Synthesis of Well-defined Azido End-functional Polystyrene via Atom Transfer Radical Polymerization
Jubaraj Chandra, Homayun Kabir, Roushown Ali and Tariqul Hasan Department of Chemistry, University of Rajshahi, Rajshahi, Bangladesh-6205 Abstract: Azide end- functional initiator 1- azidoethane-2-oxy-
2- bromopropionyl, (AEOBP) was synthesized by the esterification of 2-azidoethanol and 2- bromopropionyl bromide in the presence of Et3N in dry THF. Azido end- functional polystyrene with controlled molecular weight and narrow polydispersity was obtained by AEOBP initiated CuBr/bipyridine mediated Atom Transfer Radical Polymerization (ATRP). The yield and molecular weight of polystyrene were increased with increasing AEOBP/Styrene ratio and reaction temperature. All organic products and polystyrene obtained were characterized by NMR analysis and the molecular weight of the polystyrene was determined by Gel Permeation Chromatography (GPC).
Key Words: End-Functional Polymers, Well-defined structure, Atom Transfer Radical Polymerization (ATRP)
I. Introduction
Atom Transfer Radical Polymerization (ATRP) [1] is a controlled or living radical polymerization (CRP or LRP) technique and it has been widely used for synthesis of end functional polymers/copolymers of variety of monomers with precisely controlled architecture. In ATRP system all chain ends are retained and no new chains are formed. Chain end functionality can thus be introduced into the polymer chain by adjusting the structure of the functional initiator [2]. ATRP technique produces polymers with controlled molecular weights, narrow molecular weight distributions and high degree of chain end functionality due to small amount of terminations. It has effectively applied to the preparation of polymers with controlled functionalities, topologies and compositions. The number average molecular weight of polymers prepared by ATRP depends on the initial concentration ratio of monomer to initiator as well as the monomer conversion. The end functional initiators play a significant role in ATRP to determine the number of initiated chains. By using a functional initiator, functionalities such as azide, vinyl, allyl, hydroxyl, epoxide, cyano and other groups are incorporated at one chain end, while the other chain remains an alkyl halide[3]. Reactive end group of a polymer chain has great potential for further modification of polymer chains. Azido end of polymer chain is reactive for cycloaddition with alkyne group using Click Reaction [4]. The copper-catalyzed Huisgen dipolar cycloaddition of a terminal alkyne and an azide, [5,6] has drawn much attention because of high efficiency, technical simplicity and high specificity.
ATRP technique combined with click chemistry for the preparation of advanced macromolecular architectures [7] has a wide scope and a large range of applications. In this point of view, well-defined azido end-functional polystyrene was synthesized by ATRP”. The azido end functional polystyrene
will be used as a precursor for alkyne-azide click reaction to produce new macromolecules.
II. Experimental A. Materials
Styrene was purchased from Aldrich and it was purified by passing through an alumina column to remove stabilizer and then stirred with dry CaH2 for 7 hrs and filtered.
Finally, it was stored at 0 0C under N2 gas before use. Copper (1) bromide, bipyridine and 2-bromopropionyl bromide were purchased from Sigma Aldrich. 2-bromoethanol was purified under vacuum distillation. All solvent were purified by distillation followed by refluxed with Na & CaH2.
B. Synthesis of 2-azidoethanol
A mixture of NaN3 (32.68 g, 502.7 mmol), 2- bromoethanol (21.5 mL, 303.5 mmol) and water (102 mL) was stirred for 12 hrs at 80 0C in a 250 mL round-bottom flask. The solution was cooled to room temperature and then extracted with ether (2.50mL), dried with dry MgSO4 and filtered. Solvent was removed from the filtrate under vacuum and a colorless liquid was obtained as product (85% yield).
1H NMR (CDCl3): 4.2 ppm (s,1H, -OH) ; 3.6 ppm (t, 2H, - CH2-OH); 3.3 ppm (t, 2H, -CH2-N3).
13C NMR (CDCl3): 60.8 ppm (-CH2-OH); 53.2 ppm (-CH2- N3).
C. Synthesis of [1-azidoethane-2-oxy-(2-bromopropionyl, AEOBP)] Initiator
To a 250 mL R.B. flask, 2-azidoethanol (2.48 g, 28.5 mmol) and triethylamine (7.8 mL, 56 mmol) were dissolved in 45 mL of THF. The solution was cooled to 0 0C with an ice- bath. 2-Bromopropionyl bromide (3.1 mL, 29.6 mmol) was added drop-wise. The mixture was stirred for 7 hrs at room temperature. The product was extracted from ice-cold water with ether. The combined organic layer was washed sequentially with a saturated solution of Na2CO3, HCl solution (pH 4.5) and a second aliquot of Na2CO3. The organic layer was dried over MgSO4 and concentrated in vacuo to yield a yellow oil. Finally, the crude product was purified by silica gel column chromatography to yield a yellowish oil, yield : 80%.
1H NMR (CDCl3): 4.29 ppm (q, 1H, -CH-Br); 3.44 ppm (t, 2H, -CH2-O-); 1.8 ppm (d, 3H, -CH3); 1.19 ppm (t, 2H, -CH2- N3).
13C NMR (CDCl3): 170.24 ppm (-C=O); 64.92 ppm (-CH- Br); 49.50 ppm (-CH2-O-CO-) ; 40 ppm (-CH2-N3); 21.01 ppm (-CH3).
D. Polymerization of styrene by ATRP technique.
The polymerization of styrene was carried out at different conditions of styrene/AEOBP and as well as different temperatures. A 25 mL Schlenk flask was charged with required amount of dry CuBr and bipyridine. A required amount of degassed styrene and initiator were added via a International Conference on Materials, Electronics & Information Engineering, ICMEIE-2015
05-06 June, 2015, Faculty of Engineering, University of Rajshahi, Bangladesh www.ru.ac.bd/icmeie2015/proceedings/
ISBN 978-984-33-8940--4
syringe. The solution was degassed by three freeze-pump- thaw cycles, back filled with nitrogen, and then placed in an oil bath (preheated at the desired temperature) for various time intervals. The polymerization was quenched in MeOH. The crude polymers were dissolved in CHCl3 and passed through an alumina plug to remove the metal/ligand catalyst system.
The polymer solutions were concentrated and then precipited in methanol. Finally the polymers were dried under vacuum at 60 oC for 7 hr.
1H NMR (CDCl3): 3.30 ppm( –CH2-O-CO-), 3.9 ppm(
(
–CH2- N3), 4.50 ppm (-CH(C6H5)-Br), 7.1 ppm {b,.. (o,m)C6H5-}, 6.50 ppm{b,.. (p)C6H5-}, 1.36 ppm {b, -CH2-CH(C6H5)-}, 1.77 ppm { b, -CH2-CH(C6H5)-}.E. Analytical method
Molecular weights 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 (7.25 ppm for 1H and 77.00 ppm for 13C) was used as internal reference.
III. RESULT AND DISCUSSION
A. Synthesis of Initiator AEOBP.
At first, 2-azidoethanol was prepared from 2- bromoethanol by a simple substitution reaction with sodium azide. The esterification of 2-azidoethanol was then conducted with 2- bromopropionyl bromide to produce [1-azido-2-oxy- (2-bromopropionyl)] AEOBP initiator. The chemical equation involved in the formation of AEOBP was displayed in Scheme 1.
Br OH
N3 OH
NaN3 NaBr
2-bromoethanol Sodium azide H2O
2-azidoethanol Br
O
2-bromopropionylBr bromide Et3N,
THF
O O
N3 Br
(AEOBP) 80 0C
+ +
1-Azidoethane-2-oxy-(2-bromopropionyl) Scheme 1. Synthesis of AEOBP.
In the 1H NMR spectrum of AEOBP (Figure 1), a broad quartet at 4.28 ppm was appeared for assignable to –CH (Br) proton (denoted by A) and a doublet at 1.78 ppm was appeared for assignable to –CH3 protons (denoted by C). The presence of these two signals indicated the propionyl part in the product. A broad triplet at 3.44 ppm for N3–CH2-CH2-O- and another broad triplet at 1.18 ppm for N3–CH2-CH2-O- protons were assigned (denoted by B and D).
O O C H2 H2 C N3
C H H3C
Br
A
B
C
D D
B C
A
Figure 1. 1H NMR spectrum of 1-Azidoethane-2-oxy-(2- bromopropionyl) (AEOBP).
O O CH2 H2 C N3
CH H3C
Br E
E D
B C
A
B D A
C
Figure 2. 13C NMR spectrum of AEOBP.
The 13C NMR spectrum of AEOBP was displayed in Figure 2. The signal appeared at 170.23 ppm was assigned for carbonyl carbon of ester group -CO-O- (denoted by E), a signal at 49.5 ppm was assigned for methylene carbon attach to ester group –CH2-O-OC- (denoted by B), a signal at 40.0 ppm was assigned for methylene carbon attach to azide group N3–CH2- (denoted by D), signal at 64.92 ppm for -CH-Br (denoted by A) and 21.57 ppm for -CH3 carbons (denoted by International Conference on Materials, Electronics & Information Engineering, ICMEIE-2015
05-06 June, 2015, Faculty of Engineering, University of Rajshahi, Bangladesh www.ru.ac.bd/icmeie2015/proceedings/
ISBN 978-984-33-8940--4
C). Therefore, the 13C NMR spectrum confirmed that the structure of the initiator was 1-azidoethane-2-oxy-(2- bromopropionyl). Three weak signals observed at around 26 ppm are supposed to be the signals for grease as impurity.
B. Polymerization of styrene by ATRP using AEOBP as Initiator
The AEOBP as initiator was successfully applied for Cu(I)-bipyridine mediated ATRP of styrene at different reaction conditions such as: various feed ratio of monomer and different reaction temperatures (Scheme 2).
O O N3
Styrene
CuBr, Bipyridine
AEOBP
N3 O n-1
+
Poly(Sty-N3) O
H2 C H2
C Br
Br Feed ratio / Temperature
Scheme 2. Synthesis of Azido end-functional polystyrene
1) Effect of the ratio of styrene and initiator on polymerization
Styrene was polymerized by ATRP technique at 115
0C using three different ratio of styrene and initiator (St/AEOBP = 250, 500 and 1000) in conjugation with Cu(1)Br-Bipyridine catalyst system under nitrogen atmosphere. The results of polymerization are listed in Table 1. The catalytic system gave polymer with high molecular weight and narrow molecular weight distribution.
Table 1: Effect of the Ratio of Styrene/Initiator on polymerization of styrenea
Entry Styrene/
AEOBP (mmol)
Yield (g)
b Mn
(103)
c Mw
(103)
d Mw/Mn
1 250 0.25 7.3 9.7 1.33
2 500 0.45 15.6 21.0 1.34
3 1000 0.76 40.1 68.3 1.70
aPolymerization conditions; styrene = 3mL (26.18 mmol), CuBr : Bipyridine = 1:2, temperature =115 0C, time = 5 hrs.
bNumber average molecular weight, cweight average molecular weight and d molecular weight distribution were measured by GPC analysis using polystyrene standard.
The GPC curves of the polystyrene obtained are shown in Figure 3. The GPC curve of the polymers was shifted to higher molecular weight region with increasing the monomer/initiator feed ratio. The molecular weight distribution of the polymer was narrow (Mw/Mn= 1.3).
15 20 25 30
A B C
Log M
15 20 25 30
Log M
15 20 25 30
Log M
20 25 30
15
Figure 3. GPC curves of (A) entry 1, (B) entry 2, and (C) entry 3.
The yield and number average molecular weight (Mn) of the polymers were linearly increased with the increasing of St/AEOBP ratio (Figure 4a and 4b). These results suggested that the propagation rate was proportionally increased with increasing monomer feed.
0 250 500 750 1000 1250
0 10 20 30 40
St/In
Mn x 103
0 200 400 600 800 1000 1200 0
0.3 0.6 0.9
St/In
Yield (g)
Figure 4a. Plot of Mn vs. St/AEOBP Figure 4b. Plot of yield vs. St/AEOBP.
2) Effect of temperature on polymerization
The effect of temperature on the polymerization of styrene with AEOBP/CuBr/Bipyridine (1:1:2.1) was investigated at various temperatures (100, 115 and 130 0C) under same reaction conditions. The results obtained were listed in Table 2.
Table 2. Effect of the temperature on polymerization of styrenea
Entry Temp (oC)
Yield (g)
b Mn
(103)
c Mw
(103)
d Mw/Mn
4 100 Trace - - -
5 115 0.50 40.1 68.3 1.70
6 130 1.29 54.5 83.4 1.53
a Polymerization conditions; CuBr = 0.027 mmol, Bipyridine
= 0.056 mmol, styrene = 3 mL (26.18 mmol), styrene/ AEOBP
= 1000, time = 5 hrs. b Number average molecular weight,
c weight average molecular weight and d molecular weight distribution were measured by GPC analysis using polystyrene standard.
A trace amount of polymer was obtained at 100 0C where as the yield of polymers was increased with raising the reaction temperature at 115 0C and 130 0C. The GPC curves of the polymers obtained were displayed in Figure 5.
International Conference on Materials, Electronics & Information Engineering, ICMEIE-2015 05-06 June, 2015, Faculty of Engineering, University of Rajshahi, Bangladesh
www.ru.ac.bd/icmeie2015/proceedings/
ISBN 978-984-33-8940--4
1515 2020 2525 3030 Log M
A
20 25 30
15
Log M B
Figure 5. GPC curves of (A) entry 5 and (B) entry 6 The polymer with high molecular-weight was produced at 115 0C and 130 0C. The molecular weight of the polymers was increased and the molecular weight distribution became narrower with raising reaction temperature. These results indicated that the initiation and propagation rate were increased with increasing the reaction temperature.
3) Analysis of the structure of the polystyrene obtained The structure of polystyrene obtained was confirmed by 1H NMR spectroscopy (Figure 6).
N
3O O
H
2C H
2C Br
C
n-1 E
G J
D E
J B C
G H
B
I
A D
F
H F
Figure 6. 1H NMR spectrum of azido end-functional polystyrene.
In the 1H NMR spectrum of polystyrene (Figure 6), the signal at 3.3 ppm assignable to –CH2-O-CO- proton (denoted by H), signal at 1.3 ppm assignable to –CH2-N3
protons (denoted by G) and the signal at 4.5 ppm assignable to CH proton α- to Br (denoted by A). This result indicated that the polymerization was initiated with AEOBP initiator. A broad signal at 7.1 ppm was assigned to aromatic protons (meta- and ortho- position) of styrene unit (denoted by D and E) and another broad signal at 6.5 ppm for para-protons ( denoted by F). The signals observed at 1.36 and 1.77 ppm were assigned for the CH2 (denoted by B and C) and CH
protons (denoted by J) of main chain of polystyrene, respectively.
Conclusions
The initiator [1-azidoethane-2-oxy-(2-bromo pro- pionyl), AEOBP] was synthesized in a good yield. The structure of AEOBP was confirmed by 1H and 13C NMR analysis. The AEOBP as initiator was successfully applied for Cu(I)-bipyridine mediated ATRP polymerization of styrene.
The polymerization of styrene was investigated at various reaction conditions. A trace amount of polymer was obtained at 100 0C. On the other hand, a good amount of polymer was obtained at 115 0C and 130 0C. The yield and molecular weight of polymers were increased with raising the temperature. This result suggests that the efficiency of AEOBP as initiator for ATRP of styrene was high at above 115 0C.
The yield and the molecular weight of polymers were increased with increasing the feed ratio of styrene. This result suggests that the propagation rate was increased with increasing monomer concentration. The molecular weight distribution of polymer was narrow (Mw/Mn= 1.3). The azido end functional polystyrene obtained from this study was characterized by 1H NMR analysis. This azido end-functional polystyrene could be used as a macromolecular precursor for Click Reaction.
Acknowledgements
We especially grateful to Dr. Jun-ichi Mamiya, Assistant Professor, Chemical Resources Laboratory, Tokyo Institute of Technology, Japan for GPC analysis of polymer samples and BCSIR, Dhaka for NMR analysis of our samples.
References
1. Matyjaszewski, K.; Xia, Atom Transfer Radical Polymerization, J. Chem. Rev. 2001, 101, 2921-2990.
2. Kolb, H. C.; Finn, M. G.; Sharpless, K. B. Click Chemistry: Diverse Chemical Function from a Few Good Reations, Angew Chem Int Ed Engl 2001, 40, 2004-2021.
3. R. Huisgen, 1,3-Dipolar Cycloaddition Chemistry, Wiley, New York1984, Vol. 1.
4. Rostovtsev, V. V.; Green, L. G.; Fokin. V. V.; Sharpless, K. B., A Stepwise Huisgen Cycloaddition Process:
Copper(I)-Catalyzed Regioselective Ligation of Azides and Terminal Alkynes, Angew Chem Int Ed Engl 2002, 41, 2596-2599.
5. Tornoe, C/. W.; Christensen, C.; Meldal, M., Peptidotriazoles on Solid Phase: [1,2,3]-Triazoles by Regiospecific Copper(I)-Catalyzed 1,3-Dipolar Cycloadditions of Terminal Alkynes to Azides, J Org Chem 2002, 67, 3057-3064.
6. Moad, G.; Rizzardo, E.; Thang, S. H., Living Radical Polymerization by the RAFT process, Aust J Chem 2005, 58, 379-410.
7. Lowe, A. B.; McCormick, C. L., Homogeneous Controlled Free Radical Polymerization in Aqueous Media, Aust J Chem 2002, 55, 367-379.
International Conference on Materials, Electronics & Information Engineering, ICMEIE-2015 05-06 June, 2015, Faculty of Engineering, University of Rajshahi, Bangladesh
www.ru.ac.bd/icmeie2015/proceedings/
ISBN 978-984-33-8940--4