Novel W(II) complexes for reversible addition-fragmentation chain transfer (RAFT) polymerizations

Article abstract of DOI:10.1016/j.inoche.2010.02.013

Alkylation of the phosphorus coordination of the diphenyl(dithioformato)phosphine ligand in [W(CO)₅(PPh₂CS₂)]NEt₄ (1) at the S atom results in the formation of the novel RAFT agent S{box drawings double horizont

Full text of DOI:10.1016/j.inoche.2010.02.013

Inorganic Chemistry Communications 13 (2010) 603–605
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Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Novel W(II) complexes for reversible addition-fragmentation chain transfer
(RAFT) polymerizations
c
b
d
c,
Chuan-Lin Chen a, Yih-Hsing Lo b, , Chia-Yi Lee , Yih-Hsuan Fong , Kuo-Chen Shih , Chiung-Cheng Huang
⁎
⁎
a
Department of Biomedical Imaging and Radiological Sciences, National Yang-Ming university, Taipei, Taiwan 112, Republic of China
Department of Natural Science, Taipei Municipal University of Education, Taipei, Taiwan 100, Republic of China
Department of Chemical Engineering, Tatung University, Taipei, Taiwan 104, Republic of China
Department of Optical-Grade Organic Materials, Nanopowder and Thin Film Technologycenter, ITRI, Tainan, Taiwan, Republic of China
b
c
d
a r t i c l e i n f o
a b s t r a c t
Article history:
Alkylation of the phosphorus coordination of the diphenyl(dithioformato)phosphine ligand in [W(CO)5
(PPh2CS2)]NEt4 (1) at the S atom results in the formation of the novel RAFT agent S C[W(CO)5PPh2]S–R (2a,
R=CH2Ph; 2b, R=CH2CH CH2). These compounds have been shown to be highly effective in reversible
addition-fragmentation chain transfer (RAFT) polymerization to produce polymers (homopolymer and
diblock copolymer) of predetermined molecular weight and narrow polydispersity (b1.3). Electron-
withdrawing organometallic substituents can increase the activity of RAFT agents. To the best of our
knowledge, this is the first report of their use as RAFT agents in polymerization.
Received 21 December 2009
Accepted 20 February 2010
Available online 1 March 2010
Keyword:
W(II)
Diphenyl(dithioformato)phosphine
Reversible addition-fragmentation chain
transfer (RAFT) polymerization
Narrow polydispersity
© 2010 Elsevier B.V. All rights reserved.
Most recently, with increased ripeness of polymer technologies,
polymer applications are concerned not only in synthetic resin
industries and traditional plastics but also in high technology
industries such as electronics [1], electroluminescent materials [2],
optoelectronics [3] and biotechnologies [4]. Some relative polymer
materials with particular properties are critical for the related
industries. In 1998, CSIRO [5] disclosed an active free radical
polymerization method called reversible addition-fragmentation
chain transfer process (RAFT process) to prepare polymer products
with narrow molecular weight distribution and further control the
polymer chain length. In general, RAFT process controls most free
radical polymerizations for alkene monomers. The proposed mecha-
nism for styrene polymerization in the presence of the RAFT agent
(S C(Z)S–R) is shown in Scheme 1.
S–R (2a, R=CH2Ph; 2b, R=CH2CH CH2) could be used to confer
living character to radical polymerization.
The complex [W(CO)5(PPh2CS2)]NEt4 (1) [6] and PhCH2Br reacted
in CH2Cl2 at room temperature for 24 h, red powder was generated
and confirmed with elementary analysis and FAB mass spectrometry
as the neutral complex S C[W(CO)5PPh2]S–CH2Ph (2a) (Scheme 2).
Extraction with n-hexane followed by removal of the solvent gave the
analytically pure product 2a. This red powder was isolated in 21%
yield. Complex 2a is soluble in polar solvents such as CH2Cl2, CHCl3,
and acetone, moderately soluble in n-hexane, and stable in solution
and in air. The FAB mass spectrum of 2a shows a base peak in
Furthermore, demands and significance of polymer materials
having terminal organometallic functional groups are steadily on
the increase due to application thereof to dispersants, hybrid
materials, optoelectrical materials and nanotechnologies. However,
polymer materials having terminal organometallic functional groups
are unable to be prepared through conventional RAFT reagents.
Therefore, it is necessary to develop a novel RAFT reagent for
preparing polymer materials having terminal organometallic func-
tional groups with controllable polymerization degree. In this paper,
we demonstrated that organometallic RAFT agents S [W(CO)5PPh2]
⁎
Corresponding authors.
E-mail address: yhlo@mail.tmue.edu.tw (Y.-H. Lo).
Scheme 1. The mechanism of RAFT.
1387-7003/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2010.02.013

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C.-L. Chen et al. / Inorganic Chemistry Communications 13 (2010) 603–605
Scheme 2. Synthesis of organometallic RAFT agents.
t
agreement with the [M+] ion fragment. In the IR spectrum of 2a, three
terminal carbonyl stretchings appear at 2076 (m), 1985 (s), 1921 (vs),
a typical pattern for an LM(CO)5 unit in distorted octahedral geometry
[6]. In the 31P{1H} NMR spectrum of 2a, a resonance at δ 59.6 with a
pair of tungsten satellites (1JW–P =237.0 Hz) indicates the P-coordi-
nation of the S C(PPh2)S–CH2Ph ligand. The 1H NMR spectrum of 2a
exhibits one resonance at δ 4.54 attributed to the CH2Ph and the
corresponding resonances in the 13C NMR spectrum appear at δ 42.3.
On the basis of these spectroscopic data, it is probable that the
alkylation takes place at one of the sulfur atom. Similarly, the reaction
of 1 with ICH2(CH=CH2) in CH2Cl2 affords S C[W(CO)5PPh2]S–CH2
(CH CH2) (2b) in 76% yield. The 31P{1H} NMR resonance of 2b (δ 61.5
with 1JW–P =252.0 Hz) is close to that of 2a [7].
Styrene and BA was polymerized with the complexes 2 as RAFT
agent using thermal initiation, respectively [8]. The purity of the RAFT
agent was established by spectroscopic methods as well as elemental
analysis prior to use. RAFT polymerization was performed at 60 °C.
The molar ratio between the RAFT agent and monomer was kept
constant at 400, the AIBN and RAFT agent was kept constant at 2 in all
the polymerization processes. The molar mass characteristics of the
polymers were determined by gel permission chromatography (GPC).
In addition, in order to check their effectiveness as RAFT agents, we
also obtained the polymerization data using the organic RAFT agent
benzyl dithiobenzoate S C(Ph)S–Ph (3) [9].
The results of styrene and tBA polymerizations for 2 and 3 are
summarized in Tables 1 and 2, respectively. The evolution of
molecular weight and polydispersity vs conversion for three of the
more active RAFT agents are also summarized in Figs. 1–4 The RAFT
agent 2 and 3 are demonstrated to be an effective control on free
radical polymerization and the reaction behaves like living free radical
polymerization with a number-average molecular weight propor-
tional to conversion. We found that by varying the electronic
properties of the Z or R group (Scheme 1), the efficiency of
polymerization is affected. The increasing trends with complexes 2
were similar to those of organic RAFT agent 3. However, the
polymerization performed in the presence of the organic RAFT agent
3 was slower than that performed with 2a. In tBA and styrene
polymerization, the organic RAFT agent 3 afforded a conversion yield
12% and 16.7%, compared to over 80% and 47% for the RAFT 2a,
respectively. The relative effectiveness of the RAFT agents is
Table 1
Mn, PDI, and conversion data for the polymerization of tBA at 60 °C.
RAFT agent
Time
(h)
Conversion Mn
(%)
PDI
(g mol−1
)
1
3
6
44.1
75.4
80.5
43,894
64,416
76,705
1.19
1.21
1.26
1
3
6
19.3
46.7
77.7
10,350
23,263
40,297
1.19
1.19
1.18
1
3
6
1.76
6.2
12
1041
3434
6345
1.12
1.15
1.18
No RAFT agent
1
82.6
208,610
1.32
Table 2
Mn, PDI, and conversion data for the polymerization of styrene at 60 °C.
Fig. 1. Trend of the conversion of thermally initiated RAFT PtBA as a function of time.
RAFT agent
Time
(h)
Conversion
(%)
Mn
PDI
(g mol−1
)
8
15
25
13
32.7
47
6642
13,406
18,685
1.16
1.19
1.17
8
15
25
6
14
16.7
3674
6440
7588
1.21
1.21
1.23
Fig. 2. Trend of the number-average molar mass of thermally initiated RAFT PtBA as a
function of conversion rate.
No RAFT agent
8
16.7
102,381
1.65

C.-L. Chen et al. / Inorganic Chemistry Communications 13 (2010) 603–605
605
Acknowledgment
We gratefully acknowledge the financial support from the National
Science Council of Taiwan (NSC 97-2113-M-036-001-MY2).
References
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[7] Dichloromethane (40 mL) was added to a round-bottomed flask charged with
[Et4N] [W(CO)5(PPh2CS2)] (3.0 g, 4.2 mmol) and C6H5CH2Br (0.5 mL, 4.2 mmol)
and the mixture was stirred at room temperature for 24 h. The solvent was
removed and the residue was extracted with hexane (2×10 mL), and the extracts
were filtered through Celite. The filtrate was concentrated to ca. 5 mL and cooled
to −18 °C for 12 h to give the analytically pure product S C[W(CO)5PPh2]S–CH2Ph
(2a, 21% yield). Spectroscopic data for 2a: IR (KBr, νco): 2076 (m), 1985 (s), 1921
Fig. 4. Trend of the number-average molar mass of thermally initiated RAFT
polystyrenes as a function of conversion rate.
rationalized in terms of interaction of the electron-withdrawing
organometallic substituents (Z W(CO)5(PPh2)) with the C S double
bond to activate that group toward free radical addition (Scheme 1).
On the other hand, the tBA polymerization rate using the allyl
derivative (R=CH2CH CH2) 2b was lower than 2a with the benzyl
derivative (R=CH2Ph), which may be attributed to the more stable
benzyl radical group. The partitioning of R radical group between
adding to polymeric RAFT agent and monomer (to reinitiate) can also
have a significant effect on the rate of consumption of RAFT agent
(Scheme 1).
To verify the “livingness” of the process, we investigated the ability
to chain-extend the PS (Table 2, Mn=18,685, PDI=Mw/Mn=1.17)
to yield diblock copolymers consisting of PS and PtBA. The polymer PS
was used as a RAFT agent for the growth of the PtBA second block (PS-
block-PtBA, Mn=36,138, PDI=1.21). Intriguingly, treatment of PtBA
(Table 2, Mn=76,705, PDI=1.26) in CH3CN at room temperature in a
5-mm NMR tube causes cleavage of the W–P bond and affords W
(CO)5(CH3CN) and organic polymer PtBA. The terminal organometal-
lic functional group may be removed by subjecting the obtained
polymer to elimination to provide the corresponding organic polymer.
We synthesized W(II) complexes at high yields from [W(CO)5
(PPh2CS2)]NEt4 (1). They were very stable towards air and moisture.
To the best of our knowledge, this is the first report of their use as
RAFT agents in polymerization. In addition, the terminal organome-
tallic functional group may be removed by subjecting the obtained
polymer to elimination to provide the corresponding organic polymer.
(vs) cm−1
.
1H NMR (CDCl3): δ 7.63–7.47 (m, Ph), 4.54 (s, 2H, CH2). 13C NMR
2
(CDCl3): δ 237.4 (d, trans-CO, Jpc =7.5 Hz), 199.1 (d, CS2, Jpc =25.1).
[8] Styrene polymerizations: Styrene (28.0 mmol) were degassed via four freeze
pump–thaw cycles, transferred along with 5.5 mg (0.028 mmol) AIBN and 2a
(0.048 mmol) into a nitrogen-filled glovebox where stock solutions of monomer,
AIBN, and RAFT agent were prepared. A few drops of toluene (ca. 1.5 mL) were
added in the case of polymerization to guarantee the dissolving of the RAFT agent.
The stock solution were filled into individual glass vials and sealed with Teflon/
rubber septa. The vials were subsequently inserted into a block heater ther-
mostated at 60 °C for 25 h. The reactions were stopped by cooling the solutions in
an ice bath and quenched in CH3OH, then diluted with CH2Cl2. The PS (Mn=18685,
PDI=1.17) was purified by precipitation from CH2Cl2 solution into CH3OH. The
PtBA (Mn=76705, PDI=1.26) was prepared by using a similar procedure as that
of PS. Synthesis of Diblock Copolymer (PS-block-PtBA): PS (0.016 mmol), 2.6 mg
(0.017 mmol) AIBN and tBA (0.064 mmol) were added in a Schlenk flask. The
reaction mixture was degassed by four freeze and thaw cycles and sealed under
nitrogen. Bulk thermally initiated polymerization was conducted at 60 °C for 9 h.
The reactions were stopped by cooling the solutions in an ice bath and quenched in
CH3OH, then diluted with CH2Cl2. The diblock copolymer (Mn=36138, PDI=1.21)
was purified by precipitation from CH2Cl2 solution into CH3OH. Synthesis of
organic polymers from organometallic RAFT agent: the polymer PtBA (10 mg) in
CH3CN prepared under N2 in NMR tube was heated to reflux for 24 h. The solvent
was removed under vacuum and washed with 1 mL of CH3OH. After filtration, the
white precipitate was washed with 1 mL of n-hexane and dried under vacuum to
give the organic polymer. The filtrate was dried and extracted with 2 mL of CH2Cl2.
The extract was filtered, and the filtrate was dried under vacuum to give the
product W(CO)5(CH3CN).
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(2003) 2256.