SHORT COMMUNICATION

Photo-controlled/living radical polymerization of tert-butylmethacrylate in the presence of a photo-acid generatorusing a nitroxide mediator

Eri Yoshida

Received: 21 November 2011 /Revised: 13 January 2012 /Accepted: 3 February 2012 /Published online: 19 February 2012# Springer-Verlag 2012

Abstract The photo-controlled/living radical polymeriza-tion of tert-butyl methacrylate was performed using a(2RS,2′RS)-azobis(4-methoxy-2,4-dimethylvaleronitrile)initiator and a 4-methoxy-2,2,6,6-tetramethylpiperidine-1-oxyl (MTEMPO) mediator in the presence of a (4-tert-butylphenyl)diphenylsulfonium triflate photo-acid gener-ator. The bulk polymerization was carried out at 25 °Cby irradiation with a high-pressure mercury lamp. Where-as the polymerization in the absence of MTEMPO pro-duced a broad molecular weight distribution, theMTEMPO-mediated polymerization provided a polymerwith a comparatively narrow molecular weight distribu-tion around 1.4 without elimination of the tert-butylgroups. The living nature of the polymerization wasconfirmed on the basis of the linear correlations for thefirst-order time–conversion plots and conversion–molec-ular weight plots in the range below 50% conversion.The block copolymerization with methyl methacrylatealso supported the livingness of the polymerization basedon no deactivation of the prepolymer.

Keywords Photo-controlled/living radical polymerization .

tert-Butyl methacrylate . Nitroxide . 4-Methoxy-2,2,6,6-tetramethylpiperidine-1-oxyl . Photo-acid generator .

(4-tert-Butylphenyl)diphenylsulfonium triflate .

Molecular weight control . Block copolymerization

Introduction

Photopolymerization is an energy-saving process that uti-lizes solar energy and is also superior in local applications.Photo-controlled/living polymerization that is able to con-trol the molecular weight of a polymer is significant fromboth the viewpoints of the environmental conservation andcreation of materials with a controlled structure. In recentyears, the photo-controlled/living radical polymerization hasbeen established using a nitroxide mediator in the presenceof a photo-acid generator (PAG), such as diaryliodoniumhexafluorophosphate [1–7], triarylsulfonium triflate [8–13],and an iron–arene complex [14]. These PAGs serve as anaccelerator of the polymerization of methyl methacrylatethrough the electron transfer interaction with the mediator.

tert-Butyl methacrylate (TBMA) has often been used toprepare materials for photoresists. Some copolymers con-taining the TBMA segments were synthesized for thepurpose of making photoresists. Examples include ran-dom copolymers of adamantanemethyl methacrylate orisobornyl methacrylate with TBMA [15], random andblock copolymers of [3-(methacryloxy)-propyl]pentame-thyldisiloxane with TBMA [16], and poly(methacyloxy-propylpolyhedral oligomeric silsesquioxane-ran-TBMA)[17]. During the photoresist process, the TBMA segmentsare converted to the methacrylic acid segments by the tert-butyl elimination catalyzed by the PAG, resulting in formationof patterns. It was also confirmed that the tert-butyl groupswere easily eliminated by irradiation with a high-pressuremercury lamp [18]. The use of polymers with strictly con-trolled molecular weights in the photoresist system is impor-tant to obtain sharp patterns.

It was found that the nitroxide-mediated photoradicalpolymerization of TBMA in the presence of the PAGproceeded in accordance with a living mechanism to

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

Colloid Polym Sci (2012) 290:661–665DOI 10.1007/s00396-012-2605-2

produce a polymer with a comparatively narrow molecularweight distribution without the PAG-catalyzed elimination ofthe tert-butyl groups. This short communication describes thephoto-controlled/living radical polymerization of TBMA us-ing 4-methoxy-2,2,6,6-tetramethylpiperidiine-1-oxyl(MTEMPO) as the mediator and (4-tert-butylphenyl)diphe-nylsulfonium triflate (tBuS) as the PAG.

Experimental

Instrumentation

The photopolymerization was carried out using an Ushiooptical modulex BA-H502, an illuminator OPM2-502H

with a high-illumination lens UI-OP2SL, and a 500-W superhigh-pressure UV lamp (USH-500SC2, Ushio Co. Ltd.).Gel permeation chromatography (GPC) was performedusing a Tosoh GPC-8020 instrument equipped with aDP-8020 dual pump, a CO-8020 column oven, and aRI-8020 refractometer. Three polystyrene gel columns,Tosoh TSKGEL G2000HXL, G4000HXL, and G6000HXL,were used with THF as the eluent at 40 °C.

Materials

(2RS,2′RS)-Azobis(4-methoxy-2,4-dimethylvaleronitrile)(r-AMDV) was obtained by separation from a mixture of theracemic and meso forms of 2,2′-azobis(4-methoxy-2,4-dimethylvaleronitrile) [19]. MTEMPO was prepared as

Table 1 The photopolymeriza-tion of TBMA by r-AMDV

[r-AMDV]0045.4 mMaEstimated by GPC based onpoly(MMA) standards

MTEMPO (mM) tBuS (mM) CH3CN (mL) Time (h) Conversion (%) Mna Mw/Mna

– – – 1 100 27,700 6.50

48.3 – – 13 51 9,350 1.45

48.3 25.6 – 6.25 55 7,700 1.44

48.3 25.6 0.2 6.25 58 8,450 1.49

r-AMDV

N

CN

CN

N

O

O CN

OCOO

h

N

O

O

CN

O

COO

N

O

O

CN

O

COO

N OO

S+ CF3SO3

S+

CF3SO3

CN

O

COO

N

O

O

*

N OO

TBMA

CN

O

COO

N

O

O

n

MTEMPO

TBMA

tBuS

CN

O

COOH

N

O

O

+

Scheme 1 The mechanism of the MTEMPO-mediated photopolymerization of TBMA

662 Colloid Polym Sci (2012) 290:661–665

reported previously [20]. TBMAwas distilled over calciumhydride. Methyl methacrylate (MMA) was washed with5 wt.% sodium hydroxide solution and water and thendistilled over calcium hydride. tBuS was purchased fromSigma-Aldrich and used as received.

Photopolymerization: general procedure

TBMA (875.0 mg, 6.15 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 ampoule.After degassing the contents, the ampoule was sealedunder vacuum. The polymerization was carried out at25 °C for 3.5 h with irradiation by reflective light usinga mirror with a 500-W high-pressure mercury lamp.Dichloromethane (10 mL) was added to the product todissolve it. The solution was concentrated by an evaporator toremove the dichloromethane and unreacted monomer. Theresidue was freeze-dried with benzene (10 mL) in an oil bathat 60 °C to obtain a polymer (439.0 mg). The conversion wasestimated gravimetrically.

Block copolymerization of TBMA with MMA

TBMA (875.0 mg, 6.15 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 ampoule. After

degassing the contents, the ampoule was sealed undervacuum. The polymerization was carried out at 25 °Cfor 3.5 h with irradiation. The product was dissolved inMMA (4 mL) degassed by bubbling nitrogen for 20 min.After the product was completely dissolved in theMMA, part of the mixture (1 mL) was withdrawn usinga syringe to determine the molecular weight of thepoly(TBMA) (PTBMA) prepolymer. The solution con-taining the prepolymer was freeze-dried with benzene(10 mL) in an oil bath at 60 °C to obtain the prepol-ymer (106.8 mg). The ampoule containing the residualmixture was sealed under vacuum, after degassing thecontents. The block copolymerization was carried out at25 °C for 13.5 h with the irradiation. The product wasdissolved in dichloromethane (10 mL). The resulting polymerwas isolated by precipitation from dichloromethane intohexane. The precipitates were collected by filtration andwere freeze-dried with benzene (30 mL) in an oil bath at60 °C to obtain the block copolymer (1.9348 g).

Results and discussion

The photoradical polymerization of TBMA was performedusing r-AMDVas the initiator. The bulk polymerization wascarried out at 25 °C by irradiation with a high-pressure

5

Con

vers

ion

(%)

Time (h)

80

60

40

20

043210

ln([

M] 0/[

M] t)

0.8

0.6

0.4

0.2

0

1.0

Fig. 1 The time–conversion and its first-order plots for the TBMApolymerization. MTEMPO/r-AMDV01.06, tBuS/MTEMPO00.53

Mw

/Mn

Conversion (%)

2.0

1.6

1.4

1.2

1.0806040200

MnX

10-3

8

6

4

2

0

10

1.8

12

Theoretical

Fig. 2 The plots of the molecular weight and its distribution vs. theconversion for the TBMA polymerization. MTEMPO/r-AMDV01.06,tBuS/MTEMPO00.53

Colloid Polym Sci (2012) 290:661–665 663

mercury lamp. The results are shown in Table 1. The poly-merization in the absence of MTEMPO was completed in1 h to produce a polymer with a broad molecular weightdistribution. On the other hand, the polymerization in itspresence took a much longer time to reach a 50% conversion;however, the polymerization mediated by MTEMPO pro-vided a comparatively narrow molecular weight distribu-tion. tBuS accelerated the polymerization without anincrease in the molecular weight distribution in spite ofthe fact that tBuS was insoluble in the monomer. Thisacceleration of the polymerization by tBuS implies thattBuS served as the electron transfer photosensitizer for

MTEMPO, rather than as the photo-acid generator of thecatalyst for the elimination of the tert-butyl groups. Theplausible mechanism proposed for this MTEMPO-mediated photopolymerization based on the previousstudy on the MMA polymerization [4, 14] is shown inScheme 1. The presence of a small amount of acetonitrileadded to the system in order to dissolve tBuS had noinfluence on both the polymerization rate and molecularweight distribution. The polymerization rate was inde-pendent of the tBuS solubility.

The living nature of the polymerization mediated byMTEMPO was explored in the presence of tBuS. Figure 1shows the plots of the time–conversion and its first-orderconversion, ln([M]0/[M]t), for the polymerization. [M]denotes the monomer concentration. The conversion beganto reach its maximum in 5 h due to the bulk polymerization.However, the ln([M]0/[M]t) exhibited a linear increase up to5 h, suggesting that the number of polymer chains wasconstant throughout the course of the polymerization. Therelationship between the conversion and molecular weightrevealed the livingness of the polymerization. The plots ofthe molecular weight and its distribution vs. the conversionare shown in Fig. 2. The experimental molecular weightswere in close agreement with the theoretical values, al-though those may involve difference due to the estimationbased on poly(MMA) standards. The theoretical molecularweights were calculated using the monomer conversion and

2016 2824Retention time (min)

PrepolymerBlockcopolymer

Fig. 3 The GPC profiles of the PTBMA prepolymer and PTBMA-b-PMMA block copolymer

TMSCHCl3

01234568 7ppm

Prepolymer

Block copolymer

Fig. 4 The 1H NMR spectra ofthe prepolymer and the blockcopolymer. Solvent: CDCl3

664 Colloid Polym Sci (2012) 290:661–665

the molar ratio of the monomer to MTEMPO because thepropagating chain ends are controlled by MTEMPO. Themolecular weight linearly increased with an increase inthe conversion up to 50%, but thereafter deviated. Thepolymerization proceeded by a living mechanism up to50%. The molecular weight distributions were 1.4 or lessthroughout the polymerization.

The block copolymerization with MMA also supportedthe living nature of the polymerization. The TBMA poly-merization carried out for 3.5 h reached a 47% conversionand produced PTBMA with Mn08,410 and Mw/Mn01.34.The block copolymerization with MMA was performed for13.5 h using the PTBMA as the prepolymer. The GPCprofiles of the prepolymer and block copolymer are shownin Fig. 3. The curve for the block copolymer was completelyshifted to the higher molecular weight side and contained noprepolymer, indicating no deactivation of the prepolymer.Therefore, it can be deduced that the MTEMPO-mediatedpolymerization of TBMA proceeded by a living mechanism.The molecular weight and its distribution of the blockcopolymer were estimated by GPC to be Mn057,700 andMw/Mn01.85, respectively. However, there is doubt that thismolecular weight was overestimated because the peak startwas out of the measurable range of the columns. Accordingly,the exact molecular weight of the block copolymer was esti-mated by 1HNMR. Figure 4 shows the 1HNMR spectra of thePTBMA prepolymer and the PTBMA-b-PMMA block copol-ymer. For the spectrum of the prepolymer, the α-methyl andmethylene proton signals were observed at 0.8–1.3 and 1.7–2.3 ppm, respectively. A sharp signal of the tert-butyl protonswas observed at 1.41 ppm. The integral intensity ratio of thesesignals was in good agreement with their proton ratio, indi-cating that no elimination of the tert-butyl groups occurredduring the polymerization. These signals were also discernedfor the spectrum of the block copolymer; signals of the α-methyl and methylene protons in the main chain were ob-served at 0.6–1.3 and 1.7–2.2 ppm, and the tert-butyl protonsignal was observed at 1.46 ppm. In addition to these signals,the methyl proton signal for the ester groups of the PMMAblocks was observed at 3.70 ppm. Themolecular weight of thePMMA block was estimated to be Mn034,900 based on themolecular weight of the PTBMA prepolymer by GPC and onthe signal intensity ratio of the methyl ester protons to otherprotons containing the tert-butyl, α-methyl, and methylene

protons. The total molecular weight of PTBMA-b-PMMAwas determined to be Mn043,300 by 1H NMR.

Conclusion

The photo-controlled/living radical polymerization ofTBMA was attained using the MTEMPO mediator in thepresence of tBuS. tBuS promoted the polymerization rate inspite of its insolubility in TBMA. It was found that tBuSserved as the electron transfer photosensitizer forMTEMPO, rather than as the photo-acid generator of thecatalyst for the elimination of the tert-butyl groups. Theliving nature of the polymerization was confirmed on thebasis of the linear correlations for the first-order time–conversion plots and conversion–molecular weight plotsin the range below a 50% conversion. The block copo-lymerization with MMA also supported the livingness ofthe polymerization based on no deactivation of thePTBMA prepolymer.

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