H-transfer reaction during decomposition of N-(2-methylpropyl)- N-(1-diethylphosphono-2,2-dimethylpropyl)-N-oxyl (SG1)-based alkoxyamines

Article abstract of DOI:10.1002/pola.26500

Thermal decomposition of four tertiary N-(2-methylpropyl)-N-(1- diethylphosphono-2,2-dimethylpropyl)-N-oxyl (SG1)-based alkoxyamines (SG1-C(Me)₂-C(O)-OR, R = Me, tBu, Et, H) has been studied at different experimental conditions using ¹

Full text of DOI:10.1002/pola.26500

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H-Transfer Reaction During Decomposition of N-(2-Methylpropyl)-
N-(1-diethylphosphono-2,2-dimethylpropyl)-N-oxyl (SG1)-Based
Alkoxyamines
Mariya Edeleva,1 Sylvain R. A. Marque,2 Kuanish Kabytaev,2 Yohann Guillaneuf,2
Didier Gigmes,2 Elena Bagryanskaya1,3
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DOI: 10.1002/pola.26500
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INTRODUCTION Nitroxide-mediated polymerization (NMP)
discovered in the mid-80s by Rizzardo and coworkers1
opens an easy and convenient way to well-defined, end-func-
tionalized, low-polydispersity polymeric materials.2–6 The
kinetic scheme of NMP is based on the conventional kinetic
scheme of radical polymerization: (i) initiation stage includ-
ing homolysis and reformation of the initiating alkoxyamine
(kd and kc, respectively) and the addition of the initiating
alkyl radical onto monomer (kadd), (ii) the propagation stage
including the homolysis and the reformation of the macroal-
koxyamine (dormant species, kd,ds and kc,ds, respectively) and
the propagation of the polymeric chain (kp), and (iii) the ter-
mination stage including the self-termination reactions (dis-
proportionation and dimerization reactions) and the reaction
between alkyl and nitroxyl radicals (Supporting Information
Scheme 1).The level of self-termination reactions is usually
low in NMP, which makes the preparation of controlled and
living polymers possible.2,6,7
The first effective NMP was carried out for styrene in the
presence of commercially available nitroxide TEMPO (2,2,6,6-
tetramethylpiperidine-N-oxyl) by Georges coworkers.7 After-
ward many new nitroxides were developed as the mediating
agents for NMP.8 One of the most effective NMP mediators of
many styrenic and acrylic monomers is the N-(2-methyl-
propyl)-N-(1-diethylphosphono-2,2-dimethylpropyl)-N-oxyl
radical (SG1).9 Up to date, NMP of alkyl methacrylates, in
particular methyl methacrylate (MMA), is very demanding,
although a number of attempts using different nitroxides or
alkoxyamines have been performed.10–19 The main complica-
tion for NMP of MMA is the occurrence of side reactions,
namely, the intramolecular b-proton transfer (IPT, Cope-type
elimination20), intermolecular b-hydrogen atom transfer
(IHAT) reactions (reactions (ii) and (iv), respectively, in
Scheme 1), irreversible decomposition of dormant species
(i.e., through NO bond homolysis),21–23 and decomposition of
persistent nitroxyl radicals in the course of NMP.
A
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SCHEME 1
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Although Fischer’s diagram approach24 predicts good control
of MMA polymerization initiated by N-(2-methylpropyl)-N-
(1-diethylposphono-2,2-dimethylpropyl)-O-(2-carboxylprop-2-
yl) hydroxylamine (SG1-MAMA), it could not be reached in
experiment.25 The possible reasons of unsuccessful NMP
using SG1-MAMA have been intensively discussed in litera-
ture. On the one side, Ananchenko et al.26 concluded that
impact of H-transfer reaction is insignificant during the
2
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FIGURE 1
A
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homolysis of SG1-based alkoxyamines. On the other hand,
McHale et al.27 showed that the large excess of free SG1 in
the reaction medium can stimulate the occurrence of H-
transfer. The numerical simulation of the IHAT between SG1
and the poly(methylmethacrylyl) radical performed by Dire
et al.28 leads to conclusion that the large excess of SG1 pro-
motes the H-atom transfer reaction with the kcD values of
1.7 ꢀ 103 Mꢁ1sꢁ1. Guillaneuf et al.25 noted that the penulti-
mate and chain length effect arising from both sterically hin-
dered nitroxide (SG1) and macro radical lead to an exces-
sively large equilibrium constant K, which in combination
during the decomposition of alkoxyamines 1–3, whereas no
products of H-transfer are detected during the decomposi-
tion of 4. On the other hand, a side-product of alkoxyamine
type 1a is formed in the presence of oxygen, which is con-
firmed by numerical kinetic simulations accounting for the
hydrogen transfer. The obtained kcD value for IHAT during
the homolysis of alkoxyamine 1 is very close to that found
for the macroalkoxyamine by Dire et al.28
EXPERIMENTAL
General
with even low-level intermolecular H-transfer (kcD/kc
ꢂ
Alkoxyamines 1–3 were synthesized according to procedure
described previously31 using a reaction of SG1 radical with
corresponding alkyl bromides. Alkoxyamine 4 and nitroxide
SG1 were provided by Arkema. SG1 nitroxide was purified
by column chromatography (SiO2, pentane-ethylacetate gradi-
0.1%) makes the control of polymerization impossible. The
real breakthrough was made by Charleux et al.29,30 who have
shown that copolymerization of MMA is possible at moderate
temperature (80–90 ꢃC) with high conversion and high liv-
ingness in the presence of small amount of styrene and addi-
tional free SG1.
1
ent elution, 97% purity by H NMR after reduction). All deu-
terated reagents, solvents, and thiophenol were purchased
from Aldrich and used as received.
In this article, we report the detailed analysis of initiating
step of MMA polymerization mediated by SG1. The mecha-
nism of thermal decomposition of four SG1-based alkoxy-
amines (Fig. 1) has been investigated using the experimental
approach developed and successfully applied by us previ-
ously.22,23 In particular, for exhaustive study of processes
occurring during decomposition of SG1-based alkoxyamines
we have analyzed the reaction products and kinetics using
1H and 31P NMR spectroscopy at different experimental con-
ditions: (i) in the presence or in the absence of scavenger,
(ii) in acidic or nonacidic conditions, and (iii) in the presence
or absence of oxygen. Based on the results obtained, com-
plete decomposition mechanism has been proposed and
kinetics of alkoxyamines decomposition simulated. It has
been shown that in highly degassed solution IHAT occurs
Kinetic Experiments
Kinetic experiments were performed using Bruker Avance
200 NMR spectrometer equipped with the BVT-2000 temper-
ature unit. A typical alkoxyamine, hydroxylamine, or nitro-
xide decomposition experiment was carried out as follows:
the alkoxyamine solution (20 mM) in benzene-d6 or 1,2-
dichlorobenzene-d4 was placed in a conventional NMR tube
with an excess (7–12 equiv.) of scavenger thiophenol (PhSH).
The sample was degassed by three freeze-pump-thaw cycles
(10ꢁ3 mbar) using oil pump or by five freeze-pump-thaw
cycles (10ꢁ5 mbar) using high-vacuum pump, sealed under
vacuum, and then placed into the preheated probehead of
the NMR spectrometer. 1H NMR spectra versus time were
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H
E
M
I
S
T
R
Y
2
0
1
2
,
0
0
0
,
0
0
0
–
0
0
0
3

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1
3
1
FIGURE 2
H
N
n
M
R
(
a
,
c
)
a
n
d
P
N
M
R
(
b
,
d
)
d
e
t
e
c
t
e
d
b
e
f
o
r
e
(
l
o
w
e
r
)
a
n
d
a
f
t
e
r
d
ꢃC
e
c
o
m
3 9 , 4 0
p
o
s
i
t
i
o
n
o
f
a
l
k
o
x
y
a
m
i
n
e
1
(
0
.
0
2
M
s
o
l
u
t
i
o
n
i
n
b
e
n
z
e
n
e
,
d
)
i
t
h
e
p
r
e
s
e
n
c
e
(
a
,
b
)
a
n
d
a
b
s
e
n
c
e
(
c
,
d
)
o
f
0
.
2
M
o
f
P
h
S
H
.
T
¼
7
5
.
V
a
l
u
e
s
o
f
p
r
e
s
s
u
r
e
a
r
e
s
h
o
w
n
i
n
t
h
e
p
l
o
t
s
.
6
recorded during alkoxyamine decomposition at a given tem-
perature. 1H and 31P NMR spectra were recorded at room tem-
perature before and after kinetic experiment. PhSH, which is
the scavenger of alkyl radicals and reducing agent of nitro-
xides, has the high reaction rate constants with alkyl and
Identification of Decomposition
Products of Alkoxyamine 3
Decomposition Products of 3 after Degassing at Low
Pressure (10ꢁ3 mbar) in tert-BuOH or CDCl3 at 70 C
ꢃ
The main product observed was 3a as identified by 1H and
1
nitroxyl radicals:32,33 kPhSH((CH3)3Cꢄ) ¼ 2.5 108
M
ꢁ1sꢁ1 and
31P NMR and MS analyses: H NMR (300 MHz): 1.13 (9H, s),
k
PhSH(R1R2NOꢄ) ꢂ 100 Mꢁ1sꢁ1, respectively. It should be men-
1.20 (9H, s), 1.26–1.34 (9H, m), 3.26 (1H, d, 24 Hz), 4.06–
4.27 (6H, m), 31P NMR 24.7, M ¼ 368.2198 (MþHþ,
theor368.2197), 86%. The typical signal of acetone was also
observed by 1H NMR (d ¼ 1.55 ppm). The 14% left were
simple phosphates (31P NMR d (ppm): 9.73, 3.79–2.58), and
mainly diethyl phosphite (EtO)2P(O)H (d ¼ 3.03 ppm).
tioned that the reaction mechanism of thiophenol with nitro-
xides is rather complicated and is not shown on Scheme 1 in
details. In particular, formed PhSꢅ radical can react with nitro-
xides with formation of amines and other products.34–36
Although it does not affect the alkoxyamine decomposition
mechanism and kinetics, thiophenol reacts rapidly with alkyl
radicals (reaction vii, Scheme 1) suppressing the back recombi-
nation. The SH proton signal (3.1 ppm) does not overlap with
the vinylic or alkyl 1H signals of alkoxyamines or the thermoly-
sis products. The kinetics of alkoxyamine decomposition was
obtained by automatic integration of the NMR signals of the
alkoxyamines. The value of kd was found by linear fitting of the
decomposition kinetics in semilogarithmic coordinates.
Decomposition Products of 3 after Degassing at High
Pressure (10ꢁ5 mbar) in tert-BuOH or CDCl3 at 70 C
ꢃ
The main product observed were the diethylphosphite as
identified by 31P NMR (d ¼ 3.03 ppm, 85%), and 15% of 3a
1
(d ¼ 24.7 ppm). The typical H NMR signal of ethyl methac-
rylate (d, ppm: 6.05, 1H, br; 5.3, 1H, br; 4.25, 2H, q; 1.80,
4
J
O
U
R
N
A
L
O
F
P
O
L
Y
M
E
R
S
C
I
E
N
C
E
,
P
A
R
T
A
:
P
O
L
Y
M
E
R
C
H
E
M
I
S
T
R
Y
2
0
1
2
,
0
0
0
,
0
0
0
–
0
0
0

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2H, br, 1.25, 3H, t) was observed as well as the signal of
acetone.
decomposition of the alkoxyamines [Scheme 1, reaction (i)].
At the same time, the amount of hydroxylamine/amine41
formed was much lower than expected (ꢇ 13–16 mM). This
fact is easily understood, since, according to literature, nitro-
xide or subsequent hydroxylamine Y-H (formed after reduc-
tion of SG1 in the presence of H-donor) are unstable at ele-
vated temperatures.42,43 To verify this, we investigated the
decomposition products of SG1 in the absence/presence of
H-donor under our experimental conditions. The decomposi-
tion half-time of 0.1 mM of SG1 in benzene at 75 ꢃC was
equal to 123 h (monitored using EPR). As under our experi-
mental conditions, the stability of SG1 is sufficient not to al-
ter the alkoxyamine decomposition, the products of SG1
decomposition have not been investigated.
RESULTS
General Remarks
Scheme 1 shows the reactions taking place during decompo-
sition of alkoxyamine Y-R in different conditions. When
alkoxyamine Y-R is heated in solution, it can either (i)
decompose with formation of alkyl Rꢄ and nitroxyl Yꢄ radical
pair in the solvent cage, or (ii) form alkene R(-H) and hy-
droxylamine Y-H by ionic mechanism—intramolecular proton
transfer IPT. Alkyl/nitroxyl radical pair can recombine with
reformation of parent alkoxyamine, undergo H-transfer reac-
tion with formation of diethylphosphite and hydroxylamine
Y-H (kgem), or escape from the solvent cage producing radi-
cals in the bulk. It should be mentioned that in-cage H-trans-
fer and intramolecular reaction cannot be distinguished in
our experiment. When radicals escape from the solvent cage,
in the absence of scavenger/reductant the radicals Rꢄ and
Yꢄ can recombine (iii) or undergo H-transfer reaction by rad-
ical mechanism (iv). Furthermore, two alkyl radicals can
recombine or disproportionate (v).
Assuming that the conversion of SG1 into Y-H at room tem-
perature in the presence of PhSH is complete, the formation
of diethylphosphite at T ¼ 75ꢃC is quantitative with the rate
constant of Y-H decomposition equal to 0.03 sꢁ1 (Supporting
Information Fig. 1). Thus, in most cases (see below), the
presence of diethylphosphite is a signature of the Y-H being
an unstable intermediate. Taking into account the release of
diethylphosphite due to Y-H decomposition, the amount of
diethylphosphite (6–4 mM, entries 1–3 in Table 1) affords a
correct mass balance in phosphorus compounds.
If no H-transfer (ii) or (iv) occurs for a particular alkoxy-
amine, then the Persistent Radical Effect (PRE)24 takes place.
In this case, the kinetics of alkoxyamine decomposition is
described by eq 1:
The kinetics of alkoxyamine decay in the presence of scav-
enger is monoexponential (Supporting Information Fig. 2).
As no R(-H) has been detected and 100% of alkane RH
formed, we conclude that no reaction competes with CAON
bound homolysis. The exponential fit of experimental
kinetics provides the values of kd for three alkoxyamines
(Table 2 and Supporting Information Fig. 2).
8
9
>3k2d ꢅ 2kt>
2=3
>
>
:
>
>
;
½RYꢆðtÞ ¼ ½RYꢆðt¼0Þ ꢁ ½RYꢆðt¼0Þ
ꢅ
1=3 ꢅ t1=3 (1)
kc2
If the H-transfer reaction is present [either due to the intra-
molecular or radical mechanism, reactions (ii) and (iv)], the
quantitative formation of hydroxylamine/amine and alkene
R(-H) as final reaction products is observed. The particular
mechanism can be determined by the thermolysis of alkoxy-
amine in the presence of scavenger. The addition of scav-
enger PhSH suppresses reactions (iii)–(v), whereas reactions
of nitroxyl and alkyl radicals with thiophenol (vi) and (vii)
lead to the formation of alkane RH and hydroxylamine Y-H
or amine, respectively.34,37 If the presence of PhSH has no
effect on reaction (ii),38 the existence of alkene R(-H) in the
reaction medium during thermolysis indicates intramolecu-
lar/in-cage H-transfer reaction.
Mechanism and Kinetics of Alkoxyamines
Decomposition in the Absence of Scavenger: Evidence
of the Intermolecular H-Atom Transfer Reaction
Decomposition of Alkoxyamines 1, 2
The decomposition of 1 in solution degassed under high vac-
uum [P ¼ 10ꢁ5 mbar, entry 7, Table 1, Fig. 2(a,c,d)] yields
18 mM of alkene and 17 mM of diethylphosphite. This obser-
vation is in good agreement with almost quantitative inter-
molecular H-transfer reaction [reaction (iv) in Scheme 1]
yielding alkene R(-H) and Y-H, which afterward decomposes
into diethylphosphite (see above) due to a long duration of
experiment. The presence of SG1 (2 mM) is likely due to a
disproportionation of Y-H leading to nitroxide, water, and
amine (Supporting Information Scheme 2).44 The latter com-
pound is unstable at this temperature and is expected to
decompose into imine and diethylphosphite (retrophosphory-
lation reaction8). That is why no Y-H and phosphorylated
amine have been detected.
Mechanism and Kinetics of Decomposition Of
Alkoxyamines 1–4 in the Presence of Scavenger:
the Absence of IPT
The decomposition of 20 mM of alkoxyamines 1, 2, and 4
has been carried out in the presence of scavenger, and the
quantitative formation of alkane RH has been observed by
1H NMR [Fig. 2(a,b) and Table 1, entries 1–3]. Furthermore,
a loss of 40 mM of PhSH has been found as expected for the
scavenging of alkyl radical and the reduction of nitroxide.
The formation of the nitroxide as an intermediate during the
thermolysis of SG1-based alkoxyamines has been evidenced
using EPR previously. This highlights that the CAON
bond homolysis is the only process occurring during
Theꢃ decomposition kinetics of alkoxyamines 1 and 2 (48 h,
75 C) in the absence of scavenger in poorly degassed solu-
tions (P ¼ 10ꢁ3 mbar) and after freeze-pump-thaw degass-
ing have been measured. They are monoexponential (Fig. 3)
and have much slower rate constants (two orders of magni-
tude) compared with those obtained in the presence of PhSH
(Table 2). Independent of the experimental conditions, the
W
W
W
.
M
A
T
E
R
I
A
L
S
V
I
E
W
S
.
C
O
M
J
O
U
R
N
A
L
O
F
P
O
L
Y
M
E
R
S
C
I
E
N
C
E
,
P
A
R
T
A
:
P
O
L
Y
M
E
R
C
H
E
M
I
S
T
R
Y
2
0
1
2
,
0
0
0
,
0
0
0
–
0
0
0
5

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6
J
O
U
R
N
A
L
O
F
P
O
L
Y
M
E
R
S
C
I
E
N
C
E
,
P
A
R
T
A
:
P
O
L
Y
M
E
R
C
H
E
M
I
S
T
R
Y
2
0
1
2
,
0
0
0
,
0
0
0
–
0
0
0

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TABLE 2
K
i
n
e
t
i
c
P
a
r
a
m
e
t
e
r
s
o
f
D
e
c
o
m
p
o
s
i
t
i
o
n
o
o
f
A
l
k
o
x
y
a
m
i
n
e
s
1
–
2
,
4
U
n
d
e
r
V
a
r
i
o
u
s
C
o
n
d
i
t
i
o
n
s
a
t
T
5
7
5
8
C
D
e
c
o
m
p
o
s
i
t
i
o
n
i
n
t
h
e
D
e
c
o
m
p
o
s
i
t
i
n
i
n
D
e
c
o
m
p
o
s
i
t
i
o
n
i
n
D
e
c
o
m
p
o
s
i
t
i
o
n
i
n
a
P
r
e
s
e
n
c
e
3
o
f
S
1
c
a
v
e
n
g
e
r
,
t
h
e
P
r
e
s
e
n
c
e
o
f
S
c
a
0
v
e
n
g
e
r
s ꢁ
a
n
1
d
t
h
e
A
b
s
e
n
c
e
o
f
t
h
e
A
b
s
e
n
c
e
o
f
3
3
5
kd
ꢀ
1
0
(
sꢁ
)
T
F
A
(
1
.
1
e
q
lꢁ
u
i
v
)
,
k
ꢀ
1
0
(
)
S
c
a
v
e
n
g
e
r
(
P
¼
1
0ꢁ
m
b
a
r
)
,
S
c
a
v
e
n
g
e r
5
0
(
P
¼
1
0ꢁ
m
b
a
r
)
,
d
1
b
1
b
5
1
1
(
E
,
k
J
m
o
lꢁ
)
(
E
,
k
J
m
o
)
ko
ꢀ
1
0
(
sꢁ
)
ko
b s
ꢀ
1
(
)
0
a
a
b
s
1
2
4
a
1
3
5
5
6
1
(
1
0
8
1
1
.
0
)
1
2
6
0
6
1
(
1
0
9
1
1
.
1
)
1
2 6
8 6
2 6
1
3
–
0
.
7
6
1
.
.
5 6
5 6
0
0
.
.
5
5
(
1
1
2
0
.
.
2
9
)
)
.
.
0 6
0 6
0
0
.
.
5
(
1
1
3
.
.
8
)
1
.
.
0
0
.
5
3
(
5
(
0
6
)
2
.
.
6
6
0
.
0
5
b
1
P
h
S
H
a
s
s
c
a
v
e
n
g
e
r
(
7
–
1
2
e
q
u
i
v
.
)
.
E
r
r
o
r
c
o
m
m
o
n
l
y
a
c
c
e
p
t
e
d
i
s
1
–
2
k
J
m
o
l ꢁ
.
analysis of 1H NMR revealed the formation of alkenes R(-H)
[Fig. 1(c,d)] that cannot be ascribed to the disproportiona-
tion [reaction (v), Scheme 1], because no formation of alkane
RH has been observed. Thus, it can be concluded that R(-H)
is formed during H-atom transfer reaction (reaction (ii) in
Scheme 1).
Decomposition of Alkoxyamine 4
The decomposition of 4 (entry 8, Table 1) in solution
degassed under high vacuum leads to dramatically different
results in comparison with decomposition of 1, 2, 3. Decom-
position of 13 mM of alkoxyamine yields 7 mM of diethyl-
phosphite and only traces of alkene. These results mean that
the presence of carboxylic function in alkoxyamine involves
different pathway of decomposition due to intramolecular H-
bonding46,47 as tentatively is displayed in Supporting Infor-
mation Scheme 3. In addition, degradation of SG1 can be
induced by the acid leading to various side-products. These
degradation routes are faster than the intermolecular H-
transfer reaction, leading to diethylphosphite, and some
other phosphorus compounds formed in the process differ-
ent to those mentioned above. The amount of detected phos-
phorus compounds afforded a good mass balance (12 mM).
Note, that in poorly degassed solution, (entries 4 and 5, Table
1) the amounts of alkene were much lower (<5 mM) than
those in solution degassed under high vacuum (entries 7),
whereas the amounts of decomposed alkoxyamines were
found larger. On the other hand, a new phosphorus-containing
compound was formed (d ¼ 24.7 ppm in 31P NMR spectrum).
To identify the structure of new product, a quantitative
decomposition of alkoxyamine 3 with subsequent product
analysis using mass spectrometry has been performed. The
obtained results indicate the loss of C3H6 fragment by the
molecule 3, whereas the 31P NMR is typical for alkoxyamine.
The results obtained using 1H, 31P NMR, and mass analyses
confirm the assignment of this product to the molecule 3a.
The formation of products having similar structure (i.e.,
alkoxyamines with the carboxylic group directly bound to the
nitroxyl moiety) 1a and 2a (Scheme 1) has been expected
during decomposition of alkoxyamines 1 and 2.
In poorly degassed solution, the decomposition products of
alkoxyamine 4 show only traces of alkene. Contrary to
decomposition of alkoxyamines 1, 2, 3 the signals of 4a are
absent in NMR spectra [entry 6, Table 1, Fig. 2(c,d)], whereas
a signal around 0 ppm is observed by 31P NMR. Note that
carbonic acid derivative 4a is not expected to be stable at 75
ꢃC and should decompose into CO2 and Y-H (Supporting In-
formation Scheme 4). The latter should be reoxidized into
SG1 by the residual oxygen (entry 6, Table 1). The sum of
phosphorus compounds affords a good mass balance.
The account of phosphorus compounds [alkoxyamines 1a
and 2a, alkene R(-H)] afforded a good mass balance. For 1,
the sum of nitroxide and diethylphosphite concentrations
corresponds to the concentration of generated diethylphos-
phite (entry 4, Table 1) implying that the SG1 is observed
due to the reoxidation of Y-H by residual oxygen. For 2, the
same sum is larger, meaning that the other processes are
involved during the decomposition of this alkoxyamine. We
suppose that this difference is determined by roughly 10
times longer decomposition time of alkoxyamine 2 compared
with that of 1 (Table 2). Because of the longer time of
decomposition the tert-butoxy group of 2a can be hydro-
lyzed45 and afford 4a, which then decomposes into diethyl-
phosphite (as proposed in Supporting Information Scheme
2). Thus, a part of Y-H is reoxidized into SG1, and another
part is decomposed into diethylphosphite or some other
side-products (entry 5, Table 1).
Influence of Acid and Additional Nitroxide on the
Decomposition
To confirm the occurrence of IHAT, the decomposition of 1
and 4 has been studied in the presence of additional nitro-
xide SG1 (entries 11 and 12, Table 1). As the addition of SG1
increases the probability of alkyl radical decay via reaction
(iv) Scheme 1, it was expected that the H-atom transfer in
these reactions leads to the formation of larger amount of
alkene during the decomposition. Indeed, this trend has been
observed in the experiment: when alkoxyamines 1 and 4
decomposed in the presence of 40 mM of SG1 in poorly
degassed solution, the formation of 9 and 1 mM of alkene
was observed, respectively (entries 11 and 12 in Table 1).
This confirms that intermolecular H-transfer reaction occurs
both for ester and carboxylic fragments, and that reaction is
very slow and cannot compete with other decomposition
pathways for the carboxylic fragment.
Different behaviors depending on the experimental condi-
tions lead to the differences in kinetics exemplified for 1 in
Figure 3. This indicates the high impact of oxygen on the
decomposition mechanism of SG1-based alkoxyamines.
W
W
W
.
M
A
T
E
R
I
A
L
S
V
I
E
W
S
.
C
O
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J
O
U
R
N
A
L
O
F
P
O
L
Y
M
E
R
S
C
I
E
N
C
E
,
P
A
R
T
A
:
P
O
L
Y
M
E
R
C
H
E
M
I
S
T
R
Y
2
0
1
2
,
0
0
0
,
0
0
0
–
0
0
0
7

JOURNAL OF
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FIGURE 3
E
x
p
e
r
i
m
e
n
t
a
l
(
s
y
m
b
o
l
s
)
a
n
d
c
a
l
c
u
l
a
t
e
d
(
f
u
l
l
l
i
n
e
)
k
i
n
e
t
i
c
s
f
o
r
d
e
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o
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o
s
i
t
i
o
n
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a
l
k
o
x
y
a
m
i
n
e
1
i
n
t
h
e
a
b
s
e
n
c
e
o
f
s
c
a
v
-
e
n
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f
o
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o
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u
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i
o
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e
g
a
s
s
e
d
u
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d
e
r
h
i
g
h
v
a
c
u
u
m
(
a
)
a
n
d
f
o
r
p
o
o
r
l
y
d
e
g
a
s
s
e
d
s
o
l
u
t
i
o
n
(
)
:
(
*
)
a
l
k
o
x
y
a
m
i
n
e
,
(
h
)
a
l
k
e
n
e
,
(
þ
)
o
x
y
g
e
n
p
r
o
d
u
c
t
,
s
o
l
i
d
l
i
n
e
,
c
a
l
c
u
l
a
t
e
d
k
i
n
e
t
i
c
s
;
s
e
e
t
e
x
t
a
n
d
S
u
p
p
o
r
t
i
n
g
I
n
f
o
r
m
a
t
i
o
n
T
a
b
l
e
1
f
o
r
d
e
t
a
i
l
s
.
The difference in thermolysis of alkoxyamines 1, 2, 3, and 4
could be caused by the influence of carboxylic function in 4
the results observed under acidic conditions for 4 are likely
due to a kinetic effect and processes involving degradation
products. The occurrence of kinetic effect is supported by
the recent results reported by Ansong et al.56,57 who have
shown that the NMP of MMA is possible using TEMPO under
harsh acidic conditions. As alkoxyamine 4 carries a carbox-
ylic function on the alkyl fragment, it plays the role of extra
acid for 1, implying the absence of alkene.
on the rate of alkoxyamine homolysis. Recently, Mazarin
et al.48 have shown that the protonation of the nitroxide frag-
ment should slow down the homolysis and, hence, favor side
reactions such as the CAP bond cleavage.49 Edeleva et al.
reported that the protonation of designed nitroxide fragments
significantly decreases the CAON homolysis rate constant.50
To test the influence of carboxylic function in 4 on the rate of
alkoxyamines homolysis, decomposition of 1 in the presence
of PhSH has been investigated in the presence of 1.1 equiv. of
trifluoroacetic acid (TFA, entry 9, Table 1). It has been found
that the addition of 1.1 equiv. of TFA does not decrease kd
values for 1, 2, and 4 significantly, which should take place if
some new processes occur.51–53 This fact is in a good agree-
ment with results obtained by Marque et al.,53 that is, that Ea
values obtained in the presence of TFA are not larger than
1.6 kJ molꢁ1 in comparison with the normal ones.54
Modeling the Mechanism of Decomposition of SG1-Based
alkoxyamines
To confirm the proposed mechanism for SG1-based alkoxy-
amines, we performed simulations of experimental results
for alkoxyamine 1. The following reactions have been taken
into account for modeling the products ratios and kinetics of
alkoxyamine decomposition in highly degassed solution: the
homolysis of alkoxyamine (reaction 1 in Scheme 2), the ref-
ormation of the alkoxyamine (reaction 2 in Scheme 2), the
side reaction of intermolecular H-atom transfer (IHAT, reac-
tion 3), the decomposition of hydroxylamine Y-H (reaction 4
in Scheme 2) and the self-termination reactions (reactions 5
and 6 in Scheme 2). In our simulations, we used the values
of kd measured in this article and those listed in Table 2.
These values are in good agreement with those already
reported in the literature.31,58,59 The values of kc ¼ 1.5 ꢀ
106 Mꢁ1 sꢁ1 were assumed to be independent of the alkyl
fragment of the ester function (Supporting Information Table
In the absence of scavenger in acidic medium, the decompo-
sition of alkoxyamine 1 proceeds slower compared with the
nonacidic media, which evidently is due to the retardation of
homolysis. The amount of alkene formed in the presence of
acid is smaller compared with nonacidic conditions, showing
nearly twofold decrease for alkoxyamine 1 (Supporting Infor-
mation Fig. 4, entries 1 and 9 in Table 1). Thus, the presence
of acid decreases the impact of H-transfer reaction in the
decomposition of alkoxyamines 1, 2, 3. On the other hand,
the presence of acid shows nearly no effect on the kinetics
of the decomposition of 4 (entries 4 and 10, Table 1): as 4
carries already a carboxylic group, the addition of extra acid
has a small effect.
1).60 The kNH measured in this paper is equal to 0.03 sꢁ1
,
and the rate constant for the conventional self-termination
reactions kt ¼ 2 ꢀ 107
M
ꢁ1sꢁ1 is found in the literature.61
The value of kcD has been estimated using eqs 2 and 3:23
kcD
fD ¼
It is well known that nitroxides are unstable under acidic
condition.53,55 Consequently, the decrease of nitroxide con-
centration will decrease the contribution of the reverse reac-
tion, and hence of the side reactions. Furthermore, the
decomposition of nitroxide leads to several by-products (not
identified here) which can scavenge alkyl radicals in another
way by suppressing the formation of alkene. Consequently,
(2)
(3)
kc þ kcD
ꢀ
ꢁ
ꢀ
ꢁ
R1R2NOR ¼ R1R2NOR ꢅ expðꢁfD ꢅ kd ꢅ tÞ
t
where fD is the H-transfer factor. This factor determines the
regime of NMP (controlled character and liveliness). For fD
larger than 3% NMP experiment fails, whereas for fD smaller
8
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SCHEME 2
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than 0.5%, there should be no significant effect of reaction 3
(Scheme 2) on NMP.
amines 1 proceeds with IHAT. The analysis of our experi-
mental results allows usꢃto obtain the value of kcD equal to
1.7 ꢀ 103 Mꢁ1sꢁ1 at 75 C, which is in good agreement with
that reported by Dire et al. (kcD ¼ 1.69 ꢀ 103 Mꢁ1sꢁ1 at 70
ꢃC) for the IHAT process occurring with poly(methyl methac-
rylate) radical. For alkoxyamine 1, which is a basic model for
polymeric alkoxyamine, fD of about 0.1% has been estimated.
Thus, for alkoxyamine 1 NMP of MMA should not be spoiled
by the IHAT process. On the other hand, a very different
behavior has been observed for alkoxyamine 4, that is, only
traces of alkene have been detected, which means negligible
contribution of IHAT process. This fact reveals the strong
impact of a carboxylic function on the side reactions in
alkoxyamine decomposition. In particular, the role of IHAT
process decreases due to appearance of different pathway of
decomposition for alkoxyamine 4.
The reactions involving oxygen and alkyl radicals were
described by reactions 7–11 (Scheme 2).62,63 In particular,
alkyl radical is scavenged by oxygen with the formation of
alkyperoxyl radical (reaction 7). The latter self-terminates
yielding dialkyltetraoxide (reaction 8). The dialkyltetraoxide
collapses spontaneously into alkoxyl radicals and oxygen
(reaction 9). As there is no possibility of H-abstraction by
alkoxyl radicals, the b-fragmentation (reaction 10) could be
the main process leading to formation of acetone and alkoxy-
carbonyl radical. The latter reacts very fast by coupling with
the nitroxide (reaction 11). For the sake of simplicity, the
rate constant of reaction 11 was assumed to be similar to kc.
Reaction of oxygen with alkyl radicals is almost self-diffusion
controlled (109–107
M ) and has been assumed to
ꢁ1sꢁ1 61
occur with the rate constant k1 ¼ 107 ꢀ Mꢁ1sꢁ1. The impact
Role of Oxygen
of this reaction depends on the amount of oxygen and is
therefore given by k1 in eq 4:
*
In most polymerization experiments, degassing is usually
performed by conventional oil pump (10ꢁ3 mbar), and often
only bubbling by gases such as nitrogen or argon is used.
Therefore, to investigate the role of the residual oxygen, we
also performed the homolysis of alkoxyamine 1 and 2 in so-
lution degassed under low vacuum (10ꢁ3 mbar). It has been
found that degassing strongly affects the amount of alkene
concentration in reaction products, in particular only a very
small amount of alkene has been detected under low vac-
uum (25%) instead of 90% at high vacuum. At the same
time, the starting materials completely decomposed and the
formation of new phosphorus species identified as an ester
of hydroxylamine (1a and 2a) has been observed. We sup-
pose that the hydroxylamines 1a and 2a are formed due to
scavenging of alkyl radicals by oxygen yielding alkylperoxyl
radicals, which dimerize into tetraoxide. The latter collapse
instantaneously to yield alkoxyl radicals, which undergo
b-fragmentation and afford acyl radical scavenged by the
nitroxide. Such types of reactions of oxidation are well docu-
mented.62,63 Thus, according to the literature and our results,
Scheme 2 has been proposed to describe the effect of oxygen
on the decomposition of alkoxyamines. Good simulations
were obtained for the kinetics of alkoxyamine 1, alkene and
1a by small adjustment of the rate constants. Noteworthy,
the new alkoxyamines exhibit very close 31P NMR shifts to
those of starting materials, and this might lead to misinter-
pretation of results.
ꢈ
k1 ¼ k1 ꢅ ½O2ꢆ
(4)
Oxygen concentration in air-saturated benzene solution is
2.0 mM.65,66 Using EPR oxymetry (see Supporting Informa-
tion for experimental details) we estimated oxygen concen-
tration under our experimental conditions to be 10ꢁ4 M,
which disagrees with an important impact of oxygen on the
reactivity observed. However, taking into account the gas-so-
lution equilibrium due to the presence of atmospheric gas
above the liquid sample, we assumed constant oxygen con-
centration during the decomposition experiment. Therefore,
the reaction of alkyl radicals with oxygen was considered as
monomolecular in calculations (eq 4) with the rate constant
k1 ¼ 1.5 ꢀ 103 sꢁ1.The literature rate constant for the self-
*
termination of alkyl peroxyl is about 104
M
ꢁ1sꢁ1 and does
not depend on the alkyl fragment.63 The collapsing reaction
of the subsequent dialkyltetraoxide was assumed to occur
instantaneously. The values of k3 were estimated from the
activation energy (Ea ¼ 10–15 kJ molꢁ1) and the activation
factor A0 ¼ 1013 sꢁ1 for the b-fragmentation of tert-butoxyl
radical.67 The parameters providing for the best fit of the ex-
perimental data (listed in Supporting Information Table 1)
are close to the values reported in the literature. The results
of the calculations are shown in Figure 3.
The expected influence of oxygen concentration on decompo-
sition kinetics is shown in Figure 4(a). In agreement with ex-
perimental results, calculated kinetics depends on the
amount of residual oxygen. It varies from fast decay ([O2] ¼
10 mM, saturated solution), that is, complete scavenging, to
no effect ([O2] ¼ 0.01 mM) [Fig. 4(a)]. However, as the
amount of oxygen cannot be rigorously controlled, the effect
of alkoxyamine concentration at constant oxygen concentra-
tion has also been investigated [Fig. 5(b)]. As expected, the
experimental decay of alkoxyamine accelerates as the
concentration decreases. In experiments performed with con-
centrations of 20 and 5 mM of 1, a twofold decrease of kobs
DISCUSSION
Intermolecular H-Atom Transfer
The decomposition of SG1-based alkoxyamines in the pres-
ence of scavenger proceeds via formation of nitroxide and
alkyl radical and their subsequent reaction with PhSH. The
hydroxylamine/amine YH formed in the latter reaction
undergoes decomposition with formation of diethylphos-
phite. It should be emphasized that no intramolecular/
in-cage H-transfer has been detected during the decomposition.
In the absence of scavenger and in ‘‘highly degassed solu-
tion’’ (10ꢁ5–10ꢁ6 mbar), decomposition of SG1-based alkoxy-
10
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1
1
FIGURE 4
(
a
)
H
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has been observed as expected, which supports the robust-
ness and the validity of Scheme 2.
timate unit is accounted for.25 Besides the fact that both high
homolysis rate constant and low reformation rate constant
of alkoxyamine favor strongly the self-termination reactions
of macroalkyl radicals, which would lower the quality of
NMP, they favor also the occurrence of the IHAT side reac-
tion, dramatically spoiling any chance to perform successful
homopolymerization of MMA by NMP.
The Reasons for Uncontrolled Polymerization of MMA
in the Presence of SG1 Nitroxide
As has been found above, the kcD for SG1-based alkoxyamine
1 has been estimated as 1.7 ꢀ 103
M
ꢁ1sꢁ1 implying fD val-
ues around 0.1%, clearly below the threshold of any impact
on NMP.2,6,24 However, as was already mentioned above,
NMP of MMA in the presence of SG1 nitroxide is unsuccess-
ful.25 In fact, kc,ds for poly(methyl methacrylate) radical
recombination with SG1 has been reported as 2.0 ꢀ 104
‘‘The next question is why the NMP of MMA using SG1
becomes successful if a few percents of styrene (S) are
added?’’ A few years ago, it was shown that three dyads
MMA-MMA-SG1, S-MMA-SG1, and MMA-S-SG1 are involved in
NMP process, and the success of NMP is ensured by the dor-
mant species MMA-S-SG1. The structure reactivity relation-
ships developed by the authors of this paper68 afforded the
M
ꢁ1sꢁ1 leading to fD ¼ 10% for the polymeric species and
spoiling the NMP experiment as observed.28 These results
highlight the importance of an accurate and reliable estimate
of both kc and kc,ds values to predict NMP.
kd values of 0.01 sꢁ1, 0.003 sꢁ1, and 2.2 10ꢁ4
s
ꢁ1, and the kc
and 6 ꢀ 104
values as 2 ꢀ 104 Mꢁ1ꢅsꢁ1 69
,
105 Mꢁ1ꢅsꢁ1 70
,
‘‘Hence, what are the real reasons for the unsuccessful con-
trolled NMP of MMA using SG1?’’ In fact, few years ago,
using the Fischer’s diagram approaches, we showed that kd
and kc values of the corresponding macromolecular alkoxy-
amine are not suitable for NMP when the effect of the penul-
Mꢁ1ꢅsꢁ1 71 for MMA-MMA-SG1, S-MMA-SG1, and MMA-S-SG1,
respectively. Consequently, the relationship kd,MMA-S-
SG1<kd,MMA-MMA-SG1, kc,S-MMA-SG1>kc,MMA-MMA-SG1,fD,S-MMA-SG1
¼
2% <fD,MMA-MMA-SG1 ¼ 10%, kc,MMA-S-SG1> kc,MMA-MMA-SG1, and
72
f
D,MMA-S-SG1<<< fD,MMA-MMA-SG1
predicts that the IHAT
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FIGURE 5
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n
s
u
b
s
e
q
e
n
t
e
x
p
o
n
e
n
t
i
a
l
f
i
t
.
T
h
e
o
s
e
r
v
e
d
d
e
c
o
m
p
o
s
i
t
i
o
n
r
a
t
e
o
n
s
t
a
n
t
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e
g
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u
r
e
.
process due to the MMA moiety is dramatically lowered and
has almost no effect on the fate of the NMP experiment (liv-
ingness larger than 75%). Furthermore, these results show
that IHAT process does not depend on the effect of the pe-
nultimate unit and on the chain length of the polymeric radi-
cal. Consequently, calculations using simple models could
provide insights to the requirements for the IHAT facilitating
and, hence, to the design of the most suitable alkoxyamine
for a successful NMP of MMA.
possible side reactions that alter NMP have been evaluated.
Analysis of reaction products of decomposition in the pres-
ence of alkyl radicals scavenger or nitroxide reducing agent
has shown that these alkoxyamines undergo primary NOAC
bond homolysis, that is, no IPT occurs. When the decompo-
sition of 1 is performed in nonscavenging conditions (and
in solution degassed under high vacuum), IHAT process is
observed, despite it is often claimed to be a negligible
process. The kcD value has been estimated as 1.7 ꢀ 103
M
ꢁ1sꢁ1
.
CONCLUSIONS
On the other hand, the decomposition of SG1-based alkoxy-
amines performed in poorly degassed solution in nonsca-
venging conditions reveals high impact of oxygen on decom-
position mechanism. Indeed, nearly quantitative formation of
In this contribution, we have studied the decomposition
mechanism and kinetics of SG1 and isobutyl alkyl frag-
ments-based alkoxyamines at different conditions. The
12
J
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U
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2
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1
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,
0
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,
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–
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0

JOURNAL OF
POLYMER SCIENCE
ARTICLE
WWW.POLYMERCHEMISTRY.ORG
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oxidation product has been observed. Such degassing proce-
dure is currently applied both to investigate kinetics and to
perform polymerization experiments, and, consequently, may
cause a misleading discussion of the results. Furthermore,
the NMR shift of these new compounds is very close to those
of starting materials that may also be misleading. However,
such oxidation events are expected to occur only when IPT
or IHAT processes are low or in the absence of efficient scav-
enger (as alkene in polymerization experiments).
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The authors thank CNRS and RAS for funding the exchange of
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seille for its support. M. Edeleva and E. Bagryanskaya thank
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and RF Ministry for Education and Science (N 8436 and N
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