Phenyl Benzo[b]phenothiazine as a Visible Light Photoredox Catalyst for Metal-Free Atom Transfer Radical Polymerization

Article abstract of DOI:10.1002/chem.201605574

This paper reports use of phenyl benzo[b]phenothiazine (Ph-benzoPTZ) as a visible light-induced metal-free atom transfer radical polymerization (ATRP) photoredox catalyst. Well-controlled polymerizations of various methacrylate monomers were conducted und

Full text of DOI:10.1002/chem.201605574

A Journal of
Accepted Article
Title: Phenyl benzo[b]phenothiazine as a visible light photoredox
catalyst for metal-free atom transfer radical polymerization
Authors: Krzysztof Matyjaszewski, Sajjad Dadashi-Silab, and
Xiangcheng Pan
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To be cited as: Chem. Eur. J. 10.1002/chem.201605574
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Phenyl benzo[b]phenothiazine as a visible light photoredox
catalyst for metal-free atom transfer radical polymerization
Sajjad Dadashi-Silab,+ Xiangcheng Pan,+ and Krzysztof Matyjaszewski*
agents ATRP (SARA ATRP),[10] initiators for continuous activator
regeneration (ICAR) ATRP,[11] and electrochemically-mediated
Abstract: This paper reports use of phenyl benzo[b]phenothiazine
ATRP,[12] have been developed that use ppm levels of air-stable
(Ph-benzoPTZ) as a visible light-induced metal-free atom transfer
CuII complexes to generate in situ the activator CuI species.
radical polymerization (ATRP) photoredox catalyst. Well-controlled
Similarly, owing to the fact that CuII/L complexes are
polymerizations of various methacrylate monomers were conducted
photochemically active, CuI activators can also be (re)generated
under a 392 nm visible light LED using Ph-benzoPTZ, activation of
by photochemical means.[1, 13] Photoinduced ATRP has several
different alkyl halides. The use of the photocatalyst enabled temporal
control over the growth of polymer chains during intermittent on/off
advantages in terms of establishing spatiotemporal control over
periods. The polymerization was initiated and progressed only under
the course of the polymerization which leads to precise and
stimulation by light and completely stopped in the absence of light.
facile manipulation of the growth of polymers in space and
time.[14]
Block copolymers were synthesized to demonstrate high retention of
chain end fidelity in the polymers and livingness of the process.
Photoredox catalysis have been developed as a new class of
catalysis for RDRP techniques for the synthesis of well-defined
The emergence of photoredox catalysis in the synthetic polymer
community has provided unique opportunities for the synthesis
of well-defined polymers.[1] Photochemical processes enable
chemical reactions to be conducted under more environmentally
benign conditions as they use light as a “green” reagent to drive
a range of chemical transformations.[2] A great deal of research
in the area of reversible-deactivation radical polymerization
(RDRP), traditionally known as controlled radical polymerization
(CRP), has been directed towards developing, and implementing,
photochemical processes for the synthesis of materials with
controlled polymeric structures. In this regard, atom transfer
polymers. Advancements have been facilitated by development
of photoredox catalysis for ATRP[15] and RAFT[16]
polymerizations. In addition to transition metal-based photoredox
catalysts, such as Ir- or Ru- complexes, photoredox
organocatalysts as recently developed transition metal-free
systems are used to establish the ATRP equilibrium. In Cu-
based ATRP systems an activator L/CuI species activates alkyl
halide initiator to form initiating radicals while the L/CuI is
oxidized to L/CuII-X (where L is a ligand and X is halogen) via an
inner-sphere electron transfer process. In photoinduced metal-
free ATRP reactions, however, a photoredox catalyst, which
becomes highly reducing in its photoexcited state, is used to
activate alkyl halide initiators via outer-sphere electron transfer
reaction. The products of reduction of the alkyl halide by the
excited state photocatalyst are organic radicals that can initiate
polymerization and halide anions. At the same time, the
photocatalyst is oxidized to a radical cation. The key step in
establishing control over the process relies on the ability of the
catalyst radical cation, to deactivate propagating radicals
forming bromine-terminated polymer chains and the initial
radical
polymerization
(ATRP),[3]
reversible
addition-
fragmentation chain transfer (RAFT),[4] and nitroxide-mediated
polymerization (NMP)[5] are among the most widely used RDRP
techniques and in recent years they have all adopted
photochemical approaches.
ATRP predominately utilizes transition metal catalysis[6] to
establish and maintain an equilibrium between dormant and
growing species which in turn results in manifesting excellent
control over the molecular weight (MW) and MW distribution of
polymers, allowing access to well-defined polymers with
complex macromolecular architecture. Recent developments
have permitted diminishing catalyst concentrations to ppm levels
while maintaining control over the polymerization process. This
is of importance for economic reasons as well as expanding the
utility of ATRP to include applications targeting biologically
active materials where no, or least minimal, presence of residual
transition metal catalysts is demanded. As a result, techniques
including activators generated by electron transfer (AGET),[7]
activators regenerated by electron transfer (ARGET),[8] and use
of zero-valent metals[9] as supplemental activators and reducing
ground
state
photocatalyst,
thereby
completing
the
photocatalytic cycle.
Our group recently reported a detailed mechanistic study that
elucidated the mechanism of the steps involved in photoinduced
metal-free ATRP reactions.[17] It was proposed that the activation
step involves a photoinduced dissociative electron transfer from
the excited state catalyst to the alkyl halide initiator to form
initiating radicals, bromine anions, and catalyst radical cations.
The deactivation process proceeds favorably through an
associative electron transfer involving the radical cation, halide
anion and propagating radicals to form bromine-terminated
chains and regenerate the ground state catalyst.
Phenothiazine
derivatives
are
efficient
catalysts
for
[*]
S. Dadashi-Silab, Dr. X. Pan, Prof K. Matyjaszewski
Department of Chemistry
Carnegie Mellon University
4400 Fifth Avenue, Pittsburgh 15213, Pennsylvania, United States
E-mail: km3b@andrew.cmu.edu
S.D.-S. and X.P. contributed equally to this work.
photoinduced metal-free ATRP. Initial studies reported the use
of 10-phenylphenothiazine for controlled polymerization of
various
monomers.[18]
Dihydrophenazine[19]
-
and
phenoxazine[20]-based photoredox catalysts were subsequently
developed to tune and enhance the polymerization efficiency
[+]
Supporting information for this article is given via a link at the end of
the document.
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4000
3000
2000
1000
0
Ph-benzoPTZ
Ph-PTZ
300 350 400 450 500 550
Wavelength (nm)
Figure 1. UV-vis spectra of photocatalysts Ph-benzoPTZ and Ph-PTZ in DMA (concentration: 3.07 × 10-4 M).
in visible light regions. Polynuclear aromatic hydrocarbons
400 nm.[25] In this area Ph-PTZ is transparent and shows a very
weak absorption. The UV-vis spectra of the photocatalysts are
shown in Figure 1.
including perylene,[21] anthracene, and pyrene[22] and
carbazole[23]-based photoredox catalysts are also suitable for
photoinduced metal-free ATRP. Furthermore, organic dyes that
are oxidizing in the excited state can initiate and control the
Metal-free ATRP of methyl methacrylate (MMA) was conducted
using Ph-benzoPTZ with various light sources to investigate the
effect of light wavelength on the kinetics of ATRP. A UV light
source emitting at 365 nm with two different light intensities of
3.3 and 4.2 mW cm-2 and a visible light LED at 392 nm with the
light intensity of 0.14 mW cm-2 were used. As shown in Figure 2,
metal-free ATRP via
a different mechanistic pathway by
undergoing reductive quenching in the presence of amine
electron-donor compounds.[24] In this case, the catalyst is
reduced to a radical anion by accepting an electron from the
electron-donor in the excited state, which can then activate the
alkyl halide to form initiating radicals, a halide anion and an
amine radical cation as well as regenerating the ground state
catalyst. An electron transfer between the halide anion and
amine radical cation regenerates the initial amine and a bromine
radical that can deactivate the propagating radical.
the
[MMA]0/[EBPA]0/[Ph-benzoPTZ]0 : 100/1/0.1 in 50 v% DMA
(EBPA: ethyl α-bromophenylacetate, DMA: N,N-
kinetics
of
the
reaction
under
conditions
dimethylacetamide) followed a linear semilogarithmic plot of
monomer consumption vs. irradiation time. As expected from the
optical properties of the photocatalyst, the reaction under UV
light at 365 nm was faster with respect to visible LED light at 392
nm. The use of a UV lamp with higher light intensity (4.2 mW cm-
2) resulted in an even faster reaction. However, the use of the
In this communication we expand the scope of photoinduced
metal-free ATRP by utilizing phenyl benzo[b]phenothiazine (Ph-
benzoPTZ) as a visible light activated photoredox catalyst to
promote ATRP of methacrylate monomers under LED irradiation. UV light with higher intensity resulted in higher Ð values (1.6-2)
Due to more extended conjugation, with respect to phenyl
phenothiazine (Ph-PTZ), the Ph-benzoPTZ photocatalyst
exhibits a stronger absorption in the visible light region, around
whilst irradiation with the weaker UV lamp, or visible LED,
resulted in lower Ð values reaching to 1.3.
B
A
16000
12000
8000
4000
0
4
3
2
1
365 nm (4.2 mW cm-2
365 nm (3.3 mW cm-2
)
)
365 nm (4.2 mW/cm2)
365 nm (3.3 mW/cm2)
392 nm LED (0.14 mW cm-2
392 nm LED (0.14 mW cm-2
)
3
2
1
0
)
Mn,th
0
10
20
30
40
50
0
20
40
60
80
100
Conversion (%)
Time (h)
Figure 2. (A) Kinetics and (B) molecular weight properties of PMMA polymerized using Ph-benzoPTZ under various light sources.
Reaction conditions: [MMA]0/[EBPA]0/[Ph-benzoPTZ]0 : 100/1/0.1 in 50 v% DMA.
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Table 1. Results of photoinduced metal-free ATRP using Ph-benzoPTZ under various reaction conditions
b
c
Entry
[MMA]0/[EBPA]0/[Ph-benzoPTZ]0
Light source
Time (h)
Conversion (%) a
Mn,th
Mn
Ð c
I* d
1
2
3
4
5
6
7
100/1/0.1
100/1/0.1
100/1/0.1
100/1/0.05
100/1/0.2
200/1/0.1
50/1/0.1
365 nm (4.2 mW cm-2)
8
96
94
82
90
80
71
78
9843
9643
8443
9243
8243
14443
4143
14800
19000
13000
14700
17400
21700
7250
1.60
1.31
1.30
1.34
1.39
1.71
1.40
0.66
0.51
0.69
0.63
0.48
0.66
0.57
365 nm (3.3 mW cm-2)
392 nm LED (0.14 mW cm-2)
392 nm LED (0.14 mW cm-2)
392 nm LED (0.14 mW cm-2)
392 nm LED (0.14 mW cm-2)
392 nm LED (0.14 mW cm-2)
16
48
48
24
48
48
a Conversions were calculated using 1H NMR. b Theoretical molecular weight (Mn,th) values were calculated based on conversions obtained by 1H NMR
as: Mn,th = MEBPA + [MMA]0/[EBPA]0 × conversion × MMMA.
c Number-average molecular weight (Mn) and dispersity (Ð) were measured using GPC. d
Initiation efficiency (I*) was calculated by dividing theoretical molecular weight to experimental molecular weight measured by GPC (Mn,th/Mn).
benzoPTZ photocatalyst (Scheme 1). The polymerization results
Control experiments were carried out in order to investigate the
are summarized in Table 2. EBPA was the most efficient initiator
in terms of initiation efficiency (0.69) and formation of polymers
effect of various parameters. Loading the photocatalyst at the
ratio of [EBPA]0/[Ph-benzoPTZ]0
: 1/0.1 resulted in 82%
with low dispersity (Ð : 1.30). Similarly use of diethyl 2-bromo-2-
methylmalonate (DEBMM) resulted in a well-controlled process
with low Ð and reaction faster than with EBPA (Entry 4, Table 2).
Reactions with methyl α-bromoisobutyrate (MBiB) (Entry 2,
Table 2) or ethyl α-bromoisobutyrate (EBiB) (Entry 3, Table 2)
were not as well-controlled as they resulted in lower initiation
efficiency and high Ð values.
monomer conversion after 48 h irradiation under visible light
(Entry 3, Table 1) whereas decreasing the catalyst loading by
half, to 100/0.05, resulted in a slightly higher conversion (90%)
but lower initiation efficiency (Entry 4, Table 1). On the other
hand, increasing the concentration of the catalyst by a factor of
two accelerated the rate of the reaction, and it reached 80%
monomer conversion within 24 h. However the initiation
efficiency was lower compared to the 100/0.1 ratio (Entry 5,
Table 1). Targeting a degree of polymerization (DP) of 200
Photoinduced metal-free ATRP using Ph-benzoPTZ was also
examined with a range of alkyl methacrylate monomers. The
system displayed a good tolerance to various methacrylate
monomers including MMA, ethyl methacrylate (EtMA), benzyl
methacrylate (BzMA), and n-butyl methacrylate (n-BuMA).
Polymers were synthesized in a controlled manner with the
initiation efficiency ranging from 0.62-0.85 and low Ð.
resulted in slightly higher
Ð (Entry 6, Table 1) whereas
polymerizations targeting DPs of 100 and 50 were well-
controlled (Entries 3 and 7, Table 1).
Additionally, several alkyl halide initiators and monomers were
examined for photoinduced metal-free ATRP using Ph-
Table 2. The scope of ATRP initiator and monomer for photoinduced metal-free ATRP using Ph-benzoPTZ
photoredox catalyst under 392 nm LED irradiation a
c
d
d
Entry
Monomer
MMA
Initiator
EBPA
MBiB
Time (h)
48
Conversion (%) b
Mn,th
Mn
Ð
I* e
1
2
3
4
5
6
7
8
82
92
78
89
94
74
95
81
8443
9382
7996
9154
9535
9363
16963
11754
13000
27700
45800
13950
21950
14670
20000
17800
1.30
1.69
1.60
1.31
1.43
1.46
1.48
1.45
0.69
0.34
0.18
0.66
0.44
0.64
0.85
0.66
MMA
36
MMA
EBiB
24
MMA
DEBMM
BPN
36
MMA
36
EtMA
EBPA
EBPA
EBPA
48
BzMA
n-BuMA
36
48
a Reaction conditions: [M]0/[R-X]0/[Ph-benzoPTZ]0 : 100/1/0.1 in 50 v% DMA, irradiation under 392 nm LED. b
Conversions were calculated by 1H NMR. c Theoretical molecular weight (Mn,th) values were calculated based on
conversions obtained by 1H NMR as: Mn,th = MR-X + [Monomer]0/[R-X]0 × conversion × MMonomer d Number-average
.
molecular weight (Mn) and dispersity (Ð) were determined by GPC. e Initiation efficiency (I*) was calculated by
dividing theoretical molecular weight to experimental molecular weight measured by GPC (Mn,th/Mn).
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processes. Thus,
synthesized using
a
PMMA macroinitiator was initially
Ph-benzoPTZ
under
conditions
[MMA]0/[EBPA]0/[Ph-benzoPTZ]0 : 100/1/0.1 in 50 v% DMA. The
synthesized macroinitiator with Br chain-end functionality
(PMMA-Br; Mn: 11800, Ð: 1.41) was used as initiator in chain
extension reactions to form block copolymers in a metal-free
chain extension reaction. PMMA-Br was used in the presence of
Ph-benzoPTZ to polymerize BzMA under visible light LED. The
GPC results, shown in Figure 4, indicate the successful
formation of a P(MMA-b-BzMA) block copolymer as a clear and
unimodal shift towards higher molecular weights was observed
in the GPC traces of the polymers without any detectable dead
or deactivated chains. Furthermore, the PMMA-Br macroinitiator
was examined for
poly(methyl acrylate) (PMA) segment under reaction conditions
[MA]0/[PMMA-Br]0/[CuBr2]0/[Me6TREN]: 400/1/0.03/0.18 in
a
CuII photoinduced process to grow
Scheme 1. ATRP initiators and monomers examined in photoinduced metal-
free ATRP using Ph-benzoPTZ under visible light LED irradiation.
DMSO (Me6TREN: tris[2-(dimethylamino)ethyl]amine and
DMSO: dimethyl sulfoxide) under irradiation with UV light at 365
nm. The PMMA-Br macroinitiator was successfully chain
It was also determined that the growth of polymers can be easily
regulated by simply switching the light source on or off. For
extended to form P(MMA-b-MA) block copolymers in
a
example, exposing
a
reaction mixture consisting of
controlled manner. The resulting P(MMA-b-MA) block
copolymers showed a unimodal peak with higher molecular
weights and no dead or deactivated chains were observed.
These chain extension results confirm the high retention of chain
end functionality in the polymers synthesized by photoinduced
([MMA]0/[EBPA]0/[Ph-benzoPTZ]0 : 100/1/0.1 in 50 v% DMA) to
LED light resulted in ~20% monomer conversion after 8 h. When
the light was then switched off for 8 h no polymer growth was
detected during that period. Re-exposing the reaction to the light
re-initiated the polymerization resulting in ~40% monomer
conversion after the second 8 h illumination. This on/off cycle
was repeated several times with the reaction proceeding only
during the period light was on and completely stopping when the
light was switched off. No conversion was observed during the
dark period. Moreover, the growth of polymer chains in the light-
on periods followed a linear relationship between the monomer
consumption and reaction time, confirming a well-controlled
process proceeding only under irradiation. This behavior clearly
confirms the capability of temporal control over the system and
growth of polymer chains simply by turning light on or off. Figure
3 shows the kinetic results and molecular weight properties of
the resulting polymers during the light on/off periods.
metal-free
ATRP
using
Ph-benzoPTZ.
The
formed
macroinitiators can be readily used in different polymerization
modes to form block copolymers with various functionalities.
The proposed mechanism of the reaction is presented in
Scheme
2
and involves initial photoexcitation of the
photocatalyst Ph-benzoPTZ by visible light. In the subsequent
activation step, the photoexcited state catalyst (Cat*) is
quenched in an oxidative quenching reaction that reduces the
alkyl halide initiator to generate initiating radicals while oxidizing
the photocatalyst to a radical cation. The radicals can initiate
and propagate polymerization before being deactivated by the
photocatalyst radical cation and bromine anion which regenerate
Br-terminated polymer chains and the initial ground state
photocatalyst.
The livingness of the system was also investigated and
confirmed retention of chain end functionality for polymers
obtained using Ph-benzoPTZ through chain extension
1.6
12000
4.0
3.5
3.0
2.5
2.0
1.5
1.0
B
A
Light on
Light off
Mn,th
On Off On Off On Off On Off
10000
8000
6000
4000
2000
0
1.2
0.8
0.4
0.0
0
8
16 24 32 40 48 56 64
Time (h)
0
20
40
60
80
Conversion (%)
Figure 3. Evidence of temporal control in photoinduced metal-free ATRP using Ph-benzoPTZ under 392 nm LED irradiation through intermittent switching on/off
the light: (A) Kinetics and (B) molecular weight and dispersity of polymers. Reaction conditions [MMA]0/[EBPA]0/[Ph-benzoPTZ]0 : 100/1/0.1 in 50 v% DMA.
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PMMA-Br
P(MMA-b-MA)
PMMA-Br
P(MMA-b-BzMA)
PMMA-Br
1000
10000
100000
1000
10000
100000
1000
10000
100000
Molecular Weight
Molecular Weight
Molecular Weight
Figure 4. GPC traces of PMMA-Br macroinitiator (middle) synthesized by photoinduced metal-free ATRP using Ph-benzoPTZ (conditions: [MMA]0/[EBPA]0/[Ph-
benzoPTZ]0 : 100/1/0.1 in 50 v% DMA) and its chain extension to give P(MMA-b-BzMA) block copolymer (left) via photoinduced metal-free ATRP (conditions:
[BzMA]0/[PMMA-Br]0/[Ph-benzoPTZ]0 : 400/1/0.1 in DMA) and P(MMA-b-MA) (right) block copolymer prepared via CuII photoinduced ATRP (conditions:
[MA]0/[PMMA-Br]0/[CuBr2]0/[Me6TREN]0 : 400/1/0.03/0.18 in DMSO.
Keywords: photoinduced polymerization • atom transfer radical
polymerization • visible light photoredox catalysis • metal-free
photoredox • phenothiazine
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Acknowledgements
We thank the NSF (CHE-1400052) and members of the CRP
consortium for financial assistance.
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This article is protected by copyright. All rights reserved.

10.1002/chem.201605574
Chemistry - A European Journal
COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
A new visible light active photoredox
catalyst is reported for metal-free
atom transfer radical polymerization.
Phenyl [b]benzophenothizaine
successfully mediates controlled
synthesis of methacrylate polymers
under visible light LED illumination
with the ability to temporally control
the reaction course and synthesis of
block copolymers.
S. Dadashi-Silab, X. Pan, and K
Matyjaszewski*
Page No. – Page No.
Phenyl benzo[b]phenothiazine as a
visible light photoredox catalyst for
metal-free atom transfer radical
polymerization
This article is protected by copyright. All rights reserved.