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Chapter 15
Vinyl Ether/Vinyl Ester Copolymerization by Cationic and Radical Interconvertible Simultaneous Polymerization Kotaro Satoh,* Yuuma Fujiki, Mineto Uchiyama, and Masami Kamigaito Department of Molecular and Macromolecular Chemistry, Graduate School of Engineering, Nagoya University, Nagoya 464-8603, Japan *E-mail:
[email protected].
In this study, the concurrent cationic and radical interconvertible simultaneous copolymerization of vinyl ether and vinyl ester was accomplished by using dithiocarbamate or xanthate as the dual RAFT mediator, which has been known effective for RAFT radical polymerization of vinyl ester. The interconvertible copolymerization proceeded well in a living fashion by the combination of ZnCl2 as a Lewis acid and V-70 as a radical initiator. Especially, diphenyl dithiocarbamate as the RAFT agent afforded well-controlled polymers with relatively narrow molecular weight distributions under tuned conditions in various monomer feed ratios and catalysts loading.
Introduction Controlled/living radical polymerization has been achieved using dissociable covalent bonds as the dormant species, such as carbon-halogen, -oxygen, and -sulfur bonds, to suppress the side reactions, which is therefore now called as reversible deactivation polymerization (RDP) (1). Among them, the carbon-sulfur bond in thiocarbonylthio compounds provides superior controllability for the polymerization of numerous monomers referred to as reversible addition-fragmentation chain transfer (RAFT) radical polymerizations (2–7).
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Recently, we have found the unprecedented copolymer synthesis which could be achieved by combining dual active species, i.e., cationic and radical interconvertible simultaneous living polymerization (8, 9). In this system, the dormant species was employed not only for suppressing the side reactions but also as the intermediate between two active species to induce mechanistic transformation (10), and thus one polymer chain was formed by dual activation of the carbon-sulfur bonds in the dormant RAFT species into either carbocations or radicals (11). In the previous work, the trithiocarbonate-type RAFT agent was employed for effectively controlling the interconvertible copolymerization of vinyl ether and acrylate to afford the copolymer with various monomer sequence distributions (8, 9).
Scheme 1. Cationic/Radical Interconvertible Simultaneous Polymerization between Vinyl Ethers and Vinyl Esters
In this study, the cationic/radical interconvertible simultaneous polymerization between vinyl ester and vinyl ether was examined by designing the dual RAFT agent. The former monomer is a common and non-conjugate monomer that could be polymerized only via radical polymerization, whereas the latter is a typical cationically-polymerizable monomer. Although the preliminarily result with xanthate was already reported in the previous paper (9), the interconvertible copolymerization of various vinyl esters and vinyl ethers became possible to some extent by using dithiocarbamate as well as xanthate, both of which have been used for RAFT radical polymerization of vinyl ester (Scheme 1) (12–19). 324
Results and Discussion Cationic and Radical (Co)polymerizations of Vinyl Ether and Vinyl Ester First, the radical RAFT copolymerization was evaluated between isobutyl vinyl ether (IBVE) and vinyl acetate (VAc) as the monomers using only a low-temperature azo initiator [2,2-azobis(4-methoxy-2,4-dimethylvaleronitrile): V-70] as the radical reservoir. The radical copolymerization of IBVE and VAc was carried out at 20 °C in ethyl acetate in the presence of V-70, in which a vinyl ether-type xanthate (BEEX) or N,N-diphenyl dithiocarbamate (BEDPDC) was employed as the RAFT agent. With both RAFT agents, the radical copolymerization proceeded and the conversion of IBVE was about one third to that of VAc, which is consistent with the conventional statistical radical copolymerization according to the already-known monomer reactivity ratio (20). The SEC curves of the obtained copolymer shifted to the high molecular weight region while maintaining a relatively narrow molecular weight distributions (MWDs), which indicates that the RAFT radical copolymerization of vinyl ether and vinyl acetate was attained in a controlled manner (Figure 1A). The broadening of MWDs at the later stage of the reaction was probably due to the head-to-head addition of vinyl acetate, which caused slowdown or deactivation for the degenerative chain transfer as in the case of other controlled radical polymerizations (21). On the other hand, when the IBVE/VAc copolymerization was performed by using only a Lewis acid ZnCl2 instead of V-70 without adding any radical generator under the same conditions as above, only IBVE was consumed to result in cationic polymerization that progressed to homopolymerize the vinyl ether (Figure 1B). With both RAFT agents, the number-average molecular weights (Mn) of the produced polymer also linearly increased in proportion to the conversion of IBVE agreeing with the calculated value assuming that one molecule of the polymer is generated from one RAFT reagent molecule, and in particular, the dithiocarbamate BEDPDC afforded polymers with fairly narrow MWDs (Mw/Mn < 1.1). Therefore, these RAFT agents also effectively acted as the dual initiator both for living cationic polymerization of vinyl ether and RAFT radical copolymerization of vinyl ether and vinyl ester. Cationic and Radical Interconvertible Simultaneous Polymerization The cationic/radical interconversion polymerization of the vinyl ether and vinyl ester was then investigated by simultaneously using V-70 as the radical generator and ZnCl2 as the catalyst for cationic polymerization at 20 °C in ethyl acetate. Although there is a slight difference depending on the RAFT agents, both IBVE and VAc were simultaneously consumed at almost the same rate and the conversions of the two monomers reached around 70 to 90%. Figure 2 shows the Mn and MWDs of the resulting copolymers. In both cases, the Mn of the copolymers linearly increased with the total conversions in a good agreement with the calculated values. Moreover, the SEC curve shifted to the high molecular weight region maintaining the unimodal MWDs, indicating that the copolymerizations were still controlled. Importantly, the conversion of IBVE reached higher than that of statistical radical copolymerization, suggesting that 325
interconvertible copolymerization proceeded with the reversible conversion of the active species between cationic and radical intermediates during the formation of one controlled/living chain to form multiblock-like copolymers as in the previous cases for IBVE and acrylate, although the propagation with radical intermediates was not perfect "living" due to the fact that the non-conjugated monomer like vinyl acetate causes somehow head-to-head addition that resulted in broadening of MWDs.
Figure 1. RAFT radial copolymerization (A) and cationic polymerization (B) of IBVE and VAc mediated by xanthate (BEEX) or dithiocarbamate (BEDPDC) with V-70 or ZnCl2 in ethyl acetate at 20 °C; [IBVE]0 = [VAc]0 = 3.0 M , [RAFT]0 = 60 mM, [V-70]0 = 60 mM for (A), [ZnCl2]0 = 1.25 mM for (B).
Figure 2. Interconvertible simultaneous copolymerization of IBVE and VAc mediated by xanthate (BEEX) or dithiocarbamate (BEDPDC) in ethyl acetate at 20 °C; [IBVE]0 = [VAc]0 = 3.0 M , [RAFT]0 = 60 mM, [V-70]0 = 60 mM, [ZnCl2]0 = 1.25 mM. 326
Figure 3. 1H NMR spectra (CD2Cl2, 30 °C) of IBVE/VAc copolymers obtained with BEDPDC by radical, cationic, and interconvertible simultaneous copolymerization shown in Figures 1 and 2.
The copolymers thus obtained in radical, cationic, and interconvertible copolymerizations with BEDPDC were analyzed by 1H NMR spectroscopy (Figure 3). In all cases, the peaks of the phenyl ring originated from the RAFT agent at the growing end were observed at 7.4 ppm. In addition, the incorporation ratio of the monomers calculated from the peak intensity ratio of the main-chain methine (b) and pendent methylene protons (c) in the IBVE units observed between 3.0 and 3.7 ppm and ester methine proton (g) in the VAc units between 4.7 and 5.2 ppm almost agreed with the calculated value by the monomer consumptions. In particular, peaks b and c observed in the interconvertible copolymerization (C) were separated according to the monomer sequences, which exhibited not only those of the alternating VAc-IBVE-VAc sequences in the radical copolymerization (A) but also those of the homo-propagation of IBVE (B). These results indicate that the interconvertible copolymerization took place between IBVE and VAc as in the case with acrylate in the previous literature (8). 327
Copolymerization of Other Vinyl Ether and Vinyl Ester Another combination of vinyl ether and vinyl ester was examined for interconvertible polymerization using BEDPDC as the dual mediator. 2-Chloroethyl vinyl ether (CEVE), which has less reactivity than IBVE (22), was employed as the cationically polymerizable monomer, whereas vinyl pivalate (VPv) was also used instead of Vac (16, 23, 24). In any of the combinations of vinyl ether and vinyl ester, the simultaneous reaction at the same rate took place at 20 °C in ethyl acetate using both ZnCl2 and V-70, although CEVE required a slightly higher concentration of ZnCl2 due to its lower reactivity. The effects of monomer charge concentrations were then examined for the combination of CEVE and VPv by changing the initial monomer feed ratio from 5:1 to 1:5 at the total concentration of 6.0 M while the concentrations of ZnCl2 and V-70 were kept constant (Figure 4). The consumption rate of CEVE did not change significantly and became slightly slower when lowering the initial charge concentration. On the other hand, the consumption rate of VPv became much faster as the initial feed concentration increased. The copolymers produced at each monomer charge concentration were also analyzed by SEC, as shown at the lower part of Figure 4. In all cases, the Mn of the produced copolymer linearly increased and agreed well with the calculated value while maintaining relatively narrow MWDs in spite of the completely different relative consumption rate. Thus, the controlled/living copolymerization proceeded in the combination of CEVE and VPv, indicating successful interconversion polymerization at any initial charge ratio. This result suggests the possibility to synthesize various copolymers with arbitrary composition, with simultaneous progress of cationic polymerization of only CEVE and radical copolymerization consumed by both monomers with various apparent copolymerization rates. For the combination of CEVE and VPv, we also tried to synthesize the higher molecular weight polymers by changing the initial feed ratio of RAFT and monomers. With the lower [BEDPDC]0 (10 mM), the higher the Mn of the copolymer was (Mn = 40400), although the MWDs became broader (Mw/Mn = 1.9) probably due to the slow reversible chain transfer compared to the propagation. The structure of the copolymers obtained from CEVE and VPv at 1:1 initial charged ratio was analyzed in detail by 1H NMR (Figure 5). As in the case with IBVE and VAc, the peaks corresponding to each monomer unit were observed in the spectrum, of which the composition calculated by the peak intensity was close to the calculated value from the initial feed ratio and conversions. In addition, the growing terminal structure was revealed to be the mixture of CEVE and VPv units adjacent to the dithiocarbamate, both of which the methine peaks were observed throughout the copolymerization. To discuss the mechanism of the copolymerization, the ratio of these peaks was plotted against the total conversions on the left of the figure. Although the ratio slightly changed as the copolymerization progressed, these two types of growing ends occurred throughout the reaction. This also indicated that the interconversion of the active species took place during the polymerization reaction, and the two monomers were freely copolymerized at any initial feed ratio. 328
Figure 4. Interconvertible simultaneous copolymerization of CEVE and VPv mediated by BEDPDC in ethyl acetate at 20 °C; [CEVE]0 + [VPv]0 = 6.0 M , [BEDPDC]0 = 60 mM, [V-70]0 = 60 mM, [ZnCl2]0 = 2.5 mM.
Figure 5. 1H NMR spectra (CD2Cl2, 30 °C) of CEVE/VPv copolymers obtained with BEDPDC by interconvertible simultaneous copolymerization shown in Figure 4 at [CEVE]0 = [VPv]0 = 3.0 M. The effect of the loading concentration of the Lewis acid was also examined for tuning only the rate of cationic polymerization. The copolymerization of CEVE and VPv was carried out in the presence of dithiocarbamate at various loading concentrations of ZnCl2 from 1.0 to 5.0 mM, while the concentration of V-70 was 329
kept constant (Figure 6). The reactivity of radically polymerizable VPv was hardly affected by the Lewis acid concentration, in which VPv was consumed at almost the same polymerization rate in all cases. Meanwhile, the consumption rate of CEVE was dramatically enhanced as the Lewis acid concentration increased. Even with different relative consumption rate of the two monomers, the SEC curves of the obtained copolymers exhibited unimodal shapes and shifted with the monomer conversions retaining relatively narrow MWDs. Furthermore, with much more Lewis acid ([ZnCl2]0 = 25 mM), the difference in the rates became larger to result in a clearer block-like copolymer, in which the conversions of CEVE and VPv were 95% and 3% with Mn = 4200 at the early stage (6 h), and 99% and 64% with Mn = 6400 at the later stage (90 h), respectively. The block copolymer showed two glass transition temperatures at –8 and 35 °C for poly(VPv) and poly(CEVE), respectively, although the values became slightly closer each other compared to the corresponding homopolymers (–15 and 65 °C) due to the partial miscibility. As described above, only by changing the concentration of the Lewis acid catalyst, the consumption rate of only CEVE could be freely changed without significant loss of the controllability of the copolymerization, which indicates the interconvertible polymerization of CEVE/VPv produced controlled/living copolymers with monomer sequence distributions from statistical copolymer to block-like copolymer, i.e., the statistical RAFT radical copolymerization occurred in the absence of ZnCl2, whereas CEVE was consumed much higher than VPv at the higher concentration of ZnCl2 to form blocky homo segments of CEVE.
Figure 6. Interconvertible simultaneous copolymerization of CEVE and VPv mediated by BEDPDC in ethyl acetate at 20 °C; [CEVE]0 = [VPv]0 = 3.0 M , [BEDPDC]0 = 60 mM, [V-70]0 = 60 mM, [ZnCl2]0 = 0–5.0 mM. 330
Post-polymerization of the Obtained Copolymer Vinyl ester is an industrially important monomer as the precursor of polyvinyl alcohol using post-polymerization reaction, as represented by the saponification of polyvinyl acetate (25). Therefore, the post-polymerization reaction of pendant substituent was investigated for the copolymer obtained by interconvertible polymerization of CEVE and VPv. For the CEVE/VPv copolymers, the ester groups in the VPv units were hydrolyzed using KOH at 60 °C in the solvent mixture of THF/CH3OH, although the RAFT terminal could also not survive during the strong basic process. Figure 7 shows the 1H NMR spectra of the copolymers before and after hydrolysis reaction. The peak derived from the tertiary butyl ester (g) almost disappeared and that of the hydroxyl group (h) was observed after the hydrolysis, indicating the conversion to the copolymer of CEVE and vinyl alcohol. Since the copolymer was composed of hydrophobic CEVE and hydrophilic vinyl alcohol, it is expected to be a novel amphiphilic copolymer, of which composition can be freely changed by the cationic and radical interconvertible polymerization.
Figure 7. 1H NMR spectra (DMSO-d6, 55 °C) of CEVE/VPv copolymer obtained by interconvertible copolymerization (CEVE/VPv = 79/76, Mn = 7600, Mw/Mn = 1.41) before and after hydrolysis with KOH. In this study, the possibility of cationic/radical interconvertible simultaneous polymerization of various vinyl ethers and vinyl esters was demonstrated. Since vinyl esters were common monomers with side chain functional group, we could obtain amphiphilic copolymers with various compositions by tuning reaction condition of the copolymerization followed by hydrolysis. Especially, the interconvertible copolymerization using a xanthate or carbamate RAFT agent 331
progressed effectively for controlling the composition, which could be changed freely by changing reaction conditions, such as comonomer feed and catalyst loading.
Experimental Materials Isobutyl vinyl ether (IBVE) (TCI, >99%), 2-chloroethyl vinyl ether (CEVE) (TCI, >97%), vinyl acetate (TCI, >99%), and vinyl pivalate (TCI, >99%) were distilled on calcium hydride under reduced pressure before use. ZnCl2 (Aldrich, 1.0 M in Et2O) were used as received. 2,2-Azobis(4-methoxy-2,4-dimethylvaleronitrile) (V-70) (Wako, 95%) was purified by washing with acetone at –15 °C and evaporated until dry under reduced pressure. S-1-isobutoxyethyl O-ethyl xanthate (BEEX) and S-1-isobutoxyethyl N,N-diphenyl dithiocarbamate (BEDPDC) were synthesized according to the literatures (26). Ethyl acetate (KANTO; >99%) was distilled from calcium hydride before use. Cationic and Radical Interconvertible Simultaneous Polymerization A typical procedure for the cationic and radical interconvertible copolymerization is given below for CEVE and VPv. The reaction was initiated by addition of the prechilled ethyl acetate solution (0.5 mL) of V-70 (0.18 mmol) and ZnCl2 (0.0075 mmol) via dry syringes into the monomer solution (2.5 mL) containing VPv (1.33 mL, 9.0 mmol), CEVE (0.91 mL, 9.0 mmol), and BEDPDC (0.18 mmol) in ethyl acetate at 20 °C. The total volume of the reaction mixture was 3.0 mL. In predetermined intervals, the polymerization was terminated with methanol (1.0 mL) and then by cooling the reaction mixtures to –78 ˚C. The monomer conversions were determined from the concentration of the residual monomer measured by 1H NMR using ethyl acetate as an internal standard (CEVE; 55%, VPv; 21% for 18 h, and CEVE; 74%, VPv; 68% for 94 h, respectively). The quenched reaction mixture was washed with diluted hydrochloric acid, and distilled water to remove the catalyst residues, evaporated until dry under reduced pressure to obtain the product polymers (Mn = 4200, Mw/Mn = 1.25 for 18 h and Mn = 6400, Mw/Mn = 1.46 for 94 h). The incorporated ratio of CEVE/VPv in the copolymer was determined by 1H NMR (CEVE/VPv = 69/21 for 18 h and 52/48 for 94 h, respectively). Measurements 1H
and 13C NMR spectra were recorded on a JEOL ECS-400 spectrometer, operated at 400 and 100 MHz. The number-average molecular weight (Mn) and dispersity (Mw/Mn) of the copolymers were analyzed by size-exclusion chromatography (SEC) in THF at 40 °C through two polystyrene gel columns [Shodex KF-805 L (pore size: 20–1000 Å; 8.0 mm i.d. × 30 cm) × 2; flow rate 1.0 332
mL/min], which were calibrated against 10 standard polystyrene samples [Varian; peak-top molecular weight (Mp) = 575–2783000, Mw/Mn = 1.02–1.23], connected to a JASCO PU-2080 precision pump and a JASCO RI-2031 detector.
Acknowledgments This work was partially supported by Precursory Research for Embryonic Science and Technology (PRESTO) from the Japan Science and Technology Agency (JST) (No. JPMJPR14K8) for K.S., a Grant-in-Aid for Scientific Research (A) (No. 26248032) by the Japan Society for the Promotion of Science for K.S., and Program for Leading Graduate Schools “Integrative Graduate Education and Research Program in Green Natural Sciences”.
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