Design of Benign Initiator for Living Cationic Polymerization of Vinyl

Feb 26, 2014 - *E-mail: [email protected] (S.A.). ... A Vinyl Ether-Functional Polycarbonate as a Template for Multiple Postpolymerizatio...
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Design of Benign Initiator for Living Cationic Polymerization of Vinyl Ethers: Facile in Situ Generation of Vinyl Ether−Hydrogen Halide Adducts and Subsequent Controlled Polymerization without a Lewis Acid Catalyst Arihiro Kanazawa, Ryo Hashizume, Shokyoku Kanaoka, and Sadahito Aoshima* Department of Macromolecular Science, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan S Supporting Information *

ABSTRACT: A Lewis acid-free initiating system for cationic polymerization of alkyl vinyl ethers (VEs) was developed using CF3SO3H and tetraalkylammonium halides (nBu4NX; X = I, Br, or Cl). The reaction of CF3SO3H, nBu4NX, and an alkyl VE generated a VE−hydrogen halide adduct. The labile carbon−halogen bond, such as a carbon−iodine bond, cleaved without a Lewis acid catalyst to induce the living cationic polymerization of VEs. Combining halide anions with suitable nucleophilicities according to monomer reactivity was indispensable for controlled polymerization. Another prerequisite was that the polymerization be conducted at a suitable temperature. After the initiation step, the polymerization probably proceeded via the mechanisms similar to those for the previously reported systems using hydrogen halide and an ammonium salt with a noncoordinating anion. The present system was also useful as a facile method for the synthesis of VE−hydrogen halide adducts without the use of harmful gaseous hydrogen halides. The adducts formed were shown to function as cationogens for living cationic polymerization using Lewis acid metal catalysts.

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INTRODUCTION A wide variety of metal catalysts, from simple metal halides to metal complexes with precisely designed ligands, have supported the progress of various controlled polymerizations,1−3 and the demand for less harmful catalyst systems is increasing due to recent growing concerns about environmental problems. However, most of the metal catalysts developed tend to have drawbacks that require their residues be removed from the product polymers. In addition to this purification process, disposal of the catalyst residues is another troublesome issue. Therefore, alternative methods that require no metal catalysts have been eliciting great interest in both academia and industry. Metal catalysts have been essential for the living cationic polymerization of vinyl monomers. Since the first achievement of living systems in the 1980s,4,5 various metal halides have been used as Lewis acid catalysts.1 The main function of a metal catalyst is to generate the cationic propagating species by activating the potentially propagating (dormant) chain ends. Thus, polymerization behaviors such as reaction rates have been shown to depend heavily on the central metals and many metal catalysts have enabled the controlled polymerizations of a variety of functional monomers. A metal-free initiating system appears to be an obvious solution for reducing the environmental burden; however, its realization requires a fair degree of ingenuity. The difficulty of achieving living cationic polymerization without a metal catalyst lies in the formation of a stable propagating chain end that is © 2014 American Chemical Society

sufficiently labile to undergo a rapid equilibrium between the dormant and active (ionic) species. In fact, limited and isolated studies have been reported on the living cationic polymerization of vinyl monomers using initiating systems that were free from metal catalysts, such as hydrogen iodide−iodine (HI/ I2),4 vinyl ether−hydrogen iodide adduct (VE−HI),6 VE−HI/ alkylammonium salt,7−10 trimethylsilyl iodide/1,3-dioxolane/ tetraalkylammonium salt,11 CF3SO3H/sulfide,12 and HCl/ aliphatic ether13,14 systems. However, these initiating systems employ hazardous and/or unstable compounds. In particular, gaseous hydrogen iodide requires a complicated and troublesome technique for its generation and subsequent reaction,4 despite the effectiveness of an iodide anion as a counteranion for the propagating species in the living cationic polymerization. Consequently, we became interested in developing a new, safe, and facile method for the in situ generation of a stable yet labile bond that functions as a propagating chain end for living cationic polymerization. Our strategy includes the formation of VE−hydrogen halide adducts by a three-component reaction of CF3SO3H, an alkylammonium salt (nBu4NX; X = I, Br, or Cl), and VE (Scheme 1). A very strong protic acid, CF3SO3H, will allow reaction with nBu4NX to generate hydrogen halide and nBu4N(CF3SO3). Subsequently, the hydrogen halide may react Received: December 5, 2013 Revised: February 11, 2014 Published: February 26, 2014 1578

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Scheme 1. Postulated Reaction Mechanism for Cationic Polymerization of VEs Using the CF3SO3H/nBu4NX (X = I, Br, or Cl) Initiating System

Figure 1. 1H NMR spectra of (A) IBVE in CDCl3 at 30 °C and (B−D) the mixtures of IBVE, CF3SO3H, and nBu4NX (B) X = I, (C) X = Br, and (D) X = Cl in CD2Cl2/CH2Cl2 (1/1) at −40 °C: [IBVE]0 = 40 mM, [CF3SO3H]0 = 40 mM, and [nBu4NX]0 = 42 mM. nBu4NCl (Fluka; ≥99.0%), nBu4NBr (Acros Organics; ≥99.0+%), nBu4NI (Fluka; ≥99.0%), nBu4N(CF3SO3) (Aldrich; ≥99.0%), and CF3SO3H (Aldrich; ≥99.0%) were used without further purification, and their stock solutions in CH2Cl2 were prepared. ZnCl2 (Aldrich; 1.0 M solution in diethyl ether) was used without further purification. Dichloromethane (Wako; 99.0%) was dried by passage through solvent purification columns (Glass Contour). Polymerization Procedure. The following is a typical polymerization procedure. A glass tube equipped with a three-way stopcock was dried using a heat gun (Ishizaki, PJ-206A; the blow temperature ∼450 °C) under dry nitrogen for 10 min. A diluted solution of nBu4NI in dichloromethane (3.20 mL, 5.25 mM solution) and that of CF3SO3H (0.40 mL, 40 mM solution) were added into the tube using dry syringes. The polymerization was started by the addition of IPVE (0.40 mL) at −40 °C. After 12 h, the reaction was terminated with prechilled methanol or ethanol (3 mL) containing a small amount of aqueous ammonia solution (0.1%). The quenched mixture was washed with water. The volatiles were then removed under reduced pressure, and the residue was vacuum-dried under reduced pressure for more than 3 h at room temperature to yield a colorless gummy polymer (0.30 g). The monomer conversion was determined by gravimetry (conversion = 100%). Characterization. The molecular weight distribution (MWD) of the polymers was measured by gel permeation chromatography (GPC) in chloroform at 40 °C with three polystyrene gel columns [Tosoh; TSKgel G-4000HXL, G-3000HXL, and G-2000HXL (exclusion limit molecular weight = 4 × 105, 6 × 104, and 1 × 104, respectively; bead size = 5 μm; column size = 7.8 mm i.d. × 300 mm), or TSKgel MultiporeHXL-M × 3 (exclusion limit molecular weight = 2 × 106; bead size = 5 μm; column size = 7.8 mm i.d. × 300 mm); flow rate = 1.0 mL/min] connected to a Tosoh DP-8020 pump, a CO-8020 column oven, a UV-8020 ultraviolet detector, and an RI-8020 refractive index detector. The number-average molecular weight

smoothly with a VE to give a VE−hydrogen halide adduct with a covalent carbon−halogen bond. The labile carbon−halogen bond possibly cleaves under suitable reaction conditions and without any metal catalysts,6−10 which would lead to the living cationic polymerization of VE. The mechanisms similar to those for the systems using hydrogen iodide and an ammonium salt with a noncoordinating anion7−10 will operate during the polymerization. In contrast to past studies on living cationic polymerization using VE−hydrogen halide adducts, however, the procedure in this study does not require complicated techniques using gaseous hydrogen halides.4,15 In addition, the present method will be useful as a facile synthetic method for the formation of hydrogen halide adducts of various functional monomers that can be used as cationogens for living cationic polymerization with Lewis acid catalysts. In this article, we report the living cationic polymerization of alkyl VEs using a CF3SO3H/nBu4NX/alkyl VE initiating system. The in situ generation of alkyl VE−hydrogen halide adducts was confirmed through 1H NMR analysis. Choosing a halide anion with suitable nucleophilicity according to monomer reactivity was essential to achieving controlled polymerizations. Other factors, including the reaction temperature and the ammonium salt concentration, were also important for suppressing side reactions.

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EXPERIMENTAL SECTION

Materials. Isobutyl vinyl ether (IBVE; TCI; >99.0%), isopropyl vinyl ether (IPVE; Wako; 97.0+%), and tert-butyl vinyl ether (TBVE; Aldrich; ≥98.0%) were washed with 10% aqueous sodium hydroxide solution and then water and distilled twice over calcium hydride. 1579

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Table 1. Cationic Polymerization of VEs Using CF3SO3H/Added Salt Initiating Systema entry

monomer

added salt

temp (°C)

time

conv (%)

Mn × 10−3 (calcd)

Mn × 10−3 (GPC)b

Mw/Mnb

1 2 3 4 5 6 7 8 9 10 11 12 13

IBVE

nBu4NI nBu4NI nBu4NI nBu4NBr nBu4NCl nBu4N(CF3SO3) nBu4NI nBu4NBr nBu4NCl nBu4NI nBu4NI nBu4NBr nBu4NBr

0 −40 −78 −40 −40 −40 −40 −40 −40 −40 −78 −40 −78

18 h 600 h 360 h 381 h 384 h 1 min 12 h 144 h 144 h 20 min 3.5 h 27 h 390 h

83 96 18 6 12 95 100 51 21 100 99 99 49

15.8 18.2 3.4 1.1 2.3 18.1 18.7 9.5 3.9 19.0 18.9 18.9 9.3

2.4 9.5 14.4 1.1c 6.4 3.6 16.4 3.6c 7.8 7.3 13.9 2.0 6.1

1.54 1.39 7.68 1.16c 7.99 5.46 1.16 1.53c 7.63 1.60 1.13 1.55 1.30

IPVE

TBVE

a

[Monomer]0 = 0.76 (IBVE and TBVE) or 0.87 (IPVE) M, [CF3SO3H]0 = 4.0 mM, [added salt]0 = 4.2 (except for entries 1 and 6), 4.5 (entry 1), or 200 mM (entry 6), in CH2Cl2. bBy GPC using polystyrene calibration. cFor low-MW peaks.

Figure 2. (A) Time−conversion curves and ln([M]0/[M])−time plots, (B) Mn and Mw/Mn for the polymerization of IPVE, and (C) MWD curves ([CF3SO3H]0 = 4.0 mM and [nBu4NI]0 = 4.2 mM) for poly(IPVE) obtained using the CF3SO3H/nBu4NI initiating system in dichloromethane at −40 °C: [IPVE]0 = 0.87 M, [CF3SO3H]0 = 4.0 mM (red; circle) or 8.0 mM (blue; square); [nBu4NI]0 = 4.2 mM (red; circle) or 8.4 mM (blue; square). Values for main peaks were used for the square plots in graph B because very slight amounts of uncontrolled portions with high MW were generated at the initiation step. (Mn) and polydispersity ratio [weight-average molecular weight/ number-average molecular weight (Mw/Mn)] were calculated from the chromatographs with respect to 16 polystyrene standards (Tosoh; Mn = 577−1.09 × 106, Mw/Mn ≤ 1.1). NMR spectra were recorded using a JEOL JNM-ECA 500 spectrometer (500.00 MHz for 1H). MALDITOF-MS spectra were recorded with a Shimadzu/Kratos AXIMA-CFR spectrometer (linear mode; voltage: 20 kV; pressure: IPVE > IBVE.17 The hydrogen halide adducts of IPVE and TBVE were prepared by procedures similar to that used for IBVE. The polymerizations were initiated by the addition of a VE monomer to a solution containing CF3SO3H and nBu4NX.

RESULTS AND DISCUSSION

In Situ Generation of VE−Hydrogen Halide Adducts by CF3SO3H and nBu4NX. Prior to polymerization, in situ preparation of an isobutyl VE (IBVE)−hydrogen halide adduct was conducted by mixing almost equimolar amounts of CF3SO3H, nBu4NX (X = I, Br, or Cl), and IBVE in dichloromethane-d2 at −40 °C. The reactions were monitored using 1H NMR spectroscopy (Figure 1). When the three ingredients were mixed, the vinyl protons of IBVE (peaks a and b in Figure 1A) completely disappeared, and peaks a1−a3 and b1−b3 appeared. Peaks a1−a3 and b1−b3 were doublets and quartets, respectively, and the integral ratios between the a1−a3 peaks and the b1−b3 peaks were each equal to three, which 1580

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1. Cationic Polymerization of IBVE. The cationic polymerization of IBVE was conducted with the CF3SO3H/nBu4NX initiating system at −40 °C. When nBu4NI was used as an additive, the polymerization proceeded to yield a polymer, although the reaction was very slow, requiring 600 h to complete (entry 2 in Table 1; Figure S1). The polymers obtained at the early-to-middle stage of the polymerization had Mn values that agreed with the theoretical values and narrow MWDs. However, at the later stage, the Mn value was smaller than the theoretical value, and the MWD became broader, which indicated the occurrence of side reactions. Abstractions of protons or side chain alcohols and reactions with adventitious water both induce chain transfers, which were indicated by the 1H NMR analysis of the product polymer.18 On the other hand, polymerization at 0 °C was totally uncontrolled, which resulted in polymers with low Mn values and broad MWDs (entry 1 and Figure S1). In addition, the polymerization at −78 °C was very slow and uncontrolled, although long-lived species were partially generated (entry 3 and Figure S1). The reactions using nBu4NBr (entry 4) and nBu4NCl (entry 5) were almost inactive, resulting in very low conversions, in contrast to the efficient polymerizations with nBu4NI. These results show that the carbon−bromine or carbon−chlorine bond derived from IBVE did not cleave under the conditions examined. However, the reaction with nBu4N(CF3SO3) was completed in 1 min and yielded ill-defined products despite the use of large amounts of nBu4N(CF3SO3) (entry 6 and Figure S4). This result was most likely observed because the CF3SO3− anion is noncoordinating, and it is difficult to form a dormant species with the propagating carbocation. 2. Cationic Polymerization of IPVE. The cationic polymerization of IPVE, a more reactive VE than IBVE, proceeded faster with the CF3SO3H/nBu4NI initiating system at −40 °C to reach quantitative conversion in 12 h (entry 7 in Table 1; Figure 2). The ln([M]0/[M])−time plots were in accordance with a straight line from the origin, indicating that the concentration of the propagating species was constant during the reaction. Furthermore, the product polymers had narrow MWDs and Mn values that agreed with the theoretical values calculated from the ratios of [IPVE]0 to [CF3SO3H]0. The polymerization with twice the amounts of CF3SO3H and nBu4NI also proceeded in a controlled manner, giving polymers with theoretical Mn values (blue symbols in Figure 2). These results suggested that a proton derived from CF3SO3H (a VE− hydrogen iodide adduct) initiated the controlled polymerization. To confirm the livingness of this polymerization reaction, a fresh feed of IPVE was added at the later stage of the reaction (Figure 3). The polymerization proceeded smoothly after the addition, which resulted in polymers with unimodal and narrow MWDs.19 In addition, the Mn values increased in proportion to the monomer conversion. These results indicated that the living cationic polymerization of IPVE proceeded with the CF3SO3H/ nBu4NI initiating system without any metal catalysts. The occurrence of highly controlled polymerization was further confirmed by 1H NMR (Figure 4) and MALDI-TOFMS (Figure 5) analyses of the product polymers. The 1H NMR spectrum confirmed the presence of the ω-end derived from the methanol quencher, and no olefin peaks resulting from βproton elimination or side-chain abstraction reactions were observed. The MALDI-TOF-MS spectrum showed a single series of peaks. The m/z values agreed well with the values

Figure 3. (A) Mn and Mw/Mn for polymerization of IPVE and (B) MWD curves for poly(IPVE)s obtained using the CF3SO3H/nBu4NI initiating system in dichloromethane at −40 °C: [IPVE]0 = 0.87 M, [IPVE]added = 0.87 M, [CF3SO3H]0 = 4.0 mM, [nBu4NI]0 = 4.2 mM.

Figure 4. 1H NMR spectrum (in CDCl3 at 30 °C) of poly(IPVE) obtained using the CF3SO3H/nBu4NI initiating system in dichloromethane at −40 °C [Mn(GPC) = 1.33 × 104, Mw/Mn(GPC) = 1.10; similar polymerization conditions to those used for entry 7 in Table 1]. *Solvent, water, and vaseline.

Figure 5. MALDI-TOF-MS spectra of poly(IPVE) obtained using the CF3SO3H/nBu4NI initiating system in dichloromethane at −40 °C [Mn(GPC) = 5.2 × 103, Mw/Mn(GPC) = 1.15; similar polymerization conditions to those used for entry 7 in Table 1].

expected for the structures with the α-end derived from the proton and the ω-end derived from the methanol quencher. These data confirmed that the polymerization proceeded in a highly controlled manner without any side reactions, such as βproton elimination. Unlike the case of IBVE, nBu4NBr also allowed the polymerization of IPVE (entry 8), as shown in Figure 6. The 1581

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transfer reactions, the Mn values became smaller than the theoretical values. To suppress the side reactions, the polymerization of TBVE was conducted at a lower temperature. The reaction at −78 °C proceeded smoothly, reaching quantitative conversion in 3.5 h and producing polymers with narrow MWDs (entry 11; Figure 7). The Mn value increased in proportion to the monomer conversion even in the later stage of the polymerization process. Furthermore, structures derived from side reactions were not detected in 1H NMR (Figure 8) and MALDI-TOFMS (Figure 9) analyses of the products. Thus, highly controlled cationic polymerization was shown to occur with the CF3SO3H/nBu4NI initiating system at −78 °C. Figure 6. MWD curves for (A) poly(IPVE)s and (B) poly(TBVE)s obtained using the CF3SO3H/nBu4NBr initiating system in dichloromethane at (A) −40 °C and (B) −78 °C: [IPVE]0 = 0.87 M or [TBVE]0 = 0.76 M, [CF3SO3H]0 = 4.0 mM, [nBu4NBr]0 = 4.2 mM. † For low-MW peaks.

reaction was slower compared to that when nBu4NI was used, and only half the amount of the fed monomer was consumed in 144 h and converted into a polymer. As shown in Figure 6A, the MWD curves of the products were bimodal; however, the lower MW peak shifted toward the higher MW region, which indicated that long-lived species were partially formed. In contrast, the higher MW portion likely resulted from an uncontrolled reaction, and the 1H NMR analysis of the product polymer suggested structures derived from side reactions, such as β-proton elimination and side-chain abstraction (Figure S5). For the reaction of IPVE, nBu4NCl was also employed; however, it gave poor results (entry 9). The polymerization was very slow and yielded products with very broad MWDs. 3. Cationic Polymerization of TBVE. The most reactive monomer among the three VEs employed, TBVE, was also polymerized with the CF3SO3H/nBu4NI initiating system. Under conditions similar to those used for the living cationic polymerization of IPVE, the reaction of TBVE proceeded to completion in 20 min, much faster than that of IPVE (entry 10; Figure 7). However, the products obtained at the later stage of the reaction had broad MWDs and Mn values smaller than the theoretical values. 1H NMR analysis of the products revealed structures derived from the side-chain dealcoholization and the reaction of the eliminated alcohol with the propagating carbocation (Figure S6). Because those reactions were chain-

Figure 8. 1H NMR spectrum (in CDCl3 at 30 °C) of poly(TBVE) obtained using the CF3SO3H/nBu4NI initiating system in dichloromethane at −78 °C [Mn(GPC) = 1.01 × 104, Mw/Mn(GPC) = 1.12; similar polymerization conditions to those used for entry 11 in Table 1]. *Solvent, water, and vaseline.

Because of the high reactivity of TBVE, nBu4NBr functioned efficiently and induced controlled polymerization at −78 °C, thereby generating well-defined polymers with unimodal and relatively narrow MWDs as shown in Figure 6B (entry 13). Unlike the case of IPVE, no ill-defined portion was detected by GPC analysis. However, MALDI-TOF-MS analysis revealed small amounts of uncontrolled structures, which were derived from side reactions, such as β-proton elimination (Figure S7). Polymerization Mechanisms with CF3SO3H/nBu4NX Initiating Systems. 1. In Situ NMR Analysis of the Polymerization Reactions. To examine the polymerization mechanisms, we conducted 1H NMR analysis on the living polymerization of IPVE using the CF3SO3H/nBu4NI initiating

Figure 7. (A) Time−conversion curves, (B) Mn and Mw/Mn for the polymerization of TBVE using the CF3SO3H/nBu4NI initiating system in dichloromethane at −40 °C (circle) and −78 °C (square), and (C) MWD curves for poly(TBVE)s obtained at −78 °C: [TBVE]0 = 0.76 M, [CF3SO3H]0 = 4.0 mM, [nBu4NI]0 = 4.2 mM. 1582

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2. Relationship between nBu4NI Concentration and Polymerization Behavior. To elucidate the effect of the concentration of nBu4NI on polymerization behaviors, the cationic polymerization of IBVE was conducted using 4.0 mM CF3SO3H and 3.5−4.5 mM nBu4NI. Using 3.5 mM nBu4NI, the reaction went to completion instantaneously, yielding an illdefined polymer with a very broad MWD (Figure 11A). An increase in the concentration of nBu4NI to 3.7 mM decreased the reaction rate, which resulted in polymers with bimodal MWDs (Figure 11B). The lower-molecular-weight peak shifted toward the higher-molecular-weight region, indicating the generation of a long-lived species. The partial production of uncontrolled species with high molecular weights likely resulted from the nonquantitative formation of dormant carbon−iodine bonds. The uncontrolled species most likely possessed CF3SO3− counteranions. In contrast to the reactions where the nBu4NI concentrations were lower than the concentration of CF3SO3H, polymerizations using larger concentrations of nBu4NI proceeded in a controlled fashion, producing polymers with unimodal MWDs (Figures 11C,D). The reaction rate decreased with increasing amounts of nBu4NI, suggesting that excess I− species frequently reacted with the propagating carbocation in the dormant-active equilibrium. 3. Relationships between Monomer Reactivity and Nucleophilicity of X− in nBu4NX. The cationic polymerization utilizing the CF3SO3H/nBu4NX initiating system begins through the quantitative formation of a VE−hydrogen halide adduct. The subsequent heterolytic cleavage of the carbon− halogen bond without any catalysts generates a carbocationic species that adds to the monomer molecules. Suitable reaction conditions, including the addition of an ammonium salt, its amount, and the temperature, which all depended on the monomer reactivity, had to be employed to efficiently induce controlled polymerization that was free from side reactions. A counteranion with suitable nucleophilicity was indispensable for the controlled polymerization (Chart 1). An iodide anion, which exhibits weaker nucleophilicity than bromide or chloride anions,21 forms a carbon−iodine bond at the propagating end. This bond is relatively labile and cleaves heterolytically without any catalysts to generate a carbocation.6−10 Because a bromide anion forms a more stable bond with a carbon atom, its scission occurs less frequently, which

Figure 9. MALDI-TOF-MS spectra of poly(TBVE) obtained using the CF3SO3H/nBu4NI initiating system in dichloromethane at −78 °C [Mn(GPC) = 1.01 × 104, Mw/Mn(GPC) = 1.12; similar polymerization conditions to those used for entry 11 in Table 1].

system. The instantaneous formation of the IPVE-HI adduct (peaks p and q; Figure 10A) was observed in a mixture of equimolar amounts of CF3SO3H and IPVE (40 mM) with a slightly larger amount of nBu4NI (50 mM) at −50 °C, as was observed with the reaction of IBVE (vide supra). However, in this case, a small portion of the IPVE−HI adduct was decomposed into acetaldehyde (peak r) by adventitious water.20 The polymerization reaction was then monitored for a reaction mixture that contained a greater amount of IPVE (0.40 M, Figure 10B). In the resulting spectrum, the peaks for the IPVE−HI adduct (peaks p and q) were absent, whereas a peak (peak q′) emerged in a region slightly downfield from peak q. The new peak was assignable to the methine proton of the carbon−iodine bond at the propagating chain end. The monomer conversion was calculated to be ∼90% from the integral ratios of the monomer and polymer peaks. These results indicate that the IPVE−HI adduct was smoothly consumed and that the polymerization reaction subsequently occurred via the scission of the carbon−iodine chain end.

Figure 10. 1H NMR spectra of (A) the in situ generated IPVE−HI adduct and (B) the polymerization reaction mixture: [IPVE]0 = (A) 40 mM or (B) 0.40 M, [CF3SO3H]0 = 40 mM, and [nBu4NI]0 = 50 mM, in CD2Cl2/CH2Cl2 (6/4 v/v) at −50 °C. The monomer conversion was calculated to be ∼90% for (B). Peak p shifted to the higher magnetic field (∼1.2 ppm; the peak overlapped with another peak) after the adduct reacted with a monomer to be incorporated at the polymer chain end. 1583

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Figure 11. MWD curves for poly(IBVE)s obtained using the CF3SO3H/nBu4NI initiating system: [IBVE]0 = 0.76 M, [CF3SO3H]0 = 4.0 mM, [nBu4NI]0 = 3.5−4.5 mM, in dichloromethane at −40 °C. †For low-MW peaks.

monomer efficiently generated a VE−hydrogen halide adduct through a facile procedure. These adducts were heretofore synthesized by a direct addition of hydrogen halide with a VE, sometimes via the bubbling of gaseous hydrogen halides into a solution containing a VE monomer at a very low temperature.15 To extend the availability of the present synthetic method, adducts generated in situ were employed as cationogens for Lewis acid-catalyzed living cationic polymerizations. When ZnCl2 was used as a catalyst, the polymerization of IBVE proceeded in a highly controlled manner in conjunction with the adduct generated from CF3SO3H/nBu4NX (X = I, Br, or Cl) as the cationogen (Figure 12). The Mn values of the

Chart 1. Cationic Polymerization Behaviors with the CF3SO3H/nBu4NX Initiating Systems (*Nucleophilicity Order in Aprotic Solvents: Ref 21)

results in longer reaction times. The carbon−chlorine bond is much more stable and harder to cleave without a catalyst. On the other hand, the triflate anion leads only to an uncontrolled polymerization due to its extremely weak nucleophilicity. Monomer reactivity also affected polymerization behaviors. A highly reactive monomer forms a more labile bond with a halide anion at the propagating end, which leads to greater polymerization rates due to the frequent generation of carbocationic species. Therefore, a bromide anion is also efficient in inducing the relatively controlled polymerization of TBVE, which is a highly reactive monomer, despite the greater nucleophilicity of the anion compared to that of an iodide anion. However, a more labile carbon−halogen bond tends to induce more frequent side reactions, such as β-proton elimination. To suppress those reactions, a lower temperature is required for the reaction of TBVE compared to the cases of the less reactive monomers, IPVE and IBVE.22 A salt that was generated by the anion exchange reaction, nBu4N(CF3SO3), would be another prerequisite for the polymerization, especially in the case of a low-reactive monomer. Nuyken et al. have reported that nBu4N(ClO4) is indispensable for the polymerization of IBVE using an IBVE− HI adduct.7 Because CF3SO3− is a noncoordinating anion, similar to ClO4−, it most likely functioned as the reagent that promoted the polymerization.7−10 In fact, the IBVE polymerization was accelerated in the presence of purposely added nBu4N(CF3SO3) (4.0 or 8.0 mM; Figure S8). Use of the in Situ-Generated Adducts as a Cationogen for Lewis Acid-Catalyzed Living Polymerization. The reaction of CF3SO3H, nBu4NX, and a VE

Figure 12. (A) Time−conversion curves and (B) Mn and Mw/Mn for polymerization of IBVE using the CF3SO3H/nBu4NX [X = I (red; circle), Br (green; square), or Cl (blue; triangle)] initiating system with ZnCl2 in dichloromethane at −40 °C: [IBVE]0 = 0.76 M, [CF3SO3H]0 = 4.0 mM, [nBu4NX]0 = 4.2 mM, [ZnCl2]0 = 1.0 mM, [Et2O] = 9.6 mM.

product polymers increased linearly along the calculated line while unimodal and very narrow MWDs were maintained. In addition, the reaction rates were slightly larger for nBu4NI or nBu4NBr than for nBu4NCl. This result is most likely due to the exchange reaction that occurred between the halide anion derived from the adduct and the chloride anion of ZnCl2.16 The reaction rates likely depended on the strength of the resulting carbon−halogen bonds and the Lewis acidity of the resulting Zn halides.

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CONCLUSIONS In conclusion, the living cationic polymerization of alkyl VEs proceeded with the initiating system composed of CF3SO3H, nBu4NX, and VE. The reaction started via the in situ generation of VE−hydrogen halide adducts upon mixing the three components. The subsequent polymerization of alkyl VEs 1584

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D.; Brookhart, M. Prog. Polym. Sci. 2007, 32, 30−92. (c) Chen, E. Y.-X. Chem. Rev. 2009, 109, 5157−5214. (4) Miyamoto, M.; Sawamoto, M.; Higashimura, T. Macromolecules 1984, 17, 265−268. (5) Faust, R.; Kennedy, J. P. Polym. Bull. 1986, 15, 317−323. (6) Sawamoto, M.; Fujimori, J.; Higashimura, T. Macromolecules 1987, 20, 916−920. (7) Nuyken, O.; Kröner, H. Makromol. Chem. 1990, 191, 1−16. (8) Cramail, H.; Deffieux, A.; Nuyken, O. Makromol. Chem., Rapid Commun. 1993, 14, 17−27. (9) Cramail, H.; Deffieux, A. Macromol. Chem. Phys. 1994, 195, 217− 227. (10) Cramail, H.; Deffieux, A. Macromolecules 1994, 27, 1401−1406. (11) Van Meirvenne, D.; Haucourt, N.; Goethals, E. J. Polym. Bull. 1990, 23, 185−190. (12) Cho, C. G.; Feit, B. A.; Webster, O. W. Macromolecules 1990, 23, 1918−1923. (13) Sugihara, S.; Tanabe, Y.; Kitagawa, M.; Ikeda, I. J. Polym. Sci., Part A: Polym. Chem. 2008, 46, 1913−1918. (14) Matsuo, Y.; Yonezumi, M.; Kanaoka, S.; Aoshima, S. Polym. Prepr., Jpn. 2007, 56, 258. (15) Higashimura, T.; Kamigaito, M.; Kato, M.; Hasebe, T.; Sawamoto, M. Macromolecules 1993, 26, 2670−2673. (16) Kamigaito, M.; Sawamoto, M.; Higashimura, T. Macromolecules 1992, 25, 2587−2591. (17) Hatada, K.; Nagata, K.; Yuki, H. Bull. Chem. Soc. Jpn. 1970, 43, 3195−3198. (18) The reaction with adventitious water may have generated a hemiacetal structure at the ω-end. A peak was observed at a slightly downfield region compared to the peak of the ω-end acetal methine proton in 1H NMR (Figure S2). The peak disappeared after acid hydrolysis. In addition, the MALDI-TOF-MS analysis supported its generation because the spectrum exhibited a peak series with m/z values of −14 from the main series corresponding to the structure with methoxy ω-ends (Figure S3). TBVE and IPVE polymerizations did not include such side reactions, possibly because a less reactive VE tends to react with adventitious water more frequently [Rahman, Md. S.; Hashimoto, T.; Kodaira, T. J. Polym. Sci., Part A: Polym. Chem. 2000, 38, 4362−4372]. In addition, a very long reaction time would be responsible for the reaction of the IBVE propagating ends with adventitious water. (19) The MWD curve of the polymer obtained after the addition (Figure 3B, lower) has slight tail in the lower MW region, probably because adventitious water induced inevitable termination/transfer reactions. (20) This undesired reaction likely stemmed from the temporal but inevitable increase in temperature during the NMR experiment, which also supports the highly unstable nature of the IPVE−HI adduct. (21) Puar, M. S. J. Chem. Educ. 1970, 47, 473−474. (22) The polymerization of p-methoxystyrene (pMOS) was also conducted using the CF3SO3H/nBu4NI system. The reaction proceeded at −40 °C when the amount of nBu4NI was smaller than that of CF3SO3H ([CF3SO3H]0 = 4.0 mM and [nBu4NI]0 = 3.7 mM, [pMOS]0 = 0.38 M), but the polymerization was not controlled to give polymers with broad MWDs and uncontrolled Mn values. We are now optimizing reaction conditions in order to control the pMOS polymerization.

proceeded in a controlled fashion without any catalyst under appropriate reaction conditions. Choosing a halide anion with the appropriate nucleophilicity according to monomer reactivity was indispensable for the controlled polymerizations. The present system was also efficient as a new method for synthesizing VE−hydrogen halide adducts, the conventional synthesis of which requires toxic, gaseous hydrogen halides. Furthermore, hydrogen halide adducts of other monomers such as styrene derivatives will also be easily synthesized using our method. The easy access to carbon−iodine bonds, the bonds much more labile than carbon−chloride bonds that have mainly used for living cationic polymerization of various vinyl monomers, is another attractive feature of the present system. The extension of the scope of available monomers for the adduct synthesis and available chain end-halogen atoms will lead to the synthesis of a wide range of end-functionalized polymers, controlled polymerizations of low reactive monomers, and the development of new initiating systems for living cationic polymerization.

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ASSOCIATED CONTENT

S Supporting Information *

Figures showing time−conversion plots, Mn and Mw/Mn for polymerization, MWD curves of products, 1H NMR spectra, and MALDI-TOF-MS spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by Grant-in-Aid for Scientific Research (No. 22107006) on Innovative Areas of “Fusion Materials” (No. 2206) from MEXT and by Grant-in-Aid for Young Scientists (B) (No. 24750107) from JSPS.

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REFERENCES

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dx.doi.org/10.1021/ma402490f | Macromolecules 2014, 47, 1578−1585