Cyclopolymerization of Cleavable Acrylate-Vinyl Ether Divinyl

Jun 26, 2017 - ABSTRACT: Cyclopolymerization of a divinyl monomer, where two different vinyl groups, that is, acrylate and vinyl ether, are connected ...
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Cyclopolymerization of Cleavable Acrylate-Vinyl Ether Divinyl Monomer via Nitroxide-Mediated Radical Polymerization: Copolymer beyond Reactivity Ratio Yuki Kametani,† Marina Nakano,† Taizo Yamamoto,† Makoto Ouchi,*,†,‡ and Mitsuo Sawamoto†,§ †

Department of Polymer Chemistry, Graduate School of Engineering, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan Precursory Research for Embryonic Science and Technology (PRESTO), Japan Science and Technology Agency (JST), 4-1-8 Kawaguchi, Saitama 332-0012, Japan

‡

S Supporting Information *

ABSTRACT: Cyclopolymerization of a divinyl monomer, where two different vinyl groups, that is, acrylate and vinyl ether, are connected via an ester bond, was performed under diluted condition with nitroxide-meditated radical polymerization (NMP). Both vinyl groups were consumed at almost same rate under suitable condition, although the inherent cross-propagation ability between the two vinyl groups are pretty low in radical copolymerization. Furthermore, the polymerization was controlled to some extent to give polymers of unimodal molecular weight distributions. The results obviously differed from copolymerization and homopolymerization with vinyl monomers that constitutes the divinyl monomer, 2-methoxyethyl acrylate and 2-acetoxyethyl vinyl ether. Structural analyses indicated formation of the cyclopolymer but the cyclo-efficiency was imperfect indicating that some units of olefinic dangling were incorporated. Eventually, the ester bonds of the cyclo units were cleaved to convert into the copolymer consisting of acrylic acid and 2-hydroxy ethyl vinyl ether and the composition ratio (DPacryl/DPVE) was 55:45. The copolymer showed higher glass transition temperature than that estimated from the composition ratio and Tg values of the homopolymers, which is likely due to the formation of quasi-cyclopolymer between carboxylic acid and hydroxy groups aligned in alternating fashion.

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worthy example on a change in copolymerization behaviors is RAFT-based copolymerization of limonene and phenylmaleimide in fluoroalcohol solvent to control AAB alternating sequence, presented by Satoh and Kamigaito.3 Herein, a specific interaction of the solvent molecule with the latter monomer pendant allows control of the unique periodic sequence. As shown in the example, an ultimate control for the copolymerization process would lead to syntheses of sequencecontrolled vinyl polymers, which is a research field that has attracted attentions recently.4 Our group has designed multivinyl compounds whose linker is cleavable as the monomers for cyclopolymerization.5−7 The cleavage of the linker of the resultant cyclopolymer could lead to alternating sequence. Central to the approach is control of the selective cyclopolymerization of the designed monomers without cross-linking reaction. For example, a divinyl compound consisting of methacrylate and acrylate via hemiacetal ester bond underwent selective cyclo propagation under diluted condition with ruthenium-catalyzed living radical

n ease of copolymerization as well as wide range of applicable monomers in radical polymerization is advantageous over ionic polymerizations for syntheses of polymeric materials with various properties. For example, even a simple copolymer of styrene and methyl methacrylate (MMA) can not be synthesized via anionic polymerization but easily done via radical polymerization. In fact, more than one monomer is often used in industrial process with radical polymerization to tune the properties or functions for resultant polymers. In addition, development of living radical polymerization1 has opened the door to precise syntheses of functional copolymers of uniform composition ratio and molecular weight. The crossover propagation in radical copolymerization fairly takes place when the comonomers show similar reactivity or the electron density of the double bond is different from each other. The typical example of the latter is alternating copolymerization of electron-rich and -poor monomers (e.g., vinyl ether and maleic anhydride). The copolymerization behaviors can be changed through an addition of Lewis acid,2 but the effects are limited to an increase in the trend of alternating sequence. It is not so easy to drastically change copolymerization ability, for example, it is impossible to copolymerize a comonomer pair that is inherently not copolymerized (e.g., vinyl acetate and styrene). One note© XXXX American Chemical Society

Received: May 19, 2017 Accepted: June 21, 2017

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DOI: 10.1021/acsmacrolett.7b00368 ACS Macro Lett. 2017, 6, 754−757

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ACS Macro Letters polymerization (Figure 1a).7 The methacrylate vinyl group shows higher reactivity than the acrylate to allow the

Then, we switched the system to nitroxide-mediated radical polymerization (NMP)11 for the cyclopolymerization of 1, because it was reported that NMP is available for controlled copolymerization of a highly electron-deficient acrylate (hexafluoroisopropanol-pendant acrylate) with vinyl ether.12 BlocBuilder-MA, an adduct of methacrylic acid adduct of SG1, was used as the initiator, since it is one of most efficient initiators for controlled NMP of acrylate.13 Considering that the vinyl ether radical species is highly reactive due to the nonconjugation structure, so chlorobenzene was used as the solvent to avoid irreversible chain transfer reaction to the solvent molecule. When 1 M of 1 was polymerized with 20 mM of BlocBuilderMA at 110 °C, a gelation occurs likely due to the cross-linking reaction between the resultant chains. However, when diluting the solution ([1] = 50 mM; [BlocBuilder-MA] = 1 mM), the polymerization proceeded without any insoluble gels (Figure 2a). The conversions of the two vinyl groups were respectively

Figure 1. Cyclopolymerizations of cleavable divinyl monomers and the transformation into alternating copolymers via the pendant cleavage: (a) methacrylate-acrylate divinyl monomer connected via hemiacetal ester bond; (b) acrylate-vinyl ether divinyl monomer connected via ester bond.

preferential addition, and the intramolecular propagation occurs from methacrylate radical to acrylate double bond beyond the inherent reactivity ratio under diluted condition, which was confirmed with the model radical addition reaction. Herein, the slight difference between the two vinyl groups is important to realize the selective cyclopropagation and the alternating sequence of methacrylic acid and 2-hydroxy ethyl acrylate after the cleavage. The interesting feature of the cyclopolymerization approach with a divinyl monomer consisting of different reactivity olefins via cleavable linker encouraged us to expand the concept. We then focused on combination of acrylate and vinyl ether, instead of methacrylate and acrylate, because the pendant can be connected through cleavable linker and more importantly the double bonds show different reactivity (Figure 1b). The designed divinyl monomer is 1, where an acrylate vinyl group was connected with vinyl ether through the ester bond that is cleavable after the cyclopolymerization. Note that the monomer consists of conjugated and nonconjugated vinyl groups showing poorer copolymerizability [r1 = 3.3 r2 ∼ 0 for the combination of metyl acrylate (M1) and ethyl vinyl ether (M2)]8 than that of methacrylate and acrylate (r1 = 2.15; r2 = 0.40 for M1 = methyl methacrylate and M2 = methyl acrylate),9 indicating more difficult in realizing the cyclopolymerization. The divinyl monomer (1) was easily synthesized in one step through the reaction of acryloyl chloride with 2-hydroxyethyl vinyl ether (Figure S1). Herein the ester bond is cleavable via alkaline hydrolysis into carboxylic acid and hydroxyl groups. The resultant repeating unit is relatively compact of eightmembered ring, which would be favorable for intramolecular cyclopropagation. We first attempted the polymerization of the monomer 1 with a ruthenium-catalyzed living radical polymerization system under same condition as for polymerization of the methacrylate-acrylate divinyl monomer: [1] = 100 mM; [alkyl halide initiator] = 2 mM; [RuCp*Cl(PPh3)2] = 1 mM; Al(Ot-Bu)3 = 10 mM in toluene at 60 °C.7 However, the vinyl groups in 1 were hardly consumed and no polymer was obtained. This is likely due to that the halogen-capped vinyl ether is unstable at high temperature to be decomposed. Indeed, such a halogenbased compound is used for the dormant species in living cationic polymerization of vinyl ether but it survives only at lower temperature than 0 °C.10

Figure 2. Cyclopolymerizations of 1 with BlocBuilder-MA in chlorobenzene at 110 °C: [1]0/[BlocBuilder-MA]0 = 50/1 mM.

determined with 1H NMR, they were almost same as each other, though they did not match perfectly (Figure 2b). SEC curves of obtained polymers were not so narrow but unimodal (Mw/Mn = 1.6−1.8) and shifted to higher MW as the conversions were increased (Figure 2c). The polymerization was not satisfactory as controlled radical polymerization, but the results indicated controlled propagation as well as the progress of cyclopolymerization. Likely due to the dilute condition, the conversions reached not so high, however, relatively high molecular weight polymers were also synthesized by changing the ratio of 1 to BlocBuilder-MA (Figure S2). To confirm effects of the linker between acrylate and vinyl ether in 1 on the cyclopolymerization, 2-methoxyethyl acrylate and 2-acetoxyethyl vinyl ether, which are corresponding to the cut-out monovinyl compounds of 1, were copolymerized under the same condition (Figure 3). In contrast to the polymerization of 1, there was a big difference between conversions of the two monomers, as predicted from the reactivity ratios. Furthermore, molecular weights of the resultant copolymers were relatively low to the conversions, indicating occurrence of chain transfer reaction. Homopolymerizations were also examined with the two monomers, and no consumption was observed for the vinyl ether monomer. These control experiments suggested the consecutive propagation of acrylate 755

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ACS Macro Letters

(acrylate) and 20% (vinyl ether; Figure S3). From the results, the cyclopropagation was not perfect, though the crossover efficiency in propagation between acrylate and vinyl ether was obviously enhanced due to the connection. The ester bonds in the repeating cyclo-units of the obtained cyclopolymer were cleaved via hydrolysis under basic condition and the resultant solution was neutralized by HCl. The solubility was dramatically changed: the sample before the cleavage was soluble in common organic solvents such as THF, chloroform, and toluene, whereas the hydrolyzed sample became insoluble in such solvents but turned into soluble in alcohol and water. The structure was analyzed with 1H NMR (Figure 4b). The peaks obviously turned into sharp and the Figure 3. Control experiments for the cyclopolymerization of 1: (a) copolymerization of 2-methoxyethyl acrylate and 2-acetoxyethyl vinyl ether; homopolymerization of 2-methoxyethyl acrylate (b) and 2acetoxyethyl vinyl ether (c): [monomer]0/[BlocBuilder-MA]0 = 50/1 mM in chlorobenzene at 110 °C; 50 mM of each of the comonomers was injected for copolymerization.

monomer was not fully controlled under such a diluted condition and the cyclopolymerization of 1 likely proceeded thanks to the intramolecular vinyl ether group. Most probably, an existence of neighboring vinyl ether is rather helpful for the relatively stable “conjugated” acrylate radical species to propagate or survive even under such a diluted condition. On the other hand, the resultant “non-conjugated” vinyl ether radical species would be reactive enough to propagate with the acrylate vinyl group intermolecularly even though diluted. Another crucial is which type of dormant species is formed or which vinyl group is mainly capped with SG-1 during the polymerization. We attempted the model reaction (e.g., 1:1 reaction of BlocBuilder-MA with 1) to clarify it, but the analysis under diluted condition was very difficult. The structural analysis of the polymer, which was discussed below, likely supported the existence of both the two dormant species. However, the dissociation energy of the vinyl ether dormant is much higher than that of the acrylate, so the formation of the vinyl ether dormant VE is unfavorable. Nevertheless, the polymerization of 1 proceeded to a certain degree, which may be caused by that the dissociation energy is reduced due to the ring strain. Further analyses for mechanism study would be required as well as the condition optimization to reach higher conversion. The cyclopolymer of 1 was synthesized on a large scale for the structural analyses: [1] = 50 mM; [BlocBuilder-MA] = 1 mM for 137 mL solution. The conversions of the vinyl groups reached 28.3% (acrylate) and 25.2% (vinyl ether) in 2 h and the obtained polymer (Mn = 4000; Mw/Mn = 1.79) was purified with preparative SEC to remove low molecular weight compounds (e.g., monomer). The weight of the purified polymer (320 mg) was reasonable for the conversions and the injection ratio of 1 and BlocBuilder-MA. The structure of the purified polymer was then analyzed with 1H NMR (Figure 3a). In addition to the main broad peaks specific to the cyclopolymer (b-g), some relatively sharp peaks were also observed. Peaks around 6 ppm were likely attributed to unsaturated protons of vinyl ether and acrylate, indicating existing dangling vinyl groups by failure to cyclopropagation. On the basis of the number-averaged polymerization degree that was estimated from the acrylate conversion (28.3%), the averaged ratios of unreacted pendant vinyl groups. i.e., dangling ratios, for the two vinyl groups, were estimated as 1.2%

Figure 4. 1H NMR spectra of the cyclopolymer of 1 (a, CDCl3) and the copolymer after ester cleavage of the cyclopolymer (b, CD3OD).

spectrum surely indicated transformation of the cyclopolymer into the copolymer of acrylic acid and 2-hydroxyethyl vinyl ether. The cleavage of the ester bond was also supported by FT-IR (the peak from −OH stretch of the OH pendant) and 13 C NMR (the peak from CO of the COOH pendant) was also supported by FT-IR (Figures S4 and S5). The composition ratio of the units, which was calculated from the integration ratio, was DPacryl/DPVE = 55:45. The analysis that the composition ratio was not 1:1 is consistent with the result that the cyclopropagation was not perfect, because the dangling units would leave from the polymer. Note that such almost same incorporation of vinyl ether unit as that of acrylate is not realized via general copolymerization process. Finally, the glass transition temperature of the obtained copolymer was measured with differential scanning calorimetry (DSC). For comparison, the cyclopolymer before the cleavage was also measured as well as homopolymers of acrylic acid and 2-hydroxyethyl vinyl ether. The DSC profiles from −70 to 140 °C on the second heating process (10 °C/min) were summarized in Figure 5. There was no thermal transition in the profile of the cylcopolymer on the temperature range (a), which is attributed to the rigid cyclic structure. On the other hand, the copolymer showed the thermal transition derived from the glass transition around 90 °C (Tg = 91.7 °C, b). The value was close to that of homopolymer of acrylic acid (Tg = 111.7 °C, c) and not the averaged temperature of those for the two homopolymers despite almost 1:1 composition ratio. This is likely due to that the alternating two pendant groups interact 756

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ACS Macro Letters

Technology Agency (JST to M.O., JPMJPR13K2), Strategic International Collaborative Research Program (SICORP) from The French National Research Agency (ANR) and JST (M.O.), and KAKENHI Grant Number 15H03816 [Grant-inAid for Scientific Research (B) to M.O.].

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Figure 5. DSC profiles of cyclopolymer (a), the copolymer after ester cleavage (b), poly(acrylic acid) (c), and poly(2-hydroxyethyl vinyl ether) (d). The samples were heated from −80 to 150 °C at 10 °C/ min (second heating, see Supporting Information for the temperature program. The samples of the cyclopolymer and the copolymer are the same as the structural analyses. The sample of poly(acrylic acid) is a commercial product (Mn = 5000) and that of poly(2-hydroxyethyl vinyl ether) was synthesized (Mn = 19000, Mw = 30000, see Supporting Information).

with each other via hydrogen bonding to form quasi-cyclic repeating units. The result can be regarded as an alternating sequence effect but further study is required to clarify it in the future. In conclusion, a divinyl monomer consisting of acrylate and vinyl ether was designed to synthesize the copolymer beyond the reactivity ratios via the cyclopolymerization followed by cleavage of the cyclo ester pendant. Due to the defective cyclopropagation, sequence of the resultant copolymer was not perfectly controlled to alternating but the averaged composition ratio of the lower reactive monomer, that is, vinyl ether reached 45%. Cyclopolymerizations of such a linker-cleavable divinyl monomer would open the door to syntheses of unique copolymers as well as control of the alternating sequence.

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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.7b00368. Experimental details and some supporting results for the cyclopolymerizations of 1 and the structural analyses of the obtained polymers (Figures S1−5) (PDF).

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REFERENCES

(1) Braunecker, W. A.; Matyjaszewski, K. Controlled/living radical polymerization: Features, developments, and perspectives. Prog. Polym. Sci. 2007, 32 (1), 93−146. (2) Hirooka, M.; Yabuuchi, H.; Iseki, J.; Nakai, Y. Alternating Copolymerization through Complexes of Conjugated Vinyl Monomers-Alkylaluminum Halides. J. Polym. Sci., Part A-1: Polym. Chem. 1968, 6 (5), 1381−1396. (3) Satoh, K.; Matsuda, M.; Nagai, K.; Kamigaito, M. AAB-Sequence Living Radical Chain Copolymerization of Naturally Occurring Limonene with Maleimide: An End-to-End Sequence-Regulated Copolymer. J. Am. Chem. Soc. 2010, 132 (29), 10003−10005. (4) Lutz, J. F.; Ouchi, M.; Liu, D. R.; Sawamoto, M. SequenceControlled Polymers. Science 2013, 341, 1238149. (5) Hibi, Y.; Ouchi, M.; Sawamoto, M. Sequence-Regulated Radical Polymerization with a Metal-Templated Monomer: Repetitive ABA Sequence by Double Cyclopolymerization. Angew. Chem., Int. Ed. 2011, 50 (32), 7434−7437. (6) Hibi, Y.; Tokuoka, S.; Terashima, T.; Ouchi, M.; Sawamoto, M. Design of AB divinyl ″template monomers’’ toward alternating sequence control in metal-catalyzed living radical polymerization. Polym. Chem. 2011, 2 (2), 341−347. (7) Ouchi, M.; Nakano, M.; Nakanishi, T.; Sawamoto, M. Alternating Sequence Control for Carboxylic Acid and Hydroxy Pendant Groups by Controlled Radical Cyclopolymerization of a Divinyl Monomer Carrying a Cleavable Spacer. Angew. Chem., Int. Ed. 2016, 55 (47), 14584−14589. (8) Mayo, F. R.; Lewis, F. M.; Walling, C. Copolymerization 0.8. The Relation between Structure and Reactivity of Monomers in Copolymerization. J. Am. Chem. Soc. 1948, 70 (4), 1529−1533. (9) Zubov, V. P.; Valuev, L. I.; Kabanov, V. A.; Kargin, V. A. Effects of Complexing Agents in Radical Copolymerization. J. Polym. Sci., Part A-1: Polym. Chem. 1971, 9 (4), 833−854. (10) Kamigaito, M.; Sawamoto, M.; Higashimura, T. Living Cationic Polymerization of Isobutyl Vinyl Ether by Protonic Acid Zinc Halide Initiating Systems - Evidence for the Halogen Exchange with Zinc Halide in the Growing Species. Macromolecules 1992, 25 (10), 2587− 2591. (11) Georges, M. K.; Veregin, R. P. N.; Kazmaier, P. M.; Hamer, G. K. Narrow Molecular-Weight Resins by a Free-Radical Polymerization Process. Macromolecules 1993, 26 (11), 2987−2988. (12) Tesch, M.; Hepperle, J. A. M.; Klaasen, H.; Letzel, M.; Studer, A. Alternating Copolymerization by Nitroxide-Mediated Polymerizationand Subsequent Orthogonal Functionalization. Angew. Chem., Int. Ed. 2015, 54, 5054−5059. (13) Benoit, D.; Grimaldi, S.; Robin, S.; Finet, J. P.; Tordo, P.; Gnanou, Y. Kinetics and mechanism of controlled free-radical polymerization of styrene and n-butyl acrylate in the presence of an acyclic beta-phosphonylated nitroxide. J. Am. Chem. Soc. 2000, 122 (25), 5929−5939.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Makoto Ouchi: 0000-0003-4540-7827 Mitsuo Sawamoto: 0000-0003-0352-9666 Present Address §

Institute of Science and Technology Research, 1200 Matsumoto-cho, Kasugai, Aichi 487−8501, Japan.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We would like to thank Dr. Fumi Ariura of Arkema for providing the sample of BlocBuilder-MA. This work was partially supported by Precursory Research for Embryonic Science and Technology (PRESTO) from Japan Science and 757

DOI: 10.1021/acsmacrolett.7b00368 ACS Macro Lett. 2017, 6, 754−757