Letter pubs.acs.org/macroletters
Stereospecific Living Cationic Polymerization of N‑Vinylcarbazole through the Design of ZnCl2‑Derived Counteranions Hironobu Watanabe, Arihiro Kanazawa, and Sadahito Aoshima* Department of Macromolecular Science, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan S Supporting Information *
ABSTRACT: Highly stereospecific living polymerization of Nvinylcarbazole (NVC) successfully proceeded via a cationic mechanism as a result of the elaborate design of counteranions using an initiating system consisting of CF3SO3H, nBu4NX (X = Cl, Br, I), and a Lewis acid catalyst. The use of ZnCl2 and an appropriate amount of nBu4NCl quantitatively generated highly isotactic polymers (mm = 94%) with narrow molecular weight distributions (Mw/Mn ∼ 1.3) and molecular weights proportional to monomer conversion. In this system, a ZnCl42− species, which was formed as a counteranion of the propagating carbocation, most likely contributed to the stereoregulation of the polymers because the mm value drastically varied depending on the polymerization conditions, such as the Lewis acid catalyst and amount of added salt. Isotactic poly(N-vinylcarbazole) (PVK) showed different properties than atactic PVK based on fluorescence and differential scanning calorimetry (DSC) analysis.
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tereospecific living polymerization is a powerful method for regulating the primary structures of polymers, such as stereoregularity, molecular weight, and chain-end structure, which significantly affect polymer chemical and physical properties. Many stereoregular polymers1−4 exhibit significantly different properties from those of their nonstereoregular counterparts, as exemplified by their helix structure, crystallinity, optical activity, and superior thermal properties. Living polymerization allows us to obtain polymers with well-defined structures, such as block and star-shaped polymers, which show characteristic behaviors such as microphase separation and unique aggregation.5 Many reports on the coordination3,6,7 or anionic8,9 polymerization of vinyl monomers involve stereospecific living polymerization because the stereochemistry of the propagating species is regulated through the rational design of catalysts and additives. However, stereoregulation is difficult to achieve in cationic polymerization, most likely because of the difficulty in controlling the stereochemistry around the propagating carbocation. A successful example of a stereospecific cationic polymerization10,11 is the highly isospecific polymerization of isobutyl vinyl ether using a titanium catalyst with aryloxy substituents [TiCl2(OAr)2, mm = 83%].12 In addition, to the best of our knowledge, the study, which used benzyl vinyl ether as a monomer, only achieved both high stereoregularity and living nature in the cationic polymerization of vinyl monomers (mm = 81%, Mw/Mn ∼ 1.1).13 For vinylidene monomers, the stereospecific living cationic polymerization of α-methylstyrene has been reported.14 Poly(N-vinylcarbazole) (PVK) has recently attracted much interest because of its sensitive photoconductivity, molecular structure, and optical properties.15−20 PVK is typically synthesized by radical or cationic polymerization. The living polymerization of N-vinylcarbazole (NVC) can be achieved through radical polymerization via several mechanisms21−27 © XXXX American Chemical Society
such as atom-transfer radical polymerization and reversible addition−fragmentation chain-transfer polymerization. In contrast, only two examples using hydrogen iodide28 or iodine29 as an initiator have demonstrated the living cationic polymerization of NVC.30 In addition, past studies have suggested that isotactic and syndiotactic sequences of PVK likely had different conformations16 and excimer structures,17 although stereoregular PVK has not been previously obtained. In this study, we sought to develop a stereospecific cationic polymerization of NVC. Unlike radical polymerization, a counteranion is generated in cationic polymerization and exists near the propagating chain end; hence, the elaborate design of counteranions using a Lewis acid catalyst and a tetraalkylammonium salt may lead to stereospecific polymerization as a result of the restriction of the direction from which a monomer molecule attacks the propagating carbocation. The wide scope of Lewis acid catalysts for cationic polymerizations, such as ZnCl2, EtAlCl2, SnCl4, GaCl3, and TiCl4, is expected to enable the fine-tuning of counteranions because these Lewis acids exhibit different properties, such as the strength of Lewis acidity, coordination number, and philicity to specific atoms (e.g., oxophilicity and chlorophilicity).31 An initiating system composed of CF3SO3H and an added salt was developed by our group for the living cationic polymerization of vinyl ethers.32 This system was used to systematically investigate the polymerization of NVC by using initiating species with different halide anions (Scheme 1). In this system, the reaction of CF3SO3H, nBu4NX (X = I, Br, Cl), and NVC generates an NVC−hydrogen halide adduct as an Received: March 7, 2017 Accepted: April 5, 2017
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DOI: 10.1021/acsmacrolett.7b00175 ACS Macro Lett. 2017, 6, 463−467
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Scheme 1. Cationic Polymerization of NVC with the CF3SO3H/Added Salt Initiating Systems (1) in Conjunction with or (2) in the Absence of a Lewis Acid Catalyst (MLn: Metal Halides)
Table 1. Cationic Polymerization of NVC with the CF3SO3H/Added Salt Initiating Systemsa entry
Lewis acid
nBu4NX
[nBu4NX]0 (mM)
ratio Ab
time
conv. %
Mn × 10−3 c
Mw/Mn
mm %
1 2 3 4 5d 6e 7 8 9 10
ZnCl2 ZnCl2 ZnCl2 ZnCl2 ZnCl2 TiCl4 SnCl4 GaCl3 − −
nBu4NCl nBu4NCl nBu4NCl nBu4NI nBu4NCl nBu4NCl nBu4NCl nBu4NCl nBu4NI nBu4NCl
3.5 2.2 4.2 3.5 1.5 7.5 3.5 2.2 2.1 2.1
1.5 0.2 2.2 1.5 1.5 1.1 1.5 0.2 − −
24 h 5 min 210 h 24 h 24 h 5 min 2h 5 min 144 h 210 h
100 98 5 100 85 75 52 94 85 7
8.5 14.2 − 10.1 6.4 6.7 12.2 17.0 9.2 −
1.30 15.9 − 1.19 1.54 1.13 1.22 8.61 1.25 −
94 59 − 85 94 85 71 55 45 −
[NVC]0 = 0.20 M, [CF3SO3H]0 = 2.0 mM, [Lewis acid]0 = 1.0 mM, in CH2Cl2 at −78 °C. bRatio A = ([nBu4NX]0 − [CF3SO3H]0)/[Lewis acid]0. Determined by GPC in chloroform on the basis of polystyrene calibration. dHCl was used as a cationogen instead of CF3SO3H and nBu4NCl. e [TiCl4]0 = 5.0 mM. a c
Figure 1. (A) Time−conversion curves and ln([M]0/[M])−time plots for the polymerization using the CF3SO3H/nBu4NCl/ZnCl2 initiating system, (B) Mn and Mw/Mn values of the obtained PVKs, and (C) MWD curves of the PVKs obtained in the monomer-addition experiment ([NVC]0 = 0.20 M {for (A), (B)} or 0.10 M {for (C)}, [NVC]add = 0.10 M for (C), [CF3SO3H]0 = 2.0 mM, [nBu4NCl]0 = 3.5 mM, [ZnCl2]0 = 1.0 mM, in CH2Cl2 at −78 °C). The Mn values are determined by GPC in chloroform on the basis of polystyrene calibration.
cleaves to generate a carbocation, which induces the slow polymerization reaction (entries 9 and 10 in Table 1). The cationic polymerization of NVC was conducted with a CF3SO3H/nBu4NX initiating system in dichloromethane at −78 °C. The results are summarized in Table 1 (other results and detailed data of Table 1 are shown in Table S1 and Figure S1, respectively). The value “ratio A” in Table 1 represents the equivalency (to a Lewis acid) of nBu4NX molecules that are not consumed in the reaction with CF3SO3H. In other words, ratio A corresponds to the number of halogen anions that coordinate to one Lewis acid molecule. The reaction proceeded smoothly
initiator. The polymerization begins from this adduct in conjunction with or without a Lewis acid catalyst. When a Lewis acid catalyst is used [Scheme 1, (1)], the dormant carbon−halogen bond of the propagating chain end is activated by the Lewis acid to generate a carbocation. Since a halide anion derived from excess nBu4NX, which is not consumed in the initiation reaction, coordinates to the Lewis acid, the structure and activity of the Lewis acid can be tuned based on the amount of nBu4NX.33 When a Lewis acid catalyst is not used [Scheme 1, (2)], the carbon−halogen bond spontaneously 464
DOI: 10.1021/acsmacrolett.7b00175 ACS Macro Lett. 2017, 6, 463−467
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Figure 2. 1H NMR spectra of PVKs obtained (A) with ZnCl2 (entry 1 in Table 1) and (B) without a Lewis acid catalyst (entry 9 in Table 1); in C2D2Cl4 at 100 °C (*solvent, water, and vaseline). 2D NMR spectra were shown in Figures S4−S10. aBecause of ring current effects of neighboring rings. See refs 19, 35, and36.
value was smaller than that obtained in the nBu4NCl system. In addition, a highly isotactic polymer (mm = 94%) was obtained when using HCl as a cationogen instead of CF3SO3H and nBu4NCl (entry 5), which indicates that the CF3SO3− moiety does not necessarily control the stereoregularity and molecular weight. In contrast, when excess THF, which is a suitable additive in the living cationic polymerization of VEs, was used instead of nBu4NCl in conjunction with HCl and ZnCl2, the resultant polymer had less stereoregularity (mm = 60%, at −40 °C). These results indicate that the propagation reaction via the formation of a carbon−chlorine dormant end and catalysis by ZnCl3− is indispensable for stereospecific living cationic polymerization. The use of an appropriate Lewis acid catalyst is also significant for the stereospecific living cationic polymerization of NVC. The polymerization using TiCl4 rapidly proceeded to produce a polymer with a notably narrow MWD and relatively high stereoregularity (entry 6). In contrast, SnCl4 and GaCl3 were ineffective for living and/or stereospecific polymerization. The reaction using SnCl4 produced a polymer with a narrow MWD, but the stereoregularity was not high (entry 7). When using GaCl3, neither the molecular weight nor stereoregularity was controlled (entry 8). Further detailed investigations using TiCl4, SnCl4, and other Lewis acid catalysts are currently in progress. The living cationic polymerization of NVC also proceeded under metal-free conditions via the spontaneous cleavage of carbon−iodine chain ends [Scheme 1, (2)]. Polymerization was conducted with the CF3SO3H/nBu4NX initiating system without a Lewis acid in dichloromethane at −40 and −78 °C. The reaction proceeded when any halogen salt was used (X = I, Br, or Cl; Figure 3) at −40 °C. Under these conditions, the carbon−halogen bond spontaneously cleaved to generate a propagating carbocation and an X− counteranion. The polymerization rate decreased in the order of I− > Br− > Cl−, which is consistent with the strengths of the carbon−halogen bonds. Importantly, similar to the polymerization using HI as an initiator,28 polymerization proceeded in a living fashion
using 1.5 equiv of nBu4NCl to ZnCl2 (ratio A = 1.5) and yielded a polymer with a relatively narrow molecular weight distribution (MWD; Mw/Mn ∼ 1.3; entry 1). Moreover, the linear relationships in both the ln([M]0/[M])−time, and Mn−conversion plots (Figure 1A,B) indicate that the polymerization of NVC proceeded in a living manner.34 The occurrence of living polymerization was also confirmed by a monomer addition experiment (Figure 1C). The polymer obtained using the ZnCl2/nBu4NCl system was demonstrated to have extremely high isotacticity. Unlike atactic PVK, the isolated polymer was insoluble in common organic solvents, such as dichloromethane, chloroform, and toluene, at room temperature (Figure S2). As a result of thorough investigation using various solvents, the polymer was found to dissolve in 1,1,2,2-tetrachroloethane at 130 °C. The 1H NMR spectrum of the polymer in this solvent showed remarkably sharp peaks. Most importantly, the polymer had an extremely high mm triad value of 94%, as determined from the integral ratios of the peaks of the methine groups in the main chain18 (Figure 2). The high isotacticity was also confirmed by 13C NMR analysis (Figure S3). The results show that the stereospecific living cationic polymerization of NVC proceeded with the initiating system of CF3SO3H, nBu4NCl, and ZnCl2. The concentration and type of added salt greatly influenced the stereospecific living cationic polymerization of NVC. At a smaller concentration of nBu4NCl (ratio A < 1; entry 2 in Table 1), a notably fast reaction occurred to produce a polymer with a broad multimodal MWD and lower mm. When ratio A was less than unity, Zn species were present as ZnCl2 and ZnCl3−, the former of which is likely inappropriate for living polymerization of NVC. In contrast, polymerization with a larger amount of nBu4NCl (ratio A > 2; entry 3) did not proceed, most likely because all the coordination sites of ZnCl2 were occupied with Cl− anions, which were derived from nBu4NCl, to form ZnCl42− species. When nBu4NI was used instead of nBu4NCl (ratio A = 1.5; entry 4), the produced polymer had relatively high stereoregularity and a narrow MWD, although the mm 465
DOI: 10.1021/acsmacrolett.7b00175 ACS Macro Lett. 2017, 6, 463−467
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Figure 3. (A) Time−conversion curves for the polymerization of NVC using the CF3SO3H/nBu4NX initiating system, and (B) Mn of the obtained PVKs ([NVC]0 = 0.20 M, [CF3SO3H]0 = 4.0 mM, [nBu4NX]0 = 4.2 mM, in CH2Cl2 at −40 °C; see Figure S11 for the result of the monomer-addition experiment and Figure S12 for the experiment at a different [monomer]0/[cationogen]0 ratio). The Mn values are determined by GPC in chloroform on the basis of polystyrene calibration.
Figure 4. Vicinity of the propagating chain end of stereospecific cationic polymerization of NVC. The ZnCl42− counteranion likely exists at the front-side position of the growing carbocation.
The high stereoregularity affects the fluorescence and thermal properties of PVK. The fluorescence of PVK at 350, 370, and 410 nm is reportedly attributed to the monomer fluorescence, a partially overlapped excimer of the syndiotactic sequences, and a fully overlapped excimer of the isotactic sequences, respectively.17 Interestingly, the highly isotactic PVK (mm = 94%) in this study exhibited much more intense fluorescence at 410 nm than at 370 nm (Figure S13), unlike its atactic counterpart (mm = 50%). Differences between the isotactic and atactic PVKs were also observed in differential scanning calorimetry (DSC) analysis (Figure S14). Atactic PVK had a clear glass transition temperature (Tg) at 195 °C, whereas isotactic PVK had no distinct Tg. In addition, no melting point was observed below the decomposition temperature (approximately 330 °C). The results indicate that isotactic PVK has notably high crystallinity. In conclusion, the stereospecific living cationic polymerization of NVC was demonstrated to proceed with an initiating system consisting of CF3SO3H, nBu4NCl, and ZnCl2. Isotactic PVK with an mm value of 94% was obtained under appropriate conditions. The stereoregularity varied depending on the polymerization conditions, such as the Lewis acid catalyst and amount of added salt, which suggests that the structure of the counteranions is responsible for stereoregulation. Isotactic PVK showed remarkably different properties from atactic PVK. The results in this study will lead to the stereospecific living cationic polymerization of other monomers via the design of initiating systems and monomer structures.
when X− = I−. The absolute molecular weights of the produced polymers, determined by gel permeation chromatography (GPC) analysis using a light-scattering detector, were larger than the values determined on the basis of polystyrene standard and consistent with the theoretically calculated values from the ratios of [NVC]0 to [CF3SO3H]0 (for the 93% conversion sample, experimental absolute Mn = 8.4 × 103 to its theoretical value of 9.0 × 103). However, under any of the conditions shown in Figure 3, stereospecificity was not observed (mm = 40−50%). In addition, polymerization was significantly retarded at −78 °C (entries 9 and 10 in Table 1), which indicates that spontaneous cleavage of the carbon−chlorine bonds on the propagating chain ends was negligible at notably low temperatures. From these results, we discuss the mechanism of the stereospecific polymerization of NVC. We hypothesized several mechanisms, such as the coordination of a monomer molecule or pendant moiety of the polymer to a Lewis acid, a propagation reaction via a cyclic intermediate, and electrostatic interactions between terminal and penultimate units. However, none of these mechanisms can explain the effects of the polymerization conditions on the stereoregularity of the resulting polymer. As the most reasonable explanation, we propose that features of the counteranions such as shape, bulkiness, and charge distribution are responsible for stereoregulation. In the polymerization that had the highest stereoregularity (entry 1 in Table 1), ZnCl3− functioned as a Lewis acid catalyst and activated the carbon−chlorine bond of the propagating end to generate the carbocation and ZnCl42− counteranion. A model of the vicinity of the propagating chain end is shown in Figure 4. Considering general view of isospecificity in ionic polymerization, the ZnCl42− counteranion likely exists at the front-side position of the growing carbocation. In this situation, a monomer molecule is forced to approach the propagating carbocation from the back side, which results in isospecific polymerization. In addition, the stereoregularity of poly(tert-butyl vinyl ether) was not controlled under conditions similar to those for NVC polymerization (mm = 45% with the CF3SO3H/nBu4NCl/ ZnCl2 system at ratio A = 1.5). Thus, the structure of NVC is also highly important for achieving stereospecific polymerization. We are currently investigating the polymerization mechanism in more detail.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.7b00175. Experimental section and additional data (PDF).
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Hironobu Watanabe: 0000-0002-0911-8836 Arihiro Kanazawa: 0000-0002-8245-6014 Sadahito Aoshima: 0000-0002-7353-9272 Notes
The authors declare no competing financial interest. 466
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ACKNOWLEDGMENTS We thank the group of Prof. Hiroyasu Yamaguchi (Osaka University) for fluorescence measurements, the group of Prof. Tadashi Inoue (Osaka University) for DSC measurements, and Mr. Naoya Inazumi (Osaka University) for 2D-NMR measurements. This work was partially supported by JSPS KAKENHI Grant Numbers 26288063 and 17H03068.
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