Reusable and environmentally friendly ionic trinuclear iron complex catalyst for atom transfer radical polymerization

Article abstract of DOI:10.1039/b617722k

Ionic iron complex [(Me₃tacn)₂Fe₂Cl ₃]⁺[(Me₃tacn)FeCl₃]^- (1), which is readily soluble in methanol, acted as a powerful catalyst in controlled radical polymerization of styrene and MMA, and showed promising features of removal from the resulting polymers and was reusable after recovery from the crude products. The Royal Society of Chemistry.

Full text of DOI:10.1039/b617722k

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Reusable and environmentally friendly ionic trinuclear iron complex
catalyst for atom transfer radical polymerization{
Shota Niibayashi,a Hitoshi Hayakawa,a Ren-Hua Jina and Hideo Nagashima*b
Received (in Cambridge, UK) 5th December 2006, Accepted 17th January 2007
First published as an Advance Article on the web 15th February 2007
DOI: 10.1039/b617722k
Ionic iron complex [(Me3tacn)2Fe2Cl3]+[(Me3tacn)FeCl3]2 (1),
which is readily soluble in methanol, acted as a powerful
catalyst in controlled radical polymerization of styrene and
MMA, and showed promising features of removal from the
resulting polymers and was reusable after recovery from the
crude products.
Fig. 1 Ionic iron complex 1.
Controlled radical polymerization (CRP) is an important method
complexes with a tacn (1,4,7-trimethyltriazacyclononane) ligand
for constructing polymers with fine structure. Metal-mediated
(1) (Fig. 1); the cationic form is favorable to be dissolved in polar
atom transfer radical polymerization (ATRP) is a representative
solvents, whereas the methyl groups on the tacn ligand induced
example of CRP, in which metal complexes composed of
soluble behavior of 1 in apolar solvents.28 In this paper we report
transition metals with various auxiliary ligands promote the
that 1 is a good catalyst for well-controlled polymerization and
polymerization of vinyl monomers with living nature, leading to
copolymerization of styrene, concomitant with facile separation of
polymers with precisely controlled molecular weight and molecular
the catalyst from the polymer product via precipitation from
weight distribution and copolymers with block, graft, and star
architectures.1,2 Nevertheless ATRP has an undesirable problem
methanol. The recovered catalyst species is reusable without loss of
the catalytic activity.
for its efficiency of the metal catalyst, for example, high quantity of
The ionic complex 1 was synthesized according to a procedure
described in the literature,28 and was characterized by H NMR
metals are required to realize reasonable rates of the polymeriza-
1
tion, which results in serious contamination such as toxic and
and by ESI-MS.5 The complex 1 was used as catalyst in bulk
deep-colored heavy metals in the resulting polymer. This problem
polymerization of styrene initiated by halide compounds such as
has been noted early in the initial stage of the research of metal-
(1-chloroethyl)benzene and chlorodiphenylmethane (Scheme 1).
catalyzed ATRP. Thus, several trials for efficient removal of the
For example, a polymerization of styrene by mixing catalyst 1, the
metal residues from the crude product, including in some cases
recovery and reuse of the catalytically active metal species,3 were
chloride, and styrene with a molar ratio of 1 : initiator : monomer
provided by immobilization4–12 of the catalyst and use of biphasic
systems.13–16 The recent discovery of a highly active copper
catalyst has provided another solution of this problem, which
realizes well-controlled ATRP with low catalyst concentration.17
The simplest, but not fully investigated solution of this problem, is
attended on solubility control of the catalyst species.18–26 Typically
a catalyst with a good solubility in both polar and apolar solvents
is useful not only for polymerization of apolar vinyl monomers,
but also for separation of apolar polymer products by washing
with polar solvents. Recently, Shen et al. have reported the ATRP
of MMA with efficient removal of the copper residue from the
resulting polyMMA and catalyst recycling (three times), in which
the authors offered a solution of the solubility problem by ligand
design.27 While the challenge was successful; however, this system
still has problems of low initiation efficiency and catalyst
deactivation in the recycling experiments. Our method to over-
come the solubility problem is based on employing cationic iron
= 1 : 2 : 1000 was performed at 100 uC under nitrogen atmosphere.
The reaction mixture was heterogeneous (white suspension of the
catalyst) at the initial stage, but gradually turned homogeneous
with progress of the polymerization. This successfully produced the
corresponding polymer. As shown in Fig. 2, plots of the molecular
weight (Mn) vs. conversion (%) give a straight line. In addition, the
molecular weight distribution (Mw/Mn) was broad (.1.5) at the
initial stage of the reaction, but became narrow (around 1.2) with
increase of the conversion. These two features are in accordance
with so-called ‘‘controlled’’ atom transfer radical polymerizations.
Table 1 shows the results of polymerization of styrene performed
with alternating feeding ratios at 120 uC for 20 h. It can be seen
that the molecular weight was decreased upon increasing the ratio
of the initiator to the monomer. The polymerization proceeded
slowly in toluene (volume of toluene : styrene = 1 : 1), though the
conversion reached 100% with prolonged reaction time. In
contrast, the Mw/Mn of the product obtained in toluene was
aDainippon Ink and Chemicals, Incorporated, 631 Sakado, Sakura,
Chiba, Japan
bInstitute for Materials Chemistry and Engineering, Kyushu University,
6-1 Kasugakoen Kasuga, Fukuoka, 816-8580, Japan.
E-mail: nagasima@cm.kyushu-u.ac.jp; Fax: (+81)92-583-7819
{ Electronic supplementary information (ESI) available: Experimental
section. See DOI: 10.1039/b617722k
Scheme 1
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Chem. Commun., 2007, 1855–1857 | 1855

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Fig. 2 Plots of Mn (& and m) and Mw/Mn (% and n) vs. conversion.
Initiator: (1-chloroethyl)benzene (m and n) or chlorodiphenylmethane (&
and %). Temperature 100 uC. 1 : Initiator : St = 0.02 : 0.04 : 20 mmol.
Fig. 3 1H NMR spectra measured in CD3CN. (A) complex 1, (B)
recovered catalyst from the crude polystyrene formed by ATRP.
iron residue remained in the final product, indicating that the
amount of charged iron (2300 ppm, calculated from 0.02 mmol of
1) was almost completely removed.
somewhat narrower than that of the bulk polymerization with the
same feeding ratio of catalyst/initiator/monomer. Initiation
efficiency, which was dependent on the reaction conditions, was
over 90% in only one case (ESI{).
Another unique property of 1 is its robustness due to the good
coordination ability of triazacyclononane. This is evidenced by
spectroscopy of the iron species recovered from the precipitation
procedures described above. The colorless Fe(II) species 1 is
sensitive to air, and easily oxidized to the brown Fe(III) species.
Careful exclusion of air in the recovery process allowed the
The polymerization catalyzed by 1 gave polystyrene binding a
chlorine atom at its chain end which is able to act as a
macroinitiator. That is, the polystyrene is available in post
polymerization by adding the second monomer to a reaction
mixture. Therefore, we first polymerized styrene by the bulk
polymerization (1 : (1-chloroethyl)benzene : styrene = 1 : 2 : 500) at
120 uC for 20 h (conversion = .95%; Mn = 35 000, Mw/Mn = 1.3)
and then added MMA (10 mmol) and toluene (2 mL) to the
mixture which was stirred at 100 uC for further 40 h. This
procedure resulted in PSt-b-PMMA with Mn = 75 000, Mw/Mn =
1.4 (conversion = .95%). The SEC charts are shown in the ESI.{
The good solubility of 1 in methanol is a crucial factor in
separation of the catalyst from the polymer mixture by simple
precipitation of the polymer from methanol. As an example,
pouring a THF solution including the crude polystyrene produced
(conditions: 1 : (1-chloroethyl)benzene : styrene = 1 : 2 : 1000;
100 uC, 70.5 h, conversion = 82%) into methanol (30 mL) afforded
colorless polystyrene precipitates and a pale yellow supernatant
containing the iron species. The polystyrene (Mn = 44 000,
Mw/Mn = 1.2) purified via filtration and washing by methanol was
subjected to ICP-mass analysis. It was found that only 14 ppm
1
isolation of slightly yellow colored Fe species, of which H NMR
(Fig. 3) and ESI-mass spectra were identical to fresh 1. In Table 2,
results from reusing the catalyst recovered from the mixtures of
polymerization of St are summarized. Very interestingly, from the
first to the fourth run, the catalyst was not deactivated and thus
repeatedly promoted the polymerization to give the corresponding
PSt with nearly the same molecular weight and molecular weight
distribution. Similar recycling of the catalyst was also achieved in
the block copolymerization of styrene and MMA; the polymeriza-
tion starting from styrene (conversion .95%, PSt-Cl Mn = 35 000,
Mw/Mn = 1.2) followed by post-polymerization of MMA gave
PSt-b-PMMA (Mn = 77 000, Mw/Mn = 1.4) with 95% conversion
of MMA. The recovered catalyst via isolation treatment of the
block copolymer was reused in the second block copolymerization
of St and MMA. We found that the pre-polymerization of styrene
gave PSt-Cl (conversion 90%, Mn = 43 000, Mw/Mn = 1.2),
whereas the post-polymerization of MMA provided PSt-b-
PMMA (Mn = 116 000, Mw/Mn = 1.7) with 95% conversion of
Table 1 Polymerization of styrenea
Entry [Fe] : [I] : [St] Conversion (%) Mn(exptl.)b Mn(calc.)c Mw/Mn
Table 2 Polymerization of styrene using reused catalysta
1
2
3
4
5
6
7
8
a
1 : 1 : 1000
1 : 2 : 1000
1 : 6 : 1000
100
95
100
163 000
54 000
38 000
16 000
129 000
52 000
28 000
13 000
104 000
49 000
17 000
5 000
68 000
35 000
13 000
4 800
1.4
1.3
1.2
1.3
1.3
1.2
1.2
1.2
Conversion (%)
Mn(exptl.)c
Mn(calc.)d
Mw/Mn
1 : 20 : 1000 100
1sta
2ndb
3rdb
4thb
a
95
93
92
93
32 000
29 000
31 000
27 000
24 000
23 000
23 000
23 000
1.3
1.3
1.4
1.3
1 : 1 : 1000
1 : 2 : 1000
1 : 6 : 1000
1 : 20 : 1000
65
68
76
93
The first run of polymerization was carried out at 120 uC for 20 h
in the presence of 1 (the catalyst) and (1-chloroethyl)benzene (the
initiator) with the ratio of [1] : [I] : [St] = 0.02 : 0.04 : 10 mmol.
Polymerization was carried out at 120 uC for 20 h in the presence
1 as the catalyst and (1-chloroethyl)benzene as initiator.
of
b
Concentration of 1 was 0.01 mol L21 (for entries 1–4, bulk) and
The 2nd, 3rd and 4th polymerizations were carried out with the
recovered catalyst by the method described in the text. Other
0.005 mol L21 (for entries 5–8, in toluene), respectively.
Determined by size exclusion chromatography (SEC) calibrated by
b
c
conditions were the same as those of footnote a. Determined by
c
PSt standards. Mn(calc.) is calculated from the conversion of
monomer.
size exclusion chromatography (SEC) with PSt calibration.
Mn(calc.) is calculated from the conversion of monomer.
d
1856 | Chem. Commun., 2007, 1855–1857
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MMA. It seems that in the case of the block-copolymerization,
catalyst performance to provide a narrow molecular weight
distribution in the pre-polymerization step was reduced somewhat
at the post polymerization with MMA, although the catalyst
recovery was successful.
11 D. R. Haddleton, D. Kukulj and A. P. Radigue, Chem. Commun., 1999,
99.
In conclusion, the iron complex 1 proved to be an efficient
catalyst for ‘‘living’’ ATRP, providing a clear solution of
problematic isolation of the polymer from the catalyst residue by
simple precipitation treatment. The robustness of 1 allows us to
recover successfully it from the polymerization mixtures and to
reuse it repeatedly without loss of the catalyst efficiency. Use of
iron catalysts is one of the ultimate solutions for environmentally
benign production of chemicals. Even in the chemical processes
using iron reagents, which exhibit low biological toxicity, the
catalyst recovery and reuse are key points which should be solved.
The present work clearly demonstrates the importance of catalyst
design for the recovery and reuse of the catalyst for ATRP.
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