HCl·Et2O-Catalyzed Metal-Free RAFT Cationic Polymerization: One

Jul 30, 2015 - ACS eBooks; C&EN Global Enterprise .... Japan Science and Technology Agency, PRESTO, 4-1-8 Honcho Kawaguchi, Saitama 332-0012, Japan ...
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HCl·Et2O‑Catalyzed Metal-Free RAFT Cationic Polymerization: OnePot Transformation from Metal-Free Living Cationic Polymerization to RAFT Radical Polymerization1 Shinji Sugihara,*,†,‡ Naoto Konegawa,† and Yasushi Maeda† †

Department of Applied Chemistry and Biotechnology, Graduate School of Engineering, University of Fukui, 3-9-1 Bunkyo, Fukui 910-8507, Japan ‡ Japan Science and Technology Agency, PRESTO, 4-1-8 Honcho Kawaguchi, Saitama 332-0012, Japan S Supporting Information *

ABSTRACT: The metal-free RAFT cationic polymerization (MRCP) of vinyl ethers (VEs) mediated by HCl·Et2O and 1isobutoxyethyl ethanedithioate (IDTA) as a RAFT cationogen was demonstrated. The IDTA was efficiently catalyzed and the cationic polymerization was initiated by the HCl·Et2O, allowing good control over the molecular weight, polydispersity, and chain end structure of the resulting polyVEs. The propagation step is considered to involve a reversible addition−fragmentation chain transfer (RAFT) to the growing carbocationic species. Thus, the resulting polyVEs can exhibit high number-average end functionality at the RAFT terminal group, depending on the [IDTA]0/[HCl]0 ratio. The polymers obtained by this process could be used as macro-chain transfer agents for the RAFT radical polymerization of radically polymerizable monomers such as (meth)acrylates and styrenes to synthesize novel block copolymers. Significantly, this MRCP system allows a one-pot transformation from MRCP to RAFT radical polymerization as a result of the metal-free nature of the processes. The syntheses of block copolymers were confirmed by GPC and the formation of novel thermoresponsive micelles in water by the amphiphilic block copolymer poly(2-ethoxyethyl vinyl ether)-b-poly[poly(ethylene glycol) methyl ethyl acrylate] was observed.

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INTRODUCTION Living cationic polymerization is at present a relatively common means of designing and synthesizing new polymeric materials with control over various aspects of architecture, such as molecular weight, molecular weight distribution (MWD), endfunctional chains, and sequencing of repeating units or segments.2 Over the past decade, the development of living cationic polymerization has permitted the synthesis of macromolecules with interesting shapes and functions, including cyclopolymers,3 stereoblock copolymers,4 chemically recyclable polymers,5 biomass-derived polymers,6 stimuli-responsive polymers,7 and microspheres and capsules.8 Various initiating systems have been applied for use in living cationic polymerization systems since the first reports in the 1980s.9,10 In general, cationic polymerizations are mediated by cationogens and activators (or catalysts). Compounds with analogous structures to the monomer, such as vinyl ether−HCl adducts and −acetic acid adducts, are used as cationogens to avoid ineffective cross-transfer reactions.2 In addition, metal halides acting as Lewis acid catalysts are employed to initiate the cationogen and generate the cationic propagating species by activating dormant chain ends. Thus, combined initiating systems using cationogens and metal halide catalysts are one of © XXXX American Chemical Society

the best ways to achieve the controlled cationic polymerization of a variety of functional monomers. However, metallic residues originating from the catalysts are difficult to remove from the resulting polymers, and often cause other problems, such as degradation and pigmentation. Hence, metal-free catalysts have been developed for us in both living cationic polymerization and various precision polymerizations,11,12 as well as other processes within the field of organic chemistry.13 To the best of our knowledge, only a limited number of studies concerning the metal-free living cationic polymerization (MLCP) of vinyl ether (VE) monomers have been reported. These include the investigation of the HI/I2 living cationic polymerization initiating system,9 as well as the VE-HI/alkyl ammonium salt,14,15 1-trimethylsiloxy-4-iodo-3-oxbutane/tetraalkylammonium triflate,16 the CF3SO3H/sulfide17 or alkylammonium salts,18 and the HCl/aliphatic ether system developed by our own group.19−21 In addition, the polymerization of another vinyl monomer, N-vinylcarbazole, with HI Received: May 19, 2015 Revised: July 4, 2015

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Macromolecules Scheme 1. Transformation of MLCP Using a RAFT Cationogen (MRCP) to RAFT Radical Polymerization

alone22 and the polymerization of cyclic unsaturated ethers with AcClO423 have been reported. However, most such initiating systems employ strong protonic acids that generate significant vapor emissions and thus are challenging to work with. With regard to the HCl/aliphatic ether system, we have reported that an HCl·Et2O complex can be used as an initiating system for the MLCP of VEs. This system is versatile because it allows living cationic polymerization to proceed smoothly without using complex metallic catalysts. Furthermore, HCl· Et2O is commercially available and easy to handle. In this compound, the HCl is both dissolved and ionized in diethyl ether (Et2O), thus the HCl·Et2O complex acts as a protonogen and functions both as an activator and an initiator at comparatively mild temperatures (0−30 °C). Furthermore, Et2O is a weak Lewis base and so works as an added base to stabilize the generated carbocation. Since various aliphatic

ethers with higher boiling points, such as dibutyl ether, 1,4dioxane, tetrahydrofuran (THF), and cyclopentyl methyl ether, may be used in place of Et2O, the polymerization temperature can be varied between 0 and 65 °C. However, there are some challenges associated with the promotion of MLCP by HCl· Et2O. The α-end of the resulting poly(VE) is derived from the VE monomer-HCl adduct formed by HCl·Et2O, and so is difficult to functionalize in the same manner as polymers synthesized by other living cationic polymerization systems. Thus, neither postpolymerization modification2 nor the transformation from cationic to radical polymerization using the α-end groups of the product24,25 has yet been accomplished following MLCP in the presence of HCl·Et2O. In this article, we report the metal-free reversible addition− fragmentation chain transfer (RAFT) “cationic” polymerization (MRCP) of alkyl VEs using an HCl·Et2O/RAFT agent together B

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OCH2CH(CH3)2, J = 6.7 Hz), 1.66 (d, 3H, CH3CH, J = 6.3 Hz), 1.84 (m, 1H, OCH2CH(CH3)2), 2.82 (s, 3H, SC(S)CH3), 3.27 and 3.39 (dd, 2H, OCH2CH(CH3)2, J = 6.4 and 6.9 Hz), 5.63 (q, 1H, CH3CH, J = 6.3 Hz). 13C NMR (CDCl3, 125 MHz, 30 °C): δ 19.23 (OCH2CH(CH3)2), 22.06 (CH3CH), 28.31 (OCH2CH(CH3)2), 40.40 (SC(S)CH3), 76.46 (OCH2CH(CH3)2), 88.98 (CH3CH), 234.84 (SC(S)CH3). IR (neat, cm−1) 1053, 1090, 1116, 1195, 1470, 2957. UV/vis: λmax 305 nm in hexane (εmax: 9740). Synthesis of 1-(2-Metoxyethoxy)ethyl Acetate (MOEA). MOEA was prepared by treatment of 2-metoxyethyl vinyl ether (25.6 mL, 0.23 mol, supplied by Maruzen Petrochemical) with acetic acid (8.7 mL, 0.15 mol) at 70 °C in 10 h. Then the mixture was allowed to cool to room temperature and washed with water. The organic extracts were dried over anhydrous sodium sulfate and filtered. The crude product was distilled twice over calcium hydride under reduced pressure (58.0 °C/5.0 mmHg) to give MOEA as a colorless liquids (17.8 g, 0.11 mol, 73% isolated yield from acetic acid; purity >99%). 1H NMR (CDCl3, 500 MHz, 25 °C): δ 1.43 (d, 3H, CH3CH, J = 5.3 Hz), 2.08 (s, 3H, OC(O)CH 3 ), 3.39 (s, 3H, OCH2CHOCH3), 3.54 (t, 2H, OCH2CH(CH3)2, J = 4.4 Hz), 3.68 and 3.80 (dt, 2H, OCH2CH2OCH3, J = 5.0 and 4.6 Hz), 5.93−3.70 (q, 2H, OCH2CH2OCH3, J = 5.2 Hz). 13C NMR (CDCl3, 125 MHz, 30 °C): δ 20.11 (CH 3 CH), 20.53 (OC(O)CH 3 ), 58.32 (OCH2CH2OCH3), 67.74 (OCH2CH2OCH3), 71.03 (OCH2CH2OCH3), 95.82 (CH3CH2), 169.89 (OC(O)CH3). IR (neat, cm−1) 1252, 1737, 2938. MRCP Procedures. The polymerizations were carried out at 0 °C under a dry nitrogen atmosphere in glass tubes equipped with a threeway stopcock baked at 250 °C for 10 min before use. A typical example for the polymerization of IBVE in the presence of IDTA in hexane is given below ([IBVE]0 = 0.80 M, [IDTA]0 = 6.0 mM, [HCl]0 = 2.8 mM, [Et2O] = 0.96 M). First, distilled IDTA was diluted to 200 mM in hexane, and HCl·Et2O was diluted to 28 mM HCl in diethyl ether prior to use. Hexane (3.83 mL), IBVE (0.52 mL, 3.99 mmol), and 200 mM of IDTA in hexane (0.15 mL) were added into the glass tube using dry medical syringes. The polymerization was initiated by the addition of a prechilled 28 mM HCl·Et2O at 0 °C. After the desired time, the reaction was terminated with prechilled methanol (0.5 mL) containing a small amount of aqueous ammonia solution (0.1 wt %). The quenched mixture was diluted in hexane and was successively washed with water. The volatiles were then removed under reduced pressure, and the residue was vacuum-dried for a day at room temperature. The monomer conversion was determined by 1H NMR analysis of quenched mixture. The MWDs were assessed by gel permeation chromatography (GPC). The theoretical number-average molecular weight (Mn) on conversion is defined as follows:

with a cationogen/alkyl VE initiating system. The RAFT process is a well-known precision radical polymerization that makes use of a RAFT agent in the form of a thiocarbonylthio compound to control both the molecular weight and the molecular weight distribution (MWD) during free radical polymerization.26−30 In the present work, RAFT radical polymerization was applied to MLCP using HCl·Et2O, employing a synthesized isobutyl vinyl ether−dithioacetic acid adduct (IDTA) as the RAFT cationogen. This adduct has the advantage of a simple structure and can be purified by distillation. In our process, a small amount of HCl·Et2O was used to catalyze the living cationic polymerization of VEs initiated by IDTA. The effects of the HCl concentration on the living cationic polymerization and on the structure of the resulting polyVE are discussed in detail. The results suggest that MLCP with HCl·Et2O in the presence of IDTA includes RAFT process, which we term MRCP. The syntheses of novel block copolymers consisting of cationically and radically polymerized segments, such as VE−styrenes, VE−acrylates, and VE− methacrylates, were performed via a transformation from HCl·Et2O-induced MRCP to RAFT radical polymerization (Scheme 1). Furthermore, we enabled the one-pot transformation of MRCP to the RAFT radical polymerization without purification of the MRCP system. The synthesis of block copolymers was confirmed by GPC and by the novel formation of thermoresponsive micelles in water by the amphiphilic block copolymer.

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EXPERIMENTAL SECTION

Materials. Isobutyl vinyl ether (IBVE) (TCI; >99.0%), n-propyl vinyl ether (NPVE) (Sigma-Aldrich; ∼99%), 2-ethoxyethyl vinyl ether (EOVE) (kindly donated by Maruzen Petrochemical; >99.0%), 4-tertbutoxystyrene (tBOS) (Sigma-Aldrich; ∼99%), and styrene (St) (TCI; >99.0%) were washed with aqueous alkaline solution and then with water. These monomers were distilled twice over calcium hydride and were stored in a brown ampule under dry nitrogen in refrigerator. Ethyl acrylate (EA) (TCI; >99.0%), methyl methacrylate (MMA) (Wako; >98.0%), and poly(ethylene glycol) methyl ether acrylate (PEGA) (Mn ∼ 480, Sigma-Aldrich) were purified using inhibitor removers prepacked column (Sigma-Aldrich). For solvent, hexane (Wako; >96.0%) and toluene (Wako; > 99.5%) were washed by the usual method and then was distilled over calcium hydride just before use. Diethyl ether (Et2O) (Wako; >99.5%, super dehydrated) was used as commercially supplied. HCl·Et2O [Sigma-Aldrich; 1.0 M hydrogen chloride in diethyl ether (1.0 M HCl·Et2O)] was used as received. The HCl concentration was determined by titration with NaOH aqueous solution. 2,2′-azobis(isobutyronitrile) (Wako; AIBN) was recrystallized from diethyl ether and stored in a refrigerator. Synthesis of 1-Isobutoxyethyl Ethanedithioate (RAFT Cationogen, IDTA). IDTA was synthesized by the reaction between IBVE and dithioacetic acid prepared from carbon disulfide. For the dithioacetic acid, according to the reported method,29 methylmagnesium chloride (90 mL, 0.27 mol, 3 M solution in tetrahydrofuran (THF), Sigma-Aldrich) was diluted with THF (45 mL) and warmed to 40 °C, and carbon disulfide (Wako; >99.0%) (16.3 mL, 0.27 mol) was added dropwise over 15 min while maintaining the reaction temperature at 40 °C. After 2 h, IBVE (45 mL, 0.35 mol) was directly added into the reaction mixture and was stirred at 60 °C for 1 day. Then the mixture was allowed to cool to room temperature and poured slowly into ice water (100 mL) and extracted with diethyl ether (100 mL). The organic extracts were dried over anhydrous sodium sulfate, and filtered, and the solvent was evaporated to give the crude product (34.2 g, 66% yield). The crude product was distilled twice over calcium hydride under reduce pressure (54.0 °C/0.4 kPa) to give IDTA as a yellow liquids (15.6 g, 81.1 mmol, 30% isolated yield from carbon disulfide; purity >99%). 1H NMR (CDCl3, 500 MHz, 30 °C, see the observed spectrum in Figure 3B): δ 0.89 (d, 6H,

Mn =

[monomer]0 × M monomer × convn [RAFT cationogen]0 + MRAFT cationogen

(3)

where MRAFT cationogen and Mmonomer are the molecular weights of RAFT cationogen such as IDTA and monomer, and [monomer]0 and [RAFT cationogen]0 are the initial concentrations of monomer and RAFT cationogen, respectively. RAFT Radical Polymerization Using Macro-CTA Procedures. Poly(vinyl ether) macro-CTA with various degree of polymerizations (DPs) obtained by MRCP was mixed with AIBN and varying amount of radically polymerizable monomers such as EA, St, and MMA in a Schlenk tube. These are then diluted in toluene for the desired concentration in a Schlenk tube equipped with a magnetic stir bar. The typical contents for poly(IBVE)-b-poly(EA) are as follows: toluene (1.448 g), EA (0.274 g, 2.73 mmol), poly(IBVE) macro-CTA (DP = 25, 0.037 g, 13.7 μmol), and 1.0 wt % AIBN in toluene (0.067 g, 4.10 μmol); [EA]0 = 15 wt % and [EA]0/[poly(IBVE) macro-CTA]0/ [AIBN]0 = 200:1:0.3 molar ratio. The solution was purged with nitrogen for approximately 20 min in an ice bath and then placed in a preheated oil bath at 70 °C. The polymerization was terminated after 24 h via cooling in ice water and exposure to air. The volatiles were then removed under reduced pressure over 50 °C, and the residue was C

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Figure 1. (A) Time−conversion curves and (B) ln([M]0/[M])−time plots for Mn and Mw/Mn from the MLCP of IBVE using HCl·Et2O with either IDTA (red) or MOEA (black) in hexane at 0 °C: [IBVE]0 = 0.80 M, [IDTA or MOEA]0 = 6.0 mM, [HCl]0 = 2.8 mM, [Et2O] = 0.96 M, and (C) MWD curves of poly(IBVE) obtained by the MLCP of IBVE using IDTA during the monomer-addition experiment: [IBVE]0 = [IBVE]added.

Figure 2. MLCP of IBVE using HCl·Et2O with IDTA in hexane at 0 °C: [IBVE]0 = 0.80 M, [IDTA]0 = 4.0−32.0 mM, [HCl]0 = 2.8 mM, [Et2O] = 0.96 M. (A) Mn or Mw/Mn-[IBVE]0/[IDTA]0 plots and (B) GPC traces for the poly(IBVE) (conversion >99%). Black and red traces (for [IDTA]0 = 32.0 mM only) indicate data obtained by RI and UV at 305 nm, respectively. were doubly checked using LALS/RALS (light scattering) detector for absolute molecular weight (Viscotek 270max model with 270 dual detector, Malvern). The 1H NMR spectra to determine the detailed structure and the compositions of block copolymers were recorded on either JEOL JNM-EX300 (300 MHz) or JEOL JNM-EX500 spectrometer (500 MHz). Matrix-assisted laser desorption ionization time-of-flight mass (MALDI−TOF−MS) spectra were recorded using a Bruker Daltonics autoflex spectrometer (linear mode) using dithranol as the matrix and sodium trifluoroacetate as the ion source (polymer sample/dithranol/sodium trifluoroacetate = 1/8/1 weight ratio). Dynamic light scattering (DLS) studies for poly(EOVE)-bpoly(PEGA) were performed using a Zetasizer Nano-ZSP instrument (Malvern) at various temperatures at a scattering angle of 173°. The aqueous copolymer solution for light scattering studies was prepared at the desired concentration and filtered at 10 °C prior to use. The mean hydrodynamic diameter (Dh) and polydispersity (PDI, μ2/Γ2) of the micelle were calculated by cumulants analysis of the experimental correlation function using Zeta Software version 7.02. Atomic force microscopy (AFM) measurements were performed with SPM-9700 (Shimadzu) for micelle of poly(EOVE)-b-poly(PEGA) in water. The micellar sample for AFM imaging was prepared by placing a 20 μL droplet of 0.05 wt % diblock copolymer dispersion in water onto freshly cleaved muscovite mica (V-4 grade, ca. 1 cm ×1 cm) and allowing it to dry in air at 30 °C. The imaging was performed in

vacuum-dried for a day at room temperature. For poly(EOVE)-bpoly(PEGA), the crude product was then purified by dialysis against deionized water at room temperature using semipermeable cellulose tubing (SPECTRA/POR, corresponding to a molecular weight cutoff of 1000 Da) with at least six changes of deionized water, followed by lyophilization. One-Pot Transformation from MRCP into RAFT Radical Polymerization Procedures. First, RAFT cationic polymerization proceeds at full monomer conversion in the same way using in a Schlenk tube equipped with a magnetic stir bar. Here, toluene was used instead of hexane as a solvent for RAFT cationic polymerization. The polymerization was quenched with prechilled methanol (0.5 mL). EA and AIBN was successively added into the reaction mixture and the solution was purged with nitrogen for approximately 20 min in an ice bath. The tube was placed in a preheated oil bath at 70 °C, and the polymerization was terminated after the desired time via cooling in ice water and exposure to air. Polymer Characterization. MWDs were assessed by GPC in tetrahydrofuran (THF) at 40 °C using three polystyrene gel columns [TSK gel G-MHHR-M × 3 (exclusion limit: 4 × 106 (PSt)); 7.8 mm i.d. × 300 mm each; flow rate 1.0 mL/min] connected to a Tosoh CCPMII pump and a RI-8020 and UV-8020 for refractive index (RI) and ultraviolet (UV) detector, respectively. The RI detector was mainly used for determining of Mn and the polydispersity (Mw/Mn) calculated from GPC curves on the basis of a PSt calibration. The Mns D

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Macromolecules dynamic mode using an etched silicon tip operating at a 300 kHz resonant frequency and a 26 N/m constant force (Olympus, OMCLAC160TS-C3).

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RESULTS AND DISCUSSION HCl·Et 2O-Catalyzed MLCP of VEs with a RAFT Cationogen. IDTA was employed as a RAFT agent Scheme 2. Proposed Mechanism for Initiation of HCl·Et2OCatalyzed MLCP (MRCP)

Figure 4. MALDI−TOF−MS data for the poly(IBVE) sample in Figure 3A. The chemical structures of poly(IBVE) with various ω-end groups were estimated from the spectrum.

Figure 5. Relationship between Mn or Mw/Mn of poly(IBVE) and [HCl]0 for MLCP using HCl·Et2O with either IDTA or MOEA in hexane at 0 °C: [IBVE]0 = 0.80 M, [IDTA or MOEA]0 = 6.0 mM, and [Et2O] = 0.96 M. Figure 3. 1H NMR spectra (in CDCl3 at 25 °C) of (A) poly(IBVE) (Mn = 3000, Mw/Mn = 1.33) obtained using HCl·Et2O with IDTA in hexane at 0 °C ([IBVE]0 = 0.80 M, [IDTA]0 = 32.0 mM, [HCl]0 = 2.8 mM, [Et2O] = 0.96 M) and (B) solely IDTA as the RAFT cationogen. The inset shows the chemical structures with full peak assignments.

molecular structure and to be purified by distillation, since precision cationic polymerization requires the use of pure initiating species. The polymerization was performed using HCl·Et2O in hexane at 0 °C at a [IBVE]0/[IDTA]0/[HCl]0 ratio of 800/6.0/2.8 (mM). The concentration of HCl was thus lower than that of the IDTA, and Et2O (0.96 M, i.e., 10 vol %) was added in an amount sufficient to allow for the dissociation of HCl.19−21 Figure 1A shows the variations in the monomer conversion as functions of polymerization time. In a previous study, the MLCP of IBVE without IDTA resulted in polymerization generating quantitative conversion within 30

incorporating a cationogen (i.e., a RAFT cationogen) for the MLCP of IBVE (a common cationically polymerizable monomer) using HCl·Et2O. The term “RAFT cationogen” signifies that the compound serves both as the RAFT agent for radical polymerization and as the cationogen for cationic polymerization. The IDTA was designed to have a simple E

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Figure 6. (A) Time−conversion curves and ln([M]0/[M])−time plots, (B) Mn and Mw/Mn for the MLCP of NPVE using HCl·Et2O in hexane at 0 °C: [NPVE]0 = 0.76 M, [HCl]0 = 3.2 mM, and [Et2O] = 0.96 M. (C) Relationship between Mn or Mw/Mn of poly(NPVE) and [HCl]0 for MLCP using HCl·Et2O with IDTA in hexane at 0 °C: [NPVE]0 = 0.80 M, [IDTA]0 = 32.0 mM, and [Et2O] = 0.96 M.

Figure 7. (A) Typical result for MALDI−TOF−MS analysis of the poly(IBVE) obtained via MRCP using HCl·Et2O with IDTA in hexane at 0 °C: [NPVE]0 = 0.80 M, [IDTA]0 = 32.0 mM, [HCl]0 = 40.0 mM, [Et2O] = 0.96 M. The chemical structure of poly(IBVE) with two α-ends may be estimated from the mass spectrum. (B) Relationships between Fn and [IDTA]0/[HCl]0. The two dashed curves show Fn values calculated using eqs 7 (red) and 8 (blue), assuming that HCl and IDTA both contribute to initiation.

min.19 However, the polymerization with IDTA was found to progress slowly without an induction period and was completed in 24 h to afford soluble polymers in quantitative yields. The time−conversion plots show that IDTA retards the polymerization relative to the progress of conventional MLCP without IDTA, indicating that IDTA is involved in the polymerization. The ln([M]0/[M])−time plots (M: IBVE) generate straight lines running through the origin, demonstrating that the concentrations of the propagating species were constant throughout the polymerization. Figure 1B summarizes the evolution of the Mn and Mw/Mn values of poly(IBVE) as functions of monomer conversion. Despite the relatively slow polymerization, the Mn increases in direct proportion to the degree of conversion. The Mn values are also in good agreement with the predicted values, assuming that one polymer chain is formed per IDTA unit. Moreover, the

resulting polymers exhibited relatively narrow MWDs at higher monomer conversions (Mn = 14.1 × 103, Mw/Mn = 1.22 at 96% conversion), and the GPC traces in Figure 1C are seen to be unimodal. After a fresh feed of IBVE was added at the 90% stage of the reaction (Mn = 13.8 × 103, Mw/Mn = 1.20 at 90% conversion), the polymerization proceeded smoothly while maintaining a unimodal, narrow MWD (Mn = 27.0 × 103, Mw/ Mn = 1.28 at 200% conversion). These results indicate that the MLCP of IBVE proceeded when using the IDTA/HCl·Et2O initiating system. In general, VE-acetic acid adducts are used as initiating species in conjunction with Lewis acids such as EtAlCl2,31 ZnCl2,32 and SnBr4.33 Thus, trials were performed in which 1(2-metoxyethoxy)ethyl acetate (MOEA) was employed instead of IDTA. It was found that the polymerization using MOEA proceeded in the same manner as when using IDTA. The F

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Macromolecules Scheme 3. Proposed Mechanism for Propagation and Termination of HCl·Et2O-Catalyzed MRCP

resulting Mn values were in good agreement with the predicted values, assuming that one polymer chain is formed per unit of MOEA (also see Figure 1). Thus, both MOEA incorporating an acetate moiety and IDTA with a dithioacetate portion are applicable as RAFT cationogen initiators for the HCl·Et2Ocatalyzed MLCP of VEs. The Mn values of the polymers are plotted against the [IBVE]0/[IDTA]0 ratio over the range of 25−200 at full monomer conversion in Figure 2A, and the MWDs are shown in Figure 2B. The Mn value is seen to increase with the ratio, while the Mw/Mn value increases only slightly due to variations in initiation and/or propagation rates (Mw/Mn = 1.22 to 1.33 for [IDTA] 0 = 4.0 to 32.0 mM, respectively). The correspondence between the experimental and theoretical values of Mn indicates that IDTA functions as a cationogen (an initiating species) and that HCl·Et2O was able to catalyze the living cationic polymerization of IBVE initiated by IDTA under the conditions in Figures 1 and 2. An estimated mechanism for the HCl·Et2O-catalyzed initiating reaction (eqs 4 and 5) is shown in Scheme 2. Here, note that the addition of the proton to the VE (eq 6) is rapid.19 However, the addition to a carbonyl group seems to be faster than that to an alkenyl group under this polymerization condition. Thus, eq 6 is

considered to represent a negligible initiation process during the reactions of both cationogens at [IDTA or MOEA]0/ [HCl]0 ratios of 4.0−32.0/2.8 (mM). Results similar to those shown in Figure 2 obtained using MOEA instead of IDTA are shown in Figure S1 of Supporting Information. Figure 3 presents the 1H NMR spectra of IDTA and poly(IBVE) obtained after quenching the polymerization with methanol at full monomer conversion. The polymers obtained had a faint yellow coloration due to the presence of the thiocarbonylthio chromophores derived from IDTA. Spectroscopic evidence for this color was provided by UV (305 nm) data obtained in addition to RI detection while acquiring the MWD plots (Figure 2B), demonstrating that the SC(CH3)S− end group, the so-called RAFT end group, is retained in the polymeric product. The 1H NMR spectra confirmed the presence of the methine proton (e) of the hemiacetal dithioester (the RAFT group) at 5.8 ppm and the methyl proton (f) at the ω-end at 2.8 ppm. In addition, no olefin peaks resulting from β-proton elimination or side-chain abstraction reactions were observed. Furthermore, no peaks in the vicinity of 9−10 ppm, assignable to the aldehyde formed by the elimination of an isobutyl group from the polymer terminal, were identified. The polymer also G

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corresponding (IBVE)n-OiBu, (IBVE)n‑1-CH2CHO is thought to be at least partly derived from the hydrolysis of the hemiacetal dithioester. Thus, a value of Fn = 0.82−0.83 (ω-end) was determined from the MALDI−TOF−MS results, which is in good agreement with the value obtained by 1H NMR spectroscopy. In addition, the number-average DP was determined from the ratio of the i/2a peak intensities in Figure 3, giving a value of 24, which is also in good agreement with the targeted DP of [IBVE]0/[IDTA]0 = 25. Thus, the MLCP must have proceeded via the formation of a carbocation originating from IDTA in response to HCl·Et2O. Characterization data for the polymers obtained by HCl·Et2O-catalyzed MLCP using MOEA are provided in the Supporting Information (Figures S3 and S4 for 1H NMR and MALDI− TOF−MS analyses, respectively). When using MOEA, the Fn(α-end) value (including the MOEA moiety) can be accurately determined using MALDI−TOF−MS data and the Fn(α-end) and Fn(ω-end) were found to be 1.0 and 0.83−0.85, respectively. The poly(IBVE) product obtained using MOEA was almost completely clear since it did not contain a thiocarbonylthio chromophore, in contrast to the poly(IBVE) products obtained using IDTA. To assess the catalytic ability of HCl·Et2O, the relationship between Mn and [HCl]0 was examined at [IBVE]0 = 0.80 M and [IDTA]0 = 6.0 mM, as shown in Figure 5. At [HCl]0 ≤ 2.5 mM ([IDTA]0/[HCl]0 ≥ 2.4), the resulting Mn at almost complete conversion was higher than that calculated assuming the formation of one living polymer per chain per unit IDTA. This is due to the low initiation efficiency of IDTA with low concentration of HCl, which in turn results from the low activity of HCl·Et2O with regard to the generation of carbocations from IDTA. In contrast, at [HCl]0 ≥ 5.7 mM ([IDTA]0/[HCl]0 ≤ 1.1), the experimental Mn was lower than the calculated value, implying that excess protons also triggered conventional MLCP (eq 6). When using 2.5 mM < [HCl]0 < 5.7 mM ( CH3COOH ∼ CH3CSSH > CH3OH, the most highly nucleophilic species is obviously CH3O−. This suggests that HCl·Et2O-catalyzed MLCP in the presence of a RAFT cationogen such as IDTA proceeds through a different propagation mechanism than the HCl·Et2O-catalyzed MLCP in the absence of a RAFT cationogen. To further clarify the polymerization mechanism, including both initiation and propagation, NPVE was used as the VE monomer in place of IBVE during HCl·Et2O-catalyzed MLCP

Figure 8. MWD plots for poly(IBVE)s obtained during IDTA addition trials associated with the MLCP of IBVE using HCl·Et2O in hexane at 0 °C: [IBVE]0 = 0.80 M, [HCl]0 = 2.8 mM, [Et2O] = 0.96 M, [IDTA]add = 15.0 mM.

generated signals attributable to α-end methyl protons (a) and protons due to IBVE repeating units (c, d, and g-i). On the basis of the ratio of the peak intensities of the methine (e) and α-end methyl protons (a), the ω-end functionality [Fn(ω-end)] was calculated to be 0.84, assuming that all the polymers were initiated from IDTA catalyzed by HCl·Et2O; that is, the α-end functionality [Fn(α-end)] was 1.0. The Fn(ω-end) was 0.84, which is close to that calculated from the ratio of the peak intensities of the methine proton (e) and the methine proton next to the methoxy terminal (j) derived from methanol quenching. The value of Fn(ω-end) can also be determined by matrixassisted laser desorption−time-of-flight−mass spectrometry (MALDI−TOF−MS) analysis, using the data in Figure 4. Some of the peaks seen here are attributed to (IBVE)n‑1−CH CH(OiBu) from the decomposition of the hemiacetal dithioester due to the MALDI−TOF−MS process,34,35 while others, such as (IBVE)n−OCH3, (IBVE)n‑1−CH2CHO, and (IBVE)n−OiBu, are due to quenching by methanol containing water; these peaks are also detected following MLCP in the absence of IDTA (see Figure S2). Since no aldehydes were evident in the 1H NMR spectra in Figure 3, compared to the H

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Table 1. Results of RAFT Radical Polymerization Using Either IDTA or or PolyVE Macro-CTAs Obtained by MRCP Using HCl·Et2Oa entry

monomer

CTAb

initiatorc

time (h)

convn (%)

Mn × 10−3 (calcd)

Mn × 10−3 d

Mw/Mnd

1 2 3 4 5 6 7 8 9 10 11 12e

EA MMA St VAc NIPAM EA MMA St tBOS PEGA PEGA EA

IDTA IDTA IDTA IDTA IDTA poly(IBVE) poly(IBVE) poly(IBVE) poly(IBVE) poly(NPVE) poly(EOVE) poly(IBVE)

AIBN AIBN AIBN V-601 V-601 AIBN AIBN AIBN AIBN AIBN AIBN AIBN

15 20 20 15 15 24 24 24 24 24 24 30 + 24

70 81 59 10 trace 88 99 75 73 25 21 100 + 90

14.2 16.4 12.4 0.26 − 20.6 22.8 18.6 28.7 26.8 23.4 20.7

13.2 18.2 11.0 59.0 − 17.0 22.1 16.4 25.6 21.0 20.0 20.0

1.35 1.21 1.38 2.49 − 1.19 1.48 1.26 1.27 1.21 1.30 1.29

[monomer]0:[CTA]0:[initiator]0 = 200:1:0.3, [monomer]0 = 15.0 wt % in toluene at 70 °C (entries 1−3, 5−6, and 8), [monomer]0:[CTA]0: [initiator]0 = 300:1:0.3, bulk polymerization at 60 °C (entry 4), [monomer]0:[CTA]0:[initiator]0 = 200:1:0.3, and [monomer]0 = 15.0 wt % in toluene at 60 °C (entries 7, and 9−12). bThe isolated polyVE (or unisolated poly(IBVE) for one-pot transformation in entry 12) was prepared by MRCP of the corresponding monomer using HCl·Et2O with IDTA in hexane at 0 °C: [VE]0 = 0.80 M, [IDTA]0 = 32.0 mM, [HCl]0 = 2.8 mM, [Et2O] = 0.96 M; Mn = 3000, and Mw/Mn = 1.33 [poly(IBVE)], Mn = 2700, Mw/Mn = 1.36 [poly(NPVE)], Mn = 3100, and Mw/Mn = 1.30 [poly(EOVE)]; all Fn(RAFT end) > 0.84 by 1H NMR spectroscopy. cAIBN: 2,2′-azobis(isobutyronitrile). V-601: dimethyl 2,2′-azobis(2methylpropionate). dBy GPC using either PSt calibration (except for entries 2, 7, and 10−11) or PMMA (entries 2, 7, and 10−11). eOne-pot transformation from MRCP of IBVE (convn = 100%) for 30 h into RAFT radical polymerization of EA (convn = 85%) for 24 h. a

evident in Figure 5. In the case of the Fn(ω-end) derived for IDTA (Fn(RAFT end)), the value nearly agrees with Fn (IDTA). Since the other end groups are (NPVE)n−OCH3, (NPVE)n‑1−CH2CHO, and (NPVE)n−OiBu, due to quenching by ammoniacal methanol including water, Fn (RAFT end) gives the dormant dithioester content and 1 − Fn(RAFT end) shows the quantity of active species in the polymerization system, as summarized in eq 9 of Scheme 3. However, the RAFT end group is intact after quenching, since it is not terminated by the more nucleophilic species CH3O− or HO−, and so its more stable dormant state has to be considered. Hence, we conclude that MLCP using either IDTA or MOEA proceeds through a RAFT process (eq 10) during the propagation reaction, representing a metal-free RAFT “cationic” polymerization, or MRCP. In this process, the propagating carbocation reversibly interchanges with the dormant dithioester (dormant 1 and 2), and the intermediates are stabilized by resonance (eq 11), just as in RAFT radical polymerization.36 An IDTA addition experiment performed during the investigation of MLCP using HCl·Et2O also supports the MRCP mechanism. In the case of the MLCP of IBVE, HCl is completely consumed during the initiation reaction. 19 Throughout the polymerization, IDTA addition will generate poly(IBVE) with the RAFT end group H−(IBVE)n−SC( S)−CH3. During the course of the conventional MLCP of IBVE without IDTA, a fresh feed of IDTA was added to the reaction mixture (polymerization time = 0.17 h, convn = 38.2%, Mn = 12.0 × 103, Mw/Mn = 1.20), resulting in polymers that included the IDTA as a RAFT agent, as in Figure 8. After the addition of a fresh feed of IDTA, the polymerization further proceeded without oligomers, although the MWD plots broadened relative to the initial poly(IBVE) obtained before the addition of IDTA. The MWD plots were divided into two peaks by multipeak fitting, and both peaks a and b were found to shift toward the higher MW region as polymerization progressed, to give peaks a′ and b′, respectively. Both the peaks further shifted as polymerization further progressed to finally give peaks peaks a″ and b″. These data suggest that peaks a and b correspond to the products obtained from the propagating

in the presence of IDTA. NPVE is helpful with regard to MALDI−TOF−MS analysis because the molecular weight of the IDTA cationogen moiety is quite different from that of NPVE. Although NPVE is a more reactive VE than IBVE, MLCP using HCl·Et2O proceeded smoothly, giving the same quantitative conversion observed when using IBVE, regardless of the absence (Figure 6, parts A and B) or presence of IDTA (Figure 6C). The product polymers had narrow MWDs and Mn values that agreed with the theoretical values calculated from the ratios of [NPVE]0 to [IDTA]0 within a [IDTA]0/[HCl]0 ratio range of 1.1−2.4. Using the HCl·Et2O-catalyzed MLCP of NPVE in the presence of IDTA, the effects of the [IDTA]0/[HCl]0 ratio on the α- and ω-end groups of the resulting poly(NPVE) were investigated based on MALDI−TOF−MS, holding [NPVE]0 and [IDTA]0 constant ([NPVE]0 = 0.8 M, [IDTA]0 = 32.0 mM), while varying [HCl]0 over the range of 5.0−40.0 mM (i.e., [IDTA]0/[HCl]0 = 0.8−6.4). Figure 7 shows typical MALDI−TOF−MS data and the results of the functionality of the α- and ω-end groups derived from the RAFT cationogen in the resulting poly(NPVE). Assuming that the polymerization proceeds from both the IDTA and protons (via HCl·Et2O), the calculated Fns (α-end) of the IDTA [Fn (IDTA)] and of the protons [Fn(H+)] are defined as follows. Fn(IDTA) =

[IDTA]0 [IDTA]0 + [H+]0

Fn(H+) = 1 − Fn(IDTA)

(7) (8)

Calculated Fns (α-end) values of Fn (IDTA) > 0.9 and Fn (H+) < 0.1 were determined at [IDTA]0/[HCl]0 ≥ 1.6. These values are much larger than the calculated Fn (IDTA) obtained from eq 7 and much smaller than the calculated Fn (H+) from eq 8. These results clearly demonstrate that HCl·Et2O catalyzes the initiating reaction of IDTA, as proposed in eq 5. Furthermore, at [IDTA]0/[HCl]0 ≤ 1, the value of Fn (IDTA) decreases and approaches the calculated value due to triggering of the conventional MLCP process by excess protons (eq 6), as I

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Figure 9. Synthesis of block copolymers (entries 6−9) starting from a poly(IBVE) macro-CTA prepared by MRCP using HCl·Et2O with IDTA in hexane at 0 °C and accompanying GPC traces of various block copolymers produced by RAFT radical polymerization of different combinations of monomers (red trace: starting poly(IBVE), blue trace: block copolymer). One-pot transformation from IDTA through MRCP of IBVE using HCl· Et2O with IDTA to RAFT radical polymerization of EA (entry 12). Polymerization conditions: see the corresponding entries 6−9 and 12 in Table 1.

results, polyVEs with RAFT ends derived from IDTA were isolated and utilized as macro-CTAs for the RAFT radical polymerizations of acrylates, MMA, and styrenes (entries 6−11 in Table 1). All polymerizations proceeded readily and the resulting Mn values clearly shifted to higher molecular weights relative to those of the corresponding polyVEs, with relatively narrow molecular weight distributions. GPC traces are shown in Figure 9, summarizing the results of the RAFT radical polymerizations of EA, MMA, St, and tBOS (entries 6−9). The slight tailing seen in the GPC traces of the block copolymers is due to imperfect Fns values associated with the RAFT ends. However, the plots still exhibit near-unimodal distribution, suggesting the formation of block copolymers of polyVEs and the radically polymerizable monomers. The compositions of the block copolymers were also determined by 1H NMR spectroscopy assuming that the Fn (RAFT end) values for the MRCP and the block efficiency were both unity, and employing the characteristic peak intensities (Figure S6). For all block copolymers, the observed unit ratios of the polyVE and radically polymerized segments were in good agreement with the monomer feed ratio at the obtained conversion.

polymer generated by MLCP using HCl·Et2O, as in eq 6, and by MRCP as in eq 10 and involving IDTA, respectively. The UV absorption at 305 nm was detected for the resulting poly(IBVE) after addition of IDTA, indicating that both a and b contain RAFT end group. In practice, RAFT end groups were included in the final poly(IBVE) product, as shown by 1H NMR spectroscopy (Figure S5). This is evidence of the progression of the cationic RAFT process (i.e., MRCP). Transformation from MRCP to RAFT Radical Polymerization. Since polyVEs acting as macro-RAFT agents were obtained by MRCP using HCl·Et2O with IDTA, RAFT radical polymerizations were conducted for the synthesis of various block copolymers, based on reactions between polyVE macrochain transfer agents (CTAs) and radically polymerizable monomers. For RAFT polymerization to function effectively, the appropriate choice of RAFT agents is extremely important. Table 1 shows the results of the RAFT polymerization of various monomers using IDTA (entries 1−5). Using IDTA as the CTA provides good control and no or little retardation of the RAFT radical polymerizations of ethyl acrylate (EA), methyl methacrylate (MMA), and styrene (St). However, this approach was not effective in the case of vinyl acetate (VAc) and N-isopropylacrylamide (NIPAM). On the basis of these J

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poly(EOVE)-b-poly(PEGA) was prepared (entry 11). As the first step, the MRCP of EOVE was conducted using HCl·Et2O with IDTA in hexane at 0 °C in the same manner as had been used to prepare poly(IBVE). The resulting poly(EOVE) had Mn = 3100 and Mw/Mn = 1.30. The MRCP conditions were [EOVE]0 = 0.80 M, [IDTA]0 = 32.0 mM, [HCl]0 = 2.8 mM, and [Et2O] = 0.96 M. The poly(EOVE) generated in this manner was subsequently utilized as a macro-CTA for the following RAFT polymerization of PEGA (entry 11). The block copolymerization of PEGA using poly(EOVE) proceeded as well as that using poly(NPVE) (entry 10). The MWD of the poly(EOVE)-b-poly(PEGA) was clearly shifted toward higher molecular weights relative to that of the poly(EOVE): from Mn = 3100 for poly(EOVE) to Mn = 23,400 at 21% conversion. In addition, the MWD remained relatively narrow after RAFT radical polymerization (Mw/Mn = 1.30). The polymeric formula was determined to be EOVE30-b-PEGA21 based on the results of 1H NMR spectroscopy and the conversion of PEGA, assuming that the Fn (RAFT end) values for the MRCP and the block efficiency were both unity. The thermoresponsive behavior of the block copolymer in water was investigated in detail. Figure 10 shows dynamic light scattering (DLS) and atomic force microscopy (AFM) images (dynamic mode) of assemblies formed from poly(EOVE)-bpoly(PEGA) in water at 30 °C. The block copolymer included thermoresponsive poly(EOVE), which exhibits highly sensitive phase separation at 20−30 °C,37,38 depending on the molecular weight, and hydrophilic poly(PEGA). Below 26 °C, the poly(EOVE)-b-poly(PEGA) is molecularly dissolved and shows a mean hydrodynamic diameter (Dh) of around 4−7 nm, a relatively high polydispersity (PDI, μ2/Γ2), and a low scattering intensity (Figure 10A). Above 26 °C, a slightly opaque appearance and assemblies 116−128 nm in size are observed. The diameter change was found to be quite sensitive and reversible on heating and cooling, indicating poly(EOVE)core and poly(PEGA)-shell micelle formation above the phase separation temperature (i.e., the LCST) of poly(EOVE). AFM measurements on mica showed that the assemblies had a clear core−shell morphology with a diameter of approximately 130 nm and a height of 90 nm. These results support the successful formation of block copolymers via the transformation of MRCP to RAFT radical polymerization.

Figure 10. (A) Variations in the hydrodynamic diameter (Dh) and PDI with temperature for poly(EOVE)-b-poly(PEGA) (entry 11 in Table 1) in water. (B) AFM (height) image (dynamic mode) of poly(EOVE)-b-poly(PEGA) in water at 30 °C deposited onto mica. The cross-sectional profile obtained from the AFM image.

In the case of previous transformations from living cationic polymerization to RAFT radical polymerization, the metallic catalysts employed for cationic polymerization inhibit the radical polymerization and consequently RAFT polymerization was not sufficiently controlled as to obtain multimodal MWDs.34 However, both MRCP and RAFT radical polymerization can proceed via a metal-free process. Thus, we expected that a one-pot transformation from MRCP to RAFT radical polymerization would be successful. Initially, poly(IBVE) was polymerized via MRCP using HCl·Et2O with IDTA up to full conversion and then quenched using methanol. Without isolating the poly(IBVE), EA and AIBN were successively added into the reaction mixture and the mixture was polymerized at 60 °C. Similar to the results of the isolated poly(IBVE), RAFT radical polymerizations proceeded smoothly and the resulting MWDs were shifted toward higher molecular weights, with the especially narrow distribution seen for entry 12 (Table 1 and Figure 9). Thus, we were able to achieve the first-ever one-pot transformation from MRCP to RAFT radical polymerization. The formation of block copolymers was further confirmed using a functional polymer. Applying the study protocol described above, the novel thermoresponsive block copolymer

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CONCLUSION We have demonstrated the successful metal-free RAFT cationic polymerization (MRCP) of vinyl ethers using HCl·Et2O with IDTA. In this cationic polymerization process, MOEA can be applied to the MRCP of vinyl ethers using HCl·Et2O in place of IDTA. The MRCP proceeds via an HCl-catalyzed initiation reaction to form carbocations derived from the IDTA. In the propagation step, the possibility of RAFT processes, meaning a reversible addition−fragmentation chain transfer to the growing carbocationic species to the dormant thioester bond cannot be denied. Thus, the resulting polymers inherently have a RAFT end group obtained through the RAFT process. Using poly(vinyl ether) macro-CTAs, various block copolymers incorporating radically polymerizable monomers such as EA, MMA, St, tBOS, and PEGA were prepared via transformation from MRCP to RAFT radical polymerization. In particular, we were able to demonstrate the one-pot transformation from MRCP to RAFT radical polymerization because MRCP and RAFT radical polymerization both proceed via metal-free processes. Furthermore, this MRCP allows for the production K

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(9) Miyamoto, M.; Sawamoto, M.; Higashimura, T. Macromolecules 1984, 17, 265−268. (10) Faust, R.; Kennedy, J. P. Polym. Bull. 1986, 15, 317−323. (11) Suriano, F.; Coulembier, O.; Hedrick, J. L.; Dubois, P. Polym. Chem. 2011, 2, 528−533. (12) Treat, N. J.; Sprafke, H.; Kramer, J. W.; Clark, P. G.; Barton, B. E.; de Alaniz, J. R.; Fors, B. P.; Hawker, C. J. J. Am. Chem. Soc. 2014, 136, 16096−16101. (13) For a review on metal-free organic chemistry: Enders, D.; Grondal, C.; Hüttl, M. R. M. Angew. Chem., Int. Ed. 2007, 46, 1570− 1581. (14) Nuyken, O.; Kröner, H. Makromol. Chem. 1990, 191, 1−16. (15) Cramail, H.; Deffieux, A.; Nuyken, O. Makromol. Chem., Rapid Commun. 1993, 14, 17−27. (16) van Meirvenne, D.; Haucourt, N.; Goethals, E. J. Polym. Bull. 1990, 23, 185−190. (17) Cho, C. G.; Feit, B. A.; Webster, O. W. Macromolecules 1990, 23, 1918−1923. (18) Kanazawa, A.; Hashizume, R.; Kanaoka, S.; Aoshima, S. Macromolecules 2014, 47, 1578−1585. (19) Sugihara, S.; Tanabe, Y.; Kitagawa, M.; Ikeda, I. J. Polym. Sci., Part A: Polym. Chem. 2008, 46, 1913−1918. (20) Zaleska, I. M.; Kitagawa, M.; Sugihara, S.; Ikeda, I. J. Polym. Sci., Part A: Polym. Chem. 2009, 47, 5169−5179. (21) Sugihara, S.; Kitagawa, M.; Inagawa, Y.; Zaleska, I. M.; Ikeda, I. Polym. Bull. 2010, 64, 209−220. (22) Sawamoto, M.; Fujimori, J.; Higashimura, T. Macromolecules 1987, 20, 916−920. (23) Ogawa, Y.; Sawamoto, M.; Higashimura, T. Polym. J. 1984, 16, 415−422. (24) Ma’Radzi, A. H.; Sugihara, S.; Miura, S.; Konegawa, N.; Maeda, Y. Polymer 2014, 55, 1920−1930. (25) Ma’Radzi, A. H.; Sugihara, S.; Toida, T.; Maeda, Y. Polymer 2014, 55, 5332−5345. (26) Chiefari, J.; Chong, Y. K.; Ercole, F.; Krstina, J.; Jeffery, J.; Le, T. P. T.; Mayadunne, R. T. A.; Meijs, G. F.; Moad, C. L.; Moad, G.; Rizzardo, E.; Thang, S. H. Macromolecules 1998, 31, 5559−5562. (27) Moad, G.; Rizzardo, E.; Thang, S. H. Aust. J. Chem. 2005, 58, 379−410. (28) Moad, G.; Rizzardo, E.; Thang, S. H. Polymer 2008, 49, 1079− 1131. (29) Chiefari, J.; Mayadunne, R. T. A.; Moad, C. L.; Moad, G.; Rizzardo, E.; Postma, A.; Skidmore, M. A.; Thang, S. H. Macromolecules 2003, 36, 2273−2283. (30) Barner-Kowollik, C., Ed. Handbook of RAFT polymerization; Wiley-VCH: Weinheim, Germany, 2008. (31) Aoshima, S.; Higashimura, T. Macromolecules 1989, 22, 1009− 1013. (32) Kamigaito, M.; Sawamoto, M.; Higashimura, T. Macromolecules 1991, 24, 3988−3992. (33) Hashimoto, T.; Iwata, T.; Minami, A.; Kodaira, T. J. Polym. Sci., Part A: Polym. Chem. 1998, 36, 3173−3185. (34) Sugihara, S.; Yamashita, K.; Matsuzuka, K.; Ikeda, I.; Maeda, Y. Macromolecules 2012, 45, 794−804. (35) Sugihara, S.; Iwata, K.; Miura, S.; Ma’Radzi, A. H.; Maeda, Y. Polymer 2013, 54, 1043−1052. (36) Yamago, S. Chem. Rev. 2009, 109, 5051−5068. (37) Aoshima, S.; Oda, H.; Kobayashi, E. J. Polym. Sci., Part A: Polym. Chem. 1992, 30, 2407−2413. (38) Sugihara, S.; Hashimoto, K.; Okabe, S.; Shibayama, M.; Kanaoka, S.; Aoshima, S. Macromolecules 2004, 37, 336−343. (39) Uchiyama, M.; Satoh, K.; Kamigaito, M. Angew. Chem., Int. Ed. 2015, 54, 1924−1928.

of thermoresponsive block copolymers, such as poly(EOVE)-bpoly(PEGA). This block copolymer forms spherical micelles above the 27 °C LCST of the poly(EOVE) segments, as confirmed by DLS analysis and AFM images. During the writing of this article, Uchiyama et al. reported controlled cationic polymerization via the RAFT process using ppm concentrations of triflic acid.39 In their communication, they concluded that this new form of cationic RAFT chemistry would result in new possibilities for organic reactions, and we agree with this position. However, the HCl·Et2O/IDTA system in the present article has some advantages of a combination of a commercially available and readily useable protonic acid complex and a simple molecular structure that can be purified by distillation. On the basis of this example of facile RAFT cationic chemistry, we anticipate that many researchers will utilize RAFT cationic processes such as MRCP in both small molecule and polymer functionalization chemistry.

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ASSOCIATED CONTENT

S Supporting Information *

Further synthesis and characterization details using GPC, 1H NMR, and MALDI−TOF−MS. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b01071.

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AUTHOR INFORMATION

Corresponding Author

*(S.S.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported in part by the PRESTO program on “Molecular Technology and Creation of New Functions” from JST. We thank Maruzen Petrochemical Co., Ltd., for supplying the EOVE monomer and discussion on the MRCP.

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REFERENCES

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