One-Pot Synthesis of ABC Type Triblock Copolymers

One-Pot Synthesis of ABC Type Triblock Copolymers

(Parte 1 de 4)

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(tertbutyl 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,exceptPtBA-N3. The couplingreactionbetweenprecursorswas carriedout 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 polydispersityindices(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 homopolymerblocks,leadingto potentiallyinterestingproperties 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 monomerslimitedthe applicationsof conventionallivinganionic 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 transferradicalpolymerization(ATRP),1–14reversibleadditionfragmentation chain transfer (RAFT) polymerization,15,16 and nitroxide-mediatedpolymerization(NMP).17,18Especially,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.2 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 chemistryhas been used extensivelydue to its quantitative 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.2 They are also utilizedin functionalizedsurfaces,31–3 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-tetramethylpiperidine-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* Correspondingauthor:Fax+86-21-65640293;e-mailjlhuang@fudan.edu.cn.

10.1021/ma702404t C: $40.75 2008 American Chemical Society Published on Web 05/13/2008 radicalcoupling(ATNRC)reaction.Becausehalideand 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( -caprolactone) (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-polystyrene-block-poly(ethyleneoxide) (PtBA-PS-PEO) and poly- (tert-butyl acrylate)-block-polystyrene-block-poly( -caprolactone) (PtBA-PS-PCL) were successfully prepared with high efficienciesvia a combinationof “click chemistry”and ATNRC reaction.

Experimental Section

Materials. Ethylene oxide (EO, 9.9%, Sinopharm Chemical

Reagent) (SCR), tert-butyl acrylate (tBA, 9%, SCR), -caprolactone (CL, 9%, SCR), and propargyl alcohol (9%, SCR) were

dried by CaH2 for 48 h and distilled before use. Styrene (St; 9.5%,SCR) purchased from SCR was washed with a 15% NaOH aqueous solution and water successively for three times, dried over anhydrousMgSO4, furtherdried over CaH2, and then distilledunder reduced pressure twice before use. Diphenylmethylpotassium

(DPMK) solutionwith a concentrationof 0.630 mol/L was prepared according to the literature.42 4-Hydroxyl-TEMPO (HTEMPO) prepared according to the literature43 was purified by recrystallization with hexane. CuBr (95%, SCR) was stirred overnight in acetic acid, filtered, washed with ethanol and diethyl ether successively, and dried in vacuo. N,N,N′,N′′,N′′-Pentamethyldiethylenetriamine(PMDETA,9%),ethyl2-bromoisobutyrate(EBiB,98%), and 2-bromoisobutyrylbromide(98%)werepurchasedfromAldrich and used without further purification. Acetone (9%), tetrahydrofuran (THF, 9%), toluene (9%), pyridine (9%), N,N-dimethylformamide (DMF, 9%), 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 10 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 substituted 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-

ethylsilaneas 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 spectrometer. 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 meltingtemperatures(Tm) were calculatedas a midpoint and a peak apex of thermograms, respectively. DSC was calibrated for temperatureby 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). Propargyl alcohol (5.0 mL, 85.9 mmol) was dissolved in 40.0 mL of pyridine and cooled in an ice-water bath, and a solution of 2-bromoisobutyrylbromide (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

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 °Cf or6h , 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 precipitatedin cold methanol. The precipitate was collected and purified by dissolution/precipitation with THF/cold methanol twice and then dried at 40 °C in vacuum 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

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, 1.0 mmol), CuBr (0.144 g, 1.0 mmol), PMDETA (0.210 mL, 1.0 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 °Cf or6h , 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 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 linearPS as standard.c Mn,NMR(the NMR number-averagemolecularweight) and DP (number-average degree of polymerization of St), measured by 1H

NMR spectroscopy.

4128 Lin et al. Macromolecules, Vol. 41, No. 12, 2008

to linear PS standard); Mw/Mn ) 1.13). Then, the precipitate of PtBA-Br (3.60 g, 1.20 mmol) was dissolved in DMF (15 mL), and sodium azide (0.390 g, 6.0 mmol) was added to the solution. The reaction mixture was stirred 24 h at room temperature. Dichloromethane (25.0 mL) was added into the mixture and washed three times with distilled water. The organic layer was dried with anhydrous MgSO4, and the solvent was removed by vacuum. Then the product was collected and dried at

Synthesis of TEMPO End-Functionalized PEO (PEO-

TEMPO). PEO-TEMPO was prepared by ring-opening polymerization (ROP) of EO in THF using DPMK and HTEMPO as initiators. In an ampule, the dried HTEMPO (0.690 g, 4.0 mmol) by azeotropicdistillationin 30.0mL of THF was introduced,DPMK solution (1.51 mL, 0.950 mmol) was injected into the ampule under nitrogen by a syringe, and then EO (17.0 mL, 336 mmol) and THF (40.0 mL) were added. The reaction was allowed to proceed at 60 °C for 72 h. At the end of the polymerization, excessive methanol was added to terminate the reaction. After removing the solvent, the mixture was diluted with dichloromethane and precipitated into an excessiveamount of diethyl ether for three times. The precipitate was dried in vacuum oven at 40 °C for 24 h, and the pink powder

was obtained ([M]0/[I]0 ) 84.0; conversion ) 100%; the theoretical Mn (Mn,theo) ) 3900; Mn,NMR ) 3900 (see Figure 2 and eq 3 was used for calculation); Mn,GPC ) 3600 (relative to linear PEO standard); Mw/Mn ) 1.20). Two types of PEO-TEMPO (PEOATEMPOandPEOB-TEMPO)withdifferentMn wereprepared(listed in Table 3). 1H NMR (CD3OD, in the presence of Pd/C and HCOONH4, δ): 3.78-3.60 (CH2CH2O, repeating unit of PEO), 1.97-1.92 and 1.4-1.39 (CH2, methylene protons of TEMPO), 1.19-1.14 (CH3, methyl protons of TEMPO) (Figure 2).

Synthesis of TEMPO End-Functionalized PCL (PCLTEMPO). PCL-TEMPO was prepared by ROP of -CL in toluene solutionusing stannousoctoateSn(Oct)2 as a catalystand HTEMPO as an initiator. The dried HTEMPO (0.190 g, 1.10 mmol) by azeotropic distillation with dry toluene was dissolved in 3.50 mL of toluene, to which CL (4.32 mL, 40.0 mmol) was added. Then a givenamountof the catalyst([Sn(Oct)2]/[OH]) 0.500)was injected under nitrogen by a syringe. The reaction was allowed to proceed at 100 °C for 24 h. After cooling to room temperature, the products were dissolved in THF and precipitated into an excess amount of methanol. The precipitate was isolated by filtration and dried at

4.12-4.02 [-(CO)-CH2CH2CH2CH2CH2O, the fifth methylene group connected to carbonyl of repeating unit of

PCL], 2.35-2.24 [-(CO)-CH2CH2CH2CH2CH2O, the first methylene group connected to carbonyl of repeating unit of PCL],

Figure 1. 1H NMR spectrum of the precursor of PtBAa-N3.

Table 2. Characterization of the Synthetic PtBA-N3 a sample Mn,GPCb (g/mol) Mn,NMRc (g/mol) Mw/Mnb DPc a PtBA-N3: poly(tert-butyl acrylate) with azide end group, obtained by ATRP of tBA using EBiB (ethyl 2-bromoisobutyrate) as an initiator, CuBr/

PMDETA as a catalyst system at 60 °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 stan- dard. c Mn,NMR (the NMR number-average molecular weight) and DP (number-average degree of polymerization of tBA), measured by 1H NMR spectroscopy.

Figure 2. 1H NMR spectrum of PEOA-TEMPO (in the presence of Pd/C and HCOONH4 using CD3OD as solvent).

Table 3. Characterization of the Synthetic PEO-TEMPOa sample Mn,GPCb (g/mol) Mn,NMRc (g/mol) Mw/Mnb DPc a PEO-TEMPO: poly(ethylene oxide) with a 2,2,6,6-tetramethylpiperidine-1-oxyl end group, obtained by ROP of EO in THF using DPMK (diphenylmethylpotassium) and HTEMPO (4-hydroxyl-2,2,6,6-tetrameth- ylpiperidine-1-oxyl) as co-initiators at 60 °C. b Mn,GPC (the GPC number- average molecular weight) and Mw/Mn (molecular weight distribution), measured by GPC in distilled water with RI detector, calibration with linear

PEO as standard. c Mn,NMR (the NMR number-average molecular weight) and DP (number-average degree of polymerization of EO), measured by

1H NMR spectroscopy.

Figure 3. 1H NMR spectrum of PCL-TEMPO (in the presence of phenylhydrazine using CDCl3 as solvent).

Macromolecules, Vol. 41, No. 12, 2008 ABC Type Triblock Copolymers 4129

methylene groups connected to carbonyl of repeating unit of PCL],

1.42-1.3 [-(CO)-CH2CH2CH2CH2CH2O, the third methylene group connected to carbonyl of repeating unit of PCL], 1.28-1.17

(CH3, methyl protons of TEMPO) (Figure 3). One-Pot Synthesis of PtBA-PS-PEO Triblock Copolymer.

(Parte 1 de 4)

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