicals and the forma-
tion of alkoxyamines presenting a thermoreversible
bond. Another hypothesis can also be suggested; it
implies the nitroxide decomposition in active species.
This type of reaction occurs with a sterically encum-
bered phosphonylated nitroxide used in styrene polym-
erization.24 By an intramolecular homolysis of the C-N
bond, a nitrone and an active radical (able to initiate
styrene) are formed. If this reaction is applied in our
process, it implies that the concentration of these new
active species depends on the nitroxide concentration
in the stationary state. Therefore, for experiments that
double the persistent radicals concentration (Figure 6),
an increase of the polymerization rate should have
occurred, and this is not observed (Figure 4).
(16) Buback, M.; Gilbert, R. G.; Hutchinson, R. A.; Klumperman,
B.; Kuchta, F. D.; Manders, B. G.; O’Driscoll, K. F.; Russel,
G. T.; Schwer, J . Macromol. Chem. Phys. 1995, 196, 3267.
(17) Fukuda, T.; Terauchi, T.; Goto, A.; Ohno, K.; Tsujii, Y.;
Miyamoto, T.; Kobatake, S.; Yamada, B. Macromolecules
1996, 29, 6393.
(18) Kothe, T.; Marque, S.; Martschke, R.; Popov, M.; Fischer, H.
J . Chem. Soc., Perkin Trans. 1998, 2, 1553.
(19) Marque, S.; Fischer, H.; Baier, E.; Studer, A. J . Org. Chem.
2001, 66, 1146.
(20) Marque, S.; Le Mercier, C.; Tordo, P.; Fischer, H. Macromol-
ecules 2000, 33, 4403.
Con clu sion
This study shows that introduction of a strong polar
group such as sulfoxide, in â-position to an alkoxyamine
nitrogen, has a pronounced effect on the dissociation and
combination rate constants of the C-O bond. This new
alkoxyamine allows the synthesis, at low temperature,
of polystyrenes with polydispersity close to 1.2. The
polymerization rate was found higher than the thermal
polymerization of styrene but independent of alkoxy-
amine concentration. This particular kinetic phenom-
enon is currently under study.
(21) The equilibrium constant K was evaluated, in our laboratory,
from ESR and kinetic studies of the bulk polymerization of
styrene at 90 °C, in the presence of the alkoxyamine releasing
di-tert-butyl nitroxide and 2-phenylethyl radical; K ) 5.2 ×
10-11 mol L-1. From kd90 ) 6.4 × 10-4 s-1 20
,
it was found kc
90
) 1.2 × 107 L mol-1 s-1
.
(22) Moad, G.; Rizzardo, E. Macromolecules 1995, 28, 8722.
(23) Russell, K. B.; Tobolsky, A. V. J . Am. Chem. Soc. 1953, 75,
5052.
(24) J ousset, S.; Catala, J .-M. Macromolecules 2000, 33, 4705.
MA011063K