Improving the control of styrene polymerization at 60 °c using a dialkylated α-hydrogenated nitroxide

Article abstract of DOI:10.1002/pola.26163

A new dialkylated α-hydrogenated linear nitroxide and the corresponding 1-phenylethyl alkoxyamine were synthesized in two and three steps, respectively. The alkoxyamine was involved in the polymerization of styrene at 60 °C, and the in situ concentration of nitroxide was monitored by electron spin resonance spectroscopy. The enhanced characteristics of these new alkylated alkoxyamine and nitroxide (k d60 = 1.5 × 10^-4 s^-1 and k c60 = 5.7 × 10⁴ L mol^-1 s^-1) yielded a monomer consumption one order of magnitude higher than styrene thermal polymerization. This resulted in well-defined polystyrenes up to 70,000 g mol^-1 and the observation of a control occurring through the establishment of the radical persistent effect, that is, ln([M]₀/[M]) = t^2/3. Experimentally determined kinetic constants were involved in PREDICI modelings to investigate the influence of temperature and initial alkoxyamine concentration on the kinetics as well as on the livingness and the controlled character of the polymerization.

Full text of DOI:10.1002/pola.26163

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Improving the Control of Styrene Polymerization at 60 8C Using a
Dialkylated a-Hydrogenated Nitroxide
Yohann Guillaneuf,1 Jean-Philippe Lamps,2 Jean-Marie Catala,2 Didier Gigmes,1
Eric Drockenmuller3,4
1
ꢀ
ꢀ
ꢀ
Institut de Chimie Radicalaire, equipe CROPS, UMR 7273 CNRS, Aix-Marseille Universite, Campus de Saint Jeroˆ me,
avenue Escadrille Normandie-Niemen, Case 542, 13397 Marseille Cedex 20, France
2
ꢀ
Institut Charles Sadron, CNRS, Universite de Strasbourg, 23 rue du Loess, 67034 Strasbourg, France
3
ꢀ
ꢀ
ꢀ
Universite Claude Bernard Lyon 1, Ingenierie des Materiaux Polyme`res, UMR CNRS 5223, 15 Boulevard Latarjet,
69622 Villeurbanne Cedex, France
4Institut Universitaire de France (IUF)
Correspondence to: J.-M. Catala (E-mail: jean-marie.catala@ics-cnrs.unistra.fr) or E. Drockenmuller (E-mail: eric.drockenmuller@
univ-lyon1.fr)
Received 5 April 2012; accepted 6 May 2012; published online
DOI: 10.1002/pola.26163
ABSTRACT: A new dialkylated a-hydrogenated linear nitroxide
and the corresponding 1-phenylethyl alkoxyamine were synthe-
sized in two and three steps, respectively. The alkoxyamine was
involved in the polymerization of styrene at 60 ꢀC, and the in
situ concentration of nitroxide was monitored by electron spin
resonance spectroscopy. The enhanced characteristics of these
new alkylated alkoxyamine and nitroxide (k6d0 ¼ 1.5 ꢁ 10ꢂ4 sꢂ1
through the establishment of the radical persistent effect, that
is, ln([M]0/[M]) ¼ t2/3. Experimentally determined kinetic con-
stants were involved in PREDICI modelings to investigate the
influence of temperature and initial alkoxyamine concentration
on the kinetics as well as on the livingness and the controlled
C
V
character of the polymerization.
2012 Wiley Periodicals, Inc.
J Polym Sci Part A: Polym Chem 000: 000–000, 2012
and k6c0 ¼ 5.7 ꢁ 104 L molꢂ1
s
ꢂ1) yielded a monomer consump-
tion one order of magnitude higher than styrene thermal poly-
merization. This resulted in well-defined polystyrenes up to
70,000 g molꢂ1 and the observation of a control occurring
KEYWORDS: alkoxyamine; kinetics (polym.); nitroxide mediated
polymerization (NMP); polystyrene; radical polymerization;
styrene
INTRODUCTION With the development of controlled and/or
living radical polymerization (CLRP) techniques, the versatile
control of the structure and architecture of vinyl polymers
that exhibit specific properties and functionalities is now
well established.1 Although nitroxide mediated polymeriza-
tion (NMP) has been the first CLRP method to be developed,
atom transfer radical polymerization (ATRP)2,3 and reversi-
ble addition/fragmentation chain transfer (RAFT) polymer-
ization4–6 have rapidly demonstrated interesting advantages
and improved efficiencies such as a wider range of controlla-
ble monomers, lower polymerization temperatures, and a
broader combination with robust, efficient, and orthogonal
chemistries.1,7–10 However, NMP is one of the simplest
method to implement due to its thermally activated mono-
component control system and a simple purification process
based on precipitation to remove monomer residuals. These
features, together with the absence of cytotoxicity for nitro-
xides, alkoxyamines and polymers made by this method,11
have always clearly represented unquestionable advantages
of NMP over other CLRP techniques.12,13 In addition, in the
last 15 years, significant efforts have allowed the main weak-
nesses of NMP to be efficiently solved, or at least signifi-
cantly lowered12 as for instance the range of monomers14–19
or the range of macromolecular architecture20–23 that has
been significantly extended. In particular, the use of additives
to enhance the rate of polymerization,24,25 the design of
highly labile alkoxyamines based on highly crowded alkyl
moieties,26 and the design of new nitroxides with enhanced
structural features12 afforded a substantial decrease of the
polymerization temperature. For instance, N-tert-butyl-N-[1-
diethylphosphono-(2,2-dimethylpropyl)] nitroxide (1 or SG1,
Scheme 1)27 and 2,2,5-trimethyl-4-phenyl-3-azahexane-3-oxyl
nitroxide (2 or TIPNO)28 were shown to have the best bal-
ance between their efficiency, their good thermal stability at
elevated temperatures and a moderate difficulty for their
synthesis. Few other structures such as hindered cyclic nitro-
xides prepared by Studer et al. (3,4) are very efficient since
the successful polymerization of styrene and n-butyl acrylate
Additional Supporting Information may be found in the online version of this article.
C
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behavior stems from the different orientation of the strongly
polar sulfoxide functionality with respect to the nitroxide
moiety induced by the stereochemistry of the persistent rad-
ical. This characteristic requires, therefore, the separation of
diastereomers and thus complicates the large scale prepara-
tion of this specific mixture of diastereomers. The peculiar
skeleton of BESNO nitroxide is nevertheless highly interest-
ing and the aim of this article is to investigate the replace-
ment of the sulfoxide group by a methylene group and its
impact on the properties of the resulting 1-phenylethyl
alkoxyamine with respect to stereochemistry. We report
herein the electron spin resonance (ESR) spectroscopy and
kinetic studies of the polymerization of styrene at 60 ꢀC
mediated by the 1-phenylethyl alkoxyamine of this new alky-
lated analogue of BESNO. PREDICI modelings using the ki-
netic rate constants determined experimentally herein were
performed to explore the influence of various parameters
such as conversion, temperature, and initial alkoxyamine
concentration on the livingness and the controlled character
of the polymerization of styrene.
SCHEME 1 Chemical structures of nitroxides 1-6.
29,30
ꢀ
could be performed between 70 and 90 C.
Nevertheless,
the preparation of such nitroxides and the corresponding
alkoxyamines requires demanding multistep synthetic proce-
dures.29,30 More recently, Cameron et al.31 patented the con-
ꢀ
trolled polymerization of styrene at 60 C using N-tert-butyl-
N-(1-tert-butyl-2-methyl)propyl nitroxide (5). However, addi-
tional data regarding nitroxide’s stability, accurate determi-
nation of kinetic parameters, and extension to the prepara-
tion of high molar mass polymers would precise the real
potential of this promising persistent radical.
RESULTS AND DISCUSSION
Synthesis
Besides, one diastereomer mixture (RSRbRa/SSSbSa) of N-tert-
butyl-N-(1-tert-butyl-2-ethylsulfinylpropyl) nitroxide (6 or
BESNO), a chiral b-sulfinyl nitroxide developed by Catala
et al. was shown to efficiently control the polymerization of
Nitroxide 10 was synthesized in two steps, taking advantage
of the high reactivity of nitrones toward nucleophiles, and
the mild oxidation of the resulting hydroxylamine to afford
the corresponding nitroxide functionality (Scheme 2). This
versatile two-step strategy is particularly efficient and
requires fewer synthetic and purification steps compared to
nitroxides previously shown to control the polymerization of
styrene at moderate temperatures such as nitroxides 3, 4, or
6.29,30,32
32–34
ꢀ
ethyl acrylate, n-butyl acrylate and styrene below 90 C,
showing in the case of styrene a rate of polymerization sig-
nificantly higher than the rate of thermal polymerization.33,35
Conversely, the other BESNO diastereomers (SSRbRa/RSSbSa)
were not able to control the polymerization of acrylates and
the control of styrene polymerization was only possible at
temperatures above 110 ꢀC with a rate of polymerization
comparable to thermal polymerization of styrene. Although
the rate constant of combination (kc) was found independent
of the BESNO stereochemistry, a one order of magnitude dif-
ference was observed for the rate constant of dissociation
(kd) of the different diastereomer mixtures of the corre-
sponding 1-phenylethyl alkoxyamines.32 This particular
The nucleophilic addition of sec-pentyllithium 7 to N-tert-
butyl-a-tert-butyl nitrone 8 was performed at room tempera-
ture, thus affording all possible diastereomers of hydroxyla-
mine 9 (i.e., SaRb/RaSb and RaRb/SaSb) in 70% yield. Subse-
quent mild oxidation of the hydroxylamine intermediate by
copper(II) acetate monohydrate afforded quantitatively nitro-
xide 10, a per-alkylated analog of nitroxide 6. Conversely to
SCHEME 2 Synthesis of nitroxide 10 and the corresponding 1-phenylethyl alkoxyamine 12.
2
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strongly polar BESNO nitroxide 6 (aN ¼ 14.7 G for SSRbRa/
RSSbSa and aN ¼ 13.6 G for RSRbRa/SSSbSa diastereomers),
the different diastereomers involved in the composition of
nitroxide 10 are characterized by identical values of the
nitrogen hyperfine coupling constant (aN ¼ 13.6 G). It is
worth mentioning that this aN value matches the one
observed for the diastereomer mixture of BESNO nitroxide
able to control the polymerization of acrylates and styrene
significantly above the rate of thermal polymerization at
temperatures below 90 ꢀC. As observed previously, the
hyperfin coupling of the spin radical with the b-hydrogen
atom (aH) was not observed on the ESR spectra (Fig. S1 in
the Supporting Information). Indeed, the steric hindrance of
the a-tert-butyl substituent stabilizes the conformations
where the b-hydrogen and the nitroxide functionality do not
interact. It was verified by ESR spectroscopy that nitroxide
10 is extremely stable in benzene at elevated temperatures
(Fig. S2 in the Supporting Information).
FIGURE 1 Monomer conversion versus time for the bulk poly-
merization of styrene at 60 ꢀC: (þ, solid line) thermal polymer-
ization of neat styrene; (n, dotted line) [12]0 ¼ 14.2 ꢁ 10ꢂ3 mol
Alkoxyamine 12 was then synthesized from the coupling of
nitroxide 10 diastereomers with 1-phenylethyl radicals
issued from the decomposition of di-tert-butyl peroxalate in
ethylbenzene 11 at 35 ꢀC (Scheme 2). This particular reac-
tion allowed working at rather low temperature, which is
mandatory regarding the fast dissociation of the resulting
alkoxyamine. However, other mild methods for alkoxyamine
preparation such as atom transfer radical addition or photo-
chemical alternatives could be used.36–41 Even not structur-
ally resolved, two diastereomer mixtures of alkoxyamine 12
were partially isolated by column chromatography and their
kd values determined at 60 ꢀC in benzene by ESR spectros-
copy using a previously reported protocol based on galvi-
noxyl as spin trap.32,33 Conversely to BESNO analogs, and
consequently to the absence of strong dipole moment, the
distinct stereochemistry of the nitroxide moiety did not
induce any significant difference in the kd values of the dif-
ferent alkoxyamine diastereomers. Compared to sulfoxide-
functionalized diastereomers of nitroxide 6, this alkylated
alkoxyamine does not require tedious separation of diaster-
eomers with distinct properties, thus contributing to the
versatile nature of the synthetic approach developed herein.
Therefore, the crude diastereomer mixture of alkoxyamine
12 was involved in subsequent ESR and polymerization
experiments.
L
ꢂ1; (h, dashed line) [12]0 ¼ 8.1 ꢁ 10ꢂ3 mol Lꢂ1; (solid, dotted
and dashed lines) PREDICI modelings.
bond homolysis is the steric hindrance of both the transient
and persistent radical moieties. For the other BESNO diaster-
eomers (SSRbRa/RSSbSa), the dipole interaction of the sulfox-
ide moiety with the N-oxyl radical is marked by an increase
in the nitrogen hyperfine coupling constant (aN ¼ 14.7 G)
and a decrease in kd.
The particularly fast dissociation of alkoxyamine 12 at 60
ꢀC (k6d0 ¼ 1.5 ꢁ 10ꢂ4 sꢂ1, t60 ¼ 4621 s) prompted us to
1=2
study the bulk polymerization of styrene at this tempera-
ture. Two alkoxyamine concentrations ([12]0 ¼ 8.1 ꢁ 10ꢂ3
mol Lꢂ1 and 14.2 ꢁ 10ꢂ3 mol Lꢂ1) were used to investi-
gate the effect of the initial alkoxyamine concentration on
the rate of polymerization as well as the possibility to
reach high molar mass polystyrenes. The in situ evolution
of nitroxide concentration for each polymerization experi-
ment was also monitored by ESR spectroscopy. Interest-
ingly, the evolution of monomer conversion with time (Fig.
1) reveals a rate of polymerization about one order of
magnitude higher than styrene thermal polymerization
and that depends on the initial alkoxyamine concentration
[12]0.
ESR and Polymerization Kinetics
The accurate determination of kd by ESR spectroscopy
experiments performed at temperatures ranging from 70 to
90 ꢀC (Fig. S3 in the Supporting Information), yielded the
following Arrhenius parameters: A ¼ 1.8 ꢁ 1014 sꢂ1 and Ea
¼ 115 6 2 kJ molꢂ1 (Fig. S4 in the Supporting Information).
As it could be expected from the ESR parameters (i.e., aN
and g), the values obtained for the activation energy (Ea/kJ
molꢂ1) and the frequency parameter (A/sꢂ1) are similar to
those determined for the RSRbRa/SSSbSa BESNO-1-phenyl-
ethyl alkoxyamine and are in agreement with the general fre-
As expected for the highest initial alkoxyamine concentration
([12]0 ¼ 14.2 ꢁ 10ꢂ3 mol Lꢂ1, i.e., Mn,th ¼ 65,000 g molꢂ1
)
the number average molar mass (Mn) increases linearly with
conversion in perfect agreement with theoretical values and
-
dispersity (D) reaches values close to 1.1 (Figs. 2 and 3,
respectively). For the lowest initial alkoxyamine concentra-
tion ([12]0 ¼ 8.1 ꢁ 10ꢂ3 mol Lꢂ1, i.e., Mn,th ¼ 112,500 g
molꢂ1) a similar trend was observed that is more surprising
since it is known that obtaining Mn > 50,000 g molꢂ1 with
25
quency parameter (A ¼ 2.4 1014
s
ꢂ1) determined by Marque
D below 1.2 is very challenging. Such noteworthy experi-
-
and coworkers42 for several tailor-made alkoxyamines.
mental observations led us to investigate deeper the kinetics
involved in this system.
Indeed, the main parameter generally involved in the CAO
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FIGURE 2 Number average molar mass (Mn) versus monomer
conversion for the bulk polymerization of styrene at 60 ꢀC in
the presence of alkoxyamine 12: (solid lines) theoretical Mn val-
ues; (n, dotted line) [12]0 ¼ 14.2 ꢁ 10ꢂ3 mol Lꢂ1; (h, dashed
line) [12]0 ¼ 8.1 ꢁ 10ꢂ3 mol Lꢂ1; (dotted and dashed lines) PRE-
DICI modelings.
FIGURE 4 Double logarithmic plot of conversion index versus
time for the bulk polymerization of styrene at 60 ꢀC in the pres-
ence of alkoxyamine 12: (n) [12]0 ¼ 14.2 ꢁ 10ꢂ3 mol Lꢂ1; (h)
[12]0 ¼ 8.1 ꢁ 10ꢂ3 mol Lꢂ1; (solid lines) linear fits.
consequence of the significant additional source of transient
radicals issued from the thermal polymerization of styrene,
that is, when monomer conversion is comparable to thermal
polymerization of styrene as generally observed for previ-
ously reported alkoxyamines.44,45
For highly labile alkoxyamines, there is a constant debate
regarding the mechanism that governs the control of the
polymerization (i.e., by the persistent radical effect and then
the plot of conversion index versus time2/3 should be linear
or by a reversible termination process and then the plot of
conversion index versus time should be linear).26 This ques-
tion was assessed by plotting the double logarithmic plot of
conversion index versus time (Fig. 4). The slopes obtained
for both initial alkoxyamine concentrations are extremely
close to 2/3, characteristic of a polymerization controlled by
the persistent radical effect. It is worth mentioning that such
an ideal persistent radical effect has been scarcely experi-
mentally observed26,43 earlier in the case of styrene as a
This particular evolution of monomer conversion in the case
of styrene NMP is correlated to the steady increase of nitro-
xide concentration ([10]), which follows a t1/3 dependence
(Fig. 5) as expected from the persistent radical effect
mechanism.
Equations 1 and 2 describe the theoretical time dependence
of [10] and ln([M]0/[M]) as established by both Fischer and
Fukuda.46,47 However, both equations rely on the kc between
nitroxide 10 and 1-phenylethyl transient radicals as well as
-
FIGURE 3 Evolution of dispersity (D) with monomer conver-
FIGURE 5 Evolution of nitroxide 10 concentration as a function
sion for the bulk polymerization of styrene at 60 ꢀC in the pres-
ence of alkoxyamine 12: (n, dotted line) [12]0 ¼ 14.2 ꢁ 10ꢂ3
mol Lꢂ1; (h, dashed line); [12]0 ¼ 8.1 ꢁ 10ꢂ3 mol Lꢂ1; (dotted
and dashed lines) PREDICI modelings.
of time for the bulk polymerization of styrene at 60 ꢀC in the
presence of alkoxyamine 12: (*) [12]0 ¼ 14.2 ꢁ 10ꢂ3 mol Lꢂ1
;
(h) [12]0 ¼ 8.1 ꢁ 10ꢂ3 mol Lꢂ1; (solid lines) t1/3 fit of the experi-
mental data.
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or allows the control of the polymerization at lower temper-
atures. Nevertheless, if the K value becomes too high, the
nitroxide is not efficient enough to decrease the instantane-
ous macroradical concentration and the contribution of irre-
versible termination reactions improves significantly. That
means that below a threshold value for kc, it is not possible
to obtain a controlled polymerization.
If nitroxides 6 and 10 are compared, their kd are similar
(i.e., 1.5 ꢁ 10ꢂ4 sꢂ1 for 12 and 0.9 ꢁ 10ꢂ4 sꢂ1 for BESNO-
based 1-phenylethyl alkoxyamine) but kc is one order of
magnitude lower for 10 than for 6 leading to the possibility
to decrease the reaction temperature with the same level of
control. Moreover, kc of nitroxide 10 with 1-phenylethyl radi-
cals is two order of magnitude lower than TIPNO and such a
decrease could be detrimental to the control as observed by
Siegenthaler and Studer.30 But concomitantly, the kd value of
12 is one order of magnitude higher than the kd of 1-phenyl-
ethyl-TIPNO alkoxyamine allowing to decrease the polymer-
ization temperature since the decomposition of the (macro)-
alkoxyamine remains significant. Thus, similar values of K
(2.5 ꢁ 10ꢂ9 mol Lꢂ1 at 60 ꢀC for 10 and close to 2.0 ꢁ
10ꢂ9 mol Lꢂ1 at 120 ꢀC for TIPNO,49) are reached but at a
much lower temperature in the case of 10.
FIGURE 6 Determination of the equilibrium constant (K60) for
the bulk polymerization of styrene at 60 ꢀC in the presence of
alkoxyamine 12: (n) [12]0 ¼ 14.2 ꢁ 10ꢂ3 mol Lꢂ1; (h) [12]0
¼
8.1 ꢁ 10ꢂ3 mol Lꢂ1; (solid lines) linear fits.
on the termination rate constant (kt) of styrene. This former
value is not accurately established throughout the polymer-
ization. Therefore, the equilibrium constant (K) could be
determined using eq 3, a combination of eqs 1 and 2 that is
independent of kt:
PREDICI Modelings
To investigate further the potential of nitroxide 10, PREDICI
modelings50 were performed using a previously established
model26,51 and the kinetic rate constants (kd and kc) deter-
mined experimentally in this work. Indeed, PREDICI has pre-
viously described efficiently all CLRP techniques including
NMP,51 ATRP,52 and RAFT polymerization.53 The mechanism
scheme and the kinetic rate constants used to model the
polymerization of styrene mediated by alkoxyamine 12 are
described in the Supporting Information section. As can be
observed on Figures 1–3, the experimental data and the
PREDICI modelings are in rather good agreement for the
two [12]0 studied. Particularly, the evolutions of conversion
versus time and molar mass versus conversion are extremely
well described by the modelings on the whole range of con-
version studied herein.
!
1=3
2
3ktkd2½12ꢃ0
½10ꢃ ¼
t1=3
(1)
(2)
(3)
kc2
ꢀ
ꢁ
1=3
½Mꢃ0 3kp kd½12ꢃ0
ln
¼
t1=3
½Mꢃ
2
3ktkc
½Mꢃ0
½Mꢃ
½10ꢃ ꢁ ln
½12ꢃ0
2
¼ K ꢁ t
3kp
Relation 3 was thus plotted in Figure 6 using the experimen-
tal values of the conversion index, nitroxide concentration
[10], initial alkoxyamine concentration [12]0, and the propa-
gation rate constant (kp) of styrene that has been accurately
established earlier by the IUPAC subcommittee on polymer-
ization kinetics.48
This rather good accordance between modelings and experi-
mental results allowed the use of this model to investigate
the livingness of the polymerization. As the living fraction
(i.e., the ratio between alkoxyamine chains-ends and the total
number of chains) strongly decreases with the increase in
targeted molar mass,51 a wide range of initial alkoxyamine
concentrations ([12]0 ranging from 5.0 ꢁ 10ꢂ2 to 3.0 ꢁ
For each [12]0, the equilibrium constant (K60) was then
determined from the slopes of the resulting straight lines
(Table 1). The high average value of the equilibrium constant
(K60 ¼ 2.6 ꢁ 10ꢂ9 mol Lꢂ1) thus results from a high value
of k6d0 (determined by ESR spectroscopy) and a particularly
low value of k6c0 (k6c0 ¼ k6d0/K60 ¼ 5.7 ꢁ 104 L molꢂ1
s
ꢂ1).
TABLE 1 Kinetic Parameters for the Bulk Polymerization of
Styrene at 60 8C Mediated by Alkoxyamine 12
This result is thus very interesting and in rather good agree-
ment with the work of Siegenthaler and Studer30 that al-
ready investigated the influence of the kc value on the out-
come of a NMP process. They concluded that the lower the
kc, the lower the temperature of polymerization at a given kd
value, but this is only true above a threshold value, after
which a loss of control is obtained. Indeed for a given kd, a
lower kc increases K, shifts the equilibrium toward active
species and thus either increases the polymerization kinetics
[12]0 (mol Lꢂ1
)
k6d0 (sꢂ1 a
)
K60 (mol Lꢂ1 b
)
k6c0 (L molꢂ1 sꢂ1 c
)
8.1 ꢁ 10ꢂ3
14.2 ꢁ 10ꢂ3
a
1.5 ꢁ 10ꢂ4 2.5 ꢁ 10ꢂ9
1.5 ꢁ 10ꢂ4 2.8 ꢁ 10ꢂ9
6.0 ꢁ 104
5.4 ꢁ 104
Determined by ESR spectroscopy.
Determined from relation 3.
b
c
k6c0 ¼ k6d0/K60
.
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hydride and stored under argon. N-tert-butyl-a-tert-butyl
nitrone 8 was synthesized as described previously.33 All
other materials were used as received.
Characterization
1H NMR (200 MHz) measurements were performed on a
Bruker AC 200 spectrometer at room temperature. 1H NMR
of hydroxylamine 9 was carried out in a 1:1 D2O/C3D6O mix-
ture after in situ reduction of nitroxide 10 by sodium
dithionite. ESR spectra were recorded on a Bruker ESP-300
X-band spectrometer equipped with a HP 53150A frequency
meter and a Boonton microwatt meter. Nitroxide concentra-
tions were determined by integration of the ESR spectra and
calibration with a known TEMPO solution in benzene. Size
exclusion chromatography (SEC) was carried out at room
temperature on a Shimadzu apparatus equipped with a re-
fractometer (Shimadzu) and five columns PL GEL (10 lm
particles) (three mixted B, 103 Å, 105 Å), using THF as eluent
(flow rate: 1 mL minꢂ1). Calibration curves obtained from
FIGURE 7 PREDICI modeling of the living fraction versus mono-
mer conversion for the bulk polymerization of styrene at 60 ꢀC
at different [12]0: (solid line) [12]0 ¼ 5.0 ꢁ 10ꢂ2 mol Lꢂ1; (dotted
line) [12]0 ¼ 8.1 ꢁ 10ꢂ3 mol Lꢂ1; (dashed line) [12]0 ¼ 14.2 ꢁ
10ꢂ3 mol Lꢂ1; (dashed-dotted line) [12]0 ¼ 3.0 ꢁ 10ꢂ3 mol Lꢂ1
.
10ꢂ3 mol Lꢂ1) and, therefore, Mn,th ranging from 20,000 to
330,000 g molꢂ1 were studied (Fig. 7). It appears that the
living fraction for the whole range of alkoxyamine concentra-
tion studied is between 90 and 50% at 50% of monomer
conversion thus suggesting the preparation of high molar
mass polystyrenes with good chain-end livingness. Moreover,
the experimental molar masses obtained in this work are
rather high compared to those generally targeted in the liter-
ature. These results at 60 ꢀC are comparable to those
25
ꢀ
obtained at 90 C with SG1 nitroxide 1.
The influence of temperature on monomer conversion and
living fraction was also investigated for temperatures ranging
ꢀ
from 40 to 80 C (Fig. 8 and Figure S5 in the Supporting In-
formation). As expected, the temperature has a strong influ-
ence on the outcome of the polymerization. Indeeꢀd, whereas
600 h are needed to reach 50% conversion at 40 C, only 60
h are necessary at 60 ꢀC and 8 h at 80 ꢀC. This slowing
down of the kinetics with decreasing temperature is related
to a lower concentration of macroradicals in the medium
-
and thus led to a better livingness and a lower D at high
conversion. Conversely, an increase in temperature acceler-
ates the kinetics, does not change significantly the control of
the polymerization but is detrimental to the livingness of the
resulting polymer. Indeed, as previously described,26 an
increase in temperature results in a decrease in livingness
(e.g., at 50% conversion, 90 and 45% of living chains are
obtained at 40 and 80 ꢀC, respectively). Thus, a good com-
promise between the rate and the liꢀvingness of the polymer-
ization process was obtained at 60 C for the bulk polymer-
ization of styrene mediated by alkoxyamine 12.
FIGURE 8 PREDICI modeling of the influence of the polymer-
ization temperature on the bulk polymerization of styrene in
the presence of alkoxyamine 12 ([12]0 ¼ 14.2 ꢁ 10ꢂ3 mol Lꢂ1):
(a) conversion versus time; (b) living fraction versus conver-
sion; (short dotted lines) 40 ꢀC; (dotted lines) 50 ꢀC; (solid lines)
60 ꢀC; (dashed lines) 70 ꢀC; (dashed-dotted lines) 80 ꢀC.
EXPERIMENTAL
Materials
Benzene (Aldrich, 99%), ethylbenzene (Aldrich, 99%), and
styrene (Aldrich, 99%) were distilled twice from calcium
6
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polystyrene standards were used to calculate the number av-
Determination of the Rate Constant of Dissociation (kd)
by ESR Spectroscopy
-
erage molar masses (Mn) and dispersities (D).
A solution of alkoxyamine 12 (0.2 ꢁ 10ꢂ3 mol Lꢂ1) and gal-
vinoxyl (1 ꢁ 10ꢂ3 mol Lꢂ1) in benzene was degassed by
freeze-pump-thaw cycles, placed under argon and heated at
temperatures between 60 and 100 ꢀC. The release of the
persistent radical during alkoxyamine dissociation was moni-
tored by ESR spectroscopy. Nitroxide concentrations were
determined by integration of the first peak of nitroxide 10
and calibration with a known 2,2,6,6-Tetramethylpiperidine
1-oxyl (TEMPO) solution in benzene.
Preparation of sec-Pentyllithium 7
A dispersion of 2-chloropentane (12 mL, 98 mmol) and lith-
ium pieces (2.97 g, 429 mmol) in anhydrous benzene (100
mL) was stirred vigorously under argon during 72 h. After
filtration under argon, the crude sec-pentyllithium 7 solution
was titrated from acetanilide ([7] ¼ 0.56 M).
Synthesis of N-tert-Butyl-N-(1-tert-butyl-2-methyl)pentyl
Hydroxylamine 9
A solution of sec-pentyllithium, 7 (0.56 M, 27 mL, 15 mmol)
in benzene was added dropwise, to a solution of N-tert-
butyl-a-tert-butyl nitrone 8 (1.6 g, 10 mmol) in neat benzene
(45 mL) at room temperature and under an argon atmos-
phere. The mixture was allowed to stir at room temperature
during 6 h and was then quenched with a saturated solution
of ammonium chloride (15 mL). The aqueous phase was
extracted twice with methylene chloride (2 ꢁ 45 mL). The
organic phase was then dried over MgSO4, filtered, and
evaporated under vacuum to give the crude hydroxylamine 9
(1.6 g, 70%) as a white solid.
Typical Polymerization Experiment
A solution of alkoxyamine 12 (50 mg, 0.15 mmol) in neat sty-
rene (19.3 g, 185 mmol) was distributed among several glass
tubes. The contents were degassed by freeze-pump-thaw
cycles and the tubꢀes were sealed off under vacuum. One tube
was heated at 60 C during 99 h, cooled down, and the poly-
merization medium was dissolved in THF. After evaporation of
excess monomer and solvent, the polymer was freeze-dried in
benzene. Conversion was evaluated gravimetrically, number
average molar mass and dispersity were determined by SEC
(yield ¼ 61%, Mn ¼ 68,200 g molꢂ1, D ¼ 1.14).
-
1H NMR (200 MHz, D2O/C3D6O, d, ppm): 0.91 (m,
(CH3)3CCH, CH3CH2CH2, 12H), 1.11 (s, (CH3)3CN, 9H), 1.16–
1.47 (m, CH3CH, CH3CH2CH2, 7H), 1.82 (m, CH3CH, 1H), 2.38
(m, (CH3)3CCH, 1H).
PREDICI Simulations
Modelings have been carried out with PREDICI (CIT), version
6.34 on a 2.53 GHz Intel Core i3 Macintosh computer using
the CrossOver software. For all simulations, a PREDICI model
comprising all the reaction steps and respective rate con-
stants has been used. All the data are available in the Sup-
porting Information section.
Synthesis of N-tert-Butyl-N-(1-tert-butyl-2-methyl)pentyl
Nitroxide 10
A solution of the crude hydroxylamine 9 (0.5 g, 2.2 mmol)
and Cu(OAc)2.H2O (20 mg, 0.1 mmol) in methanol (15 mL)
was stirred for 2 h under air bubbling. Methanol was then
removed under vacuum and the crude mixture was purified
by column chromatography eluting with cyclohexane to give
nitroxide 10 (470 mg, 95%) as a red liquid. ESR (benzene):
triplet, aN ¼ 13.6 G, g ¼ 2.0061. Anal. calcd for C14H30NO: C,
73.62; H, 13.24; N, 6.13. Found: C, 73.81; H, 13.36; N, 6.08.
CONCLUSIONS
The peculiar structural and kinetic parameters of conveniently
prepared nitroxide 10 and alkoxyamine 12 afforded an excel-
lent control of the polymerization of styrene at 60 ꢀC. The
unprecendented observation of a true persistent radical effect
for styrene with a rate of polymerization one order of magni-
tude higher than thermal polymerization was highlighted and
accurately modeled using PREDICI. The results highlighted
herein make nitroxide 10 a particularly promising candidate
to investigate the control of other monomers already con-
trolled by NMP (e.g., acrylates, acrylamides, isoprene) but also
more challenging ones such as less activated monomers (e.g.,
vinyl esters, N-vinyl carbazole, …) as well as the formation of
more sophisticated macromolecular architectures. Such stud-
ies are currently under investigation.
Synthesis of 1-Phenyl-1-(N-tert-butyl-N-(1-tert-butyl-2-
methyl)pentyl nitroxide) Ethane 12
A degassed solution of nitroxide 10 (1.1 g, 5.0 mmol) and
di-tert-butyl peroxalate (1.2 g, 5.1 mmol) [Caution! Highly
energetic compound and explosive when crystals are
scratched!] in ethylbenzene (15 mL, 123 mmol) was stirred
ꢀ
at 35 C under an argon atmosphere during 4 h. Excess eth-
ylbenzene was then removed under vacuum at room temper-
ature and the residue was purified by column chromatogra-
phy eluting with cyclohexane. After evaporation of the
solvent, pure alkoxyamine 12 (890 mg, 52%) was recovered
as a pale yellow oil.
ACKNOWLEDGMENTS
E. Drockenmuller acknowledges the financial support from the
Institut Universitaire de France (IUF).
1H NMR (200 MHz, CDCl3, d, ppm): d 1.07 (s, (CH3)3CCH,
9H),
1.20
(2s,
(CH3)3CN,
9H),
0.45–1.38
(m,
REFERENCES AND NOTES
CH3CH2CH2(CH3)CH, 10H), 1.47–1.64 (m, CH3CHO, 3H), 2.41
(m, CH3CH2CH2(CH3)CH, 1H), 2.68–2.75 (2s, (CH3)3CCH, 1H),
5.00 (dq, CH3CHO, 1H), 7.13–7.46 (m, aromatics, 5H). Anal.
calcd for C22H39NO: C, 79.22; H, 11.78; N, 4.24. Found: C,
79.07; H, 11.97; N, 4.14.
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€
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8
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