SHORT COMMUNICATION

Photo-controlled/living radical polymerization mediatedby 2,2,6,6-tetramethylpiperidine-1-oxyl in inert atmospheres

Eri Yoshida

Received: 12 March 2012 /Accepted: 17 April 2012 /Published online: 4 May 2012# Springer-Verlag 2012

Abstract The photo-controlled/living radical polymerizationof methyl methacrylate using a nitroxide mediator was estab-lished in an inert atmosphere. The bulk polymerization wasperformed at room temperature using 4-methoxy-2,2,6,6-tet-ramethylpiperidine-1-oxyl as the mediator and (2RS,2′RS)-azobis(4-methoxy-2,4-dimethylvaleronitrile) as the initiator inthe presence of (4-tert-butylphenyl)diphenylsulfonium triflateas the accelerator by irradiation with a high-pressure mercurylamp. The photopolymerization in a N2 atmosphere produceda polymer with a comparatively narrow molecular weightdistribution; however, the experimental molecular weightwas slightly different from the theoretical molecular weight.The Ar atmospheric polymerization also provided a polymerwith the molecular weight distribution similar to that of thepolymer obtained by the N2 atmospheric polymerization.These inert atmospheric polymerizations more rapidly pro-ceeded to produce polymers with narrower molecular weightdistributions than the vacuum polymerization. The livingnessof the Ar atmospheric polymerization was confirmed on thebasis of the first-order time–conversion plots and conversion–molecular weight plots.

Keywords Photo-controlled/living radical polymerization .

Inert atmospheric polymerization . 4-Methoxy-2,2,6,6-tetramethylpiperidine-1-oxyl . (2RS,2′RS)-Azobis(4-methoxy-2,4-dimethylvaleronitrile) . (4-tert-Butylphenyl)-diphenylsulfonium triflate (tBuS) . Methyl methacrylate .

Deactivation

Introduction

The controlled/living radical polymerizations have beenutilized in a wide variety of industrial fields, such as photo-lithography [1–4], drug and gene deliveries [3–7], waterpurification [8], and surface modifications of wettability[9–12] and antibacterial treatment [12, 13]. The significanceof the controlled/living radical polymerization lies on thecontrol of the physical properties of the polymers throughthe molecular weight control. Many controlled/living radicalpolymerization systems have been discovered using variouscatalysts; the atom transfer radical polymerization (ATRP)[14], reversible addition–fragmentation chain transfer(RAFT) [15], nitroxide-mediated polymerization (NMP)[16], iniferter polymerization [17], iodide transfer poly-merization (ITP) [18, 19], and reverse iodine transfer po-lymerization [20]. These polymerizations also produced anumber of architectures based on novel molecular designconcepts.

Photopolymerization has advantages of being an envi-ronmentally benign technique and allows local applicationsand photospecific reactions. The combination of the photo-polymerization and the controlled/living radical polymeri-zation expands the scope of their applications, whileretaining their respective advantages. There have alreadybeen reported several photo-controlled/living radical poly-merization systems. Examples include the photo-controlledradical polymerization using bisacrylphosphine oxide andbisacrylgermanium photoinitiators [21], photoiniferter poly-merization [22–26], photoiniferter-combined RAFT [27,28], photo-induced ATRP [29], UV-induced ITP [30], andphoto-NMP [31]. In particular, the photo-NMP using2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) as themediator has been reported in recent years to produce poly-mers with the comparatively narrow molecular weight

E. Yoshida (*)Department of Environmental and Life Sciences,Toyohashi University of Technology,1-1 Hibarigaoka, Tempaku-cho,Toyohashi, Aichi 441-8580, Japane-mail: eyoshida@ens.tut.ac.jp

Colloid Polym Sci (2012) 290:1087–1091DOI 10.1007/s00396-012-2668-0

distributions of ca. 1.4 even at a high conversion [32–45].For industrial applications, like photolithography, in whichthe processes of the polymerization and shaping are simul-taneously required, an inert atmospheric polymerization ismore preferred than a vacuum polymerization. It was foundthat the photo-NMP in inert atmospheres more rapidly pro-ceeded to produce a polymer with a narrower molecularweight distribution than the vacuum photo-NMP. This shortcommunication describes the photo-NMP of methylmethacrylate (MMA) in inert atmospheres using the 4-methoxy-TEMPO (MTEMPO) mediator.

Experimental

Instrumentation The photopolymerization was carried outusing an Ushio optical modulex BA-H502, an illuminatorOPM2-502H with a high-illumination lens UI-OP2SL, anda 500-W super high-pressure UV lamp (USH-500SC2,Ushio Co. Ltd.). Gel permeation chromatography (GPC)was performed using a Tosoh GPC-8020 instrumentequipped with a DP-8020 dual pump, a CO-8020 columnoven, and an RI-8020 refractometer. Three polystyrene gelcolumns, Tosoh TSKGEL G2000HXL, G4000HXL, andG6000HXL, were used with tetrahydrofuran as the eluentat 40 °C. 1H NMR measurements were conducted using aJeol ECS400 FT NMR spectrometer.

Materials MTEMPO was prepared as reported previously[46]. (2RS,2′RS)-Azobis(4-methoxy-2,4-dimethylvaleroni-trile) (r-AMDV) was obtained by separation from a mixtureof the racemic and meso forms of 2,2′-azobis(4-methoxy-2,4-dimethylvaleronitrile) [47]. Commercial-grade MMAand isopropyl methacrylate (iPMA) were washed with5 wt% sodium hydroxide solution and water and distilledover calcium hydride. (4-tert-Butylphenyl)diphenylsulfo-nium triflate (tBuS) was purchased from Sigma-Aldrichand was used without further purification. Extrapure Argas with over 99.999 vol% purity and N2 gas with over99.9995 vol% were purchased from Taiyo Nippon SansoCorporation and used as received.

Photopolymerization: general procedure MMA (936.0 mg,9.35 mmol), r-AMDV (14.0 mg, 0.0454 mmol), MTEMPO(9.0 mg, 0.0483 mmol), and tBuS (12.0 mg, 0.0256 mmol)were placed in an ampul. The contents in the ampul weredegassed several times using a freeze–pump–thaw cycle andwere charged with argon. The bulk polymerization wascarried out in the Ar atmosphere at room temperature for5.5 h at 2,620,000 lx with irradiation by reflective lightusing a mirror with a 500-W high-pressure mercury lamp.The product was dissolved in dichloromethane (10 mL).The solution was concentrated by an evaporator to remove

the dichloromethane and unreacted monomer and wasfreeze-dried with benzene (20 mL) at 40 °C to obtain theproduct as white powder (658.8 mg). The monomer conver-sion was estimated gravimetrically. The product was dis-solved in dichloromethane (5 mL) and poured into hexane(500 mL). The precipitate was collected by filtration anddried in vacuo for several hours to be subjected to GPCanalysis.

Block copolymerization of MMA with iPMA MMA(936.0 mg, 9.35 mmol), r-AMDV (14.0 mg, 0.0454 mmol),MTEMPO (9.0 mg, 0.0483 mmol), and tBuS (12.0 mg,0.0256 mmol) were placed in an ampul. The contents inthe ampul were degassed several times using a freeze–pump–thaw cycle and were charged with argon. The bulkpolymerization was carried out in the Ar atmosphere atroom temperature for 3 h at 2,620,000 lx with irradiationby reflective light using a mirror with a 500-W high-pressure mercury lamp. The product was dissolved in iPMA(4 mL) degassed by a freeze–pump–thaw cycle and chargedwith argon. Part of the mixture (1 mL) was withdrawn usinga syringe to determine the molecular weight of the poly(MMA) (PMMA) prepolymer. The solution containing theprepolymer was freeze-dried with benzene (10 mL) at 40 °Cto obtain the prepolymer as white powder (112.3 mg). Theampul containing the mixture of which the part was with-drawn was degassed several times by a freeze–pump–thawcycle and was charged with argon. The block copolymeri-zation was carried out at room temperature for 11 h at2,620,000 lx with the irradiation. The resulting mass wasdissolved in dichloromethane (10 mL). The solution wasconcentrated by an evaporator to remove the dichlorome-thane and unreacted monomer and was freeze-dried withbenzene (30 mL) at 40 °C to obtain the product as whitepowder (2.1351 g). The product was dissolved in dichloro-methane (10 mL) and poured into hexane (500 mL). Theprecipitate was collected by filtration and dried in vacuofor several hours to be subjected to GPC and 1H NMRanalyses.

Results and discussion

The photoradical polymerization of MMA was performedusing the r-AMDV initiator and the MTEMPO mediator inthe presence of tBuS. The bulk polymerization was carried outin inert atmospheres at room temperature with irradiation. Theresults are shown in Table 1. The uncontrolled polymerizationwithout MTEMPO in an Ar atmosphere rapidly occurred toyield a polymer with a broad molecular weight distribution aswell as the polymerization in vacuo [33]. On the other hand,the MTEMPO-mediated polymerization produced a polymerwith a comparatively narrow molecular weight distribution of

1088 Colloid Polym Sci (2012) 290:1087–1091

Mw/Mn <1.4 in Ar. This Ar atmospheric polymerization wasaccelerated by tBuS, although the molecular weight distribu-tion was slightly broadened. The molecular weight of theresulting polymer was in good agreement with the theoreticalvalue calculated on the basis of the initial concentration ofMTEMPO and the monomer conversion, since the propagat-ing chain end is always accompanied by MTEMPO. The N2

atmospheric polymerization provided results similar to thoseby the Ar atmospheric polymerization, regarding the mono-mer conversion and molecular weight distribution; however,there was a slight difference in the molecular weight betweenthe experimental and theoretical. It was found that these inertatmospheric polymerizations more rapidly proceeded to pro-vide narrower molecular weight distributions than the vacuumpolymerization. This acceleration of the polymerization in theinert gases should be attributed to an increase in the diffusionrate as the pressure increases because the increase in thediffusion rate enhances the propagation rate by suppressingthe coupling between the propagating chain end andMTEMPO [48].

In order to confirm the living nature of the inert atmosphericpolymerization, the first-order time–conversion plots wereevaluated. Figure 1 shows the time–conversion and its first-order plots for the polymerization in Ar at the 1.06 MTEMPO/r-AMDVmolar ratio and 0.53 of tBuS/MTEMPO. [M] denotesthe monomer concentration. The conversion reached its maxi-mum at ca. 70 % around 6 h due to the bulk polymerization.However, the first-order conversion plots, ln([M]0/[M]t),showed a linear increase with time, suggesting the constantnumber of polymer chains throughout the course of the poly-merization. This consistent number of polymer chains impliesthe living nature of the MTEMPO-mediated polymerization inthe Ar atmosphere.

The correlation between the monomer conversion and themolecular weight of the resulting polymer verified the liv-ingness of the polymerization. Figure 2 shows the plots ofthe molecular weight and its distribution vs. the conversion.The molecular weight increased with the increasing conver-sion and was in good agreement with the theoretical

molecular weight. The molecular weight distributionremained constant at around 1.4 during the polymerization.The GPC profiles of the polymers for each conversion alsosupported the living mechanism of the polymerization(Fig. 3). As the conversion increased, the curves wereshifted to the higher molecular weight side without deacti-vation of the propagating chain ends.

The block copolymerization revealed that the stability ofthe propagating chain ends was dependent on the poly-merization time of the first monomer MMA. Figure 4shows the GPC profiles of the polymers obtained by theblock copolymerization of iPMA as the second mono-mer using the PMMA prepolymer prepared by the Ar

1.5

10

Con

vers

ion

(%)

ln([

M] 0

/[M

] t)

Time (h)86420

0

20

40

60

0.0

80

0.5

1.0

100

Fig. 1 The time–conversion plots and its first-order plots for theMTEMPO-mediated photopolymerization of MMA in Ar. MTEMPO/r-AMDV01.06, tBuS/MTEMPO00.53

Table 1 The photopolymerization of MMA

[MTEMPO]0 (mM) [tBuS]0 (mM) Atmosphere Time (h) Conversion (%) Mna (theor) Mnb (obs) Mw/Mnb

– – Ar 3 100 20,600c 46,800 6.78

48.3 – Ar 5.5 47 9,110 10,400 1.37

48.3 25.6 Ar 5.5 68 13,500 13,900 1.47

48.3 25.6 N2 5.5 68 13,500 15,500 1.41

48.3 25.6 In vacuo 5.5 63 12,200 12,800 1.59

[r-AMDV]0045.4 mMaCalculated on the basis of [MTEMPO]0b Estimated by GPC based on PMMA standardsc Calculated on the basis of [r-AMDV]0

Colloid Polym Sci (2012) 290:1087–1091 1089

atmospheric polymerization at different polymerizationtimes for the MTEMPO/r-AMDV of 1.06 and tBuS/MTEMPO of 0.53. The deactivation of the prepolymerswas detected in the curves of the copolymers for the 4-and 5.5-h polymerizations of MMA. The deactivationshould be based on the disproportionation terminationat the propagating chain ends, the disproportionationtermination that often occurs in the end stage of the

vacuum polymerization [40]. The degree of the deactiva-tion decreased as a result of shortening the polymerizationtime of MMA, and it was almost completely excluded for the3-h polymerization. The molecular weight and its distributionof the block copolymer obtained using the PMMA prepoly-mer prepared by the 3-h polymerization wereMn046,300 andMw/Mn01.77, while those of the prepolymer were Mn09,170 and Mw/Mn01.37, respectively. The absolute molecu-lar weight of the poly(iPMA) (PiPMA) block was estimated by1H NMR to be Mn025,000 based on the signal intensity at4.88 ppm for the methine protons of the propyl ester groupsfor the PiPMA block and 3.60 ppm for the methyl esterprotons of the PMMA block. The total molecular weight ofthe PiPMA-block-PMMAwas Mn034,200.

Conclusion

The photo-controlled/living radical polymerization of MMAusing the MTEMPO mediator was attained in the inertatmospheres. This inert atmospheric polymerization morerapidly proceeded to produce a polymer with a molecularweight distribution narrower than that of the vacuum poly-merization. The living nature of the inert atmospheric poly-merization was confirmed on the basis of linear correlationsfor the first-order time–conversion plots and conversion–molecular weight plots. The molecular weights of the result-ing PMMA were in good agreement with the theoreticalmolecular weights. The deactivation of the propagatingchain ends occurred during the polymerization; however, itwas suppressed by shortening the polymerization time of thefirst monomer. The establishment of the photo-NMP in theinert atmospheres should expand the scopes of the molecu-lar design and industrial applications.

Retention time (min)

18 26242220

Prepolymer

Fig. 4 GPC profiles of the polymers obtained by the block copoly-merization of iPMA using the PMMA prepolymer prepared by the Aratmospheric polymerization at different polymerization times: 5.5, 4,and 3 h from the top

Mn

Mw

/Mn

20000

100806040

Conversion (%)

2001.0

1.2

1.4

1.6

0

1.8

10000

15000

2.0

5000

Mn(theor)

Fig. 2 The plots of the molecular weight and its distribution vs. theconversion for the MTEMPO-mediated photopolymerization of MMAin Ar. [MTEMPO]0048.3 mM, MTEMPO/r-AMDV01.06, tBuS/MTEMPO00.53

Retention time (min)

2826242220

Fig. 3 GPC profiles of thePMMA for each conversion:25 % (1.75 h), 44 % (3 h),59 % (4 h), and 68 % (5.5 h)from the right

1090 Colloid Polym Sci (2012) 290:1087–1091

References

1. Werne TA, Germack DS, Hagberg EC, Sheares VV, Hawker CJ,Carter KR (2003) J Am Chem Soc 125:3831

2. Teare DOH, Schofield WCE, Garrod RP, Badyal JPS (2005)Langmuir 21:10818

3. Liu Y, Klep V, Luzinov I (2006) J Am Chem Soc 128:81064. Zhou F, Zheng Z, Yu B, Liu W, Huck WTS (2006) J Am Chem Soc

128:162535. Scales CW, Huang F, Li N, Vasilieva YA, Ray J, Convertine AJ,

McCormick CL (2006) Macromolecules 39:68716. Giacomelli C, Schmidt V, Borsali R (2007)Macromolecules 40:21487. Ting SRS, Min EH, Escalé P, Save M, Billon L, Stenzel MH

(2009) Macromolecules 42:94228. Saleh N, Phenrat T, Sirk K, Dufour B, Ok J, Sarbu T, Matyjaszewski

K, Tilton RD, Lowry GV (2005) Nano Lett 5:24899. Pyun J, Matyjaszewski K (2001) Chem Mater 13:3436

10. Andruzzi L, Hexemer A, Li X, Ober CK, Kramer EJ, Galli G,Chiellini E, Fischer DA (2004) Langmuir 20:10498

11. Idota N, Nagase K, Tanaka K, Okano T, Annaka M (2010) Lang-muir 26:17781

12. Saito J, Kawahara N, Matsuo S, Kaneko H, Matsugi T, Kashiwa N(2009) ACS Symposium Series, volume 1023, Controlled/LivingRadical Polymerization: Progress in ATRP, Chapter 25: 373

13. Zhang F, Shi ZL, Chua PH, Kang ET, Neoh KG (2007) Ind EngChem Res 46:9077

14. Matyjaszewski K, Xia J (2001) Chem Rev 101:292115. Moad G, Rizzardo E, Thang SH (2008) Acc Chem Res 41:113316. Hawker CJ, Bosman AW, Harth E (2001) Chem Rev 101:366117. Otsu T (2000) J Polym Sci Part A Polym Chem 38:212118. Tatemoto M (1988) European Patent 27269819. Tatemoto M (1992) Koubunnshirombunshu 49:77320. David G, Boyer C, Tonnar J, Ameduri B, Lacroix-Desmazes P,

Boutevin B (2006) Chem Rev 106:393621. Kruger K, Tauer K, Yagci Y, Moszner N (2011) Macromolecules

44:9539

22. Otsu T, Kuriyama A (1984) Polym Bull 11:13523. Doi T, Matsumoto A, Otsu T (1994) J Polym Sci Part A Polym

Chem 32:224124. Lalevee J, Allonas X, Fouassier JP (2006) Macromolecules

39:821625. You Y, Hong C, Bai R, Pan C, Wang J (2002) Macromol Chem

Phys 203:47726. Shin SE, Shin Y, Jun JW, Lee K, Jung H, Choe S (2003) Macro-

molecules 36:799427. Tasdelen MA, Durmaz YY, Karagoz B, Bicak N, Yagci Y (2007) J

Polym Sci Part A Polym Chem 46:338728. Yin H, Zheng H, Lu L, Liu P, Cai Y (2007) J Polym Sci Part A

Polym Chem 45:509129. Tasdelen MA, Uygun M, Yagci Y (2010) Macromol Chem Phys

211:227130. Ueda N, KamigaitoM, SawamotoM (1997) Polym Prepr Jpn 46:14931. Guillaneuf Y, Bertin D, Gigmes D, Versace D, Lalevee J, Fouassier

J (2010) Macromolecules 43:220432. Yoshida E (2008) Colloid Polym Sci 286:166333. Yoshida E (2009) Colloid Polym Sci 287:76734. Yoshida E (2009) Colloid Polym Sci 287:141735. Yoshida E (2010) Colloid Polym Sci 288:736. Yoshida E (2010) Colloid Polym Sci 288:7337. Yoshida E (2010) Colloid Polym Sci 288:23938. Yoshida E (2010) Colloid Polym Sci 288:34139. Yoshida E (2010) Colloid Polym Sci 288:90140. Yoshida E (2010) Colloid Polym Sci 288:102741. Yoshida E (2010) Colloid Polym Sci 288:163942. Yoshida E (2010) Colloid Polym Sci 288:174543. Yoshida E (2011) Colloid Polym Sci 289:83744. Yoshida E (2011) Colloid Polym Sci 289:112745. Yoshida E (2011) Colloid Polym Sci 289:162546. Miyazawa T, Endo T, Shiihashi S, Ogawara M (1985) J Org Chem

50:133247. Kita Y, Gotanda K, Murata K, Suemura M, Sano A, Yamaguchi T,

Oka M, Matsugi M (1998) Org Process Res Dev 2:25048. Nicholson AE, Norrish RGW (1956) Disc Faraday Soc 22:104

Colloid Polym Sci (2012) 290:1087–1091 1091