Synthesis of Well-defined High Molecular Weight 3-Arm Star Poly(methyl methacrylate) Using the “Core-First” Method by Atom Transfer Radical Polymerization

Masud, Roushown Ali and Tariqul Hasan Department of Chemistry, University of Rajshahi, Rajshahi, Bangladesh-6205

Abstract- A trifunctional initiator tris(ethyl 2-bromo-2-methyl propionate)amine (TrisE) was prepared by esterification of triethanolamine with 2-Bromo-2-methylpropionyl bromide. 3-arm star poly(methyl methacrylate) (PMMA) with a narrow molecular weight distribution (MWD = 1.34-1.43) was successfully synthesized using TrisE and CuBr/bipyridine catalytic system by atom transfer radical polymerization (ATRP). In the catalytic system of TrisE/CuBr/Bipyridine =1:3:6, the better catalytic activity was observed at 80 °C than that of observed at 60 °C or 70 °C. The star poly(methyl methacrylate) with hightest molecular weight and narrow MWD was also obtained at 80 0C than those of the PMMA obtained at 60 °C and 70 °C. The polymer yield and the molecular weight of the polymers were increased with increasing the concentration of MMA. 1H NMR analysis PMMA indicated that the polymerization reaction was initiated with TrisE.

Key words- Atom transfer radical polymerization, Star polymer, Trifunctional initiator.

1. INTRODUCTION

Much interest has been directed to investigate the syntheses and properties of well-defined macromolecules with complex architectures [1]. High-molecular-weight polymers with controlled architectures are desirable for applications such as rheology modifications [2-6], as well as control of crystallization characteristics, morphology development, and mechanical performance [3,7-9]. Star polymers are one example of macromolecules with precisely controlled architecture containing several linear polymer chains connected at one central core [10, 11]. Their three dimensional (3D) globular compact structure results in a unique set of physical properties, such as high functionality and low viscosity as compared to their linear analogues with similar molecular weight. This generates several potential applications for star polymers, including drug delivery, cosmetics, coatings, membrane, or lithography [12-15]. In principle, two basically different approaches are employed in the preparation of star polymers: one is by using coupling reactions or employing a linking agent (arm-first method), and the other is the core-first approach. The core-first approach in which branches grow from a predesigned multifunctional core/initiator (Scheme 1) seems to be advantageous, since no removal of linear arms from the product as in the arm-first method is necessary.

Scheme 1. Synthesis of 3-arm star polymer by “core-first” method.

Since well-defined star polymers cannot be prepared by means of conventional free radical polymerization, a large variety of star polymers has been synthesized by ionic polymerization procedures

[16]. However, these approaches are only applicable for a limited number of monomers and are sensitive to impurities. During the past couple of years, several procedures for the controlled or “living” radical polymerization have been developed and employed to prepare polymers with complex macromolecular architectures [17]. Among these methods, atom transfer radical polymerization (ATRP) is of considerable efficiency [18]. Most of the employed initiators are based on multifunctional inorganic or organic compounds with halogen or hydroxyl groups, in which latter is initially converted to chloro- or bromoesters. Three-arm star polymers have been prepared by ATRP employing polyols or phenols [19-21]. Most recently, a phenol also formed the basis of three-arm star molecular brushes with poly(n-butyl acrylate) side chains [22]. Amphilic poly(methacrylte) three-arm star block copolymer [23] based on phenols and miktoarm PS[24] (by hybrid techniques) have also been fabricated by ATRP. Four-arm stars have similarly been constructed employing either polyol- or cyclosiloxane-based initiators [20] or calix[4]arenes [25,26] and by reversible addition fragmentation chain transfer (RAFT) polymerization by using polyols or tetrabromomethyl benzene initially [27]. Five-arm PS and poly(methyl methacrylate) PMMA have been synthesized from a glucose-derived initiator [28]. The first six-arm PS prepared by ATRP in 1995 was based on hexakis (chloromethyl)benzene [29]. Later cyclotriphosphazene [20], calix[6]arenes [25,26] or hexahydroxytriphenylene [30] has been employed for both PS as well as poly((meth)acrylates) [25,26] stars; eight-arm PMMA stars have also been prepared from a sucrose-based initiator [31]. Twelve-arm PMMA stars have been synthesized by using ATRP initiated by a dendrimer-like twelve-arm initiator ester [32,33]. Additionally, various aromatic multisulfonyl chloride initiators [34] or metal complexes with polymeric ligands [35] have been employed to produce numerous star polymers.

In this contribution, a new trifunctional initiator tris(ethyl 2-bromo-2-methylpropionate)amine (TrisE) was synthesized and well-defined high molecular weight 3-arm star poly(methyl methacrylate) was prepared using TrisE by ATRP method. The effect of temperatures and initiator/monomer ratio on the polymerization was investigated.

2. EXPERIMENTAL 2.1. Materials 2-Bromo-2-methyl-propionyl bromide was purchased from Sigma-Aldrich and used without further purification. Tetrahydrofuran (THF) was purified by distillation followed by refluxing with dried CaH2 for 8 hours and stored under nitrogen gas. Triethanol amine was obtained from Sigma-Aldrich and used without further purification and triethylamine was purchased from Fisher Scientific and it was purified by distillation and stored under nitrogen gas. Methyl methacrylate (MMA) was purchased from Sigma-Aldrich. MMA was passed through a column of activated Al2O3, stirred with CaH2, and distilled under reduced pressure to remove inhibitor. Finally it was stored at 0 °C under nitrogen prior to use. CuBr was purified by

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ISBN 978-984-33-8940--4

recrystalization in methanol and washing with ether. Bipyridine was purchased from Sigma-Aldrich and used without further purification. 2.2. Preparation of tris(ethyl 2-bromo-2-methylpropionate)amine (TrisE) Triethanolamine (2.5 g, 16.76 mmol) and triethylamine (30 mL) were added to 100 mL dry THF. 2-Bromo-2-methylpropionyl bromide (6.2 mL, 11.56 g, 50.28 mmol) was added drop-wise to this mixture over 30 minute at 0 °C in inert atmosphere. After stirring for a further 4 hours, the triethylammonium hydrobromide salt was removed by filtration and the resulting clear solution was concentrated under vacuum before washing with 0.1M Na2CO4 solution. The product was extracted three times with dichloromethane and the combined organic extract was dried with magnesium sulfate. Removal of the solvent under vacuum afforded a dark-reddish brown oil residue. The residue was separated by (silica gel) column chromatograpy (EtOAc/ Petroleum ether = 1 : 8). The purity of the separated product was investigated by TLC using same solvent mixture. The expected tris(ethyl 2-bromo-2-methyl propionate)amine (TrisE) was obtained as yellowish viscous oil followed by evaporation of solvent from each eluent under vacuum. Finally the structure of TrisE was confirmed by 1H NMR analysis. 1H NMR (CDCl3): 2.90 ppm [s, 18H, {-C(Br)(CH3)2}3], 2.95 ppm [triplet, 6H, (N-CH2-CH2O-)3], 4.23 ppm [triplet, 6H, (N-CH2-CH2O-C=O)3] 2.3. Polymerization method Polymerization was carried out in a 25 mL Schelnk tube with magnetic stirrer in nitrogen atmosphere. The reactor was charged with prescribed amount of CuBr, bipyridine and a tiny magnetic capsule. Three cycles of vacuum-evacuation of reactor and fill-up with nitrogen gas were performed, and the reactor then sealed with rubber septum. A required amount of degassed methyl methacrylate and initiator TrisE were added with syringe. The reactor was placed in an oil bath to keep desired temperature by tuning thermostat and the reaction mixture was stirred for certain time using magnetic stirrer. At certain interval, the polymerization was stopped by added methanol followed by cooling the tube into ice-water and the polymer was precipitated in methanol by stirring for overnight. The polymers obtained were filtered, adequately washed with methanol and dried under vacuum at 60 °C for 6 hrs.

2.4. Analytical method Molecular weights (Mn) and molecular weight distribution (Mw/Mn) of polymer were measured by Toyo Soda HLC-802; Column GMH6 × 2 + G40000H8; eluent, CDCl3 as solvent and calibrated by polystyrene standards. NMR spectra of polymers were recorded at room temperature on a BRUKER 500 spectrometer operated at 500 MHz in pulse Fourier Transform mode with chloroform-d as solvent. The peak of chloroform-d (7.27 ppm for 1H) was used as internal reference. 3. RESULTS AND DISCUSSION Preparation of Trifunctional Initiator The preparation of α-bromoester initiator tris(ethyl 2-bromo-2-methyl propionate)amine (TrisE) was conducted by the esterification of triethanolamine using more than three equivalent of 2-bromo-2-methylpropionyl bromide according to the procedure described in

experimental section (Scheme 2). The expected product TrisE was separated by (silica gel) column chromatography (EtOAc/ Petroleum ether = 1 : 8) and the higher Rf value calculated from TLC plate was 0.80. Finally, the structure of the products TrisE was confirmed by 1H NMR analysis. The 1H NMR spectrum of TrisE initiator was displayed in figure 1. In the 1H NMR spectrum, two triplets appeared at 2.95 ppm 4.23 ppm for (N-CH2-CH2O-)3 and (N-CH2-CH2O-C=O)3 protons respectively. The singlet at 1.90 ppm was assignable for –C(Br)(CH3)2 proton. TrisE was obtained as major product and trace amount of di-ester and mono-ester were obtained.

NOH

HO

OH

NO

O

O

OBr

OBr

O

Br

Br

O

Br

THF,Et3N

NOH

O

O

OBr

O

Br

NOH

HO

OO

Br

+

Triethanolamine 2-Bromo-2-methylpropionyl bromide

Mono-esterDi-esterTri-ester (TrisE)

+ +

Scheme 2: Preparation of functional initiator.

Figure 1. 1H NMR spectrum of TrisE

ATRP of Methyl methacrylate using TrisE Initiator The synthesized trifunctional initiator, Tris(ethyl 2-bromo-2-methylpropionate)amine was employed in ATRP of MMA at various conditions. The principle route for preparation of 3-arm star PMMA by ATRP was shown in scheme 3. The effect of temperature on polymerization of MMA with TrisE/CuBr/Bipy (1:3:6) was investigated at various temperatures (60, 70 and 80 °C) under nitrogen atmosphere. The results obtained are listed in table 1. The yield of polymers was increased with raising the temperature of the polymerization.

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ISBN 978-984-33-8940--4

NO

O

O

O

O

O

Br

Br

Br

O O

O O

OO

NO

O

O

Br

O

O

O

Br

Br

CuBr/bipyridine

n

n

n

MMA

3-arm star PMMATrisE

Scheme 3. Synthesis of 3-Arm Star Poly(methyl methacrylate), PMMA

The molecular weight of the polymers obtained was measured with gel permeation chromatography (GPC) and the GPC curves obtained are displayed in figure 2. The polymers obtained with this system showed high molecular weight and narrow molecular weight distribution (Mw/Mn < 1.5). The highest catalytic activity and the PMMA with highest molecular weight and narrowest MWD were observed at 80 °C than those observed at 60 and 70 °C. Table 1: Effect of the temperature on polymerization of MMA with CuBr/Bipy as catalytic system and TrisE as initiator.

Entry Temp. (0C)

Yield (g)

bMn cMw

dMw/ Mn

01 60 0.562 47,389 67,996 1.43 02 70 0.779 95,456 131,890 1.38 03 80 1.49 232,561 311,744 1.34

aPolymerization conditions; TrisE/CuBr/Bipyridine =1:3:6, Time = 40 minutes. bNumber average molecular weight, cweight average molecular weight, and dmolecular weight distribution were measured by GPC analysis.

The effect of monomer concentration on polymerization was investigated by the polymerization of MMA at two different ratios of MMA and TrisE, where the concentration of TrisE was constant. The results obtained were listed in Table 2. The molecular weight of the polymers obtained was measured with gel permeation chromatography (GPC) and the GPC curves obtained are displayed in figure 3. Since catalyst, initiator and temperature are same in both the cases (entry 4 and 5); the number of initiating species might be same. The yield and the molecular weight of the PMMA were increased with increasing of the concentration of monomer. These results suggested that the rate of propagation is increased with increasing the concentration of monomer.

Table 2: Effect of the Ratio of MMA/TrisE on polymerization of methyl methacrylate (MMA)

Entry MMA

(mmol) MMA/TrisE

Yield (g)

bMn cMw

dMw/Mn

04 18.697 1000:1 0.779 95,456 131,890 1.38

05 37.395 2000:1 0.910 110,842 153,194 1.38

aPolymerization conditions; TrisE/CuBr/Bipyridine =1:3:6, Time = 40 minutes. bNumber average molecular weight, cweight average molecular weight, and dmolecular weight distribution were measured by GPC analysis.

[mV] [LogM]

[min]

-10.000

-5.000

0.000

5.000

0.000 10.000 20.000 30.000 40.000 50.0001

2

3

4

5

6

7

8

9

10

1 /

25.3

10 /

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保持時間ピークトップ分子量

[mV] [LogM]

[min]

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0.000

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3

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1 / 2

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122247 ﾋﾟｰｸNo

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[mV] [LogM]

[min]

-10.000

-5.000

0.000

0.000 10.000 20.000 30.000 40.000 50.0001

2

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4

5

6

7

8

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10

1 /

22.7

42 /

269138 ﾋﾟｰｸNo

保持時間ピークトップ分子量

Figure 2. The GPC curves of (A) entry 1, (B) entry 2, (C) entry 3. The blue curve represents for mV of the sample Vs min whereas the black curve for logM Vs min.

[mV] [LogM]

[min]

-10.000

0.000

0.000 10.000 20.000 30.000 40.000 50.0001

2

3

4

5

6

7

8

9

101 / 2

3.9

90 / 122247 ﾋﾟｰｸNo

保持時間ピークトップ分子量

[mV] [LogM]

[min]

-10.000

0.000

0.000 10.000 20.000 30.000 40.000 50.0001

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3

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5

6

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9

10

1 / 23.6

90 /

147064 ﾋﾟｰｸNo

保持時間ピークトップ分子量

Figure 3. The GPC curves of , (D) entry 4 and (E) entry 5 respectively. The blue curve represents for mV of the sample Vs min whereas the black curve for logM Vs min.

A

B

C

D

E

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Analysis of the structure of the poly(methyl methacrylate) obtained: The PMMA obtained was characterized by 1H NMR analysis. In the 1H NMR spectrum of PMMA (Figure 4), the signals at 0.84-1.01 ppm for methyl protones denoted as a, 1.21-1.64 ppm for methylene protones denoted as b and 3.41-3.78 ppm for methoxy protones denoted as c are assigned. These results clearly indicates the presence of –C(CH3)(COOCH3) segment of poly(methyl methacrylate).

CH2 C

CH3

C

OCH3

O

CDCl3

n

a

b

c

a

b

c

c

Figure 4. 1H NMR spectrum of star PMMA.

4. CONCLUSIONS

A new Trifunctional initiator was prepared by the reaction of triethanolamine and 2-bromo-2-methylpropionate, which was purified by column chromatography and characterized by NMR analysis. TrisE as initiator gave star PMMA with neat methyl methacrylate in a controlled manner by ATRP at 60-80 0C by using the catalytic system CuBr and bipyridine. The molecular weights and the yield of star polymers increase with increasing temperature of polymerization. A good catalytic activity was observed at 80 0C (the lowest PDI). The ratio of MMA and TrisE significantly affects on the polymerization. The linear increase of molecular weight of the polymer against MMA/TrisE ratio was observed. The star polymers formed had high molecular weights and narrow molecular weight distributions. It is expected that the obtained high molecular weight star polymers will be useful in rheological applications.

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