Improving the initiation efficiency in the single electron transfer living radical polymerization of methyl acrylate with electronic chain-end mimics

Article abstract of DOI:10.1002/pola.24543

Computational studies on the heterolytic bond dissociation energies and electron affinities of methyl 2-bromopropionate (MBP) and ethyl 2-bromoisobutyrate (EBiB) in the dissociative electron transfer (DET) step of single electron transfer living radical polymerization (SET-LRP) of methyl acrylate (MA) combined with kinetic experiments were performed in an effort to design the most efficient initiation system. This study suggests that EBiB is more effective than MBP in the SET-LRP of acrylates catalyzed by Cu(0) wire, thus being a true electronic mimic of the dormant PMA species. EBiB allows for a more predictable dependence of the molecular weight evolution and distribution. This is exemplified by the absence of a deviation in the PMA molecular weight from theoretical values at low conversions, as a result of a faster SET activation with EBiB than with MBP. The enhanced control over molecular weight evolution was also observed in the SET-LRP of MA initiated with bifunctional initiators similar in structure to MBP and EBiB, suggesting a higher reactivity than MBP in the SET activation, which matches closely that of the polymer dormant chains. The use of bifunctional initiators in conjunction with activated Cu(0) wire in SET-LRP allows for dramatically accelerated polymerizations, although still providing for exceptional control of the molecular weight evolution and distribution.

Full text of DOI:10.1002/pola.24543

Improving the Initiation Efficiency in the Single Electron Transfer
Living Radical Polymerization of Methyl Acrylate with Electronic
Chain-End Mimics
NGA H. NGUYEN, BRAD M. ROSEN, VIRGIL PERCEC
Roy and Diana Vagelos Laboratories, Department of Chemistry, University of Pennsylvania, Philadelphia,
Pennsylvania 19104-6323
Received 18 November 2010; accepted 6 December 2010
DOI: 10.1002/pola.24543
Published online 3 January 2011 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: Computational studies on the heterolytic bond dis-
sociation energies and electron affinities of methyl 2-bromo-
propionate (MBP) and ethyl 2-bromoisobutyrate (EBiB) in the
dissociative electron transfer (DET) step of single electron
transfer living radical polymerization (SET-LRP) of methyl acry-
late (MA) combined with kinetic experiments were performed
in an effort to design the most efficient initiation system. This
study suggests that EBiB is more effective than MBP in the
SET-LRP of acrylates catalyzed by Cu(0) wire, thus being a true
electronic mimic of the dormant PMA species. EBiB allows for
a more predictable dependence of the molecular weight evolu-
tion and distribution. This is exemplified by the absence of a
deviation in the PMA molecular weight from theoretical values
at low conversions, as a result of a faster SET activation with
EBiB than with MBP. The enhanced control over molecular
weight evolution was also observed in the SET-LRP of MA initi-
ated with bifunctional initiators similar in structure to MBP and
EBiB, suggesting a higher reactivity than MBP in the SET acti-
vation, which matches closely that of the polymer dormant
chains. The use of bifunctional initiators in conjunction with
activated Cu(0) wire in SET-LRP allows for dramatically acceler-
ated polymerizations, although still providing for exceptional
control of the molecular weight evolution and distribution.
VC 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem
49: 1235–1247, 2011
KEYWORDS: initiator; kinetics (polym.); methyl acrylate; radical
polymerization; SET-LRP
INTRODUCTION The precise synthesis of polymers with
well-defined compositions, architectures, and perfect struc-
tural fidelity has been of great interest in the field of poly-
Cu(I)X establishes the proper equilibrium between the active
and dormant chains, which in turn enables an effective SET-
LRP.
1
,4
1
mer chemistry. Considered as one of the most rapidly devel-
As with all metal-catalyzed LRP processes, the appropriate
choice of initiator is critical. Rapid and quantitative initiation
is required to achieve well-defined polymers with narrow
molecular weight distributions. Nevertheless, if initiation is
too fast, increased bimolecular termination of propagating
oping areas in polymer science, metal-catalyzed living radical
polymerization (LRP) has provided synthetic polymer chem-
ists with methods and strategies that can compete with or-
ganic reactions in the preparation of complex macromolecu-
lar structures otherwise inaccessible with conventional
21
radicals would result. Therefore, the initiator reactivity
2
,3
polymerization methods.
should be matched to the monomer reactivity. One strategy
is to employ initiator molecules that structurally mimic the
1
,4
Single electron transfer LRP (SET-LRP) is emerging as a
powerful tool for the rapid polymerization of functional
2
1,22
propagating dormant macroradicals.
2-Halopropionates
5
–7
6
4,8–12
acrylates,
chloride,
acrylamides, methacrylates,
and vinyl
such as methyl 2-bromopropionate (MBP) are among the
most commonly used monofunctional initiators for metal-cat-
alyzed LRP, including SET-LRP, of acrylates because of their
4
,13,14
with excellent control over the molecular
weight evolution and distribution. It allows the precise syn-
thesis of a range of polymeric structures, for example, den-
1
,23
structural resemblance to the monomers.
A variety of
2
,15
16
17
dritic macromolecules,
star polymers, telechelic func-
2-bromopropioate derivatives including bifunctional, multi-
1
7
7,18
2,15
4
tional polymers, and graft copolymers.
Mechanistically,
functional,
and macroinitiators have also been employed
the activation step in SET-LRP involves a heterolytic bond
cleavage of the carbon–halide bond via a heterogenous
in the SET-LRP of acrylates. Although structural mimicry pro-
vides a starting point for the selection of an appropriate ini-
tiator, electronic and reactivity similarities to the dormant
polymeric species must also be judiciously considered.
1
,4,19,20
Cu(0)-catalyzed outer-sphere SET process.
The ligand/
solvent mediated-disproportionation of the in situ generated
Correspondence to: V. Percec (E-mail: Percec@sas.upenn.edu)
Journal of Polymer Science Part A: Polymer Chemistry, Vol. 49, 1235–1247 (2011) V
2011 Wiley Periodicals, Inc.
C
INITIATION EFFICIENCY IN SET-LRP OF MA, NGUYEN, ROSEN, AND PERCEC
1235

JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
6
SCHEME 1 The initiator and dormant polymeric species in the SET-LRP of MA catalyzed by Cu(0) wire/Me -TREN and initiated by
ꢀ
(
a) MBP and (b) EBiB in DMSO at 25 C.
In the SET-LRP of methyl acrylate (MA) catalyzed by Cu(0)/
LRP of acrylates initiated with a-halopropionates in polar
6
,24–27
hexamethylated tris(2-aminoethyl)amine (Me -TREN), both in
solvents.
when compared with propagation or a low concentration of
in situ generated Cu(II)X similar observation was
This suggests a slow initial rate of initiation
6
the case of Cu(0) powder and nonactivated wire, an initial
deviation in the molecular weight of the polymer at conver-
2
. A
2
4–27
28
sions below 20%
was observed, indicating a M
n
of
reported previously by Sawamoto and coworkers. In the
ruthenium-mediated LRP of methyl methacrylate (MMA), the
observed deviation from theoretical molecular weight at low
conversion was attributed to the slower initiation from the
monomeric bromide (methyl 2-bromoisobutyrate, MBiB).
However, the dimeric model of dormant chain end (MMA-
MMA-Br) initiates a faster polymerization than the mono-
meric counterpart, providing poly(methyl methacrylate)
(PMMA) with lower polydispersities and with no deviation
poly(methyl acrylate) (PMA) that is higher than the one pre-
dicted by a living process. This behavior suggests a pre-equi-
librium stage, wherein there is either a lower initial rate of
initiation than propagation or
a low concentration of
2
4
Cu(II)X2 deactivator generated in situ. This indicates MBP
is less reactive than the corresponding dimeric, oligomeric,
and polymeric species derived from MA in the SET activation,
despite the fact that it is the authentic structural mimic of
the monomeric dormant species from MA.
2
8
from theoretical molecular weight. The slow initiation of
MBiB can be related to the back strain effect, that is, the
release of steric strain of dormant species from rehybridiza-
In an effort to design a more effective initiation system for
acrylates, it is crucial to correlate and evaluate the structures
of some of the most common initiators known for MA with
their electronic property and reactivity under SET activation.
Here, through computational analysis via energy profile mod-
eling of structures involved in the activation step of SET-LRP,
in conjunction with a comparative study between the Cu(0)
3
2
tion from sp to sp leading to a higher equilibrium
2
9
constant.
In the context of the SET-LRP of MA, slow initiation from
MBP can be attributed to either the slow rate of radical for-
mation or lower reactivity of MBP when compared with the
oligomeric/polymeric dormant species (MBP-MA, PMA-Br) in
the heterogeneous bond dissociation process (Scheme 1). In
search for more efficient initiators, computational analy-
wire/Me -TREN catalyzed SET-LRP of MA initiated with MBP
6
and with ethyl 2-bromoisobutyrate (EBiB), it is demon-
strated that EBiB is a more efficient initiator, allowing for an
improved dependence of the molecular weight evolution and
distribution. Computational and experimental analysis of
dibromo derivatives of MBP, dimethyl 2,5-dibromohexandio-
nate (MBHD), and 1,2-bis(bromopropionyloxy)ethane (BPE),
and of EBiB, 1,2-bis(bromoisobutyryloxy)ethane (BBiBE),
show that these bifunctional initiators offers equally excep-
tional control over the molecular weight evolution and distri-
bution in the SET-LRP of MA.
2
2,30
sis
performed on structures relevant to the dissociative
electron transfer (DET) step of SET-LRP was reevaluated,
revealing two important trends. First, in comparison with
2-halopropionates, 2-haloisobutyrates demonstrated a lower
heterolytic bond dissociation energy (BDEhetero), a decrease
in ion-radical pair formation energy (ERA), and an increase in
stability of the ion-radical pair (Estab). These should lead to
acceleration in the electron transfer step as explained
2
2,31
through the sticky dissociative model.
The rate enhance-
ment suggests 2-bromoisobutyrates, such as EBiB and MBiB
more reactive toward DET, thereby increasing the rate of the
electron transfer step (Scheme 2, eq 1). Second, it appears
from previous computational studies that the dimeric
RESULTS AND DISCUSSION
The initial deviation in the molecular weight of the polymer
at low conversions is an observable deficiency in the SET-
1236
WILEYONLINELIBRARY.COM/JOURNAL/JPOLA

ARTICLE
the BDEhetero and electron affinities between the monomeric
initiators and the dimeric models would give insight into the
relative rates of initiation and propagation.
1
,20
SCHEME 2 The initiation process in SET-LRP.
The calculated bond dissociation energies for the single unit
initiators and dimeric mimics are presented in Figure 2 and
Table 1. First, in comparison with MBP, the results for MBiB
dormant species are more reactive when compared with the
corresponding single unit initiator in the activation step
ꢁ
1
demonstrate a 4.44 kcal mol lower ERA and higher 1.57 kcal
ꢁ
1
22
2
2,30
mol Estab, in agreement with our previous study. Similarly,
under SET-LRP.
These preliminary results suggest a
ꢁ
1
in EBiB, the E is higher by 4.23 kcal mol and the Estab by
.48 kcal mol than in MBP. Increased stabilization of the ion-
closer match in reactivity between EBiB and the dormant
polymeric species (EBiB-MA, PMA-Br) derived from MA in
the activation step of the SET-LRP (Scheme 1). Thus, previ-
RA
1
ꢁ
1
radical pair results in a significant acceleration in the electron-
transfer process via the sticky dissociative model. This means
that 2-haloisobutyrates such as EBiB and MBiB are more reac-
tive during the activation step of SET-LRP when compared
with 2-halopropionates such as MBP.
2
2,30
ous computational studies
inspired us to investigate the
relative rates of radical formation from 2-bromopropionate
and 2-bromoisobutyrates in the SET-LRP of MA, and so the
potential use of 2-bromoisobutyrate as an initiator in SET-
LRP of acrylates.
Second, when comparing between MBP and the correspond-
ing dimeric species (syn and anti), there is a 2.52–3.05 kcal
ꢁ
1
ꢁ1
Comparative Analysis of BDE in Pairs of Single Unit
Initiators and the Corresponding Dimeric Models
In SET-LRP, activation of the dormant chains occurs through
the heterolytic bond cleavage of the carbon–halide bond via
mol decrease in ERA and 0.83–1.69 kcal mol increase in
stab in the dimeric mimics. This indicates the MBP-MA
E
dimers are better electron acceptor, thus more reactive to-
ward DET than the single unit MBP. Although the trend is
conserved for MBiB and EBiB, the ERAs of the radical anion
and of the dimeric model are more similar. Specifically, the
1
a heterogenous Cu(0)-catalyzed outer-sphere SET process.
Herein, the heterolytic BDEs and electron affinities of MBP,
MBiB, EBiB, and their corresponding dimeric models with
MA under outer-sphere (SET) conditions were investigated
ꢁ
1
decrease in ERA is 1.68 and 0.11 kcal mol
in MBiB and
EBiB, respectively, when compared with their corresponding
dimers. Similarly, the Estab does not increase significantly in
the dimeric models of MBiB and EBiB, demonstrated by a
2
2
using energy profile modeling (Fig. 1). The energy of elec-
trostatic ERA is calculated as the difference between the neu-
tral species and ion-radical pair at equilibrium bond length,
and the stability of ion-radical pair (Estab) as the difference
between the ion-radical pair and the completely separated
organic radical and halide leaving group. The lower is the
ERA and the stronger is the interaction of the radical with
the partial positive charge and the counter-anion halide, the
faster is the electron-transfer process. Although MBP and
EBiB were examined for direct correlation between computa-
tional and experimental analysis of the relative rate of radi-
cal formation, MBiB, monomeric mimic of MMA, was selected
ꢁ
1
difference of 0.43 and 1.43 kcal mol , respectively. Third,
our preliminary computational study of the trimer species
unveils marginal increase in the relative reactivity between
dimeric and trimeric species derived from MBP and EBiB.
For example, the results for EBiB-MA-MA show a 0.4-kcal
ꢁ
1
ꢁ1
mol lower ERA and 1 kcal mol higher Estab when com-
pared with those of EBiB-MA (Table 1). Consistent with our
expectations, these results suggest that the reactivity of 2-
haloisobutyrates matches better with the reactivity of dor-
mant species of MA on the DET step of SET-LRP of MA, with
the later being only slightly more reactive. In other words,
2
2,30
for consistency with previous work.
The comparison of
FIGURE 1 Single unit monofunctional ini-
tiators and the corresponding dimers.
INITIATION EFFICIENCY IN SET-LRP OF MA, NGUYEN, ROSEN, AND PERCEC
1237

JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
FIGURE 2 ERA and the Estab for the single unit monofunctional initiators and dimeric models.
despite the structural difference, on the basis of electronics
and reactivity, EBiB and MBiB are the true electronic chain-
end mimics of the dimeric and oligomeric dormant species
of MA.
This inspired us to investigate the formation of radicals from
MBHD, a bifunctional initiator that structurally resembles the
dimer MA-MA-Br species, [Fig. 3(b)] relative to its dimeric
species with MA [Fig. 3(a)] under SET activation. Because of
the presence of multiple stereocenters, all diastereomers of
the bifunctional initiator and its dimers with MA were sub-
jected to computational analysis to account for the effect of
stereochemistry.
Comparative Analysis of BDE of a Bifunctional Initiator
and Its Dimeric Derivatives
The results presented above suggest that the dimeric species
such as MBP-MA or EBiB-MA are better models to predict
the BDE of the polymer dormant species P–X under SET-LRP
conditions when compared with the monomer counterparts.
The relative energies of heterolytic bond dissociation ener-
gies and of the formation and decomposition of radical anion
TABLE 1 Dormant Chain-End Bond Dissociation Energies Modeled at the B3LYP/6-311G* level
ꢁ
1
ꢁ1
ꢁ1
ꢁ1
stab (kcal mol )
Entries
Compounds
E
homo (kcal mol
)
E
hetero (kcal mol
)
ERA (kcal mol
)
E
1
2
3
4
5
6
7
8
9
MBP
55.99
53.11
53.23
54.30
54.62
54.15
51.86
54.56
55.17
53.33
54.14
51.93
52.18
53.00
53.56
53.14
52.81
56.18
55.40
53.14
53.52
ꢁ26.57
ꢁ29.44
ꢁ29.32
ꢁ28.25
ꢁ27.93
ꢁ28.41
ꢁ30.70
ꢁ27.99
ꢁ27.38
ꢁ29.23
ꢁ28.41
ꢁ30.62
ꢁ30.38
ꢁ29.55
ꢁ28.99
ꢁ29.41
ꢁ29.74
ꢁ26.38
ꢁ27.16
ꢁ29.41
ꢁ29.03
ꢁ33.55
ꢁ37.99
ꢁ37.78
ꢁ36.07
ꢁ36.60
ꢁ38.22
ꢁ39.67
ꢁ37.88
ꢁ38.27
ꢁ42.95
ꢁ43.37
ꢁ51.92
ꢁ38.83
ꢁ45.90
ꢁ38.57
ꢁ44.77
ꢁ40.77
ꢁ43.96
ꢁ37.04
ꢁ44.77
ꢁ42.72
6.98
8.55
MBiB
EBiB
8.46
MBP-MA (anti)
MBP-MA (syn)
MBP-MA-MA
7.81
8.67
9.81
MBiB-MA
8.98
EBiB-MA
9.89
EBiB-MA-MA
10.88
13.72
14.95
21.29
8.45
1
1
1
1
1
1
1
1
1
1
2
2
a
0
1
2
3
4
5
6
7
8
9
0
1
MBHD-1 (S-R)
MBHD-2 (S-S)
MBHD1-MA-1 (S-S-R)
MBHD1-MA-1 (S-S-R)
MBHD1-MA-2 (S-S-S)
MBHD1-MA-2 (S-S-S)
MBHD2-MA-1 (S-R-R)
MBHD2-MA-1 (S-R-R)
MBHD2-MA-2 (S-R-S)
MBHD2-MA-2 (S-R-S)
MBHD2-MA-1 (S-R-R)
16.35
9.58
15.36
11.03
17.58
9.88
15.36
13.69
a
MBHD-MA
E
average of all diastereomers.
1238
WILEYONLINELIBRARY.COM/JOURNAL/JPOLA

ARTICLE
ꢁ
1
0
2
.073 min ; Table 2, entries 1 and 2). Thus, the choice of a
-bromopropionate or 2-bromoisobutyrate as the initiator
does not affect significantly the kinetics of the poly-
merization.
M /M versus Conversion in the SET-LRP of MA Initiated
w
n
with MBP and EBiB
The evolution of the number average molecular weight (Mn)
and of the molecular weight distribution (M /M ) versus
w n
theoretical molar mass calculated for a LRP process (M ) of
th
PMA for the Cu(0) wire/Me -TREN catalyzed SET-LRP of MA
6
initiated with MBP and with EBiB are depicted in Figure
5(b,d). For the SET-LRP of MA initiated with MBP, an initial
FIGURE 3 (a) Dimeric model of the dormant chain. (b) Single
deviation in the molecular weight of polymer is detected at
conversions below 15%. This is consistent with what was
observed previously for kinetic experiments performed with
Cu(0) powder. When using EBiB as the initiator, there is no
deviation in the polymer molar mass from theoretical values
at all conversions. This enhanced control over the molecular
weight distribution effectively overcomes the major challenge
in the SET-LRP of acrylates initiated with 2-halopropionates.
As the deviation from theoretical molecular weights could be
attributed to a lower rate of initiation than propagation, it
suggests a faster rate of activation in the SET-LRP of MA ini-
tiated by EBiB than by MBP. This is consistent with recent
findings that the use of highly activated Cu(0) wire increases
the SET activation, leading to a complete elimination of the
unit bifunctional initiator and the corresponding dimers.
intermediates for MBHD and MBHD-MA are presented in Ta-
ble 1, entries 10–20 and in Figure 4. The results for the elec-
tron affinities of the MBHD-MA dimers are averaged because
the initiator itself is not diastereomerically enriched. First,
the reactivity of MBHD is significantly higher than that of the
monofunctional initiators, MBP and EBiB under SET activa-
tion, as shown by a much lower ERA and greater Estab (Fig.
2
4
4). Second, the trend is clear that the reactivity of the bifunc-
tional initiator MBHD is very close to that of its dimers with
MA. This is illustrated by the essentially identical Ehetero and
small differences in ERA and Estab between the dimeric and
trimeric species derived from MBHD (Fig. 4).
In short, computational studies demonstrate that 2-haloiso-
butyrates, such as EBiB, are more reactive than MBP in the
activation process of SET-LRP because of a lower BDE and
better stabilization of the ion-radical pair leading to an accel-
erated electron transfer process. In addition, EBiB exhibits
similar reactivity with the corresponding dormant polymeric
species. Therefore, 2-haloisobutyrates should serve as more
efficient initiators than MBP in the SET-LRP of MA. Further,
the results suggest that a bifunctional initiator such as
MBHD could be effective in the SET-LRP of acrylates because
of its enhanced reactivity that matches well with that of the
dormant polymeric species. Hence, the Cu(0) wire/Me -TREN
6
catalyzed SET-LRP of MA initiated with EBiB and MBHD
were evaluated in comparison with the SET-LRP of MA initi-
ated with MBP.
Comparison of Apparent Rate Constants of
app
Propagation (kp ) in the SET-LRP of MA
Initiated with MBP and EBiB
Figure 5(a) shows the overlapped kinetic plots for the Cu(0)
wire/Me -TREN catalyzed SET-LRP of MA initiated with MBP
6
ꢀ
in dimethyl sulfoxide (DMSO) at 25 C under the following
conditions: [MA]/[MBP]/[Me -TREN] ¼ 222/1/0.1. Figure
6
5(c) depicts the kinetic plots for the SET-LRP of MA initiated
with EBiB under identical conditions. Both polymerizations
clearly exhibited first order kinetics, indicative of a living
polymerization. The SET-LRP of MA initiated with EBiB, with
app
ꢁ1
an apparent rate constant kp ¼ 0.069 min , proceeded
FIGURE 4 ERA and the Estab for the single unit initiators and di-
app
only slightly slower than that initiated with MBP (kp
¼
meric models.
INITIATION EFFICIENCY IN SET-LRP OF MA, NGUYEN, ROSEN, AND PERCEC
1239

JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
FIGURE 5 Kinetic plots and molecular
weight evolutions for SET-LRP of MA
initiated with (a and b) MBP and (c and
ꢀ
d) EBiB in DMSO at 25 C. Reaction
conditions: MA ¼ 1 mL, DMSO ¼ 0.5
mL, [MA]/[Initiator]/[Me
6
-TREN] ¼ 222/
1
/0.1. Cu(0) wire ¼ 12.5 cm of 20 gauge
wire.
observed deviation from theoretical molar masses at low
conversions.
Initiator Consumption
3
2
In atom transfer radical polymerization (ATRP), 2-bromoiso-
butyrates have been shown to generate initiating radicals
faster than the corresponding 2-haloproprionates because of
The enhanced control over molecular weight distribution
when using EBiB as the initiator is further demonstrated by
the fact that narrower molecular weight distribution is
achieved when EBiB is employed as the initiator, indicated
by the M /M values of <1.2 at monomer conversions of
23
the better stabilization of the generated tertiary radicals.
The ATRP equilibrium constant, K
, of EBiB is ꢂ30 times
ATRP
33
higher than for MBP. In addition, the current computational
analysis on the heterolytic BDEs involved in the activation
process of SET-LRP confirmed the higher reactivity of EBiB
as compared to MBP toward DET. To investigate the relative
rates of consumption of MBP and EBiB under SET-LRP condi-
tions the initiator consumption experiments were conducted
w
n
above 50% and of 1.11 at ꢂ89% conversion (as compared
to that of 1.17 obtained at the same monomer conversion in
the SET-LRP of MA initiated with MBP; Table 2, entries 1
and 2).
TABLE 2 SET-LRP of MA Catalyzed by Cu(0)/Me
6
-TREN in DMSO at 25 8C
app
ꢁ1
)
Entries
Initiators
Time (min)
Conversion (%)
kp (min
M
n
(GPC)
M /M
w n
Ieff (%)
a
1
MBP
33
34
42
40
38
15
16
21
89
88.5
90
89
85
82
86
88
c
0.073
0.069
0.061
0.060
0.061
0.126
0.132
0.107
18,545
20,171
38,882
36,061
35,820
30,625
30,890
36,047
1.17
1.11
1.09
1.11
1.09
1.11
1.11
1.10
91
83
96
90
97
96
95
93
a
2
EBiB
b,c
3
MBHD
BPE
b,c
4
b,c
5
BBiBE
b,d
32
6
MBHD
32
b,d
7
BPE
b,d
8
BBiBE
a
[
MA]/[Initiator]/[Me
6
-TREN] ¼ 222/1/0.1.
Commercial Cu(0) wire.
Activated Cu(0) wire.
b
d
[
MA]/[Initiator]/[Me
6
-TREN] ¼ 444/1/0.2.
1240
WILEYONLINELIBRARY.COM/JOURNAL/JPOLA

ARTICLE
1
FIGURE 6 500-MHz H NMR spectrum of PMA
at 10% conversion. Polymerization conditions:
MA ¼ 1 mL, DMSO ¼ 0.5 mL, [MA]/[MBP]/
[
Me
6
-TREN] ¼ 222/1/0.1. Cu(0) wire ¼ 12.5 cm
of 20 gauge wire.
1
using H NMR spectroscopy. The polymerization conditions
tion step was complete at ꢂ34% monomer conversion. This
is comparable with the rate of initiator consumption
observed previously in the SET-LRP of MA initiated by MBP
were as followed: MA ¼ 1 mL, DMSO ¼ 0.5 mL, [MA]/[Initia-
tor]/[Me -TREN] ¼ 222/1/0.1/1; Cu(0) wire ¼ 12.5 cm of
6
2
4
2
0 gauge wire.
and catalyzed by Cu(0) powder.
1
Figure 6 shows a 500-MHz H NMR spectrum of one of the
PMA samples isolated in the SET-LRP of MA initiated with
MBP. The comparison of the integrals of the signals of the
monomer vinyl signals (5.7–6.4 ppm) over the polymeric
AOCH3 signal (3.65 ppm) indicates 10% monomer conver-
sion after 3 min. The inset of Figure 6 shows a sharp quartet
at approximately 4.3–4.35 ppm representative of the proton
located at the a position of the bromide atom of the MBP ini-
The evaluation of initiator consumption in the SET-LRP of
MA initiated with EBiB was conducted under identical condi-
tions. DMF was added as internal standard at 1/1 molar ra-
tio to the initiator (EBiB) because of the overlapping in
chemical shift between the initiator and the polymeric pro-
1
tons. Figure 7 shows a 500-MHz H NMR spectrum of PMA
sample isolated at 4% conversion. A sharp quartet at ꢂ4.15
ppm represents the initiator AOCH CH (H ) protons which
2
3
c
tiator (H
c
) and a broad signal at 4.2 ppm corresponding to
f
overlaps with the polymer ACHA (H ) proton located at the
the proton ACHA located at the a position of the bromide
a position of the bromide chain end of PMA. A broad signal
chain end of PMA (H ). This means that at 10% monomer
at 4 ppm corresponds to the polymer AOCH CH (H ) pro-
d
2
3
d
conversion ꢂ28% of the initial initiator was consumed. The
ton. The inset from the 7.5–8.25 ppm region shows a sharp
singlet at 7.92 ppm corresponding to Hg of DMF. The
same analysis for different samples indicates that the initia-
1
FIGURE 7 500-MHz
H
NMR
spectrum of PMA at 4% conver-
sion. Polymerization conditions:
MA ¼ 1 mL, DMSO ¼ 0.5 mL,
[
MA]/[EBiB]/[Me
6
-TREN]/[DMF] ¼
2
22/1/0.1/1. Cu(0) wire ¼ 12.5 cm
of 20 gauge wire.
INITIATION EFFICIENCY IN SET-LRP OF MA, NGUYEN, ROSEN, AND PERCEC
1241

JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
FIGURE 8 Initiator consumptions in SET-LRP of MA initiated
1
ꢀ
FIGURE 9 H NMR spectrum at 500 MHz of PMA at 89% con-
version (M /M ¼ 1.17). Polymerization condi-
¼ 18545 and M
tions: MA ¼ 1 mL, DMSO ¼ 0.5 mL, [MA]/[MBP]/[Me -TREN] ¼
22/1/0.1. Cu(0) wire ¼ 12.5 cm of 20 gauge wire.
with MBP and EBiB in DMSO at 25 C. Reaction conditions: MA
n
w
n
¼
1 mL, DMSO ¼ 0.5 mL, [MA]/[Initiator]/[Me
6
-TREN] ¼ 222/1/
6
0
.1. Cu(0) wire ¼ 12.5 cm of 20 gauge wire.
2
consumption of initiator is calculated as: 2H d ꢃ 100%. Here,
Hg
this hypothesis, kinetic modeling studies may be required to
deconvolute these two processes.
ꢂ41% of the initiator was consumed at 4% monomer con-
version. The same analysis on reaction samples at later time
points in the polymerization indicated that the initiation step
is complete at ꢂ12% monomer conversion (Fig. 8).
Chain-End Analysis
Complete retention of chain-end functionality is necessary to
achieve ultrahigh molar mass polymers and allows for the
preparation of macroinitiators for block copolymerizations.
For 2-haloisobutyrates to be a suitable replacement of 2-hal-
opropionates, it is necessary to monitor and compare the
retention of chain end functionality of the PMA samples pre-
pared from the SET-LRP of MA initiated by MBP and EBiB.
Figure 8 compares the rates of initiator consumption in the
SET-LRP of MA conducted in DMSO. Consistent to the results
from computational analysis, it is clear that EBiB is con-
sumed much faster than MBP under the current SET-LRP
conditions because of its enhanced reactivity in the DET step
(
Scheme 2, eq 1).
1
Figure 9 shows the 500-MHz H NMR spectrum of the iso-
Initiator Efficiency
lated PMA sample (M
obtained at 89% monomer conversion from the Cu(0) wire/
n
¼ 18,545 and M
w
/M
n
¼ 1.17)
Despite the enhanced control over the molecular weight dis-
tribution, the initiator efficiency of EBiB (Ieff ¼ 83%) is
noticeably lower than that of MBP (Ieff ¼ 91%) [Fig. 5(b,d)].
This might be attributed to the competing effects between
DET and primary radical formation (Scheme 2). It must be
noted that although EBiB is more active than MBP in the het-
ꢀ
erogenous dissociation (Scheme 2, eq 1), the resulting 3
radical is thermodynamically more stable than the 2 radical
ꢀ
formed from the latter. As a result, it is likely that the rate
ꢄ
constant of addition of the EBiB radical to MA is slower
ꢄ
than that of the MBP radical to MA, demonstrated by a
decrease in the enthalpy of primary radical formation from
EBiB when compared with MBP (Scheme 3, eqs 3 and 4). In
theory, this would lead to an increase in the likelihood for
side reactions such as dimerization, thus, decreasing the ini-
tiator efficiency. As there are no direct experiments to test
1
FIGURE 10 H NMR spectrum at 500 MHz of PMA at 88% con-
version (M
n
¼ 20480 and M
w
/M
n
¼ 1.11). Polymerization condi-
tions: MA ¼ 1 mL, DMSO ¼ 0.5 mL, [MA]/[EBiB]/[Me
222/1/0.1. Cu(0) wire ¼ 12.5 cm of 20 gauge wire.
6
-TREN] ¼
SCHEME 3 Primary radical formation in SET-LRP.
1242
WILEYONLINELIBRARY.COM/JOURNAL/JPOLA

ARTICLE
FIGURE 11 The percentage of
bromine-functionalized chains
versus conversion (%) for the
SET-LRP of MA initiated with (a)
MBP and (b) EBiB in DMSO at 25
ꢀ
C. Reaction conditions: MA ¼ 1
mL, DMSO ¼ 0.5 mL, [MA]/[Ini-
tiator]/[Me
6
-TREN]
¼
222/1/0.1.
Cu(0) wire ¼ 12.5 cm of 20 gauge
wire.
Me -TREN catalyzed SET-LRP of MA initiated with MBP in
DMSO at 25 C. The percentage of chain-end functionality
can be estimated by a comparison of the integrals of the
c 3
peaks H (corresponding to the initiator CH A groups) and
Figure 11(b) shows that in this depicted representative poly-
merization, f is high through out the polymerization, culmi-
nating in PMA sample with f > 94%. Considering the experi-
mental error of our NMR method, this value confirms a
6
ꢀ
3
4
H (proton CH located in the a-position of the bromine chain
k
polymer with high retention of functional chain ends.
end; eq 5).
Both SET-LRP of MA initiated with MBP and EBiB allows for
high retention of functional chain ends. This suggests that
both can be used in the synthesis of functionally terminated
polymers. Further optimization of SET-LRP to ensure
ꢀ
ꢁ
Hk
f ð%Þ ¼
ꢃ 100
(5)
Hc=3
1
7,24,26
extremely high functionality may require addition of the
Consistent with previous results,
the Cu(0) wire/Me6-
16,24
external deactivator (Cu(II)X2)
or tailoring of the poly-
TREN catalyzed SET-LRP of MA initiated with MBP exhibits
very high retention of chain end functionality, indicated by
high functionality [f (%)] values, f > 96%, throughout the
polymerization [Fig. 11(a)].
3
2
merization rates via activation of Cu(0) wire, manipulation
2
6
35
of Cu(0) surface area or ligand concentration. This will
be addressed in our upcoming publication.
Structural Analysis of PMA by MALDI-TOF
MALDI-TOF analysis confirmed that the SET-LRP of MA initi-
ated with MBP or a-halopropionate derivatives produces
Similar analysis was applied for the PMA samples from the
SET-LRP of MA initiated with EBiB conducted under identical
conditions. Figure 10 shows the 500-MHz H NMR spectrum
of the PMA sample isolated at 88% monomer conversion.
The percentage of chain-end functionality can be estimated
by a comparison of the integrals of the peaks Hc (corre-
1
1
,2,15,17,24
polymers with perfect chain end functionality.
Here,
matrix-assisted laser desorption/ionization — time-of-flight
mass spectrometry (MALDI-TOF-MS) was applied to examine
low molecular weight PMA isolated at 71% in the SET-LRP
using EBiB as the initiator.
3 k
sponding to the initiator CH groups) and H (proton CH
located in the a-position of the bromine chain end; eq 6).
ꢀ
ꢁ
The MALDI-TOF spectrum (Fig. 12) exhibits only one series
of peaks of which interval was periodic at 86.2, the molar
mass of the MA monomer. The lack of a secondary mass
Hk
f ð%Þ ¼
ꢃ 100
(6)
Hc=6
FIGURE 12 MALDI-TOF-MS spec-
trum of PMA obtained at 71%
conversion by the polymerization
of MA initiated with EBiB in
ꢀ
DMSO at 25 C. Reaction condi-
tions: MA ¼ 1 mL, DMSO ¼ 0.5
mL, [MA]/[EBiB]/[Me
6
-TREN]
¼
222/1/0.1. Cu(0) wire ¼ 12.5 cm
of 20 gauge.
INITIATION EFFICIENCY IN SET-LRP OF MA, NGUYEN, ROSEN, AND PERCEC
1243

JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
FIGURE 13 MALDI-TOF-MS spec-
tra of (a) PMA obtained at 71%
conversion by the polymerization
of MA initiated with EBiB in
ꢀ
DMSO at 25 C. [MA]/[EBiB]/
[
Me
6
-TREN] ¼ 222/1/0.1; (b) PMA
end capped with thiophenol.
peak series in the spectrum indicates the absence of irre-
versible termination. From Figure 13, Mth of PMA obtained
at 71% conversion was 13,760, and MALDI-TOF analysis pro-
vided Mn of 11,833 (M /M ¼ 1.09). This result demon-
As depicted from Figure 14, the SET-LRP of MA initiated
with MBHD in DMSO at 25 C exhibits first order kinetics,
ꢀ
demonstrating a living polymerization, and most importantly,
exceptional dependence of the molecular weight evolution
and distribution with conversions. Unlike the SET-LRP of MA
initiated with MBP under identical conditions, there is no
deviation in the polymer molecular weight from theoretical
values at low conversions. This improved control, over mo-
lecular weight distribution, supports our computational
results.
w
n
strates a relatively good agreement between the theoretical
and experimental values of M . Retention of chain end func-
n
tionality was confirmed by functionalization of PMA by thio-
esterification with thiophenol. MALDI-TOF analysis was car-
ried out on PMA end capped with thiophenol to confirm the
end capping reaction and the absence of halogen loss during
polymerization (Fig. 13). It can be seen that the previous dis-
tribution corresponding to the halogen terminated PMA was
totally absent and the new distribution appears ꢂ29.3 mass
units above the previous. Thus, the thiophenol functionaliza-
tion experiment demonstrates the functional structure of the
chain end by SET-LRP using EBiB initiator.
Interestingly, the absence of an initial deviation in the poly-
mer molar mass from theory at low conversions was also
observed in the SET-LRP of MA initiated with BPE and BBiBE
under identical conditions (Figs. 15 and 16). In addition, the
polymers prepared from the SET-LRP of MA initiated with all
bifunctional initiators in this study exhibit narrow molecular
weight distribution at high conversions (M /M ¼ ꢂ1.11 at
w
n
SET-LRP of MA Initiated with Bifunctional Initiators
As predicted from our computational analysis, the reactivity
of a bifunctional derivative of 2-bromopropionate, such as
MBHD, is much higher than that of its monofunctional coun-
terpart, MBP, and closely matches that of the corresponding
polymeric dormant species in the activation process of SET-
LRP. As bifunctional initiators are particularly valuable in the
ꢂ89%; Table 2, entries 3, 4 and 5). Thus, the combined com-
putational and experimental analyses suggest that MBHD,
BPE, and BBiBE are effective initiators for the SET-LRP of
acrylates.
Toward perfection of the current SET-LRP methodology as a
catalytic platform for the synthesis of tailored polymers, it is
of great interest to combine the desirable attributes offered
by bifunctional initiators, and the dramatic rate acceleration
and improved predictability in the molecular weight evolu-
tion by activated Cu(0) wire. Consistent with our previous
1
7,36
synthesis of telechelic polymers,
molecules via TERMINI concept,
or of dendritic macro-
understanding the ki-
3
7–40
netic behaviors of the SET-LRP using MBHD as the initiator
is of great interest.
FIGURE 14 Kinetic plots and molecular
weight evolutions for SET-LRP of MA initiated
ꢀ
with MBHD in DMSO at 25 C. Reaction con-
ditions: MA ¼ 1 mL, DMSO ¼ 0.5 mL, [MA]/
[
MBHD]/[Me
6
-TREN] ¼ 444/1/0.2. Cu(0) wire ¼
1
2.5 cm of 20 gauge wire.
1244
WILEYONLINELIBRARY.COM/JOURNAL/JPOLA

ARTICLE
reactivity to the polymeric dormant species toward DET in
1
the activation process of SET-LRP. H NMR and MALDI-TOF
experiments provided structural analysis in support for
retention of chain-end functionality in the SET-LRP process
using both initiators. The combined computational and ex-
perimental analyses also demonstrated that not only EBiB,
but also MBHD, BPE, and BBiBE are effective in initiating
well-controlled SET-LRP of MA and are expected to be com-
patible for other functional acrylates. The use of activated
Cu(0) wire in the SET-LRP of acrylates initiated bifunctional
initiators, which further improve the perfection of the cur-
rent SET-LRP methodology through dramatic rate accelera-
tion, although still allowing for exceptional predictability and
dependence of molecular weight evolution and distribution
with conversion.
FIGURE 15 Bifunctional initiator derivatives of MBP and EBiB.
publication on the use of MBHD and BPE in SET-LRP in con-
3
2
junction with activated Cu(0) wire, the SET-LRP of MA ini-
tiated with BBiBE and catalyzed by activated Cu(0) wire pro-
app
ꢁ1
ceeded at a comparable rate (kp ¼ 0.107 min ) and
exhibited exceptional dependence of the molecular weight
evolution with conversion and very narrow molecular weight
distribution (Fig. 17; Table 2, entry 8).
CONCLUSIONS
EXPERIMENTAL
The Cu(0) wire/Me -TREN catalyzed SET-LRP of MA initiated
6
with a 2-bromoisobutyrate, EBiB, was evaluated in compari-
son with the SET-LRP of MA using a 2-bromopropionate,
MBP, as the initiator. The SET-LRP of MA initiated with EBiB
in DMSO proceeds at a comparable rate as that initiated
with MBP. It exhibits improved predictability and depend-
ence of molecular weight evolution and distribution as a
function of conversions, exemplified by the absence of a
deviation in the polymer molecular weight from the theoreti-
cal values at all conversions. This improved control can be
attributed to the higher reactivity of EBiB and its similar
Materials
MA (99%, Acros) was passed over a short column of basic
Al O before use to remove the radical inhibitor. Copper (0)
2
3
wire (20 gauge, Fischer), EBiB (98%, Acros), and MBP (99%,
Acros) were used as received. DMSO (99.9%, Fisher, certified
ACS) was distilled under reduced pressure before use. Me -
TREN was synthesized as described in the literature. The
bifunctional initiator BPE was synthesized by esterification
of ethylene glycol with 2-bromopropionyl bromide in the
6
4
1
1
7
presence of pyridine. BBiBE was synthesized as described
FIGURE 16 Kinetic plots and mo-
lecular weight evolutions for
SET-LRP of MA initiated with (a
and b) BPE and (c and d) BBiBE
ꢀ
in DMSO at 25 C. Reaction con-
ditions: MA ¼ 1 mL, DMSO ¼ 0.5
6
mL, [MA]/[Initiator]/[Me -TREN]
¼
444/1/0.2. Cu(0) wire ¼ 12.5 cm
of 20 gauge wire.
INITIATION EFFICIENCY IN SET-LRP OF MA, NGUYEN, ROSEN, AND PERCEC
1245

JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
FIGURE 17 Kinetic plots and mo-
lecular weight evolutions for
SET-LRP of MA initiated with
ꢀ
BBiBE in DMSO at 25 C. Reac-
tion conditions: MA
DMSO ¼ 0.5 mL, [MA]/[BBiBE]/
Me -TREN] 444/1/0.2. Acti-
¼
1 mL,
[
6
¼
vated Cu(0) wire ¼ 12.5 cm of 20
gauge wire.
4
2
in the literature. MBHD was prepared as described in a
with PMMA standards (conversion: 88.7%, M (GPC) ¼
n
4
3
previous publication.
20162, M
w
/M
n
¼ 1.17).
Computational Techniques
Techniques
All calculations were performed using Spartan 2008 Quantum
Mechanics Program (PC/X86). Full geometry optimizations
1
44
H NMR spectra at 500 MHz were recorded on a Bruker
DRX500 NMR instrument at 20 C in CDCl3 with tetrame-
ꢀ
and single-point energy calculations of all structures reported
were performed via density functional theory (DFT) with the
Berke-3-parameter Lee, Yang, Parr hybrid functional (B3LYP)
using the 6-31þG* basis set. For compounds with multiple
possible conformers, an equilibrium conformer search at the
PM3 level (before a conventional geometry optimization) was
used to determine the molecular geometry. All geometry opti-
mizations were performed without symmetry constraints. Fre-
quency calculations were performed on all optimized geome-
tries to confirm that they are true minima and to extract
thermodynamic parameters. The enthalpy H is corrected for
the zero-point vibrational energy. Energy profile calculations
for anion radicals and ion-radical pairs were performed with
a lower bound bond distance corresponding to the neutral or-
ganic halide for 4Å at intervals of 0.2 Å. All energies were con-
thylsilane as internal standard. Gel permeation chromatogra-
phy (GPC) analysis of the polymer samples were done on a
Perkin-Elmer Series 10 high-performance liquid chromatog-
raphy (HPLC), equipped with an LC-100 column oven (30
ꢀ
C), a Nelson Analytical 900 Series integration data station, a
Perkin-Elmer 785 UV–vis detector (254 nm), a Varian star
4090 refractive index detector, and three AM gel columns
4
(500 Å, 5 lm; 1000 Å, 5 lm; and 10 Å, 5 lm). THF (Fisher,
HPLC grade) was used as eluent at a flow rate of 1 mL/min.
The number-average (M ) and weight-average (M ) molecu-
lar weights of PMA samples were determined with PMMA
standards purchased American Polymer Standards. As the
hydrodynamic volume of PMA is the same as of PMMA, no
correction is needed in the determination of Mn.
n
w
ꢁ
1
verted from Hartree to kcal mol via the conversion constant
ꢁ
1
of 627.509 kcal mol
.
Typical Procedure for Polymerization Kinetics
The monomer (MA, 1.00 mL, 11.1 mmol), solvent (DMSO, 0.5
mL), initiator (MBP, 5.6 lL, 0.05 mmol), catalyst [12.5 cm of
Tabulated Values
Homolytic and Heterolytic BDE: The homolylic, Ehomo, and
heteolytic, Ehetero, bond dissociation energies were calculated
in a standard way according to:
20 gauge copper (0) wire wrapped around a Teflon-coated
stirrer bar], and the ligand (Me -TREN, 1.4 lL, 5lmol) were
6
added to a 25-mL Schlenk tube in the following order:
Cu(0), ligand, initiator, solvent, and monomer. During the
freeze-pump-thaw process the stirrer bar was held above
the reaction mixture using a small magnet. After six freeze-
pump-thaw cycles, the Schlenk tube was filled with nitrogen
Ehomo ¼ Eneutral species ꢁ Eradical ꢁ Ehalogen
(7)
and
Ehetero ¼ Eneutral species ꢁ Eradical ꢁ Ehalide
(8)
ꢀ
and placed in an oil bath thermostated at 25 6 0.1 C with
stirring. The stirrer bar with the catalyst was dropped down
to start the polymerization. The side arm of the tube was
purged with nitrogen before it was opened for samples to be
taken at different intervals throughout the reaction, with an
ERA is the difference in energy between the neutral species
and the ion-radical pair at equilibrium bond distance. It is
calculated according to eq 5.
airtight syringe. Samples were dissolved in CDCl , and the
conversion measured by H NMR spectroscopy. The polymer-
ERA ¼ Eneutral ꢁ Eionꢁradical pair
(9)
3
1
Estab is the difference in energy between the ion-radical pair
and the completely separated radical and halide. It is calcu-
lated according to eq 6.
ization mixture was passed through a small basic Al O chro-
2
3
matographic column to remove any residual Cu(II)Br
2
deacti-
vator. The solvent and residual monomer were removed
under vacuum and the samples dissolved in THF for GPC
analysis. The M and M /M values were determined by GPC
Estab ¼ Eionꢁradicalpair ꢁ Eneutral ꢁ Eanion
(10)
n
w
n
1246
WILEYONLINELIBRARY.COM/JOURNAL/JPOLA

ARTICLE
This work was supported by the National Science Founda-
tion (DMR-0548559 and DMR-0520020). The authors grate-
fully acknowledge P. Roy Vagelos, Chair at Penn.
23 Matyjaszewski, K.; Xia, J. Chem Rev 2001, 101, 2921–2990.
2
4 Lligadas, G.; Percec, V. J Polym Sci Part A: Polym Chem
2
008, 46, 6880–6895.
2
5 Nguyen, N. H.; Rosen, B. M.; Jiang, X.; Fleischmann, S.; Per-
cec, V. J Polym Sci Part A: Polym Chem 2009, 47, 5577–5590.
REFERENCES AND NOTES
2
6 Nguyen, N. H.; Rosen, B. M.; Lligadas, G.; Percec, V. Macro-
1
2
Rosen, B. M.; Percec, V. Chem Rev 2009, 109, 5069–5119.
Rosen, B. M.; Lligadas, G.; Hahn, C.; Percec, V. J Polym Sci
molecules 2009, 42, 2379–2386.
27 Jiang, X.; Fleischmann, S.; Nguyen, N. H.; Rosen, B. M.; Per-
Part A: Polym Chem 2009, 47, 3940–3948.
cec, V. J Polym Sci, Part A: Polym Chem 2009, 47, 5591–5605.
3
Ouchi, M.; Terashima, T.; Sawamoto, M. Chem Rev 2009,
28 Ando, T.; Kamigaito, M.; Sawamoto, M. Tetrahedron 1997,
109, 4963–5050.
53, 15445–15457.
4
Percec, V.; Guliashvili, T.; Ladislaw, J. S.; Wistrand, A.;
29 Matyjaszewski, K.; Wang, J.-L.; Grimaud, T.; Shipp, D. A.
Stjerndahl, A.; Sienkowska, M. J.; Monteiro, M. J.; Sahoo, S.
J Am Chem Soc 2006, 128, 14156–14165.
Macromolecules 1998, 31, 1527–1534.
3
0 Guliashvili, T.; Percec, V. J Polym Sci Part A: Polym Chem
5
Potisek, S. L.; Davis, D. A.; Sottos, N. R.; White, S. R.; Moore,
2007, 45, 1607–1618.
J. S. J Am Chem Soc 2007, 129, 13808–13809.
3
1 Cardinale, A.; Isse, A. A.; Gennaro, A.; Robert, M.; Sav e´ ant,
6
Nguyen, N. H.; Rosen, B. M.; Percec, V. J Polym Sci Part A:
J.-M. J Am Chem Soc 2002, 124, 13533–13539.
Polym Chem 2010, 48, 1752–1763.
3
2 Nguyen, N. H.; Percec, V. J Polym Sci Part A: Polym Chem
7
Zhai, S.; Wang, B.; Feng, C.; Li, Y.; Dong, Y.; Hu, J.; Lu, G.;
2010, 48, 5109–5119.
Huang, X. Polym Sci Part A: Polym Chem 2009, 48,
J
3
3 Tang, W.; Kwak, Y.; Braunecker, W.; Tsarevsky, N. V.; Coote,
647–655.
M. L.; Matyjaszewski, K. J Am Chem Soc 2008, 130, 10702–10713.
8
Fleischmann, S.; Percec, V. J Polym Sci Part A: Polym Chem
3
4 Lligadas, G.; Rosen, B. M.; Monteiro, M. J.; Percec, V. Mac-
2010, 48, 2251–2255.
romolecules 2008, 41, 8360–8364.
9
Fleischmann, S.; Percec, V. J Polym Sci Part A: Polym Chem
3
5 Nguyen, N. H.; Jiang, X.; Fleischmann, S.; Rosen, B. M.; Per-
2010, 48, 2236–2242.
cec, V. J Polym Sci Part A: Polym Chem 2009, 47, 5629–5638.
1
0 Fleischmann, S.; Percec, V. J Polym Sci Part A: Polym Chem
3
6 Moineau, G.; Minet, M.; Dubois, P.; Teyssie, P.; Senninger,
2010, 48, 2243–2250.
T.; Jerome, R. Macromolecules 1998, 32, 27–35.
1
1 Fleischmann, S.; Percec, V. J Polym Sci Part A: Polym Chem
3
7 Percec, V.; Barboiu, B.; Grigoras, C.; Bera, T. K. J Am Chem
2010, 48, 4884–4888.
Soc 2003, 125, 6503–6516.
1
2 Fleischmann, S.; Percec, V. J Polym Sci Part A: Polym Chem
3
8 Percec, V.; Grigoras, C.; Kim, H. J. J Polym Sci Part A:
2010, 48, 4889–4893.
Polym Chem 2004, 42, 505–513.
13 Sienkowska, M. J.; Rosen, B. M.; Percec, V. J Polym Sci Part
3
9 Percec, V.; Grigoras, C.; Bera, T. K.; Barboiu, B.; Bissel, P.
A: Polym Chem 2009, 47, 4130–4140.
4 Hatano, T.; Rosen, B. M.; Percec, V. J Polym Sci Part A:
Polym Chem 2009, 48, 164–172.
5 Rosen, B. M.; Lligadas, G.; Hahn, C.; Percec, V. J Polym Sci
Part A: Polym Chem 2009, 47, 3931–3939.
6 Whittaker, M. R.; Urbani, C. N.; Monteiro, M. J. J Polym Sci
Part A: Polym Chem 2008, 46, 6346–6357.
J Polym Sci Part A: Polym Chem 2005, 43, 4894–4906.
1
4
0 Percec, V.; Bera, T. K.; De, B. B.; Sanai, Y.; Smith, J.;
Holerca, M. N.; Barboiu, B.; Grubbs, R. B.; Frechet, J. M. J.
J Org Chem 2001, 66, 2104–2117.
1
4
4
4
1 Ciampolini, M.; Nardi, N. Inorg Chem 1966, 5, 41–44.
2 Kavitha, A. A.; Singha, N. K. Macromolecules,43, 3193–3205.
3 Sienkowska, M. J.; Percec, V. J Polym Sci Part A: Polym
1
17 Lligadas, G.; Percec, V. J Polym Sci Part A: Polym Chem
Chem 2009, 47, 635–652.
2007, 45, 4684–4695.
4
4 Shao, Y.; Molnar, L. F.; Jung, Y.; Kussmann, J.; Ochsenfeld, C.;
18 Feng, C.; Shen, Z.; Li, Y.; Gu, L.; Zhang, Y.; Lu, G.; Huang,
Brown, S. T.; Gilbert, A. T. B.; Slipchenko, L. V.; Levchenko, S. V.;
O’Neill, D. P., Jr., R. A. D.; Lochan, R. C.; Wang, T.; Beran, G. J. O.;
Besley, N. A.; Herbert, J. M.; Lin, C. Y.; Voorhis, T. V.; Chien, S. H.;
Sodt, A.; Steele, R. P.; Rassolov, V. A.; Maslen, P. E.; Korambath, P.
P.; Adamson, R. D.; Austin, B.; Baker, J.; Byrd, E. F. C.; Dachsel, H.;
Doerksen, R. J.; Dreuw, A.; Dunietz, B. D.; Dutoi, A. D.; Furlani, T. R.;
Gwaltney, S. R.; Heyden, A.; Hirata, S.; Hsu, C.-P.; Kedziora, G.;
Khalliulin, R. Z.; Klunzinger, P.; Lee, A. M.; Lee, M. S.; Liang, W.;
Lotan, I.; Nair, N.; Peters, B.; Proynov, E. I.; Pieniazek, P. A.; Rhee, Y.
M.; Ritchie, J.; Rosta, E.; Sherrill, C. D.; Simmonett, A. C.; Subotnik,
J. E.; Iii, H. L. W.; Zhang, W.; Bell, A. T.; Chakraborty, A. K. Phys
Chem Chem Phys 2006, 8, 3172–3191.
X. J Polym Sci Part A: Polym Chem 2009, 47, 1811–1824.
9 Percec, V.; Popov, A. V.; Ramirez-Castillo, E.; Weichold, O.
J Polym Sci Part A: Polym Chem 2003, 41, 3283–3299.
0 Percec, V.; Popov, A. V.; Ramirez-Castillo, E.; Monteiro, M.;
1
2
Barboiu, B.; Weichold, O.; Asandei, A. D.; Mitchell, C. M. J Am
Chem Soc 2002, 124, 4940–4941.
21 Odian, G. Principles of Polymerization; Wiley Interscience:
Staten Island, 2004; pp198–349.
22 Rosen, B. M.; Percec, V. J Polym Sci Part A: Polym Chem
2008, 46, 5663–5697.
INITIATION EFFICIENCY IN SET-LRP OF MA, NGUYEN, ROSEN, AND PERCEC
1247