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A Facile Strategy for Preparation of a-Heterobifunctional Polystyrenes with Well-Defined Molecular Weighta Yun Wang, Lu Lu, Hu Wang, Dairen Lu, Kang Tao, Ruke Bai*

A facile strategy for synthesis of a-heterobifunctional polystyrenes is reported. The novel functional polystyrenes have been successfully synthesized via a combination of atom transfer radical polymerization (ATRP) and chemical modification of end-functional groups. First, e-caprolactone end-capped polystyrenes with controlled molecular weight and low polydispersity were prepared by ATRP of styrene using a-bromo-e-caprolactone (aBrCL) as an initiator. Then, removal of the terminal bromine atom was performed with iso-propylbenzene in the presence of CuBr/ PMDETA. Finally, ring-opening modifications of the caprolactone group were carried out with amines, nbutanol and H2O to produce novel polystyrenes containing two different functional groups at one end.

Introduction In the past decade, increasing attention has been paid to the synthesis of well-defined polymers containing two functional groups at one chain end because they are very useful building blocks for the construction of complex polymer architectures.[1] Atom transfer radical polymerization (ATRP) provide a powerful tool for preparation of the bifunctional polymers.[1,2] Generally, two approaches are applied to prepare a- or v-bifunctional polymers by ATRP. The first is to transform the halogen end groups of the R. Bai, Y. Wang, L. Lu, H. Wang, D. Lu, K. Tao CAS Key Laboratory of Soft Matter Chemistry, Department of Polymer Science and Engineering, University of Science and Technology of China, Hefei 230026, People’s Republic of China Fax: 0086-551-3631760; E-mail: [email protected] a

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: Supporting information for this article is available at the bottom of the article’s abstract page, which can be accessed from the journal’s homepage at http://www.mrc-journal.de, or from the author.

Macromol. Rapid Commun. 2009, 30, 1922–1927 ß 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

polymer into functional groups after polymerization. For example, dihydroxyl[3] and diamino[4] groups have been directly introduced into the v-chain end via nucleophilic substitution with amino-diols and triamine respectively. The transformation of the terminal bromine into the azido group and the click reaction with trialkyne leads to vdialkynyl polystyrene.[5] The same strategy has also been used for the synthesis of v-hydroxyl-v’-alkynyl polystyrene via the click reaction of v-azido polystyrene and 3,5bis(propargyloxy)benzyl alcohol.[6] The second is to use a designed functional initiator, which is tolerant to ATRP catalysts and radicals, to produce a-functionalized polymers. A number of polymers with dihydroxyl,[7] dicarboxylic acid,[8] bis(4-fluorobenzoyl)[9] and bis(aromatic bromo)[10] have been prepared successfully with trifunctional initiators. We noticed that most papers deal with the synthesis of a- or v-homobifunctional polymers via the ATRP process,[3–5,7–10] but very few involve the preparation of a- or v-heterobifunctinal polymers.[6] Although some other living/controlled polymerization methods have been used for the synthesis of a- or v-heterobifunctinal

DOI: 10.1002/marc.200900454

A Facile Strategy for Preparation of a-Heterobifunctional . . .

polymers, most suffer from multistep synthesis of trifunctional initiators or multistep transformation of the end groups.[11] Herein, we present a facile strategy for the preparation of a-heterobifunctional polymers via a combination of ATRP and chemical modification of end-functional groups. Cyclic lactones are versatile intermediates due to their high reactivity with N- and O-nucleophiles. As a consequence, the cleavage of the lactone ring is a commonly used process in the synthesis of bifunctional compounds. Therefore cyclic lactones can be used as reactive terminal groups by incorporation into the polymer chain end. Je´roˆme et al.[11d] reported the transformation of e-caprolactone endcapped poly(ethylene oxide) into a-hydroxyl-a’-carboxylate poly(ethylene oxide) via hydrolysis, and then the hydrolyzed poly(ethylene oxide) was used as a double head macroinitiator for the sequential ring-opening polymerization of benzyl b-malolactonate and e-caprolactone to prepare ABC miktoarm star polymer. However, there are very few papers involved in the investigation of cyclic lactone derivatives as initiators for living/controlled radical polymerization. Matyjaszewski et al.[12] used a-bromo-gbutyrolactone as an initiator for the ATRP of styrene, but no detail was given. Hedrick et al.[13] prepared an e-caprolactone end-capped polymer with g-(2-bromo-2-

methyl propionyl)-e-caprolactone as an initiator via the ATRP process, and then used the resulting polymer as a macromonomer to perform ring-opening copolymerization with e-caprolactone leading to novel graft copolymers. In this paper, a-bromo-e-caprolactone (aBrCL) has been studied as an initiator for ATRP of styrene in detail and e-caprolactone end-capped polystyrene has been obtained. Moreover, the ring-opening modifications of the terminal caprolactone group have been investigated with different nucleophiles and novel a-heterobifunctional polystyrenes have been prepared, as shown in Scheme 1.

Experimental Part Materials Styrene (Shanghai Chemical Reagent Co., Ltd) was passed through a column of neutral alumina to remove the inhibitor. aBrCL was synthesized according to the literature.[14] N,N,N0 ,N00 ,N00 pentamethyldiethylenetriamine (PMDETA; 99%, Aldrich), 3-chloroperoxybenzoic acid (m-CPBA; 85%, Changzhou Baokang Pharmaceutical and Chemical Co., Ltd), N,N-dimethyl-1,3-propanediamine (DPA; 98%, Acros), and all other reagents were used as received without further purification.

Characterization NMR spectra were recorded on a Bruker DPX400 spectrometer operating at 400 MHz for 1 H and 100 MHz for 13C, using CDCl3 as a lock solvent and TMS as a standard. Infrared spectra were recorded on a Bruker Vector-22 IR spectrometer by the KBr pellet technique. Molecular weight (Mn ) and molecular weight distribution (Mw =Mn ) were determined on a Waters 150C gel permeation chromatograph ˚ Waters (GPC) equipped with 103, 104 and 105 A Ultrastyragel columns connected to a Waters refractive index detector, using tetrahydrofuran (THF) as the eluent with a flow rate of 1.0 mL min1 (30 8C), and the calibration was carried out with polystyrene standards.

Atom Transfer Radical Polymerization of Styrene

Scheme 1. Synthesis (a) and ring-opening modifications (b) of CLPSt. Macromol. Rapid Commun. 2009, 30, 1922–1927 ß 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Typically, CuBr (21.6 mg, 0.15 103 mol), 2,20 bipyridine (bpy) (70.3 mg, 0.45 103 mol), aBrCL (29.0 mg, 0.15 103 mol) and styrene (2.468 g, 23.7 103 mol) were added to a 5 mL polymerization tube equipped with a stirrer bar. After degassed by performing a freezeevacuate-thaw cycle three times under a nitrogen atmosphere, the tube was sealed under a vacuum and immersed in a 110 8C oil

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Y. Wang, L. Lu, H. Wang, D. Lu, K. Tao, R. Bai

of NaOH (0.05 mL, [NaOH] ¼ 0.1 mol L1) (CLPSt:NaOH:H2O ¼ 1:10:280 (molar ratio)) and the mixture stirred at 60 8C for 24 h. After neutralization with 1 M HCl (40 8C, overnight), the reaction mixture was concentrated and precipitated into methanol. The hydrolyzed product was obtained by filtration and dried under a vacuum.

bath for a prescribed time. The polymerization was stopped by cooling in an ice-water bath. After opening the tubes, samples were taken and conversions were determined using 1H NMR spectroscopy. The reaction mixture was diluted with THF, and passed through an alumina column to remove the copper complex. a-(eCaprolactone)-v-bromopolystyrene (CLPStBr) 2 was obtained by precipitation into methanol followed by filtration and drying under a vacuum.

Results and Discussion The ATRP of styrene was conducted in bulk using CuBr/bpy as catalyst at 110 8C and the theoretical molecular weights were calculated using Equation (1) (see Supporting Information). The polymerization results are summarized in Table 1. It shows that the molecular weights of the polystyrenes are well controlled and their molecular weight distributions are narrow. Moreover, we can see that the molecular weights obtained from GPC are close to the theoretical values and also agree well with those calculated from the 1H NMR spectra. A linear semi-logarithmic plot (Figure 1(a)) of monomer conversion vs. time shows a first-order kinetic correlation. The linear increase of molecular weight with respect to conversion and the low polydispersity indices (PDIs) can be observed throughout the polymerization (Figure 1(b)). All of the evidence indicates that the polymerization is a wellcontrolled process. The structure of the designed polymer was confirmed by 1H NMR spectroscopy. In a representative 1 H NMR spectrum of CLPStBr (Figure 2(a)), the signals at 4.05 and 3.76 ppm are characteristic for the methylene protons (Ha0 and Ha00 ) adjacent to the ester oxygen atom of the ecaprolactone group and the signal centered at 4.45 ppm is attributed to the methine proton (Hb) next to a bromine atom. Since the v-terminated bromine atoms of the polymer can react with nucleophiles such as primary amines,[15] it

Aminolysis of e-Caprolactone End-Capped Polystyrene The terminal bromine was removed using iso-propylbenzene as transfer agent to afford e-caprolactone end-capped polystyrene (CLPSt) (see Supporting Information). CLPSt (50 mg, 0.01 103 mol) was dissolved in neat amine (1.0 mL) or dissolved in solvent ([CLPSt] ¼ 0.01 mol L1) followed by addition of a 10-fold excess of amine (0.1 mol L1), and the mixture was stirred at the prescribed temperature. Then the reaction mixture was diluted with THF, and precipitated into methanol. The resulting polymer was collected by filtration and dried under a vacuum.

Alcoholysis of e-Caprolactone End-Capped Polystyrene To a solution of CLPSt (50 mg, 0.01 103 mol) and n-butanol (1.0 mL, 16.6 103 mol) in 1 mL of toluene ([CLPSt] ¼ 0.005 mol L1, [n-butanol] ¼ 8.3 mol L1, CLPSt:n-butanol ¼ 1:1660 (molar ratio)) was added one drop of concentrated H2SO4 and the mixture was stirred at 60 8C for 48 h. Then the mixture was precipitated into methanol and the product was obtained by filtration and dried under a vacuum.

Hydrolysis of e-Caprolactone End-Capped Polystyrene To a solution of CLPSt (50 mg, 0.01 103 mol) in 1 mL of 1,4dioxane ([CLPSt] ¼ 0.01 mol L1) was added a 2 M aqueous solution

Table 1. The results for ATRP of styrene with aBrCL in bulk at 110 8C.

Entry

Time

Conversiona)

M n;th b)

Mn;NMR c)

h

%

g mol1

g mol1

GPC Mn

Mw =M n

g mol1 1

1.5

13

2 360

2 320

2 310

1.10

2

3.0

31

5 290

5 390

5 180

1.15

3

4.5

45

7 580

7 320

7 410

1.10

4

6.0

51

8 570

8 510

8 410

1.09

5

7.5

60

10 060

10 230

9 790

1.09

6

9.0

63

10 690

10 950

10 500

1.06

a)

Calculated on the basis of 1H NMR spectra; b)Calculated according to Equation (1) (see Supporting Information); c)Calculated according to Equation (2) (see Supporting Information).

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DOI: 10.1002/marc.200900454


Figure 1. (a) Conversion and ln([M]0/[M]t) as a function of polymerization time. (b) Molecular weight and molecular weight distribution as a function of monomer conversion. Kinetic plot derived from 1H NMR spectroscopy, molecular weight and PDI determined from GPC. Reaction conditions: styrene:aBrCL: CuBr:bpy ¼ 158:1:1:3 (molar ratio), 110 8C.

should be removed before performing the ring-opening modification of the a-chain-end caprolactone group. The debromination was conducted using molar ratios of 1/3/3/100 for CLPStBr/CuBr/PMDETA/iso-propylbenzene at 90 8C for 2.5 h (Scheme 1(a)). In the 1H NMR spectrum (Figure 2(b)), the disappearance of the peaks at 4.45 ppm corresponding to the CH-Br proton indicates that the bromine atoms are completely removed. Modifications of the caprolactone end groups were carried out with amines, alcohol and H2O under homogeneous conditions (Scheme 1(b)) and the results are presented in Table 2. The ring cleavage reactions with amines were performed without any catalyst. We found that the reaction of CLPSt with DPA was more efficient in neat DPA than in solvent (Table 2, entries 1–4). Moreover, the aminolysis of the caprolactone group with piperidine required a higher temperature and longer time than with DPA (Table 2, entry 5); this may be due to the steric hindrance of secondary amine. Unfortunately the terminal Macromol. Rapid Commun. 2009, 30, 1922–1927 ß 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

caprolactone did not react with aniline owing to the low reactivity of the aromatic amines (Table 2, entry 6). The substitution products were analyzed using 1H NMR spectroscopy. For example, the full conversion of the aminolysis of CLPSt with DPA was confirmed by the disappearance of the signals at 4.05 and 3.76 ppm (Figure 2(c)) and the appearance of signals at 3.53 ppm corresponding to the methylene protons (Ha) adjacent to hydroxyl group. It was also confirmed by the presence of signals at 5.88–6.19, 2.80–3.40, 2.28 and 2.18 ppm corresponding to the protons (Hc, Hd, He, and Hf) from the amide moiety. Furthermore, in the 13C NMR spectrum of 4 (Figure S2(b) in the Supporting Information), it can be observed that the signals of the caprolactone group disappear with the appearance of the corresponding signals of hydroxylamide. In addition, the FT-IR spectrum (Figure S1(b) in the Supporting Information) exhibited a new peak at 1 668 cm1, which corresponds to carbonyl stretching of the amide group, with the disappearance of absorption band at 1 738 cm1 ascribed to the cyclic ester group (Figure S1(a) in Supporting Information). 1H NMR and IR spectra of the product from the reaction of CLPSt with piperidine are showed in Figure 2(d) and Figure S1(c) (in the Supporting Information), which demonstrate the successful functionalization. The alcoholysis of CLPSt was examined with n-butanol in toluene in the presence of concentrated H2SO4 (Table 2, entry 7) and completion of the reaction was confirmed by 1 H NMR spectroscopy. In Figure 2(e), we can observe that the signals at 4.05 and 3.76 ppm attributed to lactone protons disappear with the appearance of the signals at 3.53 ppm ascribed to the hydroxyl group (Ha). The peaks at 3.90 and 0.92 ppm correspond to methylene protons (Hh) adjacent to ester group and methyl protons (Hi) from the butyl moiety, respectively. Moreover, the IR spectrum shows that the carbonyl stretching band shifts from 1 738 cm1 to 1 728 cm1 (Figure S1(d) in Supporting Information), which corresponds to the cyclic ester group and acyclic ester group respectively. As water is a poor nucleophile, lactones do not undergo hydrolysis unless catalysts are present. Base-catalyzed hydrolysis of CLPSt was performed with 0.1 M NaOH in 1, 4dioxane (Table 2, entry 8) followed by acidification. In the 1 H NMR spectrum of the purified product (Figure 2(f)), the appearance of the peaks of the terminal hydroxyl at 3.53 ppm and the disappearance of the peaks at 4.05 and 3.76 ppm indicate the complete hydrolysis of the caprolactone groups. The reaction is further confirmed by the IR spectrum with the disappearance of the absorption band at 1 738 cm1 which belonged to the cyclic ester group after hydrolysis (Figure S1(e) in the Supporting Information), as well as the appearance of two new bands at 1 743 and 1 704 cm1 ascribed to the carboxyl group after acidification (Figure S1(f) in Supporting Information).

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Y. Wang, L. Lu, H. Wang, D. Lu, K. Tao, R. Bai

Figure 2. 1H NMR spectra in CDCl3 of 2 (Mn ¼ 5 330 g mol1, Mw =Mn ¼ 1.16) (a), 3 (b), 4 (c), 5 (d), 6 (e) and 7 (f).

Conclusion We have developed a facile and versatile strategy for the synthesis of a-heterobifunctional polystyrenes through the ring-opening modifications of e-caprolactone end-capped polystyrene. Well-defined e-caprolactone end-capped polystyrenes have been prepared by ATRP of styrene using

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a-bromo-e-caprolactone as the initiator, and then several new a-heterobifunctional polystyrenes have been obtained via ring-opening modifications of the terminal caprolactone with amines, alcohol and water respectively. This research demonstrates that e-caprolactone end-capped polystyrene is a very useful precursor for the synthesis of a-heterobifunctional polystyrenes. We expect that this

DOI: 10.1002/marc.200900454


Table 2. Ring-opening modifications of CLPSt (3) with nucleophiles (Nu). [CLPSt] ¼ 0.01 mol L1.

Entry

Nu

[Nu]

[3]:[Nu]

Solvent

mol L1 1

DPA

12.0

1:1200

2

DPA

0.1

1:10

3

DPA

0.1

4

DPA

0.1

5

piperidine

6

aniline

c)

7

8e)

n-BuOH H2O

d)

Temperature

Time

Conversiona)

-C

h

%

none

25

48

100

toluene

110

96

35

1:10

THF

66

72

33

1:10

DMFb)

60

48

12

14.4

1:1440

none

60

96

100

10.5

1:1050

none

80

48

0

8.3

1: 1660

toluene

60

48

100

2.8

1: 280

1,4-dioxane

60

24

100

a)

Calculated on the basis of 1H NMR spectra; b)DMF ¼ N,N-dimethylformamide; c)Under the catalysis of concentrated H2SO4, [CLPSt] ¼ 0.005 mol L1; d)n-BuOH ¼ n-butanol; e)Under the catalysis of aqueous NaOH.

approach will become a promising platform for the preparation of various a-heterobifunctional polystyrenes which can be used to prepare miktoarm star copolymers, functional macromonomers and so on. Further research is currently in progress.

[4] [5] [6]

Acknowledgements: The authors are grateful for financial support from the National Natural Science Foundation of China (No. 20474059) and Ministry of Science and Technology of China (No. 2007CB936401).

[7]

[8] [9] Received: July 3, 2009; Published online: August 27, 2009; DOI: 10.1002/marc.200900454 Keywords: atom transfer radical polymerization (ATRP); initiators; polystyrene (PS); synthesis

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