Photo-cleavage of the cobalt-carbon bond: Visible light-induced living radical polymerization mediated by organo-cobalt porphyrins

Article abstract of DOI:10.1039/c3cc41466c

This article describes the photo-induced living radical polymerization of acrylamides mediated by organo-cobalt porphyrins through photo-cleavage of the Co-C bond to give organic radicals, which propagate polymerization, and cobalt(ii), which is a persistent metal-centered radical. Photo stimulus controls the initiation steps and regulates chain growth by activating the Co-C bond.

Full text of DOI:10.1039/c3cc41466c

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Photo-cleavage of the cobalt–carbon bond: visible
light-induced living radical polymerization mediated
by organo-cobalt porphyrins†
Cite this: DOI: 10.1039/c3cc41466c
Received 26th February 2013,
Accepted 18th April 2013
Yaguang Zhao, Mengmeng Yu and Xuefeng Fu*
DOI: 10.1039/c3cc41466c
www.rsc.org/chemcomm
This article describes the photo-induced living radical polymerization
of acrylamides mediated by organo-cobalt porphyrins through photo-
cleavage of the Co–C bond to give organic radicals, which propagate
polymerization, and cobalt(II), which is a persistent metal-centered
radical. Photo stimulus controls the initiation steps and regulates chain
growth by activating the Co–C bond.
Photo-induced living radical polymerization (LRP) is receiving growing
attention because of its ‘‘green’’ character and the advantage of low
polymerization temperature.1 Efficient photo-induced LRP remains a
challenging topic although promising progress in this area has been
made.2–6 Reversible binding of metallo-radicals with the growing
polymer radicals provides one effective approach for obtaining living
radical polymerization. Inorganic–organometallic chemistry could
have major impact on controlled photo polymerization through the
development of photosensitive organo-metallic catalyst materials.
Porphyrins are highly absorbing in visible and UV light, and photo-
lysis (l Z 350 nm) of porphyrin rhodium methyl complexes in
benzene is a common method to generate porphyrin rhodium(II)
metalloradicals.7a,b An organo-cobalt compound with a much weaker
metal–carbon bond undergoes photo-cleavage of the Co–C bond7c,d to
give an organic radical, which could initiate polymerization, and
cobalt(II) which is a persistent metal-centered radical. Thus, organo-
cobalt porphyrin appears to be an ideal candidate to achieve photo-
induced LRP without addition of dyes or additives. Thermal initiated
LRP mediated by organo-cobalt porphyrins showed excellent control
through reversible termination (RT) and degenerative transfer (DT) or
mixed pathways.8 However, there has not been a report of effective
photo-induced LRP mediated by the cobalt porphyrin system or the
structurally closely related complexes including cobalt–salen and
cobaloxime9 since the debut of cobalt porphyrin mediated LRP in
Scheme 1 Visible light-induced LRP of acrylamides and preparation of block
copolymers through reversible switching from thermal to photo-induced LRP
mediated by organo-cobalt porphyrins (R = PMA or PDMA).
1994.8a Recently, Debuigne’s group reported photo-polymerization of
N-vinylpyrrolidone, n-butyl acrylate and vinyl acetate mediated by
Co(acac)2 with UV irradiation.5g,h Herein we report the visible light-
induced LRP of acrylamides through photolysis of the Co–C bond, the
preparation of functional block copolymers, and facile reversible
switching from thermal initiation to photo-induced LRP (Scheme 1).
The photo-polymerization of N,N-dimethylacrylamide (DMA) was
carried out in a J. Young valve NMR tube irradiated by a 500 W Xe
lamp equipped with a 400–800 nm filter. Organo-cobalt initiators
((TMP-OH)Co-PMA I, (TMP-OH)Co-PDMA II, (TMP)Co-PMA III)10
were prepared through thermal initiated polymerizations of methyl
acrylate (MA) and DMA following previously reported methods8b,11
and the remaining 2,20-azobissobutyronitrile (AIBN) was removed by
washing three times with methanol or diethyl ether in the glove box.
When the polymerization was performed by direct irradiation of
DMA in benzene without an organo-cobalt initiator, no polymer was
produced after 12 h (Table 1, entry 1). Heating the mixture of I and
DMA at 29 1C for 12 h gave only 8% conversion in the absence of light
(Table 1, entry 2).12 A highly controlled PDMA with high conversion
and narrow molecular weight distribution was obtained when visible
light was applied (Table 1, entry 3). Increasing the ratio of DMA/
Beijing National Laboratory for Molecular Sciences, State Key Lab of Rare Earth
Materials Chemistry and Applications, College of Chemistry and Molecular
Engineering, Peking University, Beijing, 100871, China. E-mail: fuxf@pku.edu.cn;
Fax: +86 10 6275 1708; Tel: +86 10 6275 6035
organo-cobalt initiator resulted in higher molecular weight (Mn
=
70 000–100 000) (Table 1, entries 4–6). The photo-polymerization could
be accomplished in both non-polar and polar solvents using organo-
cobalt initiator II with an amphiphilic substituent group. Well defined
PDMAs with controlled molecular weight and low PDIs were obtained
in both benzene and methanol (Table 1, entries 7 and 8).
† Electronic supplementary information (ESI) available: Synthesis of organo-
cobalt initiators (I, II and III), thermal and photo-induced polymerization
procedures, GPC profiles and 1H NMR spectra of typical block copolymers. See
DOI: 10.1039/c3cc41466c
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Table
1
Photo-induced LRP of acrylamides with different organo-cobalt
initiatorsa
Entry
I
Mn/PDIb
M (equiv.)
T/h Conv.% Mn/PDIb
1
—
I
I
I
I
—
DMA(—)
12
12
12
12
0
8
—
2c
10 300/1.25 DMA(600)
10 300/1.25 DMA(600)
10 300/1.25 DMA(900)
10 300/1.25 DMA(1200) 12
10 300/1.25 DMA(1800) 12
14 100/1.26 DMA(600)
14 100/1.26 DMA(600)
14 100/1.26 DEA(600)
14 100/1.26 AMO(600)
14 100/1.26 DMA(600)
14 100/1.26 DMA(600)
14 900/1.24
3
86
81
86
86
75
74
94
87
75
78
98
46 900/1.23
69 800/1.27
86 600/1.30
105 000/1.36
42 100/1.23
38 700/1.23
58 600/1.29
60 500/1.20
4d
5e
6f
Fig. 2 (a) Kinetic plots for the photo-induced LRP controlled by I in benzene; (b)
changes in number average molecular weight of PDMA with DMA conversion.
[DMA]0 = 1.0 M, [I]0 = 1.67 ꢀ 10ꢁ3 M.
I
7
II
II
II
II
II
II
12
12
12
6
20
20
12
8g
9
10
11h
12g,h
13
43 900/1.19 steps, but also efficiently regulated the chain growth process by
44 800/1.24
64 400/1.30
activating Co–C bonds in dormant species.
III 17 800/1.24 DMA(600)
The living character of this photo-induced LRP mediated by cobalt
a
b
[M]0 = 1.0 M. Solvent was benzene. Determined using gel permeation
porphyrins was illustrated by plots of ln([M]0/[M]t) versus exposure time
and Mn versus conversion (Fig. 2). Linear first-order kinetic behavior was
observed throughout the polymerization process indicating generation
of a constant concentration of an actively propagating chain radical
(Fig. 2a). Molecular weight of PDMAs increased linearly with conversion
of DMA, and molecular weight distribution remained around 1.2
(Fig. 2b). The GPC traces were monomodal and reasonably symmetrical
throughout the whole process (ESI,† Fig. S4). Increasing monomer
conversion led to a clear shift of GPC traces to the high molecular
weight section and the amount of dead polymers was negligible. These
results undoubtedly demonstrated the living character of this photo-
induced cobalt porphyrin system.
The significant feature of this cobalt porphyrin system is
that thermal and photo initiation could be reversibly switched
which provides potential application in production of functional
block copolymers containing monomers with low ceiling tempera-
tures. The triblock copolymers PMA-b-PDMA-b-PDEA and PMA-b-
PDMA-b-PAMO were obtained by sequential polymerization of
MA, DMA and DEA/AMO through either thermal–photo–photo
or thermal–photo–thermal process (Fig. 3).
The metal–carbon bond in organo-cobalt initiators (I, II and III)
underwent photo-cleavage upon exposure to visible light generating
an organic radical and a porphyrin cobalt(II). The photo-induced LRP
mediated by organo-cobalt porphyrins probably proceeded through
a reversible termination pathway where the photolysis generated
organic radical captured monomers forming a propagating chain
radical. Subsequently, the resulting porphyrin cobalt(II), a persistent
metallo-radical, reversibly combined with the growing chain radical
reforming dormant species (Scheme 2). Organo-cobalt initiators (I, II
and III) were the exclusive source of organic radicals and trace
amounts of (TMP-OH)CoII were detected.13
chromatography calibrated against poly(methyl methacrylate) standards.
c
Polymerization was carried out at 29 1C without visible light irradiation.
[DMA]0 = 1.5 M. [DMA]0 = 2.0 M. [DMA]0 = 3.0 M. Solvent was
d
e
f
g
h
methanol. Polymerization was carried out at 0 1C. Abbreviations: MA,
methyl acrylate; DMA, N,N-dimethylacrylamide; DEA, N,N-diethylacryla-
mide; AMO, N-acryloylmorpholine.
Acrylamides with different substituent groups (N,N-diethylacrylamide
(DEA), N-acryloylmorpholine (AMO)) were polymerized to give corres-
ponding polymers with high conversion, controlled molar mass, and
narrow molecular weight distribution (Table 1, entries 9 and 10).
Moreover, the photo-polymerization performed at 0 1C also led to well
controlled polymers (Table 1, entries 11 and 12). The organo-cobalt
initiator III which was synthesized from (TMP)CoII also exhibited
excellent control of polymerization of DMA in benzene under
visible light irradiation (Table 1, entry 13).
The effect of visible light on photo-induced LRP of DMA was
further examined by employing a periodic light-on–off process
(Fig. 1). The mixture of I and DMA in benzene was exposed to visible
light for an hour to afford approximately 30% conversion. Thereafter,
the light source was periodically turned off and on. The polymeriza-
tion stopped immediately and no monomer conversion was observed
during the light-off period, indicating the negligible concentration of
the active radical presented under dark conditions (Fig. 1). Organo-
cobalt mediated photo-induced LRP was highly responsive to external
photo stimulus. Exposure to visible light ‘‘woke up’’ the polymeriza-
tion and led to chain growth immediately. The periodic light-on–off
was repeated many times and no chain growth was observed during
the light-off periods. Photo stimulus not only controlled the initiation
In summary, we have developed a novel photo-controlled
polymerization that displays high sensitivity to external visible
light irradiation. Various acrylamide based functional diblock
and triblock copolymers can be synthesized. Reversible switch-
ing from thermal to photo-induced LRP controlled by cobalt
porphyrins provides an unprecedented opportunity to obtain
various multifunctional block copolymers. Development of
neat photosensitive organo-metal catalyst materials without
any additives for application in controlled photo polymeriza-
tion of a wide range of monomers is under investigation.
This work was supported by the Natural Science Foundation
of China (21171012).
Fig. 1 The conversion of DMA versus time during periodic light-on–off pro-
cesses. [DMA]0 = 1.0 M, [I]0 = 1.67 ꢀ 10ꢁ3 M.
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as the substituent group. (TMP-OH)Co: (5-(4-(10-hydroxyl-1-decyloxy)
phenyl)-10,15,20-tris-(2,4,6-trimethylphenyl)porphyrin) cobalt; (TMP)Co:
tetramesitylporphyrin cobalt; PMA: poly(methyl acrylate); PDMA:
poly(N,N-dimethylacrylamide). Synthesis of these three organo-cobalt
initiators is presented in ESI†.
´ˇ
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analysis. Typical resonances of (TMP-OH)CoII in C6D6: d 9.36
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c
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