First Nitroxide-Mediated Controlled/Living Free

Radical Polymerization in an Ionic Liquid

Julia Ryan,1 Fawaz Aldabbagh,*1 Per B. Zetterlund,*2a Bunichiro Yamada*2

1Department of Chemistry, National University of Ireland, Galway, IrelandFax: 00353 91 525700; E-mail: fawaz.aldabbagh@nuigalway.ie

2Department of Applied and Bioapplied Chemistry, Faculty of Engineering, Osaka City University, 3-3-138 Sugimoto,Sumiyoshi-ku, Osaka 558-8585, JapanE-mail: yamada@a-chem.eng.osaka-cu.ac.jp

Received: January 7, 2004; Revised: February 25, 2004; Accepted: March 1, 2004; DOI: 10.1002/marc.200400006

Keywords: controlled/living polymerization; ionic liquids; kinetics (polym.); nitroxides; radical polymerization

Introduction

1-Alkyl-3-methylimidazolium hexafluorophosphates are

non-volatile, room-temperature, ionic liquids. They have

been developed as air- and water-stable reaction media with

the potential to be recycled, and have been shown to accele-

rate and improve the yields of common organic reactions.[1]

The most notable are the commercially available 1-butyl-

3-methylimidazolium hexafluorophosphate [bmim][PF6]

and 1-hexyl-3-methylimidazolium hexafluorophosphate

[hmim][PF6] (Scheme 1).

Recently, the first conventional free radical polymer-

izations of methyl methacrylate (MMA) and styrene in

[bmim][PF6] were reported, and were observed to have

rates of polymerization and molecular weights approxi-

mately ten times higher than those obtained in benzene.[2]

The rate constants of propagation (kp) and termination (kt)

for the polymerization of MMA in [bmim][PF6] have been

determined by pulsed laser polymerization (PLP).[3] The

observed acceleration and higher molecular weights in

comparison to solution and bulk polymerizations were ex-

plained in terms of a combination of a larger kp and smaller

kt attributable, respectively, to the high polarity and high

viscosity of the ionic liquid.[3]

Summary: The controlled/living radical polymerizationsofmethyl acrylate in 50%v/vof an ionic liquid initiated by thealkoxyamine generated in situ from 4-oxo-2,2,6,6-tetra-methyl-1-piperidinyl-N-oxyl (4-oxo-TEMPO) and 2,20-azoi-sobutyronitrile (AIBN) at 140–155 8C are reported. Thenumber-average molecular weights increased linearly withconversion, and polydispersity indices are approximately1.4 in the best case. The rates of polymerization were greaterthan in anisole, and similar to the rate of spontaneous poly-merization in the ionic liquid.

Mn (filled symbols) andMw=Mn (open symbols) vs. conver-sion for the MA polymerization in the presence of [4-oxo-TEMPO]/[AIBN] (2.8:1) in 50% v/v anisole with 0.03 M

AIBN (squares) and 50% v/v [hmim][PF6] with 0.03 MAIBN(circles), and 0.06 M AIBN (triangles).

Macromol. Rapid Commun. 2004, 25, 930–934 DOI: 10.1002/marc.200400006 � 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

930 Communication

a Current address: Graduate School of Science and Technology,Kobe University, Kobe 657-8501, Japan;E-mail: pbzttlnd@cx6.scitec.kobe-u.ac.jp

The three most well-known controlled/living free radical

polymerization techniques are nitroxide-mediated poly-

merization (NMP), atom transfer radical polymerization

(ATRP), and reversible addition-fragmentation chain trans-

fer (RAFT).[4] There are several reported controlled/living

radical polymerizations in ionic liquids by ATRP,[5–8] re-

verseATRP,[9] and RAFT.[10] For these systems, controlled/

living polymerization characterized by the formation of

narrow polydispersity polymers and significant rate en-

hancement compared to polymerizations in volatile organic

solvents (VOS) were achieved. Here we report the first

NMP in an ionic liquid, namely the polymerization of

methyl acrylate (MA) in 50% v/v [hmim][PF6]mediated by

the commercially available nitroxide, 4-oxo-2,2,6,6-tetra-

methyl-1-piperidinyl-N-oxyl (4-oxo-TEMPO). It has pre-

viously been demonstrated that acrylate polymerizations

proceed in a controlled/living manner under appropriate

experimental conditions in the presence of TEMPO.[11,12]

MA was chosen for study because of its appreciable solu-

bility in [hmim][PF6],[6] and the fact that the resultant poly-

mer could be completely precipitated out of the reaction

medium, which was successfully re-cycled.

Experimental Part

Materials

Commercially obtained MAwas freshly distilled under reduc-ed pressure prior to all polymerizations. Anisole, 2,20-azoiso-butyronitrile (AIBN), and 4-oxo-TEMPO were obtainedcommercially, and theAIBNwas recrystallized frommethanolat 0 8C prior to use. [hmim][PF6] was prepared according to aliterature procedure from 1-methylimidazole, chlorohexane,and aqueous hexafluorophosphoric acid.[13]

Polymerizations

A typical polymerization used a stock solution of 4-oxo-TEMPO (0.114 g, 0.67 mmol), AIBN (0.039 g, 0.24 mmol),and MA (4 mL, 44 mmol). A 0.5 mL quantity of the stocksolution was transferred into a glass ampoule containing either0.5 mL of [hmim][PF6] or 0.5 mL of anisole. In both cases, onagitation, a homogeneous reaction mixture resulted,[6] whichwas degassedwith several freeze-thaw cycles and sealed undervacuum. The ampoules were submerged into an aluminiumblock at 105 8C, the temperature was raised over 30 min to

the polymerization temperature and held for prescribed times.The polymerizations were stopped by cooling the solution inan ice-bath.

Conversion and Molecular Weight Measurements

Monomer conversions during polymerization in [hmim][PF6]were continuously monitored by Fourier-transform near-infrared spectroscopy (FT-NIR; Jasco INT-400 Spectrometerequipped with an MCT detector) carried out in a 5 mm o.d.Pyrex tube in a custom-made aluminium furnace. A 0.25 mLaliquot of the same stock solution was transferred into a glassampoule containing 0.25mL of [hmim][PF6], and themixtureswere degassed with several freeze-thaw cycles and sealedunder vacuum. The ampoules were held at 105 8C for 30 minin an aluminium block, and subsequently transferred to theFT-NIR furnace maintained at the polymerization tempera-ture. The consumption of MA was obtained by monitoringthe absorbance at 6 150 cm�1, which has been assigned to theovertone absorption of nC C–H. Prior to integration, the ab-sorption of [hmim][PF6] in this region was subtracted by usinga pre-recorded spectrum of poly(MA) dissolved in [hmim]-[PF6] (poly(MA) with a concentration corresponding to100% MA conversion in the polymerization sample). For theanisole experiment, a samplewas also dissolved in CDCl3, anddirectly used for monomer conversion measurements by 1HNMR spectroscopy (Jeol 400 MHz) by monitoring the vinyl-ideneMA peak at 6.1 ppm and the anisole phenyl proton peaksat 6.8 ppm as the internal standard.

Molecular weights were measured by gel permeation chro-matography (GPC)with aTosoh8000 seriesGPCsystemequipp-edwith TSK-gel columnsG5000HHR,GMultipoerHXL-M, andGMHHR-L connected in this order, using tetrahydrofuran aseluent at 40 8C. Polystyrene standards (Mn ¼ 500–1 090 000)were used for calibration.

Recycling of the Ionic Liquid

At the end of each experiment, the residual monomer wasevaporated, and the polymer precipitated with a large excess ofmethanol. The polymer was filtered off, and the filtrate wasevaporated to dryness to give a clean sample of [hmim][PF6],as confirmed by 1H NMR spectroscopy.

Results and Discussion

The initial experimentswere carried out at 155 8Cguided by

earlier work on the n-butyl acrylate/4-oxo-TEMPO system

by Listigovers et al.[11] Following their procedure, we car-

ried out MA polymerizations containing [4-oxoTEMPO]

and [AIBN] in a 2.8:1 ratio by first heating from 105 8Cto the reaction temperature of 155 8Cover 30min in order to

form in-situ polymeric alkoxyamine adducts. Consistent

with some previous NMP work on acrylates, an excess of

nitroxide was required in order to achieve reasonable con-

trol during the polymerization,[14] although the results pre-

sented here are not optimized with respect to the nitroxide

concentration. The number-average molecular weight (Mn)

Scheme 1.

First Nitroxide-Mediated Controlled/Living Free Radical Polymerization in an Ionic Liquid 931

Macromol. Rapid Commun. 2004, 25, 930–934 www.mrc-journal.de � 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

increased close to linearly with conversion at two different

AIBN concentrations (keeping the ratio of [4-oxoTEMPO]

to [AIBN] constant) (Figure 1). At the lower AIBN concen-

tration, there is some deviation from linearity in the higher

conversion range, probably because of irreversible termina-

tion reactions of propagating species. An increase in AIBN

concentration by a factor of two resulted in a decrease in

Mn by close to a factor two as expected for a controlled/

living system. Calculation of the theoretical Mn is not

trivial in this case since the starting point was AIBN and

free nitroxide as opposed to an alkoxyamine; it is, however,

noted that the experimental Mn’s are considerably higher

than the theoretical values as calculated based on either the

initial amounts of AIBN or free nitroxide. This is in contrast

to the polymerization carried out under identical conditions

using anisole in place of the ionic liquid with an AIBN

concentration of 0.03 M, whereMn remained close to the cal-

culated values (Figure 1). Previously reported controlled/

living polymerizations using ATRP[7] and reverse ATRP[9]

in ionic liquids also exhibited low initiation efficiencies

leading to Mn values exceeding the theoretical ones. This

was attributed to low concentration of the catalyst in the

ionic liquid (two-phase system),[7] and a ‘‘cage-effect’’ (one-

phase system),[9] respectively. Somewhat low efficiency of

a RAFT agent in an ionic liquid has also been reported,[10]

and this was speculated to be caused by poor solubility of

the RAFTagent. In our case, visual observation suggested a

homogeneous reaction mixture, but it cannot be excluded

that solubility effectsmaybe at least part of the reason for the

discrepancy between experimental and theoreticalMn’s.

The polydispersity index for the sample with the higher

AIBN concentration is 1.4 at approximately 30% conver-

sion, whereas the experiment with the lower concentration

yielded higher polydispersity indices near 1.8. The poly-

dispersity indices in the anisole experiment remained near

1.2 at low conversion, but increased significantly at higher

conversions (Figure 1). Reasonable control is thus achieved

at the higher AIBN concentration in the ionic liquid, and it

may be possible to improve the level of control by fine-

tuning the nitroxide concentration.

Polymerizations carried out in [hmim][PF6] were faster

than in anisole using the AIBN concentration of 0.03 M, as

shown in Figure 2. The rate increase is consistent with

earlier observations for conventional free radical polymer-

izations[2,3] and controlled ATRP[5] and RAFT[10] systems

in ionic liquids for the reasons outlined in the introduction.

The rate of polymerization was not significantly affected by

the AIBN concentration at a constant [4-oxoTEMPO]-to-

[AIBN] ratio. Independence of the rate of polymerization on

the alkoxyamine concentration in TEMPO-based nitroxide

mediated polymerizations of styrene[15,16] is attributable to

the propagating radical concentration being governed by

the quantity (Ri/kt)0.5 after the pseudo-steady state has been

Figure 1. Evolution of Mn (filled symbols) and Mw=Mn (opensymbols) with conversion for the polymerization of MA in thepresence of a 2.8:1 ratio of [4-oxo-TEMPO]/[AIBN] at 155 8C in50% v/v anisole with [AIBN]¼ 0.03 M (&, &), and 50% v/v[hmim][PF6] with [AIBN]¼ 0.03 M (*, *), and [AIBN]¼0.06 M (~, ~).

Figure 2. ln([M]0/[M]) vs time for the polymerization of MAat 155 8Cunder various conditions, where [M] and [M]0 denotethe instantaneous monomer concentration and the initialmonomer concentration, respectively. (A)MAonly; (B)MA in50% v/v [hmim][PF6] with [4-oxoTEMPO] and [AIBN]¼ 0;(C) and (D) MA in 50% v/v [hmim][PF6] in the presence of a2.8:1 ratio of [4-oxoTEMPO]/[AIBN] with [AIBN]¼ 0.03 M

and 0.06 M, respectively; (&) in 50%v/v anisole in the presenceof a 2.8:1 ratio of [4-oxoTEMPO]/[AIBN] with[AIBN]¼ 0.03 M. Conversions obtained by FT-NIR spectro-scopy, except for the anisole experiment which were obtainedby 1H NMR spectroscopy.

932 J. Ryan, F. Aldabbagh, P. B. Zetterlund, B. Yamada

Macromol. Rapid Commun. 2004, 25, 930–934 www.mrc-journal.de � 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

reached. This is because of the thermal initiation of styrene

– in analogy with this observation, our results thus suggest

that there is a source of radicals other than the thermal

dissociation of alkoxyamines. Polymerizations ofMAwere

therefore attempted in the absence of bothAIBN and 4-oxo-

TEMPO in 50% [hmim][PF6] and forMA alone. The rate of

polymerization in the ionic liquidwithoutAIBN and 4-oxo-

TEMPO (plot B) is very similar to those carried out with

AIBN/4-oxo-TEMPO (plots C and D) in analogy with the

system styrene/TEMPO in bulk (except for some differ-

ences at very low conversion to polymerizations). How-

ever, in the absence of ionic liquid (i.e., bulk MA only,

plot A), the polymerization proceeds much faster. Sponta-

neous (thermal) polymerization of acrylates at elevated

temperatures has been reported, although it is not clear if

this is because of ‘‘true’’ thermal initiation, or whether

initiation occurs by adventitious impurities.[17]

Experiments were also carried out at 140 and 130 8C in

order to investigate whether control could be achieved at

lower temperatures with the current system. TheMn vs con-

version plots exhibited close to linear behavior at both 130

and 140 8C (Figure 3). TheMn data points at 140 and 155 8Care overlapping as expected if the number of growing chains

is the same in both cases. TheMn values at 130 8C are some-

what higher, indicating a lower number of growing chains.

This may be related to a lower rate of generation of radicals

via spontaneous initiation, although the degree of control

achievedmay suggest that the number of chains originating

from thermal initiation is small relative to that derived

from the initial alkoxyamine (as is normally the case in the

NMP of styrene[16]). It could also be a result of solubility

effects being more significant at a lower temperature. The

polydispersity indices increase markedly with decreasing

temperature, giving approximately 1.8 at 140 8C and 2.1 at

130 8C. It is apparent that the degree of control deteriorateswith decreasing temperature, most likely as a result of the

rate of dissociation of the alkoxyamines not being suffi-

ciently high. Alkoxyamine thermal-dissociation rates have

been reported to increase with solvent polarity,[18,19] and it

may therefore be anticipated that the dissociation rates in

the polar ionic liquid may be higher than in bulk/or in VOS.

However, solely based on a comparison of our polymer-

ization results with 4-oxo-TEMPO in the ionic liquid,

[hmim][PF6], and anisole, it cannot be concluded whether

this is the case or not. The polymerization proceeded only

slowly at 130 8C, the conversion reaching only about 11%

in 20 h (Figure 4).

Conclusions

Controlled/living radical polymerization ofmethyl acrylate

mediated by 4-oxo-TEMPO has been successfully carried

out in an ionic liquid for the first time. The relatively high

reaction temperature of 140 8C was required to achieve

reasonable control. The number-averagemolecular weights

(Mn) increased linearly with conversion, although they

were significantly higher than the theoretical values. An

increase in the AIBN and 4-oxo-TEMPO concentrations by

a factor of two (i.e., constant [4-oxo-TEMPO]/[AIBN]) re-

sulted in a decrease inMn by close to a factor of two, but had

Figure 3. Evolution of Mn (filled symbols) and Mw/Mn (opensymbols) with conversion for the polymerization ofMA in 50%v/v [hmim][PF6] in the presence of a 2.8:1 ratio of [4-oxoTEMPO]/[AIBN] with [AIBN]¼ 0.03 M at 155 8C (*, *),140 8C (^,}), and 130 8C (&,&).

Figure 4. ln([M]0/[M]) vs time plots obtained from FT-NIRdata for the polymerization of MA in 50% v/v [hmim][PF6] inthe presence of a 2.8:1 ratio of [4-oxoTEMPO]/[AIBN] with[AIBN]¼ 0.03 M.

First Nitroxide-Mediated Controlled/Living Free Radical Polymerization in an Ionic Liquid 933

Macromol. Rapid Commun. 2004, 25, 930–934 www.mrc-journal.de � 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

no significant effect on the rate of polymerization. The latter

finding could be accounted for by showing that the poly-

merization of MA in [hmim][PF6] and bulk MA proceed in

the absence of AIBN/4-oxo-TEMPO, revealing that signi-

ficant spontaneous polymerization occurs. In contrast, a

polymerization carried out in anisole was significantly

slower than the polymerization carried out under identical

conditions in the ionic liquid, and gave molecular weights

close to the calculated values. Upon precipitation of the

polymer, the re-cycling of the ionic liquid reaction medium

was achieved.

Acknowledgement: The authors thank the Irish ResearchCouncil for Science, Engineering & Technology (IRCSET) foran EMBARK Scholarship for Julia Ryan, and Enterprise Irelandfor an International Collaboration Award.

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Macromol. Rapid Commun. 2004, 25, 930–934 www.mrc-journal.de � 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim