Benzoyl phenyltelluride as highly reactive visible-light TERP-reagent for controlled radical polymerization

Article abstract of DOI:10.1021/ma5011588

Benzoyl phenyltelluride (BPT) is a highly reactive TERP-reagent for visible-light-induced (400-500 nm) controlled radical polymerization. The compound can be easily prepared in one step from diphenyl ditelluride and benzoyl chloride. It shows a strong abs

Full text of DOI:10.1021/ma5011588

Article
pubs.acs.org/Macromolecules
Benzoyl Phenyltelluride as Highly Reactive Visible-Light TERP-
Reagent for Controlled Radical Polymerization
Stephan Benedikt,† Norbert Moszner,‡ and Robert Liska*,†
†Institute of Applied Synthetic Chemistry, Division of Macromolecular Chemistry (part of the CD-Laboratory for Digital and
Restorative Dentistry), Vienna University of Technology, Getreidemarkt 9/163/MC, 1060 Vienna, Austria
‡Ivoclar Vivadent AG (part of the CD-Laboratory for Digital and Restorative Dentistry), Bendererstrasse 2, FL-9494 Schaan,
Liechtenstein
S
* Supporting Information
ABSTRACT: Benzoyl phenyltelluride (BPT) is a highly reactive
TERP-reagent for visible-light-induced (400−500 nm) controlled
radical polymerization. The compound can be easily prepared in one
step from diphenyl ditelluride and benzoyl chloride. It shows a
strong absorption at 407 nm that tails out to 473 nm and provides
PDIs (1.2 to 1.3) among the lowest reported in literature for
photoiniferters in general, to which our compound was compared.
PDIs obtained with BPT are much lower than those for benzyl
dithiocarbamte (BDC) (1.7 to 1.8), which was used as a reference
compound. Choice of BDC as reference is based on its property as UV-photoiniferter and on a similar initiation/control
mechanism. However, BDC does not allow living radical polymerization under visible light. The newly discovered compound
BPT provides best results with acrylamides and acrylates. Photoinitiation with styrene was ineffective, and reaction with
methacrylates is not considered living.
should be considered. One disadvantage, though, is the higher
polydispersity (PDI) obtained by the use of photoiniferters (1.5
for the best ones reported so far)13 in comparison with thermal
control agents (1.1 to 1.4).8
INTRODUCTION
■
There are three classical ways to achieve controlled living
radical polymerization (CRP): atom transfer radical polymer-
ization (ATRP), which reacts according to an atom transfer
mechanism,1 nitroxide-mediated polymerization (NMP) with a
dissociation−combination mechanism,2 and reversible addi-
tion−fragmentation chain transfer (RAFT), which is based on
degenerative transfer.3 These methods are well known and in
use for various applications, such as drug delivery systems,
synthesis of complex polymer architectures (e.g., star
polymers), and others.4−6 The concentration of active radicals
has to be kept low to avoid unwanted radical−radical
recombination reactions to ensure a linear increase in the
molecular weight with ongoing monomer double-bond
conversion (DBC) and a narrow molecular weight distribution.
While control is the main advantage, the main disadvantage is
reduced reactivity and as a result a long reaction time (usually
several hours). Furthermore, the classical living systems are
usually thermally initiated, which provides less spatio and
temporal control in comparison with photochemical systems.
Photoiniferters function as initiators that allow controlled living
polymerization to provide better control.7,8 These systems are
capable of behaving as photoinitiator, transfer agent, and
polymerization control agent in one single compound and react
based on a dissociation−combination mechanism. Applications
for photoiniferters include synthesis of complex polymer
structures (e.g., surface grafted polymers), photolithography,
photografting, and so on.9−12 In particular, for applications
where a narrow polydispersity is beneficial, photoiniferters
To date, only a few compounds have been reported in
literature, which shows the ability to function as photo-
inferters.7,14−18 Most of these compounds are based on a
dithiocarbamate structure. To the best of our knowledge, these
photoiniferters are only active in the UV region, although many
absorb up to 500−600 nm. This limits the use of photo-
iniferters to applications where UV light is not considered
problematic, but even there special light protection is usually
required along with other problems that are to overcome.19
Recently new and alternative methods to achieve controlled
radical polymerization were reported. Reversible chain transfer
catalyzed polymerization and cobalt-mediated radical polymer-
ization are only two of them.20,21 More important for our work
is the organotellurium-mediated controlled radical polymer-
ization (TERP).22,23 TERP has been established as a powerful
method to achieve controlled radical polymerization, but most
of the TERP-reagents still require additional thermal initiation
or temperatures above room temperature. An exception is the
photolabile TERP reagent (ethyl 2-phenyltellanyl-2-methylpro-
pionate),24 which allows reasonable molecular weight polymer
with low polydispersity. On the basis of these prior good
Received: June 5, 2014
Revised: July 23, 2014
Published: August 14, 2014
© 2014 American Chemical Society
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chromatography (PE/Et2O 20:1, silica gel) to yield 1.48 g (74% of
theory) of pure BPT as a bright yellow powder. m.p.: 70−72 °C. GC−
MS (THF, EI, m/z): 311.96 (M); 282.00; 206.99; 154.10; 105.03;
results, we have chosen to study a benzoyl telluride compound,
which cleaves to give a highly reactive benzoyl radical and also
allows photoinduced TERP.
1
77.06. H NMR (CD2Cl2, δ): 7.75−7.49 (m, 5H, CO−C6H5); 7.48−
We describe the newly discovered polymerization control
properties of the telluroorganic compound benzoyl phenyl-
telluride (BPT), which combines the positive characteristics of
the already mentioned organotellurium compound ethyl 2-
phenyltellanyl-2-methylpropionate with the reactivity of a
benzoyl chromophore and compare it to the most relevant
photoiniferter from literature benzyl dithiocarbamate
(BDC).7,14−16 The comparison to a photoiniferter makes
more sense than the comparison with other TERP reagents
because TERP reagents behave very similar to photoiniferters
when they are used as photoinitiator instead of thermal
initiator.24 This applies especially to the mechanism. While in
thermally initiated TERP a degenerative transfer mechanism is
exclusively responsible for the control abilities, in photoinduced
TERP, a dissociation−combination is assumed to also play an
important role, although degenerative transfer still occurs as a
competing reaction.25 This is also the case for photoiniferters,
even though the degenerative transfer is less important. It is
worth mentioning that a similar compound (benzoyl
methyltelluride) to our BPT has already been described in
literature as a thermal TERP-reagent, but this compound
showed poor control abilities because of the high C−Te-bond
dissociation energy, which should be lower in our case due to
an aryl substituent on the Te atom.26 We will show that BPT
can be used as control agent for acrylates (ACs) and
acrylamides (AMs) in the visible light (400−500 nm) region.
The ability of BPT to control the polymerization has been
confirmed by steady-state polymerization experiments, which
include measurements of the polydispersity and the combina-
tion of molecular weight and DBC, respectively. While rate
constants for reactions of dithiocarbamyl radicals were reported
in literature,27 no constants for photoinduced reactions of
TERP reagents have been reported yet. Therefore, the
comparison of control abilities will be done on the basis of
achievable PDIs. A polymerization mechanism based on
literature is proposed; photo-DSC and UV−vis measurements
are provided for comparison with other photoinitiating systems.
7.20 (m, 5H, Te−C6H5). 13C NMR (CD2Cl2, δ): 143.3; 141.1; 134.5;
129.9; 129.6; 129.3; 127.3; 125.7. IR (ATR, cm−1): 1663.47 ν(CO).
UV−vis Measurements. UV−vis measurements were carried out
on a Shimadzu UV-1800 spectrophotometer using quartz cuvettes
with 10 mm thickness. The photoiniferter samples were dissolved in
CH2Cl2 with a concentration of 1 × 10−3 mol L−1 for BDC, BPT, and
MAPO and with 1 × 10−2 mol L−1 for CQ and measured in the dark.
Kinetic Studies. The photopolymerization of the four different
monomers (BA, BMA, St, NAM) was carried out in a photoreactor
(Figure S1 in the Supporting Information). The reactor was filled to a
height of ∼25 mm with monomer formulation (monomer + BPT/
BDC), which was degassed with argon. For irradiation an OmniCure
2000 (Lumen Dynamic), mercury lamp with a filter excluding all but
400−500 nm was used. The effective irradiation intensity was adjusted
to the reactivity of the monomer (0.07 W cm−2 on the surface of the
formulation for BA, BMA, and St and 0.03 W cm−2 on the surface for
NAM). For the more reactive monomers, a lower effective irradiation
intensity and photoiniferter/TERP-reagent to monomer molar ratio
was chosen (BA: 1:200, BMA: 1:100, St: 1:100, NAM: 1:500) to make
sure the polymerization would be slow enough to take samples. These
samples were taken with a syringe over the side joint of the reactor.
Typical sample size was 0.05 mL, of which 10 mg was used for GPC
analysis and the rest of ∼40−50 mg was used for 1H NMR-
spectroscopy.
For determination of the number-average molecular weight (Mn)
and the polydispersity index (PDI), a Waters 717plus GPC with three
columns (Styragel HR 0.5 THF, Styragel HR 3 THF, and Styragel HR
4 THF) and a Waters 2410 refractive index detector were used. The
eluent was THF with a flow rate of 1.0 mL min−1, and the temperature
was set to 40 °C. For calibration, polystyrene standards were used,
which offer a molecular weight resolving range of 102 to 106 g/mol.
A Bruker AC-E-200 FT-NMR-spectrometer was used for the
samples taken from the photoreactor to determine the monomer
DBC. The solvent was deuterated chloroform (CDCl3) with a degree
of deuteration ≥99.8% D. Out of these spectra, the double-bond
conversion was calculated by comparing the integrals of the monomer
specific double bonds to the combined integrals of the side chains of
monomer and polymer.
Photo-DSC Measurements. Photo-DSC analysis was done on a
Netzsch DSC 204 F1 using an OmniCure 2000 (Lumen Dynamic)
mercury lamp light source equipped with a built-in 400−500 nm filter.
The lamp was calibrated with an OmniCure R2000 radiometer to an
effective irradiation intensity of 3.00 W cm−2. The measurements were
done with 10 1 mg of monomer formulation, which consisted of
photoinitiator/CRP-reagent and NAM in a molar ratio of 1:500 (0.3
to 0.5 wt %). As a result from the measurements, the time until the
maximum heat of polymerization (tmax) is reached can be obtained
directly. Also, the time until 95% of the maximum DBC (t95%) can be
directly acquired through integration of the resulting curves. However,
the rate of polymerization (Rp) and the DBC of the monomer need to
be calculated according to eqs 1 and 2
EXPERIMENTAL SECTION
■
Materials. The monomers styrene (St), n-butyl acrylate (BA), n-
butyl methacrylate (BMA), and 4-acryloylmorpholine (NAM) and the
photoiniferters/photoinitiators benzyl dithiocarbamate (BDC), mono-
acylphosphinoxide (MAPO) (diphenyl(2,4,6-trimethylbenzoyl)-
phosphine oxide), camphorquinone (CQ), and ethyl 4-(dimethylami-
no)-benzoate (DMAB) are commercially available (Aldrich) and were
used in the highest purities available. Benzoyl phenyltelluride (BPT)
was synthesized according to a slightly modified synthetic procedure
described in literature.28
Synthesis of Benzoyl Phenyltelluride (BPT). All synthesis was
performed under light protection in an orange light lab, which excludes
wavelengths below 520 nm. The following procedure is slightly
modified from that of Gardner et al.28 Diphenyl ditelluride (3.23
mmol, 1.32 g) was dissolved in a mixture of toluene and ethanol (5
mL, 25/75 v/v) and heated to reflux. To this solution NaBH4 (5.17
mmol, 0.20 g) dissolved in 1N NaOH (4.4 mL) was added dropwise.
During the addition, there appeared a strong formation of H2, and the
solution became colorless. Then, benzoyl chloride (7.75 mmol, 1.09 g,
0.89 mL) was added in one portion, and the warm reaction mixture
was stirred for another 5 min. Afterward, 25 mL of water was added,
and the resulting mixture was extracted with diethyl ether. The
combined organic phase was dried over anhydrous NaSO4, and after
removal of the solvent the crude product was purified via liquid
ΔHmax*ρ
Rp =
ΔH0*Mw
(1)
A
ΔH0
DBC =
*100
(2)
Herein ΔHmax is the maximum heat of polymerization, ΔH0 (NAM:
513.57 J g−1)29 is the theoretical heat of polymerization, ρ (NAM:
1114 g L−1)29 is the density of the formulation, Mw is the molecular
weight, and A is the integrated area of the DSC plot. All of these
numbers are referring to the monomer.
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RESULTS AND DISCUSSION
Scheme 1. Photoinitiator/CRP Systems Tested in the
Present Paper
■
UV−vis Spectroscopy. The UV−vis spectrum (Figure 1)
of the tellurium compound BPT is compared with the
spectrum of the dithiocarbamate BDC and a classical Type I
(MAPO) and Type II (CQ) photoinitiator.
1.8 over 50% conversion. In that paper, also a linear correlation
between increasing molecular weight and DBC has been
proven.
Figure 1. UV−vis-absorption spectra of the photoiniferters/photo-
initiators BDC, BPT, MAPO, and CQ (1 × 10−3 mol L−1 for BDC,
BPT, and MAPO and 1 × 10−2 mol L−1 for CQ in CH2Cl2).
In the present study, the experiments for the determination
of the polymerization kinetics were carried out in a photo-
reactor with a 400−500 nm light source and four different
classes of monomers (St, BMA, BA, NAM). The molar
concentration of photoiniferter/TERP-reagent in the monomer
varies between 1:100 and 1:500 (0.3 to 2.2 wt %) and was
adjusted to the reactivity of the monomer. For both the
reference BDC and the tellurium compound BPT, Mn versus
The telluride BPT has a maximum of 339 nm in the
absorption spectrum, which most likely marks the nσ*
transition of the compound. This was already shown in
literature for telluriumorganic compounds by using time-
dependent DFT calculations.24 In this band, a significant
shoulder appears at 407 nm that tails out until 473 nm, which
can be assigned to the nπ* transition of the carbonyl moiety.
This is in good accordance with other Ar−CO−X systems like
acylphosphinoxides (e.g., MAPO, λ = 380 nm) and acyl
germanium compounds (e.g., benzoyltrimethylgermane, λ =
412 nm).30 The strong shift of the nπ* transition compared
with the classical benzoyl chromophore (nπ* ∼350−360
nm) can be explained by the overlap of the d orbitals of P, Ge,
or Te with the π* orbital of the CO group, thus reducing the
necessary energy for the nπ* transition.
1
DBC plots from data of H NMR-spectroscopy and GPC
measurements were made. Additionally, the PDIs of the
synthesized polymers were measured.
In general, styrene photoinitiation with both BDC and BPT
was ineffective because styrene is more likely to quench the
initiator than to propagate under these conditions. By
comparison, TERP-reagents have been previously shown to
moderate styrene propagation under thermal initiation
conditions (Yamago 2013).35 The methacrylate BMA could
be polymerized with both BDC and BPT but not in a living
radical polymerization, and the rate for the reference BDC was
very low (Figures S2 and S3 in the Supporting Information).
This is in good accordance with literature, which highlights the
important role of dimethyl ditelluride for the controlled TERP
polymerization of methacrylates.36 The monomers BA and
NAM both met the criteria for a living radical polymerization
with our compound BPT (Figure 2). It has to be noted that
NAM was only polymerized to a DBC of ∼20% with both
photoiniferters due to its high viscosity, which made it
impossible to take samples from the photoreactor at higher
DBCs.
BDC has its maximum absorption (belonging to the nπ*
transition of the carbamate moiety) at 335 nm.31,32 Because of
the high conjugation of this functional group, the peak tails out
far into the visible region even above 500 nm. The extinction
coefficient is 50 L mol−1 cm−1, which is very low but still in the
same range of CQ.
The classical type-II photoinitiator CQ has its absorption
maximum at 468 nm. The long wavelength of the nπ*
transition can be explained by the angle between the two
carbonyl groups (Scheme 1).33
Controlled Living Radical Photopolymerization. The
living character of a polymerization can be proven with studies
of the polymerization kinetics (Figure 2).34 If the polymer-
ization occurs according to a living mechanism, the molecular
weight increases in a linear relationship with the DBC of the
monomer. Also, the PDI should be significantly below 1.5
because the control agent should theoretically provide a
homogeneous chain length. In the case of a free radical
polymerization, the overall number-average molecular weight
(Mn) will decrease before the gel point is reached. Moreover,
because of termination and unwanted transfer reactions, the
PDI will usually be above 1.5.
The increase in the PDI for NAM with BPT in the beginning
can be explained by termination reactions, which still appear in
this early phase of the polymerization.
At conversions as low as 4%, Mn has values around 4 to 5
kDa for both BA and NAM polymerized with the tellurium
compound BPT. Mn increases linearly with DBC to values
around 11 kDa. The deviation of the best-fit line from the
origin is due to radical recombination reactions, which still
appear in the early stage of the reaction.18 This causes an initial
steep increase in molecular weight as the initiator is consumed,
followed by a less steep but linear propagation phase. All of this
is in good accordance with other photoiniferter studies found in
literature.13 The PDIs are significantly below 1.5 and are as low
as 1.2 for BA over 50% DBC and 1.3 for NAM over 50% DBC
(PDI over 50% DBC for BDC according to literature: 1.7 to
The living character of the reference BDC for a polymer-
ization carried out with UV light has already been shown in
literature.13 The typical PDI for that compound as photo-
iniferter for the bulk polymerization of MMA is around 1.7 to
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Figure 2. Number-average molecular weight Mn (diamonds) and PDI (crosses) versus double-bond conversion (DBC) plots for BDC and BPT with
BA and NAM in bulk determined with photoreactor experiments.
a
Table 1. UV-vis Data and Results of the Photo-DSC Measurement with CRP-Reagent/Photoinitiator in NAM with a Molar
Ratio of 1:500 Each
Rp [10−3 mol L−1 s−1
]
DBC [%]
tmax [s]
t95% [s]
λmax [nm]
ε [L mol−1 cm−1
]
BDC
15.9
14.0
40
29
89
80
53.5
33.0
8.0
257
180
22
335
126
595
636
44
b
BPT
407
MAPO
CQ/DMAB
665.2
380
468
281.0
14.1
34
a
b
1 × 10−3 mol L−1 for BDC, BPT, and MAPO and 1 × 10−2 mol L−1 for CQ in CH2Cl2. Absorption at the shoulder of the UV−vis spectrum
determined by peak deconvolution, marking the nπ* transition of the carbonyl group
1.8).13 This fact is especially remarkable because the best
photoiniferter system so far (benzyl 9H-carbazole-9-carbodi-
thioate) can only reach a PDI of 1.5 over 40% conversion.13
In contrast with BPT, the reference BDC was not suitable for
living polymerization under visible light for any tested
monomer. Even though the PDIs were below 2 for BA and
NAM, Mn does not increase linearly for either monomer.
Photoreactivity Determined by Photo-DSC. The
reactivity of the telluride BPT was compared with the reactivity
of the reference dithiocarbamate BDC. The comparison was
done only for acrylamides because the reactivity of butyl
acrylate was too low to produce significant and accurate results
with photo-DSC. In general, the reactivity of photoiniferters is
lower than the reactivity of photoinitiators, which can be
explained by the reaction mechanism. While photoinitiators
produce radicals, which directly induce radical chain growth,
the use of photoiniferters leads to the formation of so-called
dormant species during the polymerization reaction. These
species have to be reactivated by light, which leads to a
significant decline of the reaction rate. Of course, this is a
generalization, and it also has to be mentioned that other effects
like quantum yields of the compounds play an important role as
well. Nevertheless, classical photoinitiator systems, which
initiate in the visible-light range (MAPO and CQ/DMAB)
were measured (Table 1 and Figure 3), too, to compare the
CRP-reagents among commercially used photochemical
polymerization systems.
Figure 3. Double-bond conversion DBC [%] versus time [s] for the
CRP reagents BDC and BPT and for the photoinitiator systems
MAPO and CQ/DMAB in NAM with a molar ratio of 1:500 each
(0.3, 0.4, 0.5, and 0.5 wt %, respectively).
both in a similar range of reactivity. The rate of polymerization
Rp is approximately the same (14.0 × 10−3 mol L−1 s−1 for BPT
and 15.9 × 10−3 mol L−1 s−1 for BDC), but the time until the
maximum heat of polymerization tmax is reached and the time
until 95% of the polymerization reaction is completed t95% are
significantly lower for our BPT. The reason for that might be
the formation of a highly reactive benzoyl radical in the case of
BPT (which is also the main difference to the photoinducible
TERP reagent ethyl 2-phenyltellanyl-2-methylpropionate)
instead of a benzyl radical as for BDC,8,13 which makes the
initiation step very efficient (Schemes 2 and 3, line 2). The
As expected, the reactivity (expressed by Rp, tmax, and t95%
)
and the DBC of the classical free radical photoinitiator systems
MAPO and CQ/DMAB are significantly higher than the values
for the CRP-reagents. The photoiniferters BDC and BPT are
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can be assumed that the absorption in the visible-light region
for the reagent BDC is similar to the resulting dormant species.
Therefore, cleavage can take place but is very inefficient, which
results in no control of the polymerization. In contrast, it can be
expected from the dormant species of our BPT to still
efficiently cleave in the visible light area, which is in good
accordance with literature.24 It is also worth mentioning that a
degenerative transfer mechanism may compete with the
described mechanism (Scheme 2, line 4), therefore reducing
the reactivity of BDC, but for the control of polymerization, the
dissociation−combination mechanism plays the main role.13
The tellurium compound BPT reacts via a similar
mechanism, which is already proposed in literature for another
telluroorganic compound.24 The adapted mechanism for the
specific photoiniferter is shown in Scheme 3.
Scheme 2. Mechanism for the Living Radical Polymerization
with BDC as Photoiniferter8
The cleavage of the C−Te-bond (Scheme 3, Line 3) requires
less energy (absorption above 400 nm) and is therefore still
efficient under the influence of visible light, which was already
shown in literature for TERPs.23,35 It has to be noted that also
here degenerative transfer (Scheme 3, line 4) may compete
with the dissociation−combination mechanism (Scheme 3, line
3) and may even be very important for polymerization control.
However, while degenerative transfer is the major mechanism
for thermally initiated TERP, for photoinduced TERP, the
dissociation−combination also plays a major role.24,25 For sure,
the exact mechanism needs to be elucidated in future work, but
the proposed mechanism based on literature gives us a first
plausible explanation.
Scheme 3. Proposed Living Radical Polymerization
Mechanism with BPT as TERP-Reagent24
CONCLUSIONS
■
We have shown that the telluroorganic compound BPT is a
suitable polymerization control agent for acrylates and
acrylamides, yielding polydispersities as low as 1.2 to 1.3. To
our best knowledge, BPT leads to lower polydispersities than it
was reported in literature for photoiniferters so far. The most
important benefit of BPT, though, can be found in the ability to
carry out controlled radical polymerization at room temper-
ature with a visible light (400−500 nm) radiation source. The
reference BDC also has some potential but needs UV light for
its living radical polymerization mechanism. Explanation can be
given by the UV−vis spectrum, in general, and the nπ*
transition (335 nm for BDC and 407 nm for BPT) of the two
compared compounds in particular. The subsequently formed
tellurium dormant species was still active under visible-light
irradiation. The high reactivity of the tellurium compound BPT
compared with the reference BDC has been shown by photo-
DSC experiments and can be attributed to the highly reactive
benzoyl radical. In conclusion, BPT is highly suitable for
photopolymerizations, where a narrow molecular weight
distribution is mandatory and visible light is preferred.
DBC is, with ∼30% for BPT and ∼40% for BDC for both
systems, relatively low. The higher DBC for the reference BDC
can be explained by the noncontrolled polymerization
mechanism.
Higher DBC (>50%) for BPT can be achieved by using
higher photoiniferter concentrations, which necessitates longer
polymerization times or a higher light intensity (Figure S4 in
the Supporting Information). However, the tested concen-
tration was chosen because it provides comparable results for
every tested system and is also consistent with the
concentrations used for polymerization kinetic studies.
In general, the fact that the reference BDC is slightly less
reactive does not come as a surprise. Literature already suggests
that BDC would need UV light for reinitiation8 (Scheme 2, line
3) or at least UV light would make this reinitiation much more
efficient.13
ASSOCIATED CONTENT
* Supporting Information
■
S
Figure S1: Photoreactor. Figures S2 and S3: Polymerization
kinetics of BMA with BPT and BDC. Figure S4: Photo-DSC
measurements of NAM with BPT in different concentrations
and with different light intensities. This material is available free
of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: Robert.Liska@tuwien.ac.at. Tel: +43-1-58801-163614.
Fax: +43-1-58801-16299.
Poor visible-light efficiency of BDC is explained by poor
overlap with the nπ* transition, which has its maximum in
the UV region. It thus requires UV light for efficient cleavage of
the C−S bond between polymer and chain-transfer agent. It
■
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(34) Qiu, J.; Charleux, B.; Matyjaszewski, K. Prog. Polym. Sci. 2001,
26, 2083−2134.
Notes
The authors declare no competing financial interest.
(35) Nakamura, Y.; Yamago, S. Beilstein J. Org. Chem. 2013, 9, 1607−
1612.
ACKNOWLEDGMENTS
(36) Kwak, Y.; Tezuka, M.; Goto, A.; Fukuda, T.; Yamago, S.
Macromolecules 2007, 40, 1881−1885.
■
We thank the Christian Doppler society for financial support.
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