Phosphine-ligand decoration toward active and robust iron catalysts in LRP

Article abstract of DOI:10.1021/ma400524q

Phosphine ligands were designed to enhance the catalytic activity of iron(II) complexes [FeBr₂(PR₃)₂] for metal-catalyzed living radical polymerization (LRP), and special efforts were directed to the improvement in catalyt

Full text of DOI:10.1021/ma400524q

Article
pubs.acs.org/Macromolecules
Phosphine−Ligand Decoration toward Active and Robust Iron
Catalysts in LRP
Keita Nishizawa, Makoto Ouchi,* and Mitsuo Sawamoto*
Department of Polymer Chemistry, Graduate School of Engineering, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan
S
* Supporting Information
ABSTRACT: Phosphine ligands were designed to enhance the catalytic activity of iron(II) complexes [FeBr2(PR3)2] for metal-
catalyzed living radical polymerization (LRP), and special efforts were directed to the improvement in catalytic activity and
robustness against functional monomers. Introduction of an electron donating group {methoxy [P(MeOPh)3] or N,N′-
dimethylamino [Ph2P(Me2NPh)]} onto the para position of triphenyl phosphine (PPh3) allowed active and robust Fe(II)
complexes that catalyzed LRP of poly(ethylene glycol) methacrylate (PEGMA) smoothly proceeding to high conversion
(∼90%), to form polymers of controlled molecular weights and its distributions (Mw/Mn < 1.2). In contrast, such an
enhancement was absent with the parent ligand PPh3 and those with electron-withdrawing substituents. Furthermore, the
replacement of the three methoxy groups in P(MeOPh)3 with PEG chains led to a more robust catalyst, especially tolerant of the
hydroxyl group in 2-hydroxyethyl methacrylate (HEMA). Accordingly, this catalyst enabled a four-component random living
copolymerization of HEMA, PEGMA, and two alkyl methacrylates, where all the monomers randomly copolymerized into
statistical copolymers of controlled molecular weights.
complex catalysts causes some disadvantages particularly in
practical applications: metal contamination in products,
expensiveness of metal, and low sustainability (rare and/or
toxic metals).
INTRODUCTION
■
Metal-catalyzed living radical polymerization (LRP),1,2 or atom
transfer radical polymerization,3,4 has been developed to
provide easy-to-use synthetic tools for well-defined polymers,
block copolymers, and polymer conjugates, among others.
Therein a metal complex (Mtn) is in charge of the one-electron
redox catalysis (Mtn ↔ XMtn+1) that reversibly activates the
carbon−halogen (C−X) bond of an initiator (R−X) or the
growing terminal (“dormant” species) into a carbon radical,
along with the regeneration of the dormant end. The
reversibility allows reduction of the instant concentration of
“active” radical species and thus suppresses bimolecular
termination and other unfavorable side-reactions inherent in
the conventional free radical polymerization. In addition, the
designability of initiating systems (initiators plus catalysts),
coupled with their high initiation efficiency, has now positioned
the metal-catalyzed LRP as a user-friendly, reproducible, and
extensively applicable versatile synthetic strategy in various
fields requiring well-defined polymeric architectures, often
beyond polymer chemistry toward biochemistry, medicine,
materials science, and other fields. However, the use of metal
Iron (Fe) belongs to the group 8 transition metal and mainly
assumes the +2 and +3 oxidation states, thus qualified to the
catalyst of the LRP requiring one-electron redox property. As
an element, iron should be more attractive for LRP catalysts
than often-used transition metals such as ruthenium (Ru)5 and
copper (Cu),6 because “ferrum” is abundant on Earth,
inexpensive, sustainable, and possibly safer. Indeed, industries
are ardently requiring active and useful iron catalysts that may
lead to practical application of LRP. Additionally, the important
roles of iron-mediated redox systems ubiquitously found in
biology and biological systems indicate the high biocompati-
bility and possible bioapplications of iron catalysts.
Received: March 11, 2013
Revised: April 10, 2013
© XXXX American Chemical Society
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Scheme 1. Preparation of Iron Complexes with Various Phosphine Ligands and Iron-Catalyzed LRP
Some research groups, including ours,7−17 have in fact
already developed Fe catalysts for LRPs,18−39 but they still
seem inferior in activity and functionality-robustness to Ru- and
Cu-based counterparts. A disadvantage on iron catalysts is a low
tolerance to polar groups: For example, most of the hitherto
developed iron LRP catalysts cannot survive in the presence of
polar functionality in monomer or in solvent, often transformed
into undesirable, much less active or inactive forms via ligand
dissociation or ligand exchange. To our knowledge, the
reported iron-catalyzed LRP systems, including those by us,
have in fact been still poorly versatile in terms of applicable
monomers and reaction conditions, often accessible to only
“common” monomers such as alkyl (meth)acrylates and
styrene derivatives without polar and functional pendent
groups. Although some Fe-catalyzed LRPs have recently been
reported for functional monomers, the controllability, i.e.,
livingness of growing end or efficiency in block copolymeriza-
tion, is still unsatisfactory.14,16,23,39 Given redox catalysis of
some biological iron complexes in polar and aqueous
environments, the previous efforts in ligand design have
obviously been so insufficient that we could not educe the
potential ability of iron catalysts for LRP of functional
monomers yet.
These backgrounds have encouraged us to design new
phosphine ligands for iron complexes toward really “applicable”
iron catalysis of LRP for functional monomers and in polar or
aqueous media. In this work, various phosphine ligands were
designed and selected, and simple procedures were applied
where they were mixed and aged with FeBr2 in an organic
solvent. The in situ formed phosphine-ligated Fe(II) complexes
were directly employed as catalysts for LRP of functional
monomers, such as HEMA and PEGMA [CH2C(CH3)-
COO(CH2CH2O)nCH3; n = 8.5 (number-average)], along
with MMA and other nonpolar alkyl methacrylates (Scheme 1).
Focus was directed to the electronic effects of the phosphine
ligands on catalysis and to the steric effects that may protect the
“central” iron from polar groups.
This paper is to report that the catalytic activity of iron
complex can be enhanced by the introduction of electron
donating and sometimes amphiphilic or hydrophilic groups
onto triphenylphosphine and that the ligand decoration with
polar and bulky PEG chain improves tolerance to highly polar
groups, leading to controlled polymerization of functional
monomers.
EXPERIMENTAL SECTION
■
Materials. MMA (Tokyo Kasei; > 99%) was dried overnight over
calcium chloride, and distilled from calcium hydride under reduced
pressure before use. HEMA (Aldrich; > 99%) was distilled under
reduced pressure before use. BzMA (TCI; > 98%) and PEGMA
[CH2C(CH3)COO(CH2CH2O)nCH3; n = 8.5 (number-average)]
(Aldrich) were purified by passing through an inhibitor-removal
column (Aldrich) and were subsequently degassed by three-time
vacuum-argon bubbling cycles before use. The H(MMA)2Br initiator
[H(CH2CMeCO2Me)2Br]; an MMA dimer bromide] was prepared
according to the literature.40 FeBr2 (Aldrich; 98%) and ligands [PPh3
(Aldrich; 99%), P(MeOPh)3 (Wako; > 98%), P(ClPh)3 (Aldrich;
95%), PBu3 (Strem; 99%), PCy3 (Aldrich), and Ph2P(Me2NPh)
(Aldrich; 95%)] were used as received and handled in a groove box
under a moisture- and oxygen-free argon atmosphere (H2O < 1 ppm,
O2 < 1 ppm). Toluene (Kishida Kagaku; purity 99.5%) was dried and
purified by passing through purification columns (Solvent Dispensing
System, SG Water USA, Nashua, NH; Glass Contour) and bubbled
with dry nitrogen for more than 15 min immediately before use. n-
Octane (internal standard for gas chromatography) and 1,2,3,4-
tetrahydrousnaphthalene (tetralin; internal standard for 1H NMR)
were dried over calcium chloride and distilled twice from calcium
hydride. Poly(ethylene glycol) monomethyl ether [HO-
(CH2CH2O)nMe; n = 12 on average] (Aldrich), p-toluenesulfonyl
chloride (Aldrich; > 99%), pyridine (Wako; dehydrated), H2O2 (TCI;
35% in water), BBr3 (TCI; ca. 1 M in CH2Cl2), phenylsilane (TCI; >
97%), NaOH (Wako; > 97%), HCl (Wako; 35−37% in water),
Na2SO4 (Wako; > 99%), K2CO3 (Wako; > 99.5%), NaCl (Wako; >
99.5%), distilled water (Wako), CH2Cl2 (Wako; super dehydrated),
DMF (Wako; super dehydrated), acetone (Wako; super dehydrated),
and ethyl acetate (Wako; > 99.5%) were used as received.
Synthesis of P(PEGPh)3.41 Synthesis of Tris(4-hydroxyphenyl)-
phosphine Oxide [OP(MeOPh)3]. To a solution of P(MeOPh)3
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(5.00 g, 14.2 mmol) in acetone (50 mL), water (3.33 mL) and H2O2
(35-% solution, 1.67 mL; 15 mmol) were slowly added. After stirring
at room temperature for 1 h, the acetone was evaporated and CH2Cl2
(80 mL) was added. The organic layer was washed with brine (3 × 35
mL), the aqueous solution was extracted with CH2Cl2 (2 × 25 mL),
and the combined organic solutions were dried over anhydrous
Na2SO4. After filtration, the filtrate was evaporated under vacuum to
give white solid (86% yield).
Synthesis of Tris(4-hydroxyphenyl)phosphine Oxide [OP-
(HOPh)3]. A solution of BBr3 (1 M in CH2Cl2; 50 mL, 50 mmol)
was slowly added at −78 °C to a solution of OP(MeOPh)3 (3.68 g,
10 mmol) in CH2Cl2 (35 mL) under argon. After being stirred at
room temperature for 24 h, the solution was slowly poured into cold
water (130 mL). After evaporation of CH2Cl2, the aqueous phase was
filtered and extracted with ethyl acetate (3 × 100 mL). The combined
organic phases were washed with brine (2 × 10 mL). After evaporation
of the solvent, recrystallization from ethyl acetate gave white solid
(81% yield).
particle size =10 μm; pore size =5000 A; 0.8 cm i.d. × 30 cm; flow rate,
1.0 mL min−1) connected to a PU-2080 pump and a RI-1530
refractive-index detector, and a UV-1570 ultraviolet detector (all from
Jasco). The columns were calibrated against 13 standard poly(MMA)
samples (Polymer Laboratories; Mn = 630−1 200 000; Mw/Mn
=
1.02−1.30) as well as the monomer. For poly(MMA), Mn and Mw/Mn
were measured by size exclusion chromatography at 40 °C in THF as
an eluent on three polystyrene-gel columns (Shodex LF-404; exclusion
limit = 2 × 106; particle size = 6 μm; pore size = 3000 Å; 0.46 cm i.d. ×
25 cm; flow rate, 0.3 mL min−1) connected to a DU-H2000 pump, a
RI-74 refractive-index detector, and a UV-41 ultraviolet detector (all
from Shodex), similarly calibrated against standard poly(MMA)
samples. 1H NMR and 31P NMR spectra were measured at room
temperature on a JEOL JNM-ECA500 spectrometer operating at
500.16 and 202.47 MHz, respectively. For the 31P NMR analyses, a
capillary of (C2H5O)2POH solution (50 mM in toluene-d8) was used
as an internal chemical shift standard (12 ppm for the phosphite).
Synthesis of Poly(ethylene glycol) Methyl Ether Tosylate [TsO-
PEG]. To a solution of poly(ethylene glycol) monomethyl ether (HO-
PEG) (12.5 g, 22.7 mmol) and p-toluenesulfonyl chloride (8.66 g, 45.4
mmol) in CH2Cl2 (40 mL) was slowly added pyridine (3.66 mL, 45.4
mmol). The resulting solution was stirred at room temperature for 15
h. After water (10 mL) was added, NaOH was carefully added until the
aqueous layer became neutral. The organic layer was washed with 1 N
HCl (aq) and brine, successively. The organic layer was dried over
anhydrous Na2SO4. After filtration, the filtrate was evaporated under
vacuum to give pale yellow oil (77% yield).
Synthesis of OP(PEGPh)3. A suspension of TsO-PEG (2.13 g,
3.05 mmol), OP(HOPh)3 (1.00 g, 3.05 mmol) and K2CO3 (4.21 g,
30.5 mmol) in DMF (70 mL) was stirred at 70 °C for 24 h under Ar.
After removal of volatiles under vacuum, the residure was dissolved in
CH2Cl2. The organic layer was washed with 1 N HCl (aq) and brine,
successively. The organic layer was dried over anhydrous Na2SO4.
After filtration, the filtrate was evaporated under vacuum (77% yield).
31P NMR (202 MHz, CDCl3): δ 33.9.
RESULTS AND DISCUSION
■
Effects of the Electronic Properties with Para-
Substituted Triphenyl Phosphines in PEGMA Polymer-
ization. As shown in Scheme 1, 2 equiv of various phosphines
(PR3) were mixed with anhydrous iron dibromide (FeBr2) in
toluene, and the solutions were kept (aged) at 60 °C for 12 h
hours, to in situ generate the corresponding bisphosphine iron
complexes [FeBr2(PR3)2]. The resultant iron complexes were
directly employed as a catalyst for polymerization of PEGMA
with a bromine initiator [H(MMA)2Br] in toluene.
FeBr2(PPh3)2, an iron catalyst with the most standard
phosphine ligand, triphenyl phosphine (PPh3), induced a slow
polymerization, and monomer conversion was limited to below
30% (Figure 1). A ligand carrying an electron withdrawing
Synthesis of P(PEGPh)3. A solution of OP(PEGPh)3 (2.88 g, 1.50
mmol) and phenylsilane (3.41 mL, 30.0 mmol) in toluene (10 mL)
was refluxed under argon for 24 h. After the reaction mixture was
evaporated under vacuum, the residue was extracted with CH2Cl2 and
washed with hexane. The removal of all the volatiles gave P(PEGPh)3
1
(64% yield). Finally, the structure was characterized H NMR (500
31
MHz, CDCl3: Supporting Information) and
P NMR (202 Hz,
CDCl3: δ −4.50).
Polymerization Procedures. Polymerization was carried out by
the syringe technique under dry argon in baked glass tubes equipped
with a three-way stopcock or in sealed glass vials. A typical example for
PEGMA polymerization with the H(MMA)2Br/FeBr2/P(MeOPh)3 is
given below. In a round-bottom flask (50 mL) was placed FeBr2 (4.3
mg, 0.020 mmol), P(MeOPh)3 (14.1 mg, 0.040 mmol), and toluene
(2.91 mL) under argon gas. The solution was heated to 60 °C for 12 h
to prepare bisphosphine iron complexes.42 After cooling the mixture to
room temperature, tetralin (0.06 mL), PEGMA (0.88 mL, 2.0 mmol),
and a solution of H(MMA)2Br (0.15 mL, 133.7 mM in toluene) were
added; the total volume was 4.00 mL. Immediately after mixing,
aliquots (0.50−1.0 mL each) of the solution were injected into baked
glass tubes, which were then sealed (except when a stopcock was used)
and placed in an oil bath kept at 60 °C. In predetermined intervals, the
polymerization was terminated by cooling the reaction mixture to −78
°C in dry ice−methanol. Monomer conversion was determined by 1H
NMR from the integrated peak area of the olefinic protons of the
monomer with tetralin as an internal standard. For MMA, the same
procedures as described above were applied, except that monomer
conversion was determined from residual monomer concentration
measured by gas chromatography with n-octane as an internal
standard.
Figure 1. Effects of phosphine ligand on polymerization rate of iron-
catalyzed polymerizations of PEGMA in toluene at 60 °C:
[PEGMA]0/[H(MMA)2Br]0/[Fe-complex]0 = 500/5/5 mM. The Fe
complex was prepared in prior to polymerization via aging process of
FeBr2 and 2 equiv of phosphine ligand in toluene at 60 °C in 12 h and
directly employed without purification.
substituent [P(ClPh)3] was further less effective, virtually
without monomer consumption. In contrast, when phosphines
with electron donating substituents [PBu3, PCy3, P(MeOPh)3,
and Ph2P(Me2NPh)] were employed, the polymerizations
smoothly proceeded to give higher conversions.
Molecular weight and molecular weight distribution (MWD)
of the poly(PEGMA) obtained with these phosphine-ligated
catalysts were characterized by SEC (Figure 2). For electron-
donating alkyl phosphines (PBu3 and PCy3), the SEC curves
were broad and sometimes bimodal (Mw/Mn ∼ 2), indicating
uncontrolled polymerizations. However, with triphenyl phos-
phines carrying strongly electron-donating groups, such as
Measurements. For polar (co)polymers of PEGMA or HEMA,
Mn and Mw/Mn were measured by size exclusion chromatography at
40 °C in DMF containing 10 mM LiBr as an eluent on three
polystyrene-gel columns (Shodex KF-805 L; exclusion limit =4 × 106;
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Figure 2. SEC curves of obtained polyPEGMAs to see effects of ligand on molecular weight control in iron-catalyzed radical polymerization of
PEGMA. Polymerization conditions: see caption of Figure 1.
Figure 3. Iron-catalyzed block copolymerization of MMA and PEGMA with FeBr2/P(MeOPh)3 complex. (A) Polymerization for synthesis of
PMMA-macroinitiator: [MMA]0/[H(MMA)2Br]0/[Fe-complex]0 = 4000/20/10 mM in toluene at 60 °C. (B) Polymerization for PMMA-block-
PPEGMA: [PEGMA]0/[PMMA-macroinitiator]0/[Fe complex]0 = 500/5/5 mM in toluene at 60 °C. The Fe complex was prepared in prior to
polymerization via aging process of FeBr2 and 2 equiv of P(MeOPh)3 in toluene at 60 °C for 12 h and directly employed without purification for
both polymerizations.
P(MeOPh)3 and Ph2P(Me2NPh), the polymerizations were
fairly controlled: SEC curves shifted to higher molecular weight
as conversion increased, while keeping narrow molecular weight
distributions (Mw/Mn < 1.2). Thus, enhancement of the
electron density on the iron center through triphenylphosphine
ligands was important in the iron-catalyzed LRP of PEGMA.
Please note that there are little examples of such active catalysts
to achieve higher conversions (conv. ∼90%) and narrow
molecular weight distributions (Mw/Mn < 1.20).
was also effective for the LRP of alkyl methacrylates (MMA
etc.) to give controlled polymers (Supporting Information).
This result encouraged us to apply the methoxyphenyl complex
for block copolymerization of MMA with PEGMA (Figure 3).
To synthesize AB-type block copolymers, MMA was first
polymerized. The resulting PMMA (with a halogen-capped
dormant terminal) was separated and employed as a macro-
initiator for the second-stage polymerization of PEGMA.
Thus, we first carried out MMA polymerization with FeBr2/
P(MeOPh)3 and the bromide initiator in toluene at 60 °C:
[MMA]0 = 4.0 M; [H(MMA)2Br]0 = 20 mM; [FeBr2]0 = 10
Iron-Catalyzed Block Copolymerization of MMA with
PEGMA with P(MeOPh)3. Ligation of P(MeOPh)3 on FeBr2
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Figure 4. Decoration of para-substitution of triphenylphosphine with PEG chain.
Figure 5. Conversion vs Mn and Mw/Mn plots for iron-catalyzed LRPs of PEGMA with P(PEGPh)3 or P(MeOPh)3 as a ligand in toluene at 60 °C:
[PEGMA]0/[H(MMA)2Br]0/[Fe-complex]0 = 500/5/5 mM. Fe-complex was prepared in prior to polymerization via aging process of FeBr2 and 2
equiv of phosphine ligand in toluene at 60 °C in 12 h and directly employed without purification.
mM; [P(MeOPh)3]0 = 20 mM. When conversion reached
about 60% in 2.5 h, the polymerization was quenched, and
reprecipitation into methanol gave a colorless sample of
PMMABr (Mn = 14700; Mw/Mn = 1.12; terminal Br
consecutive block copolymerization. In the SEC curves of the
final products, a very small peak was detected, indicative of
some “dead” PMMA from the first-stage polymerization, but
the main peak clearly shifted to higher molecular weight
keeping narrow MWD.
1
functionality >90% by H NMR) (Figure 3A). The Br-capped
Decoration with Polar Electron-Donating Substitu-
ents toward Robust Iron Catalyst. As shown above, the
introduction of electron-donating phosphines was effective to
enhance the catalytic activity of iron catalysts. However, these
systems were confined to the use in toluene and related
polymer was employed to initiate PEGMA polymerization with
a freshly aged catalyst [FeBr2 and 2eq P(MeOPh)3].
PEGMA conversion smoothly reached 91% in 48 h, as with
the corresponding homopolymerization with H(MMA)2Br.
Thus, the catalytic activity was well retained in the two-stage
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Figure 6. UV−vis spectra (280−600 nm) of FeBr2 complexes with PPh3, P(MeOPh)3, and P(PEGPh)3 in toluene or 5 vol % ethanol-contained
toluene at room temperature: (A) comparison among the three complexes in toluene; (B−D) effects of ethanol addition on complexation [(B)
PPh3, (C) P(MeOPh)3, (D) P(PEGPh)3]. The Fe complex was prepared via aging process of FeBr2 and 2 equiv of phosphine ligand in toluene at 60
°C in 12 h ([FeBr2]0 = 5.0 mM, [ligand]0 = 10.0 mM). The aged solutions were diluted with toluene to make 1.0 mM concentration, followed by
filtration for the measurement.
nonpolar media and seem not robust enough against polar
groups such as hydroxyl. For example, a polymerization
solution of PEGMA with FeBr2/P(MeOPh)3 immediately
faded from original yellow into colorless upon addition of a
few drops of methanol, suggesting dissociation of the ligands to
form an unidentified phosphine-free iron complex or
compound.
We thus embarked on an additional design of ligand to more
effectively “protect” the iron center from potentially poisonous
polar groups. Our focus was then directed to the replacement
of the three methoxy groups (−OCH3) in P(MeOPh)3 with
poly(ethylene glycol) chains [PEG: −O(CH2CH2O)12CH3]
into a tris(PEG-lated phenyl)phosphine P(PEGPh)3 (Figure
4). This decoration was based on the following aspects: (i)
despite its polar and polyether character, PEG would not be so
poisonous to iron complexes, as suggested by the successful
PEGMA polymerization discussed above; (ii) the bulkiness and
higher polarity of PEG would contribute to the protection of
the central iron from polar groups, sterically, electronically, or
both.
Catalytic Activity of Iron Complex with P(PEGPh)3 on
PEGMA Polymerization. To examine the catalytic activity of
the FeBr2/P(PEGPh)3 system, we first performed PEGMA
polymerization in toluene at 60 °C, under the same conditions
as with P(MeOPh)3. The aging with FeBr2 gave a yellow
homogeneous solution similar to that with the methoxy
derivative, indicating a similar diphosphine ligation. The
polymerization (Figure 5) was also comparable in terms of
reaction rate (catalytic activity) as well as molecular weight and
MWD control: Mn was linearly increased with conversion, and
Mw/Mn was kept below 1.12. Namely, the three PEG chains on
the para positions effectively enhance catalytic activity by
electron-donation and perhaps protect the iron center, without
entailing adverse effects.
Tolerance to Hydroxyl Group of Iron Catalysts: UV−
vis Analysis. When a small portion of ethanol was added to a
toluene solution of the P(PEGPh)3−iron complex, the yellow
color remained unchanged, in contrast to the fast discoloration
of the P(MeOPh)3 system (see above). The apparent
robustness of the P(PEGPh)3-complex was further evaluated
by UV−vis spectroscopy.
Figure 6 shows UV−vis spectra (280−600 nm) of FeBr2
complexes with three phosphine ligands [PPh3, P(MeOPh)3,
and P(PEGPh)3], either in the presence and the absence of
ethanol. All the complexes were prepared by mixing and aging
FeBr2 and 2 equiv of each phosphine in toluene at 60 °C for 12
h, and the solutions, all yellow independent of the ligands, were
subject to spectroscopic analysis without further purification
and treatment. The spectra generally consisted two character-
istic peaks, one at 320−400 nm most likely from charge-transfer
or d−d transition and the other at ∼300 nm, indicative of a free
phosphine (or ligand dissociation).
In the alcohol-free solutions (Figure 6A), the free ligand
signal was very strong and off-scale with FeBr2(PPh3)2, showing
dominant ligand dissociation even in the nonpolar medium and
thus the lower stability of the catalyst, as already pointed in the
literature. On the other hand, free phosphine was much less
detectable with P(MeOPh)3 or P(PEGPh)3, and the coordina-
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Scheme 2. Iron-Catalyzed Living Radical Random Copolymerization of HEMA, PEGMA, MMA, and BzMA with FeBr2−
P(PEGPh)3 Complex
Figure 7. Iron-catalyzed living radical random copolymerization of HEMA, PEGMA, MMA and BzMA with FeBr2−P(PEGPh)3 complex in toluene
at 60 °C: [HEMA]0/[PEGMA]0/[MMA]0/[BzMA]0/[H(MMA)2Br]0/[Fe complex]0 = 250/250/250/250/5/5 mM. The Fe complex was prepared
in prior to polymerization via aging process of FeBr2 and 2 equiv of P(PEGPh)3 in toluene at 60 °C in 12 h and directly employed without
purification.
tion of these para-substituted electron-donating ligands was
relatively tight and effectively enriches electron density of the
iron center.
monomers is important for industrial applications, particularly
with an iron catalyst.
The four monomers were smoothly consumed at similar
rates, although HEMA conversion was a little higher
(Supporting Information), and conversion reached over 70%
in 24 h for each component [87% (HEMA); 65% (PEGMA);
64% (MMA); 67% (BzMA)]. As shown in Figure 7, Mns of
obtained copolymers were linearly increased with copolymer
yield (calculated from conversion). At the initial stages, narrow
MWDs were observed (Mw/Mn < 1.20), but the SEC curves
broadened as the polymerization proceeded, and minor peaks
of apparently dead polymers chains were detected. However,
the Mw/Mn values were lower than with P(MeOPh)3, likely due
to higher tolerance to the hydroxyl group.
Upon addition of ethanol (5 vol %), the yellow solutions
with PPh3 and P(MeOPh)3 immediately faded into colorless,
while the free ligand signal enhanced and the red-shifted
transition signals weakened [Figure 6 (B) and (C)]. All these
indicate facile decomposition, or lesser stability, of the two
complexes. Rather antithetically, the P(PEGPh)3-complex is
apparently much more resistant to alcohol; the solution
remained yellow and the whole spectrum was hardly changed
(Figure 6D), unless the added ethanol was in excess over 5 vol
%. These analyses supported that PEG-lated phosphine-iron
complex is relatively robust and tolerant of alcohol or hydroxyl
group.
Random Copolymerization of PEGMA with HEMA.
The above results with UV−vis analyses encouraged us to
employ FeBr2/P(PEGPh)3 for polymerizations of functional
polar monomers and specifically for quaternary random
copolymerization of methacrylates [HEMA, PEGMA, MMA,
and benzyl methacrylate (BzMA): Scheme 2]. Control of such
a multicomponent random copolymerization of functionalized
CONCLUSION
■
In this paper, we have approached robust iron-catalyzed LRPs
via modification of phosphine ligand. An introduction of
electron donating (i.e., methoxy- or dimethylamino) of the
bis(phosphine)iron complex [FeBr2(PR3)2] dramatically in-
creased polymerization rate up to higher conversion for
PEGMA; nevertheless, the polymerization was well controlled
to give narrow molecular weight distributions (Mw/Mn < 1.2).
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The high catalytic activity allowed iron-catalyzed block
copolymerization of MMA with PEGMA. In addition, polar
macromolecular (i.e., PEG) substituents on ligand were
particularly effective for an enhancement of catalytic activity
of polymerization of HEMA as well as PEGMA due to
protection effect of iron center from polar groups. However, it
was still difficult to control polymerizations of more polar
monomers, such as dimethylaminoethyl methacrylate, meth-
acrylic acid, even with the iron catalyst of the PEG-decollated
ligand. Thus, our next efforts would be directed to other iron
precursors instead of FeBr2 to be combined with the related
designer phosphine ligands toward more robust and applicable
iron catalysts.
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ASSOCIATED CONTENT
* Supporting Information
■
S
1H NMR spectra for synthesis of P(PEGPh)3, iron-catalyzed
living radical polymerization of MMA with FeBr2−P(MeOPh)3
complex, and time vs conversion plots for iron-catalyzed living
radical random copolymerization of HEMA, PEGMA, MMA,
and BzMA. This material is available free of charge via the
Internet at http://pubs.acs.org/.
A: Polym. Chem. 2009, 47, 2002.
(28) Xue, Z.; He, D.; Noh, S. K.; Lyoo, W. S. Macromolecules 2009,
42, 2949.
(29) Bai, L.; Zhang, L.; Zhang, Z.; Tu, Y.; Zhou, N.; Cheng, Z.; Zhu,
X. Macromolecules 2010, 43, 9283.
(30) Wang, Y.; Matyjaszewski, K. Macromolecules 2010, 43, 4003.
(31) Guo, T.; Zhang, L.; Jiang, H.; Zhang, Z.; Zhu, J.; Cheng, Z.;
Zhu, X. Polym. Chem. 2011, 2, 2385.
(32) Kanazawa, A.; Satoh, K.; Kamigaito, M. Macromolecules 2011,
44, 1927.
AUTHOR INFORMATION
Corresponding Author
*(M.O.) Telephone: +81-75-383-7127. Fax: +81-75-383-2603.
E-mail: ouchi@living.polym.kyoto-u.ac.jp. (M.S.) Telephone:
+81-75-383-2600. Fax: +81-75-383-2601. E-mail: sawamoto@
star.polym.kyoto-u.ac.jp.
■
Notes
(33) Liu, D.; Chen, H.; Yin, P.; Ji, N.; Zong, G.; Qu, R. J Polym. Sci.,
Part A: Polym. Chem. 2011, 49, 2916.
The authors declare no competing financial interest.
(34) Wang, Y.; Matyjaszewski, K. Macromolecules 2011, 44, 1226.
(35) Wang, Y.; Zhang, Y.; Parker, B.; Matyjaszewski, K. Macro-
molecules 2011, 44, 4022.
(36) Zhu, G.; Zhang, L.; Zhang, Z.; Zhu, J.; Tu, Y.; Cheng, Z.; Zhu,
X. Macromolecules 2011, 44, 3233.
ACKNOWLEDGMENTS
■
This work was supported by Japan Society for the Promotion of
Science (JSPS) KAKENHI Grant Numbers 24245026 [Grant-
in-Aid for Scientific Research (A)] and 23655103 (Grant-in-Aid
for Exploratory Research).
(37) Mukumoto, K.; Wang, Y.; Matyjaszewski, K. ACS Macro Lett.
2012, 1, 599.
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5911.
(39) He, W. W.; Zhang, L. F.; Miao, J.; Cheng, Z. P.; Zhu, X. L.
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(41) Fujihara, T.; Yoshida, S.; Ohta, H.; Tsuji, Y. Angew. Chem., Int.
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dx.doi.org/10.1021/ma400524q | Macromolecules XXXX, XXX, XXX−XXX