A styryl TIPNO-based nitroxide as efficient mediator for the synthesis of multiblock polystyrenes and well-grafted polymers

Article abstract of DOI:10.1002/pola.26209

The chloromagnesium exchange of 4-chlorostyrene provides an easy access to a new versatile polymerizable 2,2,5-trimethyl-4-phenyl-3-azahexane-3-nitroxide (TIPNO)-based nitroxide. Indeed, first, its alkoxyamine based on the α-methyl benzyl radical fragment efficiently mediates the polymerization of styrene (respectively n-butyl acrylate) to yield branched polystyrene [respectively poly(n-butyl acrylate)] with alkoxyamine function as branch point and well-defined branches. Second, the self-condensing of this polymerizable nitroxide by manganese coupling affords a mixture of oligomeric linear polyalkoxyamines. Polymerization of styrene mediated with these polyalkoxyamines gives multiblock polystyrenes with alkoxyamine group as linker between polystyrene blocks and exhibits the following features: the synthesis of the polystyrene blocks is controlled as their average molecular weight M _n(block) increases linearly with conversion and their average dispersity M_w/M_n(block) decreases with it. At a given temperature, the molecular weight and the dispersity of the polyalkoxyamines weakly impact M_n(block) and M_w/M_n(block). In contrast, the molecular weight of the multiblock polystyrene increases linearly with conversion until reaching a constant value. The number of block is independent of the molecular weight of the polyalkoxyamines. These unusual results can be explained by the fact that during polymerization, mediating TIPNO-based polymeric nitroxides with different lengths are generated and are exchanged. Finally the dispersity of the multiblock polystyrene is quite broad and lies between 1.7 and 2.8.

Full text of DOI:10.1002/pola.26209

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A Styryl TIPNO-Based Nitroxide as Efficient Mediator for the Synthesis of
Multiblock Polystyrenes and Well-Grafted Polymers
Pauline Souchal,1,2,3 Thomas Roels,1,2,3 Patrick Hemery, Lucien Marx1,2,4
1,2
ꢀ
1CNRS, UMR 7610 and UMR 7203, Paris, France
2
ꢀ
ꢀ
UPMC Universite Paris 6, Laboratoire de Chimie des Polyme`res et Laboratoire des Biomolecules, 4 place Jussieu,
75252 Paris Cedex 05, France
3
ꢀ
Lycee Pierre-Gilles de Gennes-ENCPB, 11 rue Pirandello, 75013 Paris, France
4
ꢀ
ꢀ
ꢀ
Ecole Normale Superieure, Departement de Chimie, Laboratoire des Biomolecules, 24 rue Lhomond, 75231 Paris Cedex 05,
France
Correspondence to: L. Marx (E-mail: lucien.marx@ens.fr)
Received 2 March 2012; accepted 13 April 2012; published online
DOI: 10.1002/pola.26209
ABSTRACT: The chloromagnesium exchange of 4-chlorostyrene
provides an easy access to a new versatile polymerizable 2,2,5-tri-
methyl-4-phenyl-3-azahexane-3-nitroxide (TIPNO)-based nitroxide.
Indeed, first, its alkoxyamine based on the a-methyl benzyl radical
fragment efficiently mediates the polymerization of styrene
(respectively n-butyl acrylate) to yield branched polystyrene
[respectively poly(n-butyl acrylate)] with alkoxyamine function as
branch point and well-defined branches. Second, the self-con-
densing of this polymerizable nitroxide by manganese coupling
affords a mixture of oligomeric linear polyalkoxyamines. Polymer-
ization of styrene mediated with these polyalkoxyamines gives
multiblock polystyrenes with alkoxyamine group as linker
between polystyrene blocks and exhibits the following features:
the synthesis of the polystyrene blocks is controlled as their aver-
age molecular weight Mn(block) increases linearly with conversion
and their average dispersity Mw/Mn(block) decreases with it. At a
given temperature, the molecular weight and the dispersity of the
polyalkoxyamines weakly impact Mn(block) and Mw/Mn(block). In
contrast, the molecular weight of the multiblock polystyrene
increases linearly with conversion until reaching a constant value.
The number of block is independent of the molecular weight of
the polyalkoxyamines. These unusual results can be explained by
the fact that during polymerization, mediating TIPNO-based poly-
meric nitroxides with different lengths are generated and are
exchanged. Finally the dispersity of the multiblock polystyrene is
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quite broad and lies between 1.7 and 2.8.
2012 Wiley Periodi-
cals, Inc. J Polym Sci Part A: Polym Chem 000: 000–000, 2012
KEYWORDS: alkoxyamines; block copolymers; branched;
branched polymer; graft copolymers; inimer; living radical po-
lymerization (LRP); multiblock polystyrene; nitroxide-mediated
radical polymerization; polystyrene; radical polymerization
INTRODUCTION Controlled radical techniques (CRTs) such as
nitroxide-mediated radical polymerization (NMRP),1,2 radical
addition fragmentation transfer (RAFT)3 polymerization,
atom transfer radical polymerization (ATRP),4 and single
electron transfer living radical polymerization (SET-LRP)5 are
versatile tools in macromolecular synthesis. Indeed, because
of their living character, they allow to prepare well-defined6,7
macromolecular architectures with complex structure. Espe-
cially, NMRP, RAFT, ATRP, and SET-LRP processes were used
for the preparation of architectures whose synthesis is diffi-
cult or impossible by conventional radical polymerization.
Multiblock7 copolymers and highly branched8 polymers are
typical examples of such architectures.
when compared with linear polymers and the presence of
functional groups at their periphery as well as their potential
applications in surface modification,13 coatings,14 and drug-
delivery systems.15 As a result, tremendous efforts have
been devoted to their controlled16,17 synthesis. Indeed,
ATRP,18–21 RAFT,22,23 and NMRP24,25 have been successfully
applied to the preparation of these branched and tree-like
macromolecules. All these synthetic approaches are based on
the use of inimers,26 molecules that contain a vinyl monomer
and an initiator (halogenated alkane for ATRP or alkoxy-
amine for NMRP) or controller (dithioester group for RAFT)
site. The self-condensation vinyl polymerization of inimers
affords hyperbranched polymers.
Highly branched9,10 and hyperbranched11 polymers are the
subject of increasing attention because of their unique12
properties in solution (low viscosity) and in the solid state
As branched polymers, multiblock copolymers have also
attracted attention because of their self-assembly morpholo-
gies in solution27 and in the solid state28 and their
Additional Supporting Information may be found in the online version of this article.
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promising applications as compatibilizers,29 materials with
improved mechanically properties,30 and proton31,32
exchange fuel cells. Until the discovery of CRT, most of the
multiblock copolymers were synthesized by condensation7 of
telechelic building blocks. As a radical process tolerates
many functional groups, new well-defined telechelic building
blocks with various structures7,27,33 are now available by
capitalizing on CRT, which widens the range of accessible
multiblock copolymers. A second approach is to prepare step
by step the multiblock using sequential ATRP34 or RAFT35 or
more interestingly to build it in one pot using SET-LRP,36
which by keeping its livingness until very high conversion
makes the purification after the growing of each block
nonessential. Finally, in third general approach, RAFT37–41
agents or alkoxyamines,42–44 initiator/controllers in NMRP,
are incorporated in a linear polymer chain, which after
monomer insertion affords multiblock polymers.
ics. Methanol (MeOH), isopropanol, dichloromethane
(CH2Cl2), and toluene (>99%) were purchased from VWR.
Tetrahydrofuran (THF) was distilled under N2 from sodium
benzophenone. Silica gel for column chromatography was
Merck Kieselgel 60. Column chromatographic separations
were carried out using Merck silica gel 60 (230–400 mesh)
or alumina when it is indicated.
Size exclusion chromatography (SEC) was performed at 40ꢁC
with two columns (PSS SDV, linear MU, 8 mm ꢂ 300 mm;
bead diameter, 5 mm; separation limits, 400 to 2 ꢂ 106
g molꢀ1). The eluent was THF at a flow rate of 1 mL minꢀ1
.
A
differential refractive index detector (LDC Analytical
refracto-Monitor IV) was used, and molar mass distributions
were derived from a calibration curve based on polystyrene
standards from Polymer Standards Service. The monomer
conversion for styrene and nBA was determined by gravime-
try after drying the polymer samples under vacuum for 48 h.
In this report, a polymerizable nitroxide was easily synthe-
sized in few steps and good overall yield and was essential
in the synthesis of branched and multiblock polymers.
Indeed, its alkoxyamine based on 1-phenyl-ethyl radical has
appeared as an efficient initiator/controller in NMRP of sty-
rene and n-butyl acrylate (nBA) as well-defined branched
polystyrenes and poly(n-butyl acrylates) were obtained. In
both cases, the molecular weight and the dispersity of their
linear backbones and their branches were nearly identical.
Finally, this nitroxide underwent its self-condensation by
manganese coupling to afford linear polyalkoxyamines, which
on heating have inserted styrene to give multiblock polystyr-
enes with broad dispersity and whose molecular weight first
increases linearly with conversion and finally becomes con-
stant. In contrast, the synthesis of the polystyrene blocks
was controlled in terms of molecular weight and dispersity.
N-tert-Butyl-a-iso-propylnitrone45 and the salen–manganese
complex46 were synthesized according to literature procedures.
Synthesis of 2,2,5-Trimethyl-4-(4-vinylphenyl)-
3-azahexane-3-nitroxide (2)
Under nitrogen atmosphere, 4-chlorostyrene (2.6 mL, 18.8
mmol) was diluted in THF (25 mL) and added dropwise for
20 min to a solution of magnesium (750 mg, 22.6 mmol)
and of dibromo-1,2 ethane (0.1 mL, 1.15 mmol) in THF (4
mL) in order to have a smooth reflux. After the addition was
completed, the solution was further heated at 65ꢁC for 195
min. A solution of N-tert-butyl-a-iso-propylnitrone (2 g, 14
mmol) in THF (20 mL) was added for 10 min to the solution
of styryl magnesium chloride at 0ꢁC. When the addition is
finished, the mixture was allowed to warm to room tempera-
ture and further stirred during one night. The excess Gri-
gnard reagent was decomposed by careful addition of an
aqueous ammonium chloride solution (7 g of NH4Cl in 50
mL of water). Then 100 mL of CH2Cl2 was added. The or-
ganic layer was separated, and then the aqueous layer was
extracted with CH2Cl2 (200 mL). The organic layers were
combined, dried over magnesium sulfate, filtered, and con-
centrated. The residue obtained was then treated with a mix-
ture of MeOH (100 mL), ammonia solution (20% solution in
water, 3 mL), and Cu(OAc)2 (130 mg, 0.71 mmol) to give a
pale yellow solution. A stream of air was bubbled for 2 h.
Then the mixture was concentrated, and the crude nitroxide
was purified by flash column chromatography eluting with
pure pentane gradually increasing to 5:95 ethyl acetate/pen-
tane to afford 2.5 g (72% yield) of 2, as an orange oil that
crystallized at temperature below ꢀ25ꢁC.
EXPERIMENTAL
General Considerations
1H NMR and 13C NMR spectra were recorded at room tem-
perature on Bruker AC 200 MHz, Bruker AC 200 MHz, or
ARX 250 MHz instruments. Proton and carbon chemical
shifts are reported using the resonance of the deuterated
solvent as internal standard. Elemental analysis was per-
formed by the Service Central d’Analyses of the CNRS. Chemi-
cal ionization (CI, ammonia or methane) and electronic ioni-
zation mass spectra were obtained with
spectrometer.
a JMS-700
All reagents and chemicals were obtained from commercial
suppliers without further purification excepted when indi-
cated. nBA (99%; Aldrich) and styrene (S, 99%; Aldrich)
monomers were distilled under reduced pressure before use
and stored at ꢀ4ꢁC. 4-Chlorostyrene (technical, 90%) was
purchased from Aldrich and used without further purifica-
tion. Magnesium turnings (99%), sodium borohydride
(>98%), copper powder (99%), copper(I) bromide (98%),
MS (CI, CH4): m/z (%) 246 (M, 100) 190 (M-56, 86); HRMS
(CI, CH4) Calcd for C16H24NO (Mþ): 246.1858; found:
246.1860. Anal. Calcd for C16H24NO: C, 78.04; H, 9.75; N,
5.69. Found: C, 77.51; H, 9.50; N, 5.21.
N,N,N0,N00,N00-Pentamethyldiethylenetriamine
(PMDETA)
Synthesis of 2,2,5-Trimethyl-3-(10-(phenyl)-ethoxy)-
4-p-vinylphenyl-3-azahexane (3)
To a Schlenk flask, (1-bromoethyl)-benzene (3.66 g, 19.7
(99%), (1-bromoethyl)-benzene, 2-methyl-5-tert-butylthio-
phenol (93%), chlorobenzene (99%), and ammonia solution
(20% solution in water) were all obtained from Acros Organ-
mmol),
the
functionalized
2,2,5-trimethyl-4-phenyl-3-
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azahexane-3-nitroxide (TIPNO) 2 (4.7 g, 19.1 mmol), and 32
mL of toluene were added. To a second Schlenk flask, copper
powder (1.929 g, 30.3 mmol), copper(I) bromide (301.6 mg,
2.1 mmol), 32 mL of toluene, and PMDETA (0.8 mL, 3.8
mmol) were added. The two reaction solutions were deoxy-
genated by bubbling nitrogen for 15 min. The nitroxide solu-
tion was then added to the solution of copper derivatives
under nitrogen atmosphere, and the resulting solution was
further stirred at room temperature under inert atmosphere.
After one night, the reaction mixture was evaporated to dry-
ness. Dried silica gel soaked with the crude product was pre-
pared by dissolving and evaporating to dryness a mixture of
the crude product with silica gel in dichloromethane. The
crude product was purified by column chromatography, elut-
ing with pure pentane gradually increasing to 5:95 ethyl ace-
tate/pentane. The combined organic layers were then dried
and evaporated to dryness, and the pure alkoxyamine 3 was
obtained as a clear oil (1.72 g, 25.6% yield).
g molꢀ1, Mw/Mn ¼ 1.25), 9c (eluent: 90:10 petroleum ether/
ethyl acetate, m ¼ 139 mg, Mn ¼ 1230 g molꢀ1, Mw/Mn
¼
3.65), 9d (eluent: 70:30 until 50:50 petroleum ether/ethyl
acetate, m ¼ 204 mg, Mn ¼ 4750 g molꢀ1, Mw/Mn ¼ 1.15),
9e (eluent: 25:75 until 0:100 petroleum ether/ethyl acetate,
m ¼ 226 mg, Mn ¼ 3200 g molꢀ1, Mw/Mn ¼ 1.2), and 9f
(eluent: 100:0 ethyl acetate/MeOH, m ¼ 366 mg, Mn ¼ 1900
g molꢀ1, Mw/Mn ¼ 2.55), were separated to give oligomeric
polyalkoxyamines.
Analysis of 9d
1H NMR (300 MHz, CDCl3, d): 7.75–7 (m, H aromatic), 4.83
(s, 2H, CHAO), 3.33 (s, 1H, CHAN), 3.22 (s, 1H, CHAN), 2.25
(s, 2H, CHAiPr), 1.55 (s, 3H, CH3ACHAO), 1.45 (d, 3H,
CH3ACHAO), 1.24 (s, 3H, CH3AiPr), 0.98 (s, 9H, tBu), 0.63
(s, 3H, CH3AiPr), 0.44 (s, 9H, tBu), 0.25 (s, 3H, CH3AiPr),
and 0.0 (s, 3H, CH3AiPr). Anal. Calcd for C16H25NO: C, 77.73;
H, 10.12; N, 5.66. Found: C, 76.20; H, 10.04; N, 5.61.
1H NMR (300 MHz, CDCl3, two diastereoisomers, d): 7.38–
7.14 (m, 18H), 6.65 (dd, 1H, J ¼ 17.3 Hz and 10.9 Hz,
ACH¼¼CH2), 6.60 (dd, 1H, J ¼ 17.3 Hz and 10.9 Hz,
ACH¼¼CH2), 5.68 (d, 1H, J ¼ 17.3 Hz and 0.8 Hz, 1H,
ACH¼¼CH2 trans), 5.62 (d, 1H, J ¼ 17.3 Hz and 0.8 Hz, 1H,
ACH¼¼CH2 trans), 5.13 (d, 1H, J ¼ 10.9 Hz and 0.8 Hz, 1H,
ACH¼¼CH2 cis), 5.10 (d, 1H, J ¼ 10.9 Hz and 0.8 Hz, 1H,
ACH¼¼CH2 cis), 4.83 (q þ q, 2H, CHAO), 3.34 (d, 1H, J ¼
10.6 Hz, CHAN), 3.22 (d, 1H, J ¼ 10.8 Hz, CHAN), 2.25 (two
m, 2H, CHAiPr), 1.56 (d, 3H, J ¼ 6.6 Hz, CH3ACHAO), 1.47
(d, 3H, J ¼ 6.6 Hz, CH3ACHAO), 1.23 (d, 3H, J ¼ 6.4 Hz,
CH3AiPr), 0.98 (s, 9H, tBu), 0.84 (d, 3H, J ¼ 6.3 Hz,
CH3AiPr), 0.71 (s, 9H, tBu), 0.48 (d, 3H, J ¼ 6.6 Hz,
CH3AiPr), and 0.15 (d, 3H, J ¼ 6.6 Hz, CH3AiPr). 13C NMR
(75 MHz, CDCl3, d): 145.91, 145.13, 142.63, 142.43 (aro-
matic), 137.10 (ACH¼¼CH2), 135.75, 135.59, 131.29, 131.24,
128.27, 128.26, 127.20, 127.09, 126.85, 126.36, 126.36,
125.50, 125.32 (aromatic), 113.08, 112.94 (ACH¼¼CH2),
83.69, 83.03 (CHAO), 72.08, 71.99 (CHAN), 60.74, 60.61
(tBuAN), 32.19, 31.79 (CHAiPr), 28.66, 28.47 (tBu), 24.91,
23.34, 22.33, 22.16, 21.35, 21.24 ppm (CH3). Anal. Calcd for
General Procedure for Styrene Polymerization:
Preparation of Branched Polystyrene
A mixture of alkoxyamine 3 (191.6 mg, 0.545 mmol) and sty-
rene (17.78 g, 171 mmol) was degassed by three freeze/
thaw cycles, sealed under nitrogen, and heated at 120ꢁC for
7 h. Samples were removed at different time intervals during
polymerization, and conversion and molecular weights were
determined by gravimetry and SEC, respectively. The poly-
merization was stopped (Mn ¼ 39,800 g molꢀ1, Mw/Mn
¼
1.9, conversion ¼ 63.7%) by quenching the reaction in an
ice bath. Polystyrene was precipitated with MeOH.
General Procedure for Cleavage of the CAON Bond
of the Branched Polystyrene
The procedure has been adjusted until after post-treatment
the molecular weight of the polymer is constant. A typical
procedure is given as follows: a mixture of polymer sample
(50 mg), 2-methyl-5-tert-butylthiophenol (20 lL, 0.132
mmol), and chlorobenzene (1 mL) was degassed by three
freeze/thaw cycles, sealed under nitrogen, and heated at
125ꢁC for 7 h. The solution was evaporated to dryness under
reduced pressure, and the crude polystyrene was directly an-
alyzed by SEC.
C24H33NO: C, 82.05; H, 9.40; N, 3.98. Found: C, 81.49; H,
9.56; N, 3.63. Found: C, 81.34; H, 9.58.
General Procedure for Acrylate Polymerization:
Preparation of Branched Poly(n-butyl acrylate)
Synthesis of Linear Polyalkoxyamines
The styryl TIPNO-based nitroxide 2 (1.82 g, 12.2 mmol) was
dissolved in 8 mL of isopropyl alcohol in an open flask. The
solution was vigorously stirred, and finely ground Mn(sa-
len)Cl catalyst (724 mg, 2 mmol) was added, followed by
NaBH4 (737 mg, 19.5 mmol) in small portions. After 24 h,
the reaction mixture was evaporated to dryness. Dried silica
gel soaked with the crude product was prepared by dissolv-
ing and evaporating to dryness a mixture of the crude prod-
uct with silica gel in dichloromethane. The crude product
was purified by column chromatography, eluting with pure
petroleum ether gradually increasing first to pure ethyl ace-
tate and second to 90:10 ethyl acetate/MeOH. Six fractions,
9a (eluent: 100:0 petroleum ether/ethyl acetate, m ¼ 658
mg, Mn ¼ 1300 g molꢀ1, Mw/Mn ¼ 2.2), 9b (eluent: 100:00
petroleum ether/ethyl acetate, m ¼ 119 mg, Mn ¼ 2200
A mixture of the alkoxyamine 3 (203 mg, 0.578 mmol), the
corresponding nitroxide 2 (7.1 mg, 0.028 mmol), and nBA
(19.38 g, 151.4 mmol) was degassed by three freeze/thaw
cycles, sealed under nitrogen, and heated at 125ꢁC for 24 h.
Samples were removed at different time intervals during po-
lymerization, and conversion and molecular weights were
determined by gravimetry and SEC, respectively. The poly-
merization was stopped (Mn ¼ 54,000 g molꢀ1, Mw/Mn
¼
3.5, conversion ¼ 60%) by immersing the reaction flask in
an ice bath.
General Procedure for Cleavage of the CAON Bond
of the Branched Poly(n-butyl acrylate)
The procedure has been adjusted until after post-treatment
the molecular weight of the polymer is constant. A typical
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removed at different time intervals during polymerization,
and conversion and molecular weights were determined by
gravimetry and SEC, respectively. The polymerization was
stopped (Mn ¼ 110,000 g molꢀ1, Mw/Mn ¼ 2.11, conversion
¼ 51.7%) by quenching the reaction in an ice bath. Polysty-
rene was precipitated with MeOH.
General Procedure for Cleavage of the CAON Bond
of the Multiblock Polystyrene
The procedure has been adjusted until after post-treatment
the molecular weight of the polymer is constant. A typical
procedure is given as follows: a mixture of the polymer sam-
ple (40 mg), 2-methyl-5-tert-butylthiophenol (20 lL, 0.132
mmol), and chlorobenzene (2 mL) was degassed by three
freeze/thaw cycles, sealed under nitrogen, and heated at
125ꢁC for 3 h. The solution was evaporated to dryness under
reduced pressure, and the crude polystyrene was directly an-
alyzed by SEC.
DISCUSSION
FIGURE 1 Examples of polymerizable nitroxides and alkoxy-
amine inimers to synthesize hyperbranched architectures.
Synthesis of Branched Polymers
Synthesis of the Inimer
Several nitroxides47–50 and alkoxyamine inimers24,51–58 (Fig.
1) were dedicated to the synthesis of branched polymers by
NMRP. Most of them are based on 2,2,6,6-tetramethylpiperi-
dine-N-oxyl (Tempo) nitroxide. Recently, we have described
the inimer 125 (Fig. 1) containing the TIPNO and the styryl
radical fragment. Inimer 1 has efficiently controlled the syn-
thesis of branched polystyrene and branched poly (n-butyl
acrylate). Here, we report the synthesis of the styryl TIPNO-
based nitroxide 2 (Scheme 1) and its corresponding inimer
3 (Scheme 2) incorporating the 1-phenylethyl moiety. Its
synthesis is straightforward as reaction of styryl magnesium
chloride with N-tert-butyl-a-iso-propylnitrone followed by a
copper-catalyzed oxidation reaction by air gives 2 in overall
good yield of 72%. The inimer 3 was obtained in 25.6%
procedure is given as follows: a mixture of the polymer sam-
ple (50 mg), 2-methyl-5-tert-butylthiophenol (20 lL, 0.132
mmol), and chlorobenzene (1 mL) was degassed by three
freeze/thaw cycles, sealed under nitrogen, and heated at
125ꢁC for 7 h. The solution was evaporated to dryness under
reduced pressure, and the crude poly(n-butyl acrylate) was
directly analyzed by SEC.
General Procedure for Styrene Polymerization:
Preparation of Multiblock Polystyrene
A mixture of the alkoxyamine 9a (100 mg, n ¼ 0.405 mmol
alkoxyamine equivalents) and styrene (12.57 g, 121 mmol)
was degassed by three freeze/thaw cycles, sealed under
nitrogen, and heated at 115ꢁC for 6 h. Samples were
SCHEME 1 Synthesis of the polymerizable TIPNO-based nitroxide 2.
SCHEME 2 Synthesis of the alkoxyamine inimer 3.
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yield by nitroxide radical coupling59 reaction between 1-phe-
nylethyl bromide and 2.
Synthesis of Branched Polystyrene
We have observed that the inimer 125 efficiently initiates
and controls the bulk polymerization of styrene to yield
branched polystyrene. Furthermore, the branching was start-
ing at a conversion of around 30%.
To compare inimer 3 with inimer 1, the bulk polymerization
of styrene initiated by the inimer 3 was investigated at
120ꢁC under bulk conditions, using a ratio of 313 between
styrene and 3.
A linear relationship between ln([M]0/[M]) versus time was
observed [Fig. 2(A)] from the beginning until the end of the
experiment of polymerization, indicating that no detectable
termination occurred in this system. Moreover, the molecular
weight [Fig. 2(B)] and the dispersity [Fig. 2(C)] increase
with conversion. The polystyrene (Mn ¼ 39,800 g molꢀ1
;
Mw/Mn ¼ 1.9) was obtained after 7 h with a conversion of
63.7%. Its SEC traces (Fig. 3) have indicated a multimodal
molecular weight distribution that could be explained by the
presence of two polystyrene architectures at least, one is a
linear polystyrene macroinimer and the second is a branched
polystyrene where each branch point is an alkoxyamine func-
tion. To check this hypothesis, the polystyrene samples were
treated with 2-methyl-5-tert-butylthiophenol at 125ꢁC in
chlorobenzene during 7 h in order to cleave alkoxyamine
functions and the post-treating crude polystyrenes were ana-
lyzed by SEC. Indeed, under these conditions, the CAON
bond of any alkoxyamine function undergoes an irreversi-
ble60 homolytic cleavage as its releasing radical fragment
very quickly removes61 hydrogen atom from thiol.
After the treatment with thiol, the molecular weight Mn (Fig.
4) and the dispersity (Fig. 4) of the polystyrene architecture
decrease. For example, the polystyrene sample obtained after
7 h of polymerization has given a linear monodisperse poly-
styrene (Mn ¼ 20,400 g molꢀ1; Mw/Mn ¼ 1.11). Moreover,
Mn increases linearly with conversion [Fig. 2(B)], and the
dispersity [Fig. 2(C)] is around 1.1–1.2 at all stages of the
polymerization that are the typical features of a controlled
radical polymerization. As Yang coworkers,47 we accounted
FIGURE 2 A Kinetic plot for bulk polymerization of styrene at
120ꢁC with the inimer 3. B: Plot of the experimental molar
masses of the polystyrene architecture before (Mn; n) and after
(Mn; &) thiol treatment versus conversion for bulk polymeriza-
tion of styrene at 120ꢁC with the inimer 3. Plot of the theoreti-
cal molar masses [Mn(th); ~ versus conversion supposing the
vinyl group of 3 unreactive at all stages of the polymerization.
C: Plot of dispersity of the polystyrene architecture before (Mw/
Mn; ~) and after (Mw/Mn; ~) thiol treatment versus conversion
for bulk polymerization of styrene at 120ꢁC with the inimer 3.
Experimental conditions: [Styrene]/[3] ¼ 313; bulk; T ¼ 120ꢁC.
FIGURE 3 SEC traces of alkoxyamine-functionalized polysty-
rene architecture for 28.7% (black unbroken line), 55.7% (black
broken line), and 63.7% (black dotted line) of conversion. Ex-
perimental conditions: [Styrene]/[3] ¼ 313; bulk; T ¼ 120ꢁC.
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assuming that the vinyl group of the inimer 3 does not react.
It comes from these latter results that the alkoxyamine func-
tions of 5 located at the chain end of a branch or incorpo-
rated in it display similar reactivity.
To conclude, the polymerization of styrene is controlled as
until the end of the experiment of polymerization, the molec-
ular weight of the macroinimer 4 is growing linearly with
conversion and its dispersity is narrow. Meanwhile, the ran-
dom copolymerization of styrene with one fraction of 4 takes
place to yield a branched polystyrene with controlled length
of branch.
FIGURE 4 SEC traces of polystyrene architecture for 63.7% of
conversion before (unbroken lines) and after (dotted lines) thiol
treatment. Experimental conditions: [Styrene]/[3] ¼ 313; bulk; T
¼ 120ꢁC.
Synthesis of Branched Poly(n-butyl acrylate)
As TIPNO is an efficient nitroxide45 for the controlled and
living polymerization of nBA, 3 was also tested as initiator/
regulator of the bulk polymerization of nBA to obtain
branched poly-(n-butyl acrylate). The experiment was carried
out at 125ꢁC using a molar ratio [nBA]/[3] of 261, with 5%
of free nitroxide 2 that was added to control the propagation
rate of the polymerization. The poly(n-butyl acrylate) with
Mn ¼ 54,000 g molꢀ1 and a dispersity of 3.5 was obtained
after 24 h (conversion 60%). The apparent molecular weight
[see Supporting Information Fig. S1(B)] and the dispersity
[see Supporting Information Fig. S1(C)] increase with the
conversion. Starting from a value of 1.5 (conversion: 20%),
Mw/Mn reaches a final value of 3.5 (conversion: 60%). The
molecular weight distribution is broad, exhibiting unimodal
SEC profiles. A linear relationship between ln([M]0/[M]) ver-
sus time was again observed, indicating that no detectable
termination occurred in this system [See supporting informa-
tion: Fig. S1(A)].
for these results by the presence of branched polystyrene. At
low conversion, the inimer 3 is transformed into the linear
monodisperse polystyrene macroinimer 4 (Scheme 3). As the
conversion increases, branching occurs more and more by
random copolymerization of styrene and 4 affording a mix-
ture of 4 (narrow peak in Fig. 3) and the branched polysty-
rene 5 (Scheme 3), displaying a broad peak at higher molec-
ular weight that is growing (Fig. 3) to the detriment of the
one of 4. As for the inimer 1,25 the branching starts at a con-
version of around 30%. After thiol treatment, the architec-
tures 4 and 5 have yielded the mixture of linear polystyr-
enes 6, 7, and 8 (Z ¼ H, OH, or sulfoxide function62; Scheme
3) contaminated by molecular byproducts. The investigation
of the breaking of alkoxyamine function reveals that the
branches of 5 and the macroinimer 4 have nearly the same
molecular weight Mn [Fig. 2(B)] during the whole polymer-
ization, as it is attested by the narrow dispersity of the mix-
ture composed of 6, 7, and 8. Moreover, Mn is in good agree-
ment with the theoretical molecular weight Mn(th) calculated
As in the case of styrene, the polymer was submitted to the
treatment with thiol under the same conditions. Supporting
Information Figure S2 presents the GPC traces of one
SCHEME 3 Synthesis of the macroinimer 4 and the branched polystyrene 5 by NMRP with the alkoxyamine inimer 3.
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SCHEME 4 Synthesis of the oligomeric polyalkoxyamines
nitroxide 2.
9
by manganese self-coupling of polymerizable TIPNO-based
polymer sample before and after this treatment. As for sty-
rene, the decrease of molecular weight and the narrower dis-
persity (Mw/Mn around 1.2–1.3) is consistent with the pres-
Mn(Salen)Cl (Scheme 4). This prompted us to investigate the
self-coupling of the styryl TIPNO-based nitroxide 2.
The reaction with, as previously described, a high load-
ing46,71 of manganese catalyst to favor the formation of
alkoxyamines has been carried out in isopropanol at room
temperature for 1 day. Compound 2 has completely disap-
peared and has polymerized to afford a mixture of linear oli-
gomeric polymers 9 (Scheme 4) with a TIPNO-based alkoxy-
amine function as repeating unit according to 1H NMR
analysis (see Supporting Information Fig. S3). High conver-
sion (95%) was reached. By careful chromatography on
silica, six oligomeric and more and more polar polyalkoxy-
amines were successfully separated and characterized by
SEC. The results are given in Table 1. Compound 9a displays
a broad dispersity; however, it was possible to obtain well-
defined polyalkoxyamines like 9d (Fig. 5).
ence of
a branched poly(n-butyl acrylate) architecture.
Moreover, the synthesis of the branch is controlled as its mo-
lecular weight [Supporting Information Fig. S1(B)] is growing
linearly with conversion and its dispersity is low [Supporting
Information Fig. S1(C)]. As an example, the architecture
obtained with a conversion of 60% was converted into the
linear monodisperse poly(n-butyl acrylate) (Mn ¼ 22,000 g
molꢀ1; Mth ¼ 20,400 g molꢀ1; Mw/Mn ¼ 1.3).
It is again reasonable to assume that the molecular inimer 3
has yielded monodisperse acrylate macroinimers. Then, the
starting copolymerization of these macroinimers and nBA
affords branched poly(n-butyl acrylate) architectures and
provides a good control over the length of the branch and its
dispersity. As for the inimer 1,25 the branching starts at a
conversion of around 19%.
The 1H NMR spectrum of 9d (Supporting Information Fig.
S3) reminds the one of a molecular TIPNO-based45 alkoxy-
amine, but with broad peaks, that supports the synthesis of
a polymer chain through the formation of a CAON bond by
self-coupling of 2. Indeed, it clearly indicates the absence of
vinylic protons and displays a broad singlet at 4.83 ppm,
attributed to the proton of the HACAON fragment and at
2.25 ppm attributed to the methyne proton of the isopropyl
group of 9d. Furthermore, the diastereotopic methyls of the
CH3ACHAON fragment (1H singlet signals at 1.55 and 1.45
ppm), of the isopropyl substituent (1H singlet signals at 1.24,
0.63, 0.25, and 0 ppm), and of the tert-butyl group (1H sin-
glet signals at 0.98 and 0.44 ppm) appear in the 1H NMR
spectrum of 9d. Under the conditions of the formation of
alkoxyamines, side reactions such as epoxidation46,71 of the
styrenic group or ring opening of the prepared epoxide by
nucleophilic attack might also contribute to the total
To conclude, for styrene and nBA, as the inimer 1, inimer 3
initiates and also polymerizes to give a macromolecular
architecture whose broad dispersity is the outcome of its
branched character. The synthesis of branches, accompanied
by narrow dispersity, is controlled as their molecular weight
is a linear function of conversion that demonstrates the high
chain-end fidelity of the nitroxide 2 and nitroxides incorpo-
rated in the branch points.
Synthesis of Multiblock Polymers
Synthesis of Linear Polyalkoxyamines
The incorporation of alkoxyamine units in simple macromo-
lecular architectures enables to prepare through nitroxide
exchange reactions more sophisticated architectures such as
star polymers,63,64 polymer hybrids,65,66 macrocycles,67 and
polymer brushes68 and to control crystal assembly.69 In the
same manner, linear polymers containing several alkoxy-
amine functions have appeared as versatile polyinitiators43
to synthesize multiblock copolymers. They were obtained by
nitroxide radical self-coupling,42 from TEMPO-based diol by
polycondensation70 with adipolyl chloride, or by polyaddi-
tion44 with diisocyanate.
TABLE 1 Average molecular weights and dispersity of
synthesized polyalkoxyamines
9a
9b
9c
9d
9e
9f
a
Mn (g molꢀ1
)
1,300 2,200 1,230 4,750 3,200 1,900
2.2 1.25 3.65 1.15 1.12 2.55
a
Mw/Mn
A general method of synthesis of alkoxyamine is the radical
coupling of functionalized styrene substrates and nitroxide
using manganese-based epoxidation catalysts46,71 such as
a
Apparent values for the polyalkoxyamines from SEC analysis using
linear polystyrene standard calibrations.
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FIGURE 5 SEC traces of oligomeric polyalkoxyamines 9a (black
unbroken line) and 9d (black broken line).
FIGURE 6 SEC traces of oligomeric polyalkoxyamines 9d (black
unbroken line) and polystyrene architecture 10 before (black
broken line) and after (black dotted line) thiol treatment.
consumption of the vinyl group of polyalkoxyamine 9d.
Therefore, polyalkoxyamines with identical average degree of
polymerization can be synthesized, but with different chain
extremities displaying various polarity. As a result, these pol-
yalkoxyamines are separated by chromatography on silica
and may display similar molecular weights.
with conversion and then is nearly constant independently
of the monomer’s consumption [Fig. 7(B)]. Mw/Mn decreases
with conversion [Fig. 7(C)], and its value is between 2
and 2.8.
Two polyalkoxyamines 9a (Mn ¼ 1300 g molꢀ1 and Mw/Mn
¼ 2.2) and 9d (Mn ¼ 4750 g molꢀ1 and Mw/Mn ¼ 1.15)
were tested as polyinitiators in NMRP to prepare multiblock
polystyrenes.
A rather acceptable control [Fig. 7(B, C)] was achieved for
the growing of the polystyrene blocks: starting from a value
of 1.6 (conversion: 2.5%), Mw/Mn reaches a final value of
1.37 (conversion: 51.7%) and Mn(11, 9a) is a linear function
of the conversion.
Synthesis of Multiblock Polystyrenes
The effectiveness of the polyalkoxyamines 9a displaying a
broad molecular weight distributions and the monodisperse
9d for the living free radical polymerization of styrene was
probed at 115ꢁC in bulk under the same conditions. The
polystyrene architecture 10 (Fig. 6) with a broad dispersity
was obtained.
Using a ratio of 300 between the number of styrene mono-
mers and the number of alkoxyamine function in 9a, the
multiblock polystyrene 10 (Mn ¼ 102,500 g molꢀ1; Mw/Mn
¼ 2.05) and the mixture 11 of polystyrene blocks (Mn
¼
25,300 g molꢀ1; Mw/Mn ¼ 1.37) were obtained after 6 h
with a conversion of 51.7%.
The treatment of 10 (Scheme 5) with 2-methyl-5-tert-
butylthiophenol has given the polystyrene architecture 11.
Compound 11 displays narrower dispersity than 10, accom-
panied by a decrease of the experimental molecular weight
(Fig. 6), that confirms the synthesis of the multiblock poly-
styrene 10 (Scheme 5) in which the link between consecu-
tive polystyrene blocks is an alkoxyamine function: 11 (Z ¼
H, OH, or sulfoxide function62; Scheme 5) is a mixture of all
the polystyrene blocks formed during the styrene insertion
in 9a or 9d. The narrow molecular weight distribution [Fig.
7(C)] of 11 demonstrates that all the polystyrene blocks
have mostly the same length.
Finally, the ratio between Mn(10, 9a) and Mn(11, 9a) [Fig.
8(A)] is between 3.3 and 4.4. In other words, the multiblock
polystyrene 10 contains 3–4 blocks on average.
The seminal works of Otsuka et al.43 and Hawker et al.72 can
explain the evolution of Mn(10, 9a) with the conversion.
Indeed, Hawker et al.72 have proved that the growing macro-
alkoxyamines based on different TEMPO during polymeriza-
tion of styrene exchange their molecular nitroxides to give
macroalkoxyamines that stem from radical cross-coupling
reactions. Interestingly, Otsuka et al.70 have demonstrated
that this exchange also takes place when several alkoxy-
amine functions are incorporated in the main chain of a
linear polymer. Monodisperse polyalkoxyamines containing
TEMPO-based alkoxyamines undergo thermal reorganiza-
With 9a, a linear relationship between ln([M]0/[M]) versus
time was observed [Fig. 7(A)]. The molecular weight Mn(10,
9a) of the multiblock polystyrene 10 first increases linearly
SCHEME 5 Synthesis of the multiblock polystyrene 10 and their polystyrene blocks 11 after thiol treatment (Z ¼ H, OH, or sulfox-
ide function62).
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tion70 (exchange of TEMPO-end-capped polymers) on heating
to afford polyalkoxyamines with lower molecular weight and
broader dispersity. Moreover, a mixture of two monodisperse
distinct polyalkoxyamines66 with two different molecular
weights M1 and M2 on heating gives hybrid polyalkoxy-
amines displaying broad dispersity and a molecular weight
between M1 and M2.
In the same manner, for styrene and 9a, at all stages of the poly-
merization, the mediating nitroxides that are TIPNO-end-capped
polystyrenes with various lengths (Scheme 5) are exchanged
between the growing polystyrene chains, which induces a broad-
ening of the dispersity and a decrease of molecular weight and
function of time for the multiblock polystyrene 10.
During all the polymerization, Mn(11, 9a) is a linear function
of conversion. As a result, as in the study reported by Otsuka
and coworkers,44 the molecular weight Mn(10, 9a) of the
FIGURE 7 A: Kinetic plot for bulk polymerization of styrene at
115ꢁC with the polyalkoxyamine 9a. B: Plot of the experimental
molar masses of the polystyrene architecture before (Mn; n)
and after (Mn; &) thiol treatment versus conversion for bulk
polymerization of styrene at 115ꢁC with the polyalkoxyamine
9a. C: Plot of dispersity of the polystyrene architecture before
(Mw/Mn; ~) and after (Mw/Mn; ~) thiol treatment versus con-
version for bulk polymerization of styrene at 115ꢁC with the
polyalkoxyamine 9a. Experimental conditions: nstyrene/nnumber of
FIGURE 8 A: plot of r ¼ Mn(multiblock)/Mn(block) at 115ꢁC
(black symbol) for 9a (n) and 9d (^) and at 125ꢁC (red symbol)
for 9d (^). B: Plot of r ¼ Mn(multiblock, 125ꢁC)/Mn(multiblock,
115ꢁC) (n), Mn(block, 125ꢁC)/Mn(block, 115ꢁC) (&) for 9d and
plot of Mn(multiblock, 9d)/Mn(multiblock, 9a) (^), Mn(block,
9d)/Mn(block, 9a) (^) at 115ꢁC.
¼ 300; bulk; T ¼ 115ꢁC.
alkoxyamine function in 9a
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multiblock polystyrene 10 also increases linearly until 40%
of conversion. From this conversion, Mn(10, 9a) remains
constant despite a further increase of conversion. The
decrease of Mn(10, 9a) induced by the exchange reaction of
the mediating nitroxides offsets its increase by monomer
consumption at such extent that Mn(10, 9a) is constant even
if the conversion continues to increase. Furthermore, the
starting broad molecular weight distribution (Mw/Mn ¼ 2.8)
of the multiblock polystyrene 10 could be the result of the
relatively high dispersity (Mw/Mn ¼ 1.6) of 11 and the high
dispersity (Mw/Mn ¼ 2.2) of the polyinitiator 9a. In addition,
as the Mw/Mn of 11 decreases, Mw/Mn of the multiblock 10
also decreases.
To improve a priori the control over the Mn and Mw/Mn of
the multiblock polystyrenes 10 and to increase the number
of blocks, the monodisperse and higher molecular weight
polyinitiator 9d was tested at 115ꢁC.
For 9d, the logarithmic conversion has also exhibited a linear
relationship with time [Fig. 9(A)]. Compound 9d also pro-
vides a good control over Mn(10, 9d), as Figure 9(B) shows
a linear increase of Mn(10, 9d) with conversion during the
whole polymerization. With 9d, Mn(10, 9d) does not stag-
nate. Unlike 9d, the intrinsic broad molecular weight distri-
bution of 9a anticipates the broadening generated by the
nitroxide exchange. Thus, Mn(10, 9d) becomes constant at
lower conversion with 9a than with 9d.
Moreover, Mw/Mn of 10 decreases with conversion and its
values lie between 1.7 and 2.8. Compounds 9a and 9d give
similar values for Mw/Mn, except at conversion above 50%
[Fig. 9(C)]. Even if 9d is well defined, its impact on the dis-
persity of the multiblock polystyrene 10 is negligible.
Furthermore, the controls with 9a and 9d over the synthesis
of the polystyrene blocks are nearly of the same level except
for the dispersity. The insertion of styrene in 9d is controlled
[Fig. 9(B)] as Mn(11, 9d) increases linearly with conversion
and Mw/Mn decreases with conversion. Mw/Mn remains
lower with 9d than with 9a [Fig. 9(C)]: starting from a value
of 1.45 (conversion: 8.5%), Mw/Mn reaches a final value of
1.2 (conversion: 48.5%).
Compound 9d has given the multiblock polystyrene 10 with
Mn ¼ 82,000 g molꢀ1 and Mw/Mn ¼ 1.73 and the mixture
11 of polystyrene blocks with Mn ¼ 20,900 g molꢀ1 and
Mw/Mn ¼ 1.2 at a conversion of 48.5% after 6 h of polymer-
ization. The livingness is also established as a shift (Fig. 6)
toward higher molecular weight is observed for both multi-
block and block polystyrenes, and no traces of unreacted
polyinitiator 9d was detected by SEC. Furthermore, the ratio
between Mn(10, 9d) and Mn(11, 9d) is between 3.3 and 4
[Fig. 8(A)]. Finally, remarkably enough Mn(10, 9d)/Mn(10,
9a) is nearly equal to Mn(11, 9d)/Mn(11, 9a) [Fig. 8(B)] and
lies between 0.4 and 0.8.
FIGURE 9 A: Kinetic plot for bulk polymerization of styrene at
115ꢁC with the polyalkoxyamine 9d. B: Plot of the experimen-
tal molar masses of the polystyrene architecture before (Mn;
n) and after (Mn; &) thiol treatment versus conversion for
bulk polymerization of styrene at 115ꢁC with the polyalkoxy-
amine 9d. C: Plot of dispersity of the polystyrene architecture
before (Mw/Mn; ~) and after (Mw/Mn; ~) thiol treatment ver-
sus conversion for bulk polymerization of styrene at
115ꢁC with the polyalkoxyamine 9d. Experimental conditions:
Even if 9d has a much higher molecular weight than 9a, the
multiblock 10 synthesized with 9d contains again 3–4 blocks
on average and surprisingly Mn(10, 9d) ꢃ Mn(10, 9a).
Therefore, the molecular weight of the polyinitiator has a
n
styrene/nnumber of alkoxyamine function in 9d ¼ 300; bulk; T ¼ 115ꢁC.
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weak influence on the number of polystyrene blocks in 10.
Moreover, as Otsuka et al.,70 we can reasonably assume that
the exchange of the mediating nitroxides controls the num-
ber of blocks and the molecular weight of the multiblock
polystyrene 10.
thermore, the self-coupling of this polymerizable nitroxide by
manganese coupling has yielded a mixture of oligomers includ-
ing TIPNO-based alkoxyamine as repeating unit. These oligo-
meric polyalkoxyamines were partially separated by chromatog-
raphy on silica and have inserted styrene to give multiblock
polystyrenes. The insertion of styrene was controlled as all poly-
styrene blocks display nearly the same length increasing linearly
with conversion. The dispersity of the multiblock decreases
with conversion and its value lies between 1.7 and 2.8. Its mo-
lecular weight is first a linear function of the conversion before
reaching a constant value despite further consumption of sty-
rene. The number of blocks on which the molecular weight of
the initiating polyalkoxyamines has nearly no impact depends
on the temperature. These latter results can be explained by the
fact that on heating the multiblock polystyrene, chains grow and
meanwhile exchange their mediating macromolecular TIPNO-
based nitroxides of different lengths. This nitroxide is currently
under further investigation to get new architectures.
To check this hypothesis, the polymerization with 9d has
been repeated at 125ꢁC to favor the exchange reaction of
nitroxide and by doubling the starting quantity of styrene.
A kinetic of first order and as expected an increase of the rate
of polymerization induced by the increase in the temperature
were observed [see Supporting Information Fig. S4(A)]. The
increase of temperature also induces lower dispersity [Fig.
8(C)] for the multiblock 10 (Mw/Mn around 1.7–1.9). At low
conversion, Mw/Mn is around 2.8 at 115ꢁC, and for nearly the
same conversion at 125ꢁC, it decreases to 1.7–1.9, which is the
typical value obtained at 115ꢁC but for higher conversion.
As at 115ꢁC, the insertion of styrene in 9d is controlled [see
Supporting Information Fig. S4(B)] at 125ꢁC. The quality of
the control is good as remarkably enough, except at low con-
version, Mn(11, 125ꢁC)/Mn(11, 115ꢁC) is around 2 [Fig.
ACKNOWLEDGMENTS
The authors thank the CNRS, the university Pierre et Marie Cu-
ꢁ
ꢁ
8(B)], which is the starting ratio nSty,125 C/nSty,115 C. In con-
trast, Mn(10, 125ꢁC)/Mn(10, 115ꢁC) is around 1 except at
low conversion [Fig. 8(B)]. As with 9a at 115ꢁC, for 9d at
125ꢁC, Mn(10, 9d) increases linearly with conversion until
reaching a constant value of 90,000 g molꢀ1 from 53% of
conversion [Supporting Information Fig. S4(B)]. The rate of
exchange reaction of nitroxide was increased by the increase
in the temperature so that Mn(10, 9d) reaches a value inde-
pendent of the conversion.
ꢀ
rie (UPMC) and the Ecole Normale Superieure for financial
support.
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