Synthesis of Tadpole-Shaped Amphiphilic Cyclic PS-b-PEO via ATRP and Click Chemistry, Notas de estudo de Engenharia Elétrica

Baixe Synthesis of Tadpole-Shaped Amphiphilic Cyclic PS-b-PEO via ATRP and Click Chemistry e outras Notas de estudo em PDF para Engenharia Elétrica, somente na Docsity! Preparation of Tadpole-Shaped Amphiphilic Cyclic PS-b-linear PEO via ATRP and Click Chemistry Yong-Quan Dong, Yin-Yin Tong, Bo-Tao Dong, Fu-Sheng Du, and Zi-Chen Li* Beijing National Laboratory for Molecular Sciences (BNLMS), Key Laboratory of Polymer Chemistry & Physics of Ministry of Education, Department of Polymer Science and Engineering, College of Chemistry and Molecular Engineering, Peking UniVersity, Beijing 100871, China ReceiVed October 21, 2008; ReVised Manuscript ReceiVed February 22, 2009 ABSTRACT: Amphiphilic tadpole-shaped copolymers consisting of a polystyrene (PS) ring and a poly(ethylene oxide) (PEO) tail were synthesized via atom transfer radical polymerization (ATRP) and click chemistry. First, PEO with a propargyl group and an ATRP initiating group was prepared via click chemistry and esterification. Then, a diblock copolymer, PEO-b-PS, which contained a propargyl group at the junction point and an azide group at the PS chain end, was prepared via ATRP of styrene, followed by transformation of the PS bromo end to an azide group. Finally, cyclization of the PS segment via click chemistry in dilute solution led to the formation of cyclic PS-b-linear PEO (c-PS-b-PEO). Because both the chain length of PEO and the ring size of cyclic PS can be easily tuned, a series of c-PS-b-PEOs was prepared. All of the polymers were characterized with gel permeation chromatography, NMR spectroscopy, FTIR, and matrix-assisted laser desorption time-of-flight mass spectrometry (MALDI-TOF MS). c-PS-b-PEOs showed smaller hydrodynamic volumes compared with their linear precursors. Self-assembly of one c-PS-b-PEO sample and its linear precursor in water was preliminarily investigated by transmission electron microscopy. We found that vesicles were the main morphologies for both polymers, but they were different in size; those from c-PS-b-PEO were much larger. Introduction During the past few years, growing interest has been directed to the synthesis and properties of nonlinear copolymers, including miktoarm star, linear-dendritic, and macrocyclic-based copolymers.1-3 Compared with the corresponding linear ana- logs, the cyclic polymers exhibit distinct properties, such as different glass-transition temperatures, lower hydrodynamic volume, and reduced viscosity.2 Therefore, the macrocyclic- based homopolymers and copolymers, including cyclic block copolymers and sun-shaped, tadpole-shaped, eight-shaped, and θ-shaped polymers, have continued to attract more attention in the past decade.4-16 However, the properties of the macrocyclic- based copolymers, except for those of the cyclic block copoly- mers,17 have not been well studied experimentally because of the limited availability of such polymers. Therefore, synthesis of well-defined new cyclic polymers is still very important. The formation of a macrocycle is the key step in the synthesis of macrocyclic-based polymers. Monomer insertion6,12,14,18,19 and cyclization of linear precursors are two well-known ap- proaches20-23 to preparing well-defined cyclic polymers with narrow molecular weight distributions. The first approach is mainly applied to synthesize cyclic polyester and cyclic polyethylene. If a well-defined linear precursor with both R and ω ends is easily accessed, then the second approach can be used for many types of macrocycles with high efficiency. Anionic polymerization coupled to other reactions, such as nucleophilic substitution, metathesis reaction, and amidification, is among the most widely used strategy for preparing these polymers. Limited monomer types and rigorous conditions for the prepara- tion of polymer precursors are the main disadvantages of this synthetic method. Recent development in the controlled radical polymerization (CRP), such as atom transfer radical polymer- ization (ATRP)24 and the reversible addition-fragmentation chain transfer polymerization,25 has made it possible to prepare many kinds of well-defined linear precursors with R,ω ends. Combined with the click chemistry, a copper-catalyzed 1,3- dipolar cycloadditon reaction,26 CRP has been used to prepare many new macrocyclic polymers.13,16,27,28 The simplest example of tadpole-shaped polymer is composed of one macrocycle connected to one linear polymer chain. If the chemical composition of the macrocycle is different from that of the linear chain, then the tadpole-shaped polymer is considered to be a block copolymer. Although much effort has been devoted to the synthesis of sun-shaped and tadpole-shaped amphiphilic copolymers with more than one tail,11,15 the availability of the simplest amphiphilic tadpole-shaped block copolymers is still very limited. Beinat et al. synthesized cyclic poly(chloroethyl vinyl ether)-b-linear polystyrene (PS) by cyclization of PCEVE segment via intramolecular coupling of the block copolymer precursor.4 Very recently, Shi et al. synthesized cyclic PS-b-linear poly(N-isopropylacrylamide) by combination of click chemistry and RAFT polymerization.13 Poly(ethylene oxide)-b-PS (PEO-b-PS) is a block copolymer with known amphiphilic property and immiscibility of the two blocks. The phase morphology,29 crystalline,30 and self-assembly of PEO-b-PS in solution or air/water interfaces31-33 have been well studied. Therefore, it is of interest to synthesize cyclic PS- b-linear PEO (c-PS-b-PEO) and compare its properties with those of the linear counterparts. In this work, we report the synthesis of a new tadpole-shaped amphiphilic block copolymer consisting of one PS ring and one PEO tail. The synthetic strategy is based on two click reactions and one ATRP, as shown in Scheme 1. Both the chain length of the PS ring and the PEO tail of the tadpole block copolymer can be tuned. The preliminary self-assembly of the c-PS-b-PEO in water is presented. Experimental Section Materials. Poly(ethylene glycol) monomethyl ether (Mn ) 2000 and 5000, Fluka), propargyl alcohol (98.0%, Shenyang Chemical Works), R-bromoisobutyrate bromide (98%, Aldrich), CuBr (99.999%, Aldrich), CuBr2 (99%, Aldrich), N,N,N′,N′′,N′′-penta- methyl diethylenetriamine (PMDETA, 99%, Aldrich), and NaN3 (98.0%, Zhejiang Dongyangkaiming Chemicals Company) were * Corresponding author. E-mail: zcli@pku.edu.cn. Tel: +86-10-6275- 7155. Fax: +86-10-6275-1708. 2940 Macromolecules 2009, 42, 2940-2948 10.1021/ma802361h CCC: $40.75  2009 American Chemical Society Published on Web 03/19/2009 used as received. 2-Aminoethanol (98%, Beijing Yili Chemicals Company) was distilled under vacuum. Triethylamine (99%, Shantou Xilong Chemical Factory) was dried with KOH and distilled just before use. Styrene (98%, Beijing Chemicals Com- pany) was washed with 2 M NaOH, dried with CaCl2, and distilled over CaH2 under vacuum. Cyclohexanone (99.5%, Beijing Chemi- cals Company) was distilled over CaH2. N,N-Dimethylformamide (DMF, 99.5%, Beijing Chemicals Company) was dried with MgSO4 and distilled under vacuum. Dichloromethane (99.5%, Beijing Tongguang Chemicals Company), chloroform (99%, Beijing Tong- guang Chemicals Company), and tetrahydrofuran (THF, 99.9%, Beijing Chemicals Company) were refluxed individually in the presence of CaH2, followed by distillation. R-Methoxy-ω-azide- poly(ethylene glycol) (PEO2000-N3 and PEO5000-N3) and acryloyl chloride were synthesized according to literature methods.34,35 Propargyl acrylate was synthesized from propargyl alcohol and acryloyl chloride according to literature method.36 Characterization. Gel permeation chromatography (GPC) was carried out in THF (flow rate: 1 mL/min) at 35 °C with a Waters 1525 binary HPLC pump equipped with a Waters 2414 refractive index detector and three Waters Styragel HR columns (1 × 104, 1 × 103, and 500 Å pore sizes). Monodisperse PS standards were used for calibration. 1H NMR (400 or 300 MHz) and 13C NMR (75 MHz) spectra were recorded on a Bruker-400 spectrometer and Varian-300 spectrometer in CDCl3 with tetramethylsilane as the internal reference for chemical shifts. Matrix-assisted laser desorp- tion ionization time-of-flight mass spectrometry (MALDI-TOF MS) was performed on a Bruker Biflex III spectrometer equipped with a 337 nm nitrogen laser. R-Cyano-4-hydroxycinnamic acid was used as the matrix. Mass spectra were acquired in linear mode at an acceleration voltage of +19 kV. FT-IR spectra were recorded as KBr pellets using a Bruker VECTOR 22 FT-IR spectrometer. Preparative GPC was performed with a LC-9201 recycling prepara- tive HPLC (Japan Analytical Industry) equipped with a JAIGEL- 2.5H column (600 × 200 mm2). Chloroform was the eluent at a flow rate of 3.5 mL/min. Synthesis of N,N-Bis(2-propargyloxycarbonyl ethyl)-2-hy- droxylethyl amine (BPHA). Propargyl acrylate (27 g, 0.25 mol) was dissolved in a mixture of THF (34 mL) and tert-butanol (38 mL). 2-Aminoethanol (5 g, 0.08 mol) was then added to the mixture. The mixture was stirred for 24 h at room temperature. Solvent was removed by a rotary evaporator. The product was purified by silica gel chromatography (ethyl acetate/petroleum ether v/v ) 1/6) to produce a pale-yellow oil in 91% yield. 1H NMR (300 MHz, CDCl3, δ): 4.71 (d, 4H, HCtCCH2), 3.61 (t, 2H, CH2CH2OH), 3.3-3.1 (broad, 1H, CH2CH2OH), 2.83 (t, 4H, NCH2CH2CO), 2.61 (t, 2H, NCH2CH2OH), 2.53 (t, 4H, NCH2CH2CO), 2.52 (t, 2H, HCtC). 13C NMR (75 MHz, CDCl3, δ): 171.65 (CH2CO), 77.50 (HCtC), 74.99 (HCtC), 58.94 (CH2CH2OH), 55.91 (HCtCCH2), 52.00 (COCH2CH2), 48.98 (NCH2CH2OH), 32.42 (COCH2CH2). Synthesis of 1. Take the synthesis of 1a as an example. PEO2000-N3 (3.4 g, 1.7 mmol), BPHA (4.8 g, 17 mmol), PMDETA (700 µL, 3.5 mmol), and dichloromethane (85 mL) were added to an ampule and subjected to three freeze-pump-thaw cycles. CuBr (0.50 g, 3.5 mmol) was added when the mixture was frozen. The ampule underwent another freeze-pump-thaw cycle and was sealed under vacuum. The reaction was carried out for 4 h at room temperature. Then, copper catalyst was removed with a neutral Al2O3 column. The product (3.5 g) was obtained in 90% yield by precipitation from ethyl ether/petroleum ether (v/v 1/1). Mn,GPC ) 3430, Mw/Mn ) 1.03. Polymer 1b was synthesized in a similar way, except that PEO5000-N3 was used as the starting material. Yield: 95%, Mn,GPC ) 6600, Mw/Mn ) 1.05. Synthesis of 2. Take the synthesis of 2a as an example. Polymer 1a (3.5 g, 1.1 mmol) and triethylamine (0.20 g, 2.0 mmol) were dissolved in dichloromethane (30 mL). 2-Bromoisobutyrate bromide (2.0 mL, 16 mmol) in dichloromethane (10 mL) was added dropwise to the mixture at 0 °C. The mixture was stirred overnight at room temperature. The precipitated salt was filtered, and the solvent was removed by evaporation. The crude product was dissolved in THF, and the solution was passed through a neutral Al2O3 column. The polymer (3.3 g) was obtained in 88% yield by precipitation from ethyl ether/petroleum ether (v/v 1/1). Mn,GPC ) 3450, Mw/Mn ) 1.03. Polymer 2b was synthesized in a similar way with 1b as the starting material. Yield: 79% Mn,GPC ) 6580, Mw/Mn ) 1.04. Synthesis of 3. Take the synthesis of 3a-2 as an example. First, ATRP of St with 2a as the initiator was conducted. Following the procedure for the synthesis of 1a, polymer 2a (0.30 g, 0.13 mmol), CuBr2 (1.3 mg, 5.8 µmol), styrene (1.30 g, 0.0130 mol), PMDETA (13.5 µL, 0.0680 mmol), cyclohexanone (0.74 mL), and CuBr (9.0 Scheme 1. Synthesis of Cyclic Polystyrene-b-linear Poly(ethylene oxide) (c-PS-b-PEO)a a (a) tert-Butanol/THF, r.t.; (b) CuBr/PMDETA, dichloromethane, r.t.; (c) 2-bromoisobutyrate bromide, triethylamine, dichloromethane; (d) styrene, CuBr/PMDETA, 80 °C; (e) NaN3, DMF; (f) CuBr/PMDETA, DMF. THF: tetrahydrofuran; PMDETA: N,N,N′,N′′,N′′-pentamethyl diethylenetriamine; DMF: N,N-dimethylformamide. Macromolecules, Vol. 42, No. 8, 2009 Tadpole-Shaped Amphiphilic Cyclic PS-b-linear PEO 2941 the yield of purified tadpole copolymers is >60% for 4a with a shorter PEO chain and >45% for 4b with a longer PEO chain. Purified 4a-2 was characterized by GPC, MALDI-TOF, NMR, and IR. The GPC trace of 4a-2 (Figure 1f) was unimodal and symmetrical with narrow molecular weight distribution, but 4a-2 had a longer retention time than 3a-2, suggesting the existence of a cyclic structure. Figure 2d is the IR spectrum of 4a-2; it was the same as that of 3a-2, except for the disappear- ance of the peak at 2092 cm-1, typical absorption of azide groups. Figure 5c is the 1H NMR spectrum of 4a-2; signals of propargyl methylene protons (peak g) moved from 4.65 ppm to ∼5.07 ppm after cyclization, but signals from the newly formed triazole protons (peak h) overlapped with the first triazole protons (peak e). DPn of cyclic PS segment was determined from the integration ratio of resonance peaks p and b in the 1H NMR spectrum, which is 68 for 4a-2, a little higher than that in 3a-2. Therefore, the real number-average molecular weigh of 4a-2 was calculated to be 9500. As can be seen in the MALDI-TOF MS of 4a-2 (Figure 6), the average molecular weight was 9280, which is in agreement with the Mn estimated by NMR spectrum. For other tadpole-shaped copolymers, the GPC traces, NMR, and IR spectra are shown together with those of the linear precursors in the Supporting Information. DPn values of PS in other tadpole-shaped copolymers were calculated as for 4a-2 and collected in Table 1. For copolymers with longer PS chains, the polymer yields were low, and the calculated DPn of cyclic PS was larger than those of the precursors; we attributed this to purification by precipitation and preparative GPC. It is expected that the hydrodynamic volume of a cyclic polymer is lower than the linear one with the same molecular weight. This feature can be described using <Gc> () Mp,c/Mp,l), where Mp,c and Mp,l are peak values of the molecular weight of the cyclic polymer and linear precursor by GPC. For example, <Gc> of cyclic PS was experimentally found to be 0.78. <G> Figure 3. 1H NMR spectrum of 1a in CDCl3. Figure 4. MALDI-TOF mass spectrum of 1a: (M ) C(17+2n)H(26+4n)N4O(6+n)). Matrix: R-cyano-4-hydroxycinnamic acid; cation source: Na+ and K+ in glassware, solvents and reagents. MALDI-TOF: matrix-assisted laser desorption ionization time-of-flight. 2944 Dong et al. Macromolecules, Vol. 42, No. 8, 2009 of c-PS-b-PEO can roughly be estimated by the following equation4,13 where <Gcalcd> stands for calculated <G>; <Gc,PS> is <Gc> of cyclic PS; <Gl,PEO> ) 1; FPS ) DPPS/(DPPS + DPPEO); and FPEO ) DPPEO/(DPPS + DPPEO). The <Gcal> values of tadpole-shaped copolymers were calculated and are listed in Table 1; these data suggested that tadpole-shaped copolymers have the expected compact structure. The experimental values are lower than the calculated ones, which may imply that the calculation based on such a simple addition of cyclic and linear structures is not so reliable. The GPC, NMR, and IR results of tadpole-shaped copolymers, together with those of the linear precursors, demonstrated that tadpole-shaped copolymers consisting of a PS ring and a PEO tail were synthesized. Self-Assembly of Tadpole-Shaped Copolymers (3a-2 and 4a-2) in Water. Amphiphilic diblock copolymers can form polymer micelles in water with hydrophilic segments solvated in the coronal shell and hydrophobic segments packed in the core.31 Self-assembly of PEO-b-PS in solution has been reported in the literature.32,42 Depending on the copolymer chain length, con- centration, and preparation methods, many types of morphology have been observed. PEO-b-PS with long PEO chain and short PS chain (FEO, mole fraction of EO units, >0.8) could directly dissolve in water and form spherical micelles with PS as the core and PEO as the corona.42 Eisenberg and coworkers32 systematically studied the aggregation of PEO-b-PS with shorter PEO chain (FEO < 0.5) in water by continuous water addition method. The polymer was originally dissolved in a common solvent (for example THF or DMF); water was then added slowly to induce the aggregation of PS chains and the transformation of the morphologies; finally, a large amount of water was added to stabilize the formed morphologies. The DPn of PS was usually >100, and multiple morphologies with vesicles and rods were observed. We prepared an aggregate solution of 3a-2 and 4a-2 in water in a way similar to that reported by Eisenberg. Vesicles with diameters ranging from 50 to 100 nm and a wall thickness of 15 nm were observed for 3a-2 (Figure 7a). Micelles with diameters of ∼30 nm coexisted with vesicles. In the case of 4a-2, vesicles were the only morphologies (Figure 7b), but their size distributions were quite broad. Large vesicles (diameters ∼300 nm) coexisted with small vesicles (diameters about 60-100 nm), with the former morphologies being predominant. We further characterized the aggregates by Figure 5. 1H NMR spectra of (a) 2a, (b) 3a-2, and (c) 4a-2 in CDCl3. Table 1. Number-Average Molecular Weight (Mn) and Polydispersity Index (PDI ) Mw/Mn) Data of Linear Diblock Copolymers (3) and Tadpole-Shaped Copolymers (4) as Calculated by Gel Permeation Chromatography (GPC) and 1H NMR Spectroscopy polymera yield (%) Mpb Mnb PDIb <Gexptl>c <Gcalcd>d Mne DPPSe 3a-1 33 6250 5870 1.05 6040 35 4a-1 75 5260 5050 1.04 0.84 0.91 6140 36 3a-2 44 10 750 10 140 1.07 9160 65 4a-2 72 8700 8280 1.05 0.81 0.86 9470 68 3a-3 45 19 900 18 070 1.07 18 310 153 4a-3 61 14 860 14 720 1.07 0.75 0.83 19 770 167 3b-1 33 11 720 10 630 1.06 11 640 60 4b-1 65 10 180 9180 1.06 0.87 0.92 12 470 68 3b-2 47 19 830 17 480 1.08 21 730 157 4b-2 45 16 590 15 420 1.09 0.84 0.87 23 600 175 a 3a-1 to 3a-3 were prepared with 2a as the macroinitiator and 3b-1 and 3b-2 were prepared with 2b as the macroinitiator. b Determined from GPC. c <Gexptl> ) Mp,c/Mp,l d <Gcalcd> ) (<Gc,PS>)(FPS)+ (<Gl,PEO>)(FPEO), <Gc,PS> ) 0.78, <Gl,PEO> ) 1, FPS ) DPPS/(DPPS + DPPEO), FPEO ) DPPEO/ (DPPS + DPPEO). e Determined from 1H NMR. <Gcalcd> ) <Gc,PS>(FPS) + <Gl,PEO > (FPEO) Macromolecules, Vol. 42, No. 8, 2009 Tadpole-Shaped Amphiphilic Cyclic PS-b-linear PEO 2945 dynamic laser light scattering methods; the results revealed that for 3a-2, the average diameter of the aggregates was 70 nm, whereas that of 4a-2 was 160 nm (Supporting Information). The difference in morphology and size of the aggregates may reasonably be attributed to the cyclic PS structure, which may restrict chain stretching of PS during self-assembly. We are now systematically studying the different aggregating behavior of linear block copolymer 3 and tadpole-shaped copolymer 4. Figure 6. MALDI-TOF mass spectra of 3a-2 (Mn ) 8960, Mw/Mn ) 1.017) and 4a-2 (Mn ) 9280, Mw/Mn ) 1.015). Matrix: R-cyano-4- hydroxycinnamic acid; cation source: Na+ and K+ in glassware, solvents and reagents. MALDI-TOF: matrix-assisted laser desorption ionization time-of-flight. Figure 7. TEM graphs of aggregate of (a) 3a-2 and (b) 4a-2 in water at a concentration of 0.2 mg/mL. TEM: transmission electron microscopy. 2946 Dong et al. Macromolecules, Vol. 42, No. 8, 2009