Factors influencing C-ON bond homolysis in alkoxyamines: Unexpected behavior of SG1 (N-(2-methyl-2-propyl)-N-(1-diethylphosphono-2,2-dimethylpropyl) -N-oxyl)-based alkoxyamines

Article abstract of DOI:10.1021/jo0495586

Alkoxyamines and persistent nitroxides are important regulators of nitroxide-mediated radical polymerization (NMP). Since the polymerization time decreases with the increasing equilibrium constant K (k_dk _c), i.e., the increasing rate constant k_d of the homolysis of the C-ON bond between the polymer chain and the nitroxide moiety, the factors influencing the cleavage rate constants are of considerable interest. SG1-based alkoxyamines have turned out to be the most potent alkoxyamine family to use for NMP of various monomers. Therefore, it is of high interest to determine the factors which make SG1 derivatives better regulators than TEMPO (2,2,6,6-tetramethylpiperidine-N-oxyl) derivatives. Contrary to what we had observed with TEMPO derivatives, we observed two relationships for the plot E_a vs BDE(C-H), one for the nonpolar released alkyl radicals (E _a (kJ/mol) = -133.0 + 0.72BDE) and the other one for the polar released alkyl radicals (E_a (kJ/mol) = -137.0 + 0.69BDE). However, for both families (SG1 and TEMPO derivatives), the rate constants k_d of the C-ON bond homolysis were correlated to the cleavage temperature T _c (log(k_d(s^-1)) = 1.51 -0.058T_c). Such correlations should help to design new alkoxyamines to use as regulators and to improve the tuning of NMP experiments.

Full text of DOI:10.1021/jo0495586

F a ctor s In flu en cin g C-ON Bon d Hom olysis in Alk oxya m in es:
Un exp ected Beh a vior of SG1 (N-(2-m eth yl-2-p r op yl)-
N-(1-d ieth ylp h osp h on o-2,2-d im eth ylp r op yl)-N-oxyl)-Ba sed
Alk oxya m in es
Denis Bertin, Didier Gigmes, Christophe Le Mercier, Sylvain R. A. Marque,* and Paul Tordo
UMR 6517 case 542, CNRS-Universite´ de Provence et d’Aix-Marseille 3,
Avenue Escadrille Normandie-Niemen, 13397 Marseille Cedex 20 France
marque@srepir1.univ-mrs.fr
Received March 18, 2004
Alkoxyamines and persistent nitroxides are important regulators of nitroxide-mediated radical
polymerization (NMP). Since the polymerization time decreases with the increasing equilibrium
constant K (kd/kc), i.e., the increasing rate constant kd of the homolysis of the C-ON bond between
the polymer chain and the nitroxide moiety, the factors influencing the cleavage rate constants
are of considerable interest. SG1-based alkoxyamines have turned out to be the most potent
alkoxyamine family to use for NMP of various monomers. Therefore, it is of high interest to
determine the factors which make SG1 derivatives better regulators than TEMPO (2,2,6,6-
tetramethylpiperidine-N-oxyl) derivatives. Contrary to what we had observed with TEMPO
derivatives, we observed two relationships for the plot Ea vs BDE(C-H), one for the nonpolar
released alkyl radicals (Ea (kJ /mol) ) -133.0 + 0.72BDE) and the other one for the polar released
alkyl radicals (Ea (kJ /mol) ) -137.0 + 0.69BDE). However, for both families (SG1 and TEMPO
derivatives), the rate constants kd of the C-ON bond homolysis were correlated to the cleavage
temperature Tc (log(kd(s-1)) ) 1.51 - 0.058Tc). Such correlations should help to design new
alkoxyamines to use as regulators and to improve the tuning of NMP experiments.
SCHEME 1
In tr od u ction
Thanks to the seminal works of Rizzardo1 and Georg-
es,2 it is now possible to obtain polymers with definite
molecular weights and architectures as well as low
polydispersity indices using the process called nitroxide-
mediated radical polymerization (NMP).3 Such free radi-
cal polymerization can be performed through the revers-
ible deactivation of the growing polymeric radical by
stable or persistent radicals such as nitroxyl radicals.4
The easiest way to trigger the polymerization is to use
thermally unstable alkoxyamines prepared beforehand
(reaction 1 with n ) 0). The growth of the polymeric
radicals (reaction 3 with kp the propagation rate constant)
involves successive deactivation dissociation cycles (reac-
tions 2 and 1 with kc and kd the deactivation and
dissociation rate constants, respectively), which trans-
form them into macroalkoxyamines (called dormant
species), which are alternately reactivated into polymeric
radicals by thermal homolysis. The self-termination
reaction (reaction 4 with kt the self-termination rate
constant) is kept at a low level (Scheme 1).
SCHEME 2
Due to the persistent radical effect (PRE, Scheme 2),5
the rate of polymerization in this type of system depends
on the constant of equilibrium K ) kd/kc, kd being the
C-ON bond homolysis rate constant and kc the re-
formation rate constant. If K is too small, the polymer-
ization can be very sluggish or even inhibited, while if K
is too large, the polymerization is too rapidsthe control
is loosenedsor a noncontrolled radical polymerization
occurs.6
* To whom correspondence should be addressed. Fax: 33-4-91-98-
87-58.
(1) US Patent 4,581,429; Eur. Pat. Appl. 135280. Solomon, D. H.;
Rizzardo, E.; Cacioli, P. Chem. Abstr. 1985, 102, 221335q.
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K. Macromolecules 1993, 26, 2987.
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J .; Bosman, A. W.; Harth, E. Chem. Rev. 2001, 101, 3661 and references
therein.
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10.1021/jo0495586 CCC: $27.50 © 2004 American Chemical Society
Published on Web 06/23/2004
J . Org. Chem. 2004, 69, 4925-4930
4925

Bertin et al.
SCHEME 3
Fischer et al.6,7a-e and Fukuda et al.7f-i have shown
that, for NMP to be efficientsshort polymerization time,
low polydispersity indices PDI, and high livingnesssthe
equilibrium constant K ) kd/kc should vary roughly
between 10-7 and 10-11 M and the product kdkc should
be as large as possible to reach low PDI’s. Therefore, kd
and kc have to be in a narrow range, i.e., 10-3 s-1 < kd <
1 s-1 and 106 L mol-1 s-1 < kc < 109 L mol-1 s-1. Since kd
and kc depend on the alkoxyamine structure and kc varies
in general within a narrow range (105 to 109 L mol-1 s-1
from room temperature to 120 °C), while kd varies within
a wide range (10-9 to 1 s-1 at 120 °C),8 it is crucial to
know the effect of the alkoxyamine structure on kd.
Almost all NMPs performed with TEMPO9 or DBNO9 (di-
tert-butyl-N-oxyl)-based alkoxyamines are known to suf-
fer severe limitations.3b,10 This has led our group8c,11 to
develop a new family of alkoxyamines (Scheme 3) based
on a nitroxyl moiety carrying a phosphoryl group in the
â position, called SG1 (N-(2-methyl-2-propyl)-N-(1-dieth-
ylphosphono-2,2-dimethylpropyl)-N-oxyl).
Inspired by numerous studies on NMP using TEMPO-
based alkoxyamines, several groups8a-i,11c,d,12-18 have
undertaken thorough studies of the effect of the alkoxy-
amine structure on the values of kd. A linear plot Ea vs
bond dissociation energies BDE(C-H) or BDE(C-O) of
the released alkyl radical was obtained in the case of
TEMPO-based alkoxyamines,8b,d,19 which means that the
more stabilized the released alkyl radical, the higher kd.
However, there were some outlying alkoxyamines, due
to the anomeric effect of the heteroatom in R position to
the C-ON bond.8d,19 As for the tertiary released alkyl
radicals, they slightly deviated from the general trend.8b,d
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available and their corresponding alkoxyamines are of easy access.
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4926 J . Org. Chem., Vol. 69, No. 15, 2004

Factors Influencing C-ON Bond Homolysis in Alkoxyamines
Our group8c,11d,e,18 obtained very promising results
concerning the polymerization of styrene, acrylates, and
methacrylates. Because SG1-based alkoxyamines are the
most potent family to achieve the control of several
monomers via NMP, it is timely to check whether the
same effects occur as those observed with the TEMPO
series. Unfortunately, little is known on the effect of the
structure of SG1-based alkoxyamines on kd and thus on
K. We present hereafter the synthesis of a series of SG1-
based alkoxyamines 1-13, the values of their C-ON
bond homolysis rate constants kd and the cleavage
temperatures Tc (see the Experimental Section).
determined at only a few temperatures this average value
was used to estimate the energies of activation (Ea). The
estimated Ea values ranged within an interval of 2 kJ /
mol and their average was taken. For alkoxyamines 6,
7, and 13, kd and Ea may be subject to larger errors,
because at higher temperatures SG1 is less persistent
and the measurements were performed at very low
alkoxyamine conversions (<5%). The values of Ea, kd, A,
and Tc are collected in Table 1.
Discu ssion
In preliminary studies,8b we showed that SG1-based
alkoxyamines follow grossly the same trend as TEMPO
ones. To confirm this observation, we undertook the
synthesis of new SG1-based alkoxyamines, so that no
anomeric effect could occur, measured their kd (Table 1),
and drew a plot Ea vs BDE(C-H).24 To our great surprise,
the plot turned out to be more complicated than in the
case of the TEMPO series (eq 13).8b Indeed, the SG1-
based alkoxyamines are split into two families which are
correlated to the BDE(C-H) of the released alkyl radical
(Figure 1). Along a first line (eq 11) alkoxyamines 1-4,
6 and 7, that is, weakly or nonpolar (-0.01 < σU < 0.12)25
released alkyl radicals were gathered. A second line
(equation 12) gathered alkoxyamines 8 - 13, which carry
polar (0.30 < σU < 0.57)25 released alkyl radicals.
Furthermore, the smaller slopes (0.72 and 0.69 in eqs
11 and 12, respectively) observed for SG1 derivatives
than the slope (0.97 in eq 13) for TEMPO derivatives
point out that the reaction enthalpy is not the major
influence involved in the C-ON bond homolysis of SG1
- based alkoxyamines.
Resu lts
Due to re-formation (PRE, Scheme 2) of the alkoxy-
amine, kd cannot be measured directly from the alkoxy-
amine decay under normal conditions.8b-d,20 Instead, the
conditions have to be chosen so that either the transient
radical or the persistent species is rapidly converted into
other nonreactive species before re-formation.8b-d,20,21
We8b-d,11d,22 and others13a,23 have previously shown that
kd can be measured by ESR monitoring of the nitroxide
build-up using dioxygen, the galvinoxyl radical, TMIO
(2,2,10,10-tetramethylisoindolin-N-oxyl), and TEMPO as
alkyl radical scavengers or by 31P NMR monitoring of the
alkoxyamine evolution21 using TEMPO, phenylhydrazine,
or thiophenol as scavengers for the transient radical and/
or the nitroxyl radical. The experiments were carried out
in tert-butyl benzene, and the rate constants kd were
measured using either the plateau (eq 8) or the initial
slope (eq 9) methods8b-d,20 from the curve giving the time
dependence of the nitroxide concentration. We also
considered the decay (eq 10) of the alkoxyamine.21
Alkoxyamine 5 lies on the polar released alkyl radical
line (Figure 1) although the t-Bu group must be consid-
ered as a nonpolar group (σU ) -0.01).25 Such shift to
the nonpolar line is certainly due to the steric effect8b,d
The results were reproducible, and the rate constants
did not change upon varying alkoxyamine concentrations.
We had previously shown that the frequency factor A for
the homolysis of the alkoxyamine C-ON bond did not
vary much when the alkoxyamine structure varied and
ranged between 1013 and 1015 s-1, with an average value
(24) BDE values given for molecules with H atom instead of SG1
nitroxyl fragment. BDE(1-H) ) 357 (63) kJ /mol, BDE(2-H) ) 364.5-
(84) kJ /mol, BDE(9-H) ) 385.8(59) kJ /mol, BDE(7-H) ) 422.6(17) kJ /
mol (value of C2H6 was used): McMillen, D. F.; Golden, D. M. Annu.
Rev. Phys. Chem. 1982, 33, 493. BDE(10-H) ) 385.0(150) kJ /mol:
Zytowski, T.; Fischer, H. J . Am. Chem. Soc. 1997, 119, 12869. BDE-
(13-H) ) 406.3(105) kJ /mol: Luo, Y.-R. Handbook of Bond Dissociation
Energies in Organic Compounds; CRC Press: Boca Raton, FL, 2003;
p 66. BDE(8-H) ) 375.8 kJ /mol, BDE(12-H) ) 396.3 kJ /mol: King, K.
D.; Goddard, R. D. J . Phys. Chem. 1976, 80, 546. BDE(3-H) ) 368.8-
(88) kJ /mol, BDE(4-H) ) 370(63) kJ /mol, BDE(5-H) ) 404.0(17) kJ /
mol: Berkowitz, J .; Ellison, G. B.; Gutman, D. J . Phys. Chem. 1994,
98, 2744. BDE(11-H) ) BDE(10-H) + 9.5 kJ /mol ) 394.5 (4) kJ /mol,
increment from: Henry, D. J .; Parkinson, C. J .; Mayer, P. M.; Radom,
L. J . Phys. Chem. A 2001, 105, 6750. BDE(6-H) ) 399.6 (40) kJ /mol:
CRC Handbook of Chemistry and Physics, 76th ed.; Lide, D. R., Ed.;
CRC Press: Boca Raton, FL, 1995; pp 9-64.
of 2.4 1014 s-1 8b,d
When the dissociation rate constant was
.
(20) Kothe, T.; Marque, S.; Martschke, R.; Popov, M.; Fischer, H. J .
Chem. Soc., Perkin Trans. 2 1998, 1553.
(21) Bertin, D.; Gigmes, D.; Marque, S.; Tordo, P. e-Polymer 2003,
paper no. 2.
(22) Bertin, D.; Chauvin, F.; Marque, S.; Tordo, P. Macromolecules
2002, 35, 3790.
(23) (a) Kovtun, G. A.; Aleksandrov, A. L.; Golubev, V. A. Izv. Akad.
Nauk SSSR, Ser. Khim. 1974, 2197. (b) Kovtun, G. A.; Aleksandrov,
A. L.; Golubev, V. A. Bull. Akad. Sci. USSR: Div. Chem. Ser. 1974,
2115. (c) Howard, J . A.; Tait, J . C. J . Org. Chem. 1978, 43, 4279. (d)
Grattan, D. W.; Carlsson, D. J .; Howard, J . A.; Wiles, D. M. Can. J .
Chem. 1979, 57, 2834. (e) Stipa, P.; Greci, L.; Carloni, P.; Damiani, E.
Polymer Degrad. Stab. 1997, 55. (f) Korolev, G. V.; Berezin, M. P.;
Bakova, G. M.; Kochneva, I. S. Polym. Sci., Ser. B. 2000, 42, 339.
(25) (a) Charton, M. Prog. Phys. Org. Chem. 1981, 13, 119. (b)
Charton, M. Prog. Phys. Org. Chem. 1987, 16, 287.
J . Org. Chem, Vol. 69, No. 15, 2004 4927

Bertin et al.
TABLE 1. Activa tion P a r a m eter s, Ra te Con sta n ts kd a t 120 °C, a n d Clea va ge Tem p er a tu r es Tc for SG1-Ba sed
Alk oxya m in es
a
b
T ( 1 °C. Statistical errors smaller than a factor of 2. The value in parentheses is the average of all experimentally accessible
frequency factors. c Statistical errors around 2 kJ mol-1
.
Values calculated with parameters from columns 4 and 5. e Rescaled values of
d
Ea using an averaged frequency factor (see text) when it was necessary, i.e., A different from 2.4 × 1014 s-1
.
Both diastereoisomers
f
g
h
showed the same rate constant. The value may be affected by errors due to a side reaction; see ref 31. The two isomers were not
isolated separately. The rate constants were estimated from a plot of eq 8 showing two slopes. Only one isomer could be measured. 31P
i
NMR (C6D6) δ ) 21.8 ppm. Isomers RS and SR, 31P NMR (C6D6), δ ) 22.3 ppm. Isomers RR and SS, 31P NMR (C6D6), δ ) 23.0 ppm.
j
k
l
Not determined.
4928 J . Org. Chem., Vol. 69, No. 15, 2004

Factors Influencing C-ON Bond Homolysis in Alkoxyamines
F IGURE 2. Plot of log kd vs Tc.
F IGURE 1. Plot of Ea(CO-N) bond homolysis in SG1-based
alkoxyamines against BDE(C-H) of the corresponding re-
leased alkyl radical: (9) weakly or nonpolar, (O) polar, and
(2) tertiary alkyl fragments.
lizingshave an influence on kd. The strength of each
effect depends markedly on the structure of the released
alkyl radical as well as on the structure of the nitroxyl
moiety, which is an either strongly or weakly polar
structure. Consequently, the correlation Ea vs BDE(C-
H) cannot be used as a reliable tool to predict accurate
values of kd in the case of alkoxyamines carrying a polar
nitroxyl fragment. Consequently, more studies are re-
quired to determine accurately the influence of the
various effects (polar ground-state effect and steric effect)
involved in the homolysis of the alkoxyamine C-ON
bond, and they will be published in forthcoming papers.
of the second methyl group (t-Bu is the only tertiary
group of the series) which increases the steric hindrance
and thus enhances kd, i.e., lowers Ea. However, at the
moment, such assumption is hard to verify because only
a few tertiary SG1-CMe2COOR alkoxyamines have been
prepared.26 And due to the size effect of the R group, the
Ea’s of such alkoxyamines cannot be directly correlated
to the BDE(C-H).27 It is now clear that the structures
of both nitroxyl radical and released alkyl radical strongly
influence the values of kd in a more complex way than
earlier expected. So far, it is not clear whether the polar
effect destabilizes the ground state or stabilizes the
transition state although it is established that the
stabilization of the nitroxyl radical decreases with the
increasing polarity of the substituents.28 Consequently,
the predictive tables built in a previous work8b to give kd
as function of the nitroxyl radical and the released alkyl
radical have to be used with care. It seems that the
nitroxyl moiety family should be split into two families,
i.e., one for the polar structures (SG1 family and homo-
logues) and one for the nonpolar nitroxyl structures
(TEMPO, DBNO...). These results show that the trend
observed for TEMPO derivatives cannot be directly
extended to any type of alkoxyamine.
Exp er im en ta l Section
Alkoxyamines 1-4 and 8-13 were synthesized following a
modified11d procedure (GP1) described by Matyjazewsky.29
Alkoxyamines 5-7 were made following the procedure (GP2)
established by Curran.30
Gen er a l P r oced u r e (GP 1) for th e Alk oxya m in e P r ep a -
r a tion Usin g th e ATRA Meth od . A deoxygenated solution
of SG1 and alkyl bromide in benzene was added under inert
atmosphere to a deoxygenated suspension of Cu(0), CuBr, and
ligand [2,2′-dipyridyl (bpy) or N,N,N′,N′,N′′-pentamethyldi-
ethylenetriamine (PMDETA)] in benzene. The resulting reac-
tion mixture was stirred between rt and 80 °C for 2-48 h.
The solids were then removed by filtration on Celite and
washed with Et2O. When the ligand was 2,2′-bipyridyl (bpy),
the filtrate was washed with a 5% w/w aqueous solution of
CuSO4 and then with water. With PMDETA as ligand the
filtrate was only washed with water. The organic phase was
dried over MgSO4 and evaporated under reduced pressure.
Purification by silica gel column chromatography afforded pure
alkoxyamine.
We have recently shown11d that the values of kd of
various alkoxyamines can be correlated to the cleavage
temperature Tc (eq 14). In Figure 2, only new and old
SG1-based alkoxyamines are reported, in good agreement
with the correlation observed in a previous work (line in
Figure 2).11d The new molecules lie close to the straight
line and thus, the relationship (eq 14) can be used as a
predictive tool.
Gen er a l P r oced u r e (GP 2) for th e Alk oxya m in e P r ep a -
r a tion via a n Or ga n om eta llic Com p ou n d . The appropriate
magnesium or lithium organometallic compound was added
dropwise to a deoxygenated solution of SG1 (0.11 M) in
anhydrous THF at -90 °C and under inert atmosphere. The
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P. Org. Biomol. Chem. 2004, 2, 709.
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J . Polym. Sci. Part A: Polym. Chem. 2004 in press.
(28) Marque, S. J . Org. Chem. 2003, 68, 7582.
(29) Matyjaszewski, K.; Woodworth, B. E.; Zhang, X.; Gaynor, S.;
Metzner, Z. Macromolecules 1998, 31, 5955.
(30) Nagashima, T.; Curran, D. P. Synlett 1996, 330.
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Con clu sion
The observations made with the TEMPO series cannot
be directly extrapolated to any type of alkoxyamine. This
work shows that three effectsspolar, steric, and stabi-
J . Org. Chem, Vol. 69, No. 15, 2004 4929

Bertin et al.
reaction temperature was allowed to rise slowly to room
temperature. The reaction mixture was then hydrolyzed with
a saturated NH4Cl aqueous solution and extracted three times
with Et2O. The organic layers were dried over MgSO4 and
evaporated under reduced pressure. Purification of the crude
material by silica gel column chromatography afforded pure
alkoxyamine.
the ESR signal of the SG1 appears. Then the second probe
was inserted in the ESR cavity (CW-ESR EMX Bruker
equipped with a temperature setup Bruker BV 2000) pre-
heated at a temperature 30 °C below the rough Tc. The
temperature was increased stepwise by 5 °C, and Tc corre-
sponds to the temperature where the ESR signal recorded at
short intervals (1-min) showed a steady increase.
Kin etic Mea su r em en ts. Measurements of kd were carried
out following previously described experimental conditions
(CW-ESR EMX Bruker8b-d,20 and 300 MHz Avance Bruker21
spectrometers).
Ack n ow led gm en t. We thank the Centre National
de la Recherche Scientifique and the University of
Provence for their financial support.
Mea su r em en t of th e Clea va ge Tem p er a tu r e Tc.11d Two
ESR probes were filled with 0.01 M tert-butylbenzene solution
of SG1-based alkoxyamine and degassed with several freeze-
pump-thaw cycles. With the first sample, a rough estimation
of Tc was made. The temperature was increased stepwise by
5 °C and the rough Tc corresponds to the temperature where
Su p p or tin g In for m a tion Ava ila ble: Preparation and
characterization of alkoxyamines 1-13. This material
is available free of charge via the Internet at http://
pubs.acs.org.
J O0495586
4930 J . Org. Chem., Vol. 69, No. 15, 2004