Communication Preparation of Star-Shaped ABC Copolymers of Polystyrene-Poly(ethylene oxide)-polyglycidol Using Ethoxyethyl Glycidyl Ether as the Cap Molecule Guowei Wang, Junlian Huang* The 3-miktoarm star-shaped ABC copolymers of polystyrene poly(ethylene oxide) poly(ethoxyethyl glycidyl ether) (PS-PEO-PEEGE) and polystyrene poly(ethylene oxide) polyglycidol (PS-PEO-PG) with low polydispersity indices (PDI 1.12) and controlled molecular weight were synthesized by a combination of anionic polymerization with ring- opening polymerization. The polystyryl lithium (PS Li þ )was capped by EEGE firstly to form the functionalized polystyrene (PS A ) with both an active v-hydroxyl group and an v 0 - ethoxyethyl-protected hydroxyl group, and then the PS-b-PEO block copolymers, star(ps-peo- PEEGE) and star(ps-peo-pg) copolymers were obtained by the ring-opening polymerization of EO and EEGE respectively via the variation of the functional end group, and then the hydrolysis of the ethoxyethyl group on the PEEGE arm. The obtained star copolymers and intermediates were characterized by 1 HNMR spectroscopy and SEC. Introduction G. Wang, J. Huang The Key Laboratory of Molecular Engineering of Polymers, State Education Ministry of China, Department of Macromolecular Science, Fudan University, Shanghai 200433, China. E-mail: jlhuang@fudan.edu.cn The ABC 3-miktoarm star-shaped copolymers have attracted much attention due to their unique asymmetric structure with three different homopolymer arms connected at one central junction point, [1] and leading to potentially interesting properties for possible future applications. [2] However, over the past years, the investigation on microdomain morphologies in bulk [3] and self-assembly behavior in solution [4,5] of ABC 3-miktoarm star-shaped copolymers was limited due to the difficulties in their synthesis. Generally, there were four main strategies developed to synthesize the ABC 3-miktoarm starshaped copolymers: (a) The linking agent approach; [6,7] (b) the non-homopolymerizable macromonomer approach with the 1,1-diphenylethylene group; [8 10] (c) using 298
Preparation of Star-Shaped ABC Copolymers of Polystyrene-Poly(ethylene oxide)-polyglycidol Using... a multifunctional small molecule as initiator [11] and; (d) a multifunctional macroinitiator strategy by cap molecules. [12 14] In the literature, the PS and PEO chains were always used as arms of star-shaped copolymers because of their typical hydrophobicity and hydrophilicity. [15] The polyglycidol (PG) derived from poly(ethoxyethyl glycidyl ether) (PEEGE) is another kind of polyoxirane with a primary hydroxyl group on each repeat unit which could also be further chemically modified. [16,17] In this work, a novel and universal route was put forward to synthesize star(ps- PEO-PEEGE) and star(ps-peo-pg) using ethoxyethyl glycidyl ether as the cap molecule. Experimental Part Materials Styrene (St) was washed with 10% NaOH aqueous solution followed by water three times successively, dried over CaH 2 and distilled under reduced pressure. Ethylene oxide (EO) was dried over CaH 2 and then distilled. Diphenylmethylpotassium (DPMK) solution was prepared according to the literature, [18] at a concentration of 0.61 mol L 1. Ethoxyethyl glycidyl ether (EEGE) was synthesized from 2,3-epoxy-1-propanol (glycidol) and ethyl vinyl ether according to Fitton et al. [16b,19] Butyllithium (BuLi) was prepared according to the literature, [7,20] at a concentration of 1.57 mol L 1. All other reagents were commercially available and purified using standard procedures. Instruments Size-exclusion chromatography (SEC) was performed in THF at 35 8C at an elution rate of 1.0 ml min 1 on an Agilent1100 with a G1310A pump, a G1362A refractive index detector and a G1314A variable wavelength detector, and the polystyrene was used as standards. 1 H NMR spectra were obtained on a DMX500 MHz spectrometer with tetramethylsilane (TMS) as the internal standard and CDCl 3 as the solvent. cold EO (142.27 mmol), the solution was heated to 50 8C and stirred for 96 h. After completion of polymerization, an additional amount of DPMK solution was injected into the mixture to guarantee complete protonation of all hydroxyl groups, then terminated with excess bromoethane (67.05 mmol). The crude powder of (PS-b-PEO) p was purified by extraction with cyclohexane twice to remove the homopolymer PS A, then precipitated in cold petroleum ether and dried under vacuum at 45 8C. Preparation of PS-b-PEO Block Copolymers with Deprotected Hydroxyl Group at the Junction Point (Denoted as (PS-b-PEO) d ) The (PS-b-PEO) d copolymers were prepared by the hydrolysis of ethoxyethyl group on (PS-b-PEO) p block copolymer according to the literature. [17d] Typically the (PS-b-PEO) p copolymer (6.60 g) was dissolved in 40 ml THF and 80 ml of formic acid, stirred at 35 8C for 5 h and then the formic acid and THF were evaporated off completely. After the residue was dissolved in 40 ml THF, the KOH (2 N) aqueous solution was added until the ph reached 12.0. Then, the mixture was refluxed at 65 8C for 24 h and neutralized with HCl aqueous solution. The product was obtained by removing the formed salts and precipitating in cold petroleum ether. Preparation of Star(PS-PEO-PEEGE) Copolymers To a 100 ml ampoule, the dried (PS-b-PEO) d (0.17 mmol, M nðnmrþ ¼ 7 100 g mol 1 ) in 5 ml THF was charged, then the required amount of DPMK solution and EEGE monomer (6.16 mmol) were added, the solution was heated to 60 8C and stirred for 48 h. After termination with methanol and precipitation in cold petroleum ether twice, a light red product star(ps-peo-peege) was obtained and dried under vacuum at 45 8C. Preparation of Star(PS-PEO-PG) Copolymers The star(ps-peo-pg) copolymers were obtained by the hydrolysis of ethoxyethyl group on the PEEGE arm, the procedure was similar to the hydrolysis of ethoxyethyl group on (PS-b-PEO) p. Preparation of EEGE-Functionalized Polystyrene (PS A ) The anionic polymerization of St was carried out in a mixed solvent of cyclohexane and THF using BuLi as the initiator [7,21], and the EEGE was used as the cap molecule. Preparation of PS-b-PEO Block Copolymers with Ethoxyethyl-Protected Hydroxyl Group at the Junction Point (Denoted as (PS-b-PEO) p ) The dried PS A (1.40 mmol, M nðsecþ ¼ 2 800 g mol 1 ) in 100 ml THF was charged into a 250 ml dried ampoule, then the required amount of DPMK solution was added. After the addition of the Results and Discussion Preparation of EEGE-Functionalized Polystyrene (PS A ) In our work, the living PS Li þ was capped by a seven-fold excess of EEGE in non-polar cyclohexane with a small amount of THF (Scheme 1). Once the EEGE was added, the characteristic red of PS Li þ immediately turned to a light yellow color characteristic of alkoxides. The SEC results of functionalized PS A with both an active v-hydroxyl group and an v 0 -ethoxyethyl- protected hydroxyl group at one end were shown in Figure 1 (A 1, A 2 ), which gave the monomodal and low PDI. www.mrc-journal.de 299
G. Wang, J. Huang Scheme 1. Preparation of star(ps-peo-peege) and star(ps-peo-pg) copolymers and their intermediates. The 1 H NMR spectrum of functionalized PS A was shown in Figure 2(A). Besides the resonance signal at 0.80 ppm ( a ) attributed to the a-methyl group protons ( CH 3 ) of PS A introduced by the initiator BuLi and the signal at 6.30 7.30 ppm ( f ) assigned to the aromatic protons ( C 6 H 5 )onps A chain, the appearance of the resonance signal for the v 0 -ethoxyethyl group proton ( CH(CH 3 ) ) at 4.65 4.76 ppm ( j ) and the signal for the methyne group proton ( CH(OH) ) connected to v-hydroxyl group at 3.52 ppm ( h ) proved the successful addition of EEGE to Figure 1. SEC curves of star(ps-peo-peege) (C 1, Polymer S 28 EO 95 G 41 ; C 2, Polymer S 72 EO 307 G 44 ) and their corresponding intermediates of functionalized PS A (A 1,A 2 ) and (PS-b-PEO) p (B 1,B 2 ). (SEC-RI Signal, solid line; SEC-UV Signal, dashed line). the end of PS chain. The number-average molecular weight M nðnmrþ of functionalized PS A and the efficiency of functionalization (E. F.) between EEGE and PSLi could be determined by 1 H NMR spectroscopy (see Table 1). Preparation of Block Copolymers (PS-b-PEO) p The block copolymers of (PS-b-PEO) p with an ethoxyethylprotected hydroxyl group at the junction point were prepared using PS A as the macroinitiator to initiate the ROP of EO in the presence of DPMK (Scheme 1). In the preparation of (PS-b-PEO) p, there was always some homopolymer PS left over. However, the homopolymer PS was easy to by remove from the (PS-b-PEO) p extraction with cyclohexane. [22] The SEC results of (PS-b-PEO) p after extraction with cyclohexane were achieved both by using a refractive index (RI) and UV detector (at 254 nm) and shown in Figure 1 (B 1,B 2 ), and the superimposable curves confirmed that no PEO homopolymers existed because the PEO homopolymers had no absorbance in the UV region. Figure 2(B) shows the 1 H NMR spectrum of pure (PS-b-PEO) p. Besides the resonance signal at f for aromatic protons on PS block, the appearance of resonance signal at 300
Preparation of Star-Shaped ABC Copolymers of Polystyrene-Poly(ethylene oxide)-polyglycidol Using... Figure 2. 1 H NMR spectra (CDCl 3 )of(a) Functionalized PS A (M nðsecþ ¼ 2 800 g mol 1 ); (B) (PS-b-PEO) p copolymer (M nðnmrþ ¼ 7 200 g mol 1 ) and (C) (PS-b-PEO) d copolymer (M nðnmrþ ¼ 7 100 g mol 1 ). 3.53 3.70 ppm ( n ) for the characteristic methylene group protons ( CH 2 CH 2 O ) on the PEO block confirmed the successful polymerization of EO by the initiation of PS A,and the proton of the ethoxyethyl group proton at j was still at the junction point after the ROP of EO. The most reliable method to determine the actual molar mass of (PS-b-PEO) p copolymers was using 1 H NMR spectroscopy. [23] The molecular weight of (PS-b-PEO) p (M nðnmrþðps-b-peoþp) were determined according to the already known M nðsecþðpsþ and M nðnmrþps of PS A using Equation (1). obtained by hydrolysis of the ethoxyethyl group at the junction point of (PS-b-PEO) p. See Figure 2(C) for the 1 H NMR spectrum of (PS-b-PEO) d. Comparing Figure 2(C) with the Figure 2(B), it was observed that only the resonance signal of the ethoxyethyl group proton at j disappeared completely in Figure 2(C), and no other signals for PS and PEO blocks changed, which confirmed that the ethoxyethyl group was removed efficiently under these experimental conditions. The molecular weight M nðnmrþ of (PS-b-PEO) d (M nðnmrþðps-b-peoþd) was also determined by 1 H NMR spectroscopy using Equation (2). M nðnmrþðps-b-peoþp ¼ 5 A n M nðsecþps 4 104 A f 44 þ M nðnmrþps þ 28 (1) Here, A n represents the integral area of the peaks at n for methylene group protons on the PEO block, the value 44 was the molecular weight of the EO unit and 28 was the difference between the mass of the ethyl group and that of a proton ( H). See Table 1 for definition of other symbols. Preparation of Block Copolymers (PS-b-PEO) d After the PEO anions at the (PS-b-PEO) p end were blocked by bromoethane, the (PS-b-PEO) d block copolymers were M nðnmrþðps-b-peoþ d ¼ 5A nm nðsecþps 4104A f 44þM nðnmrþps þ28 72 (2) Here, the value 72 was the mass difference between the ethoxyethyl group and a proton ( H). For definition of other symbols see Equation (1) and Table 1. The value of M nðnmrþðps-b-peoþd was very close to its corresponding M nðnmrþðps-b-peoþp, which meant no PEO chain degradation on (PS-b-PEO) d was observed during the acidolysis in formic-acid/thf and saponification in KOH solution of the water/thf (ph ¼ 12.0) system. www.mrc-journal.de 301
G. Wang, J. Huang Table 1. Polymerization data of star(ps-peo-peege), star(ps-peo-pg) copolymers and the intermediates. Polymer a) PS PS-b-PEO Star (PS-PEO-PEEGE) Star (PS-PEO-PG) M nðsecþ b) PDI b) M nðnmrþ c) E.F. d) (PS-b-PEO) p (PS-b-PEO) d M nðnmrþ g) PDI b) M nðtheo:þ h) M nðnmrþ e) PDI b) M nðnmrþ f) g mol S1 g mol S1 % g mol S1 g mol S1 g mol S1 g mol S1 S 28 EO 93 G 22 2 800 1.04 2 900 94.2 7 100 1.05 7 000 10 200 1.05 8 600 S 28 EO 95 G 41 7 200 1.06 7 100 13 100 1.07 10 100 S 28 EO 95 G 47 14 000 1.07 10 600 S 72 EO 307 G 44 7 500 1.03 7 500 92.1 21 100 1.04 21 000 27 400 1.12 24 200 S 72 EO 245 G 66 18 500 1.03 18 300 28 000 1.11 23 200 S 72 EO 245 G 81 30 100 1.12 24 300 S 138 EO 409 G 95 14 600 1.04 14 400 96.0 32 300 1.06 32 300 46 200 1.10 39 400 S 138 EO 632 G 71 42 400 1.04 42 200 52 600 1.12 47 500 a) The subscripts represent the monomer units on each arm (S-styrene, EO-ethylene oxide, G-glycidol); b) Determined by SEC-RI with THF as solvent using PS standards; c) M nðnmrþ of PS A (denoted as M nðnmrþps ) was determined by 1 H NMR spectroscopy by end group analysis using Equation: M nðnmrþps ¼ð3104 A f Þ=ð5 A a Þþ57 þ 147, where the A f and A a were the integral area of the aromatic protons at f and the a-methyl group protons at a respectively. The values 104 and 57 were the molecular weights of St unit and the mass of the butyl group, the value 147 was the sum of the mass of EEGE and that of a proton( H) [see Figure 2(A)]; d) E. F. (efficiency of functionalization) was determined by 1 H NMR spectroscopy using the Equation: E:F: ¼ð3A j Þ=A a 100%, where the A j was the integral area of the v(-ethoxyethyl group proton at j, the A a was the same as the one in c [see Figure 2(A)]; e) M nðnmrþ of (PS-b-PEO) p (denoted as M nðnmrþðps-b-peo)p ) was determined according to Equation (1); f) M nðnmrþ of (PS-b-PEO) d (denoted as M nðnmrþðps-b-peo)d ) was determined according to the Equation (2); g) M nðnmrþ of star(ps-peo-peege) (denoted as M nðnmrþstarðps-peo-peegeþ) was determined according to Equation (3); h) M nðtheo:þ of star(ps-peo-pg) (denoted as M nðtheo:þstarðps-peo-pgþ) was calculated using Equation (4). Preparation of Star(PS-PEO-PEEGE) and Star(PS-PEO-PG) Copolymers The (co)polymerization of EEGE initiated by several initiators [24] has already been investigated. In our work, the third arm of PEEGE was prepared by ROP of EEGE using (PS-b-PEO) d and DMPK as the co-initiator (Scheme 1). The polymerization of EEGE initiated by (PS-b-PEO) d showed a similar phenomenon to that of EO initiated by PS A. The small quantity of uninitiated (PS-b-PEO) d precursor could be removed efficiently with fractional precipitation by adding petroleum ether drop-wise into a polymer solution of methanol. The SEC results of the final product were shown in Figure 1 (C 1, C 2 ). The superimposable curves obtained by the RI and UV detector confirmed that no PEEGE homopolymer occurred (the PEEGE did not absorb UV light at 254 nm). The purity of the expected copolymers could be estimated according the shape of the SEC curves, while the actual molecular weight was determined by 1 H NMR spectroscopy. Figure 3(D) shows the 1 H NMR spectrum of star(ps- PEO-PEEGE). The appearance of the resonance signal at j (the ethoxyethyl group proton on the PEEGE arm), at f (aromatic protons on PS arm) and at 3.43 3.83 ppm n, q, r (the methylene group protons: CH 2 CH 2 O on PEO arm, and methylene group, methyne group protons ( CH 2 CH ) on PEEGE arm, respectively) confirmed the successful synthesis of the star(ps-peo-peege). The molecular weight of star(ps-peo-peege) (M nðnmrþstarðps PEO PEEGEÞ ) was determined by 1 H NMR spectroscopy using Equation (3). M nðnmrþstarðps-peo-peegeþ ¼ 5 A j M nðsecþps 4 104 A f 146 þ M nðnmrþðps-b-peoþ (3) Here, the value 146 was the molecular weight of EEGE unit and for definition of other symbols refer to Table 1. The star(ps-peo-pg) copolymers were obtained by the hydrolysis of ethoxyethyl group on the PEEGE arm in 302
Preparation of Star-Shaped ABC Copolymers of Polystyrene-Poly(ethylene oxide)-polyglycidol Using... Figure 3. 1 H NMR spectrum (CDCl 3 ) of (D) star(ps-peo-peege) copolymer, (M nðnmrþ ¼ 46 200 g mol 1 ); (E) star(ps-peo-pg) copolymer, (M nðthero:þ ¼ 39 400 g mol 1 ). formic acid/thf and KOH solution of water/thf solution successively. From the 1 H NMR spectrum in Figure 3(E), the ethoxyethyl group proton of PEEGE arm at j on PEEGE arm had completely disappeared, which showed that all the PEEGE arms had converted to PG arms. Correspondingly, the molecular weight of the PG arm could be calculated from the PEEGE arm using Equation (4). ring-opening polymerization using ethoxyethyl glycidyl ether as the cap molecule. The structure of the target copolymers and the intermediates were characterized by 1 H NMR spectroscopy and SEC. Our work provided a versatile and efficient route to the synthesis the ABC 3-Miktoarm star-shaped copolymers. M nðtheo:þstarðps-peo-pgþ ¼ 5 A j M nðsecþps ð146 72Þ 4 104 A f þ M nðnmrþðps-b-peoþ (4) Here the values 72, 146 and others were the same as defined in Equation (2), Equation (3) and in the footnotes of Table 1. Acknowledgements: We appreciate the financial support of this research from the Natural Science Foundation of China (No:205740100). Received: October 24, 2006; Revised: November 17, 2006; Accepted: November 22, 2006; Keywords: ethoxyethyl glydicyl ether; poly(ethoxyethyl glycidyl ether); poly(ethylene oxide); polystyrene; star polymers Conclusion The ABC 3-Miktoarm star-shaped copolymers star(ps- PEO-PEEGE) and star(ps-peo-pg) were prepared successfully by a combination of anionic polymerization with [1] N. Hadjichristidis, M. Pitsikalis, S. Pispas, H. Iatrou, Chem. Rev. 2001, 101, 3747. [2] N. Hadjichristidis, S. Pispas, M. Pitsikalis, H. Iatrou, C. Vlahos, Adv. Polym. Sci. 1999, 142, 71. [3] K. Yamauchi, K. Takahashi, H. Hasegawa, H. Iatrou, N. Hadjichristidis, T. Kaneko, Macromolecules 2003, 36, 6962. www.mrc-journal.de 303
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