High Efficiency Coupling of Polymers with TEMPO and Halide Groups, Notas de estudo de Engenharia Elétrica

Baixe High Efficiency Coupling of Polymers with TEMPO and Halide Groups e outras Notas de estudo em PDF para Engenharia Elétrica, somente na Docsity! One-Pot Synthesis of ABC Type Triblock Copolymers via a Combination of “Click Chemistry” and Atom Transfer Nitroxide Radical Coupling Chemistry Wencheng Lin, Qiang Fu, Yi Zhang, and Junlian Huang* Key Laboratory of Molecular Engineering of Polymer, State Education Ministry of China, Department of Macromolecular Science, Fudan UniVersity, Shanghai 200433, China ReceiVed October 30, 2007; ReVised Manuscript ReceiVed March 21, 2008 ABSTRACT: A new strategy for one-pot synthesis of ABC type triblock copolymers via a combination of “click chemistry” and atom transfer nitroxide radical coupling (ATNRC) reaction was suggested, and poly(tert- butyl acrylate)-block-polystyrene-block-poly(ethylene oxide) (PtBA-PS-PEO) and poly(tert-butyl acrylate)- block-polystyrene-block-poly(
-caprolactone) (PtBA-PS-PCL) were successfully prepared by this method. The precursors with predetermined number-average molecular weight and low polydispersity indices, such as PS with R-alkyne and ω-bromine end groups, PtBA with azide end group, PEO and PCL with a 2,2,6,6- tetramethylpiperidine-1-oxyl end group, were directly prepared by living polymerization technique using the compounds with corresponding functional groups as initiators, and no further modifications of the end groups were needed, except PtBA-N3. The coupling reaction between precursors was carried out in the CuBr/N,N,N′,N′′,N′′- pentamethyldiethylenetriamine system with high efficiencies. The obtained polymers were characterized by FT- IR, 1H NMR, differential scanning calorimetry, and gel permeation chromatography in detail. Introduction Molecular design of block copolymers with well-defined architectures is a very important research field.1 Traditionally, block copolymers with predetermined number-average molec- ular weight (Mn) and low polydispersity indices (PDI) are always synthesized by two strategies: sequential living polymerizations of different monomers2,3 or coupling reaction of polymers with preformed functional groups.4,5 Among various kinds of block copolymer, the ABC type triblock copolymers have attracted much attention for their unique structure with three different homopolymer blocks, leading to potentially interesting properties for possible further applications.6–9 In order to prepare well- defined ABC type triblock copolymers with predetermined Mn and PDI, living anionic polymerizations are always used.2,3 However, the rigid conditions and only a few available monomers limited the applications of conventional living anionic polymerizations. As the development of “controlled/living” radical polymerization (CRP), a novel route for synthesis of well-defined triblock copolymer has arisen. In recent years, CRP techniques have developed rapidly for facile preparation of a variety of polymeric materials with predetermined Mn, low PDI, and high degrees of chain-end functionalization.10 Compared with conventional living anionic polymerizations, CRP techniques have the advantage of the variety of applicable monomers and more tolerant experimental conditions. The most widely used CRP methods are atom transfer radical polymerization (ATRP),11–14 reversible addition- fragmentation chain transfer (RAFT) polymerization,15,16 and nitroxide-mediated polymerization (NMP).17,18 Especially, ATRP and NMP have proved useful in the synthesis of triblock copolymers.19–21 The polymers contained terminal halogen groups synthesized by ATRP can be successfully converted to various desired functional chain-end groups through appropriate transformations.22 For example, a halogen end group of the polymer could be successfully converted to an azide group and then further react with alkynes to form a substituted triazole group, which is termed “click chemistry”.23 Click chemistry has been used extensively due to its quantita- tive yields, high tolerance of functional groups, and insensitivity of the reaction to solvents.24 The reaction between a terminal alkyne and an azide groups to form a triazole group is the most popular one, which was first studied by Huisgen.25 Nowadays, click reactions have already been widely used in polymeric science and material,24 such as the synthesis of linear,26,27 dendritic,28,29 cyclic,30 and star polymers.22 They are also utilized in functionalized surfaces,31–33 sugars,34 probe biological systems,35,36 and synthesis of synthesize analogues of vitamin D.37 The great potential of this coupling procedure for the construction of well-defined (functional) polymer architectures was quickly recognized, and it is the subject of intensive research.38 The fact has showed that it was a wonderful route to use click reaction in synthesis of ABC triblock copolymer.39 However, the polymers with azide groups are difficult to be reserved because of their photosensitivity, thermal instability, and shock sensitivity. Thus, in the operations of “click” chemistry, special care should be taken. It is obvious that there is a need to look for a strategy to prepare well-defined copolymers with complex structure by the coupling reaction of the more stable and more reactive functional groups than azide. Matyjaszewski et al.40 reported the synthesis of several alkoxyamines derived from organic halides and 2,2,6,6-tetram- ethylpiperidine-1-oxyl (TEMPO) or TEMPO derivatives using copper systems. The alkoxyamines bearing different functional groups have been prepared in one simple step with high yield. Now, we are trying to introduce this reaction to the polymer field, and what we care about is whether this coupling reaction with high efficiency would be realized when a polymer containing TEMPO group is mixed with another polymer contained halide group in the presence of CuBr. In this coupling reaction, the terminal bromine groups of the polymers served as oxidant are reduced to bromine anions, and then carbon radicals of polymers are formed; CuBr is served as reductant, and the Cu1+ is oxidized to Cu2+. The formed free carbon radicals are immediately captured by the TEMPO radical of another polymer, and a stable bond -C-O- is obtained between the two polymers. This oxido-reduction process is irreversible. The reaction is termed as atom transfer nitroxide* Corresponding author: Fax+86-21-65640293; e-mail jlhuang@fudan.edu.cn. 4127Macromolecules 2008, 41, 4127-4135 10.1021/ma702404t CCC: $40.75  2008 American Chemical Society Published on Web 05/13/2008 radical coupling (ATNRC) reaction. Because halide and TEMPO are typical groups for ATRP and NMP, the ATNRC reaction could be widely used in synthesis of a variety of polymers. Several months ago, Durmaz41 reported a one-pot synthesis of ABC type triblock copolymers via a combination of click [3 + 2] with Diels-Alder [4 + 2] reactions between maleimide and anthracene groups. However, since the required functional anthryl and maleimide groups for his one-pot reaction are not easy to be prepared, the modification of end groups of each precursor polymer blocks is a time-consuming procedure. Herein, a new strategy for preparation of the ABC triblock copolymers by the one-pot method is provided, some precursor polymers as poly(ethylene oxide) (PEO) and poly(
-caprolac- tone) (PCL) with a TEMPO end group, polystyrene (PS) with R-alkyne and ω-bromine end groups, could be prepared simply by the initiators with corresponding functional groups, and the final triblock copolymer poly(tert-butyl acrylate)-block-poly- styrene-block-poly(ethylene oxide) (PtBA-PS-PEO) and poly- (tert-butyl acrylate)-block-polystyrene-block-poly(
-caprolac- tone) (PtBA-PS-PCL) were successfully prepared with high efficiencies via a combination of “click chemistry” and ATNRC reaction. Experimental Section Materials. Ethylene oxide (EO, 99.9%, Sinopharm Chemical Reagent) (SCR), tert-butyl acrylate (tBA, 99%, SCR),
-caprolac- tone (CL, 99%, SCR), and propargyl alcohol (99%, SCR) were dried by CaH2 for 48 h and distilled before use. Styrene (St; 99.5%,SCR) purchased from SCR was washed with a 15% NaOH aqueous solution and water successively for three times, dried over anhydrous MgSO4, further dried over CaH2, and then distilled under reduced pressure twice before use. Diphenylmethylpotassium (DPMK) solution with a concentration of 0.630 mol/L was prepared according to the literature.42 4-Hydroxyl-TEMPO (HTEMPO) prepared according to the literature43 was purified by recrystalli- zation with hexane. CuBr (95%, SCR) was stirred overnight in acetic acid, filtered, washed with ethanol and diethyl ether succes- sively, and dried in vacuo. N,N,N′,N′′,N′′-Pentamethyldiethylen- etriamine (PMDETA, 99%), ethyl 2-bromoisobutyrate (EBiB, 98%), and 2-bromoisobutyryl bromide (98%) were purchased from Aldrich and used without further purification. Acetone (99%), tetrahydro- furan (THF, 99%), toluene (99%), pyridine (99%), N,N-dimethyl- formamide (DMF, 99%), and other reagents were all purchased from SCR and purified by standard methods before use. Measurements. Gel permeation chromatography (GPC) was performed on an Agilent 1100 with a G1310A pump, a G1362A refractive-index detector, and a G1314A variable-wavelength detector with THF as the eluent at a flow rate of 1.0 mL/min at 35 °C. One 5 µm LP gel column (500 E, molecular range 500-2 × 104 g/mol) and two 5 µm LP gel mixed bed column (molecular range 200-3 × 106 g/mol). Polystyrene standards were used for calibration. For PEO, GPC was performed in distilled water at 40 °C with an elution rate of 0.5 mL/min with the same instruments, except that the G1314A variable-wavelength detector was substi- tuted by a G1315A diode-array detector, and PEO standards were used for calibration. 1H NMR spectra were recorded at room temperature by a Bruker (500-MHz) spectrometer using tetram- ethylsilane as the internal standard and CDCl3 as the solvent, except for PEO; the latter was determined in deuterated methanol in the presence of stoichiometric ammonium formate (HCOONH4) and the catalyst palladium on carbon (Pd/C). All of the samples were scanned for 128 times, and the sensitivity of the instrument was 0.1% ethylbenzene; NS ) 1, LB ) 1; S/N ) 300:1. FT-IR spectra were obtained on a Magna-550 Fourier transform infrared spec- trometer. The differential scanning calorimetry (DSC) analysis was carried out with a Perkin-Elmer Pyris 1 DSC instrument under a nitrogen flow (10 mL/min); all samples were heated from -70 to 140 at 10 °C/ min under a nitrogen atmosphere. The glass transition (Tg) and the melting temperatures (Tm) were calculated as a midpoint and a peak apex of thermograms, respectively. DSC was calibrated for temperature by indium (theoretical: 156.6 °C; measured: 158.344 °C) and zinc (theoretical: 419.47 °C; measured: 423.4666 °C). Heat flow was calibrated by indium (theoretical: 28.45 °C; measured: 26.914 °C; weight: 3.6 mg). Synthesis of Propargyl 2-Bromoisobutyrate (PgBiB). Prop- argyl alcohol (5.00 mL, 85.9 mmol) was dissolved in 40.0 mL of pyridine and cooled in an ice-water bath, and a solution of 2-bromoisobutyryl bromide (10.6 mL, 85.9 mmol) in pyridine (10.0 mL) was slowly added under stirring. Then the system was stirred continuously in the cooling bath for 2 h and then at room temperature for another 24 h. After the precipitated pyridine salts were filtered, the solvent was removed on a rotary evaporator. The crude product was distilled under reduced pressure to give the clear liquid of PgBiB with yield of 85.3%. 1H NMR (CDCl3, δ): 4.71 (2H, CH2O), 2.47 (1H, CtCH), and 1.90 (6H, (CH3)2C). IR spectrum (neat liquid, KBr plates): 3296 cm-1 (ν≡C-H), 2132 cm-1 (νC≡C), and 1743 cm-1 (νCdO). Synthesis of PS with r-Alkyne and ω-Bromine (Alkyne-PS- Br). Alkyne-PS-Br was prepared by ATRP of St using PgBiB as an initiator and CuBr/PMDETA as a catalyst. A typical example is as follows: PgBiB (0.180 mL, 1.20 mmol), CuBr (0.0860 g, 0.0600 mmol), PMDETA (0.130 mL, 0.0600 mmol), and St (30.0 mL, 262 mmol) were added to a dry ampule. The reaction mixture was degassed by three freeze-pump-thaw cycles and purged with nitrogen. The ampule was immersed in oil bath at 90 °C for 6 h, then taken from the oil bath, and dipped in liquid nitrogen to stop the polymerization. The products were diluted with THF, passed through a column chromatograph filled with neutral alumina to remove the copper complex, and precipitated in cold methanol. The precipitate was collected and purified by dissolution/precipitation with THF/cold methanol twice and then dried at 40 °C in vacuum oven for 4 h ([M]0/[I]0 ) 220; conversion ) 29.8%; the number- average molecular weight from 1H NMR (Mn,NMR) ) 6900 (see Figure 4a, eq 1 was used for calculation); the number-average molecular weight from GPC (Mn,GPC) ) 7300 (relative to linear PS standard); molecular weight distribution (Mw/Mn) ) 1.10). Two types of alkyne-PS-Br (alkyne-PSA-Br and alkyne-PSB-Br) with different Mn were prepared (listed in Table 1). 1H NMR (CDCl3, δ): 7.23-6.30 (phenyl protons of PS), 4.60-4.30 (CH(Ph)-Br, end group of PS), 4.17-3.90 (CHtC-CH2 of initiator PgBiB), 2.50-1.20 (CH2CH(Ph), repeating unit of PS), 1.14-0.80 (-(CO)-C(CH3)2 of initiator PgBiB) (Figure 4a). Synthesis of PtBA with Azide End Group (PtBA-N3). PtBA with bromine end group (PtBA-Br) was prepared by ATRP of tBA in acetone, using EBiB as an initiator and CuBr/PMDETA as a catalyst. EBiB (0.150 mL, 1.00 mmol), CuBr (0.144 g, 1.00 mmol), PMDETA (0.210 mL, 1.00 mmol), and tBA (18.0 mL, 126 mmol) were dissolved in acetone (18.0 mL). The reaction mixture was degassed by three freeze-pump-thaw cycles and left under nitrogen. The ampule was immersed in oil bath at 60 °C for 6 h, then taken from the oil bath, and dipped in liquid nitrogen to stop the polymerization. The products were diluted with THF, passed through a column chromatograph filled with neutral alumina to remove the copper complex, and precipitated in a cold mixture solution of methanol and H2O (1/1 v/v). The precipitate was collected and dried at 40 °C in vacuum oven for 4 h ([M]0/[I]0 ) Table 1. Characterization of the Synthetic Alkyne-PS-Bra sample Mn,GPCb (g/mol) Mn,NMRc (g/mol) Mw/Mnb DPc alkyne-PSA-Br 7 300 6 900 1.10 64 alkyne-PSB-Br 10 600 10 700 1.08 101 a Alkyne-PS-Br: polystyrene with R-alkyne and ω-bromine end groups, obtained by ATRP of St using PgBiB (propargyl 2-bromoisobutyrate) as an initiator, CuBr/PMDETA as a catalyst system at 90 °C. b Mn,GPC (the GPC number-average molecular weight) and Mw/Mn (molecular weight distribution), measured by GPC in THF with RI detector, calibration with linear PS as standard. c Mn,NMR (the NMR number-average molecular weight) and DP (number-average degree of polymerization of St), measured by 1H NMR spectroscopy. 4128 Lin et al. Macromolecules, Vol. 41, No. 12, 2008 determined by the ratio of the integrated signals at 3.78-3.60 ppm (CH2CH2O, repeating unit of PEO) to 1.19-1.14 ppm (CH3, methyl protons of TEMPO), using eq 3. Mn,NMR ) 12Ae 4Aa × 44+ 172 (3) Here, Ae represents the integral area of the peaks at “e” for CH2CH2 methylene group protons on PEO repeating unit, Aa represents the integral area of the peaks at “a” for methyl group protons on PEO end group, and the value 172 is the molecular weight of the initiator HTEMPO. The Mn,theo, Mn,NMR, and Mn,GPC were in good agreement. PCL-TEMPO was prepared by ROP of CL in toluene using Sn(Oct)2 as a catalyst and HTEMPO as an initiator at 100 °C. TEMPO end group displays a characteristic signal of CH3 (methyl protons of TEMPO) at 1.28-1.17 ppm in the 1H NMR spectrum (Figure 3). For the TEMPO radicals, no reliable information could be derived from the 1H NMR measurement because of its paramagnetism. In this work, the 1H NMR spectrum was carried out in deuterated methanol in the presence of stoichiometric HCOONH4 and catalytic Pd/C (or phenylhy- drazine). TEMPO radicals on the copolymers were reduced to the corresponding oximes, and clear 1H NMR spectra were then obtained.44 The Mn,NMR of PCL-TEMPO was 4400, which was determined by the ratio of the integrated signals at 2.35-2.24 [-(CO)-CH2CH2CH2CH2CH2O, the first methylene group connected to carbonyl, repeating unit of PCL] to 1.28-1.17 ppm (CH3, methyl protons of TEMPO), using eq 4. Mn,NMR ) 12Af 2Aa × 114+ 172 (4) Here, Af represents the integral area of the peaks at “f” for -(CO)-CH2CH2CH2CH2CH2O (the first methylene group connected to carbonyl) on PCL repeating unit, Aa represents the integral area of the peaks at “a” for methyl group protons on PCL end group, and the value 172 is the molecular weight of the initiator HTEMPO. The Mn,GPC of PCL-TEMPO was derived as 7700 g/mol; however, the chain structure of PCL is quite different from the PS standard. Thus, the more precise Mn obtained by GPC should use a correction formula:40,45 Mn,PCL ) 0.259Mn,GPC1.073. The calibrated value Mn,PCL was 3800, which was almost consistent with the Mn,theo and Mn,NMR of PCL-TEMPO. One-Pot Synthesis of PtBA-PS-PEO Triblock Copoly- mer. PtBA-N3, alkyne-PS-Br, and PEO-TEMPO were reacted in one pot to obtain the corresponding triblock copolymer PtBA-PS-PEO, as shown in Scheme 1. Click reaction between azide end group of PtBA-N3 and alkyne functional group of alkyne-PS-Br was performed in the presence of CuBr/PMDETA in DMF at 90 °C. Simultaneously, ATNRC reaction between TEMPO end group of PEO-TEMPO and bromine functional group of alkyne-PS-Br was conducted at the same condition; the obtained PtBA-PS-PEO triblock copolymer is listed in Table 4. The synthesis of PtBA-PS-PEO triblock copolymer was characterized by 1H NMR and IR spectra. The crude product of the one-pot reaction was purified by dissolution/precipitation with THF/methanol because the excessive PEO-TEMPO and PtBA-N3 could be dissolved in methanol, but PtBA-PS-PEO cannot. Comparing the 1H NMR spectra of alkyne-PS-Br with that of PtBA-PS-PEO triblock copolymer (Figure 4), the characteristic peak of CHtC-CH2 (end group of PS) at 4.17-3.90 ppm disappeared, and a new signal related to triazole ring at 7.60-7.41 ppm appeared, which proved the click reaction accomplished between azide group and alkyne group. Besides, the characteristic peak of CH(Ph)-Br (end group of PS) at 4.60-4.30 ppm disappeared because the signal of the methine group proton shifted to upfield when the Br atom broke off from the alkyne-PS-Br chain. This indicated the ATNRC reaction occurred between bromine group and TEMPO group. Figure 4b also shows the characteristic peaks of phenyl group (repeating unit of PS) at 7.26-6.30, -CH2CH2O (repeating unit of PEO) Table 4. One-Pot Synthesis of PtBA-PS-PEO/PCLa efficiency (%)e sample Mn,GPCb(g/mol) Mn,actc(g/mol) Mn,theod(g/mol) Mw/Mnb yield (%) ATNRC click PtBAa-PSA-PEOA 11 500 13 800 14 700 1.08 82.0 86.0 89.6 PtBAb-PSB-PEOB 15 800 19 600 21 100 1.13 80.9 80.9 87.9 PtBAb-PSA-PCL 14 900 15 800 17 500 1.23 84.4 81.6 85.9 PtBAb-PSB-PCL 17 700 19 200 21 300 1.13 85.1 80.1 80.7 a PtBA-PS-PEO/PCL: poly(tert-butyl acrylate)-block-polystyrene-block-poly(ethylene oxide)/poly(
-caprolactone), obtained in the presence of CuBr/ PMDETA in DMF at 90 °C. b Mn,GPC (the GPC number-average molecular weight) and Mw/Mn (molecular weight distribution), measured by GPC in THF with RI detector, calibration with linear PS as standard. c Mn,act (the actual molecular weight), measured by the ratio of integrated values of the PS to the PtBA and the PEO/PCL segments. d Mn,theo (the theoretical molecular weight), measured by eq 5. e Efficiency of ATNRC (atom transfer nitroxide radical coupling) and click reaction, measured by 1H NMR spectrum of the triblock copolymers (see Supporting Information). Scheme 1. One-Pot Synthesis of PtBA-PS-PEO Macromolecules, Vol. 41, No. 12, 2008 ABC Type Triblock Copolymers 4131 at 3.82-3.48 ppm, and -CH2CH (repeating unit of PtBA) at 2.32-2.16 ppm, which confirmed the successful coupling of PtBA-N3 and PEO-TEMPO with alkyne-PS-Br chain. This one-pot reaction was further supported by the IR spectrum (Figure 5). Compared with the IR spectrum of alkyne- PS-Br, the characteristic signal of -HCtC at 3295 cm-1 disappeared, and a new signal related to triazole ring appeared at 1684 cm-1, which indicated the alkyne group was transformed to triazole group. Furthermore, a characteristic signal of ether bond (-C-O-C-, belong to PEO chain) at 1174 cm-1 appeared while the signal of carbonyl (due to PtBA chain) at 1727 cm-1 enhanced, which was another evidence of the PEO and the PtBA chains coupled to PS chain. In this one-pot reaction, the feeded PtBA-N3 and PEO- TEMPO were slightly excessive, compared to that of alkyne- PS-Br. The molar ratio of PtBA-N3 to alkyne-PS-Br and PEO-TEMPO was about 1.2:1:1.2. Because PtBA-N3 and PEO- TEMPO could be dissolved in methanol completely, the excess amounts of PtBA-N3 and PEO-TEMPO could be easily removed from the mixture by precipitation in cold methanol after the one-pot reaction was carried out. The GPC curves of the triblock copolymer and the corresponding precursors showed singlet and low PDI (Figure 6). All the data of the triblock copolymer are summarized in Table 4. The theoretical molecular weight (Mn,theo) of PtBA-PS-PEO could be calculated by the sum of the separate Mn,NMR of precursors participated in the reaction, using eq 5. Mn,theo )Mn,NMR(PtBA-N3)+Mn,NMR(alkyne-PS-Br)+Mn,NMR (PEO-TEMPO)- 80 (5) Here Mn,NMR(PtBA-N3), Mn,NMR(alkyne-PS-Br), and Mn,NMR- (PEO-TEMPO) represented the Mn,NMR values of corresponding precursors PtBA-N3, alkyne-PS-Br, and PEO-TEMPO which participated in the one-pot reaction; the value 80 was the molecular weight of the leaving group Br atom. The actual molecular weight (Mn,act) of triblock copolymer was determined by a ratio of the integration of PS segment to PtBA and PEO segment at 7.26-6.30 (phenyl protons, repeating unit of PS) to 2.32-2.16 (CH2CH, repeating unit of PtBA) and 3.82-3.48 (CH2CH2O, repeating unit of PEO) in NMR using eq 6. Mn,act )Mn,NMR(PtBA)+Mn,NMR(PEO)+Mn,NMR(PS) ) 5Ae Af × Mn,NMR(PS) 104 × 128+ 5Ab 4Af × Mn,NMR(PS) 104 × 44+ Mn,NMR(PS) (6) Here, Mn,NMR(PtBA), Mn,NMR(PEO), and Mn,NMR(PS) represent the actual Mn,NMR of PtBA, PEO, and PS segment in the PtBA-PS-PEO triblock copolymer, respectively, where the Mn,NMR(PS) was equal to Mn,NMR of the precursor alkyne-PS- Br which participated the reaction. Ae represents the integral area of the peaks at “e” for CH2CH methine group proton on PtBA segment, Af represents the integral area of the peaks at “f” for phenyl group protons on PS segment, and Ab represents the integral area of the peaks at “b” for CH2CH2 methylene group protons on PEO segment; the values 104, 128, and 44 were the molecular weights of repeating unit of PS, PtBA, and PEO segment, respectively. The efficiencies of the ATNRC and click reactions (listed in Table 4) were calculated by the ratio of PEO and PtBA segments to PS segment in the 1H NMR spectrum of triblock copolymer, respectively (see Supporting Information). Compared with the efficiency of click reaction (89.6% and 87.9%, respectively), the efficiency of ATNRC reaction (86.0% and 80.9%, respectively) was a little lower. Figure 5. IR spectrum of PtBAa-PSA-PEOA and the corresponding precursor alkyne-PSA-Br. Figure 6. GPC curves of PtBAa-PSA-PEOA and the corresponding precursors PtBAa-N3, PEOA-TEMPO, and alkyne-PSA-Br by using RI detector. Figure 7. 1H NMR spectra of (a) alkyne-PSA-Br and (b) PtBAb-PSA- PCL (CDCl3 as solvent). 4132 Lin et al. Macromolecules, Vol. 41, No. 12, 2008 And it was very clear that the efficiency for both of ATNRC and click reactions was reduced with the increasing of the molecular weight (compared the efficiencies of the two triblock copolymers (PtBAa-PSA-PEOA and PtBAb-PSB-PEOB)). The mechanism of the ATNRC reaction is shown in Scheme 2. The terminal bromine group of alkyne-PS-Br served as oxidant was reduced to bromine anions, and then secondary carbon radicals of PS were formed; CuBr was served as reductant, the Cu1+ was oxidized to Cu2+, and the CuBr2 was formed. The formed carbon-centered radical was immediately capturedbytheTEMPOradicalofPEO-TEMPO,andalkoxyamines were formed between the two polymers. In the ATNRC reaction, CuBr participated in the reaction as reactant, and the reaction is irreversible, which is quite different from the common ATRP. One-Pot Synthesis of PtBA-PS-PCL Triblock Copoly- mer via a Combination of Click and ATNRC Reaction. PtBA-N3, alkyne-PS-Br, and PCL-TEMPO were reacted to synthesize corresponding triblock copolymer PtBA-PS-PCL in one-pot using a combination of click and ATNRC reaction as we described before. Figure 7 shows the characteristic peak of CHtC-CH2 (end group of PS) at 4.17-3.90 ppm disap- peared, and a new signal related to the triazole ring at 7.60-7.41 ppm appeared. Besides, the characteristic peak of CH(Ph)-Br (end group of PS) at 4.60-4.30 ppm disappeared as the result of ATNRC reaction. Moreover, the characteristic peaks of the phenyl group (repeating unit of PS), -(CO)-CH2CH2- CH2CH2CH2O (the fifth methylene group connected to carbonyl, repeating unit of PCL), and CH2CH (repeating unit of PtBA) at 7.26-6.27, 4.17-3.96, and 2.50-2.13 ppm, respectively, were observed, which confirmed the successful coupling of PtBA-N3 and PCL-TEMPO with alkyne-PS-Br chain in one pot. This one-pot reaction was further supported by IR spectra (Figure 8). Compared with the IR spectra of alkyne-PS-Br, the characteristic signal of -HCtC at 3295 cm-1 disappeared and a new signal related to triazole ring appeared at 1649 cm-1, which indicated the alkyne group was transformed to triazole group. Furthermore, a characteristic signal of the ether bond (-C-O-C-, belong to PCL chain) at 1151 cm-1 appeared while the signal of carbonyl group at 1731 cm-1 enhanced due to the successful coupling of PtBA-N3 and PCL-TEMPO with alkyne-PS-Br chain. The Mn,theo of PtBA-PS-PCL triblock copolymer was calculated by the sum of the separate Mn,NMR of the precursors which participated in the reaction, using eq 7. Mn,theo )Mn,NMR(PtBA-N3)+Mn,NMR(alkyne-PS-Br)+Mn,NMR (PCL-TEMPO)- 80 (7) Here, Mn,NMR(PCL-TEMPO) was the Mn,NMR value of the corresponding precursor PCL-TEMPO which participated in the one-pot reaction. The Mn,act of triblock copolymer was determined by a ratio of the integration of PS segment to PtBA and PCL segment, using eq 8. Mn,act )Mn,NMR(PtBA)+Mn,NMR(PCL)+Mn,NMR(PS) ) 5(Ae+p -Ab) Af × Mn,NMR(PS) 104 × 128+ 5Ab 2Af × Mn,NMR(PS) 104 × 114+Mn,NMR(PS) (8) Here, Mn,NMR(PCL) represents the actual Mn,NMR of PCL segment in PtBA-PS-PCL triblock copolymer, Ae+p represents the integral area of the peaks at “e” and “p” for the CH2CH methine group proton on PtBA segment and -(CO)- CH2CH2CH2CH2CH2O (the first methylene group connected to carbonyl) on PCL segment, respectively, Af represents the integral area of the peaks at “f” for phenyl group protons on PS segment, Ab represents the integral area of the peaks at “b” for -(CO)-CH2CH2CH2CH2CH2O (the fifth methylene group protons connected to carbonyl) on PCL segment, the value 114 is the molecular weight of repeating unit of PCL segment, and others are the same as defined in eq 6. The efficiencies of the ATNRC and click reactions (listed in Table 4) for the synthesis of PtBA-PS-PCL were also calculated by the ratio of PCL and PtBA segments to PS segment in the 1H NMR spectra of triblock copolymers, respectively (see Supporting Information); the efficiency of ATNRC reaction (81.6% and 80.1%, respec- tively) was a little lower than that of click reaction (85.9% and 80.7%, respectively). Comparing the efficiencies of the two type triblock copolymers (PtBAb-PSA-PCL and PtBAb-PSB- PCL), it also showed the efficiency, no matter ATNRC or click reactions, was reduced with the increasing of molecular weight. The GPC curves of the triblock copolymer and the correspond- ing precursors showed singlet and low PDI (Figure 9). All the data of the triblock copolymers are summarized in Table 4. Variation of the Thermal Transition Temperature of Triblock Copolymers. Thermal transitions of the triblock copolymers were determined by DSC at a heating rate of 10 °C/min under a nitrogen atmosphere (Figure 10). In the case of Scheme 2. Mechanism of the ATNRC Reaction Figure 8. IR spectrum of PtBAb-PSA-PCL and the corresponding precursor alkyne-PSA-Br. Macromolecules, Vol. 41, No. 12, 2008 ABC Type Triblock Copolymers 4133