Synthesis and characterization of a novel bisnitroxide initiator for effecting "outside-in" polymerization

Article abstract of DOI:10.1021/ma051217m

A new bis-α-hydrogen nitroxide based initiator for use in nitroxide-mediated free radical polymerization (NMRP) has been synthesized and is demonstrated to effect controlled polymerizations. The bisnitroxide forms "living" systems; the resulting polymers are formed with low polydispersities. The addition of a second block is facile, forming symmetrical ABA triblock copolymers. Monomer addition occurs at the center of the living polymer. This "outside-in" strategy overcomes previous restrictions in sequence specificity encountered in preparing ABA triblock copolymers using NMRP by the more traditional, complementary "inside - out" bidirectional nitroxide initiators.

Full text of DOI:10.1021/ma051217m

9066
Macromolecules 2005, 38, 9066-9074
Synthesis and Characterization of a Novel Bisnitroxide Initiator for
Effecting “Outside-In” Polymerization
Nicole L. Hill and Rebecca Braslau*
Department of Chemistry and Biochemistry, University of California, Santa Cruz, California 95064
Received June 9, 2005; Revised Manuscript Received August 19, 2005
ABSTRACT: A new bis-R-hydrogen nitroxide based initiator for use in nitroxide-mediated free radical
polymerization (NMRP) has been synthesized and is demonstrated to effect controlled polymerizations.
The bisnitroxide forms “living” systems; the resulting polymers are formed with low polydispersities.
The addition of a second block is facile, forming symmetrical ABA triblock copolymers. Monomer addition
occurs at the center of the living polymer. This “outside-in” strategy overcomes previous restrictions in
sequence specificity encountered in preparing ABA triblock copolymers using NMRP by the more
traditional, complementary “inside-out” bidirectional nitroxide initiators.
Introduction
2 based on di-tert-butyl nitroxide radical with a styryl-
like linker in 1996 and demonstrated the “living”
polymerization of styrene. Also in 1996, Priddy et al.7
reported a similar initiator 3 built with TEMPO. Cleav-
able carbonate and dicarbonate linked bis-TEMPO
initiators 4 and 5 were developed by Knauss et al.8 and
Korn and Gagne,9 respectively. Gnanou et al.10 devel-
oped the SG1-based bisnitroxide initiator 6 containing
central hydrolyzable ester linkages. Schmidt-Naake et
al.11 developed both bis-TEMPO 7a and bis-TIPNO 7b
N-alkoxyamine initiators. All of these initiators grow
from the “inside-out”, resulting in polymers bearing
nitroxide caps at both chain termini.
The complementary “outside-in” approach generates
polymers in which a bisnitroxide resides at the center
of the resulting polymer chain. Charleux et al.12 and
Rassat et al.13 have synthesized and studied the polym-
erization of initiators 8 and 9, respectively, and Long
et al.14 have developed initiator 10, which is similar to
8 but contains carbamate instead of ester linkages
(Figure 1). All of these “outside-in” bisnitroxide initia-
tors are TEMPO-based and are therefore limited to the
use of styrenic monomers.
The distinction between the “inside-out” vs the “out-
side-in” approach is very important, as there is a
limiting sequence specificity observed in NMRP.15 The
use of TIPNO16 or SG117 type nitroxides allows the use
of a variety of monomers to design copolymers with
highly tunable properties; however, the order of block
construction is important. It is possible to add either a
polystyrene or a polyisoprene B block to a polyacrylate
or polyacrylamide macroinitiator, but a polystyrene or
polyisoprene macroiniator will not readily add a poly-
acrylate or polyacrylamide B block with adequate
control. Thus, the development of an “outside-in” initia-
tor based on an R-hydrogen-containing nitroxide makes
it possible to prepare amphiphilic ABA triblock copoly-
mers with a hydrophilic A block derived from a wide
variety of monomers including acrylates or acrylamides.
Herein we present a novel bis-N-alkoxyamine 1 that
polymerizes in an “outside-in” fashion built on a TIPNO
type of bisnitroxide (Figure 1), therefore permitting
access to a wide variety of monomer building blocks.
Triblock copolymers that were previously unaccessible
by the traditional NMRP bisnitroxide initiators are now
easily prepared.
“Living” polymerization techniques including nitrox-
ide-mediated radical polymerization1 (NMRP), atom
transfer radical polymerization2 (ATRP), and reversible
addition fragmentation transfer3 (RAFT) have evolved
as excellent methods for the controlled growth of
polymeric materials. These methods are often comple-
mentary, as each has advantages and disadvantages.
For example, functionality such as nitriles and free
amines, which can act as ligands on copper, are compat-
ible with NMRP. ATRP has the advantage that it
functions at lower temperatures, while RAFT provides
controlled polymerization of monomers such as vinyl
acetate that are not compatible with NMRP and ATRP.
A disadvantage of NMRP is that it must be run at
relatively high temperatures. ATRP and RAFT systems
may be contaminated with components of the initiators
after the polymerization; polymers produced with ATRP
usually contain trace metal contaminates while poly-
mers constructed with RAFT may be contaminated with
highly colored sulfur byproducts. All of these systems
are commonly considered “living” as once an initial
polymer is generated, the entire chain can be utilized
as a macroinitiator for subsequent polymerizations.4
Block copolymers have specialized and highly tunable
properties; they are often constructed via “living” po-
lymerization techniques. One class of block copolymers
of particular interest are ABA triblock copolymers in
which the A blocks are hydrophilic and the B block is
hydrophobic. With judicious choice of block length and
monomer selection, these polymers can self-assemble to
form vesicles and membranes.5 Using NMRP, tradi-
tional N-alkoxyamine initiators can generate triblock
copolymers in a linear, three-step fashion. An attractive
alternative is the development of bidirectional initiators
capable of growing in two directions simultaneously. In
addition to being convergent, the use of a bidirectional
initiator ensures that both outside blocks are of the
same length and that the polymer chain ends are
identical, forming symmetrical polymers (Scheme 1).
A number of bisnitroxide initiators have been devel-
oped (Figure 1). Catala et al.6 first presented initiator
* To whom correspondence should be addressed: Tel (831) 459-
3087; Fax (831) 459-2935; e-mail braslau@chemistry.ucsc.edu.
10.1021/ma051217m CCC: $30.25 © 2005 American Chemical Society
Published on Web 10/07/2005

Macromolecules, Vol. 38, No. 22, 2005
A Novel Bisnitroxide Initiator 9067
Figure 1. Bisnitroxide initiators that polymerize “inside-out” or “outside-in”.
Scheme 1. “Inside-Out” vs “Outside-In” Polymerization
coated with silica gel (0.25 nm thick). Reactions were carried
out under a nitrogen atmosphere unless otherwise noted.
Polymerizations were carried out in sealed ampules.
Experimental Section
General Materials and Methods. Styrene (St) (99%,
Acros Organics), n-butyl acrylate (nBA) (99+%, Acros Organ-
ics), tert-butyl acrylate (tBA) (98%, Aldrich), and dimethy-
lacrylamide (DMA) (99%, Aldrich) were distilled immediately
before use under vacuum. Trimethylsilyl chloride (Acros, 98%)
was freshly distilled from poly(vinylpyridine). 2,5-Dimethyl-
hexane-2,5-diol (97%), chloroacetonitrile (99%), isopropyl-
magnesium chloride, and sodium borohydride were purchased
from Aldrich. Benzaldehyde (98%) was purchased from Alfa
Aesar. Cupric acetate was purchased from Mallinchkrodt
Chemical Works. All otherssthiourea, m-chloroperbenzoic acid
(70-75%), 1,4-diazobicyclo[2.2.2]octane (97%), and Jacobsen’s
catalyst (R,R)swere purchased from Acros. Unless otherwise
noted, all other reagents were used as received. Tetrahydro-
furan (THF) was distilled from sodium/benzophenone when
anhydrous conditions were required. Acetonitrile was distilled
over calcium hydride. Water was deionized. Flash chromatog-
raphy was performed using EM Science Silica Gel 60. Analyti-
cal TLC was performed using commercial Whatman plates
Analytical Techniques. NMR spectra were recorded at
250 MHz (Bruker ACF dual probe 250 MHz) or 500 MHz
(Varian 500 MHz) as noted in CDCl3. Mass spectra were
obtained on an electrospray ionization time-of-flight (ESITOF)
mass spectrometer (Mariner Biospectrometry workstation from
Applied Biosciences). Melting points are uncorrected. FTIR
spectra were recorded in deuteriochloroform on a Perkin-Elmer
1600 FTIR spectrometer. Elemental analysis was performed
by M-H-W Laboratories in Phoenix, AZ. Gel permeation
chromatography (GPC) was performed using a Waters ap-
paratus equipped with five Styragel columns (300 × 4.6 mm,
5 µm bead size), HR 0.5 (pore size 50 Å, 0-1000 Da), HR 1
(pore size 100 Å, 100-5000 Da), HR 2 (pore size 500 Å, 500-
20 000 Da), HR 4 (pore size 10 000 Å, 50-100 000 Da), HR
5E (linear bed, mixed pore sizes, 2000-4 × 106 Da). Tetra-
hydrofuran (THF) was used as the eluent at a flow rate of 0.35
mL/min at ambient temperature. A refractive index detector

9068 Hill and Braslau
Macromolecules, Vol. 38, No. 22, 2005
was used, and the molecular weights were calibrated against
seven polystyrene standards ranging from 2000 to 156 000 Da.
Bis-N,N′-(2-chloroacetyl)-2,5-dimethylhexane-2,5-di-
amine (12). 2,5-Dimethylhexane-2,5-diol, 11 (10.00 g, 68.39
mmol), was combined with chloroacetonitrile (20.51 g, 273.5
mmol) in 22 mL of glacial acetic acid. The solution was placed
in an ice bath, and 22 mL (410 mmol) of concentrated sulfuric
acid was added dropwise, causing the solution to turn amber
in color. The reaction was allowed to warm to room temper-
ature and stirred overnight. Next, 275 mL of ice-chilled water
was added, forming a white precipitate. The precipitate was
filtered and washed twice with 140 mL of saturated NaHCO3
solution and three times with 140 mL of water. The solid was
recrystallized from aqueous ethanol, affording 13.68 g (46.21
mmol, 68% yield) of white crystals; mp ) 153 °C. TLC: 1:1
hexanes:EtOAc, UV, molybdate, Rf ) 0.39. IR (CDCl3): 3413
(N-H), 1682 cm-1 (CdO). 1H NMR (250 MHz, CDCl3) δ: 6.28
(br s, 2H), 3.94 (s, 4H), 1.73 (s, 4H), 1.32 (s, 6H) ppm. 13C NMR
(62.5 MHz, CDCl3) δ: 165.2, 54.0, 43.0, 33.2, 27.0 ppm.
HRMS: M + 1 (C12H23N2O2Cl2) 297.113 calcd; 297.112 obsd.
2,5-Dimethylhexane-2,5-diamine (13). Bis-N,N′-(2-chlo-
roacetyl)-2,5-dimethylhexane-2,5-diamine, 12 (13.68 g, 46.21
mmol), was dissolved in 275 mL of ethanol, and 8.44 g (111
mmol) of thiourea was added. Glacial acetic acid (55 mL) was
added, and the solution was refluxed overnight, forming a
white precipitate. Ice-cold water (650 mL) was added to the
mixture, and the precipitate was filtered. The filtrate was
made basic by the addition of 6 M NaOH until the pH was
between 10 and 11. The filtrate was extracted twice with 700
mL of CHCl3. The organic layers were combined and dried over
MgSO4, and the volatiles were removed in vacuo affording 4.53
g (31.0 mmol, 67% yield) of a clear yellow oil. TLC: 1:1
methanol:EtOAc, UV, cobalt TLC dip, Rf ) 0.13. IR (CDCl3):
3358 (N-H), 2855 (C-H), 1368 cm-1 (C-N). 1H NMR (250
MHz, CDCl3) δ: 1.38 (s, 4H), 1.27 (br s, 4H), 1.09 (s, 12H) ppm.
13C NMR (62.5 MHz, CDCl3) δ: 49.2, 39.2, 30.3 ppm. HRMS:
M + 1 (C8H21N2) 145.170 calcd; 145.170 obsd.
N,N′-Dibenzylidene-2,5-dimethylhexane-2,5-diamine
(14). Benzaldehyde (4.60 mL, 45.2 mmol), 2,5-dimethylhexane-
2,5-diamine, 13 (3.00 g, 20.5 mmol), and activated 4 Å
molecular sieves were added to a flame-dried round-bottom
flask with stir bar. Anhydrous THF (20 mL) was added via
syringe, and the solution was refluxed overnight. After cooling,
the resulting mixture was filtered through a sintered glass
funnel, and the filter cake was washed with diethyl ether until
it appeared white in color. The filtrate was concentrated in
vacuo, affording 7.43 g of a yellow solid which was recrystal-
lized from hexanes to give 5.88 g (18.4 mmol, 89% yield) of
pale yellow crystals; mp ) 93 °C. TLC: CH2Cl2, UV, Rf ) 0.60.
IR: (CDCl3) 2947 (C-H), 1642 cm-1 (CdN). 1H NMR (250
MHz, CDCl3) δ: 8.23 (s, 2H), 7.75 (m, 4H), 7.39 (m, 6H), 1.59
(s, 4H), 1.26 ppm (s, 12H). 13C NMR (125 MHz, CDCl3)
δ: 155.5, 137.4, 130.2, 128.6, 128.0, 59.5, 37.7, 27.4 ppm.
HRMS: M + 1 (C22H29N2) 321.233 calcd; 321.236 obsd.
N,N′-Dibenzylidene-2,5-dimethylhexane-2,5-bisoxa-
ziridine (15). N,N′-Dibenzylidene-2,5-dimethylhexane-2,5-
diamine, 14 (694.1 mg, 2.167 mmol), was combined with 11
mL of CHCl3, followed by 919 mg (8.67 mmol) of Na2CO3, and
the solution was placed in an ice bath. An anhydrous solution
of m-chloroperbenzoic acid (mCPBA) was made by dissolving
1.28 g (5.20 mmol) of mCPBA in 43 mL of CHCl3, washing
with brine, drying over MgSO4, and filtering. This mCPBA
solution was added via an oven-dried addition funnel to the
ice-cooled reaction flask, and the mixture was stirred overnight
at room temperature. The reaction mixture was washed twice
with saturated Na2CO3 solution prior to being run three times
through a 2 cm plug of basic alumnia. The solution was dried
over MgSO4 and filtered, and the volatiles were removed in
vacuo, giving 458 mg of 15 as a clear blue oil (1:1 mixture of
diastereomers). This residue solidified and was recrystallized
from hexanes to afford white crystals in 44% yield (335 mg,
0.950 mmol); mp ) 71-73 °C. TLC: 9:1 hexanes:EtOAc, UV,
molybdate, Rf ) 0.27. IR: (CDCl3) 3114 (C-H aromatic) 2933
cm-1 (C-H). 1H NMR (500 MHz, CDCl3, inseparable mixture
of diastereomers) δ: 7.38 (m, 10H), 4.67 (s, 1H, diast A), 4.65
(s, 1H, diast B), 1.68 (m, 4H), 1.14 (s, 6H, diast B), 1.13 (s,
6H, diast A), 1.07 (s, 6H, diast A), 1.06 ppm (s, 6H, diast B).
13C NMR (62.5 MHz, CDCl3) δ: 158.4, 129.7, 128.4, 127.5, 70.1,
34.9, 22.9, 21.1 ppm. HRMS: M + 1 (C22H29N2O2) 353.222
calcd; 353.221 obsd.
N,N′-Dibenzylidene-2,5-dimethylhexane-2,5-bisnitro-
ne (16). N,N′-Dibenzylidene-2,5-dimethylhexane-2,5-bisox-
aziridine, 15 (2.60 g, 7.38 mmol), was added to a flame-dried
round-bottom flask with a stir bar. Anhydrous acetonitrile (25
mL) was added via syringe, and the mixture was refluxed for
3 days. During this time the reaction turned dark brown. Upon
cooling, light-colored crystals formed which were recrystallized
from acetonitrile to give the dinitrone as light-brown crystals
in 54% yield (1.41 g, 3.99 mmol), mp ) 187 °C. TLC: 1:1
hexanes:EtOAc, UV, Rf ) 0.43. IR (CDCl3): 2951 (C-H), 1579
1
(nitrone), 1190 cm-1 (nitrone). H NMR (250 MHz, CDCl3) δ:
8.29 (m, 4H), 7.5 (s, 2H), 7.41 (m, 6H), 1.86 (s, 4H), 1.58 (s,
12H) ppm. 13C NMR (62.5 MHz, CDCl3) δ: 158.6, 130.8, 130.2,
128.8, 128.4, 72.9, 34.2, 26.6 ppm. HRMS: M + 1 (C22H29N2O2)
353.222 calcd; 353.220 obsd.
2,5,5,8,8,11-Hexamethyl-3,10-diphenyl-4,9-diazadode-
cane-4,9-bishydroxylamine (17). N,N′-Dibenzyl-2,5-di-
methylhexane-2,5-bisnitrone, 16 (2.00 g, 5.67 mmol), 1,4-
diazobicyclo[2.2.2]octane (DABCO) (1.28 g, 11.4 mmol), and
100 mL of THF were added to a flame-dried round-bottom flask
with stir bar. The solution was sonicated for 30 min to help
solubilize the nitrone. Trimethylsilyl chloride was added (1.45
mL, 11.4 mmol), and the mixture was cooled in an ice bath.
Isopropylmagnesium chloride (17 mL, 2.0 M solution in THF)
was added, and the solution was allowed to warm to room
temperature and stirred for 3.5 h. Afterward, 100 mL of a
saturated NH4Cl solution and 45 mL of concentrated NH4OH
were added, and the solution was extracted twice with 100
mL of CH2Cl2. The organic layers were combined, dried over
MgSO4, and filtered, and the volatiles were removed in vacuo.
The product was purified via flash chromatography (hexanes
to 10:1 hexanes:ethyl acetate), affording 1.80 g (4.10 mmol,
72% yield) of the hydroxylamine. TLC: 4:1 hexanes:EtOAc,
UV, molybdate, Rf ) 0.60. 1H NMR (500 MHz, CDCl3) δ: 7.44-
7.27 (m, 10H), 5.22 (br s, 2H), 3.41 (d, 2H, J ) 9.5 Hz), 2.40
(m, 2H), 2.06 (m, 2H), 1.16 (d, 6H, J ) 6 Hz), 1.06 (s, 6H),
1.02 (m, 2H), 0.64 (d, 6H, J ) 6 Hz), 0.59 ppm (s, 6H). 13C
NMR (125 MHz, CDCl3) δ: 4° ipso: 141.8, CH (aromatic):
130.4, 128.0, 126.9, CH-N: 70.9, 4°: 61.4, CH2: 35.3, CH
(iPr): 32.0, CH3: 24.8, 23.0, 22.1, 20.9 ppm.
2,5,5,8,8,11-Hexamethyl-3,10-diphenyl-4,9-diazadode-
cane-4,9-bisnitroxide (18). 2,5-Dimethyl-N,N′-bis(2-methyl-
1-phenylpropyl)hexane-2,5-bishydroxylamine, 17 (1.70 g, 3.89
mmol), was dissolved in 8 mL of MeOH, 16 mL of EtOH, and
1 mL of 2-propanol. Concentrated NH4OH (574 µL) and
catalytic Cu(OAc)2 (39 mg, 0.19 mmol) were added. Air was
bubbled through the suspension until it turned blue. The solid
yellow dihydroxylamine 17 did not completely dissolve. Vola-
tiles were removed in vacuo; the residue was dissolved in 8
mL of CHCl3 and 2.3 mL of saturated NaHSO4 solution. The
organic layer was washed with 8 mL of saturated NaHCO3
solution and dried over MgSO4, and volatiles were removed
in vacuo, affording 1.57 g (3.60 mmol, 92% yield) of 18 as a
bright yellow solid; mp ) 98-109 °C (decomposition). TLC:
4:1 hexanes:EtOAc, UV, p-anisealdehyde, Rf ) 0.29. IR:
(CDCl3) 2872 (C-H), 1387 cm-1 (N-O). HRMS: M + 1
(C28H43N2O2) 439.332 calcd; 439.337 obsd.
2,5,5,8,8,11-Hexamethyl-4,9-(1-phenylethoxy)-3,10-di-
phenyl-4,9-diazadodecane (1). 2,5-Dimethyl-N,N′-bis(2-
methyl-1-phenylpropyl)hexane-2,5-bisnitroxide, 18 (500 mg,
1.14 mmol), was dissolved in 5.7 mL of toluene and 5.7 mL of
EtOH followed by the addition of styrene (317 µL, 2.74 mmol).
Jacobsen’s catalyst (R,R) (290 mg, 0.460 mmol) was added
followed by sodium borohydride (129 mg, 3.40 mmol); the
reaction was stirred overnight open to the atmosphere. Vola-
tiles were removed in vacuo, and the residue was dissolved in
6 mL of CHCl3 and 6 mL of H2O. A small amount of a 10%
HCl solution was added to reduce emulsions within the
separatory funnel. The organic layer was dried over MgSO4,
and volatiles were removed in vacuo. The product was purified

Macromolecules, Vol. 38, No. 22, 2005
A Novel Bisnitroxide Initiator 9069
by three consecutive flash chromatography columns in hexanes
to give the product as a highly viscous green oil in 43% yield
(316 mg, 0.490 mmol) as a mixture of multiple diastereomers.
TLC: 10:1 hexanes:EtOAc, UV, p-anisealdehyde, Rf ) 0.46.
IR: (CDCl3) 2956 (C-H), 1452 (N-O), 1364 (N-O), 1064 cm-1
(C-N). Elemental analysis: C44H60N2O2 calcd: C, 81.43; H,
9.32; N, 4.32. Found: C, 81.65; H, 9.43; N, 4.14. 1H NMR (500
MHz, CDCl3, mixture of diastereomers) δ: 7.45-7.13 (m, 20H),
4.92 (m, 2H, 2 diast), 4.81 (m, 2H, 2 diast) [3.42 (d, 2H, J )
11 Hz), 3.29 (d, 2H, J ) 10.5 Hz), 3.25 (d, 2H, J ) 10.5 Hz),
3.16 (d, 2H, J ) 10.5 Hz) 4 diast], 2.33 (m, 2H) [note: other
diast d of sept. is buried around 1.4 ppm], [1.64 (d, 6H, J )
6.5), 1.57 (d, 6H, J ) 7 Hz), 1.55 (d, 6H, 8 Hz), 1.48 (d, 6H, J
) 7 Hz) 4 diast], [0.76 (s, 6H), 0.75 (s, 6H), 0.68 (s, 6H), 0.67
(s, 6H) 4 diast], [0.55 (d, 6H, J ) 6.5 Hz), 0.51 (d, 6H, J ) 5.5
Hz), 0.20 (d, 6H, J ) 6.5 Hz), 0.17 (d, 6H, J ) 6.5) 4 diast],
1.52-0.84 ppm (m, 18H). 13C NMR (125 MHz, CDCl3, mixture
of 4 diastereomers) δ [4° ipso: 146.2, 146.0, 145.3, 143.1, 142.8]
[CH (aromatic): 130.9, 128.2, 127.63, 127.58, 127.5, 127.4,
127.2, 127.1, 126.8, 126.6, 126.4, 126.3, 126.2, 126.1] [CH-O:
83.5, 83.3, 83.1, 83.0] [CH-N: 71.9, 71.7] [4°: 63.3, 63.2] [CH2:
35.7, 35.5, 35.2, 34.9] [CH3: 32.4, 31.9, 31.8, 26.5, 26.4, 26.2,
26.1, 25.8, 25.7, 25.6, 25.5, 25.3, 23.7, 23.6, 22.5, 22.3, 22.2,
21.5, 21.3] ppm.
General Polymerization Procedure. A representative
example is as follows: A mixture of N-alkoxyamine (0.087
mmol, 1 equiv), styrene (17.46 mmol, 200 equiv), and bisni-
troxide (when added) (0.0044 mmol, 0.05 mol equiv) was
degassed in an ampule by three consecutive freeze/pump/thaw
cycles and sealed under argon. The vial was heated to 120-
125 °C until the solution became just slightly viscous. After
cooling, a small aliquot was taken for a crude 1H NMR to
calculate percent conversion based on integrating the remain-
ing monomer vs polymer peaks, which indicated 72% monomer
conversion in this case. The remainder of the polymer sample
was dissolved in a minimum amount of CH2Cl2 (unless
otherwise noted below) and the ice-chilled precipitation solvent
(6-10 mL) was added dropwise. The precipitated polymer was
separated by decantation, and volatiles were removed in vacuo.
This precipitation procedure was repeated two times to give
1.4 g of purified polymer which was analyzed by 1H NMR and
GPC (Mn ) 16 200 g/mol, polydispersity index (Mw/Mn) ) 1.21).
Polymers were precipitated as follows: PS with methanol,
PDMA with hexanes, PtBA was dissolved in THF and pre-
cipitated with 50% aqueous methanol, and PnBA was dissolved
in THF and precipitated with methanol.
PtBA-PS-PtBA Triblock Copolymer. The procedure to
make the ABA triblock copolymer PtBA-PS-PtBA (second
entry Table 4) is typical using 100 mg (0.0052 mmol) of the
PtBA macroinitiator with 400 equiv of styrene for 1 h. The
ABA polymer was dissolved in THF and precipitated with
methanol (Mn ) 41 300, polydispersity index (Mw/Mn) ) 1.25).
PtBA-PnBA-PtBA Triblock Copolymer. The procedure
to make the ABA triblock copolymer PtBA-PnBA-PtBA
(fourth entry Table 4) is typical using 470 mg (0.030 mmol) of
the PtBA macroinitiator with 200 equiv of nBA for 3 h. The
polymer was dissolved in CH2Cl2 and precipitated with metha-
nol (Mn ) 28 700, polydispersity index (Mw/Mn) ) 1.26).
PDMA-PnBA-PDMA Triblock Copolymer. The proce-
dure to make the ABA triblock copolymer PDMA-PnBA-
PDMA (sixth entry Table 4) is typical using 514 mg (0.035
mmol) of the PDMA macroinitiator with 400 equiv of nBA in
500 µL of DMF as solvent. The polymerization was run for 4
h, and the polymer was dissolved in CH2Cl2 and precipitated
with diethyl ether (Mn ) 46 200, polydispersity index (Mw/Mn)
) 1.43).
mg. 0.0873 mmol, 16 equiv), and 8 mL of anisole were
combined and placed in a 100 °C oil bath for 16 h open to the
atmosphere. The majority of the anisole was then removed by
vacuum distillation. A 50 µL aliquot of the residue was taken
for GPC analysis (Mn original ) 11 200 g/mol; Mn after
cleavage ) 5900 g/mol).
Example Procedure for Thermal Decomposition. Poly-
mer sample 8 (50.0 mg, 0.0055 mmol) was dissolved in 2 mL
of p-xylene and heated to 130 °C for 16 h. The majority of the
p-xylene was removed by vacuum distillation. A 50 µL aliquot
of the residue was taken for GPC analysis (Mn original )
16 200 g/mol; Mn after cleavage ) 10 900).
Cleavage of PDMA with Zn°/HOAc. Polymer sample 4
(300.0 mg, 0.026 mmol, 1 equiv) and freshly activated zinc
metal (washed sequentially three times each with 5% HCl,
water, methanol, and diethyl ether) (900.0 mg, 13.77 mmol)
were combined in 100 mL of 60% aqueous acetic acid. The
solution was heated to 60 °C open to the atmosphere for 16 h.
After cooling, 100 mL of 5 M KOH was added followed by
addition of KOH pellets until the pH was between 11 and 13.
This mixture was extracted twice with 150 mL of CH2Cl2,
washed with brine, and dried over MgSO4, and volatiles were
removed in vacuo yielding 30 mg (0.0055 mmol) of cleaved
polymer, which was analyzed by GPC (Mn original ) 11 500
g/mol; Mn after cleavage ) 7000 g/mol).
Results and Discussion
The synthesis of initiator 1 began with a modified
Ritter reaction18 on the tertiary bisdiol 11 using chlo-
roacetonitrile under strongly acidic conditions to give
the R-chloro amide 12. Chloride displacement by thio-
urea followed by intramolecular transamidification in
acidic ethanol gave bisamine 13 (Scheme 2). This
tertiary amine was condensed with benzaldehyde to give
bisimine 14 in 89% yield. Oxidation of the imine bond
was performed with mCPBA to form bisoxaziridine 15
as a mixture of diastereomers. Thermolysis19 in reflux-
ing acetonitrile provided bisnitrone 16 in 54% yield. The
next step of the synthesis required the nucleophilic
introduction of an isopropyl group to the nitrone func-
tionality, which proved challenging. After examining the
addition of isopropyl Grignard with several Lewis acids
(CeCl3, ZnCl2, and various copper reagents), it was
found that addition of freshly distilled TMS-Cl resulted
in a satisfying 72% yield of the adduct 17 after purifica-
tion. Oxidation of the hydroxylamine species with air
and catalytic Cu(OAc)2 gave the bisnitroxide 18. Last,
formation of the N-alkoxyamine initiator using styrene
and Jacobsen’s catalyst following the method of Hawker20
gave bidirectional initiator 1 in 43% yield after purifica-
tion (Scheme 2).
An alternative approach to the preparation of a
bisnitroxide similar to TIPNO was envisioned to begin
with a nitro compound. Bisnitro 19 is commercially
available and was examined as a possible starting
material following the standard procedure developed in
these labs for the preparation of TIPNO.16 Thus, reduc-
tion with zinc and acid converted the nitro groups to
hydroxylamines in situ, which condensed with isobuty-
laldehyde. However, instead of forming the desired
bisnitrone 21, the cyclic aminal 20 was formed; the
oxidation product led to the formation of a brillant pink
reaction mixture (Scheme 3).21 This undesired cycliza-
tion was probably enhanced by the gem dimethyl
effect.22 Thus, the route beginning with this nitro
starting material was abandoned.
[PtBA-r-PDMA]-PnBA-[PtBA-r-PDMA] Triblock Co-
polymer. The procedure to make the ABA triblock copolymer
[PtBA-r-PDMA]-PnBA-[PtBA-r-PDMA] (eighth entry Table
4) is typical using 260 mg (0.029 mmol) of the PtBA/PDMA
macroinitiator with 200 equiv of nBA for 4 h. The polymer
was dissolved in CH2Cl2 and precipitated with diethyl ether
(Mn ) 29 200, polydispersity index (Mw/Mn) ) 1.27).
With bisnitroxide initiator 1 in hand, polymerizations
were carried out with styrene (St), tert-butyl acrylate
(tBA), n-butyl acrylate (nBA), and dimethylacrylamide
(DMA) (Table 1). The molecular weights predicted on
Example Procedure for Nitroxide Exchange. The poly-
mer sample 8 (50.0 mg, 0.0055 mmol, 1 equiv), TEMPO (13.7

9070 Hill and Braslau
Macromolecules, Vol. 38, No. 22, 2005
Scheme 2. Synthetic Route to the “Outside-In” Bidirectional Initiator 1
Scheme 3. Attempted Route to a Bisnitroxide Using
the Commercially Available Bisnitro Compound 19
relationship between the predicted molecular weights
based on 1H NMR data and the experimental molecular
weights obtained from GPC during the polymerization
of both styrene and tert-butyl acrylate with initiator 1,
indicating an absence of chain transfer events (Table
2). The polydispersities are also presented in these
figures. During the polymerization of styrene, a low
molecular weight shoulder is observed in the GPC traces
as the polymerization proceeds (vide infra). During the
polymerization of tert-butyl acrylate, the polydispersities
continue to improve throughout the polymerization,
which is consistent with statistical predictions. From
these data, it is clear that initiator 1 behaves as a
“living” system. While clearly initiator 1 effects polym-
erization in a living manner and polymerizes a variety
Table 1. Polymerization of Various Monomers Using
Bidirectional Initiator 1 at 120-125 °C
b
c
d
time
Mn,calc
Mn
Mw
entry monomera (min) (g/mol) (g/mol) (g/mol) PDe
1
2
3
4
St
120
180
105
105
15 600 16 200 19 600 1.21
19 100 22 600 27 600 1.22
16 300 17 100 20 700 1.21
16 100 11 500 13 700 1.19
tBAf
nBAf
DMAf
a 200 equiv was used, all polymerizations were run neat.
b Number-average molar mass calculated from percent conversion
determined by 1H NMR; see Supporting Information for an
example calculation. c Number-average molar mass measured by
GPC. d Weight-average molar mass measured by GPC. e Polydis-
persity index (Mw/Mn) by GPC. f 5% of bisnitroxide 18 was added
(St ) styrene, tBA ) tert-butyl acrylate, nBA ) n-butyl acrylate,
DMA ) dimethylacrylamide).
the basis of 1H NMR percent conversion correlate with
those observed via gel permeation chromatography
(GPC), and good polydispersities were obtained. A linear
relationship exists between ln([M]0/[M]) and time in the
polymerization of styrene (Figure 2) and tert-butyl
acrylate (Figure 3), indicating no significant termina-
tion. Figures 4 and 5 graphically represent the linear
Figure 2. Styrene polymerization with initiator 1, neat at
120 °C, as a function of ln([M]0/[M]) and time.

Macromolecules, Vol. 38, No. 22, 2005
A Novel Bisnitroxide Initiator 9071
Figure 3. tert-Butyl acrylate polymerization with initiator
1, neat at 120 °C, as a function of ln([M]0/[M]) and time.
Figure 6. Exchange of internal bisnitroxide in polysytrene
sample 22 with excess TEMPO in anisole at 100 °C: the
resulting polymer is approximately half the original molecular
weight. Bisnitroxide 22: Mn ) 11 200 g/mol; TEMPO-23:
Mn ) 5900 g/mol.
cess.23 Thus, heating a polymer prepared with bidirec-
tional initiator 1 in the presence of a large excess of
TEMPO should result in nitroxide exchange. As R-hy-
drogen-substituted nitroxides such as TIPNO and bis-
nitroxide 18 decompose upon prolonged heating, whereas
TEMPO is stable at polymerization temperatures, over
time all polymer chains should be capped by TEMPO
and should be half the length of the initial polymer
(Scheme 4). Thus, a sample of polystyrene 22 prepared
with initiator 1 (entry 8 from Table 2) (Mn ) 11 200,
PD ) 1.18) was heated with 16 equiv of TEMPO at 100
°C for 16 h. The resulting polymer 23 showed a single
peak by GPC (Mn ) 5900, PD ) 1.21) (Figure 6).
Another way to remove the nitroxide moiety from the
polymer is by extended heating to effect nitroxide
thermal decomposition following the method of Long.14
Thus, each of the polymer samples 1, 2, 3, 4, and 8 from
Table 2 were subjected to heating at 130 °C in p-xylene
for 16 h. As can be seen in Table 3, each sample was
cleaved to form a polymer of approximately half the
original molecular weight. Finally, it is common to
reductively cleave nitrogen-oxygen bonds using zinc
and acetic acid.24 However, solubility problems arise
when this procedure is applied to hydrophobic polymer
samples. Although polystyrene and poly(n-butyl acry-
late) samples were recovered unchanged upon treatment
with activated zinc and acetic acid, the water-soluble
poly(dimethyacrylamide) was successfully cleaved (last
entry of Table 3).
Figure 4. Evolution of Mn (molecular weight as calculated
1
by H NMR) and polydispersity (Mw/Mn) as a function of Mn
determined by GPC for the polymerization of styrene neat at
120 °C with bidirectional initiator 1.
Figure 5. Evolution of Mn (molecular weight as calculated
1
by H NMR) and polydispersity (Mw/Mn) as a function of Mn
Convinced that initiator 1 does indeed grow polymer
chains in both directions, the formation of block copoly-
mers was investigated (Table 4). An inside B block of
styrene or n-butyl acrylate was added to A block
macroinitiators consisting of poly(tert-butyl acrylate),
poly(dimethyacrylamide), and a random 1:1 copolymer
of poly(tert-butyl acrylate)-r-poly(dimethylacrylamide)
determined by GPC for the polymerization of tert-butyl acry-
late neat at 120 °C with bidirectional initiator 1.
of monomers, it was still necessary to demonstrate
growth at both ends of the initiator.
Crossover experiments have clearly demonstrated
that nitroxide exchange during NMRP is a facile pro-
Scheme 4. Exchange of Internal Bisnitroxide with Excess TEMPO

9072 Hill and Braslau
Macromolecules, Vol. 38, No. 22, 2005
Table 2. Polymerization with Bidirectional Initiator 1 of
Styrene and tert-Butyl Acrylate
c
d
time monomer Mn,calc
Mn
entry monomera (min)
% convb
(g/mol) (g/mol) PDe
5
6
St
15
30
19
32
39
44
62
70
70
80
13
21
29
27
36
47
83
87
4 600
7 300
8 800
4 600 1.30
7 600 1.22
9 100 1.19
7
45
8
9
60
9 200 11 200 1.18
13 600 13 600 1.20
15 200 14 200 1.20
15 200 14 900 1.21
17 300 16 500 1.21
4 000
6 000
8 000 11 900 1.28
7 600 10 000 1.31
9 900 11 500 1.28
12 700 14 600 1.26
21 900 21 700 1.23
22 900 25 000 1.23
75
10
11
12
13
14
15
16
17
18
19
20
90
105
120
45
tBAf
4 200 1.52
7 300 1.36
60
75
90
105
120
210
300
Figure 7. GPC trace of PtBA-PnBA-PtBA triblock copoly-
mer. Mn of PtBA ) 18 900 g/mol; Mn of PtBA-PnBA-PtBA )
28 700 g/mol.
a 200 equiv was used. b Calculated by 1H NMR. c Number-
average molar mass calculated from percent conversion deter-
mined by 1H NMR. d Number-average molar mass measured by
GPC. e Polydispersity index (Mw/Mn) from GPC. f 5% of bisnitrox-
ide 18 was added (St ) styrene, tBA ) tert-butyl acrylate).
of the peak.12,14 If one of the chains of the bidirectional
nitroxide terminates as the polymerization proceeds, the
resulting unidirectional polymer chains would be ap-
proximately half the molecular weight of the rest of the
sample. All of the previously developed bisnitroxide
initiators are TEMPO based; therefore, the polymeriza-
tions were performed with styrenic monomers. Bidirec-
tional initiator 1 also displays this shoulder in the GPC
traces of polystyrene starting at ∼60% conversion
(Figure 8), however not in the GPC traces of acrylate
or acrylamide polymers, even at high conversions
(Figure 9). This is a particularly interesting observation;
further work is necessary to understand this phenom-
enon. Another interesting property of initiator 1 is that
(Table 4). An example GPC trace is shown in Figure 7.
To ensure the B blocks were adding to both internal
C-O N-alkoxyamine bonds of the macroinitiators, three
of the triblock copolymers were subjected to the nitrox-
ide crossover reaction with excess TEMPO (Table 5). In
each case, the resulting AB diblock copolymers were of
significant lower molecular weight and showed similar
polydispersities to the original ABA triblocks.
Other groups who have developed “outside-in” bisni-
troxide initiators have found that the GPC traces for
these polymers often display a shoulder on the right side
Table 3. Central Cleavage of Polymers Prepared with Bidirectional Initiator 1
monomer
original Mna (g/mol) Mna after cleavage (g/mol) PDb
11 200 5 900 1.21
sample
method
8
1
2
3
4
8
4
St
St
100 °C, TEMPO
130 °C, 16 h
130 °C, 16 h
130 °C, 16 h
130 °C, 16 h
130 °C, 16 h
60 °C, Zn°/HOAc
16 200
22 600
17 100
11 500
11 200
11 500
10 900
14 600
14 000
8 000
1.14
1.32
1.33
1.43
1.22
1.42
tBA
nBA
DMA
St
5 600
DMA
7 000
a Number-average molar mass measured by GPC. b Polydispersity index (Mw/Mn) from GPC after cleavage (St ) styrene, tBA ) tert-
butyl acrylate, nBA ) n-butyl acrylate, DMA ) dimethylacrylamide).
Table 4. Addition of B Blocks To Form ABA Triblock Copolymers
Mn,calca (g/mol)
Mnb (g/mol)
PDc
A block
A-B-A triblock
A block
A-B-A triblock
A block
A-B-A triblock
A block
A-B-A triblock
PtBA
19 100
29 500
15 800
28 100
14 800
61 700
9100
22 600
41 300
18 900
28 700
14 800
46 200
9400
1.22
1.25
1.22
1.26
1.28
1.43
1.29
1.27
PtBA-PS-PtBA
PtBA
PtBA-PnBA-PtBAd
PDMA
PDMA-PnBA-PDMAd
[PtBA-r-PDMA]d,e
[PtBA-r-PDMA]-PnBA-[PtBA-r-PDMA]d
29 600
29 200
a Number-average molar mass calculated from percent conversion determined by 1H NMR. b Number-average molar mass measured
by GPC. c Polydispersity index (Mw/Mn) from GPC. d 5% of bisnitroxide 18 was added. e 100 equiv of each monomer was used. For equivalents
of monomer used and polymerization times, please see Experimental Section (St ) styrene, tBA ) tert-butyl acrylate, nBA ) n-butyl
acrylate, DMA ) dimethylacrylamide).
Table 5. Nitroxide Exchange to Cleave ABA Triblock Copolymers to AB Diblock Copolymers with Excess TEMPO
sample
original Mna (g/mol)
Mna after cleavage (g/mol)
PDb original
PDb cleaved
PtBA-PnBA-PtBA
28 700
46 200
29 200
20 000
31 100
19 300
1.26
1.43
1.27
1.24
1.58
1.26
PDMA-PnBA-PDMA
[PtBA-r-PDMA]-PnBA-[PtBA-r-PDMA]
a Number-average molar mass measured by GPC. b Polydispersity index (Mw/Mn) from GPC.

Macromolecules, Vol. 38, No. 22, 2005
A Novel Bisnitroxide Initiator 9073
Supporting Information Available: Spectra with full
assignments and example calculations. This material is avail-
able free of charge via the Internet at http://pubs.acs.org.
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Conclusion
The new “outside-in” bisnitroxide initiator 1 was
synthesized, beginning with the conversion of a tertiary
bisalcohol to a tertiary bisamine and proceeding through
a key oxaziridine to nitrone rearrangement, to provide
a new synthetic route to N-alkoxyamines. This new
bidirectional initiator effects polymerization of several
types of monomers in a “living”, well-controlled manner.
Symmetrical ABA triblock copolymers are easily pre-
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Acknowledgment. The authors thank Research
Corporation (RA0336) and NSF (CHE0078852) for
financial support.

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MA051217M