Nitroxide-mediated polymerization of methyl methacrylate and styrene with new alkoxyamines from 4-nitrophenyl 2-methylpropionat-2-yl radicals

Article abstract of DOI:10.1021/ma1018044

The N-phenylalkoxyamine N-(1-methyl-(1-(4-nitrophenoxy)carbonyl)ethoxy)-N- (1-methyl-(1-(4-nitrophenoxy)carbonyl)ethyl)benzenamine (1) was prepared by the addition of 4-nitrophenyl 2-methylpropionat-2-yl radicals across the double bond of nitrosobenzene a

Full text of DOI:10.1021/ma1018044

10320 Macromolecules 2010, 43, 10320–10325
DOI: 10.1021/ma1018044
Nitroxide-Mediated Polymerization of Methyl Methacrylate and Styrene
with New Alkoxyamines from 4-Nitrophenyl 2-Methylpropionat-2-yl
Radicals
Anna C. Greene† and Robert B. Grubbs*,†,‡
†Department of Chemistry, Dartmouth College, 6128 Burke Laboratory, Hanover, New Hampshire 03755,
United States, and ‡Department of Chemistry, State University of New York, Stony Brook,
New York 11794, United States
Received August 6, 2010; Revised Manuscript Received November 9, 2010
ABSTRACT: The N-phenylalkoxyamine N-(1-methyl-(1-(4-nitrophenoxy)carbonyl)ethoxy)-N-(1-methyl-(1-
(4-nitrophenoxy)carbonyl)ethyl)benzenamine (1) was prepared by the addition of 4-nitrophenyl 2-methyl-
propionat-2-yl radicals across the double bond of nitrosobenzene and evaluated as an initiator for nitroxide-
mediated polymerization (NMP). N-Phenylalkoxyamines have not been extensively studied in NMP, though
they have shown promise in controlling methyl methacrylate (MMA) polymerization to moderate conver-
sions. It is thought that delocalization of the nitroxide radical through the N-phenyl substituent may minimize
cross-disproportionation of the nitroxide with the chain end that has made NMP of MMA difficult. Here, we
show that alkoxyamine 1 is capable of controlling the NMP of MMA to 50% conversion while maintaining
narrow molecular weight distributions (Mw/Mn=1.12-1.30). Additionally, chain extension from the result-
ing PMMA macroinitiators with MMA or styrene allows the formation of diblock copolymers. The corre-
sponding N-tert-butylalkoxyamine, 2,2-dimethyl-3-(1-methyl-(1-(4-nitrophenoxy)carbonyl)ethoxy)-4-methyl-(4-
(4-nitrophenoxy)carbonyl)ethyl-3-azapentane (2), was synthesized by the addition of 4-nitrophenyl
2-methylpropionat-2-yl radicals across the double bond of 2-methyl-2-nitrosopropane. Polymerization of
MMA with alkoxyamine 2 was uncontrolled, which suggests the paramount importance of the N-phenyl
group for MMA polymerizations.
Scheme 1. Activation-Deactivation Equilibrium for NMP Where
Introduction
P=Polymer Chain and T=Nitroxide
Nitroxide-mediated polymerization (NMP) is one of the
most widely employed controlled radical polymerization tech-
niques.1-6 It operates by the reversible trapping of growing poly-
merchainswithnitroxidesandiscontrolled by the persistent radical
effect, which ensures efficient cross-coupling of polymeric radi-
cals with nitroxides instead of terminating with themselves.7,8
NMP has been utilized to control the polymerization of styrenics,
dienes, acrylates, acrylamides, and, to some extent, methacry-
lates.9-11,13
Because methacrylates comprise about 1 billion pounds of
polymeric products produced in the United States each year, it is
desirable to extend NMP to enable greater control over metha-
crylate polymerization.12 This has proven difficult because cross-
disproportionation and a large activation-deactivation equilib-
rium constant, Keq (kd/kc, Scheme 1), are two problematic issues
with the NMP of methyl methacrylate (MMA).13,14 Polymeriza-
tion of MMA with the widely available nitroxide 2,2,6,6-tetra-
methyl-1-piperidinyloxy (TEMPO) fails because of the cross-
disproportionation side reaction, which occurs when TEMPO
abstracts a hydrogen atom from the PMMA chain end to form a
saturated polymer chain end and a hydroxylamine.15-18 With
newer R-hydrido nitroxides such as 1-(diethoxyphosphinyl)-2,2-
dimethylpropyl-1,1-dimethylethylnitroxide (SG1), cross-dispro-
portionation side reactions have not been observed in some
studies, and the uncontrolled nature of MMA polymerization
has been attributed solely to the large activation-deactivation
equilibrium constant (Keq, Scheme 1).17,19 The large Keq for MMA
is attributed to a large kd due to steric destabilization of the
ground state, as well as a small kc because of steric hindrance as
the nitroxide approaches the bulky tertiary propagating radical
center.19 However, cross-disproportionation has been observed
during the polymerization of MMA with an excess of SG1,18 and
Charleux and co-workers detected the occurrence of both cross-
disproportionation and bimolecular termination in SG1-mediated
MMA polymerizations.14
Because of both cross-disproportionation and the large Keq for
MMA, the design of an initiator that can control the homo-
polymerization of MMA has been difficult. The most success-
ful route for MMA homopolymerization has been the use of N-
arylnitroxides. It is thought that these nitroxides allow radical
delocalization through the N-aryl moiety, perhaps preventing
the cross-disproportionation side reaction and allowing better
control over MMA polymerization.20,21 An alternative route
involves the copolymerization of a small percentage of styrene
(4-10%) with MMA to form copolymers dense with metha-
crylate functionality.5,21-29 This copolymerization method allows
high MMA conversions to afford PMMA-rich copolymers with
low polydispersity indices.
Guillaneuf and co-workers utilized 2,2-diphenyl-3-phenylimi-
no-2,3-dihydroindol-1-yloxyl (DPAIO)-based alkoxyamines that
*To whom correspondence should be addressed. E-mail: robert.grubbs@
stonybrook.edu.
pubs.acs.org/Macromolecules
Published on Web 11/19/2010
r 2010 American Chemical Society

Article
Macromolecules, Vol. 43, No. 24, 2010 10321
contained either a phenyl 2-methylpropionat-2-yl initiating frag-
ment or a 4-nitrophenyl 2-methylpropionat-2-yl initiating fragment
for the polymerization of MMA.20 Because the DPAIO nitroxide
allows delocalization of the free radical through the indolinoxyl
ring, it was reasoned that the extent of cross-disproportionation
would be minimized, and the polymerization would be better
controlled. They observed better control for MMA polymeriza-
tion with the alkoxyamine containing the 4-nitrophenyl 2-
methylpropionat-2-yl initiating fragment and ascribed this result
to a long-range polar effect.20 According to studies based on sim-
ilar initiating fragments trapped with the nitroxide SG1, the disso-
ciation rate constant, kd (Scheme 1), was increased for the nitro
derivative as a result of the electron withdrawing nature of
the nitro group (t1/2=120 s) when compared to the SG1 deriva-
GPC software (Polymer Laboratories) calibrated with narrow
molecular weight polystyrene standards with molecular weights
in the range of 580-400 000 g/mol (EasiCal PS-2, Polymer Lab-
oratories). All reported Mn,SEC values are based upon compar-
ison to polystyrene standards. Initiating efficiencies for poly-
styrene samples were calculated based on the following formula:
I
34
eff = Mn,th/Mn,SEC
.
Elemental analysis data were obtained
from Schwarzkopf Microanalytical Laboratory and Galbraith
Laboratories, Inc.
Synthesis of N-(1-Methyl-(1-(4-nitrophenoxy)carbonyl)ethoxy)-
N-(1-methyl-(1-(4-nitrophenoxy)carbonyl)ethyl)benzenamine (1).
In a nitrogen-filled glovebox, 4-nitrophenyl 2-bromo-2-methyl-
propionate (4) (8.00 g, 27.7 mmol) was combined with nitroso-
benzene (1.78 g, 16.6 mmol) and dissolved in toluene (8 mL) in a
Schlenk tube equipped with a magnetic stir bar. CuBr (0.993 g,
6.90 mmol), Cu(0) (1.80 g, 28.4 mmol), and PMDETA (1.20 g,
6.9 mmol) were added sequentially to form a dark green hetero-
geneous mixture. The tube was sealed under N2, removed from
the glovebox, and placed in an oil bath at 60 °C. The mixture was
allowed to stir for 24 h and was then flushed through a column of
basic alumina. The solution was concentrated to a dark brown
oil and placed on the Schlenk line for further drying, which
resulted in a dark powder. The material was recrystallized from
methanol to afford alkoxyamine 1 as a fine white powder (2.60 g,
36%). 1H NMR (CDCl3, 500 MHz) δ 8.29 (d, J=7 Hz, Ar-H,
2H); 8.15 (d, J=7 Hz, Ar-H, 2H); 7.41-7.22 (m, Ar-H, 7H); 6.84
(d, J=7 Hz, Ar-H, 2H); 1.63 (s, CH3, 3H); 1.57 (s, CH3, 3H); 1.54
(s, CH3, 3H); 1.50 (s, CH3, 3H). 13C NMR (CDCl3, 125 MHz): δ
171.27, 170.60, 155.83, 155.40, 147.68, 145.51, 145.40, 128.44,
127.11, 126.15, 125.43, 125.08, 122.49, 122.26, 82.11, 69.05,
24.94, 24.87, 24.11, 21.75. Elem. Anal. Calcd. for C26H25N3O8:
C, 59.68%; H, 4.97%; N, 8.03%. Found: C, 59.53%; H, 5.08%,
N, 7.84%.
Synthesis of 2,2-Dimethyl-3-(1-methyl-(1-(4-nitrophenoxy)-
carbonyl)ethoxy)-4-methyl-(4-(4-nitrophenoxy)carbonyl)ethyl-3-
azapentane (2). A Schlenk tube with a magnetic stir bar was
charged with 4-nitrophenyl 2-bromo-2-methylpropionate (4)
(1.93 g, 6.67 mmol) and 2-methyl-2-nitrosopropane (0.350 g,
4.00 mmol) in anhydrous toluene (10 mL). CuBr (0.240 g, 1.67
mmol), Cu(0) (0.430, 6.84 mmol), and PMDETA (0.290 g, 1.67
mmol) were added to the tube sequentially. The contents of the
tube were degassed by three freeze-pump-thaw cycles and
backfilled with N2. The tube was placed in an oil bath at 60 °C
and allowed to stir for 42 h. The contents of the tube were
passed through a plug of basic alumina and then purified by
flash chromatography (SiO2, 12.34: 1 hexanes-ethyl acetate) to
afford alkoxyamine 2 as bright yellow needles (0.17 g, 10% yield).
1H NMR (CDCl3, 500 MHz) δ 8.33 (d, Ar-H, 4H); 7.36 (d, J=
9 Hz, Ar-H, 4H); 1.80 (s, CH3, 3H); 1.76 (s, CH3, 3H); 1.65 (s,
CH3, 3H); 1.60 (s, CH3, 3H); 1.30 (s, CH3, 9H). 13C NMR
(CDCl3, 125 MHz): δ 174.12, 172.94, 156.05, 155.85, 145.69,
145.51, 125.58, 125.55, 122.51, 122.27, 83.31, 68.06, 62.59, 30.45,
28.78, 25.65, 23.61, 21.85. Elem. Anal. Calcd. for C24 H29 N3O9:
C, 57.27%; H, 5.77%; N, 8.35%. Found: C, 56.32%; H, 5.89%,
N, 8.13%.
tive with the phenyl propionat-2-yl initiating fragment (t1/2
=
197 s).20,30,31 They postulated that a faster homolysis rate for the
nitrophenyl ester-substituted DPAIO derivative led to faster
initiation and better control from the early stages of the poly-
merization. These results suggest that the more general problem
of ensuring that initiation is rapid relative to propagation is also
relevant to the NMP of methacrylates.
Previously, we prepared N-phenylalkoxyamines by the addi-
tion of 1-phenylethyl or phenyl 2-methylpropionat-2-yl radicals
to nitrosobenzene that were able to control MMA polymeriza-
tion to moderate conversions (32-41%) to yield PMMA with
narrow molecular weight distributions.21 In thiswork, we describe
the one-step preparation of a new N-phenylalkoxyamine (1), anal-
ogous to the nitrophenyl ester-based DPAIO initiators reported
by Guillaneuf and co-workers,20 by the addition of 4-nitrophenyl
2-methylpropionat-2-yl radicals across the nitroso group of nitroso-
benzene, and evaluate its effectiveness for the polymerization of
styrene and MMA. These results indicate that a small struc-
tural change in the initiator, such as the addition of para-nitro
groups on the phenyl rings, can lead to valuable improvements
in the polymerization kinetics, affording control to higher con-
versions (49%) with narrow molecular weight distributions (Mw/
Mn =1.12-1.30). In order to assess the effect of the N-phenyl
moiety on polymerization kinetics, the N-tert-butyl analog (2) of
alkoxyamine 1 was also synthesized and evaluated for use in the
polymerizations of styrene and MMA.
Experimental Section
Materials. 2-Methyl-2-nitrosopropane, 2-bromo-2-methylpro-
pionyl bromide, 4-nitrophenol, Cu(0), Cu(I)Br, nitrosobenzene,
N,N,N0,N0,N00-pentamethyldiethylenetriamine (PMDETA), and
triethylamine were used as purchased from Fisher Scientific or
Sigma-Aldrich. ACS-grade solvents were used as received unless
ultrapure solvent was needed in which case it was dried by passage
through basic alumina under positive N2 pressure.32 Monomers
were passed through a column of basic alumina to remove inhib-
itors before use. Alkoxyamine 3 was synthesized as previously
reported.21 4-Nitrophenyl 2-bromo-2-methylpropionate (4) was
synthesized according to a literature procedure.33
1
General. H and 13C NMR spectra were recorded on a 500
Polymerization of Styrene. In a typical procedure, alkoxya-
mine 2 (0.018 g, 0.036 mmol) and styrene (0.75 g, 0.082 mL,
7.2mmol) weresealed in a Schlenk tube, degassed by three freeze-
pump-thaw cycles, backfilled with N2 and placed in an oil bath
set at 125 °C for 7.5 h. After the specified amount of time, the
polymerization was stopped by placing the tube in an ice bath.
The contents of the tube were dissolved in CH2Cl2 and purified
by precipitation into methanol. The solution was decanted
yielding the white polymer, which was subsequently dried and
analyzed by 1H NMR and SEC (Mn,NMR=8.5 kg mol-1, con-
version =39%; Mn,SEC=9.0 kg mol-1, Mw/Mn=1.11).
Polymerization of MMA. In a typical procedure, alkoxyamine
1 (0.022 g, 0.042 mmol) and MMA (1.26 g, 1.35 mL, 12.6 mmol)
were sealed in a side arm Schlenk tube, degassed by three freeze-
pump-thaw cycles, and placed under N2. The tube was placed
MHz Varian Unity spectrometer using CDCl3 or CD2Cl2 as
solvent with the solvent peak as reference. Monomer conversion
was calculated by H NMR by comparison of the integration
1
values for monomer peaks to polymer peaks using the following
equation: p=[mol polymer]/[mol monomer þ mol polymer].
Size exclusion chromatography (SEC) was performed at 40 °C
with HPLC-grade tetrahydrofuran (THF) as eluent at a flow
rate of 1.0 mL/min on a system consisting of a K-501 pump
(Knauer), a K-3800 Basic Autosampler (Marathon), a set of two
PLgel 5 μm Mixed-D columns (300 ꢀ 7.5 mm2, rated for molec-
ular weights from 200 to 400 000 g/mol, Polymer Laboratories),
and a PL-ELS 1000 Evaporative Light Scattering Detector (Polymer
Laboratories). SEC data were acquired through a PL Data-
stream unit (Polymer Laboratories) and analyzed with Cirrus

10322 Macromolecules, Vol. 43, No. 24, 2010
Greene and Grubbs
Figure 1. (a) Evolution of Mn and Mw/Mn with conversion for [MMA]:[1] = 300:1 at 125 °C; (b) Evolution of Mn and Mw/Mn with conversion for
[MMA]:[1] = 1000:1 at 125 °C. The molecular weights for PMMA are relative to polystyrene standards used in SEC.
Scheme 2. Synthesis of Alkoxyamines 1 and 2
in an oil bath thermostatted at 125 °C for 5 h, and then the poly-
merization was quenched by placing the tube in an ice bath. The
contents of the tube were dissolved in CD2Cl2, and an aliquot
was withdrawn for 1H NMR and SEC analyses. The remainder
of the material was purified by precipitation into hexanes and
weights increased with conversion to the highest observed con-
version of 49%, and the polydispersity indices remained low
throughout the polymerization (Mw/Mn = 1.12-1.30)
(Figure 1a). However, the highest observed conversion for
1
these polymerizations was 49%. The H NMR spectrum of
analyzed by SEC against polystyrene standards. (Mn,NMR
=
,
PMMA synthesized from alkoxyamine 1 revealed vinylic reso-
nances (δ 6.2 and 5.5 ppm) indicative of alkene-terminated
chain ends, suggesting that cross-disproportionation occurred
during the course of the polymerization. This process leads to
dead polymer chains and to the build up of free nitroxide, both
of which would contribute to halting the polymerization at
lower conversion. On the basis of the kinetic data (Figure 1a),
initiator 1 appears to be better than alkoxyamine 3 as an
initiator for the polymerization of MMA, as it shows a faster
rate of polymerization and gives slightly narrower molecular
weight distributions (Table 1).
The preparation of higher molecular weight PMMA
was examined by polymerizing MMA (1000 equiv) with alko-
xyamine 1. The molecular weights of the polymers increased
with conversion to the highest observed conversion of 42% at
6 h (Figure 1b). The molecular weight distributions remained
narrow during the polymerizations (Mw/Mn=1.21-1.28), and
molecular weights as high as 65 kg/mol (against polystyrene
standards) were observed at 42% conversion with Mw/Mn=
1.27, indicating good control with alkoxyamine 1.
Since alkoxyamine 1 was an efficient initiator for the homo-
polymerization of MMA, the effect of the N-phenyl group
was probed by exchanging the phenyl moiety for the t-butyl
group (2; Scheme 2). Alkoxyamine 2 was utilized in the
polymerization of 300 equiv of MMA, and the effect of this
structural change was quite pronounced. There was no con-
trol exhibited over MMA polymerization using initiator 2.
The polymerizations stopped at low conversions (19-27%
15.1 kg mol-1, conversion=49%; Mn,SEC=22.4 kg mol-1
Mw/Mn=1.30).
Results and Discussion
Synthesis of Alkoxyamines. Alkoxyamine 1 was synthe-
sized by the addition of 4-nitrophenyl 2-methylpropionat-2-
yl radicals generated from 4-nitrophenyl 2-bromo-2-methyl-
propionate33 across the double bond of nitrosobenzene and
obtained as a white powder in 36% overall yield after recrys-
tallization (Scheme 2). We postulated that alkoxyamine 1
might provide better control over the polymerization of MMA
than was achieved with the phenyl ester-functional alkox-
yamine N-(1-methyl-1-(phenoxycarbonyl)ethoxy)-N-(1-methyl-
1-(phenoxycarbonyl)ethyl)benzenamine 321 (Scheme 2) due to
a potentially larger initiating dissociation rate constant, kd,
for alkoxyamine 1 than for 3 because of the presence of the
para-nitro groups in 1. The N-tert-butyl analog of initiator 1,
2,2-dimethyl-3-(1-methyl-(1-(4-nitrophenoxy)carbonyl)ethoxy)-
4-methyl-(4-(4-nitrophenoxy)carbonyl)ethyl-3-azapentane (2),
was also investigated in order to make a direct comparison
between the two different N-substituents for both styrene
and MMA polymerization. Alkoxyamine 2 was isolated as
bright yellow needles after the addition of 4-nitrophenyl 2-
methylpropionat-2-yl radicals across the nitroso group of 2-
methyl-2-nitrosopropane (Scheme 2).
Polymerization of MMA. Alkoxyamine 1 was used to initiate
the polymerization of MMA (300 equiv). The molecular

Article
Macromolecules, Vol. 43, No. 24, 2010 10323
Table 1. Comparison of Alkoxyamines 1 and 3 for the Polymerization
of MMAa
conv.b/%
Mw/Mn
c
entry
alkoxyamine
time/h
1
2
3
4
3
1
3
1
4
4
6
5
31
39
38
49
1.29
1.24
1.34
1.30
a Polymerizations run at 125 °C under N2 atmosphere. [MMA]/
[1 or 3] = 300:1. b Conversion determined by 1H NMR. c Determined by
SEC against polystyrene standards.
Table 2. Polymerization of Styrene with Alkoxyamine 1a
d
entry time/h conv.b/% Mn,theorc/kg/mol Mn,SECd/kg/mol Mw/Mn
1
2
3
4.5
7
15
4.3
23.5
40
1.4
5.4
8.9
7.2
11.5
15.1
1.50
1.57
1.43
a Polymerizations run at 125 °C under N2 atmosphere. [Styrene]/[1] =
200:1. b Conversion determined by 1H NMR. c Calculated based on
conversion. d Determined by SEC against polystyrene standards.
conversion) with most chains appearing to have undergone
irreversible termination reactions in less than 1 h, and the
molecular weight distributions remained broad for the dif-
ferent times examined (Mw/Mn=1.58-1.63). This suggests
that the N-phenyl group plays an important role in the
successful homopolymerization of MMA.
Polymerization of Styrene. Polymerization of styrene (200
equiv) with 1was uncontrolled, with large discrepancies observed
between the theoretical and calculated molecular weights
and broad molecular weight distributions (Mw/Mn=1.43-
1.57) (Table 2). We have previously observed similar results
with the arylnitroxide-based alkoxyamine 3, which produced
polystyrene with broad molecular weight distributions over
the conversion range studied (conversion=20-66%; Mw/
Mn=1.36-1.50), along with inconsistent theoretical and cal-
culated molecular weights.21 The polymerization of styrene
with alkoxyamine 1 proceeded more slowly than with alkox-
yamine 3, although initiating efficiencies were similar for 1
(Ieff = 0.47-0.59) and 3 (Ieff = 0.50-0.53) for conversions
from 20 to 40%. Molecular weight distributions were also
slightly broader for polystyrene prepared from 1 (Mw/Mn=
1.43-1.57) than polystyrene prepared from 3 (Mw/Mn =
1.36-1.50).
Initiator 2 was also employed in the polymerization of
styrene (200 equiv). While alkoxyamine 2 was not able to
control the homopolymerization of MMA, presumably due
to steric and electronic effects of the N-t-butyl moiety, it was
able to effect a controlled polymerization of styrene. The molec-
ular weights evolved linearly with conversion, up to the highest
observed conversion of 73%, and the theoretical and calculated
molecular weights correlated well to at least 66% conversion.
The molecular weight distributions remained narrow through-
out the polymerization (Mw/Mn=1.08-1.22) suggesting a
controlled process (Figure 2a). The pseudo-first-order kinetic
plot exhibited a smooth consumption of monomer over time
(Figure 2b).
Figure 2. (a) Evolution of Mn and Mw/Mn with conversion; (- - -) =
theoretical Mn and (b) pseudo-first-order kinetic analysis for [styrene]:
[2] = 200:1 at 125 °C; (;) = best fit line.
prepared by the polymerization of MMA (300 equiv) from
alkoxyamine 1 to 17% conversion. For MMA chain exten-
sion, purified PMMA-1 was redissolved in MMA (2000
equiv) and allowed to react at 125 °C for 6 h. The resulting
PMMA-PMMA-1 showed a slightly higher molecular weight
(Mn,SEC =14.0 kg/mol) and broader molecular weight dis-
tribution (Mw/Mn=1.37) (Figure 3). This indicates that the
macroinitiator PMMA-1 had living chain ends that could be
extended with additional MMA, although chain extension to
high enough molecular weights to show significant shifts in
SEC traces was not achieved.
Macroinitiator PMMA-1 was also employed for chain
extension with styrene to examine the feasibility of diblock
copolymer formation. PMMA-1 macroinitiator was dissolved
in styrene (2000 equiv) and allowed to react at 125 °C for 6 h
to 24% conversion. The resulting PMMA-PS diblock (PMMA-
PS-1a; Figure 4a) had a substantially higher molecular weight
by SEC (Mn=61.8 kg/mol). The molecular weight distribution
was significantly broadened, however, from homopolymer to
the diblock (Mw/Mn=1.28 for PMMA-1a; Mw/Mn=1.97 for
PMMA-PS-1a). This result is not unusual since N-phenyl initi-
ators, such as alkoxyamine 1, do not appear to be capable of
controlling the polymerization of pure styrene. The substantial
increase in molecular weight seen in this polymerization shows
that there is a low molecular weight shoulder present in the SEC
trace of the diblock, which was not visible in the MMA chain
extension experiments (Figure 3) and most likely results from
dead homopolymer that was not able to chain extend with styrene
(Figure 4a).
Nitroarenes, while known to be relatively innocuous addi-
tives in the radical polymerization of MMA, are known to
retard the polymerization of styrene.35 However, the ability
of alkoxyamine 2 to control styrene polymerizations suggests
that the lack of control observed with alkoxyamine 1 and
styrene results largely from the N-aryl substituent and not
the nitro group.
Chain Extension Studies. Chain extension polymerizations
from a PMMA macroinitiator made from alkoxyamine 1
were conducted with both MMA and styrene. Macroinitia-
tor PMMA-1 (Mn,SEC =11.4 kg/mol; Mw/Mn =1.28) was
After precipitation of PMMA-PS-1a, the resultant diblock
(PMMA-PS-1b, Figure 4b) showed a higher molecular weight

10324 Macromolecules, Vol. 43, No. 24, 2010
Greene and Grubbs
Figure 3. Chain extension from PMMA made from 1 with MMA. PMMA-1: reaction time 2.5 h; 1H NMR: Mn=5.6 kg/mol, conversion =17%; SEC:
Mn=11.4 kg/mol, Mw/Mn=1.28; PMMA-PMMA-1: reaction time 6 h; SEC: Mn=14.0 kg/mol, Mw/Mn=1.37. The polymer was precipitated once
into hexanes from dichloromethane before SEC analysis.
Figure 4. Chain extension from PMMA made from 1 with styrene. (a) PMMA-1: reaction time 2.5 h; 1H NMR: Mn=5.6 kg/mol, conversion=17%;
SEC: Mn=11.4 kg/mol, Mw/Mn=1.28; PMMA-PS-1a: reaction time 6 h; 1H NMR: Mn=60.8 kg/mol, conversion=24%; SEC: Mn=61.8 kg/mol, Mw/
Mn=1.97. The polymer was not precipitated prior to SEC analysis. (b) PMMA-1: reaction time 2.5 h; 1H NMR: Mn=5.6 kg/mol, conversion=17%;
SEC: Mn=11.4 kg/mol, Mw/Mn=1.28; PMMA-PS-1b: reaction time 6 h; 1H NMR: Mn=60.8 kg/mol, conversion=24%; SEC: Mn=94.3 kg/mol, Mw/
Mn=1.42. The polymer was precipitated once into methanol from dichloromethane before SEC analysis.
(Mn,SEC=94.3 kg/mol) than that seen with the unprecipitated
PMMA-PS-1a diblock copolymer (Mn,SEC = 61.8 kg/mol),
and the molecular weight distribution was narrower for the
precipitated polymer as a result of fractionation of shorter
polymer chains (Mw/Mn=1.42 for PMMA-PS-1b compared
to Mw/Mn=1.97 for PMMA-PS-1a). The 1H NMR spectrum
of the precipitated diblock PMMA-PS-1b showed incorpo-
ration of both PS and PMMA blocks, which, in conjunction
with the monomodal SEC trace, corroborates formation of
the diblock.
appeared to allow greater control over MMA polymerization than
the previously examined phenyl ester-functional alkoxyamine 3.
As had previously been observed with other arylnitroxide-based
alkoxyamines,21 PS prepared with alkoxyamine 1 showed large
discrepancies between the theoretical and calculated molecular
weights and broad molecular weight distributions, suggesting
that N-phenylalkoxyamines are not broadly applicable to a wide
range of monomers for NMP. However, they are able to control
MMA polymerization to moderate conversions and allow chain
extension for the formation of block copolymers. A recent report
also suggests that N-arylalkoxyamines can also afford some degree
of control over the polymerization of acrylate monomers.36
The effect of the N-phenyl group of initiator 1 was probed by
the synthesis of the corresponding N-alkylnitroxide-based initia-
tor 2, which contained an N-tert-butyl group. Alkoxyamine 2 was
not able to control the polymerization of MMA, but controlled
the polymerization of styrene. These findings are corroborated by
our previous work, which showed that arylnitroxides are not
Conclusion
Alkoxyamine 1 was synthesized to ascertain the effect of the
para-nitrophenyl ester substituents on the polymerization of MMA
and styrene. Alkoxyamine 1 was able to control the polymeriza-
tion of MMA (300-1000 equiv) at conversions up to 50%
and molecular weights up to 65 kg/mol (vs PS standards) with
narrow molecular weight distributions (Mw/Mn=1.12-1.30) and

Article
Macromolecules, Vol. 43, No. 24, 2010 10325
capable of controlling the polymerization of styrene, but are
successful at controlling MMA polymerizations to moderate
conversions, while the corresponding N-tert-butylnitroxides con-
trol styrene polymerization but not methacrylate polymeriza-
tion.21 This indicates that the steric and electronic effects afforded
by the N-phenyl moiety in alkoxyamine 1 are paramount to achiev-
ing success with the NMP of MMA.
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Acknowledgment. The authors of this paper thank the NSF
(DMR-0239697, DMR-0804792) for partial financial support of
thiswork. The authorsadditionally thankDartmouthCollegefor
their support during the conduct of all of the research presented
here.
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