New sterically hindered nitroxides for the living free radical polymerization: X-ray structure of an α-H-bearing nitroxide

Article abstract of DOI:10.1021/ma035128q

The synthesis of three new sterically hindered Hawker-Braslau type alkoxyamines for the nitroxide-mediated living radical polymerization is described. Efficient polymerizations can be performed at 105°C. With alkoxyamine 11 controlled polymerization of n-

Full text of DOI:10.1021/ma035128q

Macromolecules 2004, 37, 27-34
27
New Sterically Hindered Nitroxides for the Living Free Radical
Polymerization: X-ray Structure of an R-H-Bearing Nitroxide
Ar m id o Stu d er ,* Kla u s Ha r m s, Ch r istop h Kn oop , Ch r istop h Mu1 ller , a n d
Tobia s Sch u lte
Department of Chemistry, Philipps-University Marburg,
Hans-Meerwein-Strasse, D-35032 Marburg, Germany
Received August 4, 2003; Revised Manuscript Received October 23, 2003
ABSTRACT: The synthesis of three new sterically hindered Hawker-Braslau type alkoxyamines for
the nitroxide-mediated living radical polymerization is described. Efficient polymerizations can be
performed at 105 °C. With alkoxyamine 11 controlled polymerization of n-butyl acrylate and styrene is
possible even at 90 °C. AA and AB diblock copolymers can be prepared in a controlled manner using
alkoxyamine 11. Kinetic EPR experiments for the determination of the C-O bond activation energies of
the new alkoxyamines are described. Furthermore, 1H NMR alkoxyamine decomposition studies are
presented. These kinetic experiments are used for the discussion of the polymerization results.
Furthermore, the X-ray structure of a Hawker-Braslau type nitroxide is presented. X-ray data and ESR
data are used to explain the high stability of these nitroxides.
In tr od u ction
Controlled free radical polymerizations have gained
renewed interest during the past few years. This is due
to the development of nitroxide-mediated polymeriza-
tions (NMP),1 atom transfer radical polymerizations
(ATRP),2 and RAFT polymerizations,3 which allow the
preparation of polymers with well-defined architectures.
The molecular weight can be controlled, and small
polydispersities are generally obtained. Furthermore,
block copolymers can be prepared using these new
methods. These polymerization techniques can also be
used to prepare new interesting materials.1,2 The control
of the polymerization in NMP and ATRP relies on the
principle of the persistent radical effect (PRE).4 For
example, in the NMP, reversible formation of a dormant
alkoxyamine from the corresponding nitroxide and the
chain growing polymer radical ensures a low concentra-
tion of free radicals during the entire poly-
merization. Low radical concentrations provide a low
fraction of irreversible termination via polymer radical
dimerization/disproportionation processes.
Alkoxyamines are generally used as initiators/regula-
tors in NMP.1 Many nitroxides have successfully been
used as regulators/initiators for controlled styrene po-
lymerization. However, controlled NMP of acrylates is
still difficult to achieve. Only a few reports on successful
acrylate polymerizations using alkoxyamines have ap-
peared to date. Tordo and co-workers introduced
alkoxyamine 1 as a highly efficient initiator/regulator
for the polymerization of acrylates (Figure 1).5,6 Hawker
and Braslau showed that readily prepared alkoxyamine
2 is also suitable for the controlled n-butyl acrylate
polymerization.7 In their detailed investigations they
studied the effect of the variation of the substituents
R1 and R2 (see 3). A tert-butyl group as in 2 was chosen
as the R3 substituent for most compounds. We assume
that the R3 group may heavily influence the efficiency
of the polymerization (kinetics, polydispersity, etc.).
Herein, we report the synthesis of new Hawker/Braslau
type alkoxyamines bearing bulky tertiary R3 substitu-
F igu r e 1. Alkoxyamines 1 and 2 successfully used for the
controlled acrylate polymerization.
ents. Their efficiency as regulators for the polymeriza-
tion of styrene and n-butyl acrylate will be discussed.
Furthermore, the X-ray structure of a Hawker/Braslau
type nitroxide will be presented.8,9 In addition, we will
discuss kinetic data of the C-O bond homolysis and
alkoxyamine decomposition studies.
Exp er im en ta l Section
Ma ter ia ls. Benzaldehyde (Fluka, 98+%), (1-bromoethyl)-
benzene (Aldrich, 97%), 2-bromopropane (Aldrich, 98%), 4,4′-
di-tert-butyl-2,2′-bipyridyl (Aldrich, 98%), N,N-dimethylacryl-
amide (Fluka, 98+%), hydrazine monohydrate (Fluka, 99%),
isobutyraldehyde (Aldrich, 98%), 2-nitropropane (Fluka, 96+%),
pyridinium toluene-4-sulfonate (Fluka, 99+%), (trimethylsilyl)-
diazomethane (Aldrich, 2 M in hexane), and Triton B (Fluka,
96+%) were used as received. Styrene (BASF) and n-butyl
acrylate (Fluka, 99+%) were both distilled under reduced
pressure from CaH2 to remove the stabilizer. Et2O was distilled
over K/Na, benzene and THF were distilled from sodium, and
CH2Cl2, 1,2-dimethoxyethane, and n-hexane were distilled
from CaH2 before use. All other chemicals were used as
received.
1
Gen er a l. H NMR and 13C NMR spectra were recorded on
a AMX 500 (500 MHz, Bruker), ARX 300 (300 MHz, Bruker),
or ARX 200 (200 MHz, Bruker). Chemical shifts δ in ppm
relative to SiMe4 as internal standard. TLC: silica gel 60 F254
plates (Merck); detection with UV or dipping into a solution
of KMnO4 (1.5 g in 333 mL of 1 m NaOH) or a solution of Ce-
(SO4)2‚H2O (10 g), phosphormolybdic acid hydrate (25 g),
concentrated H2SO4 (60 mL), and H2O (940 mL), followed by
heating. Flash chromatography: silica gel 60 (40-63 µm,
Merck or Fluka); at ca. 0.4 bar. Melting points: 510 apparatus
(Bu¨chi); uncorrected. IR spectra of the new compounds were
recorded on a IR 750 (Nicolet Magna) or a IFS-200 (Bruker).
* Corresponding author: e-mail studer@mailer.uni-marburg.de.
10.1021/ma035128q CCC: $27.50 © 2004 American Chemical Society
Published on Web 12/10/2003

28 Studer et al.
Macromolecules, Vol. 37, No. 1, 2004
Mass spectra were recorded as electrospray mass spectrometry
(ESI-MS) or field desorption mass spectrometry (FD-MS) on
a CH7 (Varian) and as HRMS on a 95S (MAT). Size exclusion
chromatography (SEC) was carried out with THF as eluent
at a flow rate of 1.0 mL/min at room temperature on a system
consisting of a L6200A Intelligent Pump (Merck Hitachi), a
set of two Plgel 5 µm MIXED-C columns (300 × 7.5 mm, 5 µm
pore size, molecular range 200-2 000 000 g/mol, Polymer
Laboratories), and a RI-101 detector (Shodex). Data were
acquired through a PL Datastream unit (Polymer Laborator-
ies) and analyzed with Cirrus GPC software (Polymer Labor-
atories) based upon calibration curves built upon polystyrene
and poly(methyl methacrylate) standards (Polymer Labora-
tories Polystyrene Medium MW Calibration Kit S-M-10 to
determine the molecular weight of polystyrene and Polymer
Laboratories Poly(methyl methacrylate) Medium MW Calibra-
tion Kit M-M-10 to determine the molecular weight of n-butyl
acrylate) with peak molecular weights ranging from 500 to
3 000 000 g/mol. EPR spectra were recorded on a ESP 300 E
(Bruker) equipped with a Nicolet Cavity (Bruker) and a B-TC
80/15 (Bruker). The nitroxide concentrations were determined
by double integration of the EPR spectra and calibration with
a TEMPO solution in tert-butylbenzene. Elemental analyses
were performed on a CHN-Rapid-Elementaranalysator (Her-
aeus) at the University of Marburg.
N,N-4-Tr im eth yl-4-n itr op en ta n a m id e (4). 2-Nitropro-
pane (15.2 mL, 168 mmol) was dissolved in 1,2-dimethoxy-
ethane (100 mL) and heated to 70 °C. Triton B (2.3 mL, 13
mmol) and N,N-dimethylacrylamide (14.8 mL, 143 mmol) were
added dropwise to the stirred solution in three portions. The
reaction mixture was stirred at 70 °C for 3 h. 1 N HCl was
added at room temperature, and the aqueous layer was
extracted with CH2Cl2 (3×). The collected organic layers were
washed with brine and dried over MgSO4. The solvent was
removed under reduced pressure, and the crude product was
dried in vacuo to afford 4 (26.87 g, 85%) as a green oil which
was used without further purification. IR (film): 3478 w, 2989
w, 2939 w, 1648 s, 1536 s, 1499 m, 1472 m, 1458 m, 1399 s,
1373 m, 1348 m, 1263 w, 1143 w, 1062 w, 845 w cm-1. 1H NMR
(200 MHz, CDCl3): δ ) 3.00 (s, 3H, N(CH3)2), 2.95 (s, 3H,
N(CH3)2), 2.29 (s, 4H, CH2), 1.62 (s, 6H, C(CH3)2). 13C NMR
(50 MHz, CDCl3): δ ) 171.3 (CO), 88.0 (C(CH3)2), 37.4 (CH2),
36.1 (N(CH3)2), 35.8 (CH2), 29.0 (N(CH3)2), 26.2 (C(CH3)2). MS
(EI): 188 (2, [M]+), 142 (48), 72 (100), 44 (26). HRMS (EI) Calcd
for C8H16N2O3 (M+): 188.1161. Found: 188.1159.
phenylmagnesium bromide (prepared using Mg (1.17 g, 48.2
mmol) and phenyl bromide (2.54 mL, 24.1 mmol) in Et2O (25.5
mL)) was slowly added to the nitrone solution at 0 °C. The
reaction mixture was allowed to warm to room temperature,
and stirring was continued for 12 h. Saturated NH4Cl and H2O
were added at 0 °C, and the mixture was extracted with Et2O
(3×). The collected organic layers were dried over MgSO4, and
the solvent was removed under reduced pressure. Recrystal-
lization from pentane/CH2Cl2 afforded the hydoxylamine (3.64
g, 54%) as colorless plates; mp 111 °C. IR (KBr): 3333 w, 2988
w, 2971 w, 2953 w, 2922 w, 2866 w, 1618 s, 1488 m, 1468 m,
1454 m, 1423 m, 1404 m, 1358 m, 1305 m, 1289 m, 1266 m,
1134 m, 1073 m, 951 w, 733 s, 702 s cm-1. 1H NMR (200 MHz,
CDCl3): δ ) 7.43-7.22 (m, 5H, CH arom), 3.14 (d, J ) 10.0
Hz, 1H, NCH), 3.06 (s, 3H, N(CH3)2), 2.99 (s, 3H, N(CH3)2),
2.62-2.02 (m, 4H, CH2), 1.15 (dd, J ) 5.5 Hz, 1H, CH(CH3)2),
1.12 (s, 3H, C(CH3)2), 0.93 (d, J ) 6.8 Hz, 3H, CH(CH3)2), 0.53
(d, J ) 6.8 Hz, 3H, CH(CH3)2), 0.47 (s, 3H, C(CH3)2). 13C NMR
(75 MHz, CDCl3): δ ) 169.3 (CO), 143.5 (C arom), 130.4 (CH
arom), 128.0 (CH arom), 126.8 (CH arom), 71.5 (NCH), 60.6
(C(CH3)2), 37.7 (N(CH3)2), 36.5 (CH2), 36.3 (N(CH3)2), 32.0 (CH-
(CH3)2), 27.8 (C(CH3)2), 24.9 (CH2), 22.5 (CH(CH3)2), 21.2
(C(CH3)2), 21.0 (CH(CH3)2). MS (EI): 306 (7, [M]+), 263 (20),
247(10), 218 (40), 142 (100), 91 (38), 87 (42), 46 (49). Anal.
Calcd for C18H30N2O2 (306.443): C, 70.55; H, 9.87; N, 9.14.
Found: C, 70.17; H, 9.92; N, 9.05.
The hydroxylamine (1.70 g, 5.54 mmol) was treated with a
mixture of MeOH (23 mL), concentrated NH4OH (0.9 mL), and
Cu(OAc)2 (55.0 mg, 0.28 mmol). A stream of oxygen was
bubbled through the solution for 1 h. The reaction mixture
was treated with 2 N NH4OH (143 mL) and extracted with
CH2Cl2 (4×). The combined organic layers were dried over
MgSO4, and the solvent was removed under reduced pressure.
Drying in vacuo afforded the pure nitroxide 6 as an orange
solid in quantitative yield; mp 64 °C. IR (KBr): 3441 w, 2976
w, 2964 w, 2913 w, 2871 w, 1634 s, 1492 m, 1453 m, 1415 m,
1396 m, 1365 m, 1128 m, 1068 m, 741 s, 700 s cm-1. EPR
data: RN ) 14.44 G, RHâ ) 2.40 G, g ) 2.006. MS (EI): 305 (2,
[M]+), 218 (3), 142 (100), 133 (17), 91 (20), 87 (61), 72 (44), 46
(32). HRMS (EI) Calcd for
Found: 305.2233.
C
18H29N2O2 (M+): 305.2229.
N,N-4-Tr im eth yl-4-[(2-m eth yl-1-p h en ylp r op yl)(1-p h en -
yleth oxy)a m in o]p en ta n e Am id e (7). (1-Bromoethyl)benzene
(0.49 mL, 3.57 mmol), nitroxide 6 (1.20 g, 3.93 mmol), Cu
powder (238 mg, 3.75 mmol), Cu(OTf)2 (11.0 mg, 0.04 mmol),
and 4,4′-di-tert-butyl-2,2′-bipyridyl (19.0 mg, 0.14 mmol) were
suspended under argon in benzene (14 mL). In a sealed tube,
the reaction mixture was stirred at 65-70 °C for 20 h. The
solids were removed by filtration over silica gel (washing with
CH2Cl2), and the solvent was removed under reduced pressure.
Purification by flash chromatography (pentane/ethyl acetate,
3:1) afforded 7 (968 mg, 66%) as a 1:1 mixture of diastereo-
isomers as a colorless oil. IR (film): 3469 w, 3026 w, 2974 w,
2868 w, 1650 s, 1492 m, 1453 m, 1396 m, 1382 m, 1137 m,
[4-(Dim eth yla m in o)-1,1-d im eth yl-4-oxobu tyl][2-m eth -
ylp r op ylid en e]a m m on iu m ola te (5). Compound 4 (10.0 g,
53.1 mmol), isobutyraldehyde (9.6 mL, 0.1 mol), and NH4Cl
(3.12 g, 58.4 mmol) were dissolved in a mixture of Et2O (95
mL) and H2O (257 mL) and cooled to 0 °C. Zinc powder (13.89
g, 0.212 mol) was added in small portions over a period of 1 h
at 0 °C, and the reaction mixture was vigorously stirred at
room temperature for 3 days. The heterogeneous mixture was
filtered through a sintered glass filter, and the residue was
washed with MeOH (4×). The solution was extracted with CH2-
Cl2 (5×), and the collected organic layers were dried over
MgSO4. The solvent was removed under reduced pressure and
the crude product was dried in vacuo. Purification by flash
chromatography (CH2Cl2/MeOH, 19:1) afforded 5 (8.68 g, 72%)
as a colorless oil. IR (film): 3474 w, 2964 w, 2935 w, 2872 w,
1645 s, 1577 m, 1499 m, 1467 m, 1397 s, 1357 m, 1263 w, 1169
1059 m, 761 s, 702 s cm-1 1H NMR (300 MHz, CDCl3, both
.
diastereoisomers): δ ) 7.50-7.17 (m, 10H, CH arom), 4.96-
4.87 (m, 1H, CH), 3.42 (d, J ) 10.5 Hz, 1H, CH), 3.28 (d, J )
10.7 Hz, 1H, CH), 2.95 (s, 3H, CH3), 2.89 (s, 3H, CH3), 2.84 (s,
3H, CH3), 2.43-1.78 (m, 4H, CH2), 1.65 (d, J ) 6.6 Hz, 3H,
CH3), 1.57 (d, J ) 6.6 Hz, 3H, CH3), 1.35 (d, J ) 6.3 Hz, 3H,
CH3), 1.06 (s, 3H, CH3), 1.00 (s, 3H, CH3), 0.80 (s, 3H, CH3),
0.72 (s, 3H, CH3), 0.57 (d, J ) 6.6 Hz, 3H, CH3), 0.24 (d, J )
6.3 Hz, 3H, CH3). 13C NMR (75 MHz, CDCl3, both diastereo-
isomers): δ ) 173.9 (C), 142.6 (C), 131.3 (CH), 131.2 (CH),
128.5 (CH), 127.8 (CH), 127.7 (CH), 127.3 (CH), 126.8 (CH),
126.6 (CH), 126.5 (CH), 83.7 (CH), 83.2 (CH), 72.3 (CH), 72.2
(CH), 62.8 (C), 62.7 (C), 37.4 (CH3), 36.7 (CH2), 36.5 (CH2),
35.8 (CH3), 32.6 (CH3), 32.2 (CH3), 28.7 (CH2), 28.5 (CH2), 27.2
(CH3), 26.9 (CH3), 26.0 (CH3), 25.6 (CH3), 25.1 (CH3), 23.7
(CH3), 22.5 (CH3), 22.4 (CH3), 21.6 (CH3), 21.4 (CH3), 20.0
(CH2). MS (FD): 410 (100, [M]+, 305 (60), 105 (20). MS (ESI):
328 (100, [M-C8H9 + Na]+), 285 (30), 195 (45), 165 (10), 142
(40), 109 (10). HRMS (ESI) Calcd for C18H29N2O2 (M-C8H9+):
305.2229. Found: 305.2230.
m, 1141 w, 11103 m, 1063 w cm-1 1H NMR (200 MHz,
.
CDCl3): δ ) 6.61 (d, J ) 7.0 Hz, 1H, NCH), 3.15 (hept, J )
7.0 Hz, 1H, CH(CH3)2), 3.01 (s, 3H, N(CH3)2), 2.92 (s, 3H,
N(CH3)2), 2.30-2.06 (m, 4H, CH2), 1.48 (s, 6H, C(CH3)2), 1.05
(d, J ) 6.8 Hz, 6H, CH(CH3)2). 13C NMR (50 MHz, CDCl3): δ
) 172.5 (CO), 141.3 (NCH), 71.1 (C(CH3)2), 37.6 (CH2), 35.7
(N(CH3)2), 35.6 (CH2), 28.7 (N(CH3)2), 26.6 (C(CH3)2), 26.4 (CH-
(CH3)2), 19.3 (CH(CH3)2). MS (EI): 228 (8, [M]+), 183 (18), 142
(74), 112 (12), 97 (10), 72 (100), 46 (22). HRMS (EI) Calcd for
C
12H24N2O2 (M+): 228.1833. Found: 228.1835.
N,N-4-Tr im eth yl-4-[(2-m eth yl-1-p h en ylp r op yl)a m in o]-
p en t a n a m id e-N-oxyl R a d ica l (6). Nitrone 5 (5.00 g, 21.9
mmol) and LiBr (19.0 g, 218 mmol) were dissolved in THF (70
mL), and the solution was cooled to 0 °C. A solution of

Macromolecules, Vol. 37, No. 1, 2004
New Sterically Hindered Nitroxides 29
(1,1-Diet h ylp r op yl)[p h en ylm et h ylid en e]a m m on iu m -
ola te (9). 3-Ethyl-3-nitropentane (8)10 (1.22 g, 8.41 mmol),
benzaldehyde (1.34 g, 12.6 mmol), and NH4Cl (0.50 g, 9.25
mmol) were dissolved in a mixture of Et2O (5 mL) and H2O
(10 mL) and cooled to 0 °C. Zinc powder (2.20 g, 33.6 mmol)
was added in small portions over a period of 1 h, and the
reaction mixture was vigorously stirred at room temperature
for 19 h. The heterogeneous mixture was filtered through a
sintered glass filter, and the residue was washed with Et2O.
The solution was extracted with Et2O (3×), and the collected
organic layers were dried over MgSO4. The solvent was
removed under reduced pressure, and the crude product was
dried in vacuo. Purification by flash chromatography (pentane/
MTBE, 9:1) afforded 9 (0.98 g, 53%) as colorless crystals; mp
38 °C. IR (KBr): 3447 w, 3019 w, 2968 w, 2944 w, 2879 w,
1574 s, 1550 s, 1447 s, 1405 s, 1378 m, 1322 m, 1124 s, 799 s,
atmosphere at 65-70 °C for 15 h. The solids were removed by
filtration over silica gel (washing with CH2Cl2). Purification
by flash chromatography (pentane/MTBE, 200:1) afforded 11
(1.66 g, 72%) as a 1:1 mixture of diastereoisomers as a colorless
oil. IR (film): 3085 w, 3061 w, 3026 w, 2971 w, 2878 w, 1944
w, 1873 w, 1803 w, 1601 s, 1493 m, 1453 s, 1381 s, 1057 s,
1028 m, 923 m, 910 m, 890 m, 759 s, 700 s cm-1 1H NMR
.
(200 MHz, CDCl3, both diastereoisomers): δ ) 7.95-7.13 (m,
10H, CH arom), 4.88 (q, J ) 6.6 Hz, 1H, CH), 3.45 (d, J )
10.8 Hz, CH), 3.32 (d, J ) 10.8 Hz, CH), 2.44-2.32 (m, CH),
1.63 (d, J ) 6.6 Hz, CH3), 1.54 (d, J ) 6.6 Hz, CH3), 1.49-1.26
(m, CH2, CH3), 1.23 (d, J ) 6.3 Hz, CH3), 1.19-1.01 (m, CH2,
CH3), 0.95 (d, J ) 6.3 Hz, CH3), 0.76 (t, J ) 7.5 Hz, CH3), 0.55
(t, J ) 7.5 Hz, CH3), 0.53 (d, J ) 6.3 Hz, CH3), 0.21 (d, J ) 6.3
Hz, CH3). 13C NMR (75 MHz, CDCl3, both diastereoisomers):
δ ) 145.81 (C), 143.31 (C), 131.04 (CH), 128.03 (CH), 127.21
(CH), 127.01 (CH), 126.87 (CH), 126.23 (CH), 83.94 (CH), 83.27
(CH), 71.39 (CH), 71.28 (CH), 67.61 (C), 32.77 (CH3), 27.66
(CH2), 24.89 (CH3), 22.36 (CH3), 21.53 (CH3), 8.94 (CH3). MS
(EI): 367 (0.2, [M]+), 264 (58), 262 (89), 235 (24), 221 (21), 220
(72), 174 (32), 165 (20), 164 (47), 134 (50), 133 (86), 130 (29),
122 (51), 105 (46), 99 (46), 91 (39). HRMS (ESI) Calcd for
755 m, 695 s cm-1 1H NMR (200 MHz, CDCl3): δ ) 8.34-
.
8.27 (m, 2H, CH arom), 7.45-7.36 (m, 4H, CH arom), 1.91 (q,
J ) 7.4 Hz, 6H, CH2CH3), 0.85 (t, J ) 7.4 Hz, 9H, CH2CH3).
13C NMR (75 MHz, CDCl3): δ ) 131.85 (C(CH2CH3)3), 129.95
(C arom), 128.86 (CH arom), 128.41 (CH arom), 80.10 (NCH),
26.58 (CH2CH3), 8.39 (CH2CH3). MS (ESI): 220 (20, [M + H]+),
195 (10), 176 (15), 163 (10), 154 (20), 144 (10), 123 (12), 122
(100), 99 (10), 57 (15). HRMS (ESI) Calcd for C14H22NO (M+):
219.1622. Found: 219.1624.
C
25H37NO (M+): 367.2875. Found: 367.2865.
1-(2-Meth yl-1-p h en ylp r op yl)h yd r a zin e (12). (1-Bromo-
2-methylpropyl)benzene11 (7.32 g, 34.4 mmol) was dissolved
at room temperature in MeOH (75 mL). Hydrazine monohy-
drate (200 mL, 4.12 mol) was added dropwise to the stirred
solution, and the reaction mixture was stirred for 12 h at room
temperature. The solution was extracted with Et2O (2×), and
the collected organic layers were washed with brine and were
dried over MgSO4. The solvent was removed under reduced
pressure, and the crude product was dried in vacuo to afford
pure 12 (5.48 g, 97%) as a pale yellow oil. The product obtained
was directly used in the next step. 1H NMR (200 MHz,
CDCl3): δ ) 7.33-7.26 (m, 5H, CH arom.), 3.36 (d, J ) 7.2
Hz, 1H, CH(NHNH2)), 3.08 (bs, 3H, NHNH2), 1.87 (oct, J )
7.0 Hz, 1H, CH(CH3)2), 1.00 (d, J ) 6.6 Hz, 3H, CH(CH3)2),
0.75 (d, J ) 6.8 Hz, 3H, CH(CH3)2). MS (EI): 164 (1, [M]+),
133 (31), 121 (6), 107 (96), 91 (22), 77 (41).
N-(1,1-Dieth ylp r op yl)-R-(1-m eth yleth yl)ben zen em eth -
a n a m in e-N-oxyl Ra d ica l (10). Nitrone 9 (3.60 g, 16.4 mmol)
was dissolved in Et2O (40 mL) and cooled to 0 °C. A Grignard
solution (prepared with Mg (1.40 g, 57.5 mmol) and 2-bro-
mopropane (3.84 mL, 41.0 mmol) in Et2O (38 mL)) was added
dropwise to the solution at 0 °C. The reaction mixture was
stirred for 2 h under reflux. Saturated NH4Cl was added, and
the aqueous layer was extracted with Et2O (3×). The combined
organic layers were washed with brine and dried over MgSO4.
The solvent was removed under reduced pressure, and the
crude product was dried in vacuo. Purification by flash
chromatography (pentane/ethyl acetate, 9:1) afforded the pure
hydroxylamine (2.60 g, 60%) as an orange oil. IR (film): 3465
w, 3062 w, 3026 w, 2967 w, 2938 w, 2879 w, 1643 s, 1491 m,
1455 s, 1382 m, 1166 w, 1073 w, 1029 w, 748 s, 719 w, 701 s
cm-1. 1H NMR (200 MHz, CDCl3): δ ) 8.19 (s, 1H, CH arom.),
7.76-7.74 (m, 2H, CH arom), 7.40-7.39 (m, 2H, CH arom),
3.35 (d, J ) 9.6 Hz, 1H, NCH), 2.37-2.27 (m, 1H, CH(CH3)2),
1.62 (q, J ) 7.3 Hz, 4H, CH2CH3), 1.36-1.19 (m, 2H, CH2-
CH3), 1.13 (d, J ) 9.9 Hz, 3H, CH(CH3)2), 0.83-0.74 (m, 9H,
CH2CH3), 0.57 (d, J ) 9.9 Hz, 3H, CH(CH3)2). 13C NMR (75
MHz, CDCl3): δ ) 143.6 (C(CH2CH3)3), 131.1 (C arom), 130.5
(CH arom), 129.3 (CH arom), 128.9 (CH arom), 128.1 (CH
arom), 127.4 (CH arom), 126.9 (CH arom), 70.4 (NCH), 66.1
(CH(CH3)2), 32.6 (CH(CH3)2), 28.9 (CH2CH3), 27.7 (CH2CH3),
26.9 (CH2CH3), 22.0 (CH2CH3), 21.2 (CH2CH3), 9.4 (CH2CH3),
8.3 (CH2CH3). MS (EI): 262 (2, [M-H]+), 134 (5), 133 (37),
122 (17), 98 (14), 89 (29), 61 (100), 54 (10). HRMS (EI) Calcd
for C17H29NO (M+): 263.2249. Found: 263.2242.
1-{2-Me t h yl-1-[(2-m e t h yl-1-p h e n ylp r op oxy)(1,1,3,3-
tetr a m eth ylbu tyl)a m in o]p r op yl}ben zen e (14). A suspen-
sion of PbO2 (5.85 g, 24.4 mmol) in hexane (20 mL) was treated
for 1 h by ultrasonic. A solution of nitroso-tert-octane 1312 (1.00
g, 6.98 mmol) in hexane (20 mL) was stirred for 1 h and added
to the stirred PbO2 suspension, and the reaction mixture was
cooled to 0 °C. Hydrazine (12) (4.01 g, 24.4 mmol) was added
dropwise, and the suspension was stirred for 18 h at room
temperature. The mixture was filtered, and the solvent was
removed under reduced pressure. Purification by flash chro-
matography (pentane/MTBE, 200:1) afforded 14 (1.99 g, 70%)
as a 1.4:1 mixture of diastereoisomers as a colorless viscous
oil. IR (film): 2955 w, 1469 m, 1452 m, 1382 m, 1364 m, 1241
1
w, 1072 w, 1000 m, 758 m, 733 w, 701 s cm-1. H NMR (300
MHz, CDCl3, both diastereoisomers): δ ) 7.48-7.13 (m, 10H,
CH arom), 4.77 (bs, CH) 4.66 (d, J ) 6.3 Hz, CH), 3.46 (d, J )
10.3 Hz, CH), 3.27 (d, J ) 10.6 Hz, CH), 2.74-2.64 (m, CH),
2.59-2.46 (m, CH), 2.37-2.26 (m, CH), 1.90 (s, CH2), 1.86 (s,
CH2), 1.66 (s, CH2), 1.61-0.79 (m, CH3), 0.63 (s, 3H, CH3), 0.57
(d, J ) 6.6 Hz, 3H, CH3), 0.15 (d, J ) 6.6 Hz, CH3), 0.07 (s,
CH3). 13C NMR (75 MHz, CDCl3, both diastereoisomers): δ )
142.09 (C), 139.18 (C), 131.5 (CH), 131.05 (CH), 129.57 (CH),
129.20 (CH), 129.09 (CH), 128.88 (CH), 128.54 (CH), 127.94
(CH), 127.80 (CH), 127.72 (CH), 127.55 (CH), 127.48 (CH),
126.91 (CH), 126.79 (CH), 126.50 (CH), 126.09 (CH), 93.08
(CH), 91.98 (CH), 72.59 (CH), 72.42 (CH), 66.39 (C), 65.38 (C),
53.44 (CH), 51.31 (CH), 51.16 (CH), 46.18 (CH), 33.04 (CH3),
32.94 (CH3), 32.70 (CH3), 32.61 (CH3), 32.35 (CH3), 29.15 (CH3),
29.07 (CH3), 28.78 (CH3), 28.27 (CH3), 23.07 (CH3), 22.87 (CH3),
22.56 (CH3), 22.10 (CH3), 21.64 (CH3), 21.15 (CH3), 21.08 (CH3),
18.02 (CH3), 16.73 (CH3). MS (ESI): 410 (8, [M + H]+), 298
(49), 242 (67), 227 (48), 196 (6), 195 (100), 179 (6), 163 (13),
133 (15), 65 (10). Anal. Calcd for C28H43NO (409.647): C, 82.09;
H, 10.58; N, 3.42. Found: C, 82.38; H, 10.40; N, 3.55.
The hydroxylamine (2.53 g, 9.60 mmol) was treated with a
mixture of MeOH (40 mL), concentrated NH4OH (1.5 mL), and
Cu(OAc)2 (96.0 mg, 0.48 mmol). A stream of oxygen was
bubbled through the solution for 1 h. The reaction mixture
was treated with 2 N NH4OH (288 mL) and extracted with
CH2Cl2 (3×). The combined organic layers were dried over
MgSO4, and the solvent was removed under reduced pressure.
Drying in vacuo afforded the pure nitroxide 10 (2.19 g, 87%)
as a red oil. IR (film): 3062 m, 3026 m, 2967 w, 2879 w, 1705
m, 1643 m, 1491 m, 1454 w, 1382 s, 1155 m, 1072 m, 915 m,
748 s, 719 m, 701 s cm-1. EPR data: RN ) 14.21 G, RHâ ) 2.62
G, g ) 2.006. MS (EI): 262 (2, [M]+), 134 (6), 133 (50), 122 (8),
98 (24), 89 (36), 61 (100), 54 (12). HRMS (EI) Calcd for C17H28
-
NO (M+): 262.2171. Found: 262.2165.
1-{1-[(1,1-Die t h ylp r op yl)(1-p h e n yle t h oxy)a m in o]-2-
m et h ylp r op yl}ben zen e (11). (1-Bromoethyl)benzene (0.85
mL, 6.25 mmol), nitroxide 10 (1.80 g, 6.88 mmol), Cu powder
(417 mg, 6.57 mmol), Cu(OTf)2 (20.0 mg, 0.06 mmol), and 4,4′-
di-tert-butyl-2,2′-bipyridyl (34.0 mg, 0.25 mmol) were sus-
pended under an argon atmosphere in benzene (15 mL). In a
sealed tube, the reaction mixture was stirred under an argon
Typ ica l P r oced u r e for th e P olym er iza tion of Styr en e.
A Schlenk tube was charged with the alkoxyamine initiator

30 Studer et al.
Macromolecules, Vol. 37, No. 1, 2004
11 (53.0 mg, 0.14 mmol) and styrene (1.65 mL, 14.4 mmol).
The tube was subjected to three freeze-thaw cycles and sealed
off under argon. The polymerization was carried out under
argon at 105 °C for 15 h. The resulting mixture was cooled to
room temperature, dissolved in CH2Cl2, and poured into an
aluminum dish, and residual monomer was removed in a
vacuum-drying cabinet at 60 °C for 12 h. Conversion was
evaluated gravimetrically; molecular weight and polydispersity
index (PDI) were determined by size exclusion chromatography
(SEC). Conversion ) 76%; Mn ) 7500 g mol-1; PDI ) 1.12.
Sch em e 1. Syn th esis of th e Alk oxya m in es 7, 11,
a n d 14
Typ ica l P r oced u r e for th e P olym er iza tion of n -Bu tyl
Acr yla te. A Schlenk tube was charged with the alkoxyamine
initiator 11 (12.0 mg, 0.03 mmol), nitroxide 10 (0.40 mg, 1.60
µmol), and n-butyl acrylate (1.18 mL, 8.25 mmol). The tube
was subjected to three freeze-thaw cycles and sealed off under
argon. The polymerization was carried out under argon at 105
°C for 14 h. The resulting mixture was cooled to room
temperature, dissolved in CH2Cl2, and poured into an alumi-
num dish, and residual monomer was removed in a vacuum-
drying cabinet at 60 °C for 12 h. Conversion was evaluated
gravimetrically; molecular weight and polydispersity index
(PDI) were determined by size exclusion chromatography
(SEC). Conversion ) 57%; Mn ) 14 500 g mol-1; PDI ) 1.19.
Resu lts a n d Discu ssion
Syn th esis of th e Alk oxya m in es 7, 11, a n d 14. The
tertiary nitroalkane 4 was readily prepared from 2-ni-
tropropane and N,N-dimethylacrylamide.13 Reduction
of the nitroalkane to the corresponding hydroxylamine
and subsequent nitrone formation using i-PrCHO af-
forded 5 in 72% yield (Scheme 1). Phenyl Grignard
addition and oxidation provided nitroxide 6 (54%).
Alkoxyamine formation according to a literature pro-
cedure14 yielded alkoxyamine 7 (66%). Alkoxyamine 11
was prepared using a similar strategy. Triethylni-
tromethane (8) was readily prepared via a Ritter reac-
tion from the corresponding tertiary alcohol.10 Nitrone
formation was achieved with activated Zn and nitroal-
kane 8 in the presence of benzaldehyde (f 9, 53%).
Grignard addition and oxidation (f 10, 52%) followed
by alkoxyamine formation afforded initiator/regulator
11 (72%). Alkoxyamine 14 was prepared in a single
operation by the reaction of hydrazine (12)11 with nitroso
compound 13 in the presence of PbO2 according to Corey
et al. (70%).12,15,16 Nitroxide 16 was prepared according
to literature procedures,7 and nitroxide 15 was prepared
in analogy.
P olym er iza tion Stu d ies. The polymerization of
styrene was studied first. The initial polymerizations
were conducted in sealed tubes using 1% of the
alkoxyamine initiator at 105 and 125 °C and were
stopped after 6 or 15 h, respectively. The conversion was
determined gravimetrically. The PDI and the molecular
weight of the polymers were analyzed using SEC. The
results are summarized in Table 1.
Styrene polymerization using alkoxyamine 7 at 125
°C afforded PS with a narrow PDI in 53% conversion
(entry 1). Polymerization can also be performed at 105
°C with initiator/regulator 7 (entry 2, 56%, PDI ) 1.13).
With alkoxyamine 11 polymerization at 125 °C for 6 h
was fast (87% conversion), however, not well controlled
(PDI ) 1.32, entry 3). Probably, nitroxide 10 or dormant
polymer alkoxyamines derived from 10 are not suf-
ficiently stable at 125 °C. Indeed, polymerization at 105
°C afforded PS with narrow PDI and high conversion
(entry 4). Styrene polymerization using tert-octyl alkoxy-
amine 14 at 125 °C afforded PS with a PDI of 1.28 (entry
9). Again, lowering the reaction temperature to 105 °C
provided a better control of the polymerization (PDI )
1.16, entry 10). With known7 alkoxyamine 2 good control
of the polymerization was obtained at 105 and 125 °C;
however, compared with 11 lower conversions were
obtained (entries 11 and 12).
We can conclude from these initial studies that
similar results in terms of conversion and control were
obtained for the Hawker/Braslau alkoxyamine 27 and
the amide containing alkoxyamine 7 (compare entries
1, 2 with 11, 12). The replacement of the tert-butyl group
(R3 substituent in 3) by a 3-dimethylaminocarbonyl-1,1-
dimethylpropyl group has no significant effect on the
styrene polymerization. However, replacing the tert-
butyl group with the 1,1-diethylpropyl group in going
from alkoxyamine 2 to 11 heavily influences the poly-
merization outcome. As compared to the 2-mediated
styrene polymerizations, reactions with alkoxyamine 11
are faster and higher conversions can be obtained.

Macromolecules, Vol. 37, No. 1, 2004
New Sterically Hindered Nitroxides 31
Ta ble 1. Bu lk Styr en e P olym er iza tion Usin g
Alk oxya m in es 7, 11, a n d 14a
alkoxyamine temp time conv Mn,th
Mn,exp
entry
(mol %)
(°C)
(h)
(%) (g/mol) (g/mol) PDI
1
2
3
4
5
6
7
8
7 (1%)
7 (1%)
125
105
125
105
105
105
105
90
125
105
125
105
6
15
6
15
15
15
15
53
6
53
56
87
77
74
73
58
73
79
59
63
51
5500
5800
3600 1.15
6600 1.13
11 (1%)
11 (1%)
11 (0.6%)
11 (0.4%)
11 (0.1%)
11 (1%)
14 (1%)
14 (1%)
2 (1%)
9100 10800 1.32
8000 9800 1.14
12800 16700 1.14
19000 21500 1.13
60400 68200 1.28
7600
9800 1.09
9
8200 11700 1.28
10
11
12
15
6
15
6100
6600
5300
8800 1.16
5000 1.16
6400 1.17
2 (1%)
a
For comparison, some polymerizations were conducted using
known7 alkoxyamine 2.
However, an increase of the polydispersity was observed
if the polymerization was conducted at 125 °C. There-
fore, polymerizations using alkoxyamine 11 are best
conducted at 105 °C. At 105 °C the tert-octyl alkoxyamine
14 is a more efficient initiator/regulator than the parent
compound 2, however, not as efficient as regulator 11.
We therefore decided to continue our studies with the
most efficient initiator 11. We first attempted to prepare
higher molecular weight polystyrene (entries 5-7).
Polystyrene with an Mn of 68 000 g/mol and satisfactory
PDI was obtained. We found that styrene polymeriza-
tion can also be performed at 90 °C (entry 8, 53 h, 73%
conversion, PDI ) 1.09).
To check the controlled/living character of the styrene
polymerization with alkoxyamine 11, conversion was
plotted as a function of time, and the molecular weight
was plotted as a function of monomer conversion (Figure
2). The linear increase of ln([M0]/[M]) vs time and
molecular weight vs monomer consumption proves the
controlled character of the 11-mediated styrene poly-
merization.
We then started to look at the controlled polymeri-
zation of n-butyl acrylate using the new alkoxyamines
7, 11, and 14. The polymerizations were conducted at
125 and 105 °C in neat n-butyl acrylate in the presence
of the corresponding nitroxide (5% with respect to the
alkoxyamine). The addition of nitroxide improves the
control of the acrylate polymerization for other alkoxy-
amines.7,17
n-Butyl acrylate polymerization using 7 at 125 °C
afforded a 41% conversion (entry 1, PDI ) 1.20). With
alkoxyamine 11 a higher conversion and slightly broader
PDI was obtained (entry 2). Decreasing the initiator
concentration under otherwise identical conditions pro-
vided poly(n-butyl acrylate) with a Mn of 33 400 g/mol
and a PDI of 1.22 (entry 3). With the tert-octyl alkoxy-
amine 14 good control and acceptable conversion were
obtained (entry 8, 49% conversion, PDI ) 1.18). Poly-
merization using the known7 alkoxyamine 2 provided
a similar result (entry 9, 48% conversion PDI ) 1.21).
In analogy to the styrene polymerizations described
above, alkoxyamines 7 and 14 and the parent alkoxyamine
2 provide similar results. With the triethyl derivative
11 as an initiator/regulator higher conversions were
obtained. Therefore, some additional polymerizations
were performed using the most efficient regulator 11.
First we tested whether polymer formation can be
conducted at lower temperature. Indeed, polymerization
at 105 °C for 14 h provided poly(n-butyl acrylate) with
a narrow PDI (1.19, 57% conversion, entry 4). Decreas-
F igu r e 2. (a) Monomer conversion vs time (styrene, bulk, 105
°C, 1 mol % 11). (b) Molecular weight (experimental and
theoretical) vs monomer conversion (styrene, bulk, 105 °C, 1
mol % 11, 15 h).
ing the amount of added nitroxide resulted in a faster
but still controlled polymerization (entry 5). A broader
PDI was obtained upon targeting higher molecular
weight poly(n-butyl acrylate) (entry 6). Controlled po-
lymerization of n-butyl acrylate was achieved at 90 °C
(56% conversion, 72 h, PDI ) 1.16, entry 7). With the
alkoxyamines 7 and 14 n-butyl acrylate polymerization
could not be conducted at 105 °C. Conversions were low
and results were not reproducible.
We then focused on the formation of block copolymers
using alkoxyamine 11. Poly(n-butyl acrylate) prepared
with 11 (Mn ) 15 100 g/mol, PDI ) 1.15) was used as
an initiator (0.2 mol %) for the neat styrene polymeri-
zation. Reaction for 15 h at 105 °C afforded poly(n-butyl
acrylate)-block-polystyrene with good control (Mn
)
40 800 g/mol, PDI ) 1.26, 58% conversion). Further-
more, polystyrene (Mn ) 7500 g/mol, PDI ) 1.12) was
successfully used as a macroinitiator for the neat
styrene polymerization (105 °C, 15 h, 0.4 mol %) to
provide polystyrene with a Mn of 21 800 g/mol and a
PDI of 1.17. However, the formation of polystyrene-
block-poly(n-butyl acrylate) using a polystyrene macro-
initiator failed. Similar observations have previously
been described for the diblock copolymer preparation
using the parent alkoxyamine 2.7
Kin etics of th e C-O Bon d Hom olysis a n d De-
com p osition Ra te Con sta n ts. Kinetic experiments to
determine the C-O-bond homolysis rate constants were
conducted in tert-butylbenzene at 403 K. Oxygen was
used to scavenge the styryl radical, and the concentra-
tion of the released nitroxide was measured by EPR

32 Studer et al.
Macromolecules, Vol. 37, No. 1, 2004
Ta ble 2. Bu lk n -Bu tyl Acr yla te P olym er iza tion Usin g Alk oxya m in es 7, 11, a n d 14a
entry
alkoxyamine (mol %)
nitroxide (mol %)
temp (°C)
time (h)
conv (%)
Mn,th (g/mol)
Mn,exp (g/mol)
PDI
1
2
3
4
5
6
7
8
9
7 (1%)
11 (1%)
11 (0.4%)
11 (1%)
11 (1%)
11 (0.6%)
11 (1%)
14 (1%)
2 (1%)
6 (0.05%)
10 (0.05%)
10 (0.02%)
10 (0.05%)
10 (0.02%)
10 (0.03%)
10 (0.05%)
15 (0.05%)
16 (0.05%)
125
125
125
105
105
105
90
6
6
6
14
15
14
72
6
41
84
82
57
84
89
56
49
48
5300
10500
26300
7300
10800
19000
7200
8000
18000
33400
14500
15100
43400
10200
13500
9500
1.20
1.23
1.22
1.19
1.15
1.45
1.16
1.18
1.21
125
125
6300
6200
6
a
For comparison, a polymerization was conducted using known7 alkoxyamine 2.
Ta ble 3. Ra te Con sta n ts for th e C-O Bon d Hom olysis a n d Decom p osition Ra te Con sta n ts of Alk oxya m in es 7, 11, a n d 14
a n d EP R P a r a m eter s of th e Cor r esp on d in g Nitr oxid es
a
c
entry
alkoxyamine
kd (s-1
)
Ea (kJ /mol)b
k
dec (s-1
)
nitroxide
RN (G)d
R
Hâ (G)d
g
1
2
3
7
11
14
9.9 × 10-3
4.0 × 10-2
1.1 × 10-2
126.4
121.7
126.2
4.2 × 10-5
6
10
14.44
14.21
14.86
2.40
2.62
2.79
2.006
2.006
2.006
9.6 × 10-5
Measured at 403 K. EA was calculated using A ) 2.4 × 1014 s-1; see ref 18. Statistical errors between 2 and 3 kJ /mol. c Measured
a
b
d
at 398 K. Measured at room temperature.
spectroscopy, as previously described.6,18-21 The experi-
Sch em e 2. Self-Rea ction of Dia lk yl Nitr oxid es
mental cleavage rate constants kd were calculated using
eq 1 (conversions up to 30%).
[nitroxide]∞ - [nitroxide]t
ln
) -kdt
(1)
(
)
[nitroxide]∞
The activation energies Ea were estimated from the rate
constants using A ) 2.4 × 1014 s-1 6,18-20
.
The data are
presented in Table 3.
Furthermore, we also determined the rate constant
for the thermal decomposition of the alkoxyamines 7
and 11 to provide styrene and the corresponding hy-
droxylamine. The decomposition rate constant is an
important parameter to evaluate the efficiency of a new
alkoxyamine initiator for the NMP.22 The alkoxyamine
was dissolved in a NMR tube in perdeuterio-p-xylene
(0.03-0.06 M). The degassed sample was heated to 398
K within the cavity of a 500 MHz 1H NMR spectrometer,
and the decomposition was followed by monitoring the
decrease of the alkoxyamine signals and the increase
of the styrene resonances. The signal of the benzylic H
atom at around 4.7 was used to estimate the alkoxyamine
concentration. Similar experiments have previously
been described.20,22,23 Spectra were recorded every 5
min. The decomposition rate constants (kdec) for the
various alkoxyamines were determined using eq 2
alkoxyamines are not sufficiently stable at 125 °C. The
same was observed for the n-butyl acrylate polymeriza-
tions where best results have been obtained at 105 °C.
Replacement of one methyl group of the tert-butyl
group in alkoxyamine 2 by larger alkyl substituents (f
7, 14) does not alter the C-O homolysis rate constant
significantly (entries 1 and 3). As expected from the
homolysis rate constants, alkoxyamine 7 decomposes
slower than alkoxyamine 11.
X-r a y Str u ctu r e of Nitr oxid e 6. In general, nitrox-
ides with a H atom R to the N atom are not stable and
readily undergo disproportionation to form the corre-
sponding hydroxylamine and nitrone. Ingold et al.
carefully studied the self-reaction of dialkyl nitroxides
and suggested a two-step mechanism depicted in Scheme
2.24 Nitroxide dimerization in the first step is followed
by syn-elimination to give the nitrone and the hydroxy-
lamine. Therefore, syn-orientation of the R-H atom with
respect to the nitroxide O atom is important for the
decomposition; R-H-bearing nitroxides where the H
atom is located anti to the nitroxide O atom should be
stable. We obtained crystals of nitroxide 6 suitable for
X-ray analysis. The structure is depicted in Figure 3.25
As expected for a stable nitroxide, the H atom is located
anti to the nitroxide O atom. The O(2)-N(2)-C(9)-H
torsion angle in 6 is 170.8°, close to 180° where highest
stability is expected. A similar result has previously
been obtained by the groups of Volodarsky and Tordo
for two other R-H-bearing stable nitroxides.8 Further-
more, the C(5)-methyl group in 6 is positioned syn to
the nitroxide O atom and is therefore the most impor-
tant substituent for shielding of the nitroxide O atom.
ln([S]/[A] + 1) ) kdec
t
(2)
according to Fukuda’s work23 ([S] ) styrene concentra-
tion; [A] ) alkoxyamine concentration) and are sum-
marized in Table 3.
In agreement with the polymerization results de-
scribed above, the lowest activation energy (Ea) for the
C-O bond homolysis was measured for alkoxyamine 11
(entry 2). The replacement of the tert-butyl group by the
larger 1,1-diethylpropyl group leads to a decrease of Ea
by about 5 kJ /mol (Ea for 2 ) 127.1 kJ /mol).18 This is
due to the larger steric repulsion by the 1,1-diethylpro-
pyl group compared to the repulsion by the tert-butyl
group. However, the sterically hindered alkoxyamine 11
turned out to be less stable than the parent regulator 2
(kdec for 2: 8.8 × 10-6 s-1). Therefore, the best styrene
polymerization results with 11 were obtained at 105 °C.
Alkoxyamine 11 and the corresponding polymeric

Macromolecules, Vol. 37, No. 1, 2004
New Sterically Hindered Nitroxides 33
butyl at the N atom in the parent alkoxyamine 2 was
replaced with sterically more demanding substituents
such as the 1,1-diethylpropyl group (f 11). The in-
creased steric demand of the R3 substituent in alkoxy-
amines of type 3 decreases the activation energies for
C-O bond homolysis, as measured by kinetic ESR
experiments. The C-O bond homolysis activation en-
ergy is an important parameter in the nitroxide-medi-
ated polymerization. Compared to the parent regulator
2, our new alkoxyamine 11 has higher activity as
regulator/initiator for the controlled polymerization of
styrene. Efficient polymerization could be accomplished
at 105 °C. Controlled styrene polymerization can even
be performed at 90 °C. However, the increased steric
demand of the R3 substituent also decreases the stability
as determined by 1H NMR spectroscopy. Therefore,
polymerizations worked better at lower temperature.
The same was also observed for the n-butyl acrylate
polymerization where best results with alkoxyamine 11
were obtained at 105 °C. With alkoxyamine 11 con-
trolled polymerization of n-butyl acrylate is possible
even at 90 °C. In addition, AA and AB diblock copoly-
mers have successfully been prepared using polystyrene
and poly(n-butyl acrylate) macroinitiators derived from
11.
F igu r e 3. X-ray structure of nitroxide 6.
Furthermore, we presented an X-ray structure of a
Hawker-Braslau type nitroxide. As expected for a
stable nitroxide, the R-H atom is located away from the
nitroxide O atom. The O(2)-N(2)-C(9)-H torsion angle
in 6 is 170.8°, close to 180° where highest stability is
expected (Figure 3). Conformational analysis using ESR
spectroscopy showed that this anti arrangement of the
R-H atom with respect to the nitroxide O atom is also
conserved in solution. The X-ray structure revealed that
for perfect shielding of the nitroxide all three methyl
groups of the tert-butyl group in the parent Hawker-
Braslau nitroxide have to be replaced with larger
substituents. Indeed, replacement of only one methyl
group with larger alkyl substituents such as in 7 and
in 14 has only a small effect on the polymerization
regulator/initiator efficiency.
F igu r e 4. Room temperature ESR spectrum of nitroxide 6
(RHâ ) 2.40 G; RN ) 14.44 G, in t-BuPh) and the dihedral angle
æ which can be estimated from the Heller-McConnell equa-
tion.
The larger 3-dimethylaminocarbonyl-1,1-dimethylpropyl
is located away from the nitroxide and has therefore no
influence on the shielding of the nitroxide. This is
probably the reason why alkoxyamines 7 and the parent
2 provided similar polymerization results. Thus, for
efficient shielding all three methyl groups of the tert-
butyl substituent in 2 have to be replaced. Indeed,
alkoxyamine 11 bearing ethyl groups instead of methyl
groups at the given position turned out to be the most
efficient polymerization initiator/regulator in this series.
We decided to further study the conformation of
nitroxides 6 and 10 in solution using ESR spectroscopy.
Similar studies on stable R-H-bearing nitroxides have
previously been published.6,26 The hyperfine coupling
constant RHâ can be used to determine the dihedral
angle æ (see Figure 4, Table 3) using a simplified
Heller-McConnell27 equation (RHâ ) BN cos2(æ)) with
an estimated BN of 26.55 G.28 At room temperature, 2.40
G was measured for RHâ, which leads to a dihedral angle
æ of 72.5°. The room temperature ESR spectrum of
nitroxide 6 is depicted in Figure 4. Thus, also in solution
the R-H atom adopts near perfect anti alignment with
the nitroxide O atom (O(2)-N(2)-C(9)-H torsion angle
in solution ) 162.5°). Unfortunately, because of line
broadening, it was not possible to measure the RHâ of 6
at higher temperatures (90-125 °C). A dihedral angle
æ of 71.7° was calculated from the ESR hyperfine
coupling constant for the solution conformation of
nitroxide 10. As expected, a similar dihedral angle was
also obtained for the nitroxide derived from alkoxyamine
14 (æ ) 71.1°).
Ack n ow led gm en t. We thank the Deutsche Fors-
chungsgemeinschaft (STU 280/1-1) for funding and Dr.
Olaf Burghaus for assistance during the EPR experi-
ments. J ens Tra¨ger is acknowledged for conducting
some experiments during the advanced organic chem-
istry lab course.
Refer en ces a n d Notes
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34 Studer et al.
Macromolecules, Vol. 37, No. 1, 2004
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