New seven- and eight-membered cyclic alkoxyamines for the living free radical polymerization

Article abstract of DOI:10.1021/ma025806t

A straightforward synthesis of new seven- and eight-membered cyclic alkoxyamines from the corresponding lower homologous keto-alkoxyamines via ring-enlargement using TMS-diazomethane is described. The use of these ring-enlarged cyclic alkoxyamines as regulators/initiators for the radical polymerization of styrene and n-butyl acrylate is presented. Efficient controlled and living styrene polymerization (molecular weight of up to 40 000) can be obtained using the seven- and eight-membered alkoxyamine initiators. The influence of the ring-enlargement on the quality of the polymerization process (polymerization time, livingness, PDI) is discussed. The rate constant of the C-O bond cleavage of these new alkoxyamines has been measured. In addition, the thermal decomposition of the alkoxyamines has been studied. Furthermore, EPR data of the corresponding new nitroxides are presented.

Full text of DOI:10.1021/ma025806t

3078
Macromolecules 2003, 36, 3078-3084
New Seven- and Eight-Membered Cyclic Alkoxyamines for the Living
Free Radical Polymerization
Tobia s Sch u lte a n d Ar m id o Stu d er *
Fachbereich Chemie der Universita¨t Marburg, Hans-Meerwein-Strasse, D-35032 Marburg, Germany
Received November 5, 2002; Revised Manuscript Received March 4, 2003
ABSTRACT: A straightforward synthesis of new seven- and eight-membered cyclic alkoxyamines from
the corresponding lower homologous keto-alkoxyamines via ring-enlargement using TMS-diazomethane
is described. The use of these ring-enlarged cyclic alkoxyamines as regulators/initiators for the radical
polymerization of styrene and n-butyl acrylate is presented. Efficient controlled and living styrene
polymerization (molecular weight of up to 40 000) can be obtained using the seven- and eight-membered
alkoxyamine initiators. The influence of the ring-enlargement on the quality of the polymerization process
(polymerization time, livingness, PDI) is discussed. The rate constant of the C-O bond cleavage of these
new alkoxyamines has been measured. In addition, the thermal decomposition of the alkoxyamines has
been studied. Furthermore, EPR data of the corresponding new nitroxides are presented.
over CaH2 to remove the stabilizer. Benzene was distilled over
sodium and CH2Cl2 was distilled over CaH2 before use. All
other chemicals were used as received.
In tr od u ction
Well-defined polymers with polydispersities below the
theoretical limit for a conventional radical polymeriza-
tion (1.5) can nowadays be prepared by living free
radical polymerization. Nitroxide-mediated polymeriza-
tions (NMP),1 atom transfer radical polymerizations
(ATRP),2 and RAFT polymerizations3 belong to this
category. The NMP and ATRP processes are controlled
by the persistent radical effect (PRE).4 The control of
the polymerization in the NMP is based on the revers-
ible formation of a dormant alkoxyamine from the
corresponding nitroxide and the chain growing polymer
radical. The concentration of free radicals remains low
during the entire polymerization, thus ensuring a very
low fraction of irreversible termination via polymer
radical dimerization/disproportionation. The equilibri-
um constant between the nitroxide-capped polymer and
the free nitroxide and polymer radical, respectively, is
therefore of great importance in these processes.4 Vari-
ous parameters influence the equilibrium.5 On the basis
of experimental results and on calculations, it has been
predicted that in cyclic nitroxides the ring size influ-
ences the C-O bond dissociation energy (BDE) of the
corresponding alkoxyamines and hence the equilibrium.
The BDE increases from seven- to six- to five-membered
cyclic nitroxides.6
Gen er a l Da ta . 1H 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 (FC): silica gel 60
(40-63 µm, Merck or Fluka); at ca. 0.4 bar. Melting points:
510 apparatus (Bu¨chi); uncorrected. IR spectra were recorded
on a IR 750 (Nicolet Magna) or a IFS-200 (Bruker). 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,
Polymer Laboratories), and a RI-101 detector (Shodex). Data
were acquired through a PL Datastream unit (Polymer Labo-
ratories) and analyzed with Cirrus GPC software (Polymer
Laboratories) based upon calibration curves built upon poly-
styrene and poly(methyl methacrylate) standards (Polymer
Laboratories 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. EPR spectra were recorded on a ESP 300 E (Bruker)
equipped with a high-temperature 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.
Herein we present our results on the synthesis of new
alkoxyamines formally deriving from seven- and eight-
membered cyclic nitroxides.7 Their efficiency as regula-
tors for the polymerization of styrene and n-butyl
acrylate will be discussed. Furthermore, rate constants
for the C-O bond homolysis of these new alkoxyamines,
decomposition studies and EPR data of the correspond-
ing nitroxides will be presented.
2,2,6,6-Tet r a m et h yl-1-(1-p h en ylet h oxy)-4-p ip er id in o-
n e (1). (1-Bromoethyl)benzene (6.69 mL, 49.02 mmol), 2,2,6,6-
tetramethyl-4-piperidinone-N-oxyl8 (10.00 g, 58.82 mmol), Cu
powder (3.26 g, 51.46 mmol), Cu(OTf)2 (182 mg, 0.49 mmol),
and 4,4′-di-tert-butyl-2,2′-bipyridyl (264 mg, 1.97 mmol) were
suspended under Ar in benzene (200 mL). The reaction
mixture was stirred under Ar at 65-70 °C for 20 h. The solids
were removed by filtration over SiO2. Purification by FC
(pentane/ethyl acetate, 14:1) afforded 1 (10.73 g, 79%) as a
colorless solid. Mp: 72 °C. IR (KBr): 3000 s, 2969 w, 2958 w,
2923 w, 1715 s, 1448 m, 1364 m, 1305 s, 1229 m, 1196 m, 1058
Exp er im en ta l Section
Ma ter ia ls. (1-Bromoethyl)benzene (Aldrich, 97%), 4,4′-di-
tert-butyl-2,2′-bipyridyl (Aldrich, 98%), (trimethylsilyl)diaz-
omethane (Aldrich, 2 M in hexane) and pyridinium toluene-
4-sulfonate (Fluka, 99+%) were used as received. Styrene
(BASF) and n-butyl acrylate (Fluka, 99+%) were both distilled
* Corresponding author. E-mail: studer@mailer.uni-marburg.de.
10.1021/ma025806t CCC: $25.00 © 2003 American Chemical Society
Published on Web 04/11/2003

Macromolecules, Vol. 36, No. 9, 2003
s, 1025 m, 957 m, 764 s, 705 s cm-1
Living Free Radical Polymerization 3079
.
1H NMR (200 MHz,
39.5, 34.9, 34.5, 32.7, 32.1, 31.8, 27.4, 23.6, 21.2, 20.9, 17.8.
MS (ESI): 304 (12, [M + H]+), 290 (10), 221 (28), 200 (56),
168 (48), 99 (28), 74 (12). HRMS (ESI): calcd for C19H29NO2
(M+), 303.2198; found, 303.2185.
CDCl3): δ ) 7.35-7.26 (m, 5 H, CH), 4.87 (q, J ) 6.6 Hz, 1 H,
CH), 2.65-2.46 (m, 2 H, CH2), 2.27-2.05 (m, 2 H, CH2), 1.54
(d, J ) 6.6 Hz, 3 H, CH3), 1.44 (s, 3 H, CH3), 1.23 (s, 3 H,
CH3), 1.06 (s, 3 H, CH3), 0.77 (s, 3 H, CH3). 13C NMR (75 MHz,
CDCl3): δ ) 208.9, 145.0, 128.6, 127.7, 127.2, 83.8, 63.4, 54.1,
34.3, 34.0, 23.3, 23.1, 23.0. MS (ESI): 298 (100, [M + Na]+),
276 (80, [M + H]+), 274 (24), 225 (24), 193 (20), 178 (24), 172
(84, [M - C8H7]+), 105 (32, (M - C9H17NO2]+), 74 (40). HRMS
(ESI): calcd for C17H25NO2 (M+), 275.1885; found, 275.1894.
Gen er a l P r oced u r e (GP 1) for t h e R in g E xp a n sion
Rea ction s. The alkoxyamine was dissolved in CH2Cl2, and
molecular sieve (MS) 3 Å was added. The solution was cooled
to ca. -78 °C. After addition of BF3‚OEt2, (trimethylsilyl)-
diazomethane (TMSCHN2, 2 M in hexane) was slowly added.
The resulting yellow solution was stirred for 20 min at ca. -78
°C. The mixture was allowed to warm to room temperature,
saturated NaHCO3 and pentane were added, and the phases
were separated. The organic layer was washed with H2O and
brine and dried over MgSO4. The solvent was removed under
reduced pressure and the yellow crude product was dissolved
in MeOH. Pyridinium toluene-4-sulfonate (PPTS) was added
and the solution was stirred for 12 h at room temperature.
NEt3 was added, and the solvent was removed under reduced
pressure. Purification by FC (SiO2) afforded the ring-enlarged
alkoxyamine.
Gen er a l P r oced u r e (GP 2) for th e Red u ction Rea c-
tion s. The alkoxyamine was dissolved in MeOH at 0 °C, and
NaBH4 was added in small portions. After 30 min, the
suspension was allowed to warm to room temperature. Then
MeOH was added until the residue was dissolved. After being
stirred for 12 h at room temperature, the reaction mixture was
treated with saturated NH4Cl and a white precipitate was
formed. H2O was added and the mixture was extracted with
diethyl ether (4×). The combined organic layers were dried
over MgSO4. The solvent was removed under reduced pressure
and the crude product was dried in vacuo.
2,2,7,7-Tetr a m eth yl-1-(1-p h en yleth oxy)-4-a zep a n ol (6).
6 was obtained according to GP2 with 2 (930 mg, 3.20 mmol),
MeOH (5 mL), NaBH4 (448 mg, 11.84 mmol), and saturated
NH4Cl (5 mL) to afford the product (932 mg, 99%) as a 3:1
mixture of diastereoisomers. IR (film): 2974 w, 2934 w, 1478
m, 1452 m, 1362 s, 1160 s, 1058 m, 1034 m, 927 w, 762 s, 700
s cm-1. H NMR (300 MHz, CDCl3): δ ) 7.31-7.25 (m, 5 H,
1
CH), 4.84-4.73 (m, 1 H, CH), 4.06-4.01 (m, 1 H, CH), 2.17-
2.10 (dd, J 1 ) 6.6 Hz, J 2 ) 13.9 Hz, 1 H, CH2), 2.10-1.10 (m,
15 H, CH2, CH3), 0.84 (s, CH3, both isomers), 0.76 (s, CH3,
minor isomer), 0.67 (s, CH3, major isomer). 13C NMR (75 MHz,
CDCl3, both isomers): δ ) 144.0, 128.7, 128.6, 128.5, 128.0,
127.9, 127.7, 127.6, 84.2, 84.0, 83.6, 69.4, 67.9, 64.5, 64.3, 64.0,
49.1, 45.2, 45.0, 37.7, 37.0, 36.8, 34.0, 33.5, 33.2, 33.1, 32.8,
32.6, 31.6, 30.5, 27.7, 27.5, 26.4, 26.1, 25.5, 22.9, 22.8. MS
(ESI): 292 (100, [M + H]+), 274 (26, [M - CH3]+), 236 (16),
209 (35), 188 (80, [M - C8H7]+), 170 (46), 156 (80), 137 (58),
132 (22), 121 (58), 105 (68, [M - C10H20NO2]+). HRMS (ESI):
calcd for C18H29NO2 (M+), 291.2198; found, 291.2189.
2,2,7,7-Tetr am eth yl-1-(1-ph en yleth oxy)-4-azepan on e (2).
2 was prepared according to GP1 with 2,2,6,6-tetramethyl-1-
(1-phenylethoxy)-4-piperidinone (1) (3.00 g, 10.89 mmol), CH2-
Cl2 (100 mL), MS 3 Å (20.00 g), BF3‚OEt2 (1.51 mL, 11.98
mmol), TMSCHN2 (7.10 mL, 14.16 mmol), saturated NaHCO3
(110 mL), MeOH (130 mL), PPTS (336 mg, 1.34 mmol), and
NEt3 (0.66 mL). Purification by FC (pentane/ethyl acetate, 9:1)
afforded 2 (2.35 g, 75%) as a colorless solid. Mp: 54 °C. IR
(KBr): 3003 w, 2876 w, 2924 w, 1710 s, 1361 m, 1301 s, 1061
2,2,8,8-Tetr a m eth yl-1-(1-p h en yleth oxy)-5-a zoca n ol (7).
7 was prepared according to GP2 with 3 (300 mg, 1.00 mmol),
MeOH (2 mL), NaBH4 (139 mg, 3.7 mmol) and saturated NH4-
Cl (3 mL) to afford the product (287 mg, 95%) as a viscous
colorless oil. IR (film): 3438 w, 2975 w, 2929 w, 1451 w, 1363
w, 1059 w, 1029w cm-1. 1H NMR (200 MHz, CDCl3): δ ) 7.26-
7.16 (m, 5 H, CH), 4.81 (q, J ) 6.8 Hz, 1 H, CH), 3.87-3.66
(m, 1 H, CH), 2.54-1.56 (m, 6 H, CH2), 1.49-1.09 (m, 14 H,
CH2, CH3), 0.77 (s, 3 H, CH3). 13C NMR (75 MHz, CDCl3): δ )
144.4, 128.4, 128.2, 128.0, 127.3, 127.3, 127.0, 125.3, 83.1, 67.7,
65.8, 64.9, 64.6, 63.7, 41.0, 31.9, 31.4, 25.8, 25.2, 23.5, 15.2.
MS (ESI): 306 (100, [M + H]+), 169 (20), 138 (18), 105 (6, [M
- C11H22NO2]+). HRMS (ESI): calcd for C19H31NO2 (M+),
305.2355; found, 305.2358.
1
m, 761 s, 704 s cm-1. H NMR (200 MHz, CDCl3): δ ) 7.32-
7.23 (m, 5 H, CH), 4.85-4.73 (m, 1 H, CH), 2.89 (dd, J 1 ) 12.0
Hz, J 2 ) 19.7 Hz, 1 H, CH2), 2.51-2.05 (m, 4 H, CH2), 1.94-
1.66 (m, 1 H, CH2), 1.49-1.45 (m, 3 H, CH3), 1.38-1.35 (m, 3
H, CH3), 1.18 (d, J ) 6.0 Hz, 3 H, CH3), 0.91 (s, CH3), 0.77 (s,
CH3). 13C NMR (75 MHz, CDCl3): δ ) 212.2, 145.1, 130.0,
128.6, 127.5, 127.4, 127.2, 83.5, 83.4, 63.8, 63.7, 63.5, 54.3, 54.2,
38.4, 36.8, 36.5, 34.5, 34.3, 33.3, 32.8, 25.9, 25.8, 24.9, 23.1,
22.9. MS (FD): 289 (36, [M]+), 184 (96, [M - C8H9]+), 105 (100,
[M - C10H18NO2]+). HRMS (ESI): calcd for C18H27NO2 (M+),
289.2024; found, 289.2029.
2,2,8,8-Tetr am eth yl-1-(1-ph en yleth oxy)-5-azocan on e (3)
a n d 2,2,8,8-tetr a m eth yl-1-(1-p h en yleth oxy)-4-a zoca n on e
(4). These compounds were prepared according to GP1 with
2,2,7,7-tetramethyl-1-(1-phenylethoxy)-4-azepanone 2 (3.00 g,
10.37 mmol), CH2Cl2 (40 mL), MS 3 Å (30 g), BF3‚OEt2 (1.43
mL, 11.40 mmol), TMSCHN2 (5.7 mL, 11.40 mmol), saturated
NaHCO3 (40 mL), MeOH (45 mL), PPTS (318 mg, 1.27 mmol),
and NEt3 (0.60 mL). Purification by FC (pentane/ethyl acetate,
9:1) afforded 3 (316 mg, 10%) as a colorless solid and 4 (714
mg, 23%) as a viscous colorless oil. Analytical data for 3 are
as follows. Mp: 50 °C. IR (KBr): 3443 w, 2976 w, 1691 s, 1495
m, 1364 m, 1213 m, 1130 m, 766 m, 704 s cm-1. 1H NMR (300
MHz, CDCl3): δ ) 7.38-7.22 (m, 5 H, CH), 4.76 (q, J ) 10.1
Hz, 1 H, CH), 2.75-2.27 (m, 5 H, CH2), 1.70-1.49 (m, 3 H,
CH2), 1.44 (d, J ) 10.1 Hz, 3 H, CH3), 1.39 (s, 3 H, CH3), 1.28
(s, 3 H, CH3), 1.18 (s, 3 H, CH3), 0.77 (s, 3 H, CH3). 13C NMR
(75 MHz, CDCl3): δ ) 210.6, 144.6, 128.4, 127.7, 127.6, 82.7,
64.2, 64.1, 40.4, 40.2, 39.0, 38.3, 31.8, 31.5, 26.1, 25.6, 22.4.
MS (FD): 303 (100, [M]+), 198 (36, [M - C8H9]+), 105 (80, [M
- C11H20NO2]+). HRMS (ESI): calcd for C19H29NO2 (M+),
303.2198; found, 303.2198. Analytical data for 4 are as follows.
IR (film): 2976 w, 2932 w, 1713 s, 1452 s, 1379 s, 1363 s, 1174
2,2,8,8-Tetr a m eth yl-1-(1-p h en yleth oxy)-4-a zoca n ol (8).
8 was obtained ccording to GP2 with 4 (310 mg, 1.02 mmol),
MeOH (2 mL), NaBH4 (143 mg, 3.8 mmol), and saturated NH4-
Cl (3 mL). Purification by FC (pentane/ethyl acetate, 5:1)
afforded 8 (98 mg, 31%) as a viscous colorless oil. IR (film):
3378 w, 2975 w, 2932 w, 1451 m, 1363 m, 1157 m, 1059 m,
928 m, 762 m, 699 s cm-1 1H NMR (300 MHz, CDCl3, both
.
isomers): δ ) 7.26-7.17 (m, 5 H, CH), 4.83-4.66 (m, 1 H, CH),
4.06-3.75 (m, CH), 2.45-1.66 (m, CH2), 1.63-0.70 (m, CH2,
CH3), 0.60 (s, CH3). 13C NMR (75 MHz, CDCl3, both isomers):
δ ) 144.3, 128.1, 128.0, 127.4, 127.2, 127.1, 126.8, 126.6, 83.5,
83.1, 68.9, 67.7, 44.5, 41.0, 37.2, 36.5, 32.6, 31.9, 31.7, 31.3,
30.0, 25.8, 25.0, 23.5, 22.3, 20.2, 17.5, 17.3. MS (ESI): 306 (20,
[M + H]+), 262 (12), 202 (16), 169 (48), 151 (24), 138 (56), 121
(100), 105 (68, [M - C11H22NO2]+). HRMS (ESI): calcd for
C
19H31NO2 (M+), 305.2355; found, 305.2351.
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 2
(38 mg, 0.131 mmol) and styrene (1.5 mL, 13.106 mmol). The
tube was subjected to three freeze-thaw cycles and sealed off
under argon. The polymerization was carried out under argon
at 125 °C for 6 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
s, 1061 s, 930 m, 762 s, 700 s cm-1 1H NMR (300 MHz,
.
CDCl3): δ ) 7.28-7.22 (m, 5 H, CH), 4.82-4.72 (m, 1 H, CH),
2.71-2.53 (m, 1 H, CH2), 2.31-2.11 (m, 3 H, CH2), 1.82-1.51-
(m, 2H, CH2), 1.50-1.41 (m, 5 H, CH2, CH3), 1.36, 1.30 (2s,
CH3), 1.22, 1.18 (2s, CH3), 1.07, 1.04 (2s, CH3), 0.67 (s, 3 H,
CH3). 13C NMR (75 MHz, CDCl3): δ ) 210.9, 145.7, 128.7,
128.5, 127.4, 127.2, 127.0, 83.9, 83.7, 62.3, 62.1, 60.4, 60.2, 60.1,

3080 Schulte and Studer
Macromolecules, Vol. 36, No. 9, 2003
Ta ble 1. P olym er iza tion of Styr en e Usin g Alk oxya m in es
Sch em e 1. Va r iou s Alk oxya m in es Stu d ied
1-8 a s Regu la tor s/In itia tor s
alkoxyamine time temp
conversion
(%)
entry
(amount)
(h) (° C) Mn,exp
Mn,th PDI
1
2
3
4
5
6
7
8
1 (1%)
1 (0.2%)
2 (1%)
2 (0.4%)
2 (0.2%)
2 (0.1%)
2 (0.2%)
2 (0.1%)
2 (0.2%)
3 (1%)
3 (0.2%)
4 (1%)
4 (0.2%)
5 (1%)
5 (0.2%)
6 (1%)
6 (0.2%)
7 (1%)
6
6
6
6
6
6
125
3000
2900 1.23
28
43
84
83
66
71
73
89
88
54
78
58
68
26
37
30
52
49
80
36
46
125 15 200 22 400 1.37
125 9000 8700 1.10
125 18 600 21 600 1.20
125 32 000 34 400 1.21
125 45 700 73 900 1.38
25 105 24 900 38 000 1.18
25 105 52 000 92 700 1.39
115
9
85 22 200 45 800 1.53
125 5000 5600 1.17
125 31 500 40 600 1.34
125 5600 6000 1.29
125 23 700 35 400 1.33
125 2200 2700 1.27
125 13 200 19 300 1.38
125 3300 3100 1.23
125 17 000 27 100 1.36
125 6400 5100 1.41
125 31 000 41 700 1.62
125 3600 3700 1.23
125 19 000 24 000 1.32
10
11
12
13
14
15
16
17
18
19
20
21
3
6
6
6
6
6
6
6
6
6
6
6
7 (0.2%)
8 (1%)
8 (0.2%)
P olym er iza tion s. The polymerization of styrene was
studied first. The initial polymerizations were conducted
in sealed tubes using 0.2 or 1% of the alkoxyamine
initiator at 125 °C and were stopped after 6 h. 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.
With the six-membered ketone 1 a low conversion was
obtained (entry 1, 28%). Decreasing the initiator con-
centration afforded PS with a higher PDI (1.37) and a
higher conversion (entry 2). We were pleased to find that
the ring-enlarged seven-membered ketone 2 provided
better results. Using 1% of initiator 2, a high conversion
was obtained and polystyrene (PS) with a narrow PDI
was formed (84%, PDI ) 1.10, entry 3). Decreasing the
initiator amount afforded PS with slightly increased
PDI (entry 5, PDI ) 1.21). The eight-membered sym-
metrical ketoalkoxyamine 3 afforded even better results.
Quantitative conversion was obtained in 6 h. After 3 h
a conversion of 54% was already obtained and PS with
a narrow PDI was formed (entry 10, PDI ) 1.17). As
for the seven-membered ring, a slightly increased PDI
value was observed upon decreasing the initiator con-
centration (entry 11, PDI ) 1.34). The isomeric unsym-
metrical eight-membered alkoxyamine 4 provided simi-
lar results (entries 12 and 13).
(PDI) were determined by size exclusion chromatography
(SEC). Conversion ) 84%. Mn ) 9000 g mol-1. PDI ) 1.10.
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 2 (61 mg, 0.210 mmol) and n-butyl acrylate (3 mL,
21.019 mmol). The tube was subjected to three freeze-thaw
cycles and sealed off under argon. The polymerization was
carried out under argon at 125 °C for 14 h. The resulting
mixture was cooled to room temperature, dissolved in CH2-
Cl2, 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 ) 59%. Mn
)
11 200 g mol-1. PDI ) 1.44.
We then turned to the question whether the keto
group is important for the polymerization process. To
this end, alcohols 5-8 were used as regulators/initiators
for the polymerization of styrene. As in the keto series,
the lowest conversion was obtained with the six-
membered cyclic alkoxyamine 5 as initiator (entries 14
and 15). Surprisingly, the homologous hydroxyal-
koxyamine 6 turned out to be a far less efficient
initiator/regulator than the corresponding ketone 2
(compare entries 16 and 17 with entries 3 and 4).
Similar results were obtained for the eight-membered
cyclic alkoxyamines 7 and 8 (entries 18-21). From these
initial polymerization studies we can conclude that
indeed, as predicted by theory,6 ring-enlargement posi-
tively influences the regulator/initiator efficiency of
cyclic alkoxyamines. We found that a keto function
within the ring is far superior than a hydroxymethylene
unit. Transannular interactions of the keto group with
the alkoxyamine oxygen atom can be excluded, since IR-
Resu lts a n d Discu ssion
Syn th esis of th e Alk oxya m in es. Alkoxyamine 18
was prepared from phenethyl bromide and 4-Oxo-
TEMPO using the Matyjaszewski protocol.9 Alkoxyamine
2 was readily obtained starting from the six-membered
alkoxyamine 1 in a one-step reaction via ring homolo-
gization using commercially available TMS-diazometh-
ane (75%, Scheme 1).10
In analogy, ring-homologization of alkoxyamine 2
provided the two separable regioisomeric eight-mem-
bered alkoxyamines 3 (10%) and 4 (23%). Alcohol 5 is a
known compound11 and was included in the present
study for comparison. Reduction of ketone 2 with NaBH4
in MeOH afforded the seven-membered cyclic alcohol 6
as a 3:1 mixture of diastereoisomers in quantitative
yield. The alcohols 7 (95%) and 8 (31%, diastereoiso-
meric ratio ) 1:1) were obtained from the corresponding
ketones 3 and 4 using the same method.

Macromolecules, Vol. 36, No. 9, 2003
Living Free Radical Polymerization 3081
stretching frequencies of the CdO double bond in these
ketones did not show any indication of transannular
interactions: 1, ν ) 1715 cm-1; 2, ν ) 1710 cm-1; 3, ν )
1691 cm-1; 4, ν ) 1715 cm-1. In addition, also for the
free nitroxides no indications of transannular interac-
tions were found (vide infra). The effect of the keto group
will be further discussed below.
The seven-membered cyclic ketone 2 was used for
further styrene polymerization studies (entries 4, 6-9).
Decreasing the amount of initiator to 0.1% afforded PS
with a higher molecular weight (Mn ) 45 700); however,
the PDI also slightly increased (PDI ) 1.38, entry 6).
The polymerization was also conducted at 105 °C
(entries 7 and 8). Controlled polymerization was ob-
tained using 0.2% of initiator 2 (Mn ) 24 900; PDI )
1.18). Again, reducing the initiator concentration pro-
vided PS with increased PDI. Polymerization at 85 °C
for 115 h was not controlled (entry 9, PDI ) 1.53). Thus,
alkoxyamine 2, which is readily prepared on a large
scale, is an efficient regulator/initiator for the prepara-
tion of PS with a molecular weight of up to 40 000.
In all the experiments we found that decreasing the
initiator concentration leads to an increase of the PDI.
This can be understood by the increase of the influence
of the thermal polymerization of styrene by autoinitia-
tion. Indeed, theory predicts higher PDI’s in NMP if
additional initiation is considered.12d In most of our
experiments, the difference between Mn and the theo-
retical number-average molecular weight (Mn,th) in-
creases upon decreasing the initiator concentration (see
Table 1). This indicates an increased fraction of ad-
ditional chains deriving from autoinitiation of styrene.5m
According to theory,12d additional initiation should lead
to higher conversion in NMP. This is in agreement with
most of our experiments, since as mentioned above,
decreasing the initiator concentration should lead to a
higher fraction of additional initiation, and this in turn
will lead to higher conversion. For initiator 2, however,
a decrease of conversion upon decreasing the initiator
concentration was obtained. A similar decrease of
conversion upon decreasing the initiator concentration
has previously been reported for NMP of styrene.5m
Currently, we do not understand why initiator/regulator
2 showed concentration effects on the polymerization
conversions different from those of the other systems
studied.
F igu r e 1. (a) Monomer conversion vs time for the polymer-
ization of styrene using initiator 2 (1 mol % 2, 125 °C, 6 h).
(b) Molecular weight vs monomer conversion for the poly-
merization of styrene using initiator 2 (1 mol % 2, 125 °C, 6
h).
F igu r e 2. GPC traces for the bulk polymerization of styrene
using alkoxyamines 2 and 3 before (traces a and c) and after
reinitiation (traces b and d): (a) ) PS-2 (PDI ) 1.15, Mn
)
7900); (b) ) PS-2 (PDI ) 1.33, Mn ) 25 100); (c) ) PS-3 (PDI
) 1.17, Mn ) 5000); (d) ) PS-3 (PDI ) 1.23, Mn ) 18 500)).
Ta ble 2. P olym er iza tion of Styr en e Usin g Va r iou s
Ma cr oin itia tor s a t 125 °C
To check whether the styrene polymerization using
initiator 2 is a living/controlled process, we determined
the conversion as a function of time and we analyzed
the molecular weight as a function of monomer conver-
sion (Figure 1). The linear increase of ln([Mo]/[M]) vs
time and molecular weight vs monomer consumption
proves the controlled character of the polymerization.
PS-alkoxyamine
(Mn/PDI/amount)
time
(h)
conversion
(%)
entry
Mn
PDI
1
2
3
4
5
6
PS-2 (7900/1.15/0.4%)
PS-3 (5000/1.17/0.4%)
PS-4 (5600/1.29/0.4%)
PS-6 (3300/1.23/0.4%)
PS-7 (6400/1.41/0.4%)
PS-8 (3600/1.23/0.4%)
6
3
6
6
6
6
25 100 1.33
18 500 1.23
15 600 1.42
13 500 1.59
16 800 1.65
9900 1.64
83
60
52
43
54
23
We further tested whether the polymerization is
living. To this end, PS prepared using alkoxyamine 2
(PDI ) 1.15, Mn ) 7900) was used as macroinitiator for
the bulk polymerization of styrene (250 equiv styrene
were used). Reaction was conducted at 125 °C for 6 h
providing PS with a slightly larger PDI (Mn ) 25 100,
83% conversion, PDI ) 1.33, Table 2, entry 1, Figure
2). This shows that the 2-mediated styrene polymeri-
zation is living. However, the non symmetrical GPC
trace b in Figure 2 indicates loss of control during the
polymerization. Probably, decomposition of the dormant
alkoxyamine is occurring which leads to an increased
PDI (see below). We repeated the experiment using PS
prepared with alkoxyamine 3 (entry 2). Reinitiation
could be accomplished and controlled polymerization
was obtained (PDI ) 1.23). With the regioisomeric eight-
membered alkoxyamine 4 macroinitiation was again
successful and PS with a PDI of 1.42 was formed. The
position of keto group in the eight-membered ring
influences the polymerization process. In general, the
symmetrical ketoalkoxyamine 3 provided slightly better
results than alkoxyamine 4.
Macroinitiation using PS prepared with the seven-
and eight-membered alcohols 6-8 provided PS with
broad polydispersities (PDI > 1.5, entries 4-6). In

3082 Schulte and Studer
Macromolecules, Vol. 36, No. 9, 2003
Ta ble 3. P olym er iza tion of n -Bu tyl Acr yla te Usin g
Alk oxya m in es 1-4 a n d 6-8 a t 125 °C
K. Oxygen was used to scavenge the styryl radical and
the concentration of the released nitroxide was mea-
sured by EPR spectroscopy, as previously described.5e,5
h,5i,14 The experimental cleavage rate constants kd were
calculated using eq 1. The activation energies Ea were
estimated from the rate constants using A ) 2.6 × 1014
alkoxyamine
(amount)
%
time
conversion
(%)
entry
nitroxide (h)
Mn
PDI
1
2
3
4
5
6
7
8
1 (1%)
2 (1%)
14
2100 1.60
13
59
48
40
34
59
55
67
71
55
16
36
17
14 11 200 1.44
20 15 600 1.53
20 23 700 1.52
20 52 500 1.57
20 25 100 2.25
20 22 300 2.23
20 21 700 2.51
14 13 800 1.44
14 10 600 1.80
s-1 5i
.
2 (0.4%)
2 (0.2%)
2 (0.1%)
2 (0.4%)
2 (0.4%)
2 (0.4%)
3 (1%)
4 (1%)
6 (1%)
7 (1%)
8 (1%)
[nitroxide]∞ - [nitroxide]t
ln
) -kdt
(1)
0.01
0.02
0.03
(
)
[nitroxide]∞
The smallest rate constant was measured for the six-
membered ketoalkoxyamine 1 (entry 1). As expected,
ring-enlargement (f 2) reduces the activation energy
for the C-O bond homolysis: A 4-fold increase of the
rate constant was obtained (entry 2). The seven-
membered ketoalkoxyamine 2 and both regioisomeric
eight-membered alkoxyamines 3 and 4 showed very
similar rate constants (entries 2-4). Thus, going from
the seven- to the eight-membered ring does not alter
the rate constant. These kinetics are in agreement with
the polymerization results presented above, where slow
polymerization was obtained using six-membered alkoxy-
amine initiator/regulator 1 (28% conversion, 6 h) and
efficient polymerizations were observed using alkoxy-
amines 2 and 3 (84%, 6 h; 54%, 3 h).
9
10
11
12
13
14
14
14
3400 1.38
5300 1.56
3500 1.42
contrast to the keto series, polymerizations of styrene
using the alkoxyamines 6-8 are not living.
We next focused on the polymerization of n-butyl
acrylate (Table 3). The reactions were conducted at 125
°C using the alkoxyamines 1-4 and 6-8 as regulators/
initiators under different conditions. With the six-
membered ketone 1 only 13% conversion was obtained
after 14 h (entry 1). A better result was obtained using
alkoxyamine 2 (59% conversion, PDI ) 1.44, entry 2).
With alkoxyamine 3 a similar result was observed (entry
9). A larger PDI was measured for poly butyl acrylate
prepared with alkoxyamine 4 (entry 10). Polymeriza-
tions with the alcohol initiators 6-8 were not efficient,
and low conversions were obtained (entries 11-13). A
decrease of the initiator concentration did not provide
better results, as shown for the polymerization using
alkoxyamine 2 (entries 3-5). The addition of free
nitroxide has been shown to improve the polymerization
process.5g,12 We therefore studied the polymerization of
n-butyl acrylate with alkoxyamine 2 in the presence of
various amounts of nitroxide 10 (entries 6-8). In
contrary to our expectation, conversion and PDI in-
creased. We can conclude that our new alkoxyamines
are not perfect regulator/initiators for the acrylate
polymerization. With the seven- and eight-membered
keto-alkoxyamines 2 and 3, acceptable conversions
were obtained; however, only modest control of the
polymerization was observed (PDI around 1.4).
In the “alcohol series” the enlargement from the six-
to the seven-membered ring influences the C-O bond
homolysis to an even greater extent. Alkoxyamine 6
reacts about 14-times faster than alkoxyamine 55l
(entries 5 and 6). Interestingly, for the eight-membered
alkoxyamines 7 and 8 slightly smaller rate constants
were measured as compared to the lower homologue 6
(entries 7 and 8). In general, homolysis of the reduced
alkoxyamines occurs faster than homolysis of the cor-
responding ketones. This is in contrary to the poly-
merization results where the keto series always pro-
vided better results. Probably, ketoalkoxyamines are
more stable than the corresponding alcohols and are
therefore better initiators/regulators (see below).
In Table 4, the EPR data of the new nitroxides
depicted in Figure 3 are summarized. The seven-
membered ketonitroxide 10 and the corresponding hy-
droxy derivative 14 have a very similar nitrogen hy-
perfine coupling constant aN (entries 2 and 6). Similar
aN values were also measured for the unsymmetrical
eight-membered cyclic ketonitroxide 12 and the corre-
sponding reduced congener 16 (entries 4 and 8). The
symmetrical ketonitroxide 11 has a smaller aN value
than the corresponding alcohol 15 (entries 3 and 7).
Stabilization of the seven- and eight-membered ketoni-
troxides by transannular interactions as depicted for 10
f 10′ and 11 f 11′ can be excluded, since these
Kin etics of th e C-O Bon d Hom olysis. In nitrox-
ide-mediated living radical polymerizations the rate
constant kd for the C-O bond homolysis is an important
parameter affecting the conversion times and polydis-
persities.4,5,13 We therefore determined the rate constant
kd for the C-O bond homolysis of the various alkoxy-
amines presented in Figure 1 (Table 4). The kinetic
experiments were conducted in tert-butylbenzene at 407
Ta ble 4. Ra te Con sta n ts for th e C-O Bon d Hom olysis a n d Decom p osition Ra tes of Alk oxya m in es 1-4 a n d 6-8 a n d EP R
P a r a m eter s of th e Nitr oxid es 9-12 a n d 14-16
a
c
entry
alkoxyamine
kd (s-1
)
Ea (kJ /mol)b
k
dec (s-1
)
nitroxide
aN (G)d
g
1
2
3
4
5
6
7
8
1
2
3
4
5
6
7
8
5.7 × 10-4
2.0 × 10-3
1.7 × 10-3
1.1 × 10-3
3.3 × 10-3 e
2.7 × 10-2
1.3 × 10-2
7.4 × 10-3
137.6
133.3
133.8
135.3
133.6f
124.6
127.0
128.9
5.9 × 10-6
5.2 × 10-5
1.6 × 10-5
3.1 × 10-5
1.3 × 10-5 g
1.3 × 10-4
5.7 × 10-5
4.8 × 10-5
9
15.09
15.52
14.94
15.87
2.006
2.006
2.006
2.006
10
11
12
13
14
15
16
15.23
15.51
15.73
2.006
2.006
2.006
Measured at 407 K. EA was calculated using A ) 2.6 × 1014 s-1, see ref 5i. Statistical errors between 2 and 3 kJ /mol. c Measured at
a
b
398 K. EPR data were recorded in t-BuPh saturated with O2 and are given with an error of ( 0.14 G. e Determined at 413 K (ref 5l).f EA
d
was calculated using A ) 2.6 × 1014 s-1 and Fukuda’s kd value which was determined at 413 K. Measured by Fukuda at 413 K in
g
deuterated toluene (ref 5l).

Macromolecules, Vol. 36, No. 9, 2003
Living Free Radical Polymerization 3083
F igu r e 4. 1H NMR spectra (in the range of 4.5-6.7 ppm)
recorded during the decomposition study of alkoxyamine 2.
F igu r e 3. Various nitroxides studied.
interactions would lead to a higher spin density at the
nitrogen atom (see 10′ and 11′) and therefore to an
increase of aN.15
Th er m a l Sta bility of th e Alk oxya m in es. An im-
portant side reaction in NMP is the decomposition of
the alkoxyamine leading to a terminally unsaturated
polymer and the corresponding hydroxylamine. This
process can occur via a â-hydrogen atom transfer from
the transient polymer radical to the nitroxide or via a
nonradical direct ionic elimination.5l,16 We decided to
study the thermal decomposition of the various alkoxy-
amines to provide styrene and the corresponding hy-
droxylamine. To this end, the alkoxyamine was dis-
solved in a NMR tube in perdeutero-p-xylene (0.03-0.06
M). The degassed sample was heated to 125 °C within
the cavity of a 500 MHz 1H NMR spectrometer, and the
decomposition was followed by monitoring the decrease
of the alkoxyamine signals as well as 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.5l,16 Spectra were recorded every 5 min.
F igu r e 5. Plot of ln([S]/[A] + 1) vs time for the decomposition
of the alkoxyamines 2 and 6.
decomposition rate constants obtained from the slopes
are summarized in Table 4.
As shown in Figure 5, hydroxyalkoxyamine 6 decom-
poses faster than the corresponding keto derivative 2
(6, kdec ) 1.3 × 10-4 s-1; 2, kdec ) 5.2 × 10-5 s-1).17 The
same trend was observed also for the six- and eight-
membered alkoxyamines, where the hydroxyalkoxyamine
always provided a larger kdec than the corresponding
ketoalkoxyamine (compare entries 1 and 5; 4 and 8; and
3 and 7 in Table 4). This is in agreement with the
experimental polymerization results, where the keto
series always provided better results. Thus, although
homolysis of the hydroxyalkoxyamines occurs faster
than homolysis of the corresponding ketones, polymer-
ization is more efficient for the ketones due to increased
thermal stability of the ketoalkoxyamines.
1
In Figure 4, a series of H NMR spectra (in the range
Con clu sion s
of 4.5-6.7 ppm) recorded during the decomposition
study of alkoxyamine 2 are presented as a representa-
In this work we presented a straightforward synthesis
for the preparation of seven- and eight-membered cyclic
alkoxyamines. The new alkoxyamines, six-, seven- and
eight-membered cyclic systems, were tested as regula-
tors/initiators for the polymerization of styrene and
n-butyl acrylate. We showed that ring-enlargement
positively influences the quality of the polymerization
process (polymerization time, livingness, PDI). The
seven-membered cyclic alkoxyamines are far superior
than the corresponding six-membered alkoxyamines.
tive example. The decomposition rate constants (kdec
)
for the various alkoxyamines were determined using eq
2, according to Fukuda’s work5l ([S] ) styrene concen-
tration; [A] ) alkoxyamine concentration).
ln([S]/[A] + 1) ) kdec
t
(2)
In Figure 5, a plot of ln([S]/[A] + 1) vs time is depicted
for the decomposition of the alkoxyamines 2 and 6. The

3084 Schulte and Studer
Macromolecules, Vol. 36, No. 9, 2003
Fukuda, T. Macromol. Chem. Phys. 2000, 201, 2138. (g)
Benoit, D.; Grimaldi, S.; Robin, S.; Finet, J .-P.; Tordo, P.;
Gnanou, Y. J . Am. Chem. Soc. 2000, 122, 5929. (h) Marque,
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provided similar results. Efficient controlled and living
styrene polymerization (molecular weight of up to
40 000) was achieved using ketoalkoxyamines 2 and 3.
In addition, the kinetics of the C-O bond homolysis of
the alkoxyamines were presented. We found that the
seven- and eight-membered cyclic alkoxyamines have
similar rate constants, whereas the six-membered sys-
tems are homolysing slower. This is in agreement with
the polymerization results. Furthermore, we found that
a keto function within the ring in these cyclic alkoxy-
amines is important (“keto effect”). Although C-O bond
homolysis of the hydroxyalkoxyamines occurs faster
than homolysis of the corresponding ketones, the ke-
toalkoxyamines are better initiator/regulators. The
reason for the improved efficiency of the keto-alkoxy-
amines as compared to the corresponding hydroxy
derivatives lies in the increased thermal stability of the
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Ack n ow led gm en t . We thank the Deutsche For-
schungsgemeinschaft (STU 280/1-1) and the Swiss
Federal Office for Education and Science (COST D12
action, D12/0005/98) for funding. Dr. Olaf Burghaus is
acknowledged for assistance during the EPR experi-
ments and Meike Cordes and Christoph Knoop for
conducting some experiments. Furthermore, we thank
two reviewers for helpful suggestions.
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(17) We also studied the thermal stability of the free nitroxides
10 and 14 in t-BuPh (0.06 mM) at 407 K using EPR
spectroscopy. Experiments were stopped after 3 h. No de-
composition was observed (within the error limits of the
integration of the EPR signals) during that time. The t-BuPh
solution of nitroxide 10 (0.06 mM) was further heated at 393
K for 9 days. EPR analysis revealed that still 5% of the
nitroxide is present in the solution after 9 days, proving the
high thermal stability of 10.
MA025806T