Direct Transformation of Living Anionic Polymerization into RAFT

Article pubs.acs.org/Macromolecules

Direct Transformation of Living Anionic Polymerization into RAFTBased Polymerization Chao Zhang, Yuliang Yang, and Junpo He* The State Key Laboratory of Molecular Engineering of Polymers, Department of Macromolecular Science, Fudan University, Shanghai 200433, China S Supporting Information *

ABSTRACT: A method of direct switching from anionic polymerization into RAFT-based polymerization was developed. The transformation involved in situ addition of living carbanionic species, end-capped with DPE, toward CS2 and subsequent reaction with alkyl bromides, resulting in macro-RAFT agents (macro-CTAs). The macro-CTAs were used to mediate RAFT polymerization of (functional) vinyl monomers. These processes were performed in a continuous way without isolation of the intermediate. Diblock or ABA-type triblock copolymers, composed of polydiene segment and functional acrylate (2-hydroxyethyl acrylate, HEA) or acrylamide (N-isopropylacrylamide, NIPAM), were synthesized by RAFT polymerization mediated by PI macro-CTAs. The successful preparation of block copolymers was demonstrated by the self-assembly of amphiphilic diblock copolymer PI75-b-PNIPAM199 in water (selective solvent for PNIPAM) or heptane (selective solvent for PI). The obtained block copolymers are cleavable into homopolymer components due to the presence of the thiocarbonylthio moieties at their joint points. The block copolymers are also “clickable” as a whole onto Au nanoparticles through Au−sulfur complexation.

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INTRODUCTION Living polymerizations play a central role in the synthesis of polymers with well-defined structures such as block, star, and dendrimer-like polymers.1 Anionic polymerization is a classic, truly living system suitable for styrene, diene, (meth)acrylates, and cyclic monomers but intolerant of monomers with active hydrogen, vinyl acetate, and vinyl ethers. Controlled/“living” radical polymerizations based on reversible deactivation reactions are applicable for, in principle, all monomers that can be polymerized by radical polymerization. There is, however, a rather rigorous selectivity between monomers and mediating agents. For instance, in the radical polymerization mediated by the reversible addition−fragmentation chain transfer (RAFT) process, the selection of RAFT agents for a specific monomer should take into account the stabilizing effect of the Z-group and the leaving ability of the R-group of the RAFT agent.2 In many synthetic studies, it is necessary to combine two different polymerizations in order to make block copolymers that cannot be obtained using a single mechanism. The publications in this area have been reviewed extensively.3,4 In the following, we will focus on combination of RAFTbased polymerization and other systems. There are generally two approaches, i.e., the system using a dual initiator, or inifer,5,6 and the system employing end-group transformation. On one hand, inifers composed of thiocarbonylthio and alkyl halides,7−10 alkoxyamine,11 hydroxyl,12−17 carboxyl,18 and acyclic olefin19 groups have been proved very useful for combining RAFT and atom transfer radical polymerization (ATRP), nitroxide-mediated radical polymerization (NMRP), ring-opening polymerization (ROP), cationic polymerization, and ring-opening metathesis polymerization (ROMP), respec© XXXX American Chemical Society

tively. On the other hand, end-group transformation approach can be performed without a dual functional initiator precursor. The propagating center of the first block is converted into a functionality that is capable of initiating the polymerization of the second monomer. Thus, block copolymers are synthesized from an intermediate denoted as macroinitiator or macrochain-transfer agent (macro-CTA). For this purpose, the terminal halides of ATRP products were subject to nucleophilic attack by thiocarbonylthio salts to form macro-CTAs that could further mediate RAFT polymerizations.20−22 End-group modification of products prepared from ATRP, NMP, and cobalt-mediated polymerization with di(thiobenzoyl) disulfide afforded similar macro-RAFT agents.23−25 A mechanistic switch from RAFT to ROP was achieved using either a unique radicalinduced hydroxylation process26 or insertion reaction of a single molecule of hydroxyethylene cinnamate.27 An inverse process that switched from ROP to RAFT was accomplished using esterification of hydroxyl terminus into thiocarbonylthio moiety.28 Switching from cationic into RAFT polymerizations was also reported for the synthesis of block copolymers containing segment of isobutylene or cyclic monomers, such as cyclic thiourethane and oxazoline.29−31 A very special case was the direct transformation of the propagating radical center of RAFT polymerization into cationic center by in situ addition of a Lewis acid catalyst.32 This approach facilitated the synthesis of (meth)acrylates−vinyl ether block copolymers. Received: March 28, 2013 Revised: March 30, 2013

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dx.doi.org/10.1021/ma4006457 | Macromolecules XXXX, XXX, XXX−XXX

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TSK-GEL guard column, a Waters 515 pump, a Waters 2414 RI detector, and a Waters 2487 UV detector, using THF and THF with 0.25% tetrabutylammonium bromide as the eluents for PI CTA and PI-b-PNIPAM, respectively, at a flow rate of 1 mL/min at 40 °C. For PI-b-PHEA block copolymers, DMF with 0.2% LiBr served as the eluent at a flow rate of 1 mL/min at 80 °C for GPC characterization. Two TSK-GEL α-type columns (particle size: 7.0 μm, Mw ranges: 0− 5000 and 0−9 × 104 g/mol) and a TSK-GEL guard column were calibrated by polystyrene standards (Mw range: 682−1.4 × 106 g/mol). The absolute molecular weights (Mw) were measured by employing GPC-MALS equipped with a Waters 515 pump, a multiangle (14°− 145°) laser light scattering (MALS) detector (Wyatt Technology, DAWN HELLOS) with the He−Ne light wavelength at 658.0 nm, and a RI detector (λ = 658 nm, Wyatt Technology, OptilabrEX). THF was used as the eluent at 35 °C at a flow rate of 1 mL/min. The refractive index increment dn/dc was measured off-line by OptilabrEX at 25 °C. 1 H and 13C NMR spectra in CDCl3, THF-d8, or DMSO-d6 were collected on a Bruker 500 MHz spectrometer using tetramethylsilane (TMS) as the internal standard. Spectrophotometer Lambda 35 (PerkinElmer) was used for UV−vis measurements. The IR spectrum was recorded on a Magna-550 FTIR instrument (KBr cast). Thermogravimetric analysis (TGA, TA Q5000) was conducted under nitrogen atmosphere from ambient temperature to 700 °C at a heating rate of 20 °C/min. Transmission electron microscopy (TEM) images were recorded on a Tecnai G2 20 Twin TEM (FEI). The specimens for TEM observations were prepared by depositing a drop of the sample onto a carbon-coated copper grid. Model Reaction. DPE (1.02 g, 5.67 mmol) was dissolved in dry THF (30 mL) under a N2 atomosphere, and the solution was cooled to ca. −80 °C. s-BuLi (5 mL, 5.65 mmol) was added using a syringe. After 15 min, CS2 (0.44 g, 5.80 mmol) was added, and the system was stirred for another 20 min at ca. −80 °C. After addition of benzyl bromide (0.98 g, 5.73 mmol) using a syringe, the reaction mixture was slowly warmed up to room temperature and stirred for 30 min. Then, the mixture was concentrated by rotatory evaporation, and an orange oil was obtained. Characterizations were performed without further purification. Purity by HPLC = 99%. Yield: 2.76 g (97.9%). 1H NMR (DMSO-d6): δ (ppm) = 0.43 (d, 3H, CH3−CH), 0.57 (t, 3H, CH3− CH2), 0.81 (m, 2H, CH3−CH2), 1.13 (s, 1H, CH3−CH), 2.62 (q, 1H, CH−CH2), 2.69 (q, 1H, CH−CH2). 4.36 (s, 2H, CH2−Ph), 7.16− 7.46 (m, 15H, ArH). 13C NMR (DMSO-d6): δ (ppm) = 11.53 (CH3− CH2), 20.57 (CH3−CH), 30.66 (CH3−CH), 31.25 (CH−CH2), 41.87 (CH2−Ph), 49.00 (CH−CH2), 72.14 (C−Ph), 127.36−130.25 (15C, o-, m-, p-Ar), 135.15 (1C, phenyl attached to methylene), 143.26 (2C, phenyl attached to tertiary carbon), 245.59 (CS). FT-IR: ν (cm−1) = 1097 and 1238 (CS). UV−vis max (THF): 250 and 325 nm. Transformation of Anionic Living Center into Macro-CTAs, PI16-CSSR (R = Phenylethyl). Isoprene (5 mL, 3.41 g, 50.1 mmol) was dissolved in THF (30 mL) under a N2 atmosphere to form a solution, to which s-BuLi (2.2 mL, 2.22 mmol) was injected to initiate the polymerization. After the reaction was stirred at 0 °C for 20 min, an aliquot of the solution was taken and terminated for GPC analysis. The reaction mixture was cooled to ca. −80 °C. DPE (0.41 g, 2.28 mmol) was injected, and the reaction stood for 15 min; then CS2 (0.17 g, 2.24 mmol) was injected, and the reaction stood for another 30 min. An equimolar amount of 1-phenylethyl bromide (0.41 g, 2.22 mmol) was added using a syringe, and the reaction mixture was stirred for 5 h at 50 °C. An orange-colored solution was obtained which was used directly, or after simple concentration by evaporation, for the subsequent RAFT polymerizations either in solution or in bulk. For analysis, an aliquot of solution was concentrated, and the viscous residue was precipitated in methanol thrice and dried under vacuum. In the case of experiments using chelating agents, a double molar equivalent of ligands such as TMEDA, 12-crown-4 ether, or DEGDME was added before addition of phenylethyl bromide. PI16-CSSR (R = phenylethyl, the subscript denotes the degree of polymerization (DP) of PI segment calculated from Mn,NMR): Mn,GPC = 2100 g/mol, Mw/Mn = 1.08; Mn,NMR = 1500 g/mol. The procedure for synthesis of telechelic bifunctional macro-CTA, RSSC-PI41-CSSR (R = phenylethyl, DP from Mw,MALS), was similar to

There was a desire to combine anionic and RAFT polymerizations to prepare block copolymers of diene or ethylene oxide (EO) and functional vinyl monomers. A number of papers reported the transformation of hydroxyl or amino terminus of PEO, an anionic polymerization product, either commercially available or homemade, into thiocarbonylthio moieties through esterification using carboxyl- or acid chloridefunctionalized thiocarbonylthio compounds,33−40 addition of dithiobenzoic acid to the double bond after attachment of a single maleic acid,41 or amidation using succinimidyl esterfunctionalized dithioesters.42 Besides, sequential conversion of −OH into alkyl halides and then dithioesters or xanthates was also achieved through various organic reactions.43−47 Polymers of vinyl monomers prepared by anionic polymerization, such as (hydrogenated) polydienes and polystyrene functionalized with hydroxyl termini, were used as macro-CTA precursors in block copolymerization with N,N-dimethylacrylamide (DMA), Nisopropylacrylamide (NIPAM), glucose-functionalized methacrylamide, and vinylpyridines, etc.48−50 All these transformation processes involved two or more steps of reactions at the chain ends. In the present work, we report a direct method to convert the propagating center of anionic polymerization into thiocarbonylthio moiety that facilitates continuous synthesis of block copolymers. The conversion of the living anionic species into thiocarbonylthio moiety may be as high as 95%. Amphiphilic block copolymers of isoprene and NIPAM or 2hydroxyethyl acrylate (HEA) are prepared using the direct transformation method. In these products, the thiocarbonylthio moiety is located in the middle of the two block chains. Therefore, the diblock copolymers are cleavable by nucleophilic reaction and clickable onto Au nanoparticle surfaces via sulfur− Au complexation.

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EXPERIMENTAL SECTION

Materials. Styrene (Shanghai Linfeng, 99%), isoprene (TCI, 99%), 1,1-diphenylethylene (DPE) (Alfa Aesar, 98%), benzyl bromide (Sinopharm Chemical Reagent Co., 99%), 1-phenylethyl bromide (Aldrich, 97%), N,N,N′,N′-tetramethylethylenediamine (TMEDA) (J&K, 99%), diethylene glycol dimethyl ether (DEGDME) (J&K, 99%), and 12-crown-4 ether (TCI, 97%) were distilled over CaH2 and stored at −10 °C. Styrene and isoprene were distilled over di-nbutylmagnesium (Bu2Mg) (Aldrich, 1.0 M in heptane) before use. NIsopropylacrylamide (NIPAM) (Shanghai Wujin, 99%) was recrystallized twice from hexane prior to use. 2-Hydroxyethyl acrylate (HEA) (Shanghai Dibai, 98%) was distilled under vacuum. 2,2′-Azobis(isobutyronitrile) (AIBN) (Shanghai Fourth Factory of Chemicals, 99%) was recrystallized twice from methanol. Carbon disulfide (CS2) (Sinopharm Chemical Reagent Co., 99%) was purified by vigorously shaking with KMnO4 (0.4 wt % based on CS2) followed by filtration and distillation to collect a colorless fraction. n-Butyllithium (n-BuLi) (Aldrich, 2.5 M solution in cyclohexane/heptane) was used as received. sec-Butyllithium (s-BuLi) (Aldrich, 1.3 M solution in cyclohexane) was titrated before use. Lithium naphthalenide was prepared according to the literature,51 and the concentration was 1.05 M. Tetrahydrofuran (THF) (Shanghai Feida, 99.5%) was distilled from 1,1-diphenylhexyllithium (DPHLi, adduct of s-BuLi and DPE) on the vacuum line before use. All other reagents were used as received unless otherwise indicated. Measurement. High-performance liquid chromatography (HPLC) was performed on an instrument composed of a Waters 515 pump, a C-18 column (Symmetry ShieldP RP-18, 5.0 μm, 4.6 × 250 mm), a Waters 486 UV detector, and a Waters 410 RI detector. Gel permeation chromatography (GPC) was performed on a Waters system equipped with three TSK-GEL H-type columns (particle size: 5.0 μm, Mw ranges: 0−1000, 0−2 × 104, and 0−4 × 105 g/mol) and a B


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that for PI16-CSSR (R = phenylethyl), except that lithium naphthalenide was used to initiate the polymerization of isoprene. Mn,GPC = 4400 g/mol, Mw/Mn = 1.05, Mw,MALS = 3500 g/mol. RAFT Polymerization of Styrene Mediated by PI16-CSSR (R = Phenylethyl). The PI-based macro-RAFT agent obtained in the above process, PI16-CSSR (R = phenylethyl) (0.4 g, 0.19 mmol, Mn,GPC = 2100 g/mol), and AIBN (15.6 mg, 0.1 mmol) were dissolved in styrene (6.5 mL, 5.92 g, 56.9 mmol) (molar ratio of AIBN/PI16CSSR/styrene = 1/2/600). The reaction mixture was degassed by three cycles of freeze−pump−thaw, backfilled with nitrogen, and then heated at 70 °C for a predetermined period. Samples were taken at various reaction times via syringe for analysis. After 12 h, the reaction was quenched into liquid nitrogen to stop the polymerization. Mn,GPC = 18 000 g/mol, Mw/Mn = 1.21, Mn,NMR = 20 200 g/mol, conversion = 58.4%. RAFT Polymerization of NIPAM Mediated by PI16-CSSR (R = Phenylethyl). PI16-CSSR (R = phenylethyl) (0.2 g, 0.1 mmol, Mn,GPC = 2100 g/mol), AIBN (8.2 mg, 0.05 mmol), and NIPAM (3.39 g, 30 mmol) were dissolved in THF (15 mL) (molar ratio of AIBN/PI16CSSR/NIPAM = 1/2/600). The reaction mixture was degassed by three cycles of freeze−pump−thaw, backfilled with nitrogen, and then heated at 70 °C for a predetermined period. Samples were taken at various reaction times via syringe for analysis. After 75 min, the reaction was quenched into liquid nitrogen to stop the polymerization. Mn,GPC = 24 800 g/mol, Mw/Mn = 1.45, Mn,NMR = 13 300 g/mol, conversion = 50%. RAFT Polymerization of HEA Mediated by PI16-CSSR (R = Phenylethyl). PI16-CSSR (R = phenylethyl) (0.2 g, 0.1 mmol, Mn,GPC = 2100 g/mol), AIBN (8.2 mg, 0.05 mmol), and HEA (3.2 mL, 3.55 g, 30.6 mmol) were dissolved in a mixture solvent of dioxane/methanol (4/1, v/v) (molar ratio of AIBN/PI16-CSSR/HEA = 1/2/600). The reaction mixture was degassed by three cycles of freeze−pump−thaw, backfilled with nitrogen, and then heated at 70 °C for predetermined period. Samples were taken at various reaction times via syringe for analysis. After 3 h, the reaction was quenched into liquid nitrogen to stop the polymerization. Mn,GPC = 26 000 g/mol, Mw/Mn = 1.36, conversion = 33.6%. Cleavage of Block Copolymers. PI75-b-PS68 diblock copolymer (0.1 g, Mn,GPC = 21 000 g/mol) was mixed with s-BuLi (1 mL, 1.01 mmol) in THF (20 mL) under a N2 atmosphere. The mixture was stirred at room temperature for 3 h, quenched by methanol, then concentrated, and precipitated in methanol. The collected powder was dried under vacuum. Mn,GPC = 10 000 g/mol, Mw/Mn = 1.20. Self-Assembly of PI75-b-PNIPAM199 in Selective Solvents. PI75-b-PNIPAM199 diblock copolymer was purified by washing with nhexane twice (∼150 mL × 2) to remove the residual PI-based macroCTA. Then the purified product (10 mg, Mn,GPC = 47 000 g/mol, PDI = 1.39) was dissolved in THF (1 mL), and the solution was stirred for 24 h; then deionized water (9 mL) or heptane (9 mL) was added droply using syringe pump under vigorous stirring within 3 h. After stirring overnight, the blue opalescent suspension was dialyzed against deionized water or heptane for 3 days using a dialysis bag with a 14 kDa cutoff molecular weight. “Click” Reaction between Diblock Copolymers and Au Nanoparticles (AuNPs). Citrate-stabilized AuNPs were synthesized by reduction of HAuCl4·4H2O (50 mg, 0.12 mmol) using sodium citrate (0.20 g, 0.68 mmol) in aqueous solution (500 mL).52 The average particle size determined by TEM was 22.8 nm in diameter. The aqueous dispersion with a wine red color was concentrated by centrifugation (12 000 rpm, 30 min) and then mixed with a THF solution of PI75-b-PS68 diblock copolymer (0.2 g, Mn,GPC = 21 000 g/ mol, 10 wt %). After vigorous stirring for 24 h at ambient temperature, the mixture was centrifuged (12 000 rpm, 30 min), and the solid was collected, followed by eight cycles of washing with THF (10 mL × 8) and centrifugation to remove the free copolymers in solution. The purified AuNPs grafted with copolymer, PI75-g-AuNPs-g-PS68, were vacuum-dried overnight. Polymer content (by TGA): 26%.

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RESULTS AND DISCUSSION The transformation of anionic living center into thiocarbonylthio moiety is achieved through nucleophilic addition of anionic species toward carbon disulfide and subsequent reaction of the resulting dithiocarboxylates with alkyl bromides.53 It was reported that the nucleophilic reaction of CS2 and carbanion may proceed via both C-addition and Saddition (thiophilic addition) modes depending on reaction conditions and molecular structures.54−60 We achieved selective C-addition with the aid of 1,1-diphenylethylene (DPE) to modify the activity of the carbanion. This is demonstrated by a small molecular model reaction among sec-butyllithium (sBuLi), DPE, CS2, and benzyl bromide, as discussed in the following section. Model Reaction. The reaction between s-BuLi and DPE, followed by nucleophilic addition toward CS2, and then nucleophilic replacement on benzyl bromide were performed at low temperature in a continuous way without isolation of the intermediates (except for analysis), as shown in Scheme 1. All reactants were fed in a stoichiometric ratio. The obtained orange-colored oil was directly analyzed after simple evaporation concentration. Scheme 1. Model Reactions Converting Anionic Species into Thiocarbonylthio Moiety

Figure 1 shows the HPLC, FTIR, and NMR results of the product. The content of the main product is 99% as detected by UV detector at double wavelengths connected to HPLC. The reaction is found to be quantitative in a short reaction time with only one peak observed in HPLC diagram (Figure 1a). In the FTIR spectrum (Figure 1b), the strong absorption at 1097 cm−1 is assignable to CS stretch vibration. In the 1H NMR spectrum (Figure 1c), the chemical shifts of methyl protons derived from the sec-butyl group, methylene protons from DPE, and benzyl methylene protons from benzyl bromide are at 0.5, 2.7, and 4.3 ppm, respectively. The integration ratio, 3:1:1, agrees well with the anticipated structure. The 13C NMR spectrum (Figure 1d) shows a signal at δ = 245.6 ppm for the thiocarbonyl group. These results demonstrate that the reaction of CS2 with DPE-modified carbanion occurs exclusively in Caddition mode and is nearly quantitative. However, measurement of molecular weight by mass spectrometry failed, possibly due to the susceptibility of the thiocarbonylthio moiety to MS conditions. Synthesis of Mono- and Difunctional Macro-CTAs. The procedure for synthesis of PI- and PS-based macro-CTAs is very similar to that of model reaction, as shown in Scheme 2. The polymerization medium can be either cyclohexane or THF in order to adjust the content of 1,4-microstructure of PI segment. Nonetheless, the end-capping with DPE and the subsequent addition to CS2 must be taken in THF or mixture of cyclohexane/THF (v/v: 1/3). It is also important to use crown ether as chelating agent to promote the reaction between the intermediate dithiocarboxylic anion and alkyl bromides. The three-step end-group transformation is conducted in a continuous process. Figure 2 shows the results of PI-based macro-CTA synthesis, using 1-phenylethyl bromide as the C


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Figure 1. High performance liquid chromatography (HPLC) diagram (a), FTIR (b), 1H NMR (c), and 13C NMR (in DMSO-d6) (d) spectra of the model reaction product.

Unfortunately, no mass spectrum was acquired for PI macroCTAs under various MALDI-TOF conditions due to instability of thiocarbonylthio moiety. Two key factors determine the efficiency of the end-group transformation: the end-capping with DPE in the presence of THF and the nucleophilic reaction of dithiocarboxylates with alkyl bromides with the aid of 12-crown-4 ether as the chelating agent. Without DPE end-capping, coupling products were always observed, no matter how CS2 was added, e.g., in onebatch or slow addition or titration (Supporting Information). The coupling products may be formed via dimerization of dithiocarboxylate functionalized PI through Yates’ process.61 The presence of crown ether in the reaction of the intermediate dithiocarboxylic anion and alkyl bromides guarantees 95% and 93% end-group functionality, respectively, for PS- and PI-based macro-CTA. Without crown ether the conversion of anionic species into thiocarbonylthio moiety is as low as 74% (determined by NMR), as shown in Table 1. This difference may be ascribed to the increasing nucleophilicity of dithiocarboxylic anion after the counterion, Li+, was chelated

Scheme 2. Synthesis of Macro-CTAs, PS-CSSR and PICSSR, through a Three-Step Continuous Process

precursor for the leaving group. The three-step reaction causes only a slight shift in GPC elution curves while keeping a narrow distribution (Figure 2a). The obtained macro-CTA is orange in color, showing a maximum absorption at around 329 nm in the UV/vis spectrum (Figure 2b), which is typical for dithioesters. The 1H NMR spectrum in Figure 2c shows signals at δ = 7.0− 7.5 ppm for aromatic protons derived from DPE moiety and phenylethyl leaving group. The methyl protons derived from the initiator s-BuLi show signals at around 0.75 ppm. The content of 1,4-microstructure is 90% as estimated from NMR. In addition, the 13C NMR spectrum in Figure 2d shows a clear signal at δ = 244.3 ppm attributable to CS in the terminus.

Figure 2. Results of GPC (a), UV−vis (b), 1H NMR (c), and 13C NMR (d) in THF-d8 spectra for PI10-CSSR (R = phenylethyl) prepared with 12crown-4 ether as chelating agent. D


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Table 1. Macro-CTAs Synthesized from Living Anionic Species sample c, d

PS10-CSSR (R = phenylethyl) PI10-CSSR (R = phenylethyl)c,d PI10-CSSR (R = phenylethyl)c PI9-CSSR (R = benzyl) PI16-CSSR (R = phenylethyl) PI75-CSSR (R = phenylethyl) RSSC-PI41-CSSR (R = phenylethyl)

solvent for transformation

Mn,GPC (Da)

Mw/Mn (GPC)

Mn,NMRa (Da)

f (%)b (NMR)

cyclohexane/THF (1/3, v/v) cyclohexane/THF (1/3, v/v) THF THF THF THF THF

1500 1400 1400 1300 2100 11000 4400

1.06 1.08 1.08 1.08 1.08 1.05 1.05

1400 1100 1100 1000 1500 5500

95 93 90 74 74 70

Determined by 1H NMR. For example, for PI-based macro-CTA, Mn,NMR = [(68 (C5H8) × A1/2)/(A2/6)] + 57 (C4H9) + 180 (C14H12) + 76 (CS2) + MR, where A1 and A2 are the areas of the vinyl protons of PI and the methyl protons from initiator (s-BuLi), respectively. MR is the mass of leaving group. bf (%) (functionality) is determined by 1H NMR. For PI-based macro-CTA, f (%) = (A3/15)/(A2/6) × 100%, where A3 is the area of the aromatic protons (−C6H5 × 3) at 7.0−7.5 ppm; for PS-based macro-CTA, f (%) = A4/(A2/6) × 100%, where A4 is the area of the methine proton in R group (−CH(CH3)−Ph) at 4.86 ppm. c12-crown-4-ether was added as the chelating agent, other systems were conducted without any chelating agent. dSolvent for polymerization is cyclohexane. a

Figure 3. Kinetic results of styrene RAFT polymerization mediated by PI16-CSSR (R = phenylethyl): (a) GPC traces of samples taken during the polymerization; (b) reaction kinetics; (c) dependence of molar mass (g/mol) on monomer conversion.

by crown ether. Other additives, such as TMEDA and DEGDME, only slightly increase the transformation efficiency to ∼78%. In the case of benzyl bromide, TMEDA caused prompt precipitation due to quaternization reaction. Dilithium species Li−PI−Li was prepared from anionic polymerization of isoprene initiated by lithium naphthalenide in THF. However, the end-group functionality cannot be determined by NMR due to the lack of distinct proton in the initiator fragment. The successful synthesis of bifunctional macro-CTA is indicated by a clear shift of monomodal peak of GPC diagrams before and after the chain extension of the second block (Supporting Information). Synthesis of Block Copolymers. PI-based macro-CTAs were used to mediate radical polymerizations of styrene, NIPAM, and HEA to prepare block copolymers. These reactions were carried out either in bulk (for styrene) or in solution (for NIPAM and HEA). The RAFT polymerizations of these monomers generally exhibit living character although the Z-groups of macro-CTAs are attached to a polymer chain.

Nonetheless, different behaviors are observed for different monomers and RAFT agents. The results of styrene RAFT polymerizations mediated by PI-CSSR (R = benzyl and phenylethyl) are presented in Figure 3 and Supporting Information, respectively. Figure 3 shows a linear kinetics for the system mediated by PI16-CSSR (R = phenylethyl), indicating a stationary concentration of the propagating radicals which is caused by a dynamic balance between radical generation and termination after the main equilibrium of the RAFT process has been built. GPC curves gradually shift to high molecular weight region with increasing monomer conversions (Figure 3a). The molecular weight distributions are narrow, with the polydispersity indices (PDIs) being around 1.2 (Figure 3c). It is noted, however, that benzyl as the leaving group leads to a slow consumption of macroCTA (Supporting Information), whereas phenylethyl results in a fast depletion of the initial RAFT agent. This difference is clear by inspecting the evolution of GPC peak intensities of the macro-CTA precursors in the two systems. E


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Figure 4. Kinetic results of NIPAM RAFT polymerization mediated by PI16-CSSR (R = phenylethyl): (a) GPC traces of samples taken during the polymerization; (b) reaction kinetics; (c) dependence of molar mass (g/mol) on monomer conversion.

In a similar way, ABA-type triblock copolymer of styrene and isoprene, PS133-b-PI41-b-PS133 (SIS), was prepared from RSSCPI41-CSSR (R = phenylethyl) mediated RAFT polymerization of styrene. A product of SIS with molar mass Mn,GPC = 17 600 g/mol and PDI = 1.16 was obtained (Supporting Information). RAFT polymerizations of NIPAM are also performed using mono- and bifunctional macro-CTAs as the mediating agents, in which phenylethyl is the leaving group. In both systems, an induction period of ∼40 min is observed, and GPC curves indicate a slow conversion of macro-CTAs, as shown in Figure 4 and the Supporting Information. The slow conversion is attributable to slow cross-initiation of NIPAM by the phenylethyl radical. For monofunctional macro-CTA-mediated system in Figure 4, a bimodal distribution is observed at the reaction time of 30 min due to the presence of residual initial macro-RAFT agent and possibly unfunctionalized polyisoprene precursor. After the induction period, the polymerization proceeds rather fast. The dependence of molecular weight on monomer conversion (Figure 4c) exhibits an abrupt increase in the initial period and then levels off in middle and later stages. Thus, the RAFT polymerization process of NIPAM is complicated by the slow initiation. This results in a relatively broader distribution of the final product (Figure 4c). The system using bifunctional macro-CTA gives very similar results except that the final product is an ABA-type triblock copolymer, PNIPAM113-b-PI41-b-PNIPAM113, with molar mass Mn, GPC = 36 000 g/mol and PDI = 1.37 (Supporting Information). Well-defined block copolymer containing PI and PNIPAM segments was previously synthesized by using single anionic mechanism, yet NIPAM needs to be protected.62 RAFT polymerization of HEA mediated by PI-based macroCTAs shows strict demands upon solvent selection. This is because HEA, like 2-hydroxyethyl methacrylate (HEMA),63 tends to form gel during polymerization in some organic solvents. The cosolvent for PI and PHEA is quite limited. Thus, a mixture solvent of dioxane/methanol (4/1, v/v) is used for the synthesis of block copolymer, PI16-b-PHEA211 and PHEA67b-PI41-b-PHEA67. These products can be analyzed by GPC using DMF as eluent only for products with high contents of HEA units. As shown in Figure 5a, monomodal distributions

Figure 5. (a) GPC profiles of block copolymer products, PI16-bPHEA211 and PHEA67-b-PI41-b-PHEA67; (b) GPC evolution for RAFT polymerization of HEA mediated by PS20-CSSR (R = phenylethyl).

with relatively low polydispersities are observed. Nevertheless, samples taken during the polymerization are not well soluble in DMF due to high content of PI fraction, which impedes GPC analysis. In order to trace the reaction process by GPC, we used a polystyrene-based macro-CTA, PS20-CSSR (R = phenylethyl), instead of PI counterparts, to mediate the RAFT polymerization of HEA. GPC measurement is performed using DMF as eluent with 0.2% LiBr as additive. As shown in Figure 5b, GPC curves shift to high molecular weight region during the polymerization, but slow initiation is again observed from the residual peak of the initial macro-RAFT agent and unfunctionalized PS precursor. The polymerization proceeds without occurrence of cross-linking reactions. Diblock copolymer product, PS20-b-PHEA292, with a relatively low polydispersity (PDI < 1.3) was obtained after purification by extraction with methanol. Characterizations of all block copolymers are summarized in Table 2. It should be pointed out that PDI values in Table 2 and Figures 3−5 were obtained from GPC integrations excluding the peaks of the residual macro-CTA precursors. As a demonstration of amphiphilic block copolymer formation, we carried out the self-assembly of PI75-bPNIPAM199 in water (selective solvent for PNIPAM) or heptane (selective solvent for PI) at room temperature (∼10 °C). PI75-b-PNIPAM199 was purified by washing with n-hexane F


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Table 2. Characterization Results of Block Copolymers macro-CTAs

block copolymers

sample

Mn,GPC (Da)

Mn,NMR (Da)

Mw/Mn (GPC)

Mn,GPC × 10−3 (Da)

Mn,NMRc × 10−3 (Da)

Mw/Mn (GPC)

conv

PI9-b-PS139a PI16-b-PS180 PI16-b-PNIPAM117 PI16-b-PHEA211 PS20-b-PHEA292 PI75-b-PS68 PI75-b-PNIPAM199 PS133-b-PI41-b-PS133 PHEA67-b-PI41-b-PHEA67 PNIPAM113-b-PI41-b-PNIPAM113

1300 2100 2100 2100 2300 11000 11000 4400 4400 4400

1000 1500 1500 1500 2500 5500 5500 3500b 3500b 3500b

1.08 1.08 1.08 1.08 1.08 1.05 1.05 1.05 1.05 1.05

13.8 18.0 24.8 26.0 34.2 21.0 47.0 17.6 19.0 36.0

15.5 20.2 14.7

1.19 1.21 1.45 1.36 1.25 1.13 1.39 1.16 1.46 1.37

0.45e 0.58e 0.50e 0.34d 0.58f 0.21d 0.20d 0.52e 0.23d 0.44e

36.4 12.6 28.0 31.3 29.0

a

Benzyl as the leaving group, phenylethyl as the leaving group in all other systems. bMw,MALS. cDetermined by 1H NMR. For example, Mn,NMR of PIb-PS could be calculated: Mn,NMR = [(104 (C8H8) × A4/5 × Mn,NMR,PI)/(68 (C5H8) × A1/2)] + Mn,NMR,PI macro‑CTA, where A4 is the area of aromatic protons from polystyrene segment. dDetermined by gravimetry. eDetermined by GPC. fDetermined by 1H NMR.

base, e.g., s-BuLi, is used to cleave a model diblock copolymer, PI75-b-PS68 with Mn,GPC = 21 000 g/mol, prepared from PI75CSSR (R = phenylethyl) of Mn,GPC = 11 000 g/mol in a similar procedure. As shown in Figure 7, the copolymer is cleaved into

twice to remove the residual PI-based macro-CTA. The purified sample showed monomodal distribution of GPC diagram. Typically, PI75-b-PNIPAM199 was dissolved in THF, and then water was added to the solution using a syringe pump under vigorous stirring. During water addition the clear solution became turbid with precipitation. The obtained mixture was dialyzed against water to remove THF. Further precipitation developed during dialysis. The precipitation was redispersed by mechanical shaking and collected for TEM measurement. As shown in Figure 6a, cylindrical micelles of microns in length are

Figure 7. GPC graphs of PI75-b-PS68 before and after cleavage reaction.

species of Mn,GPC ≈ 10 000 g/mol after treatment with excess sBuLi in THF at room temperature. The peak after cleavage may be overlapping of the two components. Besides, a small shoulder appears at the same position to the precursor, which may be ascribed to addition product of s-BuLi toward thiocarbonylthio moiety. The possible reaction mechanism is shown in Scheme 3.69

Figure 6. TEM images for self-assembly of amphiphilic diblock copolymer PI75-b-PNIPAM199 after dialysis in water (a) and heptane (b) for 3 days.

observed, which may be composed of PI as the core and PNIPAM as the shell. On the other hand, when heptane was used instead of water, a stable opalescent solution was formed. The TEM image in Figure 6b shows spherical micelles formed with PNIPAM as the core and PI as the corona. These micelles tend to aggregate on copper grid before TEM measurement. Cleavage and “Click” Reactions of Block Copolymers. One feature of the block copolymers in the present work is that the thiocarbonylthio moiety is located at the junction of the two segments. This renders the copolymer products degradability into parts of component homopolymers and affinity to gold surfaces as a whole. It was reported that thiocarbonylthio termini of the RAFT-based polymerization products can be hydrolyzed, aminolyzed, or reduced into thiocarbonyl species and thiols.64−68 However, the dithioesters produced in the present work, either small molecule or macro-CTAs, are difficult to be completely hydrolyzed, aminolyzed, or reduced in the presence of primary amines, potassium tert-butoxide (tBuOK), KOH, and NaBH4, respectively (Supporting Information). A possible reason is that the Z-group, α,α-diphenylalkyl, may stabilize the thiocarbonylthio moiety. Therefore, a stronger

Scheme 3. Mechanism for Cleavage Reaction of Block Copolymer

Dithioesters and trithiocarbonates are able to coordinate with gold.70,71 Polymers prepared from RAFT polymerization were directly attached onto gold nanoparticles (AuNPs).72−75 In the present work, the block copolymers are attached to AuNPs by means of simply mixing a diblock copolymer, PI75-b-PS68, with citrate-stabilized AuNPs in THF/H2O (5/1, v/v). Since the complexation occurred at the junction point on block copolymers, AuNPs were covered by mixed brushes of polystyrene and polyisoprene after grafting. TGA shows a G


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polymer content of 26% in Figure 8. The hybrid material forms stable dispersion in toluene due to the stabilization effect of

Article

AUTHOR INFORMATION

Corresponding Author

*Phone +86-21-65643509; e-mail [email protected] (J.H.). Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We are grateful for the financial support by the National Nature Science Foundation of China (NSFC) (Grants 21074024 and 21174030). J.H. also thanks the Research Fund for the Doctoral Program of Higher Education of China (20100071110013).

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Figure 8. TGA curve of PI75-g-AuNPs-g-PS68. Insets: (a) solution of PI75-g-AuNPs-g-PS68 in toluene and its TEM image; (b) Pickering emulsion of DMF in n-hexane stabilized by PI75-g-AuNPs-g-PS68.

polymer brushes (inset a in Figure 8). In mixed solvents of DMF and n-hexane (1/3, v/v, DMF/n-hexane), which are selective solvents for PS and PI, respectively, the hybrid material forms Pickering emulsion with large sized oil-in-oil (o/ o) droplets due to self-assembly of the particles at the interfaces (inset b of Figure 8). Thus, the current study explores a facile approach to functionalize Au surface with mixed polymer brushes, which has been attracting continuous attention in the scientific community.76−80 Scheme 4. “Click” Reaction of Diblock Copolymers onto Au Nanoparticles and Assembly of the Resulting Hybrid Particles at n-Hexane/DMF Interface

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CONCLUSIONS The model reaction indicates that the addition reaction of carbanion, end-capped with DPE, toward CS2 is quantitative and exclusively in C-addition style. This facilitates direct transformation of the living anionic propagating center into dithioester moiety that can subsequently mediate RAFT polymerization. Both monofunctional and bifunctional macroCTAs (PS-CSSR, PI-CSSR, and RSSC-PI-CSSR) are prepared by this method. The end-group functionality can be as high as ca. 95% with the aid of crown ether as the chelating agent in the reaction of dithiocarboxylic anion with alkylbromides. Amphiphilic block copolymers of mechanistically incompatible monomers, such as isoprene/NIPAM and isoprene/HEA, are successfully synthesized. These block copolymers, carrying a thiocarbonylthio moiety at the connecting point of the chains, are not only cleavable by strong base but also clickable as a “Vshaped” precursor to Au surface through sulfur−Au complexation chemistry.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

Figures S1−S6. This material is available free of charge via the Internet at http://pubs.acs.org. H


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Direct Transformation of Living Anionic Polymerization into RAFT

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