Article pubs.acs.org/Macromolecules
Structure Effects of Lewis Acids on the Living Cationic Polymerization of p‑Methoxystyrene: Distinct Difference in Polymerization Behavior from Vinyl Ethers Arihiro Kanazawa,† Shota Shibutani,† Nobuto Yoshinari,‡ Takumi Konno,‡ Shokyoku Kanaoka,† and Sadahito Aoshima*,† †
Department of Macromolecular Science, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan Department of Chemistry, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan
‡
S Supporting Information *
ABSTRACT: A new design perspective on initiating systems for living cationic polymerization was gained by thorough examination of various metal chlorides as catalysts in conjunction with a weak Lewis base in the cationic polymerization of p-methoxystyrene. The Lewis acids clearly differed in controllability, in contrast to the findings of a previous report on the polymerization of isobutyl vinyl ether (IBVE) using various catalysts (all the metal chlorides used in the present study induced the controlled polymerization of IBVE, although the reaction rates depended on the chlorophilic and oxophilic nature of the central metals: Macromolecules 2009, 42, 3965). Metal tetrachlorides and dichlorides such as SnCl4, TiCl4, ZrCl4, HfCl4, and ZnCl2 induced controlled polymerizations to produce polymers with predetermined molecular weights and very narrow molecular weight distributions (MWDs). In contrast, frequent side reactions (β-proton elimination and the FriedelCrafts reaction) occurred with the trichlorides FeCl3 and GaCl3, yielding polymers with molecular weights lower than the theoretical values and with broad MWDs. Another trichloride, AlCl3, produced polymers with very high molecular weights owing to its very low initiation efficiency. NbCl5, a pentachloride, was also unable to control the polymerization. The structures of the counteranions with or without a coordinating weak Lewis base were shown to be responsible for the difference in the controllability between the metal chlorides.
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INTRODUCTION Lewis acids have been extensively employed and studied as catalysts in various fields, including pharmaceutical chemistry and polymer synthesis, since the early development of petroleum chemistry. Metal halides are representative Lewis acids for organic reactions: these halides promote reactions such as the FriedelCrafts, aldol, DielsAlder, and Mannich reactions by accepting an electron pair from basic compounds into the vacant coordination site of the central metal.1 Cationic polymerization, another reaction catalyzed by Lewis acids, can proceed in a living fashion with an appropriate additive, yielding polymers with well-defined primary structures.2 Despite numerous studies, no absolute scale of Lewis acidity has been established for a wide range of metal halides. A preference for specific atoms3 (for example, oxophilicity or chlorophilicity) is a useful measure of acidity and is correlated with the principle of hard and soft acids and bases (HSAB). However, even HSAB theory is not capable of fully explaining the interaction preferences between Lewis acids and bases.4 The difficulty in evaluating Lewis acidity merely on the basis of HSAB is attributed to another influence: the structure dependence of Lewis acidity, which is usually determined by the coordination numbers or coordination modes. Some metal © 2012 American Chemical Society
halides accept more than one basic molecule using plural coordination sites, whereas others can coordinate only one molecule.5 Living cationic polymerization systems are suitable for detailed comparisons of different metal halides' catalytic activities because the polymerization rates and side reactions, which are often specific to the nature of the central metals, are determined in a straightforward fashion. Our recent findings6 that a variety of metal halides could function as catalysts for the living cationic polymerization of isobutyl vinyl ether (IBVE) in combination with a weak Lewis base, such as an ester or ether, indicated that the polymerization activity was governed by the balance between the oxophilicity and chlorophilicity of the metal chlorides. For example, the reactions were completed in 20 seconds with FeCl3/1,4-dioxane, a few minutes with SnCl4/ ethyl acetate, several days with TiCl4/ethyl acetate, and weeks with SiCl4/ethyl acetate. These trends are consistent with studies of metal halide activity in FriedelCrafts reactions7 and their interactions with Lewis basic compounds.8,9 Received: July 19, 2012 Revised: September 6, 2012 Published: September 20, 2012 7749
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tetrahydrofuran (THF; Wako; >99.5%) were distilled over calcium hydride and then lithium aluminum hydride. Dichloromethane (Wako; 99.0%), hexane (Wako; 96.0%), and toluene (Wako; 99.5%) were dried by passage through solvent purification columns (Glass Contour). Toluene was further purified by distillation over metallic sodium. The adduct of IBVE with HCl (IBVEHCl) was prepared from the addition reaction of IBVE with HCl.19 Commercially available SnCl4 (Aldrich; 1.0 M solution in heptane), ZnCl2 (Aldrich; 1.0 M solution in diethyl ether), EtAlCl2 (Wako; 1.0 M solution in hexane), and TiCl4 (Aldrich; 1.0 M solution in toluene or dichloromethane) were used without further purification. For the following metal chlorides, stock solutions in diethyl ether, hexane, ethyl acetate, or ethyl acetate/dichloromethane were prepared: FeCl3 (Aldrich; 99.99%; 200 mM in diethyl ether), GaCl3 (Aldrich; >99.999%; 200 mM in hexane), InCl3 (Strem; 99.999%; 100 mM in ethyl acetate), AlCl3 (Aldrich; 99%; 500 mM in ethyl acetate), HfCl4 (Strem; >99.9%; 50 mM in ethyl acetate), ZrCl4 (Strem; >99.95%; 500 mM in ethyl acetate), TaCl5 (Strem; >99.99%; 200 mM in ethyl acetate), and NbCl5 (Strem; 99.99%; 200 mM in dichloromethane with 400 mM of ethyl acetate). For nBu4NCl, a stock solution in dichloromethane was prepared from anhydrous nBu4NCl (Fluka; >99.0%). All chemicals except for dichloromethane, toluene, and hexane were stored in brown ampules under dry nitrogen. Polymerization Procedures. The following is a typical polymerization procedure (using the SnCl4/ethyl acetate system as an example). 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. Toluene (2.7 mL), ethyl acetate (0.40 mL; 4.1 mmol), pMOS (0.20 mL; 1.5 mmol), and 53 mM IBVEHCl solution in toluene (0.30 mL; 1.6 × 10‑2 mmol) were added successively into the tube using dry syringes. The polymerization was started by the addition of a prechilled 100 mM SnCl4 solution in toluene/heptane (9/1 v/v; 0.40 mL; 4.0 × 10−2 mmol) at 0 °C. After 20 min, the reaction was terminated with prechilled methanol (3 mL) containing a small amount of aqueous ammonia solution (0.1%). The quenched mixture was washed with dilute hydrochloric acid, an aqueous NaOH solution, and then water to remove the initiator residues. The volatiles were then removed under reduced pressure at 50 °C, and the residue was vacuum-dried for more than 3 h at 60 °C to yield a white solid polymer (0.189 g). The monomer conversion was determined by gravimetry (conversion = 94%). The GPC analysis data were as follows: Mn = 1.13 × 104; Mw/Mn = 1.13. 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 mmI.D. × 300 mm) or TSKgel MultiporeHXL-M × 3 (exclusion limit molecular weight = 2 × 106; bead size = 5 μm; column size =7.8 mmI.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 a RI-8020 refractiveindex detector. The number-average molecular weight (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 = 5771.09 × 106, Mw/Mn ≤ 1.1). NMR spectra were recorded using a JEOL JNMECA 500 spectrometer (500.00 MHz for 1H and 125.65 MHz for 13 C). X-ray Crystallographic Analysis. A crystal of the Sn compound was prepared as follows: 3.0 mL of hexane and 0.5 mL of toluene were gently poured into 2.0 mL of a solution containing 200 mM of nBu4NCl, 200 mM of SnCl4, and 3.0 M of THF in CH2Cl2/heptane (3/1 v/v) at room temperature. A colorless block crystal precipitated slowly. A crystal of the Al compound was prepared as follows: 4.0 mL of hexane and 2.0 mL of toluene were gently poured into 2.0 mL of a solution containing 200 mM of nBu4NCl, 200 mM of AlCl3, and 2.0 M of THF in CH2Cl2/ethyl acetate (1/1 v/v) at room temperature. A colorless block crystal precipitated slowly.
Other living systems would be more helpful for investigating the differences in polymerization behavior, especially controllability differences, because the polymerization of vinyl ether with a base is highly controlled irrespective of the metal halides used. On the basis of these studies, we refocused our attention on the cationic polymerization of styrene derivatives, which can be accompanied by characteristic side reactions, such as the electrophilic reaction of a carbocation with the aromatic ring. Differences in the types of side reactions depending on the metal halide are expected to reveal the structural effects of the metal halides. Among various styrene derivatives that polymerize cationically, p-methoxystyrene (pMOS) was employed in the present study. Although the cationic polymerizability of pMOS is much lower than that of IBVE,10 pMOS has much higher reactivity than other styrene derivatives11 such as p-methylstyrene, styrene, and p-chlorostyrene due to the large electron-donating ability of the p-methoxy group. The relatively high reactivity of pMOS is expected to facilitate the comparison of the reactions using various Lewis acids with a wide range of reactivity. In fact, its living cationic polymerization can be readily conducted using certain initiating systems, including HI/ZnI2,12 HI/I2,13 IBVEHCl/ZnX 2 , 14 alcohol/BF 3 OEt 2 , 15 pMOS−HCl/ SnBr 4 , 16 pMOS−OH/B(C 6 F 5 ) 3 , 17 and IBVEAcOH/ EtAlCl2/SnCl4.18 However, no systematic studies comparing the cationic polymerization behaviors of various Lewis acids have been reported. Thus, the cationic polymerization of pMOS was examined using a variety of metal chlorides as Lewis acid catalysts in the presence of weak Lewis bases (Scheme 1). The Lewis acids Scheme 1. Cationic Polymerization of pMOS
differed in their ability to control the polymerization of pMOS, in contrast to the polymerization of IBVE. Metal tetrachlorides and dichlorides such as SnCl4 and ZnCl2 induced controlled polymerizations, whereas frequent side reactions (β-proton elimination and the FriedelCrafts reaction) occurred with the trichlorides FeCl3 and GaCl3. Another trichloride, AlCl3, showed a very low initiation efficiency, resulting in polymers with very high molecular weights. The differences in the polymerization behavior will be discussed in terms of the structures of the metal chlorides, focusing in particular on the counteranions generated in the activating step.
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EXPERIMENTAL SECTION
Materials. pMOS (Wako; >97.0%) was dried over sodium sulfate overnight and distilled twice over calcium hydride under reduced pressure. IBVE (TCI; >99.0%) was washed with 10% aqueous sodium hydroxide solution and then water, and distilled twice over calcium hydride. Ethyl acetate (Wako; >99.5%) was distilled twice over calcium hydride before use. Diethyl ether (Wako; >99.5%) and 7750
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Table 1. Cationic Polymerization of pMOS with Various Metal Chlorides in the Presence of Weak Lewis Basea entry
metal chloride
solvent
time
convn (%)
Mn × 10−3 (calcd)
Mn × 10−3 b
Mw/Mnb
1 2c 3 4d 5 6d 7 8d 9 10f 11 12 13c 14 15g 16
SnCl4 ZnCl2 TiCl4
toluene toluene toluene CH2Cl2 toluene CH2Cl2 toluene CH2Cl2 toluene toluene toluene toluene toluene toluene CH2Cl2 toluene
20 min 3h 100.5 h 46 h 20.5 h 5h 60 h 7h 20 min 20 min 2 min 5 min 15.5 h 17 min 30 s 10 min
94 95 63 91 87 86 96 84 98 93 79 88 89 94 91 24
11.9 12.0 7.9 11.5 11.1 10.9 12.2 10.6 12.4 11.9 9.5 11.1 11.2 12.0 11.5 1.9
11.3 10.6 3.2 9.7 6.5 10.2 6.6 10.2 11.6 13.3 69.6 8.9 6.8 5.6 7.0 25.1
1.13 1.10 6.99 1.04 2.36 1.13 1.21 1.30 1.25 (1.09)e 1.07 2.16 1.73 1.34 1.88 1.45 1.85
ZrCl4 HfCl4 InCl3 AlCl3 GaCl3 FeCl3 NbCl5 TaCl5
a
[pMOS]0 = 0.38 M, [IBVEHCl]0 = 4.0 mM, [metal chloride]0 = 5.0 (for entries 12 and 13) or 10 mM (except for entries 12 and 13), [ethyl acetate] = 0.5 M (for entry 4, 6, and 8) or 1.0 M (except for entries 4, 6, 8, 12, 13, and 15), [THF] = 1.1 M (for entries 12 and 13), in toluene or CH2Cl2 at 0 °C. bBy GPC (polystyrene calibration). cContaining diethyl ether (100 mM for entry 2 and 250 mM for entry 13). d[DTBP] = 5.0 mM. e For the main peak. f[DTBP] = 1.0 mM. g[nBu4NCl]0 = 9.5 mM, [ethyl acetate] = 20 mM. Single-crystal X-ray analyses of the Sn and Al compounds were performed with a Rigaku R-AXIS VII imaging plate area detector with a graphite monochromated MoKα radiation. The intensity data were collected by the ω scan mode. The intensities were corrected for Lorentz and polarization effects. Empirical absorption corrections were also applied. The structures were refined with full-matrix least-squares on F2. All calculations were performed using Yadokari-XG 2009 software package20 except for the solution with SIR9721 or SHELXS9722 and the refinement with SHELXL-97.22 Crystal data are summarized in Tables S1 and S2 in the Supporting Information. H atoms were located in calculated positions. All non-hydrogen atoms were refined anisotropically. For the crystal of the Sn compound, one of the carbon atoms of nBu4N+ cation was disordered in two positions (C20 and C21) with an occupancy of 0.5.
ZnCl2 proceeded smoothly to achieve monomer conversions of over 90% in the presence of ethyl acetate in toluene at 0 °C (entries 1 and 2 in Table 1; red and orange symbols in Figure 1A). The reactions produced polymers with very narrow, unimodal MWDs, as shown in Figure 1C. In addition, the Mn values of the product polymers increased in a manner that was directly proportional to the monomer conversion along the calculated line, assuming that one initiator molecule produces one polymer chain (red and orange symbols in Figure 1B). These results show that both polymerizations proceeded in a highly controlled manner. The polymerization rates were much slower than those of IBVE with SnCl4 and ZnCl2.23 The polymerization using SnCl4 was also examined in the absence of an added base. The reaction without a base was completed in a few seconds, resulting in polymers with broad MWDs (conversion = 90% in 1 s, Mn = 5.0 × 104, Mw/Mn = 1.92). This result showed that a weak Lewis base is indispensable for controlling the polymerization, as is the case with IBVE under similar conditions. In polymerizations with metal tetrachlorides of Group 4 elements, TiCl4, ZrCl4, and HfCl4, the reaction solvents were crucial for controlling the reactions. The polymers obtained with TiCl4 in toluene had unimodal but broad MWDs, indicating that no long-lived species existed (entry 3 in Table 1). Although long-lived species were formed in the reactions with ZrCl4 and HfCl4 in toluene, the products had bimodal MWDs with uncontrolled portions in the higher MW region (entries 5 and 7 in Table 1; Figure 2A for ZrCl4). However, the polymerizations with the three Group 4 metal chlorides proceeded in controlled fashions in dichloromethane (entries 4, 6, and 8 in Table 1; Figure 2B for ZrCl4). The obtained polymers possessed narrow MWDs and Mn values that agreed with the theoretical values. To control the polymerizations with these catalysts, a small amount of DTBP was required as a proton trap in addition to the ethyl acetate, probably because the Group 4 metal chlorides are prone to causing protic initiation because of their strong oxophilicity. Indeed, the reactions without DTBP produced polymers with a lower MW or products with small amounts of uncontrollable parts. Most
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RESULTS AND DISCUSSION 1. Polymerization Results. The cationic polymerization of pMOS was performed using a variety of metal chlorides in conjunction with an IBVEHCl adduct as a cationogen in the presence of an added base in toluene or dichloromethane at 0 °C. Interestingly, the polymerization behaviors differed among the Lewis acids used (Table 1), in contrast to the polymerization of IBVE.6 The tetra- and dichlorides, SnCl4, TiCl4, ZrCl4, HfCl4, and ZnCl2, induced controlled reactions, whereas the tri- and pentachlorides, AlCl3, GaCl3, FeCl3, NbCl5, and TaCl5 (but not InCl3), produced ill-defined polymers (Table 2). The polymerization results will be explained in detail in the following subsections. (a). Controlled Polymerization Using Tetra- and Dichlorides (and InCl3). The polymerizations of pMOS with SnCl4 or Table 2. Cationic Polymerization of pMOS with Various Metal Chlorides uncontrolled controlled
high MW
ZnCl2, SnCl4, TiCl4, ZrCl4, HfCl4, InCl3
AlCl3, TaCl5
[di- and tetrachlorides (except for InCl3)]
low MW
GaCl3, FeCl3, NbCl5 [tri- and pentachlorides]
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Figure 1. (A) Timeconversion curves, (B) Mn and Mw/Mn for the polymerization of pMOS, and (C) MWD curves for poly(pMOS)s obtained using SnCl4, ZnCl2, GaCl3, and AlCl3 ([pMOS]0 = 0.38 M, [IBVEHCl]0 = 4.0 mM, [metal chloride]0 = 5.0 mM (GaCl3) or 10 mM (SnCl4, ZnCl2, and AlCl3), [ethyl acetate] = 1.0 M for SnCl4, ZnCl2, and AlCl3, [THF] = 1.1 M for GaCl3, in toluene at 0 °C) Key: red, SnCl4; orange, ZnCl2; green, GaCl3; blue, AlCl3.
(b). Uncontrolled Polymerization Using Tri- and Pentachlorides. In contrast to the tetra- and dichlorides (except for InCl3), the metal trichlorides AlCl3, GaCl3, and FeCl3 induced uncontrolled polymerizations of pMOS in the presence of an added base. When the polymerization of pMOS was conducted with AlCl3 in conjunction with ethyl acetate in toluene at 0 °C, the polymers produced from the early stage of polymerization had molecular weights much higher than the calculated values and had broad MWDs (entry 11 in Table 1; blue symbols in Figure 1). Moreover, the polymerization rate was much higher than that for the polymerization of IBVE under similar conditions, which was different from the reactions with SnCl4 and ZnCl2.23 The polymerizations using FeCl3 or GaCl3 in the presence of THF also produced polymers with broad MWDs; however, in contrast to AlCl3, the molecular weights of the polymers were lower than the theoretical values (entries 12 and 13 in Table 1; green symbols in Figure 1 for GaCl3). These results suggested the frequent occurrence of side reactions, such as the chain transfer reaction. In addition, polymerizations with these trichlorides in dichloromethane were also not controlled under similar conditions.24 To elucidate the side reactions that occurred during polymerizations with these trichloride catalysts, the 1H NMR spectra of the obtained poly(pMOS)s were recorded as shown in Figure 3. The products with uncontrolled MW were shown to have undesired structures derived from side reactions. For example, several peaks due to irregular structures were observed in the spectra of the products generated by catalysis with GaCl3 and FeCl3, in addition to the peaks assignable to the structures with the initiator fragment at the α-ends and the methoxy ωends from the quencher. The peaks at approximately 6.0 ppm
Figure 2. MWD curves for poly(pMOS)s obtained using ZrCl4 in (A) toluene and (B) CH2Cl2 ([pMOS]0 = 0.38 M, [IBVEHCl]0 = 4.0 mM, [ZrCl4]0 = 10 mM, [ethyl acetate] = (A) 1.0 M or (B) 0.5 M, [DTBP] = 5.0 mM for part B, at 0 °C).
likely, DTBP trapped the protons from adventitious water and/ or interacted with the catalysts to moderate the acidity. In addition to the tetra- and dichlorides that induced controlled polymerizations, InCl3 successfully catalyzed the reaction to yield well-defined polymers (entry 9 in Table 1). The polymerization of pMOS with InCl3 proceeded smoothly in the presence of ethyl acetate in toluene. The Mn of the product polymers increased along the theoretical line, although the higher MW region exhibited a small amount of uncontrolled parts. The generation of the uncontrolled portion was suppressed when DTBP was used (entry 10 in Table 1), as was the case for the group 4 catalysts. 7752
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Figure 3. 1H NMR spectra of poly(pMOS)s obtained with (A) SnCl4, (B) ZnCl2, (C) GaCl3, and (D) FeCl3 (polymerization conditions: [pMOS]0 = 0.38 M, [IBVEHCl]0 = 4.0 mM, [metal chloride]0 = 5.0 mM (GaCl3 and FeCl3) or 10 mM (SnCl4 and ZnCl2), [ethyl acetate] = 1.0 M for SnCl4 and ZnCl2, [THF] = 1.1 M for GaCl3 and FeCl3, in toluene at 0 °C; peak v: vaseline).
Scheme 2. Plausible Mechanisms for Cationic Polymerization of pMOS with Various Metal Chlorides
(peaks p and p′) were assigned to the ω-end olefin structures stemming from β-proton elimination reactions. The peaks at 4.14.3 ppm (peaks n and o) are evidence of the occurrence of intra- and/or intermolecular FriedelCrafts reactions. Because these two types of reactions generated protons as well as dead chains, new chains formed by the protons, i.e., by the transfer reactions, produced polymers with a hydrogen−pMOS bond at the α-end, leading to a MW lower than the theoretical value. A
peak assignable to such a structure was confirmed at 0.81.1 ppm (peak m).15−17 In contrast, the 1H NMR spectra of the poly(pMOS) generated by catalysis with SnCl4 or ZnCl2 exhibited peaks exclusively assignable to the structure resulting from polymerization without side reactions. In addition, the molecular weights determined by the peak intensity ratios of the chain end-moieties (peaks b, g, and l) and the phenyl protons (j) 7753
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Tetrabutylammonium chloride was used as a chloride anion source instead of the chain-end chlorine atom in the polymerization. The chemical shifts of the carbonyl carbon of ethyl acetate are summarized in Figure 4. Obvious shifts toward
were in good agreement with those determined by GPC [Mn(NMR) = 3.9 × 103 and Mn(GPC) = 3.9 × 103 for the polymer produced by catalysis with SnCl4]. The pentachlorides NbCl5 and TaCl5 also induced ill-defined polymerization in the presence of ethyl acetate (entries 14 and 16 in Table 1), similar to the trichlorides. With NbCl5, polymers were produced with Mn values lower than the theoretical values and broad MWDs, indicating the frequent occurrence of chain transfer reactions. The 1H NMR analysis (Figure S1, Supporting Information) suggested that primarily FriedelCrafts-type side reactions occurred during the polymerization. In contrast, polymers with very high molecular weights were produced with TaCl5 regardless of the monomer conversion, similar to the reaction using AlCl3. The polymerization of pMOS with NbCl5 was also examined in the presence of nBu4NCl because an added salt was suitable for controlling the polymerization of IBVE when the two pentachlorides were employed.6 The polymerization proceeded smoothly to produce polymers with relatively narrow MWDs (Mw/Mn < 1.5; entry 15 in Table 1). However, the polymers obtained in the middle and the later stages of polymerization had similar Mn values (Mn = 6.2 × 103 for 44% conversion and Mn = 7.0 × 103 for 91% conversion). 2. Counteranion Influence on the Polymerization Behavior: Counteranions with or without a Coordinating Lewis Base. The structure of the counteranion derived from the metal halide may play a key role in controlling the polymerization. The Lewis acids used in the present study are classified into two groups by coordination mode: tetrachlorides (SnCl4, TiCl4, ZrCl4, and HfCl4) and dichlorides (ZnCl2), which have two sites for the coordination of Lewis bases, and trichlorides (AlCl3 and GaCl3) and pentachlorides (NbCl5 and TaCl5), which have only one site.5,25 In cationic polymerization using a weak Lewis base, the Lewis acidity of a metal chloride catalyst is moderated by forming a complex with the weak Lewis base (the left parts in Scheme 2) to suitably establish the dormant−active equilibrium.26 For the polymerizations of monomers with large cationic polymerizability, such as alkyl vinyl ethers and alkoxystyrenes, very large amounts of the Lewis base are necessary to keep the concentration of the active species very low.2 Under such conditions, a Lewis acid catalyst cleaves the carbon−halogen bond (C−Cl bond in this study) of the initiating/propagating end to generate a carbocationic species and a counteranion after the release of a weak Lewis base (the abstraction of the halogen atom and the release of a Lewis base may occur associatively in some cases). Thus, the counteranions from metal chlorides with two coordination sites that induced living polymerizations are expected to possess one Lewis base, such as an ester or ether, in addition to the chlorine atom derived from the chain end (Scheme 2A). In contrast, the counteranions from metal chlorides with one coordination site that resulted in ill-defined polymerizations may lack both ester and ether molecules (Scheme 2B). Indium appears to have only one coordination site, as it belongs to the same group as aluminum and gallium. However, InCl3 was reported to coordinate three Lewis bases.27,28 To confirm whether the counteranions with coordinated ester or ether molecules are actually generated, we performed two model experiments as described below. First, the interaction in solution between ethyl acetate and the anionic species generated from an equimolar amount of the metal chloride (SnCl4, TiCl4, or GaCl3) and tetrabutylammonium chloride was determined using a 13C NMR analysis.
Figure 4. 13C NMR analysis of interaction between ethyl acetate and nBu4NCl/metal chloride complexes {TiCl4 (triangle): [ethyl acetate] = 100 or 200 mM, [TiCl4]0 = 100 mM, [nBu4NCl]0 = 100 mM, in CDCl3/CH2Cl2 (4/1 v/v); SnCl4 (circle): [ethyl acetate] = 100, 200, or 300 mM, [SnCl4]0 = 100 mM, [nBu4NCl]0 = 100 mM, in CDCl3/ CH2Cl2/heptane (8/1/1 v/v/v); GaCl3 (square): [ethyl acetate] = 40 or 80 mM, [GaCl3]0 = 40 mM, [nBu4NCl]0 = 40 mM, in CDCl3/ CH2Cl2/hexane (13/7/5 v/v/v); recorded at 25 °C}.
lower magnetic fields were observed in the spectra with SnCl4 and TiCl4. These results indicated that anionic species generated by mixing the metal chlorides and nBu4NCl, such as SnCl5− and TiCl5−, still exhibited Lewis acidity to further interact with the carbonyl carbon of ethyl acetate. The decrease in the degree of the shifts when adding two or three times the amount of ethyl acetate suggested a fast exchange among the pentachloride anions and the excess ethyl acetate molecules. In contrast, no shift of the carbonyl carbon of ethyl acetate was observed with GaCl3. This is probably because no Lewis acid site remained on the GaCl4− anion generated from GaCl3 and nBu4NCl. For the second model experiment, the X-ray crystal structures of the compounds produced by the reaction of metal chlorides, tetrabutylammonium chloride, and THF were analyzed. Equimolar amounts of SnCl4 or AlCl3 and nBu4NCl were mixed in the presence of a very large excess of THF in CH2Cl2. After the addition of toluene and hexane, colorless crystals slowly precipitated from both solutions. The crystal structures determined by X-ray analyses are shown in Figure 5. The tin compound has an octahedral structure, in which a tin center is coordinated by five chloride ions and one oxygen atom from a THF molecule. In contrast, the aluminum compound is transformed into a highly symmetrical tetrahedral structure in AlCl4− without the coordination of any THF molecule. The counteranions that coordinate with the Lewis bases are likely to have large interactions with the propagating carbocations because the counteranions have sufficient electron densities through the donation of the electrons from the 7754
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Table 3. Cationic Polymerizations of IBVE and pMOS with Various Metal Chlorides
pMOS IBVEb
dichloride (ZnCl2)
trichlorides (AlCl3, GaCl3, FeCl3, InCl3a)
tetrachlorides (SnCl4, TiCl4, ZrCl4, HfCl4)
pentachlorides (NbCl5, TaCl5)
controlled controlled
uncontrolleda controlled
controlled controlled
uncontrolled controlledc
a
InCl3 was reported to coordinate three Lewis bases, and induced controlled polymerization of pMOS. bReference 6. cIn the presence of an added salt (nBu4NCl).
between the various metal chlorides. In general, a carbocation with an adjacent group with a stronger electron-donating ability or a larger resonance effect shows lower activity. These effects stabilize the carbocation by the moderation or delocalization of the positive charge, leading to fewer side reactions such as βproton elimination. A stronger carbocation−counteranion interaction is required to suppress the side reactions that result from a less stable carbocation. In the case of IBVE and pMOS, the isobutoxy group stabilizes the carbocations with stronger electrondonating abilities,32 whereas the p-methoxy group does so with a larger resonance effect. Because of these different effects, however, it is difficult to accurately compare the stability of the carbocations. Furthermore, a relatively small difference in the reactivity between IBVE and pMOS23 makes the matter complex; a monomer with a higher reactivity or nucleophilicity generally forms a more stable carbocation, but the electrondonating ability of the adjacent group offers much greater stabilization to the carbocation than to the double bond.11 Meanwhile, the polymerization results show that the nucleophilicity of the counteranions without a coordinating weak Lewis base, such as AlCl4− and GaCl4−, was low but sufficient to stabilize the carbocation from IBVE, whereas it was insufficient to stabilize the carbocation from pMOS. In addition, a side reaction unique to styrene derivatives, i.e., the Friedel−Crafts reaction, may be another factor that differentiated the polymerization behaviors. It should be noted that the polymerization of pMOS performed in this study revealed for the first time the importance of the counteranions in controlling the cationic polymerization. At this moment, this structural effect can fully account for the difference in terms of controllability, although there may be other minor factors responsible for the difference among various metal chloride catalysts. To further understand the roles of Lewis acid catalysts, polymerizations of less active styrene derivatives using various metal chlorides are currently being investigated in our group.
Figure 5. ORTEP drawings of the crystals obtained from solutions containing (A) nBu4NCl, SnCl4, and THF or (B) nBu4NCl, AlCl3, and THF. Displacement ellipsoids are drawn at the 50% probability level. Hydrogen atoms and labels for carbon atoms are omitted for clarity.
coordinating weak Lewis base. This interaction not only suppresses side reactions by moderating the acidity of the carbocation but also contributes to the fast exchange in the dormantactive equilibrium by rapidly returning the chloride anion to the propagating cation.29−31 The concept of ion-pairs and free ions may explain the controllability difference; i.e., ionpairs were almost exclusively generated in the polymerizations with tetra- or dichlorides.2 The counteranions that do not coordinate with Lewis bases probably have weaker interactions with the propagating carbocations. This weak interaction resulted in the very high polymerization rate with AlCl323 and decreased the deactivation of active species into dormant species, which resulted in products with very high molecular weights. In the reactions using other trichlorides or pentachlorides such as FeCl3 and GaCl3, side reactions, including β-proton elimination and FriedelCrafts alkylation, were confirmed by the low Mn values and the 1H NMR spectra. The high acidity or electrophilicity of the propagating carbocations are likely responsible for the side reactions due to weaker interactions with the counteranions. These results suggested that the counteranions generated by FeCl3 and GaCl3 function differently from the counteranion generated by AlCl3 despite having similar structures. 3. Comparison with the Polymerization of Vinyl Ether. All of the metal chlorides used in this study induce the controlled cationic polymerization of IBVE as reported in our previous study,6 in sharp contrast to the polymerization of pMOS (Table 3). The differences in the activities of the propagating carbocations generated from IBVE and pMOS are most likely responsible for the differences in the controllability
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CONCLUSION The detailed comparison of various metal chlorides as catalysts with a Lewis base in the cationic polymerization of pMOS indicated decisive structure effects of counteranions on the controllability of the polymerization. The polymerizations proceeded in a controlled fashion with the metal tetrachlorides and dichloride SnCl4, TiCl4, ZrCl4, HfCl4, and ZnCl2 in the presence of ethyl acetate or THF. In contrast, the reaction was not controlled with the metal trichlorides and pentachlorides AlCl3, FeCl3, GaCl3, NbCl5, and TaCl5 because of low initiation efficiencies or the occurrence of side reactions, such as β-proton elimination or FriedelCrafts reactions. We suggest that the difference in behavior between the Lewis acids stems from the 7755
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Higashimura, T.; Gührs, K.-H.; Heublein, G. Polym. J. 1985, 17, 929−933. (11) Kolishetti, N.; Faust, R. Macromolecules 2008, 41, 3842−3851. (12) Higashimura, T.; Kojima, K.; Sawamoto, M. Polym. Bull. 1988, 19, 7−11. (13) Kojima, K.; Sawamoto, M.; Higashimura, T. Macromolecules 1990, 23, 948−953. (14) Kanaoka, S.; Higashimura, T. J. Polym. Sci., Part A: Polym. Chem 1999, 37, 3694−3701. (15) Satoh, K.; Kamigaito, M.; Sawamoto, M. Macromolecules 2000, 33, 5830−5835. (16) De, P.; Faust, R. Macromolecules 2004, 37, 7930−7937. (17) Kostjuk, S. V.; Radchenko, A. V.; Ganachaud, F. Macromolecules 2007, 40, 482−490. (18) Ashida, J.; Yamamoto, H.; Yonezumi, M.; Kanaoka, S.; Aoshima, S. Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 2009, 50, 156. (19) Higahimura, T.; Kamigaito, M.; Kato, M.; Hasebe, T.; Sawamoto, M. Macromolecules 1993, 26, 2670−2673. (20) Kabuto, C.; Akine, S.; Kwon, E. J. Cryst. Soc. Jpn. 2009, 51, 218− 224. (21) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G. L.; Giacovazzo, C.; Guagliardi, A. G. G.; Polidori, G.; Spagna, R. J. Appl. Crystallogr. 1999, 32, 115−119. (22) Sheldrick, G. M. Acta Crystallogr., Sect. A 2008, 64, 112−122. (23) The polymerization rates of IBVE6 and pMOS with SnCl4, ZnCl2, and AlCl3 were roughly compared as follows: The apparent rate constants (kapp) defined as ln([M]0/[M]) = kappt were determined for the polymerizations. The reaction conditions were slightly different between the two monomers in concentrations of monomers and metal chlorides ([IBVE]0 = 0.76 M, [metal chloride]0 = 5.0 mM for IBVE; [pMOS]0 = 0.38 M, [metal chloride]0 = 10 mM for pMOS). The kapp values were calculated to be 3.4 × 10−2 s−1 (IBVE) and 2.3 × 10−3 s−1 (pMOS) for SnCl4, 2.2 × 10−3 s−1 (IBVE) and 3.0 × 10−4 s−1 (pMOS) for ZnCl2, and 1.7 × 10−4 s−1 (IBVE) and 1.3 × 10−2 s−1 (pMOS) for AlCl3. From these results, the ratios of the polymerization rate of pMOS toward that of IBVE were 0.067 for SnCl4, 0.14 for ZnCl2, and 76 for AlCl3. (24) An IBVEAcOH/EtAlCl2 combination was used as a cationogen instead of IBVEHCl for the polymerizations with GaCl3 or FeCl3 in dichloromethane. The combination generates IBVEHCl in situ by the exchange of the AcO group and the Cl atom. (25) Good, R.; Merbach, A. E. Helv. Chim. Acta 1974, 57, 1192− 1198. (26) Although some Lewis acids such as AlCl3 and FeCl3 generally exists as a dimer in nonpolar organic solvents, those dimer are most likely dissociated by a weak Lewis base such as ethyl acetate, diethyl ether, or THF, transformed into monomeric complexes under the conditions for the cationic polymerization. (27) Denmark, S. E.; Almstead, N. G. J. Am. Chem. Soc. 1993, 115, 3133−3139. (28) Pietrangelo, A.; Knight, S. C.; Gupta, A. K.; Yao, L. J.; Hillmyer, M. A.; Tolman, W. B. J. Am. Chem. Soc. 2010, 132, 11649−11657. (29) The polymerization behaviors depended on the solvents used in the polymerizations with the Group 4 metal chlorides, TiCl4, ZrCl4, and HfCl4, as described in the previous section. The reactions were controlled in CH2Cl2 but not in toluene. In past studies, TiCl4 was reported to easily dimerize to produce species such as Ti2Cl9− as a result of the reaction with a chloride anion source.30,31 Thus, different counteranions may have been generated depending on solvent polarity, i.e., TiCl5(EA)− in CH2Cl2 and Ti2Cl9− in toluene, which likely contributed to the difference in controllability. (30) Creighton, J. A.; Green, J. H. S. J. Chem. Soc. A 1968, 808−813. (31) Kistenmacher, T. J.; Stucky, G. D. Inorg. Chem. 1971, 10, 122− 132. (32) Hammett constants (σ), representative indexes for electrondonating and -abstracting ability, of 4-methoxy, 4-ethoxy, and 4-(pmethoxyphenyl) groups were reported to be −0.268, −0.25, and −0.088, respectively, in the following studies: (a) Hammett, L. P. J.
structures of the counteranions. Counteranions with a coordinating weak Lewis base were able to control the reaction, whereas those without a coordinating base were insufficient for moderating the activity of the propagating carbocations. These results contribute to both realizing precise polymerizations of various monomers via cationic pathways and establishing a new index for the Lewis acidity of various metal chlorides.
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ASSOCIATED CONTENT
S Supporting Information *
NMR spectrum, crystal data, selected bond distances and angles, and X-ray crystallographic data in cif format. 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]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was partially supported by a Grant-in-Aid for Scientific Research (No. 22107006 and 23107517) on Innovative Areas of “Fusion Materials: Creative Development of Materials and Exploration of Their Function through Molecular Control” (No. 2206) from the Ministry of Education, Culture, Sports, Science and Technology, Japan (MEXT), and by Grant-in-Aid for Young Scientists (B) (No. 24750107) from Japan Society for the Promotion of Science (JSPS).
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REFERENCES
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