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Preparation of dendron-like polystyrenes from atom transfer radical polymerization (ATRP) and direct chain-end functionalization
Reactive & Functional Polymers 69 (2009) 424–428
Contents lists available at ScienceDirect
Reactive & Functional Polymers journal homepage: www.elsevier.com/locate/react
Preparation of dendron-like polystyrenes from atom transfer radical polymerization (ATRP) and direct chain-end functionalization Ying-Ling Liu *, Meng-Hsin Chen, Keh-Ying Hsu Department of Chemical Engineering and R&D Center for Membrane Technology, Chung Yuan University, Chungli, Taoyuan 32023, Taiwan
a r t i c l e
i n f o
Article history: Available online 5 April 2009 Keywords: Atom transfer radical polymerization ATRP Dendron-like
a b s t r a c t Dendron-like branched polystyrenes are prepared using atom transfer radical polymerization (ATRP) and chain-end functionalization. The growing chain ends of polystyrene from ATRP are end-capped with 1,4benzoquinone to incorporate branching points and ATRP initiating groups to the chain ends. Sequential ATRP of styrene initiated from the chain ends builds up the polystyrene arms. The number-averaged molecular weights of the first, second, and third generations of dendron-like polystyrene polymers calculated from their respective 1H NMR spectra are 5960, 6850, and 9150 g/mol, respectively. The NMR-data are consistent with the molecular weights determined by GPC. An approach to prepare dendron-like polymers through ATRP and chain-end functionalization is developed. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Dendrimers have received much attention in recent years as they possess highly ordered, regularly branched, and globular structures on the nanometer scale and exhibit unique properties [1–4]. Preparation of dendrimers, however, usually requires repeated protection, condensation, and de-protection. The tedious cycles required for preparation motivates the development of dendron-like polymers, which contain large dendritic structures and can be obtained by much simple means [5–25]. Controlled living polymerization is a useful technique to prepare dendron-like polymers. A common divergent approach is to endcap the living polymerization chains by incorporating a di-functional moiety to create branching points and then to chemically modify the chain ends to introduce the initiating sites of polymerization for growth of the next generation. The Gnanou group prepared dendron-like poly(ethylene oxide) by anionic polymerization [7], poly(ethylene oxide)/polystyrene dendron-like block copolymers by atom transfer radical polymerization (ATRP) and anionic polymerization [8,10], and dendron-like polystyrene by ATRP [14]. The divergent approach was also used by Hedrick et al. [5,6] and Perce et al. [9] to prepare dendron-like polyesters and their copolymers as well as polymethacrylate, respectively. Hirao et al. [20] reported the preparation of dendritic poly(methyl methacrylate) through an interactive divergent approach. Alternatively, anionic polymerization and coupling reactions were used
by Hadjichristidis et al. to prepare dendritic polybutadienes using a convergent method [22]. ATRP is a convenient approach to prepare well-defined polymers. The critical step of applying ATRP to the preparation of dendron-like polymers is the incorporation of two or more ATRP initiating groups to the polymer chain ends [14]. In this work, we report a facile approach to perform this critical step and to simplify the preparation of dendron-like polymers. The divergent methodology was applied to prepare dendron-like polystyrene by means of chain-end functionalization and ATRP. Benzoquinone, which is a radical trap, is used to react with the ATRP chain ends to endcap and to provide branching points for ATRP growing chain ends in one step [26]. Dendron-like polystyrenes are consequently obtained by repeated incorporation of ATRP initiators followed by ATRP of styrene. The prepared dendron-like polystyrenes are characterized by infrared and 1H NMR spectroscopy. 2. Experimental 2.1. Materials Styrene (purchased from Acros Organics Co.) was distilled out under reduced pressure prior to use. 1-Bromoethylbenzene, copper (I) chloride (CuCl), bipyridine (bPy), 1,4-benzoquinone (BQ), and 2bromo-isobutyryl bromide (BIBB) were purchased from Aldrich Chemical Co. and used as received. 2.2. Characterization
* Corresponding author. Tel.: +886 3 2654130; fax: +886 3 2654199. E-mail address: [email protected] (Y.-L. Liu). 1381-5148/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.reactfunctpolym.2009.03.014
Fourier transform infrared (FTIR) spectra were measured with a Perkin–Elmer Spectrum One FTIR spectrometer. 1H NMR spectra
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2.3. Preparation of end-capped polystyrene Styrene (9 g, 86.5 mmol), copper (I) bromide (0.1 g, 0.7 mmol), and bipyridine (0.33 g, 2.1 mmol) were added to a round-bottom flask. The system was purged with argon for 15 min, and then 0.13 g of 1-bromoethylbenzene (0.7 mmol) was added. The reaction mixture was de-gassed with three freeze-pump-thaw cycles and then reacted at 80 °C for 17 h, after which BQ (2.5 g, 23 mmol) was added and the reaction mixture was kept at 80 °C for another 12 h. The product was solubilized by the addition of 20 mL of THF. The product was then precipitated with methanol and collected
CH3 H C Br
PS
Transmittance (%)
were recorded with a Brüker MSL-300 (300 MHz) NMR spectrometer using CDCl3 as a solvent. The molecular weights of polymers were determined by gel permeation chromatography (GPC) using a Lab Alliance Series III pump, an RI 2000 refractive index detector, and a Plgel Mixed-D column. Tetrahydrofuran (THF) was used as the mobile phase at a flow rate of 1.0 mL/min and with an oven of 40 °C. Monodispered polystyrenes of various molecular weights were used as standards.
PS-Br2
-CH3
C=O -1 1742 cm
-1
1372 cm
PS-(PSBQ)2 -CH3 -1
4000
3500
3000
1800 1600 1400 1200 1000
O
O CH3 Br C O C Br CH3
O
CH3 H C
CHCH2
OH n HO
PS-BQ
triethylamine
Br O CH3 O C O C Br n CH3 O CH3 O C O C Br CH3
ATRP CH=CH2 O
O Br
CH3 O O C O C CH3
Br CH3 H C
CHCH2
OH
CHCH2 m HO
n CH3 O O C O C CH3
Br OH
CHCH2 m HO
(1) CH3 O Br C O C Br CH3
PS-(PSBQ)2 (2) CH=CH2
ATRP
Br CH3 O O C O C CH3
Br CH3 H C
CHCH2
CHCH2 m
n O
CH3 O C O C CH3
400
Fig. 2. FTIR spectra of polystyrenes. The BIBB absorption peaks appeared with polystyrene after BIBB end-capping. The observed characteristic absorptions of BIBB decreased in intensity with PS–(PSBQ)2.
CuBr, 2,2-bipyridine
PS-Br2
600
Wavelength (cm )
80 oC, 17 h
CHCH2
800
-1
+
CH3 H C
C-Br -1 622 cm
C-O-C -1 1103 cm
1372 cm
Br
CH=CH2
C-Br -1 622 cm
C-O-C -1 1103 cm
Br
CH3 O O C O C CH3
CH3 O O C O C CH3 CH3 O O C O C CH3
CHCH2 m
CH3 O O C O C CH3
CHCH2
CHCH2
CHCH2
CHCH2
Br k
Br k
Br k
Br k
PS-PS2-PS4
Fig. 1. Preparation of dendron-like polystyrenes using BQ as a di-functional end-capping agent.
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with filtration. The solubilization in THF and reprecipitation with methanol was repeated three times, and the resultant polymer was washed with methanol and hot water several times to remove any adsorbed compounds, and then dried under vacuum to give BQ capped PS–BQ (6.8 g, yield: 76 wt%).
2.4. Incorporation of ATRP initiators to PS–BQ chain end In a 100 mL round-bottom flask, PS–BQ (4.0 g) was dissolved in 60 mL of dry THF. After the addition of 18 g of triethylamine and 6 g of BIBB, the reaction mixture was kept at room temperature
Fig. 3. 1H NMR spectra of end-functionalized polystyrenes.
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427
for 16 h with stirring. The product was then precipitated with methanol and collected with filtration. After repetition of dissolution in THF and precipitation with methanol two additional times, the polymer was washed with methanol and hot water, and then dried under vacuum to give the product of PS–Br2 (3.8 g). 2.5. Preparation of the second generation of polystyrene In a similar procedure for preparation of PS–BQ, the second generation of polystyrene (PS–(PSBQ)2) was obtained by using PS–Br2 to replace 1-bromoethylbenzene in the reaction system. 2.6. Preparation of the third generation of polystyrene PS-(PSBQ)2 was reacted with BIBB in the same procedure used for the preparation of PS–Br2 to give the product of PS–PS2–Br4. The third generation of polystyrene was prepared with a similar procedure for the preparation of the second generation of polymer, but BQ was not added to the reaction after styrene polymerization. Therefore, the obtained third generation polystyrene (PS–PS2–PS4) is Br-terminated.
3. Results and discussion In the ATRP reaction of styrene, the growing PS chain ends could react with another styrene monomer to extend the chain length through the radical addition. The growing chain ends should also be reactive toward a radical-capturing molecule to terminate the chain-growing reaction. Consequently, the polymer chains were end-capped with radical-capturing molecules. In this work, BQ was used as the end-capping agent. Reaction of BQ with the PS chain ends in the styrene ATRP system resulted in PS chains containing dihydroxylbenzene end groups (PS–BQ). As shown in Fig. 1, further reaction of PS–BQ with BIBB incorporates two 2-bromoisobutyrate groups, which can serve as ATRP initiating sites, to PS chain ends (PS–Br2). The obtained PS–Br2 is a bifunctional macro-initiator for ATRP. Sequential ATRP of styrene using PS–Br2 as an initiator results in a 3-armed star-like polystyrene. Here, we prepared a 3-armed star-like polystyrene with BQ end groups PS– (PSBQ)2. PS–(PSBQ)2 is a second generation (G2) of dendron-like polystyrene. By repeating the end-modification with BIBB on PS(PSBQ)2 and ATRP of styrene, the third generation (G3) of dendron-like polystyrene (PS–PS2–PS4) could be prepared. Fig. 2 shows the FTIR spectra of PS, PS–Br2, and PS-(PSBQ)2 to demonstrate the successful end-capping and functionalization on the PS chains. The 2-bromoisobutyrate groups at the ends of the PS–Br2 chains, which serve as ATRP initiating sites for sequential styrene polymerization, were characterized by the absorption peaks at 1742 (–C@O stretch), 1372 (–CH3 asymmetric stretch), 1103 (–C–O– stretch), and 622 (C–Br) cm 1. The absorptions did not appear in the FTIR spectrum of the un-capped polystyrene PS–BQ. A decrease in the relative intensities of the absorptions of the 2-bromoisobutyrate groups appears with the FTIR spectrum of PS–(PSBQ)2, indicating the increase in the PS chain lengths of PS–(PSBQ)2 compared to that of PS–Br2. The end-capping of BIBB also brings bromide atoms to PS chains, resulting in the increase in the bromide/carbon atomic ratios from 0.04% (measured with the un-capped polystyrene) to 0.052% (measured with BIBB-caped PS–Br2). Moreover, the increase in the oxygen atom content of PS–Br2 also supports the success of BIBB end-capping on the PS–BQ chains. Therefore, the performance of the BQ and BIBB end-capping on the PS chain ends is also supported with the oxygen signals detected in SEM-EXD analysis of PS–BQ and PS–Br2. The chemical structures of the first generation (PS–Br2), second generation (PS–(PSBQ)2), and third generation (PS–PS2–PS4) den-
Fig. 4. GPC chromatograms of polystyrenes of different generations.
dron-like PSs were also characterized by 1H NMR (Fig. 3). PS chains presented broad resonances at about d = 6.35–7.00 ppm (aromatic protons), 1.41–1.45 ppm (–CH2–CH–), and 1.82 ppm (–CH2–CH–). After chain-end functionalization with BIBB, the resonance arising from the methyl groups of BIBB appeared at 1.94 ppm in the 1H NMR spectrum of PS–Br2. As the PS chain lengths increase in PS– (PSBQ)2, the intensity of the methyl resonance decreases. Since the chain ends of PS–PS2–PS4 are not functionalized with BIBB, the peak intensity of the methyl groups of BIBB decreases further. The molecular weights and polydispersity indexes (PDI) of the polymers were measured by GPC. As shown in Fig. 4, PS–Br2 showed a number-averaged molecular weight (Mn) of approximately 5500 g/mol and a PDI of 1.23. The molecular weights of the G2 (PS–(PSBQ)2) and G3 (PS–PS2–PS4) dendron-like PSs increased to 6450 and 8200 g/mol, respectively. The PDIs of the higher generation dendron-like PSs are still as low as approximately 1.20. The low PDI value of PS–PS2–PS4 polymer indicates that the polymer can reasonably be considered to be dendron-like. The number-averaged molecular weights of the polymers can also be obtained from their 1H MNR spectra. The calculated number-averaged molecular weights (Mn) from 1H NMR are 5960, 6850, and 9150 g/mol for the first, second, and third generations of dendron-like polystyrenes, respectively. The data are consistent with the molecular weight data obtained by GPC. The molecular weights of the obtained PSs are relatively low when compared to other previously reported dendron-like polymers. End-functionalization on ATRP polymer chains is performed through the condensation reactions of the C–Br groups. This approach requires a two-step process in which the ATRP polymer is collected and then subjected to an end-functionalizing reaction. This method is therefore limited by the incomplete conversion of the end-functionalization reaction [15]. Because ATRP is a radical polymerization, the coupling reactions utilized in conventional radical polymerization are also applicable to ATRP systems. One example of this is that ATRP chain ends show reactivity toward the surfaces of carbon nanotubes [27]. Our previous work demonstrated the utilization of BQ as a functionalization agent for polymer modification through the radical-capturing reaction [26]. Therefore, uses of BQ as an end-capping and functionalization agent in ATRP polymerization systems could directly incorporate the diol groups, which serve as a branching point to polymer chain ends. The high reactivity of BQ toward radicals also causes the high efficiency of end-capping polymers. Therefore, the synthetic route reported in this work provides numerous conveniences and advantages in the molecular design and preparation of dendron-like
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polymers. Besides dendron-like polymers, sequential ATRP of PS– Br2 by employing another monomer in polymerization is also effective for the preparation of AB2 miktoarm star block copolymers [28,29]. 4. Conclusions In conclusion, 1,4-benzoquinone is an effective end-capping agent for ATRP to incorporate a di-functional diol end to polymer chains. The chain ends can be converted to be ATRP initiators and serve as a branching point for further chain extension. Consequently dendron-like polymers can easily be prepared with this end-functionalization approach and ATRP. Acknowledgements The authors appreciate the financial support on this work from the Ministry of Education, Taiwan (The Center-of-Excellence Program 2008–2010) and from Chung Yuan Christian University (Taiwan) under the project of Toward Sustainable Green Technology (Grant No. CYCU-97-CR-CE). References [1] D.C. Tully, K. Wilder, J.M. Fréchet, A.R. Trimble, C.F. Quate, Adv. Mater. 11 (1999) 314. [2] S.M. Grayson, J.M. Fréchet, Chem. Rev. 101 (2001) 3919. [3] C.P. Chen, S.A. Dai, H.L. Chang, W.C. Su, R.J. Jeng, J. Polym. Sci. Part A: Polym. Chem. 43 (2005) 682. [4] C. Cao, D. Yan, Prog. Polym. Sci. 29 (2004) 183.
[5] M. Trollsås, J.L. Hedrick, J. Am. Chem. Soc. 120 (1998) 4644. [6] J.L. Hedrick, M. Trollsås, C.J. Hawker, B. Atthoff, H. Claesson, A. Heise, R.D. Miller, D. Mecerreyes, R. Jérôme, Ph. Dubois, Macromolecules 31 (1998) 8691. [7] S. Angot, D. Taton, Y. Gnanou, Macromolecules 33 (2000) 5418. [8] S. Hou, E.L. Chaikof, D. Taton, Y. Gnanou, Macromolecules 36 (2003) 3874. [9] V. Perce, B. Barboiu, C. Grigoras, T.K. Bera, J. Am. Chem. Soc. 125 (2003) 6503. [10] R. Fancis, D. Taton, J.L. Logan, P. Masse, Y. Gnanou, R.S. Duran, Macromolecules 36 (2003) 8253. [11] M. Trollsås, B. Atthoff, H. Claesson, J. Hedrick, J. Polym. Sci. Part A: Polym. Chem. 42 (2004) 1174. [12] A. Matsuo, T. Watanabe, A. Hirao, Macromolecules 37 (2004) 6283. [13] C.Y. Hong, Y.Z. You, J. Liu, C.Y. Pan, J. Polym. Sci. Part A: Polym. Chem. 43 (2005) 6379. [14] B. Lepoittevin, R. Matmour, R. Francis, D. Taton, Y. Gnanou, Macromolecules 38 (2005) 3120. [15] R. Matmour, B. Lepoittevin, T.J. Joncheray, R.J. El-khouri, D. Taton, R.S. Duran, Y. Gnanou, Macromolecules 38 (2005) 5459. [16] B. Luan, C.Y. Pan, Eur. Polym. J. 42 (2006) 1467. [17] X. Feng, D. Taton, R. Borsali, E.L. Chaikof, Y. Gnanou, J. Am. Chem. Soc. 128 (2006) 11551. [18] T.J. Joncheray, S.A. Bernard, R. Matmour, B. Lepoittevin, R.J. El-Khouri, D. Taton, Y. Gnanou, R.S. Duran, Langmuir 23 (2007) 2531. [19] D.M. Knauss, T. Huang, Macromolecules 36 (2003) 6036. [20] A. Hirao, A. Matsuo, T. Watanabe, Macromolecules 38 (2005) 8701. [21] I. Chalari, N. Hadjichristidis, J. Polym. Sci. Part A: Polym. Chem. 40 (2002) 1519. [22] K. Orfanou, H. Iatrou, D.J. Lohse, N. Hadjichristidis, Macromolecules 39 (2006) 4361. [23] A. Hirao, K. Sugiyama, Y. Tsunoda, A. Matsuo, T. Watanabe, J. Polym. Sci. Part A: Polym. Chem. 44 (2006) 6659. [24] N. Hadjichristidis, S. Pispas, Adv. Polym. Sci. 200 (2006) 37. [25] N. Hadjichristidis, H. Iatrou, M. Pitsikalis, J. Mays, Prog. Polym. Sci. 31 (2006) 1068. [26] Y.L. Liu, G.C. Lin, C.S. Wu, J. Polym. Sci. Part A: Polym. Chem. 45 (2007) 949. [27] Y.L. Liu, W.H. Chen, Macromolecules 40 (2007) 8881. [28] R. Francis, B. Lepoittevin, D. Taton, Y. Gnanou, Macromolecules 35 (2002) 9001. [29] A. Mavroudis, A. Avgeropoulos, N. Hadjichristidis, E.L. Thomas, D.J. Lohse, Chem. Mater. 18 (2006) 2164.
Contents lists available at ScienceDirect
Reactive & Functional Polymers journal homepage: www.elsevier.com/locate/react
Preparation of dendron-like polystyrenes from atom transfer radical polymerization (ATRP) and direct chain-end functionalization Ying-Ling Liu *, Meng-Hsin Chen, Keh-Ying Hsu Department of Chemical Engineering and R&D Center for Membrane Technology, Chung Yuan University, Chungli, Taoyuan 32023, Taiwan
a r t i c l e
i n f o
Article history: Available online 5 April 2009 Keywords: Atom transfer radical polymerization ATRP Dendron-like
a b s t r a c t Dendron-like branched polystyrenes are prepared using atom transfer radical polymerization (ATRP) and chain-end functionalization. The growing chain ends of polystyrene from ATRP are end-capped with 1,4benzoquinone to incorporate branching points and ATRP initiating groups to the chain ends. Sequential ATRP of styrene initiated from the chain ends builds up the polystyrene arms. The number-averaged molecular weights of the first, second, and third generations of dendron-like polystyrene polymers calculated from their respective 1H NMR spectra are 5960, 6850, and 9150 g/mol, respectively. The NMR-data are consistent with the molecular weights determined by GPC. An approach to prepare dendron-like polymers through ATRP and chain-end functionalization is developed. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Dendrimers have received much attention in recent years as they possess highly ordered, regularly branched, and globular structures on the nanometer scale and exhibit unique properties [1–4]. Preparation of dendrimers, however, usually requires repeated protection, condensation, and de-protection. The tedious cycles required for preparation motivates the development of dendron-like polymers, which contain large dendritic structures and can be obtained by much simple means [5–25]. Controlled living polymerization is a useful technique to prepare dendron-like polymers. A common divergent approach is to endcap the living polymerization chains by incorporating a di-functional moiety to create branching points and then to chemically modify the chain ends to introduce the initiating sites of polymerization for growth of the next generation. The Gnanou group prepared dendron-like poly(ethylene oxide) by anionic polymerization [7], poly(ethylene oxide)/polystyrene dendron-like block copolymers by atom transfer radical polymerization (ATRP) and anionic polymerization [8,10], and dendron-like polystyrene by ATRP [14]. The divergent approach was also used by Hedrick et al. [5,6] and Perce et al. [9] to prepare dendron-like polyesters and their copolymers as well as polymethacrylate, respectively. Hirao et al. [20] reported the preparation of dendritic poly(methyl methacrylate) through an interactive divergent approach. Alternatively, anionic polymerization and coupling reactions were used
by Hadjichristidis et al. to prepare dendritic polybutadienes using a convergent method [22]. ATRP is a convenient approach to prepare well-defined polymers. The critical step of applying ATRP to the preparation of dendron-like polymers is the incorporation of two or more ATRP initiating groups to the polymer chain ends [14]. In this work, we report a facile approach to perform this critical step and to simplify the preparation of dendron-like polymers. The divergent methodology was applied to prepare dendron-like polystyrene by means of chain-end functionalization and ATRP. Benzoquinone, which is a radical trap, is used to react with the ATRP chain ends to endcap and to provide branching points for ATRP growing chain ends in one step [26]. Dendron-like polystyrenes are consequently obtained by repeated incorporation of ATRP initiators followed by ATRP of styrene. The prepared dendron-like polystyrenes are characterized by infrared and 1H NMR spectroscopy. 2. Experimental 2.1. Materials Styrene (purchased from Acros Organics Co.) was distilled out under reduced pressure prior to use. 1-Bromoethylbenzene, copper (I) chloride (CuCl), bipyridine (bPy), 1,4-benzoquinone (BQ), and 2bromo-isobutyryl bromide (BIBB) were purchased from Aldrich Chemical Co. and used as received. 2.2. Characterization
* Corresponding author. Tel.: +886 3 2654130; fax: +886 3 2654199. E-mail address: [email protected] (Y.-L. Liu). 1381-5148/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.reactfunctpolym.2009.03.014
Fourier transform infrared (FTIR) spectra were measured with a Perkin–Elmer Spectrum One FTIR spectrometer. 1H NMR spectra
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2.3. Preparation of end-capped polystyrene Styrene (9 g, 86.5 mmol), copper (I) bromide (0.1 g, 0.7 mmol), and bipyridine (0.33 g, 2.1 mmol) were added to a round-bottom flask. The system was purged with argon for 15 min, and then 0.13 g of 1-bromoethylbenzene (0.7 mmol) was added. The reaction mixture was de-gassed with three freeze-pump-thaw cycles and then reacted at 80 °C for 17 h, after which BQ (2.5 g, 23 mmol) was added and the reaction mixture was kept at 80 °C for another 12 h. The product was solubilized by the addition of 20 mL of THF. The product was then precipitated with methanol and collected
CH3 H C Br
PS
Transmittance (%)
were recorded with a Brüker MSL-300 (300 MHz) NMR spectrometer using CDCl3 as a solvent. The molecular weights of polymers were determined by gel permeation chromatography (GPC) using a Lab Alliance Series III pump, an RI 2000 refractive index detector, and a Plgel Mixed-D column. Tetrahydrofuran (THF) was used as the mobile phase at a flow rate of 1.0 mL/min and with an oven of 40 °C. Monodispered polystyrenes of various molecular weights were used as standards.
PS-Br2
-CH3
C=O -1 1742 cm
-1
1372 cm
PS-(PSBQ)2 -CH3 -1
4000
3500
3000
1800 1600 1400 1200 1000
O
O CH3 Br C O C Br CH3
O
CH3 H C
CHCH2
OH n HO
PS-BQ
triethylamine
Br O CH3 O C O C Br n CH3 O CH3 O C O C Br CH3
ATRP CH=CH2 O
O Br
CH3 O O C O C CH3
Br CH3 H C
CHCH2
OH
CHCH2 m HO
n CH3 O O C O C CH3
Br OH
CHCH2 m HO
(1) CH3 O Br C O C Br CH3
PS-(PSBQ)2 (2) CH=CH2
ATRP
Br CH3 O O C O C CH3
Br CH3 H C
CHCH2
CHCH2 m
n O
CH3 O C O C CH3
400
Fig. 2. FTIR spectra of polystyrenes. The BIBB absorption peaks appeared with polystyrene after BIBB end-capping. The observed characteristic absorptions of BIBB decreased in intensity with PS–(PSBQ)2.
CuBr, 2,2-bipyridine
PS-Br2
600
Wavelength (cm )
80 oC, 17 h
CHCH2
800
-1
+
CH3 H C
C-Br -1 622 cm
C-O-C -1 1103 cm
1372 cm
Br
CH=CH2
C-Br -1 622 cm
C-O-C -1 1103 cm
Br
CH3 O O C O C CH3
CH3 O O C O C CH3 CH3 O O C O C CH3
CHCH2 m
CH3 O O C O C CH3
CHCH2
CHCH2
CHCH2
CHCH2
Br k
Br k
Br k
Br k
PS-PS2-PS4
Fig. 1. Preparation of dendron-like polystyrenes using BQ as a di-functional end-capping agent.
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with filtration. The solubilization in THF and reprecipitation with methanol was repeated three times, and the resultant polymer was washed with methanol and hot water several times to remove any adsorbed compounds, and then dried under vacuum to give BQ capped PS–BQ (6.8 g, yield: 76 wt%).
2.4. Incorporation of ATRP initiators to PS–BQ chain end In a 100 mL round-bottom flask, PS–BQ (4.0 g) was dissolved in 60 mL of dry THF. After the addition of 18 g of triethylamine and 6 g of BIBB, the reaction mixture was kept at room temperature
Fig. 3. 1H NMR spectra of end-functionalized polystyrenes.
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for 16 h with stirring. The product was then precipitated with methanol and collected with filtration. After repetition of dissolution in THF and precipitation with methanol two additional times, the polymer was washed with methanol and hot water, and then dried under vacuum to give the product of PS–Br2 (3.8 g). 2.5. Preparation of the second generation of polystyrene In a similar procedure for preparation of PS–BQ, the second generation of polystyrene (PS–(PSBQ)2) was obtained by using PS–Br2 to replace 1-bromoethylbenzene in the reaction system. 2.6. Preparation of the third generation of polystyrene PS-(PSBQ)2 was reacted with BIBB in the same procedure used for the preparation of PS–Br2 to give the product of PS–PS2–Br4. The third generation of polystyrene was prepared with a similar procedure for the preparation of the second generation of polymer, but BQ was not added to the reaction after styrene polymerization. Therefore, the obtained third generation polystyrene (PS–PS2–PS4) is Br-terminated.
3. Results and discussion In the ATRP reaction of styrene, the growing PS chain ends could react with another styrene monomer to extend the chain length through the radical addition. The growing chain ends should also be reactive toward a radical-capturing molecule to terminate the chain-growing reaction. Consequently, the polymer chains were end-capped with radical-capturing molecules. In this work, BQ was used as the end-capping agent. Reaction of BQ with the PS chain ends in the styrene ATRP system resulted in PS chains containing dihydroxylbenzene end groups (PS–BQ). As shown in Fig. 1, further reaction of PS–BQ with BIBB incorporates two 2-bromoisobutyrate groups, which can serve as ATRP initiating sites, to PS chain ends (PS–Br2). The obtained PS–Br2 is a bifunctional macro-initiator for ATRP. Sequential ATRP of styrene using PS–Br2 as an initiator results in a 3-armed star-like polystyrene. Here, we prepared a 3-armed star-like polystyrene with BQ end groups PS– (PSBQ)2. PS–(PSBQ)2 is a second generation (G2) of dendron-like polystyrene. By repeating the end-modification with BIBB on PS(PSBQ)2 and ATRP of styrene, the third generation (G3) of dendron-like polystyrene (PS–PS2–PS4) could be prepared. Fig. 2 shows the FTIR spectra of PS, PS–Br2, and PS-(PSBQ)2 to demonstrate the successful end-capping and functionalization on the PS chains. The 2-bromoisobutyrate groups at the ends of the PS–Br2 chains, which serve as ATRP initiating sites for sequential styrene polymerization, were characterized by the absorption peaks at 1742 (–C@O stretch), 1372 (–CH3 asymmetric stretch), 1103 (–C–O– stretch), and 622 (C–Br) cm 1. The absorptions did not appear in the FTIR spectrum of the un-capped polystyrene PS–BQ. A decrease in the relative intensities of the absorptions of the 2-bromoisobutyrate groups appears with the FTIR spectrum of PS–(PSBQ)2, indicating the increase in the PS chain lengths of PS–(PSBQ)2 compared to that of PS–Br2. The end-capping of BIBB also brings bromide atoms to PS chains, resulting in the increase in the bromide/carbon atomic ratios from 0.04% (measured with the un-capped polystyrene) to 0.052% (measured with BIBB-caped PS–Br2). Moreover, the increase in the oxygen atom content of PS–Br2 also supports the success of BIBB end-capping on the PS–BQ chains. Therefore, the performance of the BQ and BIBB end-capping on the PS chain ends is also supported with the oxygen signals detected in SEM-EXD analysis of PS–BQ and PS–Br2. The chemical structures of the first generation (PS–Br2), second generation (PS–(PSBQ)2), and third generation (PS–PS2–PS4) den-
Fig. 4. GPC chromatograms of polystyrenes of different generations.
dron-like PSs were also characterized by 1H NMR (Fig. 3). PS chains presented broad resonances at about d = 6.35–7.00 ppm (aromatic protons), 1.41–1.45 ppm (–CH2–CH–), and 1.82 ppm (–CH2–CH–). After chain-end functionalization with BIBB, the resonance arising from the methyl groups of BIBB appeared at 1.94 ppm in the 1H NMR spectrum of PS–Br2. As the PS chain lengths increase in PS– (PSBQ)2, the intensity of the methyl resonance decreases. Since the chain ends of PS–PS2–PS4 are not functionalized with BIBB, the peak intensity of the methyl groups of BIBB decreases further. The molecular weights and polydispersity indexes (PDI) of the polymers were measured by GPC. As shown in Fig. 4, PS–Br2 showed a number-averaged molecular weight (Mn) of approximately 5500 g/mol and a PDI of 1.23. The molecular weights of the G2 (PS–(PSBQ)2) and G3 (PS–PS2–PS4) dendron-like PSs increased to 6450 and 8200 g/mol, respectively. The PDIs of the higher generation dendron-like PSs are still as low as approximately 1.20. The low PDI value of PS–PS2–PS4 polymer indicates that the polymer can reasonably be considered to be dendron-like. The number-averaged molecular weights of the polymers can also be obtained from their 1H MNR spectra. The calculated number-averaged molecular weights (Mn) from 1H NMR are 5960, 6850, and 9150 g/mol for the first, second, and third generations of dendron-like polystyrenes, respectively. The data are consistent with the molecular weight data obtained by GPC. The molecular weights of the obtained PSs are relatively low when compared to other previously reported dendron-like polymers. End-functionalization on ATRP polymer chains is performed through the condensation reactions of the C–Br groups. This approach requires a two-step process in which the ATRP polymer is collected and then subjected to an end-functionalizing reaction. This method is therefore limited by the incomplete conversion of the end-functionalization reaction [15]. Because ATRP is a radical polymerization, the coupling reactions utilized in conventional radical polymerization are also applicable to ATRP systems. One example of this is that ATRP chain ends show reactivity toward the surfaces of carbon nanotubes [27]. Our previous work demonstrated the utilization of BQ as a functionalization agent for polymer modification through the radical-capturing reaction [26]. Therefore, uses of BQ as an end-capping and functionalization agent in ATRP polymerization systems could directly incorporate the diol groups, which serve as a branching point to polymer chain ends. The high reactivity of BQ toward radicals also causes the high efficiency of end-capping polymers. Therefore, the synthetic route reported in this work provides numerous conveniences and advantages in the molecular design and preparation of dendron-like
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polymers. Besides dendron-like polymers, sequential ATRP of PS– Br2 by employing another monomer in polymerization is also effective for the preparation of AB2 miktoarm star block copolymers [28,29]. 4. Conclusions In conclusion, 1,4-benzoquinone is an effective end-capping agent for ATRP to incorporate a di-functional diol end to polymer chains. The chain ends can be converted to be ATRP initiators and serve as a branching point for further chain extension. Consequently dendron-like polymers can easily be prepared with this end-functionalization approach and ATRP. Acknowledgements The authors appreciate the financial support on this work from the Ministry of Education, Taiwan (The Center-of-Excellence Program 2008–2010) and from Chung Yuan Christian University (Taiwan) under the project of Toward Sustainable Green Technology (Grant No. CYCU-97-CR-CE). References [1] D.C. Tully, K. Wilder, J.M. Fréchet, A.R. Trimble, C.F. Quate, Adv. Mater. 11 (1999) 314. [2] S.M. Grayson, J.M. Fréchet, Chem. Rev. 101 (2001) 3919. [3] C.P. Chen, S.A. Dai, H.L. Chang, W.C. Su, R.J. Jeng, J. Polym. Sci. Part A: Polym. Chem. 43 (2005) 682. [4] C. Cao, D. Yan, Prog. Polym. Sci. 29 (2004) 183.
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