Article pubs.acs.org/Macromolecules
Thioether-Mediated Degenerative Chain-Transfer Cationic Polymerization: A Simple Metal-Free System for Living Cationic Polymerization Mineto Uchiyama,† Kotaro Satoh,*,†,‡ and Masami Kamigaito*,† †
Department of Applied Chemistry, Graduate School of Engineering, Nagoya University, 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: Cationic degenerative chain-transfer polymerization of vinyl ethers and p-alkoxystyrenes was investigated using a series of thioethers as a reversible chain-transfer agent via the equilibrium between a growing carbocationic species and the resulting sulfonium intermediate in the presence of a small amount of triflic acid (TfOH) as a cationogen. The stable thioether, which was easily prepared from isobutyl vinyl ether (IBVE) and n-butanethiol, efficiently controls the molecular weight of the resulting poly(IBVE) up to Mn ∼ 1 × 105 with narrow molecular weight distributions (MWDs) (Mw/Mn ∼ 1.2). Upon increasing the bulkiness of the alkyl substituents in the thiols (R−SH; R: n-Bu < s-Bu < t-Bu) or those in the monomers (CH2CHOR′, R′: ethyl < isobutyl < cyclohexyl), the MWDs became broader due to the slower formation of the sulfonium intermediate for the degenerative chain-transfer reaction. For pmethoxystyrene, thioethers derived from bulkier alkylthiols or more electron-rich thiophenols are more effective. A silylprotected difunctional dithioether produced telechelic polymers possessing hydroxyl groups at both chain ends and stable thiol linkers in the middle of the polymer chains. These polymers were subsequently used in chain-extension reactions in conjunction with diisocyanates and diols as chain extenders to be converted into high molecular weight polymers linked via urethane linkages.
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INTRODUCTION Living polymerization is one of the most effective and useful methods for not only controlling the molecular weight of synthetic polymers but also enabling the synthesis of complex polymeric architectures such as block, end-functional, telechelic, and star polymers by design.1 Recently, there has been great progress in controlled/living polymerization techniques, most of which are based on the transient reversible deactivation of the propagating chain end into a dormant species, and as such, IUPAC recommends the name reversible deactivation polymerization (RDP).2 This class of polymerization has been most actively studied in radical polymerization systems and can be classified into at least three different mechanisms,3 i.e., dissociation−combination,4−7 atom transfer,8−15 and degenerative chain transfer,16−23 according to the kinetic treatments. The concept of RDP originates from several living ionic polymerizations24−27 such as group transfer polymerization (GTP) of methacrylates28,29 and living cationic polymerization of vinyl ethers and isobutene,30−35 which were accomplished in the mid-1980s. Most living cationic polymerizations rely on a type of atom transfer mechanism, in which the dormant covalent bonds, such as carbon−halogen and carbon−oxygen ester bonds, are reversibly activated by Lewis acid catalysts into a carbocationic species (Scheme 1B).30−35 The Lewis acid catalysts are generally metal halides with a few exceptions such as iodine,34 while the dormant species is derived from a © XXXX American Chemical Society
relatively weak protonic acid generating a nucleophilic counteranion that can form a covalent bond via an addition reaction with a monomer. In these Lewis-acid-catalyzed living cationic polymerizations, the contamination of metal residues is generally inevitable. Another living cationic polymerization, which is based on a dissociation−combination mechanism, was achieved using triflic acid (TfOH) in combination with thioethers in the 1990s (Scheme 1A).36,37 Although the superstrong acid, TfOH, generates a free carbocation via a reaction with vinyl ether, it is immediately converted into a stable and dormant sulfonium ion in the presence of the added thioethers. The formed sulfonium chain ends can transiently dissociate into the carbocationic species, which then induces the living cationic polymerization via a dissociation−combination mechanism. Although this system is free from metal, it needs one molecule of TfOH per polymer chain and a large molar excess of thioether with respect to TfOH or the polymer chain. Recently, we reported a new living cationic polymerization that proceeds via a reversible addition−fragmentation chaintransfer (RAFT) mechanism,38 a type of degenerative chaintransfer polymerization (Scheme 1C). In this cationic RAFT Received: June 20, 2015 Revised: July 28, 2015
A
DOI: 10.1021/acs.macromol.5b01341 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules Scheme 1. Reversible Deactivation Mechanisms for Living Cationic Polymerization
Scheme 2. Thioether-Mediated Degenerative Chain-Transfer Cationic Polymerization
polymerization, similar to the radical RAFT polymerization,17−21 the covalent C−SC(S)Z bond derived from the thiocarbonylthio compound employed as a chain-transfer agent is reversibly activated by a very small amount of the growing carbocationic species originating from TfOH as cationogen. This system is not only free from metal but also needs only one chain-transfer agent per chain and very small quantities of TfOH relative to the RAFT agent (ppm range or smaller). The key to the cationic RAFT polymerization is a high affinity of the sulfur atom to the carbocation, which contributes to the formation of the intermediate and then enables the rapid interchange reaction between the growing carbocation and the dormant thiocarbonylthio terminal. In this study, we developed another novel living cationic polymerization that proceeds via a degenerative chain-transfer mechanism using a thioether as a reversible chain-transfer agent in place of the thiocarbonylthio compound used in conventional RAFT polymerizations (Scheme 2). The thioether is more easily prepared and possesses a simpler structure with a more stable C−S bond. If the thioether forms a stable sulfonium intermediate via reaction with the propagating carbocationic species and the resulting intermediate rapidly dissociates into the carbocation and thioether via an interchange reaction, the chain length of the resulting polymer
can be controlled by the proposed degenerative chain-transfer mechanism. Although a competing degenerative mechanism has been proposed in living cationic polymerizations involving C−I bonds in the presence of ammonium salts that generate less nucleophilic anions such as perchlorates and triflates, it has not been fully elucidated.39,40 In addition, the iodide compound is less stable and difficult to synthesize and to purify. Here, we synthesized various adducts of vinyl ether and thiols with different steric and electronic substituents via a simple addition reaction followed by purification under mild conditions to use as chain-transfer agents for controlling the cationic polymerization of various vinyl ethers and p-methoxystyrene in the presence of a small amount of TfOH as a cationogen. Furthermore, telechelic polymers were synthesized by preparing stable difunctional thioether compounds and used in subsequent chain-extension reactions.
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RESULTS AND DISCUSSION Thioether-Mediated Cationic Polymerization of IBVE via a Degenerative Chain-Transfer Mechanism. The thioether or thioacetal 1 was easily synthesized by the reaction of isobutyl vinyl ether (IBVE) and n-butanethiol in the presence of a small amount of p-toluenesulfonic acid as a catalyst. It is stable and can be distilled and handled in air. B
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Macromolecules Table 1. Cationic Polymerization of IBVE with Various Transfer Agentsa entry 1 2f 3 4f 5g 6g 7 8 9 10 11 12 13
transfer agent (IBVE-X) none −Sn-Bu (1)
−Ss-Bu (2) −St-Bu (3) −SC6H4Cl (4) −SC6H5 (5) −SC6H4OCH3 (6) −SC(S)SEt (7) −OCH3 (8)
[M]0/[I]0
time
convb (%)
Mnc
Mn(calcd)d
Mw/Mnc
− 25 50 100 500 1000 50 50 50 50 50 50 50
1 min 1 min 1 min 1 min 45 min 1h 1 min 2 min 1 min 1 min 1 min 25 s 30 s
98 93 95 92 99 99 91 98 94 93 91 90 89
24600 3300 5100 10600 56500 117400 4600 6100 5500 4900 5300 5000 5800
2500 4800 9400 50300 100400 4800 5100 5000 4900 4800 4700 4800
3.58 1.18 1.23 1.27 1.19 1.24 1.68 1.84 1.93 1.90 1.52 1.18 1.82
Ctre
5.31
1.81 1.31 1.23 1.32 2.41 7.68 1.55
a Polymerization condition: [M]0/[transfer agent]0/[TfOH]0 = 500/10/0.05 mM in n-hexane/CH2Cl2/Et2O (80/10/10) at −40 °C. bDetermined by 1H NMR. cDetermined by SEC. dMn(calcd) = MW(IBVE) × ([M]0/[transfer agent]0) × conv + MW(transfer agent). eChain transfer constant. f [IBVE]0/[1]0/[TfOH]0 = 500/20 or 5/0.05 in n-hexane/CH2Cl2/Et2O (80/10/10) at −40 °C. g[IBVE]0/[1]0/[TfOH]0 = 600/1.2 or 0.6/0.03 in n-hexane/CH2Cl2/Et2O (80/10/10) at −78 °C.
The cationic polymerization of IBVE was then examined using 1 as a possible chain-transfer agent in conjunction with a trace amount of TfOH as a cationogen ([IBVE]0/[1]0/ [TfOH]0 = 500/10/0.05 mM) in a solvent mixture of nhexane/CH2Cl2/Et2O (80/10/10 vol %) at −40 °C. The polymerization proceeded rapidly and was complete within 1 min. Irrespective of the very fast polymerization, the obtained polymers showed controlled number-average molecular weights (Mn) that agreed well with the calculated values assuming that one thioether generates one polymer chain and were characterized by relatively narrow molecular weight distributions (MWDs) (Mw/Mn ∼ 1.2) (entry 3 in Table 1 and Figure S1 in the Supporting Information). To confirm the molecular weight control by the thioether, feed ratios of IBVE to 1 were changed ([M]0/[1]0 = 25−1000). The Mn values were directly proportional to [M]0/[1]0 and were in good agreement with the theoretical values (entries 2−6 in Table 1 and Figure S2) in all cases. Even at a high feed ratio targeting a 1000-mer of IBVE, a high molecular weight was obtained that agreed well with the expected molecular weight (Mn ∼ 1 × 105) accompanied by a narrow MWD (entry 6 in Table 1). These results indicate that the thioether, which is easy to prepare and to handle, efficiently works as a degenerative chain-transfer agent in the metal-free cationic polymerization of IBVE in the presence of a small amount of TfOH. Furthermore, a fresh feed of IBVE was added to the reaction mixture after the initial charge of monomer was almost completely polymerized. The additional portion of IBVE was also consumed smoothly. As shown in Figure 1, even after the monomer addition, the Mn of the obtained polymers increased in direct proportion to the monomer conversion and agreed well with the calculated values. The MWDs remained similarly narrow (Mw/Mn ∼ 1.2). Thus, the 1/TfOH initiating system mediates the cationic polymerization in a living manner without any significant irreversible termination and chain-transfer reactions during the chain propagation, even after the monomer is almost depleted. Mechanistic Investigation To Elucidate the Effect of Triflic Acid on the Resulting Polymers. To elucidate the mechanism of the thioether-mediated cationic polymerization, the concentration of TfOH ([TfOH]0) was changed, keeping all other parameters constant. As [TfOH]0 increased, the Mn of
Figure 1. Mn, Mw/Mn, and SEC curves for monomer-addition experiment in cationic polymerization of IBVE in the presence of 1 in n-hexane/CH2Cl2/Et2O (80/10/10 vol %) at −40 °C: [M]0/ [M]add/[1]0/[TfOH]0 = 500/10/0.05 mM.
the products decreased due to an increase in the cationogen and thereby the polymer chain (Figures S3 and S4). Almost all the polymers obtained using a small amount of TfOH ([1]0/[TfOH]0 = 10/0.05 mM) had thioacetal ω-chain ends (Figure S4), which are polymeric analogues of 1 and are dormant species. The MALDI-TOF-MS spectrum also revealed that nearly one series of peaks each separated by the molar mass of the IBVE units (100.2 Da) are observable and can be assigned to the thioether-ended polymers (Figure 2A). It also indicates the high stability of the thioether terminal even during the work-up procedures and MALDI-TOF-MS analysis unlike the thioester terminals obtained by the cationic RAFT polymerization of IBVE.38 Upon increasing the concentration of TfOH, the acetal terminals originating from the methanol used for quenching the polymerization became visible in both the 1H NMR and MALDI-TOF-MS spectra (Figure 2B). A further increase of [TfOH]0 makes these peaks larger (Figure 2C). These results indicate that the concentration or number of cationic propagating species increased as [TfOH]0 was increased and that the cationic propagating species is terminated as the acetal group upon quenching with methanol. Thus, TfOH works as a cationogen and generates the carbocationic species that interchanges with the thioether C
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Figure 2. Effects of [TfOH]0 on MALDI-TOF-MS spectra of the polymers obtained in the polymerization of IBVE at [M]0/[1]0/[TfOH]0 = 500/ 10/0.05−10.0 mM in n-hexane/CH2Cl2/Et2O (80/10/10 vol %) at −78 °C.
Figure 3. Mn, Mw/Mn, and SEC curves for cationic polymerization of IBVE in the presence of various aliphatic thioethers in n-hexane/CH2Cl2/Et2O (80/10/10 vol %) at −40 °C: [M]0/[thioether]0/[TfOH]0 = 500/10/0.05 mM.
sec-butanethiol (2), and tert-butanethiol (3), and were employed to polymerize IBVE (entries 3, 7, and 8 in Table 1, respectively). In all cases, the polymerization proceeded smoothly and was complete in 1−2 min. The Mn of the polymers obtained with a bulkier thioether 2 increased almost linearly with IBVE conversion and were close to the theoretical values assuming that one thioether generates one polymer chain (Figure 3). The bulkiest thioether 3 with tert-butanethiol resulted in higher molecular weights than the calculated values. Furthermore, the MWDs became broader as the thioether substituent bulkiness increased. These results indicate that a
chain end via the sulfonium intermediate, resulting in controlled molecular weights and the retention of chain-end groups via a degenerative chain-transfer mechanism. Various Thioethers and Related Compounds for the Cationic Polymerization of IBVE. To elucidate the thioether structure that can effectively control the cationic polymerization via a degenerative chain-transfer mechanism, various thioethers and related compounds were employed for the cationic polymerization of IBVE under the same conditions. First, a series of thioethers of different bulkiness were synthesized from various butanethiols, i.e., n-butanethiol (1), D
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chain-transfer reactions. The least bulky thioether 1 with nbutanethiol is therefore effective for at least the cationic polymerization of IBVE via a degenerative chain-transfer mechanism. Another series of thioethers (4, 5, 6) were prepared from IBVE and thiophenols with electron-withdrawing and -donating substituents and used for the cationic polymerization of IBVE under the same conditions (entries 9, 10, and 11 in Table 1). The Mn determined by SEC were all in good agreement with the calculated values, although the MWDs were relatively broad for 4 and 5 (Mw/Mn ∼ 1.9) (Figure S5). However, the MWD of the polymers prepared with 6, which has an electrondonating p-methoxy group on the thiophenol, was narrower (Mw/Mn ∼ 1.5). These results suggest that a more electrondonating substituent on the thioether group results in a more stable sulfonium intermediate, which results in a faster degenerative chain-transfer reaction. To evaluate the rate of the degenerative chain-transfer process, the chain-transfer constants (Ctr) were calculated for the various chain-transfer agents under the same conditions used in Table 1 according to this simple equation Mw/Mn = 1 + 1/Xn + (2 − c)/(cCtr), where Xn is the number-average degree of polymerization and c is the monomer conversion.3 A relatively large Ctr was obtained with 1 (Ctr = 5.31, entry 3), which gave the narrowest MWD among the various thioethers examined, whereas it was lower than that with the trithiocarbonate RAFT agent (7; Ctr = 7.68, entry 12) previously reported by us.38,41 A similar oxygen ether or acetal compound (8) that is the adduct of methyl vinyl ether and methanol resulted in a higher molecular weight, a broader MWD (Mw/Mn ∼ 1.8), and, thus, a low constant (Ctr = 1.55, entry 13). Thus, the sulfur atom is necessary for the effective degenerative chain-transfer process via the formation of the sulfonium intermediate. However, the thioether is no longer effective in the radical mechanism as indicated by the control experiments for the radical copolymerization of IBVE and methyl acrylate (MA) in the presence of 2,2-azobis(4-methoxy-2,4-dimethylvaleronitrile) (V-70), where copolymers with uncontrolled high molecular weight (Mn > 105) and broad MWDs (Mw/Mn ∼ 2.5) were obtained (Figure S6). This is in sharp contrast with the trithiocarbonate (7), which controlled the molecular weight even in the radical copolymerization (Figure S7).38 Cationic Polymerization of Other Vinyl Ethers. To confirm the versatility of this polymerization system for other vinyl ethers, ethyl (EVE) and cyclohexyl vinyl ether (CHVE)
bulky substituent around the thioether retards the formation of the sulfonium ion for the effective degenerative chain-transfer mechanism, resulting in higher and uncontrolled molecular weights and broader MWDs. The 1H NMR analysis of the obtained polymers all showed the presence of thioether ω-chain ends, which is characterized by a methine proton (b′) adjacent to a sulfur atom and the corresponding butanethiol moiety ( f) (Figure 4). The Mn
Figure 4. 1H NMR spectra (in CDCl3 at 55 °C) of poly(IBVE) obtained in the same experiments as for Figure 3. (A) Mn(NMR) = 2200, Mn(SEC) = 2700, Mw/Mn = 1.27, conversion = 35%. (B) Mn(NMR) = 2700, Mn(SEC) = 3000, Mw/Mn = 1.82, conversion = 38%. (C) Mn(NMR) = 5500, Mn(SEC) = 6000, Mw/Mn = 1.67, conversion = 37%.
[Mn(NMR)] determined from the integration ratios of the repeat units (e) to the ω-end (b′) were close to those obtained by SEC [Mn(SEC)] in all cases, indicating that the resulting polymers all possessed the thioether chain ends irrespective of the bulkiness of the substituents. This again shows that less controlled molecular weights with bulkier substituents are not due to irreversible side reactions but due to slower degenerative
Table 2. Cationic Polymerization of Various Vinyl Ethers with Thioethersa entry 1 2 3 4 5 6 7 8 9
monomer EVE
CHVE
CEVE
transfer agent (IBVE-X)
time (s)
convb (%)
Mnc
Mn(calcd)d
Mw/Mnc
−Sn-Bu (1) −Ss-Bu (2) −St-Bu (3) −Sn-Bu (1) −Ss-Bu (2) −St-Bu (3) −Sn-Bu (1) −Ss-Bu (2) −St-Bu (3)
30 30 15 12 2 2 40 30 30
97 96 94 99 98 99 94 93 92
4700 4000 4800 6600 8900 37200 3700 3200 3200
3700 3700 3600 6400 6400 6400 5200 5100 5100
1.11 1.38 1.71 1.63 2.47 4.05 1.37 1.52 2.19
a
Polymerization condition: [monomer]0/[transfer agent]0/[TfOH]0 = 500/10/0.05 in n-hexane/CH2Cl2/Et2O (80/10/10) (EVE and CHVE) or [monomer]0/[transfer agent]0/[TfOH]0 = 500/10/0.50 toluene/CH2Cl2/Et2O (80/10/10) (CEVE) at −40 °C. bDetermined by 1H NMR. c Determined by SEC. dMn(calcd) = MW(monomer) × ([M]0/[transfer agent]0) × conv + MW(transfer agent). E
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Figure 5. SEC curves of the obtained polymers for cationic polymerization of various monomers in the presence of various aliphatic thioethers in the same experiments as for Tables 1 and 2.
Table 3. Cationic Polymerization of pMOS with Thioethersa entry 1 2 3 4 5 6 7 8 9 10 11 12 13 14
transfer agent none IBVE-Sn-Bu (1) IBVE-Ss-Bu (2) IBVE-St-Bu (3) IBVE-SC6H4Cl (4) IBVE-SC6H5 (5) IBVE-SC6H4OCH3 (6) pMOS-SC6H4ClI (9) pMOS-SC6H5 (10) pMOS-SC6H4OCH3 (11)
TfOH (mM)
time
convb (%)
Mnc
0.05 0.50 0.05 0.50 0.05 0.50 0.05 0.50 0.05 0.05 0.50 0.05 0.05 0.05
40 min 5 min 30 h 24 h 30 h 80 h 140 h 42 h 30 min 14 h 1h 80 min 1h 6h
90 90 0 0 1 1 5 99 91 97 96 94 93 95
119800 58800
5500 5100 5700 5200 6500 6000 5100
Mn(calcd)d
Mw/Mnc 2.02 2.05
6800 6300 6700 6700 6600 6500 6600
1.55 1.47 1.35 1.31 1.41 1.28 1.27
a Polymerization condition: [M]0/[transfer agent]0/[TfOH]0 = 500/10/0.05 or 0.50 mM in CH2Cl2/Et2O (90/10) at 0 °C. bDetermined by 1H NMR. cDetermined by SEC. dMn(calcd) = MW(pMOS) × ([M]0/[transfer agent]0) × conv + MW(transfer agent).
with different bulkiness were polymerized using 1, 2, and 3 under the same conditions used for IBVE (Table 2). These alkyl vinyl ethers were similarly and quantitatively polymerized within 1 min. The molecular weights and MWDs depended on the bulkiness of the substituents in both the monomers and the thioethers. As shown in Figure 5, the MWDs became narrower as the substituents in the monomers and thioethers became smaller. Thus, the narrowest MWD was obtained (Mw/Mn = 1.11) with 1 for EVE. However, for CHVE, i.e., the bulkiest monomer around the ether oxygen, the MWD obtained, even with 1, was relatively broad (Mw/Mn ∼ 1.6). Furthermore, the bulkiest thioether, 3, derived from tertbutanethiol resulted in much higher and uncontrolled molecular weights. These results indicate that bulky substituents around the thioacetals make the sulfonium inter-
mediate difficult to form due to steric repulsion, which results in poorer control over the molecular weight. Another vinyl ether with a chlorine containing substituent, i.e., 2-chloroethyl vinyl ether (CEVE), was also examined using 1, 2, and 3 under the same conditions (entries 7−9 in Table 2). Although the MWDs were slightly broader than those of IBVE, the molecular weight can be controlled by the thioethers (Figure S8). Thus, thioethers or thioacetals are effective for the controlled cationic polymerization of various vinyl ethers via a degenerative chain-transfer mechanism triggered by a small amount of triflic acid. The steric hindrance around the ether group of the monomer and the thioether group of the chaintransfer agent is crucial for control because the sulfonoium ion intermediate is surrounded by two vinyl ether units and one thiol substituent. F
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Figure 6. Time−conversion curve and SEC curves of the obtained polymers in cationic polymerization of pMOS in the presence of aromatic thioethers in CH2Cl2/Et2O (90/10 vol %) at 0 °C: [pMOS]0/[thioether]0/[TfOH]0 = 500/10/0.05 mM.
Figure 7. SEC curves of (A) silyl-protected telechelic poly(IBVE), (B) deprotected telechelic poly(IBVE), (C) products obtained with with 2 equiv of 1,4-phenylene diisocyanate, and (D) products obtained via further reaction with 1 equiv of 1,4-butanediol.
Cationic Polymerization of p-Methoxystyrene. To further expand the scope of the thioether-mediated degenerative cationic polymerization, a styrene derivative with an electron-donating group, p-methoxystyrene (pMOS), was polymerized using various thioethers in the presence of a trace amount of TfOH in a CH2Cl2/Et2O (90/10 vol %) mixture at 0 °C (Table 3). Without thioethers, the polymerization proceeded very quickly and reached 90% conversion within 5 min, resulting in high molecular weight polymers (Mn > 5 × 104). Almost no polymerization occurred with 1 and 2, which were effective for IBVE, even in the presence of a larger amount of TfOH ([TfOH]0 = 0.50 mM) (Figure S9). The polymerization of pMOS using 3 with a bulky tert-butyl group occurred slowly to give polymers with controlled molecular weights closer to the calculated values and tert-butanethiol moiety at the chain end (Figure S10). These results can be attributed to the difference in the stability of the carbocationic species derived from styrene derivatives and vinyl ether. The equilibrium between the carbocationic species of pMOS and its resulting sulfonium intermediate is expected to be shifted more to the sulfonium
ion than that for IBVE which results in a more stable carbocationic species. Especially in the case where pMOS is polymerized in combination with a less bulky thioether such as 1 and 2, the equilibrium is highly shifted to the sulfonium intermediate, which makes the dissociation into the carbocationic species more difficult, resulting in almost no polymerization. However, the bulkier thioether, 3, with a tert-butyl group, promotes the dissociation of the sulfonium ion into the carbocationic species due to steric hindrance to induce an effective degenerative chain-transfer reaction for controlling the molecular weight in the cationic polymerization of pMOS. A series of aromatic thioethers, i.e., adducts of pMOS and pchloro (9), nonsubstituted (10), and p-methoxy (11) thiophenols, were also used in the cationic polymerization of pMOS to investigate the electronic effects on the controllability of the polymerizations. The polymerizations all proceeded quantitatively even in the presence of a lower amount of TfOH ([TfOH]0 = 0.05 mM) when employing any aromatic thioethers. The polymerizations proceeded at almost the same rate in the presence of 9 and 10, whereas the polymerization became slower using 11 that bears a more G
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Macromolecules electron-donating methoxy substituent. This substituent results in a more stable sulfonium intermediate and therefore a lower concentration of the propagating cationic species in the polymerization equilibrium. As shown in Figure 6, with all aromatic thioethers, the SEC curves are relatively narrow and shifted to high molecular weight. Furthermore, the Mn increased directly with the monomer conversion and were close to the calculated values (entries 9, 10, and 11 in Table 3). The MWDs were narrower using aromatic thioethers derived from nonsubstituted (10) or p-methoxy-substituted (11) thiophenols than from an electron-withdrawing p-chlorosubstituted thioether (12). Similar results were obtained with adducts of IBVE and a series of thiophenols (4−6) for the cationic polymerization of pMOS (Figure S11). Thus, the cationic polymerization of pMOS can also be controlled via a thioether-mediated degenerative chain-transfer mechanism using appropriate thioethers, particularly those derived from thiophenols. These results indicate that for the design of thioethers for good polymerization control it is crucial to consider both the steric and electronic nature of the thioether. Difunctional Dithioethers for Telechelic Polymers and Postpolymerization Reactions. One of the most notable features of thioether-based chain-transfer agents is their high stability and accessibility in comparison to other initiators or reversible chain-transfer agents for living cationic polymerizations thus far reported. To evaluate the applicability of the thioether-mediated polymerization for precision polymer design and synthesis, a difunctional dithioether (12) was synthesized from a dithiol and functionalized vinyl ether and was used for the synthesis of telechelic polymers followed by a postpolymerization reaction. The difunctional dithioether with silyl-protected hydroxyl groups was easily synthesized from the lithium salt of 1,4butanedithiol and 2 equiv of the hydrogen chloride adduct of tert-butyldimethylsiloxyethyl vinyl ether. As with the monothioethers above, the polymerization also proceeded smoothly, resulting in polymers with controlled molecular weights and narrow MWDs (Mw/Mn < 1.2) (Figure 7A). The Mn agreed well with the calculated values, assuming that one dithioether forms one polymer chain (Figure S12). The polymers obtained with the difunctional dithioether were analyzed by 1H NMR spectroscopy (Figure 8A). The spectrum showed the main signals of the IBVE repeating units and the characteristic signals derived from the difunctional dithioether, such as the methyl protons (i) of the protected silyl groups, methyl protons ( f) at the initiating α-end, and methylene (k) and methine (b′) protons adjacent to the sulfur atoms in the middle of polymer chain. The α-end functionality (1.81) was close to two, indicating that this difunctional dithioether works as a difunctional reversible chain-transfer agent for the living cationic polymerization of IBVE. The silyl groups were then deprotected and converted to the hydroxyl groups using tetrabutylammonium fluoride (TBAF) in THF at 40 °C. There were almost no changes in the SEC curves (Figure 7B), indicating that the mid thioacetal group is stable even during the deprotection. The 1H NMR spectrum after the deprotection showed that the silyl groups completely disappeared and that the adjacent methylene signals (g) slightly shifted upfield (Figure 7B) whereas almost no changes were observed for the other signals. The functionality of the α-end group (1.80) remained almost the same. These results indicate that telechelic polymers with hydroxyl groups at both chain
Figure 8. (A) 1H NMR spectrum (in CDCl3 at 55 °C) of silylprotected poly(IBVE) obtained in the polymerization of IBVE with difunctional dithioether at [M]0/[12]0/[TfOH]0 = 500/10/0.05 mM in n-hexane/CH2Cl2/Et2O (80/10/10 vol %) at −78 °C and (B) the telechelic poly(IBVE) obtained polymers by deprotection with TBAF. (A) Mn(NMR) = 3200, Mn(SEC) = 2900, Mw/Mn = 1.22, Fn(α) = 1.81. (B) Mn(NMR) = 3000, Mn(SEC) = 2700, Mw/Mn = 1.23, Fn(α) = 1.80.
ends were obtained via the difunctional dithioether-mediated cationic polymerization. The chain extension reaction using the hydroxyl groups of the telechelic poly(IBVE) was carried out in conjunction with 1,4-phenylene diisocyanate via the formation of urethane linkages. The telechelic poly(IBVE) was first reacted with 2 equiv of 1,4-phenylene diisocyanate in the presence of dibutyltin dilaurate (DBTDL) as a catalyst in n-hexane at 20 °C to be converted into diisocyanate chain ends. After 2 h, 1 equiv of 1,4-butanediol as the chain extender was further added to the reaction solution. During these reactions, the SEC curves shifted to high molecular weight, and the precursor telechelic poly(IBVE) peak decreased (Figure 8D). These results indicate that the telechelic poly(IBVE) was efficiently converted into high molecular weight polyurethane via the reaction between the hydroxyl and isocyanate groups. This success is owed primarily to the high functionality of the telechelic polymers as well as the high stability of the mid thioether groups. This approach for telechelic polymers based on difunctional compounds is not suitable for the other living cationic polymerizations, which are based on less stable covalent bonds. Thus, the thioether-mediated cationic polymerization system offers a different approach for precision polymer synthesis of novel well-defined polymer architectures.
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CONCLUSIONS A series of thioethers, which can be easily synthesized and are easy to handle, effectively mediate the degenerative cationic chain-transfer polymerization of vinyl ethers and p-alkoxystyrenes in conjunction with a trace amount of triflic acid to result in polymers with a controlled molecular weight and stable H
DOI: 10.1021/acs.macromol.5b01341 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
reduced pressure (1.13 mmHg, bp = 46 °C) to give 1 as colorless liquid (4.0 g, 21 mmol, 30% yield). The experimental details for the other thioethers (2−6, 9−12) are described in the Supporting Information. Cationic Polymerization with Thioether. The cationic polymerization of IBVE was carried out by the syringe technique under dry nitrogen in baked glass tubes equipped with a three-way stopcock. A typical example for the polymerization procedure is given below. The reaction was initiated by addition of TfOH (0.30 mL of 0.50 mM in Et2O, 1.5 × 10−4 mmol) via dry syringe into monomer solution (2.70 mL) containing IBVE (0.20 mL, 1.53 mmol) and 1 (0.05 mL of 564 mM in toluene, 0.03 mmol) in the n-hexane/CH2Cl2 mixture (80/10) at −40 °C. In predetermined intervals, the polymerization was terminated with methanol (1.0 mL) containing a small amount of triethylamine. The monomer conversion was determined from the concentration of residual monomer measured by 1H NMR with toluene as an initial standard (25 s, 90%). The quenched reaction mixture was washed with distilled water to remove initiator residues, evaporated to dryness under reduced pressure, and vacuum-dried to give the product polymers (Mn = 5000, Mw/Mn = 1.18). Radical Copolymerization of IBVE and MA. The polymerization was carried out by syringe techniques under dry argon or nitrogen in baked glass tubes equipped with three-way stopcock. A typical example for radical copolymerization of IBVE and MA is given below. In a 25 mL round-bottomed flask was placed IBVE (1.77 mL, 13.6 mmol), MA (1.25 mL, 13.6 mmol), 1 (0.48 mL of 564 mM in toluene, 0.27 mmol), toluene (1.94 mL), and solution of V-70 (1.36 mL of 50 mM solution in toluene, 0.068 mmol) at room temperature. The total volume of the reaction mixture was 6.80 mL. After mixing, the solution was charged in eight glass tubes, and the tubes were sealed by flame under a nitrogen atmosphere. The tubes ware immersed in thermostatic oil bath at 20 °C. In predetermined intervals, the polymerization was terminated by cooling of the reaction mixtures to −78 °C. The monomer conversion was determined from the concentration of residual monomer measured by the 1H NMR using EtOAc as internal standard (20 h, IBVE: 41%, MA: 95%). The quenched reaction mixture was evaporated to dryness under reduced pressure and vacuum-dried to give the product polymers (Mn = 163 300, Mw/Mn = 2.53). Deprotection of Silyl-Protected Telechelic Polymer. The deprotection of silyl-protected telechelic polymer was carried out under a dry nitrogen atmosphere in 50 mL round-bottom flask equipped with a three-way stopcock. For a typical example, a THF solution of TBAF (1.0 M, 5.20 mL, 5.2 mmol) was added to the silylprotected telechelic poly(IBVE) (0.15 g, Mn = 2900, Mw/Mn = 1.22, 0.05 mmol). The mixture was stirred for 20 h at 40 °C. The reaction was quenched by adding methanol (5.0 mL). The reaction solution was diluted with CHCl3, washed with distilled water, and then dried under vacuum to give telechelic polymers with hydroxyl groups at both chain ends (0.15 g, Mn = 2700, Mw/Mn = 1.23). Chain Extension Reaction. The chain extension reaction via the formation of urethane linkages was carried out under a dry nitrogen atmosphere in baked 25 mL glass tubes equipped with a three-way stopcock. For a typical example, toluene solution of 1,4-phenylene diisocyanate (1.88 mL of 36.3 mM solution in toluene, 0.068 mmol) was added to the mixture solution of telechelic poly(IBVE) (93.5 mg, Mn = 2700, Mw/Mn = 1.23, 0.034 mmol), DBTDL (0.54 mL of 12.7 mM solution in n-hexane, 6.8 × 10−4 mmol), and n-hexane (1.0 mL). The mixture was stirred for 2 h at 20 °C. Then, 1,4-butanediol (0.13 mL of 260 mM solution in THF, 0.034 mmol) was added to the reaction solution. After 2 h, the reaction was quenched by adding methanol to give the poly(IBVE) linked via urethane linkages (Mw = 20 400, Mw/Mn = 3.03). Measurements. Monomer conversion was determined from the concentration of residual monomer measured by 1H NMR spectroscopy with toluene or 1,2-dichlororbenzene as an internal standard. 1H NMR spectra were recorded on a JEOL ECS-400 spectrometer, operating at 400 MHz. MALDI-TOF-MS spectra were measured on a Shimazu AXIMA-CFR Plus mass spectrometer (linear mode) with dithranol as the ionizing matrix and sodium trifluoroacetate as the ion
thioether chain ends with high fidelity. This system is not only based on a novel relevant degenerative chain-transfer mechanism but is also metal-free34,36−39,42−48 and can be distinguished from other controlled or living cationic polymerizations thus far reported. Thus, this study offers a novel synthetic route to prepare well-defined polymers and can be used for bioapplications in which the polymers should be metalfree.
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EXPERIMENTAL SECTION
Materials. Isobutyl vinyl ether (IBVE) (TCI, 95%), ethyl vinyl ether (EVE) (TCI, 98%), cyclohexyl vinyl ether (CHVE) (Aldrich, 98%), 2-chloroethyl vinyl ether (CEVE) (TCI, 97%), p-methoxystyrene (pMOS) (TCI, 95%), acetaldehyde dimethylacetal (8) (Aldrich, 95%), and 1,4-butanediol (TCI, 99%) were distilled over calcium hydride under reduced pressure before use. Trifluoromethanesulfonic acid (TfOH) (TCI, >98.0%), n-butanethiol (Aldlich, 99%) 2butanethiol (sec-butanethiol) (TCI, 93%), 2-methyl-2-propanethiol (tert-butanethiol) (Aldrich, 99%), benzenethiol (TCI, 98%), 4chlorobenzenethiol (TCI, 98%), 4-methoybenzenethiol (TCI, 96%), 1,4-butanedithiol (TCI, 95%), p-toluenesulfonic acid monohydrate (TCI, 98%), sodium hydrate (TCI, 60%), hydrogen chloride solution (Aldrich, 1.0 M in Et2O), dibutyl dilaurate (TCI, 95%), and 1,4phenylene diisocyanate (TCI, 98%) were used as received. 2,2Azobis(4-methoxy-2,4-dimethylvaleronitrile) (V-70) (Wako, 95%) was purified by washing with acetone at −15 °C and was evaporated to dryness under reduced pressure. S-1-Isobutoxyethyl S′-2-ethyltrithiocarbonate (7),49 1-(isobutoxy)ethyl chloride (IBVE-HCl),50 and 2(tert-butyldimethylsiloxy)ethyl vinyl ether51 were synthesized according to the literature. Toluene (KANTO, >99.5%; H2O 96%; H2O 99.5%; H2O 99.5%; H2O
