Letter pubs.acs.org/macroletters
Diversifying Cationic RAFT Polymerization with Various Counteranions: Generation of Cationic Species from Organic Halides and Various Metal Salts Mineto Uchiyama,† Kotaro Satoh,*,†,‡ and Masami Kamigaito*,† †
Department of Applied Chemistry, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan ‡ Precursory Research for Embryonic Science and Technology, Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan S Supporting Information *
ABSTRACT: A combination of hydrogen chloride adduct of isobutyl vinyl ether (IBVE−HCl) and various metal salts (AgOTf, AgPF6, AgSbF6, NaBArF (BArF: [3,5-(CF3)2Ph]4B)) efficiently generated initiating cationic species with different low, weakly, or non-nucleophilic counteranions (OTf−, PF6−, SbF6−, BArF−) and induced a cationic reversible-addition− fragmentation chain-transfer (RAFT) or degenerative chaintransfer (DT) polymerization of IBVE, ethyl vinyl ether, pmethoxystyrene, and α-methylstyrene (αMS) in the presence of appropriate thiocarbonylthio compounds or thioethers as reversible chain-transfer agents. The polymerization behaviors in terms of polymerization rate, polymer molecular weight, terminal structure, and stereochemistry were affected by the counteranions, whereas the molecular weight control was achieved by appropriate RAFT or DT agents for all of these counteranions. With a weakly or noncoordinating BArF anion and a trithiocarbonate, a nearly atactic poly(IBVE) with a narrow molecular weight distribution (Mw/Mn < 1.1) was obtained. When using nonoxoanions, such as SbF6−, PF6−, and BArF−, in the presence of thioethers, controlled cationic polymerization of αMS was achieved while frequent irreversible β-proton elimination occurred using TfO−. Thus, this method widens the scope of living or controlled cationic polymerizations with various counteranions.
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compounds,10 thioethers,11 and phosphoric acid esters,12 all of which induce fast reversible interchange reactions with the growing cationic species with triflate counteranion via transient formation of the stable sulfonium and phosphonium intermediate. The various aspects of this cationic DT or RAFT chemistry can be further developed. In this study, we used a series of low, weakly, or nonnucleophilic counteranions, including PF6−, SbF6−, and BArF− ([3,5-(CF3)2Ph]4B−), in conjunction with sulfur-based RAFT or DT agents in cationic polymerization of isobutyl and ethyl vinyl ether (IBVE and EVE), p-methoxystyrene (pMOS), and α-methylstyrene (αMS). Although these low nucleophilic counteranions induce “free” cationic polymerization, resulting in polymers with uncontrolled molecular weights, the polymerization should be changed into “living” or controlled polymerization with the aid of RAFT or DT agents. Furthermore, differences in the structure and nucleophilicity of the counteranions may affect the polymerization behavior, leading to the appearance of polymer microstructures, whereas the
n cationic polymerization, a counteranion is the partner species that can affect the fate of the growing cationic species. 1−7 Namely, the counteranion can dictate the elementary reactions, that is, initiation, propagation, termination, and chain-transfer reactions, during the polymerization and may affect the molecular weights and the terminal and stereochemical structures of the resulting polymers. To achieve living cationic polymerization, appropriate nucleophilic counteranions have been employed for stabilizing the cationic species or inducing controlled propagation via an interaction with the growing cationic species. With the exception of a few examples, non-nucleophilic or weakly nucleophilic anions that show little interaction with the carbocation have thus been considered inappropriate in living or controlled cationic polymerization.8,9 Indeed, the use of a low nucleophilic trifluoromethanesulfonate (triflate) anion (OTf−) usually results in an uncontrolled high-molecular-weight polymers from vinyl ethers. However, we recently found that this process can be changed into controlled/living polymerization using degenerative chaintransfer (DT) or reversible-addition−fragmentation chaintransfer (RAFT) process,10−13 which has been established in controlled radical polymerization.14 The effective DT or RAFT agents for cationic polymerization include thiocarbonylthio © XXXX American Chemical Society
Received: July 13, 2016 Accepted: September 27, 2016
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DOI: 10.1021/acsmacrolett.6b00541 ACS Macro Lett. 2016, 5, 1157−1161
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counteranion was transferred without isolation into the solution of IBVE and 1. The polymerization occurred smoothly whereas the overall polymerization rate was smaller than that with TfOH under the same conditions due to an incomplete formation or a slight loss of the initiating cationic species or residual metal salts (Figure S1). Irrespective of the differences in the rates, the numberaverage molecular weight (Mn) of the polymers obtained with in situ generated CH3CH(OiBu)+OTf− increased in direct proportion to monomer conversion and was in good agreement with the calculated value assuming that one polymer chain is generated by a single RAFT molecule (Figure 1). In addition,
molecular weight control can rely on the degenerative chaintransfer agents. To generate carbocationic species with various counteranions, here we used hydrogen-chloride adduct of IBVE (IBVE−HCl) as a cationogen in the presence of silver or the sodium salts of OTf−, PF6−, SbF6−, and BArF−. The mixture supposedly results in the carbocation of IBVE associated with these counteranions via the precipitation of silver or sodium chloride. A similar method has already been used for the generation of the initiating cationic species from benzylic halides and silver salts of PF6− and ClO4− in living cationic polymerization of tetrahydrofuran (THF)15 and quite recently for vinyl ethers.16,17 However, the effects of the counteranions on the polymerization and the polymer structures have not been a focus of previous studies. We, therefore, investigated cationic RAFT or DT polymerization using a small amount of IBVE cation accompanying various counteranions that may affect the polymer structures, in the presence of thiocarbonylthio compounds or thioethers as the chain transfer agents that can control the polymer molecular weights (Scheme 1). Scheme 1. Cationic RAFT or DT Polymerization with Various Counteranions Generated from Mixture of Organic Halide and Various Metal Salts
Figure 1. Mn, Mw/Mn, and SEC curves of the polymers obtained in cationic RAFT polymerization of IBVE using IBVE−HCl + AgOTf or TfOH as cationogen in n-hexane/CH2Cl2/Et2O (80/10/10 vol %) at −40 °C: [IBVE]0/[1]0/[IBVE−HCl + AgOTf]0 = 500/10/0.50 mM and [IBVE]0/[1]0/[TfOH]0 = 500/10/0.50 or 0.05 mM.
the molecular weight distribution (MWD) was very narrow (Mw/Mn < 1.1) throughout the polymerization. The molecular weights were almost identical to those obtained with TfOH as the cationogen, indicating that the mixture of IBVE−HCl and AgOTf works sufficiently well for the generation of the cationic species with the triflate anion in a similar manner to TfOH to induce the cationic RAFT polymerization in the presence of 1. A high molecular weight (Mn > 105) polymer was also successfully synthesized at a high feed ratio of IBVE to RAFT agent ([IBVE]0/[RAFT]0 = 1000/1; Figure S2). Other metal salts with different anions were then similarly employed in conjunction with IBVE−HCl to investigate their effects on the cationic RAFT polymerization of IBVE in the presence of 1. We used a series of commercially available silver or sodium salts with low, weakly, or non-nucleophilic anions, including silver hexafluorophosphate (AgPF6) and hexafluoroantimonate (AgSbF 6 ) and sodium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (NaBArF). With all metal salts, the monomers were quantitatively consumed even though the reaction rates were different (Figure 2A). The rate was decreased in the order of AgOTf ∼ AgPF6 > AgSbF6 > NaBArF. The order may reflect the concentration of the cationic propagating species rather than the nucleophilicity of the counteranions because the nucleophilicity, coordinating ability, basicity, or donor numbers of these anions decreases in the order of OTf− > PF6− ∼ SbF6− > BArF−.18−22 Irrespective of differences in the counteranions, Mn values of the polymers obtained in the presence of 1 increased in direct proportion to monomer conversion and were in good agreement with the calculated values assuming that one polymer chain is generated
We first examined RAFT cationic polymerization of IBVE using a small amount of an equimolar mixture of silver triflate (AgOTf) and IBVE−HCl as a cationogen in the presence of large amount of the dithiocarbamate-type RAFT agent (1) ([M]0/[1]0/[IBVE−HCl]0/[AgOTf]0 = 500/10/0.50/0.50 mM) in n-hexane/CH2Cl2/Et2O (80/10/10 vol %) at −40 °C and compared the results with those for the previously reported RAFT polymerization using TfOH as the alternative cationogen.10 Here, the organic chloride and silver salt were premixed in n-hexane and CH2Cl2 (50/50 vol %) at −78 °C for 5 min prior to the polymerization. Although AgOTf was insoluble in the organic solvents, an apparently different white precipitate that was most likely AgCl formed during the premixing to obtain CH3CH(OiBu)+OTf−. The supernatant solution containing the in situ generated cation with the triflate 1158
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Figure 2. Time−conversion (A), Mn and Mw/Mn (B), and SEC curves (C) obtained in cationic RAFT polymerization of IBVE in the presence of 1 using various metal salts in n-hexane/CH2Cl2/Et2O (80/10/10 vol %) at −40 °C: [IBVE]0/[1]0/[IBVE−HCl + metal salt]0 = 500/10/0.50 mM.
from one molecule of 1 (Figure 2B). In addition, the MWDs were narrow, although they were slightly broader for AgPF6 and AgSbF6, especially at low monomer conversions (Figure 2B,C). The chain-transfer constants (Ctr) were estimated from the changes of Mw/Mn values along with the monomer conversions according to the equations in the experimental section. All the Ctr values were relatively large and showed a slight dependence on the counteranions (Table S1). To further evaluate the effects of the counteranions on the RAFT cationic polymerizations, a different type of RAFT agent, that is, trithiocarbonate (2), was employed.10,23 A similar dependence on the polymerization rate was observed (Figure S3A). Although the Mn values similarly increased in direct proportion to monomer conversion (Figure S3B), the dependence of MWDs on the counteranions was more noticeable (Figure S3B,C). The MWDs were broader (Mw/Mn ∼ 1.5) with PF6− and SbF6−, while those for OTf− and BArF− were still narrow. Similarly, the Ctr values calculated from the dependence of MWDs on the monomer conversion became larger in the order of PF6− < SbF6− < OTf− < BArF− (Table S1). These results suggest that the counteranions affect the chain-transfer and propagation reactions to affect the molecular weight of the resulting polymers. The terminal and main-chain structures of the obtained polymers were analyzed by 1H and 13C NMR (Figure 3). The 1 H NMR spectra of the polymers obtained with all metal salts in the presence of 2 showed the characteristic peak of the methine proton adjacent to the trithiocarbonate at the ω-chain end (b′) as observed for the polymers obtained with TfOH as the cationogen (Figure 3A−C). In addition, there are almost no peaks corresponding to the acetal chain end that was formed by quenching the reaction with methanol as a terminator, under the condition of a small amount of the metal salts in comparison to 2. The Mn(NMR) value calculated from the ω-chain end (b′) and the main-chain repeating units (e) was in good agreement with that obtained by SEC. These results indicate that the mixture of metal salts and IBVE−HCl successfully functions as a cationogen and induces cationic RAFT polymerization in the presence of appropriate thiocarbonylthio compounds. The effects of counteranions on polymer tacticity were evaluated by 13C NMR. The polymers obtained with OTf−, PF6−, and SbF6− are relatively isotactic-rich (m ∼ 70%; Figure 3E and Table S1), similar to those obtained in most of living cationic polymerizations of IBVE reported to date.24−29 This result suggests some nucleophilic interactions of the counter-
Figure 3. 1H (A−C) and 13C (D−F) NMR spectra (in CDCl3 at 55 °C) of poly(IBVE) obtained in cationic RAFT polymerization of IBVE in the presence of 2 using IBVE−HCl and various metal salts in nhexane/CH2Cl2/Et2O (80/10/10 vol %) at −78 °C. (A, D) NaBArF, (B, E) AgOTf, (C, F) TfOH. [IBVE]0/[2]0/[IBVE−HCl + metal salt]0 or [TfOH]0 = 500/10/0.50 or 0.05 mM.
anions with the propagating vinyl ether cation although the molecular weight control is difficult without the RAFT agents due to the lack of appropriate dormant species. In contrast, the use of weakly or non-nucleophilic anion, BArF− resulted in nearly atactic polymers (m = 52%), most likely due to little interaction with the growing carbocationic species. Generally, with such a low nucleophilic counteranion, the control of molecular weight is similarly difficult due to the absence of dormant species. However, such control can now be achieved using cationic RAFT process, where the dormant species can be generated from the RAFT agent. This result indicates that in addition to the molecular weight control by the RAFT agent, another approach to achieving control can be provided by the counteranions. 1159
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Figure 4. Mn, Mw/Mn, and SEC curves of the polymers obtained in cationic DT polymerization of αMS in the presence of 4 using various metal salts in CH2Cl2 at −78 °C. [αMS]0/[4]0/[IBVE−HCl + metal salt]0 = 500/10/0.50 mM.
to the thioether ω-terminal and methoxy (g) protons at the αterminal both originating from the DT agent (Figure S5A−C). Thus, cationic polymerization of αMS can be controlled using appropriate counteranions and appropriate reversible chaintransfer agents. In conclusion, in the presence of appropriate RAFT or DT agents, the combination of organic halide and various metal salts with low, weakly, or non-nucleophilic counteranions efficiently generates the initiating cationic species and successfully induces cationic RAFT and DT polymerizations of various monomers, such as vinyl ethers, alkoxystyrenes, and αMS (Table 1). This method allows the use of various
Similar molecular weight control was achievable for EVE and pMOS with various counteranions (OTf−, PF6−, SbF6−, BArF−) in the presence of 2 (Table S2 and Figure S4), even though the MWDs showed a slight dependence on the counteranions. These results show that the combination of a metal salt and IBVE−HCl provides appropriate initiating cationic species for the cationic RAFT polymerizations of EVE and pMOS with various counteranions. The system that can generate various counteranions works more efficiently and meaningfully for αMS, the living cationic polymerization of which has been attained only by a few systems with Lewis acid catalysts.30−34 It is known that in cationic polymerization of αMS with oxoacids such as TfOH, the oxoacid-derived counteranion frequently induces irreversible chain transfer reactions via β-proton elimination from the α-methyl group to produce low molecular-weight oligomers only.2,35 Indeed, cationic polymerization with AgOTf and IBVE−HCl gave low-molecular-weight products (Mn ∼ 2000), even in the absence of any chain-transfer agents in CH2Cl2 at −78 °C (entry 1 in Table S3). The 1H NMR of the products also showed the exomethylene chain end formed via β-proton elimination (Figure S5D). In contrast, with AgSbF6 and NaBArF, which generate nonoxoanions, high molecular weight polymers (Mn = 60000−70000) were obtained under the same conditions (entries 2 and 3 in Table S3). This indicates that irreversible chain-transfer reactions depend on the counteranions. To control the cationic polymerization of αMS by reversible chain-transfer agents, various RAFT or DT agents were added. Using a trithiocarbonate (2) and xanthate (3) that are effective for cationic RAFT polymerization of pMOS and p-hydroxystyrene, the polymerization stopped at low conversion (entries 4 and 5 in Table S3). However, with a thioether (4) that is also effective for cationic DT polymerization of pMOS,11 the polymerization with nonoxoanions (PF6−, SbF6−, BArF−) proceeded smoothly (entries 7−9 in Table S3). The Mn increased in direct proportion to monomer conversion in good agreement with calculated values assuming that one polymer chain is generated from one molecule of 4 (Figure 4). In contrast, the molecular weight of the products obtained with OTf− in the presence of 4 was very low (Mn ∼ 400; entry 6 in Table S3). The 1H NMR spectra of poly(αMS) obtained with nonoxoanions (PF6−, SbF6−, BArF−) in the presence of 4 all showed characteristic peaks of methylene (a′) protons adjacent
Table 1. Effective Systems for Cationic RAFT or DT Polymerization of VE, pMOS, and αMS monomer VE
pMOS αMS
RAFT or DT agent
counteranion
dithiocarbamate trithiocarbonate thioether phosphorus ester trithiocarbonate thioether thioether
OTf−, NTf2−, PF6−, SbF6−, BArF−
OTf−, NTf2−, PF6−, SbF6−, BArF− PF6−, SbF6−, BArF−
counteranions, while the molecular weight control can be achieved by the reversible chain-transfer agents. Thus, this method widens the scope of counteranions that have not been employed in living or controlled cationic polymerization and will lead to further developments in cationic polymerization.
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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.6b00541. Experimental details and supplementary data (PDF).
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AUTHOR INFORMATION
Corresponding Authors
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[email protected]. Notes
The authors declare no competing financial interest. 1160
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(33) Kwon, Y.; Cao, X.; Faust, R. Macromolecules 1999, 32, 6963− 6968. (34) Fodor, Z.; Faust, R. J. Macromol. Sci., Part A: Pure Appl.Chem. 1998, 35, 375−394. (35) Kawakami, Y.; Toyoshima, N.; Yamashita, Y. Chem. Lett. 1980, 9, 13−16.
ACKNOWLEDGMENTS This work was supported in part by a Grant-in-Aid for Scientific Research (A) (No. 23685023) for K.S. and Grant-in-Aid for Scientific Research Activity Start-up (No. 16H06863) for M.U. by the Japan Society for the Promotion of Science and Program for Leading Graduate Schools “Integrative Graduate Education and Research Program in Green Natural Sciences”.
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