Letter Cite This: ACS Macro Lett. 2018, 7, 997−1002
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Acrylate-Selective Transesterification of Methacrylate/Acrylate Copolymers: Postfunctionalization with Common Acrylates and Alcohols Daiki Ito,†,‡ Yusuke Ogura,†,‡ Mitsuo Sawamoto,†,§ and Takaya Terashima*,† †
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Department of Polymer Chemistry, Graduate School of Engineering, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan § Institute of Science and Technology Research, Chubu University, 1200 Matsumoto-cho, Kasugai, Aichi 487-8501, Japan S Supporting Information *
ABSTRACT: Acrylate-selective transesterification of methacrylate/acrylate copolymers with alcohols was developed for a site-selective postfunctionalization technique of polymers without using specific monomers. Importantly, a common methyl acrylate efficiently works as a selective modification unit via transesterification coupled with a titanium alkoxide catalyst. The acrylate-selective transesterification is achieved owing to less steric hindrance of the carbonyl groups that are attached to the main chain without an α-methyl group. Typically, the acrylate pendants of dodecyl methacrylate/methyl acrylate (MA) random copolymers were selectively transesterified with benzyl alcohol (BzOH). The conversion of the pendent esters into benzyl esters proportionally increased with MA contents. Additionally, various alcohols were applicable to this MAselective transesterification system.
P
the pendent esters are intact, to give chlorine-capped telechelic polymethacrylates with terminal esters derived from the alcohols.19 In contrast, the pendent esters of homopolyacrylates were partially transesterified with similar conditions.21,34 These results imply that the polymethacrylate backbones bearing α-methyl groups sterically hinder the Ti catalyst from accessing and activating the pendant carbonyl groups. Given such a proposed reason, we noticed the possibility that the acrylate pendants of methacrylate/acrylate copolymers could be selectively transesterified because of the less steric hindrance of the carbonyl groups that are attached in the main chains without α-methyl groups. Herein, we report acrylate-selective transesterification of methacrylate/acrylate random copolymers coupled with Ti(Oi-Pr)4 and various alcohols (Scheme 1). This system efficiently realizes site-selective transformation of copolymers using a common methyl acrylate (MA) as a transforming unit, although efficient postfunctionalization of poly(meth)acrylates generally requires activated ester monomers such as pentafluorophenyl or N-hydroxysuccinimide (meth)acrylates.5,6,14−16 This is the first example of acrylate-selective transesterification of methacrylate/acrylate copolymers with common metal alkoxide catalysts, while a similar site-selective transesterification of those copolymers with an amine-based organocatalyst was recently reported.22 For example, the MA
olymer reaction and postfunctionalization play important roles to efficiently produce functional polymers with welldefined structures and distinct properties in laboratory and industry.1−24 As a synthetic strategy of polymers, postmodification involves the following advantages: (1) easy modulation of physical properties and functions of common polymers into diverse functional polymer materials with distinct properties; (2) introduction of functional units that are not compatible with polymerization systems. Recently, siteselective postfunctionalization methods of polymers have been developed using efficient organic reactions (e.g., reactions with azide/alkyne,7−10 thiol/ene,11−13 activated esters/alcohols or amine,14−16 and alcohol/isocyanate17,18). In particular, the combination of such postfunctionalization and precision polymerization (e.g., living radical polymerization: LRP25−27) is regarded as a powerful strategy to design functional polymers with controlled primary structures and complex architectures. Transesterification draws much attention as one of the efficient and versatile organic reactions to produce ester derivatives and polymer materials owing to the wide applicability of the starting ester compounds and alcohols.16,19−22,28−40 So far, we have originally developed efficient synthetic systems of gradient, telechelic, and pinpointfunctionalized polymers by metal alkoxide [e.g., Ti(Oi-Pr)4]catalyzed transesterification coupled with LRP.19,33−37 In metal alkoxide-mediated transesterification, reactivity and selectivity of ester compounds (units) highly depend on the steric hindrance around the carbonyl groups.19,28,29,34 For example, the terminal esters of chlorine-capped poly(methacrylate)s are selectively transesterified with alcohols and Ti(Oi-Pr)4, while © XXXX American Chemical Society
Received: July 4, 2018 Accepted: July 26, 2018
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DOI: 10.1021/acsmacrolett.8b00502 ACS Macro Lett. 2018, 7, 997−1002
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ACS Macro Letters
efficiently transesterified into corresponding benzyl esters, whereas isopropyl or tertiary butyl esters (IPr, TPr) were not so transesterified.29,34 In detail, the reactivity of methyl or ethyl esters further depended on the R1 structure. Conversion of these substrates decreased in this order: Conv. = 59% (MPr, EPr), 44% (MI), and 29% (MPi). These results indicate that the steric hindrance of both substituents around carbonyl groups affects the reactivity in transesterification: the reactivity decreased with increasing the steric hindrance. MI and MPi are regarded as model compounds of methyl acrylate (MA) and methyl methacrylate (MMA) units built in (co)polymers, respectively. Thus, reactivity of MI larger than that of MPi suggests the possibility of acrylate-selective transesterification of methacrylate/acrylate copolymers. We thus synthesized a MA homopolymer (P1), a MMA homopolymer (P2), and methacrylate/acrylate copolymers (P3−P9) as substrates by Ru-catalyzed living radical (co)polymerization (Scheme 1). The copolymers consist of different monomers (MMA, MA, DMA, dodecyl acrylate: DA) and composition (molar ratio): P3 (DMA/MA = 34/66), P4 (DMA/MA = 53/47), P5 (DMA/MA = 71/29), P6 (MMA/DA = 47/53), P7 (MMA/MA = 35/65), P8 (MMA/ MA = 55/45), P9 (MMA/MA = 75/25). The dodecyl-bearing copolymers (P3−P6) are designed for efficient and convenient characterization of acrylate-selective transesterification by 1H NMR spectroscopy. Typically, DMA and MA were smoothly copolymerized with RuCp*Cl(PPh3)2/n-Bu3N and a bromide initiator [H-(MMA)2-Br] in toluene at 80 °C to produce wellcontrolled DMA/MA copolymers with narrow molecular weight distribution [P3−P5: Mn = 7200−15700, Mw/Mn = 1.13, calculated degree of polymerization: 36−72]. The characterization of P1−P9 is summarized in Table S1. Transesterification of PMA (P1: Mn = 9800, Mw/Mn = 1.21) and PMMA (P2: Mn = 11500, Mw/Mn = 1.20) was conducted with Ti(Oi-Pr)4 in anisole/1-dodecanol (1/1, v/v) at 130 °C in the presence of molecular sieves (MS) 4A to investigate the effect of α-methyl groups of the polymer backbone on transesterification of the pendants. Confirmed by 1H NMR, the methyl esters of P1 was efficiently transformed into dodecyl esters to give a poly(dodecyl acrylate) slightly containing unreacted MA units (96% conversion in 48 h, Figure S3). Analyzed by size exclusion chromatography (SEC), the product had molecular weight larger than the original P1 and kept narrow molecular weight distribution (Mn = 19900, Mw/Mn = 1.24). In contrast, the methyl esters of PMMA were hardly transesterified (5% conversion in 48 h, Figure S3). These results indicate that polymer backbones bearing αmethyl groups sterically hinder the pendent esters from being transesterified. Given these results, we investigated the transesterification of the pendants of DMA/MA copolymers (P3−P5) using Ti(OiPr)4 and benzyl alcohol (BzOH). P4 (Mn = 11600, Mw/Mn = 1.13, DMA/MA = 53/47, [polymer chain]0 = 10 mM) was treated with Ti(Oi-Pr)4 (160 mM) and MS 4A (0.33 g/mL) in anisole/BzOH (1/1, v/v, [BzOH]0 = 4800 mM) at 120 °C for 48 h (Table 1, entry 6). The product was analyzed by 1H NMR (Figure 2a,b). The methoxy protons of MA units (b: 3.7−3.5 ppm) clearly decreased, whereas the methylene and phenyl protons of benzyl esters (c, c′: 5.1−4.8 ppm, d, d′: 7.4− 7.2 ppm) newly appeared. The methylene proton signal of DMA units (a: 4.1−3.8 ppm) was changed by introducing benzyl units: The shoulder peak at 4.0 ppm observed in P4 disappeared and another shoulder peak in turn appeared at 3.8
Scheme 1. Acrylate-Selective Transesterification of Methacrylate/Methyl Acrylate (R1MA/MA) Copolymers with Alcohols (R2OH) into R1MA/R2A Copolymers
units of dodecyl methacrylate (DMA)/MA copolymers were more selectively transesterified with Ti(Oi-Pr)4 and alcohols (R2OH) into corresponding ester pendants (−COOR2) than the DMA units. Simple yet versatile Ti-mediated transesterification of common methacrylate/acrylate copolymers would open new vistas in postfunctionalization and polymer reactions applicable to various research fields. Reactivity of model ester compounds related to (meth)acrylate units of copolymers was first examined in metal alkoxide-mediated transesterification, focused on the steric effects of the substrates around the carbonyl groups on reactivity. Six ester compounds (R1COOR2) were treated with Ti(Oi-Pr)4 (catalyst) in toluene/benzyl alcohol (BzOH) (1/1, v/v) at 80 °C (Figures 1, S1, and S2). The ester substrates consist of different substituents (R1 and R2): methyl propionate (MPr), methyl isobutyrate (MI), methyl pivalate (MPi), ethyl propionate (EPr), isopropyl propionate (IPr), and tert-butyl propionate (TPr). Confirmed by 1H NMR spectroscopy, methyl or ethyl esters (MPr, MI, MPi, EPr) were
Figure 1. Transesterification of ester compounds [(a) methyl propionate, MPr; methyl isobutyrate, MI; methyl pivalate, MPi; (b) ethyl propionate, EPr; isopropyl propionate, IPr; t-butyl propionate, TPr] with Ti(Oi-Pr)4 and benzyl alcohol (BzOH): [substrate]0/ [Ti(Oi-Pr)4]0 = 2000/10 mM in toluene/BzOH (1/1, v/v) at 80 °C ([BzOH]0 = 3300−3800 mM). 998
DOI: 10.1021/acsmacrolett.8b00502 ACS Macro Lett. 2018, 7, 997−1002
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ACS Macro Letters Table 1. Acrylate-Selective Transesterification of Copolymersa entry
polymer
[Ti]0 (mM)
temp (°C)
R2OH
M/A/R2 M/R2A compositionb (%)
Mnc
Mw/Mnc
selectivityd (%)
1 2 3 4 5 6 7 8 9 10 11 12
P3 P3 P3 P4 P4 P4 P4 P5 P5 P5 P5 P6
80 160 240 160 160 160 160 160 160 160 160 160
120 120 120 100 110 120 130 120 120 120 120 120
a a a a a a a a b c d a
24/10/10/56 20/7/14/59 20/6/14/60 50/20/3/27 49/18/4/29 42/11/11/36 36/5/17/42 63/11/8/18 65/12/6/17 70/8/1/21 71/19/0/10 41/25/6/28
7300 7100 7000 11400 11300 11400 11100 15700 16400 17600 16700 10300
1.12 1.12 1.13 1.11 1.12 1.12 1.12 1.13 1.11 1.10 1.13 1.12
85 81 81 90 88 77 71 69 74 96 >99 82
a
Conditions: [P3, P4, P5, P6]0/[Ti(Oi-Pr)4]0 = 10/80, 160, and 240 mM with MS 4A (entries 1−11) or MS 13X (entry 12) (0.33 g/mL) in anisole/R2OH (1/1, v/v) at 100−130 °C for 48 h. R2OH: benzyl alcohol (a), 2-(2-methoxyethoxy)ethanol (b), 4,4,5,5,5-pentafluoropentanol (c), and 2-naphtharenemethanol (d). P3: Mn = 7200, Mw/Mn = 1.13. P4: Mn = 11600, Mw/Mn = 1.13. P5: Mn = 15700, Mw/Mn = 1.13. P6: Mn = 11800, Mw/Mn = 1.11. bCopolymer composition of M (DMA or MMA), A (MA or DA), R2 M (methacrylate), and R2A (acrylate) determined by 1 H NMR. cDetermined by SEC in THF with PMMA standard calibration. dSelectivity defined as R2A (acrylate) content of transesterified units in copolymers: 100 × R2A/(R2M + R2A).
weight distribution (Mn = 11400, Mw/Mn = 1.12) after transesterification (Table 1, entry 6). Thus, it was revealed that acrylate-selective transesterification of a DMA/MA random copolymer was achieved with Ti(Oi-Pr)4 and BzOH without any side reactions. Transesterification of DMA/MA copolymers with different MA contents (P3: MA = 66%, Mn = 7200, Mw/Mn = 1.13, P5: MA = 29%, Mn = 15700, Mw/Mn = 1.13) was carried out with Ti(Oi-Pr)4 and BzOH at 120 °C under the same conditions as that of P4 (Table 1, entries 2 and 8). The content of benzyl ester units in products proportionally increased from 26% to 73% with increasing MA contents of the substrates (Figures 3 and S4). This supports the transesterification occurs selectively for MA units. Actually, BzA content in the benzyl ester units of products was ranged between 69% and 81%.
Figure 2. Transesterification of a DMA/MA random copolymer (P4) with Ti(Oi-Pr)4 and BzOH: [P4]0/[Ti(Oi-Pr)4]0 = 10/160 mM with MS 4A (0.33 g/mL) in anisole/BzOH (1/1, v/v) at 120 °C. 1H NMR spectra (left) and SEC curves (right) of (a) P4, (b) the product, and (c) DMA/benzyl acrylate (BzA) copolymer (DMA/BzA: 49/51).
ppm, while the peak at 3.9 ppm was intact. The proton signal of the DMA units was close to that of DMA units of a model DMA/benzyl acrylate copolymer (Figure 2c). The conversion of the pendent esters into benzyl esters [benzyl (meth)acrylate units] was determined to be 47% from the area ratio of the pendant protons. The composition of DMA, MA, benzyl methacrylate (BzMA), and benzyl acrylate (BzA) was estimated as 42%/11%/11%/36%, assuming that the consumption of DMA and MA corresponds to the yield of BzMA and BzA, respectively. If transesterification of pendent esters takes place without selectivity between acrylate and methacrylate units, BzA content in benzyl ester units [100BzA/ (BzMA + BzA)] corresponds to the original MA content in P4 (47%). However, the BzA content defined as acrylate selectivity in transesterification was 77%, meaning that the transesterification is 1.6 times more selective for MA units against DMA units. The product maintained narrow molecular
Figure 3. Benzyl esters in products transesterified with benzyl alcohol as a function of acrylate contents of precursor copolymers (P3−P5: DMA/MA, P6: MMA/DA, P7−P9: MMA/MA). [polymer]0/[Ti(Oi-Pr)4]0 = 10/160 mM with MS 4A or 13X (0.33 g/mL) in anisole/BzOH (1/1, v/v) at 120 °C for 48 h.
Other methacrylate/acrylate copolymers (P6−P9) were also employed for transesterification with Ti(Oi-Pr)4 (160 mM) in anisole/BzOH (1/1, v/v) at 120 °C for 48 h to examine the effects of the ester pendants on reactivity and selectivity. The dodecyl acrylate units of P6 (DA = 53%, Mn = 11800, Mw/Mn = 1.11) were also transesterified to result in almost the same selectivity of BzA (82%) as that with DMA/MA copolymers 999
DOI: 10.1021/acsmacrolett.8b00502 ACS Macro Lett. 2018, 7, 997−1002
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ACS Macro Letters
Figure 4. Effects of temperature, catalyst concentration, and alcohols on transesterification of DMA/MA random copolymers (P3, P4, and P5) with Ti(Oi-Pr)4 and alcohols (R2OH): [polymer]0/[Ti(Oi-Pr)4]0 = 10/80, 160, and 240 mM with MS 4A (0.33 g/mL) in anisole/R2OH (1/1, v/ v) at 100−130 °C. R2OH: benzyl alcohol (a), 2-(2-methoxyethoxy)ethanol (b), 4,4,5,5,5-pentafluoropentanol (c), and 2-naphtharenemethanol (d).
(Figure S5). This result importantly demonstrates that selectivity of acrylate units in transesterification is controlled by the α-methyl groups of the backbones. The conversion of P6 (34%) was lower than that of a corresponding DMA/MA copolymer (P4: MA = 47%, Conv. = 47%). This is probably due to low efficiency to remove 1-dodecanol generating from P6 during transesterification. Acrylate-selective transesterification of MMA/MA copolymers (P7−P9: Mn = 5500−7800, Mw/Mn = 1.13−1.17) was also confirmed by 1H NMR (Figures S6 and S7). The conversion into benzyl esters increased with increasing MA units (Figure 3), while the benzyl ester contents were larger than the MA contents due to partial transformation of MMA units (Figure S7). In P8, all the MA units are transformed into benzyl esters. The methacrylate units placed in methacrylate/acrylate copolymers are partially transesterified although MMA homopolymers are hardly transesterified. This is because methacrylate units located beside or between acrylate units are less sterically hindered than sequenced methacrylate units of MMA homopolymers. Transesterification of P3−P5 was systematically investigated by changing Ti catalyst concentration, temperature, and alcohols (Table 1, Figure 4). MA-selective transesterification of the copolymers was achieved under all conditions examined in Table 1. However, conversion into benzyl esters and acrylate selectivity depended on the reaction conditions. In P4 (MA: 47%), the conversion increased from 30% to 59% with increasing temperature from 100 to 130 °C. Transesterification was selective for acrylate even at 130 °C (high temperature) although the selectivity decreased from 90% to 71%. In P3, the conversion slightly increased from 66% to 74% with increasing
Ti concentration (80, 160, and 240 mM), while acrylate selectivity was independent of the concentration (∼80%). Various alcohols (R2OH) were also available for the MAselective transesterification of P5 with a Ti catalyst: benzyl alcohol, 2-(2-methoxyethoxy)ethanol, 4,4,5,5,5-pentafluoropentanol, and 2-naphtharenemethanol (Figure S8), as reported in the synthesis of gradient, end, and pinpoint-functionalized (co)polymers via titanium-catalyzed transesterification.19,33−37 In conclusion, we have successfully developed acrylateselective transesterification of methacrylate/acrylate random copolymers as a novel postfunctionalization technique using a titanium alkoxide catalyst and alcohols. Importantly, this system affords efficient and site-selective transformation of the copolymers just using a common acrylate as a transforming segment although efficient postfunctionalization of poly(meth)acrylates often requires specific monomers typically bearing activated esters. The selective transesterification of acrylate units is achieved owing to less steric hindrance of the carbonyl groups that attached to polyacrylate backbones without α-methyl groups. Thus, the acrylate-selective transesterification systems developed herein would open new avenue to create various functional polymers via postmodification coupled with common copolymers and diverse alcohols, far more conveniently than conventional postfunctionalization methodologies. 1000
DOI: 10.1021/acsmacrolett.8b00502 ACS Macro Lett. 2018, 7, 997−1002
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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.8b00502. Experimental details, characterization by SEC, 1H NMR (PDF).
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Mitsuo Sawamoto: 0000-0003-0352-9666 Takaya Terashima: 0000-0002-9917-8049 Author Contributions ‡
These authors contributed equally.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by JSPS through Grants-in-Aid for Scientific Research KAKENHI Grant Numbers JP26410134, JP17H03066, and JP17K19159, by The Mazda Foundation, by The Sumitomo Electric Group Social Contribution Foundation, by The Ogasawara Foundation for the Promotion of Science and Engineering, and by The Noguchi Institute.
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