Electron Spin Resonance Study of Monomeric, Dimeric, and Polymeric Acrylate Radicals Prepared Using the Atom Transfer Radical Polymerization Technique - Direct Detection of Penultimate-Unit Effects

Article abstract of DOI:10.1021/ma035516s

Model radicals generated through the reaction of organotin compounds with radical precursors prepared by atom transfer radical addition (ATRA) or atom transfer radical polymerization (ATRP) were studied by electron spin resonance (ESR). Dimers of various (meth)acrylate units, methyl acrylate (MA), tert-butyl acrylate (tBA), and methyl methacrylate (MMA), MA-MA-Br, MA-tBA-Br, tBA-tBA-Br, MMA-tBA-Br, and MA-MMA-Br, were prepared by ATRA. These dimers provided radicals mimicking the active species in a radical copolymerization of MA, MMA, and tBA. Well-resolved ESR spectra of the dimeric radicals confirm electronic and steric effects of the penultimate unit on the propagating radical. Hyperfine splitting constants observed for monomeric, dimeric, oligomeric, and polymeric tert-butyl acrylate radicals depend on the polymer chain length. The variation of ESR spectra with the chain length and penultimate unit was compared with the rate constants of activation in an ATRP using a CuBr/bpy catalyst system.

Full text of DOI:10.1021/ma035516s

1
378
Macromolecules 2004, 37, 1378-1385
Electron Spin Resonance Study of Monomeric, Dimeric, and Polymeric
Acrylate Radicals Prepared Using the Atom Transfer Radical
Polymerization TechniquesDirect Detection of Penultimate-Unit Effects
,
†
‡
‡
Atsu sh i Ka jiw a r a ,* Aja ya Ku m a r Na n d a , a n d Kr zysztof Ma tyja szew sk i
Department of Materials Science, Nara University of Education,
Takabatake-cho, Nara 630-8528, J apan, and Center for Macromolecular Engineering, Department of
Chemistry, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, Pennsylvania 15213
Received October 8, 2003; Revised Manuscript Received November 26, 2003
ABSTRACT: Model radicals generated through the reaction of organotin compounds with radical
precursors prepared by atom transfer radical addition (ATRA) or atom transfer radical polymerization
(ATRP) were studied by electron spin resonance (ESR). Dimers of various (meth)acrylate units, methyl
acrylate (MA), tert-butyl acrylate (tBA), and methyl methacrylate (MMA), MA-MA-Br, MA-tBA-Br, tBA-
tBA-Br, MMA-tBA-Br, and MA-MMA-Br, were prepared by ATRA. These dimers provided radicals
mimicking the active species in a radical copolymerization of MA, MMA, and tBA. Well-resolved ESR
spectra of the dimeric radicals confirm electronic and steric effects of the penultimate unit on the
propagating radical. Hyperfine splitting constants observed for monomeric, dimeric, oligomeric, and
polymeric tert-butyl acrylate radicals depend on the polymer chain length. The variation of ESR spectra
with the chain length and penultimate unit was compared with the rate constants of activation in an
ATRP using a CuBr/bpy catalyst system.
In tr od u ction
Sch em e 1. Gen er a tion of Ra d ica ls fr om
Cor r esp on d in g Alk yl Br om id es
The well-resolved spectra obtained from electron spin
resonance (ESR) spectroscopy provide information not
only on the structure, properties, and concentration of
radicals but also on the initiating and propagating
(
oligomeric and polymeric) of radicals in radical poly-
1
-5
merizations.
Well-resolved ESR spectra in the radical
polymerizations of styrene and its derivatives, diene
compounds, methacrylates, and vinyl esters were stud-
ied in benzene or toluene solution under usual polym-
erization conditions.6
-13
All of these spectra were rea-
sonably well simulated as the corresponding propagating
radicals. ESR spectra of primary propagating radicals
were observed in a styrene polymerization initiated with
an azo initiator (dimethyl 2,2′-azobisisobutyrate, MAIB)
examine the potential intermediates in ATRP and to
provide a deeper understanding of the ATRP process,
including identification of the structure and concentra-
1
2
under irradiation and have been observed in other
1
2,13,23-32
1
4,15
tion of the paramagnetic species involved.
The
systems.
The values of the hyperfine splitting con-
polymers formed in ATRP contain terminal carbon-
halogen bonds, and Giese et al. (Scheme 1)33 have
reported that these bonds can be homolytically cleaved
by reaction with organotin compounds. Accordingly,
various radicals that model the end groups in an ATRP
can be formed from the corresponding precursors pre-
pared by atom transfer radical addition (ATRA) and
ATRP, and the generated radicals can be studied by
ESR spectroscopy. Variation of the chain length and
composition of polymeric radical precursors elucidates
of the affect of chain length and penultimate units on
the ESR spectra of the formed radicals. It was previ-
ously reported that the ESR spectra of propagating
tert-butyl methacrylate radicals show chain length
dependency.
ATRP is based on a sequence of ATRA reactions, and
accordingly, ATRA can be used to make various dimeric
species modeling the expected chain ends in a copolym-
erization reaction. An ESR study of radicals generated
from these mixed dimers can be used to clarify the
penultimate-unit effects relevant to copolymerization.
Direct ESR studies of penultimate-unit effects in real
stants (hfc) of the radicals were slightly different from
those determined for polymeric propagating radicals.
The difference originated from a penultimate-unit effect
from the initiating radical, which had a methyl meth-
acrylate structure (H-MMA-) attached to one styrene
unit. In another case, large amounts of midchain
radicals in acrylate polymerizations were detected by
ESR spectroscopy. The terminal propagating radicals
had readily rearranged to form a midchain radical via
a 1,5-hydrogen shift.13 Accordingly, ESR spectroscopy
has given an unambiguous proof of several reactions
involved in radical polymerization.
3
4
Atom transfer radical polymerization (ATRP) is one
of the most widely applied polymerization techniques
in the field of controlled/living radical polymerization.
ATRP allows the preparation of a wide range of poly-
meric materials with controlled molecular weights and
1
6-22
well-defined architectures.
ESR was employed to
*
Corresponding author. E-mail: kajiwara@nara-edu.ac.jp.
Nara University of Education.
Carnegie Mellon University.
†
‡
1
0.1021/ma035516s CCC: $27.50 © 2004 American Chemical Society
Published on Web 01/24/2004

Macromolecules, Vol. 37, No. 4, 2004
Monomeric, Dimeric, and Polymeric Acrylate Radicals 1379
Sch em e 2. Elem en ta r y Rea ction s in ATRP
Ch a r t 1
copolymerization reactions are extremely difficult due
to the presence of many coexisting radical species. ESR
spectra of monomeric radicals of (meth)acrylates were
3
5-37
previously investigated by Fischer et al.,
Gilbert et
3
8,39
40,41
al.,
and Matsumoto et al.
However, no ESR study
has so far been made for dimeric radicals of (meth)-
acrylates, mainly due to the difficulty of obtaining clean
spectra in the experiments.
Recently, the rate constants of activation of mono-
meric and dimeric alkyl bromides with a CuBr/bpy
complex as activator were determined. ATRP relies on
the reversible activation of a dormant alkyl halide
through halogen abstraction by a transition-metal com-
plex to form a radical that participates in the classical
free-radical polymerization scheme (Scheme 2) prior to
•
deactivation. In this equilibrium, the alkyl radical (Pm )
is formed in an activation process, with a rate constant
kact, by the homolytic cleavage of an alkyl halogen bond
(
Pm-Z) catalyzed by a transition-metal complex in its
I
lower oxidation state (Cu ). The relative values of kact
of the alkyl bromides were determined for CuBr/bpy
catalyst systems in acetonitrile at 35 °C, and they
followed the order ethyl 2-bromoisobutyrate (EBriB) (30)
.
methyl 2-bromopropionate (MBrP) (3) > tert-butyl
2
-bromopropionate (tBBrP) (1) for monomeric initia-
4
2,43
tors
>
tors.
and MMA-MMA-Br (100) . MA-MMA-Br (20)
into a predried round-bottom flask which was then tightly
sealed with a rubber septum. Deoxygenated acetone (50 mL)
MMA-MA-Br (5) > MA-MA-Br (1) for dimeric initia-
44
-3
followed by MA (430 mg, 5 × 10 mol/L) and PMDETA (956
In this article, various dimeric acrylate and meth-
acrylate model compounds were prepared by ATRA
reactions as shown in Chart 1. Well-resolved ESR
spectra were obtained, not only for the dimeric radicals
but also monomeric and polymeric radicals, as shown
in Chart 2. ESR was used to detect any chain length
dependent spectroscopic changes for acrylate radicals
and also the penultimate effect of different acrylate and
methacrylate monomer units. The structure of the
remote (penultimate) ester group does not affect ESR
and that for the simplicity both ethyl methacrylate
-3
mg, 5.5 × 10 mol/L) were introduced into the flask through
a nitrogen-purged syringe and stirred until the system became
homogeneous. Three “freeze-pump-thaw” cycles were per-
formed to remove any oxygen from the reaction mixture.
-
3
Finally, degassed MBrP (1.245 g, 7.5 × 10 mol/L) in 10 mL
of acetone was introduced using a nitrogen-purged syringe and
then the flask was placed in a thermostated oil bath at 35 °C.
As soon as the initiator was added, the system turned dark
green, indicating the progress of the reaction. After 2 h, the
reaction was stopped by exposure to air and diluted with 20
mL of diethyl ether. The mixture was passed through an
alumina column to remove the copper complex. The excess
solvent was evaporated using a rotor evaporator. The excess
MBrP was removed by vacuum. The dimer, H-MA-MA-Br, was
collected by distilling the remaining mixture under a vacuum
at ∼1 Torr at 75 °C. H-MA-MA-Br was purified by two
distillations, which gave 370 mg (25%) of the product. Purity
was 99% by gas chromatography (GC). In GC, two signals were
(EMA) and MMA were referred to as MMA.
Exp er im en ta l
I
Ma ter ia ls. Cu Br (99%, Aldrich) was used after purification
to remove any copper(II) species. EBriB (99%, Aldrich), ethyl
2
-bromopropionate (EBrP) (99%, Aldrich), MBrP (99%, Ald-
rich), tert-butyl 2-bromopropionate (tBBrP) (99%, Aldrich), and
N,N,N′,N′′,N′′-pentamethyldiethylenetriamine (PMDETA) (99%,
Aldrich) were distilled under reduced pressure before use. tert-
Butyl acrylate (tBA) was distilled over calcium hydride under
reduced pressure and stored in the freezer before use. Di-
methyl 2,2′-azobis(isobutyrate) (MAIB) (>97%, Wako), di-tert-
butyl peroxide (tBPO) (>98%, Aldrich), and bis(tri-n-butyltin)
obtained associated to the presence of two diastereomers (i.e.,
1
a mixture of the recemic and meso forms of the dimer).
H
NMR (600 MHz, CDCl ): δ 4.37 (ddt, J ) 5.6, 4.8, 7.5 Hz, 1H,
3
CHBr), 3.80 (ss, 3H, (CH-Br)COOCH ), 3.71 (ss, 3H, (CH)-
3
COOCH
.26 (dd, J ) 7.8, 7.2 Hz, 3H, CH
Similarly, H-MA-tBA-Br was synthesized using MBrP as the
3
2
), 2.65-2.38 (m, 1H, CH), 2.20-2.03 (m, 2H, CH ),
1
3
).
(
3 2
(nBu) Sn) ) (>90%, TCI) were used as received. All other
initiator in the presence of tBA. Purity was 99% by GC. In
reagents were purified before use.
GC, two signals with nearly similar retention times were
Syn th esis of Dim er s. Dimers were synthesized via an
atom-transfer radical-addition reaction using a molar excess
1
obtained for the two diastereomers. H NMR (600 MHz,
I
CDCl
3H, COOCH
CH ), 1.49 (ss, 9H, COOC(CH
H, CH ).
H-tBA-tBA-Br was synthesized using tBBrP as the initiator
in the presence of tBA. Purity was 99% by GC. In GC, two
3
): δ 4.24 (ddt, J ) 4.9, 4.9, 8.1 Hz, 1H, CHBr), 3.71 (ss,
), 2.64-2.32 (m, 1H, CH), 2.16-1.98 (m, 2H,
), 1.25 (dd, J ) 7.0, 7.0 Hz,
of initiator compared to monomer, in the presence of a Cu /
II
44
3
Cu PMDETA complex, to avoid polymerization.
2
3 3
)
All synthetic steps were carried out under nitrogen atmo-
sphere. In a typical experiment, H-MA-MA-Br was synthesized
3
3
I
-3
II
as follows: Cu Br (720 mg, 5 × 10 mol/L), Cu Br
2
(112 mg,
-
4
5
× 10 mol/L), and a magnetic stirring bar were introduced

1
380 Kajiwara et al.
Macromolecules, Vol. 37, No. 4, 2004
Ch a r t 2
1
signals were obtained for the two diastereomers. H NMR (600
MHz, CDCl ): δ 4.27 (ddt, J ) 4.9, 5.2, 7.3 Hz, 1H, CHBr),
.49-2.25 (m, 1H, CH), 1.21-1.93 (m, 2H, CH ), 1.51 (ss, 9H,
CHBr)COOC(CH ), 1.47 (ss, 9H, (CH)COOC(CH ), 1.21 (dd,
J ) 7.0, 6.3 Hz, 3H, CH ).
CH
3
), 1.9 (s, 3H, CH
3
), 2.8 (s, 2H, CH
3
).
2 3
), 3.7 (s, 3H, COOCH ),
3
3.80 (s, 3H, COOCH
2
(
2
P r ep a r a tion of Mod el Ra d ica l P r ecu r sor of p oly-tBA.
A model radical precursor of a polymeric tBA with a degree of
3
)
3
3 3
)
3
polymerization (DP ) ) 15 was prepared by ATRP according
n
46
-4
H-MMA-tBA-Br was synthesized using EBriB as the initia-
tor in the presence of tBA. Purity by GC was 99%. In GC, only
to reported procedures. CuBr (16 mg, 1.2 × 10 mol), tBA
-
2
-
3
(3.00 g, 2.3 × 10 mol), EBrP (0.21 g, 1.2 × 10 mol),
1
-4
one signal was seen. H NMR (600 MHz, CDCl
3
): δ 4.19 (dd,
),
), 1.48 (s, 9H,
), 1.23 (s, 3H,
PMDETA (20 mg, 1.2 × 10 mol), and degassed acetone (1.4
J ) 4.5, 4.5 Hz, 1H, CH-Br), 4.15 (q, J ) 7 Hz, 2H, OCH
2
mL) were mixed under nitrogen in a Schlenk flask. The flask
was flushed with nitrogen by standard Schlenk techniques.
The resulting mixture was placed in an oil bath at 55 °C for
17 h. Formed oligomeric tBA was separated from the mixture
2
.51 (dddd, J ) 9.1, 8.7, 4.5, 4.5 Hz, 2H, CH
COOC(CH ), 1.27 (t, 7.3 Hz, 3H, (OCH )CH
CH ), 1.18 (s, 3H, CH ).
2
3
)
3
2
3
3
3
H-MA-MMA-Br was synthesized under UV irradiation of
dimethyl 2,4-dimethylglutarate (5.0 mL, 0.69 M) in the pres-
ence of N-bromosuccinimide (5.0 g, 0.69 M) and benzoyl
peroxide (0.25 g, 0.026 M) at 30 °C for 4 h. The compound was
purified by filtration followed by fractional distillation. In GC,
and purified by repeated precipitations. Yield was 2.65 g. M
) 2000, M /M ) 1.11.
n
w
n
Gen er a tion of Mod el Ra d ica ls. Monomeric and Dimeric
Model Radicals. Model radicals were generated by reduction
of the corresponding alkyl bromides. For example, generation
of the radical from H-tBA-tBA-Br was performed according to
two signals with nearly similar retention times were obtained
1
for the two diastereomers. H NMR (600 MHz, CDCl
3
): δ 3.78
), 2.78-
), 1.9 (ss, 3H, (CBr)-
).
H-MMA-MMA-Br was synthesized using the procedure
-5
the following procedure. H-tBA-tBA-Br (20 mg, 5.9 × 10 mol)
(
ss, 3H, (CHBr)COOCH
.67 (m, 1H, CH), 2.24-2.20 (m, 2H, CH
CH , 1.25 (dd, J ) 7.0, 7.7 Hz, 3H, CH
3
), 3.69 (ss, 3H, (CH)COOCH
3
-3
and ((nBu)
3
Sn)
2
(20 mg, 1.2 × 10 mol) were dissolved in
2
2
toluene (200 mg) in a quartz tube (o.d. 5.0 mm). The tube was
flushed with argon. After the mixture was set in an ESR
cavity, it was irradiated with UV light at 30 °C. Irradiation
was carried out with a 500-W ultrahigh-pressure mercury
3
3
4
5 1
3
reported earlier. H NMR δ ) 1.10 (s, 3H, CH ), 1.25 (s, 3H,

Macromolecules, Vol. 37, No. 4, 2004
Monomeric, Dimeric, and Polymeric Acrylate Radicals 1381
F igu r e 2. ESR spectra of monomeric methacrylate model
•
•
radicals observed at 30 °C: (a) H-EMA and (b) H-MMA .
bromides were determined by the method described in the
previous paper.43
Resu lts
Ob ser va t ion of E SR Sp ect r a . Monomeric Model
Radicals. Radicals were generated by three methods:
homolytic cleavage of carbon-bromine bonds of the
alkyl bromides with hexabutyldistannane, photodecom-
position of an azo initiator, and radical polymerization
performed directly in a sample cell.
F igu r e 1. ESR spectra of generated radicals from correspond-
•
•
ing alkyl bromides observed at 30 °C: (a) H-MA ; (b) H-EA ;
•
and (c) H-tBA .
lamp (Ushio USH-500D) at a distance of ca. 15 cm from the
tube in the cavity. An IR-cut filter (Toshiba IRA-25S) was
used.
The ESR spectra of the radicals generated from
monomeric alkyl bromides were studied first. MBrP,
EBrP, and tBBrP provided radicals that model mono-
Polymeric Model Radicals. PtBA (P
n
) 15) (50 mg, ca.
•
•
-
5
-5
meric methyl acrylate (H-MA ), ethyl acrylate (H-EA ),
2
.5 × 10 mol), ((nBu)
3
Sn)
5
2
(4.5 mg, 2,5 × 10 mol), and
•
-
and tert-butyl acrylate (H-tBA ), respectively, by reac-
tBPO (3.7 mg, 2.5 × 10 mol) were dissolved in toluene (150
mg) in a quartz tube (o.d. 5.0 mm). The solution was flushed
with dry argon for several minutes and was placed in an ESR
cavity. Radicals were generated under UV irradiation at
tions with organotin compounds. As shown in Figure 1,
clear and well-resolved spectra were observed. Values
of hfc were precisely determined from the spectra, and
these spectra were reasonably simulated with the
obtained hfc values. The values of hfc from these
radicals and the other model radicals are summarized
in Table 1. Two sets of quartets were observed in each
spectrum, indicating a presence of three equivalent
protons and one more proton with a different coupling
constant.
-
30 °C.
ESR Detection of P r op a ga tin g Ra d ica ls of tBA in situ .
-
4
-5
tBA (30 mg, 2.3 × 10 mol) and tBPO (2.0 mg, 1.4 × 10
mol) were dissolved in toluene (170 mg) in a quartz tube (o.d.
mm). The solution was irradiated with UV light in an ESR
5
cavity to initiate polymerizations at -30 °C. Formed propagat-
ing radicals were observed by ESR. ESR spectra of propagating
radicals of MMA were also observed in a similar manner at 0,
•
•
In the cases of H-MA and H-EA , each spectroscopic
line has an additional splitting in the form of a fine
quartet (0.145 mT) and triplet (0.138 mT), respectively,
due to methyl and methylene protons in the ester side
groups. It means that the spin density is delocalized
3
0, 60, and 90 °C.
ESR Sp ectr oscop y. The ESR spectra of the radicals were
recorded on a J EOL J ES RE-2X spectrometer operating in the
X-band, utilizing a 100-kHz field modulation and a microwave
power of 1 mW. A TE011 mode cavity was used. Temperature
was controlled by a J EOL DVT2 variable-temperature acces-
sory. ESR measurements were performed at 90, 60, 30, 0, -15,
and -30 °C in toluene. Spectroscopic simulations were carried
out by a J EOL IPRIT Data Analysis System.
Molecular weights and molecular weight distributions were
roughly estimated using a TOSOH CCP&8020 series GPC
system with TSK gel columns. Polystyrene standards were
used to calibrate the columns.
•
through the ester bonds. In the case of H-tBA , splitting
due to methyl groups in the tert-butyl group is too small
to be observed. They are included in the line width.
These findings indicate the formation of radicals with
the expected structures, as shown in Chart 1.
The hydrogenated monomeric model radical of ethyl
•
methacrylate (H-EMA ) was formed in the reaction of
Activa tion Ra te Con sta n ts Mea su r em en ts. The activa-
tion rate constants for the various monomeric and dimeric
EBriB with organotin compounds. A clear and well-
resolved ESR spectrum was observed (Figure 2a), and

1
382 Kajiwara et al.
Macromolecules, Vol. 37, No. 4, 2004
Ta ble 1. Hyp er fin e Sp littin g Con sta n ts of Ra d ica ls Mea su r ed a t 30 °C
hfc, mT
radicals
source
MBrP
EBrP
tBBrP
EBriB
MAIB
MA-tBA-Br
tBA-tBA-Br
MA-MA-Br
MA-MMA-Br
R
â
γ
ester
line widtha
•
H-MA
H-EA
2.130
2.134
2.090
2.555
2.547
2.550
2.220
2.225
2.270
2.270
2.29
0.145
0.138
0.051
0.040
0.040
0.047
0.039
0.040
0.047
0.039
0.055
•
•
H-tBA
•
H-EMA
0.128
0.128
•
H-MMA
•
H-MA-tBA
2.070
2.090
2.07
0.078
0.078
0.08
•
H-tBA-tBA
•
H-MA-MA
0.1
0.128
•
b
H-MA-MMA
2.260 (CH3)
.480 (CH2)
0.080
1
•
H-MMA-tBA
MMA-tBA-Br
polytBA-Br
MMA (in situ)
2.090
2.28
2.210
2.06
0.047
0.12
0.050
•
c
polytBA
•
b
polyMMA
2.22 (CH3)
0.11
1
0
.44 (â-CH)
.97 (â-CH)
a
Line width at half height. b Measured at 90 °C. c Measured at -30 °C.
interpretation of the spectrum confirms the forma-
tion of a radical with the structure shown in Chart 1.
and two methylene protons. An additional very small
splitting of a doublet (0.080 mT) due to the γ proton in
the MA moiety can also be observed. The signal inten-
sity of each spectroscopic line displays a temperature-
dependent change due to hindered rotation around the
The hydrogenated monomeric methyl methacrylate
•
(
H-MMA ) radical was generated through photodecom-
position of MAIB under irradiation. The ESR spectra
of monomeric methacrylate radicals are shown in Figure
3
,12,34
C -C bond.
R
â
At 90 °C, the bond rotates freely and
the spectrum was simulated as shown in Figure 4.
2
b. The values of hfc are summarized in Table 1.
•
•
•
ESR spectra of H-EA , H-EMA , and H-MMA were
ESR spectra of such model dimeric radicals with well-
defined structures have not been observed before.
Analyses of the values obtained from the hfc clearly
show the dimeric radicals have the structures indicated
in Chart 1.
reported previously and the values of hfc in that study
are comparable to the results obtained in the present
study.4
0,47
Dimeric Model Radicals. The dimeric model com-
pounds studied can be classified into two groups. The
terminal radical can have either an acrylate or a
methacrylate structure. Dimeric model radicals with an
acrylate terminal group were generated in the reaction
of the corresponding alkyl bromides (tBA-tBA-Br, MA-
tBA-Br, MMA-tBA-Br, and MA-MA-Br) with an orga-
notin compound under irradiation. The resulting radi-
cals had structures of hydrogenated dimeric radicals,
Polymeric Radicals. The ESR spectrum of the propa-
gating radical for tBA polymerization was measured in
situ at -30 °C. The reaction was conducted at -30 °C
since, at temperatures higher than -30 °C, a 1,5-
hydrogen shift chain-transfer reaction occurs forming
a large amount of midchain radicals. ESR spectra of
propagating radicals were obtained at -30 °C, although
the line width of the spectra is greater than those seen
in monomeric and dimeric model radicals. A doublet of
triplets, which should be due to an R proton and
â-methylene protons, can be observed in the spectra
allowing the precise determination of the hfc values. The
splitting from the γ proton, which was observed in the
spectra of dimers, disappeared within the line width.
Oligomeric model radicals, with 15 monomer units of
tBA (DPn )15), were also generated from corresponding
oligomeric alkyl bromide prepared by ATRP. The radi-
cals were also studied by ESR at -30 °C.
•
•
•
i.e., H-tBA-tBA , H-MA-tBA , H-MMA-tBA , and H-MA-
•
MA , respectively. Observed ESR spectra of these radi-
cals are presented in Figure 3. As shown in the figure,
clear and well-resolved spectra were observed and
precise values for hfc can be determined from the
spectra. The hfc values are included in Table 1. A
doublet of triplets was observed in each spectrum and
can be assigned to the R proton and two equivalent
â-methylene protons, respectively.
•
The model dimeric H-MA-MMA radicals were gener-
ated by reacting the corresponding alkyl bromide (MA-
MMA-Br) with an organotin compound under irradia-
tion. ESR spectra were performed at 30, 60, and 90 °C.
Radical polymerization of MMA initiated with di-tert-
butyl peroxide (tBPO) was performed in an ESR sample
cell in the cavity under irradiation at 90 °C. An ESR
spectrum of the propagating radicals of MMA was
recorded. A clear and well-resolved spectrum was
obtained. Values of hfc for these polymeric radicals are
summarized in Table 1.
•
The spectrum of H-MA-MMA at 90 °C, along with its
simulation, is shown in Figure 4. A 12-line spectrum
with a very small quartet in each spectroscopic line was
detected. The 12-line spectrum is caused by a quartet
of triplets (4 × 3) from three equivalent methyl protons

Macromolecules, Vol. 37, No. 4, 2004
Monomeric, Dimeric, and Polymeric Acrylate Radicals 1383
F igu r e 3. ESR spectra of dimeric model radicals generated from corresponding alkyl bromides observed at 30 °C: (a) H-tBA-
•
•
•
•
tBA ; (b) H-MA-tBA ; (c) H-MMA-tBA ; and (d) H-MA-MA . Spectroscopic lines appearing at the center of the spectra (indicated by
dashed line) are due to radicals of tin compounds.
Discu ssion
Dim er ic Ra d ica ls a n d P en u ltim a te-Un it Effect.
Penultimate-unit effects were observed in the ESR
spectra of model dimeric radicals. The ESR spectra of
dimeric radicals with acrylate terminal units will be
discussed first. When we compare the spectrum of
•
•
•
H-MMA-tBA with that of H-MA-tBA or H-tBA-tBA ,
we can see the effect of a methacrylate penultimate unit
(this is the effect of the presence of a methyl group at
the γ position). The presence of an MMA unit shows only
small variations in the couplings of the R proton.
Obviously, the spin density at the R carbon atom does
not change appreciably by â substitution. On the other
hand, the couplings of â-methylene protons vary con-
siderably with the presence of an MMA penultimate
unit as shown in Table 1. The value of hfc at â-meth-
•
ylene protons for H-MMA-tBA (2.210 mT) is smaller
•
•
than that of H-MA-tBA (2.270 mT) or of H-tBA-tBA
2.270 mT). ESR spectra of monomeric acrylic acid
(
radicals with various substituents, i.e., HO-, NH2-,
CH3-, HO-CH2-, were previously discussed by Fischer
et al.,35 and the reported values of hfc’s of the â protons
varied considerably with the substituents. J udging from
the reported results, the electronic effect of a penulti-
mate MMA unit is larger than those of substituents.
These findings show an electron-withdrawing effect of
the MMA unit. Steric effect of the presence of the γ
methyl group may have appeared in the broader line
F igu r e 4. Observed (a) and simulated (b) ESR spectra of
H-MA-MMA observed at 90 °C.
•
•
width than that in the case of H-MA-tBA . In compari-
•
son with ESR spectrum of H-MA-tBA , the steric effect
methyl group by a tert-butyl group is very small.
Sterically, the small ester methyl group of MA caused
less hindrance to the rotation of the Câ-CR bond. As a
•
of H-tBA-tBA appeared in the line width of the spec-
trum, although the electronic effect of replacement of a

1
384 Kajiwara et al.
Macromolecules, Vol. 37, No. 4, 2004
•
F igu r e 5. Observed and simulated ESR spectra of tBA radicals with various chain lengths observed at 30 °C: (a) H-tBA
•
•
(
monomeric); (b) H-tBA-tBA (dimeric); and (c) H-(tBA)
appearing at the center of the spectra (indicated by dashed line) are due to radicals of tin compounds.
n
-tBA (polymeric model radicals, n ∼ 15 (DP
n
) 15)). Spectroscopic lines
Ta ble 2. Hyp er fin e Sp littin g Con sta n ts of tBA Ra d ica ls w ith Va r iou s Ch a in Len gth s
hfc (mT)
radicals
source
R
â
γ
reference
H-tBA•
H-tBA-tBA
H-(tBA)n
poly A (poly(acrylic acid))
poly B (poly(acrylic acid))
tBBrP
tBA-tBA-Br
(tBA)n-Br
acrylic acid (in situ)
acrylic acid (in situ)
2.090
2.090
2.28
2.067
2.262
2.550
2.270
2.06
2.206
2.134
this work
this work
56
37
37
•
0.078
•
result, a clear separation of spectroscopic lines, due to
narrower line width, was observed. This difference in
ESR spectra between H-MMA-tBA and H-tBA-tBA
suggests that the reactivity of the terminal tBA radical
may depend on the penultimate unit in copolymeriza-
tions of MMA and tBA.
of DPn ) 15.13,56 This can indicate either a pen-
penultimate-unit effect or another chain-length effect.
Therefore the origin of the chain length dependent
variation of the ESR spectra is not yet explained.
•
•
Fischer et al. reported ESR spectra of polymeric
radicals of acrylic acid by a flow method and detected
two kinds of radical species with different coupling
constants. They named them as polymeric radicals A
and B, respectively. The coupling constants for R and â
protons were reported to be 2.067 and 2.206 mT for A
and 2.262 and 2.134 mT for B. In the case of radical A,
although they were different from those of monomeric
radicals, the coupling constant for the R proton was
smaller than that for â protons. In the case of radicals
B, the value of the hfc for the R proton was larger than
that seen for â-methylene protons. A similar dependence
was found for tBA-derived radicals in this work. These
results could indicate that radical A was an oligomeric
radical (longer than dimeric) and that radical B had a
longer chain length than radical A.
Radical copolymerization of MA (M1) and MMA (M2)
was investigated and r1 and r2, copolymer reactivity
ratios, were determined to be 0.4 and 2.15, respec-
4
8
tively. However, these results were obtained with-
out considering the penultimate-unit effect. The pres-
•
•
ent ESR results for H-MA-MMA , H-MA-MA , and
•
H-MMA-tBA suggest the presence of penultimate-unit
effects on the acrylate terminal unit.
Thus, electronic and steric penultimate effects influ-
4
9-55
ence not only radical reactivity
spectra.
but also their ESR
P olym er ic Ra d ica ls a n d Ch a in Len gth Dep en d -
en t Ch a n ge in ESR Sp ectr a . The ESR spectra of
•
•
•
H-tBA , H-tBA-tBA , and polytBA (DPn ) 15), where
the radicals were generated from the corresponding
alkyl bromides, are shown in Figure 5 along with their
simulations.
Chain length dependent ESR spectra were also
observed for methacrylate radicals. Apparently, the
coupling constant of two â-methylene protons changed
from equivalent to nonequivalent as the chain length
increased. However, coupling constants of protons in
methyl group at R position do not show any chain
length dependent change. The exchange of the mag-
nitude of hfc values between R and â protons in
acrylates is therefore solely characteristic to the acry-
lates. Chain length dependent spectroscopic change in
In the monomeric and dimeric radicals, the coupling
constants for the R proton are smaller than those for
the â protons. However, in the case of polymeric
radicals, with DPn ) 15, the coupling constant for the
R proton was larger than those for the â protons as
shown in Figure 5c and Table 2. The ESR spectra of
tBA polymeric radicals with DPn ) 50, DPn ) 100, and
propagating radicals in situ were very similar to that

Macromolecules, Vol. 37, No. 4, 2004
Monomeric, Dimeric, and Polymeric Acrylate Radicals 1385
methacrylates will be discussed in detail in the following
(13) Kajiwara, A.; Kamachi, M. In Advances in Controlled/ Living
Radical Polymerization; Matyjaszewski, K., Ed.; ACS Sym-
posium Series 854; American Chemical Society, Washington,
DC., 2003; Chapter 7, pp 86-100.
article.57
Activa tion Ra te Con sta n ts for Mon om er ic a n d
Dim er ic Alk yl Br om id es a n d ESR Sp ectr a . The
relative values of kact for alkyl bromides follow the order
EBriB (30) . MBrP (3) > tBBrP (1). This order not only
indicates higher thermodynamic stability of the tertiary
(
(
(
14) Tanaka, H.; Sasai, K.; Sato, T. Macromolecules 1988, 21,
3534.
15) Sato, T.; Inui, S.; Tanaka, H., Ota, T.; Kamachi, M.; Tanaka,
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•
radicals (H-EMA ) but also lower reactivity of the
5
614.
secondary radical with a bulkier tert-butyl group rather
than the smaller methyl group. This was explained by
suggesting steric hindrance in the atom -transfer pro-
cess where the catalyst complex must approach the
halogen atom. However, judging from the values of hfc
for H-tBA , H-MA , and H-EA , electronic effect from the
ester side groups seems negligibly small. Thus, replace-
ment of a methyl group (MBrP) by a tert-butyl group
(17) Wang, J .-S.; Matyjaszewski, K. Macromolecules 1995, 28,
7901.
(
(
18) Patten, T. E.; Matyjaszewski, K. Adv. Mater. 1998, 10, 901.
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•
•
•
3
689.
(
(
22) Davis, K. A.; Matyjaszewski, K. Adv. Polym. Sci. 2002, 159,
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(
tBBrP) has no significant effect on the values of the
23) Knuehl, B.; Pintauer, T.; Kajiwara, A.; Fischer, H.; Matyjas-
zewski, K. Macromolecules 2003, 36, 8291.
hfc determined in the ESR spectra, but the steric effect
in the transition state for atom transfer leads to a 3-fold
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(
(
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5
48.
The values of kact for the dimeric bromides follow
the order MMA-MMA-Br (100), MA-MMA-Br (20), MMA-
MA-Br (5), and MA-MA-Br (1). Differences between
MMA-MMA-Br and MA-MMA-Br, and between MMA-
MA-Br and MA-MA-Br, may be explained similarly
to MA-tBA and MMA-tBA systems. When a MMA unit
is located in the penultimate position, values of hfcs
of â protons of tBA were smaller than with MA as
the penultimate unit. These electronic effect and
steric effects manifest themselves in different values
of kact.
We anticipate that a combination of ESR analysis and
kinetic studies will lead to a better understanding of
both the structure and the reactivity of the radicals.
Such information is very important for basic research,
not only for ATRP but also for conventional radical
polymerization processes.
(
26) Kajiwara, A.; Matyjaszewski, K. Macromol. Rapid Commun.
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Ack n ow led gm en t . This research was supported
in part by a Grant-in-Aid (No. 15550107) from the
Ministry of Education, Culture, Sports, Science, and
Technology of J apan. Financial support from the Na-
tional Science Foundation (CHE-0096601) is gratefully
acknowledged.
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MA035516S