New selenium-based iniferter agent for living free radical polymerization of styrene under UV irradiation

Article abstract of DOI:10.1002/pola.25994

P,P-Diphenyl phosphinodiselenoic acid benzyl ester was synthesized and used as a mediator for the polymerization of styrene under UV-vis irradiation. Moderately controlled evidence was found: linear polymerization kinetics, linear evolution of molecular w

Full text of DOI:10.1002/pola.25994

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New Selenium-Based Iniferter Agent for Living Free Radical
Polymerization of Styrene Under UV Irradiation
Jindong Zeng, Jian Zhu, Zhengbiao Zhang, Xiangqiang Pan, Wei Zhang,
Zhenping Cheng, Xiulin Zhu
Jiangsu Key Laboratory of Advanced Functional Polymer Design and Application, Department of Polymer Science and
Engineering, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123,
People’s Republic of China
Correspondence to: J. Zhu (E-mail: chemzhujian@suda.edu.cn) or X. Zhu (E-mail: xlzhu@suda.edu.cn)
Received 14 September 2011; accepted 30 January 2012; published online 23 February 2012
DOI: 10.1002/pola.25994
ABSTRACT: P,P-Diphenyl phosphinodiselenoic acid benzyl ester
was synthesized and used as a mediator for the polymerization
of styrene under UV–vis irradiation. Moderately controlled evi-
dence was found: linear polymerization kinetics, linear evolu-
tion of molecular weight with monomer conversion, and
relatively narrow molecular weight distribution (1.5–2.0). The
structure of the obtained polymers was characterized using
NMR and oxidative eliminaton. Based on polymerization
results, an iniferter mechanism was proposed for the current
C
V
polymerization system.
2012 Wiley Periodicals, Inc. J Polym
Sci Part A: Polym Chem 50: 2211–2218, 2012
KEYWORDS: iniferter; living polymerization; photopolymeriza-
tion; radical polymerization; selenium; UV–vis irradiation
INTRODUCTION In the past decades, the growing demand
for functionalized, structurally well-defined, polymeric mate-
rials has been a driving force for the development of con-
trolled/‘‘living’’ radical polymerization (CRP) techniques. Sev-
eral CRP methods have been well developed, such as atom
transfer radical polymerization (ATRP),1,2 stable free radical
polymerization,3,4 and reversible addition–fragmentation
chain transfer (RAFT) polymerization, and so on.5,6 The key
step for realizing CRP process is the establishment of a fast
equilibrium between dormant species and active free radi-
cals, which ensures the low concentration of propagating
free radical to reduce the termination reaction while keeping
considerable polymerization rate.7,8 Many compounds, such
as dithiocarbonyl compounds,5 transition metals,9 and stable
free radicals,3 have been developed as the mediator to estab-
lish such equilibrium. Among them, dithiocarbonyl com-
pounds have proven as the most versatile mediator. It has
been firstly used as an iniferter agent by Otsu and Yoshida
in 1982,10 followed as RAFT agent by Rizzardo and co-
workers in 1998 (Scheme 1).5 The key step for iniferter and
RAFT polymerization is shown in Scheme 1. In iniferter pro-
cess, the dithiocarbonyl compounds acted as initiator, chain
transfer agent, and terminal group at the same time.10 For
the RAFT polymerization, dithiocarbonyl compounds acted
as a chain transfer agent, and through such reversible trans-
fer and fragmentation, the polymerization process proceeded
in a controlled way.5 Further investigation proved that
dithiocarbonyl compounds also can be used as an ATRP me-
diator.11 Based on these earlier reports, selenium- and tellu-
rium-containing organic compounds, which showed similar
outer side electronic structure as sulfur, have been also
investigated as the mediator for CRP.12–17
According to the mechanism of CRP, the moiety of mediator
can be introduced into polymer chain end.8,18,19 Thus, sele-
nium or tellurium elements can be attached to the polymer
chain ends by using selenium- or tellurium-containing medi-
ators. It was already known that organoselenium compounds
play important roles in organic synthesis and biological sys-
tem.20–22 Selenium-containing compounds have been widely
used in the pharmacochemistry as antioxidants for the well-
known glutathioneperoxidase activity.23–25 Over the last sev-
eral years, various unusual and attractive compounds and
polymers containing selenium moieties have been prepared.
Han and coworkers developed a near-IR reversible fluores-
cent probe containing an organoselenium functional group.26
Zhang and coworkers synthesized selenide-containing block
copolymers and investigated their dual redox responsive dis-
assembly.27,28 Lee and cooperators synthesized phosphinodi-
selenoic esters and used it as the mediator for styrene poly-
merization. They suggested a RAFT mechanism for the
phosphinodiselenoic ester-mediated polymerization.29,30
In this work, based on potential applications of polymers
containing selenium moieties, we combined benzylseleno
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SCHEME 1 Respective key step in iniferter and RAFT polymerization using dithiocarbonyl compound as the mediator.
group and P,P-diphenyl phosphinoseleno group together to
compose P,P-diphenyl phosphinodiselenoic acid benzyl ester
(DPPSB). The polymerization behavior of styrene using
DPPSB as the mediator under different conditions was inves-
tigated. The mechanism of the polymerization was proposed
based on the polymerization behavior and polymer
structures.
of 10 min at 0 ꢀC with vigorous stirring. This reaction mix-
ture was heated to room temperature and stirred for 1 h. Af-
ter removal of the solvent, the mixture was purified by col-
umn chromatography on silica gel to afford crude product.
The crude product was then purified by recrystallization
from n-hexane, and a light yellow solid was obtained.
1H NMR (CDCl3, d, ppm): d 7.92–7.82 (m, 4H), 7.48–7.36 (m,
6H), 7.25 (m, 2H), 7.16 (m, 3H), 4.24 (d, J ¼11.4 Hz, 2H);
13C NMR (CDCl3, d, ppm) d 133.24 (d, J ¼ 133.2 Hz), 131.82
(d, J ¼ 11.5 Hz), 131.67 (d, J ¼ 3.2 Hz), 129.29, 128.48 (d, J
¼ 5.7 Hz), 128.34, 128.05, 127.15, 35.97 (d, J ¼ 2.9 Hz); 31P
NMR (CDCl3, d, ppm) d 42.45; 77Se NMR (CDCl3, d, ppm)
370.68, ꢁ165.61; MS (EI) m/z: 435.9393.
EXPERIMENTAL
Materials
Chlorodiphenylphosphine (ClPPh2, 98%) was purchased
from Subaosheng Chemical Reagents, China. Se powder
(99.95%) was purchased from Shanghai Meixing Chemical
Reagents, China. All other chemicals (analytical grade) used
in this study were purchased from Shanghai Chemical
Reagents. Styrene was washed with an aqueous solution of
sodium hydroxide (5 wt %) three times and then with deion-
ized water until neutralization. After being dried with anhy-
drous magnesium sulfate, styrene was distilledꢀ under
reduced pressure and kept in a refrigerator under 0 C. 2,20-
Azoisobutyronitrile (AIBN, 98%) was recrystallized from
ethanol and kept in a refrigerator under 4 ꢀC. Diethyl ether
(Et2O) and tetrahydrofuran (THF) were distilled from so-
dium/benzophenone ketyl prior to use. Other chemicals
were used as received.
Typical Polymerization Procedure
A dry 1 mL ampule was filled with styrene (0.6 mL, 5.24
mmol) and DPPSB (11.4 mg, 0.026 mmol). The solution was
deoxygenated with three standard freeze–pump–thaw cycles.
The ampoule was then flame sealed and placed under UV–
vis radiation of a 300 W high-presꢀsure mercury vapor lamp
with distance of 50 cm at 30 6 1 C. At timed intervals, the
ampule was immersed in ice water and then opened. The
contents were dissolved in 1 mL of THF and then precipi-
tated into 200 mL of methanol. The polymer was obtained
by filtration and dried at room temperature under vacuum
to constant weight. Conversion of styrene was determined
gravimetrically.
Synthesis of DPPSB
The DPPSB was synthesized according to the modified litera-
ture procedure (Scheme 2).29–31 To a suspension of elemen-
tal selenium (0.8 mg, 10.0 mmol) in Et2O (30 mL), 5.0 mL of
benzylmagnꢀesium chloride (2.0 M in Et2O, 10 mmol) was
added at 0 C, and the mixture was stirred for 30 min. A 10
mL THF solution of diphenylphosphinoselenoic chloride (3.0
g, 10 mmol), obtained from the reaction of ClPPh2 with sele-
nium in toluene at 120 ꢀC for 3 h in almost quantitative
yield, was then added dropwise to this mixture over a period
Typical Chain Extension Experiment
The procedure of chain extension experiment was similar to
those described above except that the DPPSB was replaced
by the obtained polystyrene.
Oxidative Elimination of Terminal Group
A solution of polystyrene (PS) (0.05 g, Mn,GPC ¼ 11,400 g/
mol, polydispersity index (PDI) ¼ 2.12) and 0.25 g hydrogen
peroxide (30 wt %) in 2.0 mL of THF was stirred for a week
at room temperature. The solution was poured into methanol
to precipitate polymer. The polymer was collected by filtra-
tion and was dried in vacuum at room temperature.
SCHEME 2 Synthetic route of DPPSB.
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reported by Lee and coworkers31 who used phenyl-type
groups in their compounds. Normally, the benzyl group was
superior to phenyl group as initiating free radical.
TABLE 1 Polymerization of Styrene with DPPSB Under Heating
Entry Polymerization Condition Con. (%) Mn,GPC Mn,th PDIa
a
b
1c
2c
3c
4d
5e
T ¼ 80 ꢀC, t ¼ 3.5 h
T ¼ 80 ꢀC, t ¼ 7.0 h
T ¼ 60 ꢀC, t ¼ 22.0 h
T ¼ 60 ꢀC, t ¼ 5.3 h
T ¼ 120 ꢀC, t ¼ 8.0 h
35.2
45.8
27.5
39.8
5.4
36,400 12,200 2.23
42,200 15,900 2.72
46,300 9,500 2.51
9,300 1,400 1.77
6,100 1,100 1.64
The polymerization results are summarized in Table 1. The
polymers with uncontrolled molecular weights and board
molecular weight distributions were obtained in all of these
attempts. These results were similar to the literature,29 how-
ever, differed far away from typical RAFT polymerization. In
the case of thermal-initiated polymerization, the GPC of
obtained polymer even showed double peaks with one peak
at 6100 g/mol and another peak at 13,230 g/mol. These
results indicated that under heating conditions, DPPSB was
not an effective mediator for the controlled polymerization
of styrene.
13,230
1.84
a
Determined by GPC.
b
c
Mn,th ¼ ([monomer]0/[DPPSB]0) ꢂ Mmonomer ꢂ Conversion þ M
[St]0:[DPPSB]0:[AIBN]0 ¼ 1,000:3:1.
.
DPPSB
d
e
[St]0:[DPPSB]0:[AIBN]0 ¼ 100:3:1.
[St]0:[DPPSB]0 ¼ 200:1, St ¼ 0.5 mL, dimethylformamide (DMF) ¼ 0.25 mL.
Characterization
Polymerization of Styrene With DPPSB Under UV–vis
UV–vis was another initiating source for free radical poly-
merization. One of the first living/controlled free radical po-
lymerization examples was conducted under the irradiation
of UV–vis, reported by Otsu et al. in 1980s.33,34 Since then,
many dithiocarbamate compounds have been used to medi-
ate CRPs under UV–vis irradiation. In this work, we investi-
gated the polymerization behavior of styrene with the pres-
ence of DPPSB under UV–vis irradiation.
The number-average molecular weight (Mn) and molecular
weight distribution (Mw/Mn) of the resultant polymers were
determined using a Waters 1515 gel permeation chromato-
graph (GPC) equipped with
a refractive-index detector
(Waters 2414), using HR 1, HR 2, and HR 4 (7.8 mm ꢂ 300
mm, 5 lm beads size) columns with molecular weights rang-
ing from 102 to 5 ꢂ 105 g/mol. THF was used as the eluent
at a flow rate of 1.0 mL/min and 30 ꢀC. GPC samples were
injected using a Waters 717 plus autosampler and calibrated
with PS standards purchased from Waters. Matrix-assisted
laser desorption/ionization time of flight (MALDI-TOF) mass
spectrometry measurement was performed using a Bruker
Autoflex III (MALDI-TOF) mass spectrometer equipped with
a 337 nm nitrogen laser. Both matrice 4-hydroxy-a-cyanocin-
namic acid and sample were dissolved in 1:1 (v/v) acetoni-
trile: water with 1% trifuoroacetic acid. The mixture solution
(0.5 lL) was placed on a metal sample plate. The sample
was air-dried at ambient temperature. The 1H NMR, 31P
NMR, and 13C NMR spectra were recorded on an INOVA 400
MHz nuclear magnetic resonance instrument using CDCl3 as
the solvent.
The polymerization was performed using 300 W high-pres-
sure mercury lamp as the light source with distance of 50
cm. The light power at 365 nm was 282 lW/cm2.
Figure 1 shows the polymerization kinetics in different con-
ditions. As shown in Figure 1, styrene could be polymerized
under the 365 nm UV irradiation with the apparent rate con-
stant kpapp ¼ 1.68 ꢂ 10ꢁ3 hꢁ1 of the polymerization. After
introducing DPPSB, the polymerization rate was enhanced
significantly. The kappp increased to 6.83 ꢂ 10ꢁ3 hꢁ1 in the
RESULTS AND DISCUSSION
Polymerization with DPPSB Under Heating
Gigmers et al. reported that dithiophosphinate esters can be
used as RAFT agent to conduct styrene polymerization in liv-
ing free radical polymerization manner. Following such
result,32 Lee and coworkers synthesized DPPSB and demon-
strated the polymerization behavior of styrene in thermal
initiating.29,30 They proposed a RAFT polymerization mecha-
nism for such polymerization.29 The polymerization showed
linear kinetics. However, the polymers with broad molecular
weight distribution were obtained in their results. Actually, it
seems to be dangerous to draw a conclusion of living behav-
ior due to the lack of the key data, such as the relationship
between measured molecular weight and theoretical value,
and characterization of chain ends.29 In this work, we first
investigated the DPPSB polymerizations of styrene both in
cases of thermal initiation and AIBN initiation. It should be
noted that the structure of DPPSB with benzyl group
attached with selenium (Scheme 2), which differs with that
FIGURE 1 ln([M]0/[M]) versus time for the polymerization of
styrene in the presence of DPPSB in bulk under UV–vis irradia-
tion at 30 ꢀC with the molar ratios of [St]0:[DPPSB]0 ¼ 800:0,
800:1, 400:1, and 200:1, respectively.
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TABLE 2 Polymerization of Styrene Under UV–Vis Irradiation
With the Molar Ratio of [St]0/[DPPSB]0 5 200/1 at 30 8C in
Various Solvents
Item
Solvent
t (h)
Con. (%)
Mn,GPC
Mn,th
PDI
0
N.A.
THF
61
61
61
61
61
61
61
85
85
85
42.3
8.1
11,500
2,900
4,900
4,900
5,200
6,400
6,800
6,200
8,700
9,800
8,800
1,700
4,500
3,700
4,400
5,000
5,200
5,000
7,100
8,300
1.91
1.44
1.68
1.58
1.68
1.67
1.65
1.75
2.05
2.18
1a
2a
3a
4a
5a
6a
7a
8b
9c
a
CH2Cl2
CH3Cl
21.7
17.7
21.3
24.2
25.1
24.2
34.1
40.0
Toluene
DMF
Pyridine
Toluene
Toluene
Toluene
Solvent ¼ 0.25 mL.
Toluene ¼ 0.10 mL.
Toluene ¼ 0.05 mL.
b
c
FIGURE 2 Dependence of Mn and PDI on conversion of styrene
in the presence of DPPSB in bulk under UV–vis irradiation at
30 ꢀC with the molar ratios of [St]0:[DPPSB]0 ¼ 800:1, 400:1,
and 200:1, respectively.
polymerization behavior was found, for example, moderately
controlled molecular weight with broad molecular weight
distribution as shown in Table 2. These results indicated
that the current polymerization system can be applied to
control the molecular weight of polystyrene in various condi-
tions, both in bulk and in solutions. The polymerization rate
in solution was lower than that in bulk, and the polymeriza-
tion rate was significantly reduced in THF, which may due to
the high chain transfer effect occurred in THF.35 In other sol-
vents, the polymerization showed comparable polymerization
rate.
case of [styrene]0:[DPPSB]0 ¼ 800:1, which implied that
DPPSB was involved in the initiating process of the polymer-
ization. This hypothesis was confirmed by the increasing of
polymerization rate to 7.49 ꢂ 10ꢁ3 hꢁ1 when the ratio of
[styrene]0:[DPPSB]0 was changed to 400:1. Similar polymer-
ization rate was observed by further increasing the concen-
tration of DPPSB to [styrene]0:[DPPSB]0 ¼ 200:1. The kinetic
results differed with the characters of RAFT polymerization.
In RAFT polymerization, the increase of RAFT agent concen-
tration could not result in an elevated polymerization rate.
Investigation of the End Groups to Clarify the
Polymerization Mechanism
The relationships of molecular weight and PDI of the poly-
mer with conversion are shown in Figure 2. The molecular
weight of the polymer increased linearly with the conver-
sion in all cases and the value determined by GPC agreed
well with the theoretical one, especially in cases of [styre-
ne]0:[DPPSB]0 ¼ 800:1 and 400:1. The molecular weight
determined by GPC differed with theoretical one in case of
[styrene]0:[DPPSB]0 ¼ 200:1, which should be due to the
accumulation of side reactions, such as coupling termina-
tion at high concentration of DPPSB. It was interesting that
the relationships between molecular weight distribution of
these polymers and conversion showed inverse tendency.
The polymers obtained by DPPSB produced moderately con-
trolled molecular weights (high PDI values), and the poly-
merization rate increased with the increase of DPPSB
The PDI values obtained in case of [styrene]0:[DPPSB]0
200:1 were in the range of 1.60–1.80, which were lower
than those obtained in cases of [styrene]0:[DPPSB]0
¼
¼
400:1 and 800:1, for example, in the range of 1.6–3.2. It
was worthy to note that the molecular weight distribution
of the polymer did not change with conversion in case of
[styrene]0:[DPPSB]0 ¼ 200:1, whereas it increased with
conversion in other two cases. The reason is still unclear at
present.
The polymerization was further investigated in various sol-
vents. Different solvents, such as THF, CH2Cl2, CH3Cl, toluene,
DMF, and pyridine, were used in the polymerization. Similar
FIGURE 3 1H NMR spectrum of PS-DPPSB (Mn, GPC ¼ 9900, PDI
¼ 1.85).
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FIGURE 5 UV–vis spectra in CH2Cl2 solution with 10ꢁ4 M con-
centration at room temperature. (1) DPPSB. (2) PS obtained
from DPPSB-mediated polymerization under UV–vis irradiation
(Mn,GPC ¼ 19,000, PDI ¼ 2.62). (3) PS obtained from dibenzyl-
diselenide-mediated polymerization with the molar ratio
[St]0:[Dibenzyl diselenide]0:[AIBN]0 ¼ 400:1:0.5 under heating at
60 ꢀC (Mn,GPC ¼ 19,100, PDI ¼ 2.44 Con. ¼ 48.5%). (4) p-Styryl
phenyl selenide.
FIGURE 4 31P NMR spectrum of polystyrene obtained from
DPPSB-mediated polymerization. (Mn,GPC ¼ 17,000 g/mol, PDI
¼ 2.62) obtained in toluene under UV–vis irradiation at 30 ꢀC
with the molar ratio of [DPPSB]0:[St]0 ¼ 1:400 with CDCl3 as
the solvent.
concentration. This result differed far from those from RAFT
polymerization, as in RAFT polymerization, the polymeriza-
tion rate usually decreased with the increase of RAFT agent.
It would be valuable to investigate the chain end groups of
the polymer for clarifying the polymerization mechanism.
or the polymer connected with selenium atom. The final
structure would be decided by the bonding energy difference
between CASe and PASe. The bonding energy of CASe is
362 kJ/mol, whereas the bonding energy of PASe is 324 kJ/
mol,36 which is more close to the energy of 365 nm light
(328 kJ/mol). Thus, under the irradiation of 365 nm of UV
light, DPPSB prefers to decompose by the cleavage of PASe
bond, which evidently resulted in the phosphorus atom
attached directly with polymer chain (structure shown in
Fig. 3).36
First of all, PS-DPPSB was characterized with the 1H NMR
spectrum, which was shown in Figure 3. There are two sig-
nals at d ¼ 3.90–4.15 ppm (integral ratio ¼ 1/2), which are
ascribed to the protons of methyne (d) and methylene (f)
neighboring at the Se atom. The signal at about d ¼ 3.25–
3.53 ppm is ascribed to the protons of the methylene (c)
neighboring at the P atom. No signal at d ¼4.24 ppm (as
DPPSB) or lower field can be found in the spectrum.
The UV–vis absorptions of (1) DPPSB, (2) PS-DPPSB, (3)
BzSe-PS-SeBz, and (4) p-styryl phenyl selenide were com-
pared in Figure 5. The maximum adsorption peak of DPPSB
was appeared at wavelength lower than 230 nm. However,
in the case of PS-DPPSB, it showed maximum adsorption
peak at 262 nm, which was very similar to the adsorption
peak of BzSe-PS-SeBz and p-styryl phenyl selenide. The
results implied that the Ph2P(¼¼Se)SeA moiety of DPPSB
may decompose during the polymerization resulting in the
formation of ACASeACA moiety in the polymer. Such result
agreed with the structure deduced from NMR analysis.
Meanwhile, we determined the 31P NMR of the polymer and
DPPSB. The spectra are shown in Figure 4.
The DPPSB shows 31P resonance signal at d ¼ 42.45 ppm.
This 31P resonance signal shifts to 32.45–34.10 ppm in the
polymer, which implies that (1) the phosphorus structure
remained intact in the polymer; (2) the chemical environ-
ment around phosphorus atom was different in DPPSB and
the polymer. There are two possibilities for phosphorus
structure being attached to the polymer, for example, the
polymer connected with phosphorus atom (showed in Fig. 4)
SCHEME 3 Oxidative elimination of DPPSB moiety from polystyrene by H2O2.
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FIGURE 6 31P NMR spectrum of DPPSB-PS (Mn,GPC ¼ 17,000 g/
mol, PDI ¼ 2.62) obtained in toluene under UV–vis irradiation
at 30 ꢀC with the molar ratio of [St]0:[DPPSB]0 ¼ 400:1 after oxi-
dative elimination with CDCl3 as the solvent.
FIGURE 7 1H NMR spectrum of DPPSB-PS (Mn,GPC ¼ 5200 g/
mol, PDI ¼ 1.72) obtained in toluene under UV–vis irradiation
at 30 ꢀC with the molar ratio of [DPPSB]0:[St]0 ¼ 1:200 after oxi-
dative elimination with CDCl3 as the solvent and tetramethylsi-
lane as the internal standard.
The CASe structure could be cleaved under oxidation condi-
tion,12 which was used to confirm the hypothetical structure
(Scheme 3). The polymer reacted with H2O2 in room temper-
ature for 1 week to eliminate CASe structure. The 31P reso-
nance signal in oxidized polymer shifted to 30.67–31.72 ppm
(Fig. 6) from the original 32.45–34.10 ppm, which was the
result of oxidation of the remained selenium (structure
reacting with monomer to form the radical (3),38,39. Species
3 reacted with monomer forming the propagating radical 4.
The polymerization was controlled by reversible termination
of active propagating radicals 4 with benzyllseleno radical
(2) to form dormant species 5. Control experiment showed
that benzyllseleno radical (2) cannot initiate styrene to poly-
merize (Table 3). It shows that dibenzyl diselenide cannot
initiate the polymerization within 120 h under the irradia-
tion of 365 nm light. Therefore, the current polymerization
was supposed to be via an iniferter process. The results
from the polymerization profiles with the variation of DPPSB
also supported the iniferter process (Fig. 1), for example, lin-
ear relationship between molecular weight and conversion,
1
shown in Fig. 6). The H NMR spectrum of oxidized polymer
(Fig. 7) further confirmed the cleavage of CASe bond to
form carbon–carbon double bond in the chain end. In Figure
5, the signals at about d ¼ 7.32–7.70 ppm (a) were ascribed
to the protons of the phenyl from oxydic P,P-diphenyl phos-
phinoseleno group.28 The signals at d ¼ 3.06 ppm (b) corre-
sponded to the protons of methylene neighboring at P,P-di-
phenyl phosphinoseleno group. The signals at d ¼ 3.74 ppm
(c) were ascribed to the proton of methylene neighboring at
the C–C double bond. The signals at d ¼ 6.00–6.20 ppm (d,
f) corresponded to the vinylene protons.12
The molecular weight and molecular weight distribution of
the polymers with their GPC traces before and after oxida-
tion are shown in Figure 8. The molecular weight of oxida-
tive polymer was 11,000 g/mol, which was very close to the
molecular weight of the polymer before oxidative, for exam-
ple, 11,400 g/mol. This result indicated that the CASe bond
was connected on the terminal rather than in the middle of
the polymer chain. The unimodal GPC trace of the polymer
before and after oxidation further confirmed that the CASe
bond was attached on the chain end of the polymer.
Based on aforementioned investigations, the mechanism of
the polymerization is assumed as shown in Scheme 4. DPPSB
decomposed under UV–vis irradiation into two parts by the
cleavage of PASe bond to form P,P-diphenyl phosphinoseleno
radical (1) and benzyllseleno radical (2).37 P,P-Diphenyl
phosphinoseleno radical (1) initiated the polymerization by
FIGURE 8 (a) GPC curves before oxidative elimination with
H2O2. (b) GPC curves after oxidative elimination with H2O2.
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SCHEME 4 The proposed mechanism of the iniferter polymerization with DPPSB as an iniferter agent.
polymers showed higher molecular weight than theoretical
value and broad molecular weight distribution.10
light were carried out to confirm the mechanism. Figure 9
shows the respective GPC curves of DPPSB-PS before (a) and
after (b) the chain extensions with styrene. There was an
obvious peak shift from the macroiniferter agent to the
chain-extended product. The molecular weight increased
from 5200 g/mol (PDI ¼ 1.68) to 27,100 g/mol (PDI ¼ 3.36,
time ¼ 102 h, and conversion ¼ 33.8%). PDI value of the
chain extension product was larger than that of the original
polymer, which might be caused by a fraction of the dead
polymer chains in the original polymer and the side reac-
tions during the chain extension. It can be concluded from
the above results that most chains of DPPSB-PS were still liv-
ing, and the polymerizations complied with the classical
iniferter polymerization mechanism.
Chain Extension and Polymerization of Methyl
(Meth)acrylate Under UV–vis Irradiation
Based on the iniferter mechanism, the polymers can be used
as microiniferter agent to conduct chain extension reaction.
The chain extension experiments of the polystyrene obtained
by using DPPSB as mediator under irradiation of 365 nm
TABLE 3 Polymerization of Styrene With Dibenzyl Diselenide
Under UV–Vis Irradiation at 30 8C
Entry
t (h)
Con. (%)
1a
2b
a
120
85
0
Similar polymerization procedure can be applicable to other
monomers to investigate the universality of polymerization
mediated by DPPSB. The monomers of methyl methacrylate
(MMA) and methyl acrylate (MA) were polymerized under
same conditions. The polymerization results summarized in
Table 4 indicated poor controllability of DPPSB for these
monomers with largely deviated molecular weights.
24.2
[St]0:[Dibenzyl diselenide]0 ¼ 200:1, St ¼ 0.5 mL, toluene ¼ 0.5 mL.
[St]0:[Diphenyl diselenide]0 ¼ 200:1, St ¼ 0.5 mL, toluene ¼ 0.25 mL.
b
CONCLUSIONS
In this work, DPPSB was synthesized and used as a mediator
for the polymerization of styrene. Through variation of poly-
merization condition, ultimately, the polymerization of sty-
rene with DPPSB as the iniferter under UV–vis irradiation
proceeded via
a
living radical mechanism. The
TABLE 4 Polymerization of MMA and MA Under UV–Vis
Irradiation at 30 8C
Item
Monomer
t (h)
Con. (%)
Mn,GPC
Mn,th
PDI
1a
2
3b
MMA
MA
38
38
58
86
38.9
22.1
50.9
45.4
26,700
23,300
28,200
22,300
19,400
9,500
1.70
2.31
1.73
1.29
MMA
MA
10,200
7,800
4
FIGURE 9 GPC curves before (a) and after (b) chain extension
a
1–2: [Monomer]0:[DPPSB]0 ¼ 500:1, monomer ¼ 0.5 mL, THF ¼ 0.25
with polystyrene as the macroiniferter agent under UV–vis irra-
mL.
b
diation at 30 ꢀC. [St]0:[macroiniferter]0
¼
800:1, 38.8%
3–4: [Monomer]0:[DPPSB]0 ¼ 200:1, monomer ¼ 0.5 mL, toluene ¼
conversion.
0.25 mL.
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14 Patwa, A. N.; Tomer, N. S.; Singh, R. P. J. Mater. Sci. 2004,
39, 1047–1049.
polymerization mediated by DPPSB was mildly controlled.
Based on polymerization results, an iniferter mechanism was
suggested. Despite of the not so perfect control over the po-
lymerization, the polymerization from selenium-containing
mediator provided an alternative method to synthesis sele-
nium-containing polymers, which may have potential applica-
tion in biological and biomedical area.
15 Yamago, S. J. Polym. Sci. Part A: Polym. Chem. 2006, 44,
1–12.
16 Yamago, S.; Ukai, Y.; Matsumoto, A.; Nakamura, Y. J. Am.
Chem. Soc. 2009, 131, 2100–2101.
17 Kitayama, Y.; Moribe, H.; Minami, H.; Okubo, M. Polymer
2011, 52, 2729–2734.
The financial support from the National Nature Science Foun-
dation of China (No. 21074082), the Specialized Research Fund
for the Doctoral Program of Higher Education contract grant
(Nos. 200802850005 and 20103201110005), Science and
Technology Development Planning of Suzhou (No.
SYG201026), the Priority Academic Program Development of
Jiangsu Higher Education Institutions, the Qing Lan Project, and
the Program of Innovative Research Team of Soochow Univer-
sity are gratefully acknowledged.
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