Abstract
Postpolymerization modifications of poly(2-methoxyethoxycarbonylmethylene) (pMEDA’) and poly(2-phenoxyethoxycarbonylmethylene) (pPEDA’) are described. The reactions of these polymers with mixtures of chlorotrimethylsilane (Me3SiCl) and lithium diisopropylamide (LDA) efficiently transformed the alkoxycarbonylmethylene repeating units to ketene silyl acetals to yield a product with up to 93 mol% composition of the latter unit. The ketene silyl acetal composition of the product was controlled by changing the feed ratio of Me3SiCl/LDA with respect to the alkoxycarbonylmethylene unit. Tetrabutylammonium fluoride (TBAF)-mediated benzylation of the highly silylated polymer with benzyl bromide yielded a polymer containing side chain O (major)- and main chain C (minor)-benzylated units along with the unreacted ketene silyl acetal unit.
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Introduction
C1 polymerization of diazoacetates is an effective method for preparing C–C main chain polymers in which an alkoxycarbonyl group (ester) is located on each main chain carbon atom [poly(alkoxycarbonylmethylene)s, pACMs] [1,2,3,4,5,6,7,8]. Polymers with high Mn values were obtained with Rh- and Pd-based initiating systems, and a variety of polymer structures have been realized with respect to tacticity [9,10,11,12,13,14,15,16], ester substituents [17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32], and polymeric architectures [33,34,35,36,37,38,39,40,41]. On the other hand, because the polymerizable monomer is limited to diazoacetates, thus far, all of the polymers obtained via C1 polymerization of diazocarbonyl compounds have H and an alkoxycarbonyl group on each main chain carbon atom. Because the replacement of H with other organic substituents is expected to result in improved physical properties such as a higher Tg, we have attempted to introduce alkyl groups (benzyl and methyl) to the main chain carbon atoms of poly(ethoxycarbonylmethylene) [pEDA’ (EDA = ethyl diazoacetate)] and poly(benzyloxycarbonylmethylene [pBDA’ (BDA = benzyl diazoacetate)] via the formation of a ketene silyl acetals by reacting the polymers with a mixture of chlorotrimethylsilane (Me3SiCl) and lithium diisopropylamide (LDA), followed by tetrabutylammonium fluoride (TBAF)-mediated substitution with electrophiles such as benzyl bromide (BnBr) and methyl iodide (MeI) [Scheme 1(a)] [42]. For example, after postpolymerization modifications of pEDA’ and pBDA’, 26 and 52 mol% of their repeating units were silylated, followed by 7.4 mol% benzylation and 13 mol% methylation of the main chain carbon atoms, respectively, as reported in our previous publication [42].
Postpolymerization modifications of poly(alkoxycarbonylmethylene)s
These results indicated the potential of ketene silyl acetal formation as a method for postpolymerization modification of pACMs to afford C–C main chain polymers with unprecedented chemical structures. In this paper, we applied the sequence of silylation and alkylation to poly(2-methoxyethoxycarbonylmethylene) [pMEDA’ (MEDA = 2-methoxyethyl diazoacetate)] [20] and poly(2-phenoxyethoxycarbonylmethylene) [pPEDA’ (PEDA = 2-phenoxyethyl diazoacetate)], expecting that the chelation effect derived from the presence of the additional methoxy or phenoxy side chain terminal would enhance the reactivities of the postpolymerization modification processes [Scheme 1(b)]. Indeed, we observed that the silylation of pMEDA’ and pPEDA’ proceeded much more efficiently than that of pEDA’ and pBDA’, resulting in up to 93 mol% incorporation of ketene silyl acetal units into these polymer main chains. In addition, more than 50 mol% of the ketene silyl acetal units were benzylated, although benzylation unexpectedly occurred predominantly on the silylated acetal oxygen, affording repeating units with O-benzylated ketene acetals in the main chain. The unique reactivity of pMEDA’ and pPEDA’ in these postpolymerization modifications could be utilized for the syntheses of new polymer structures based on pACM frameworks.
Results and Discussion
Postpolymerization modification of pMEDA’ and pPEDA’ with Me3SiCl/LDA: efficient conversion of the repeating unit to a ketene silyl acetal framework
First, we tried to transform the repeating units of pMEDA’ into ketene silyl acetals by reacting the precursor polymer with a mixture of Me3SiCl and LDA ([Me3SiCl]/[LDA] = 2:1) under the same conditions employed for the silylation of pEDA’ and pBDA’ in our previous publication [42] (Scheme 2); the results are summarized in Table 1. The precursor pMEDA’s were prepared by C1 polymerization of MEDA initiated by the π-allylPdCl/NaBPh4 system [20, 43] used in our standard polymerization procedure.
Preparation of the ketene silyl acetal unit-containing polymer
As shown in run 1, with the use of a large excess of LDA (10 eq. with respect to the MEDA’ repeating unit), the product exhibited 90 mol% incorporation of the ketene silyl acetal unit, which was much greater than those observed with pEDA’ and pBDA’ under the same condition as shown in a comparison of runs 1 to 3. The highest incorporation of silyl groups was apparent in the 1H NMR spectrum of the product in Fig. 1, where a large broad signal assignable to the Me3Si groups appeared at approximately 0 ppm; the composition was calculated from the ratio of the Me3Si peak intensity to those of other signals except the peak intensities of contaminants such as Me4Si and silicone grease. The origins of the unexpected broad signals at 5.1 and 5.6 ppm will be discussed later. The apparent enhancement in the reactivity of the repeating unit toward Me3SiCl/LDA was ascribed to the presence of an additional oxygen in the side-chain terminal MeO-group, which enabled chelation of the side chains with the Li+ in LDA together with its neighboring side chain, as illustrated in Scheme 3. The resulting somewhat naked diisopropylamide moiety should become more reactive, as commonly observed for crown-ether-type chelating agents, leading to more effective silylation of the pMEDA’ repeating units (Scheme 3, path A).
1H NMR spectra of pMEDA’ (upper) and the product of silylation (lower) (run 1 in Table 1)
Enhanced reactivity of LDA derived from chelation by the side chains leading to ketene silyl acetal formation and main chain cleavage
On the other hand, compared to those of pEDA’ and pBDA’, the yield of the highly silylated polymer was significantly lower (29%). The low yield was ascribed to the formation of low Mn oligomers via main chain cleavage during the silylation, which were removed by preparative SEC purification of the polymer sample. These results indicated that in addition to the above-described increase in the silylation reactivity, efficient generation of the intermediate lithium enolate framework promoted main chain cleavage via a retro-Michael addition reaction, as shown in Scheme 3, path B [42].
We found that the extent of ketene silyl acetal incorporation could be controlled by changing the relative amount of Me3SiCl/LDA with respect to the MEDA’ repeating unit. As shown in runs 4 to 7, as the amounts of the reagents employed were decreased, the amount of ketene silyl acetal incorporated decreased to 3.0 mol%. Considering the increased formula weight of the repeating unit from 116.12 to 188.30 with silylation, the almost constant Mn values for the silylated products indicated less main chain cleavage with decreasing amounts of Me3SiCl/LDA.
With the use of a Ph group instead of Me at the side chain terminal in pPEDA’ (run 8), a much greater yield (51%) of the highly silylated polymer (93 mol% silylation) with Mn (9800) close to that of the precursor pPEDA’ was achieved under the same conditions. These results indicated that the main chain cleavage was significantly suppressed, probably because of the steric effect of the side chain Ph groups.
Characterization of the ketene silyl acetal repeating unit
Although the appearance of a large broad signal at approximately 0 ppm in the 1H NMR spectrum of the silylated polymer indicated the formation of the expected ketene silyl acetal in the polymer main chain, the two broad signals at 5.1 and 5.6 ppm in the spectrum could not be attributed to the expected structure; these signals were not observed in the case of silylated pEDA’ and pBDA’ in our previous publication [42]. To clarify the origins of the two broad signals, we conducted a 1H-13C HSQC NMR analysis of a silylated sample (86 mol% silylation, Mn = 11900, Đ = 1.78). As a result, the two broad 1H NMR signals were correlated with the 13C NMR signal at 116 ppm, which was subsequently identified as a CH2 carbon atom by DEPT135 measurement (Fig. 2A and Figure S4 in the SI). Accordingly, these two broad signals were assigned to -OCH2CH2O-, which was the only CH2-containing group in both the original and silylated repeating units. Our assumption is that the C=C π-electrons of the ketene silyl acetal caused downfield shifts in the signals of some Hs in their proximity to each other; another possible explanation for the downfield shift is that some of the LiCl generated during ketene silyl acetal formation remained among the methoxyethyl side chains due to strong chelating interactions, thus affecting the chemical shifts of the CH2 signals.
A 1H-13C HSQC NMR spectrum of silylated pMEDA’ and B FT-IR spectra of the silylated pMEDA’ and its precursor
Additional evidence for ketene silyl acetal formation was obtained with FT-IR measurements. As shown by the comparison of the IR spectra in Fig. 2B for the precursor (Mn = 9900, Đ = 1.59) and the silylated polymer sample (86% silylation, Mn = 11900, Đ = 1.78), a strong absorption appeared at approximately 1600 cm–1, which was assigned to the stretching vibrations of conjugated C=C bonds. The strong absorption indicated that because of the high proportion of silylation, most of the C=C bonds in the polymer were conjugated with the C=Cs of the adjacent ketene silyl acetal units.
Reaction of the ketene silyl acetal units with benzyl bromide in the presence of TBAF
In our previous publication, some of the ketene silyl acetal units incorporated in pEDA’ were reacted with BnBr in the presence of TBAF to afford quaternary main chain carbon atoms bearing both alkoxycarbonyl and benzyl groups [42]. Compared to the silylated pEDA’ (25% silylated polymer leading to 7.4% benzylation) employed for the previous benzylation attempts, we expected that the greater silylation level of pMEDA’s would result in greater benzylation. The benzylation of 90 mol% silylated pMEDA’ (Mn = 10600, Đ = 1.87) was conducted with 3 eq. (with respect to the ketene silyl acetal units) of TBAF and 9 eq. of BnBr at 50 °C in THF for 18 h. As shown in Fig. 3, the 1H NMR spectrum of the product (Mn = 7500, Đ = 1.51, yield = 67% based on the assumption that 54 mol% of the ketene silyl acetal units were benzylated, as described below) exhibited a broad signal in the aromatic region, which indicated the incorporation of benzyl groups into the product. On the other hand, there was a broad signal at 0 ppm, indicating the presence of unreacted ketene silyl acetal units. More importantly, the intensity of the CH2 signal at 5.1 ppm relative to that at 5.6 ppm had apparently increased, suggesting that benzylation had occurred primarily at the acetal oxygen sites via substitution of Me3Si by the benzyl group; the initially expected main chain benzylation process was a minor modification mode (Scheme 4). The preference for O-benzylation over C-benzylation was ascribed to the higher hydrophilicity of the polymer main chain imparted by the hydrophilic MeOCH2CH2O side chains, which prevented the hydrophobic BnBr from approaching the main chain carbon atoms as illustrated in Scheme 5. On the basis of the relative signal intensities in the spectrum, the ratio of [unreacted ketene silyl acetal]/[ketene acetal (O-benzylated unit)]/[main chain C-benzylated unit] was 46:47:7 (it was not possible to accurately determine the composition of the original MEDA’ unit). Even though benzylation did not proceed exclusively on the main chain, the reaction afforded an unprecedented C–C main chain polymer structure containing significant amounts of ketene silyl acetal and ketene acetal units.
1H NMR spectra of the polymer before (upper) and after (lower) benzylation
Benzylation of the ketene silyl acetal units
Preferred O-benzylation over C-benzylation
Reactivity of the ketene silyl acetal unit toward H+/TBAF
The unexpected benzylation of the ketene silyl acetal units prompted us to examine the reactivities of the silylated polymers toward H+/TBAF in converting them to their pMEDA’ precursors; for silylated pEDA’ and pBDA’, this reaction proceeded as expected. After silylation, pMEDA’ samples with silylation proportions of 24 mol% (run 6 in Table 1, Mn = 7500, Đ = 1.54) and 3 mol% (run 7 in Table 1, Mn = 9100, Đ = 1.63) were reacted with HCl aq. (1.0 N, 9.0 eq. to the silylated repeating unit) in the presence of TBAF (3 eq.), and pMEDA’ was obtained (Mn = 6000, Đ = 1.82, 44% yield, and Mn = 11400, Đ = 1.59, 65% yield; see Figure S5 for the 1H NMR spectra), providing indirect evidence for the presence of ketene silyl acetals in the starting silylated polymers (Scheme 6). On the other hand, protonation of a pMEDA’ sample with a much greater silylation percentage of 79 mol% (Mn = 9500, Đ = 1.60) under similar conditions yielded low Mn oligomers (Mn < 600, yield = 83%), suggesting that the high content of the ketene silyl acetal unit resulted in the main chain cleavage upon treatment with H+/TBAF.
Protonation of the ketene silyl acetal unit
Benzylation without a ketene silyl acetal intermediate
Although the original objective in this study was to alkylate the main chain carbon atoms through ketene silyl acetal intermediates, the results described above revealed that the transformation leading to main chain alkylation did not proceed effectively for pMEDA’. Then, we attempted to examine alkylation of the main chain via reaction with BnBr/LDA, expecting that the reactivity would be enhanced by the presence of the -OCH2CH2O- side chains as in the aforementioned ketene silyl acetal formation.
The reactions of pMEDA’ (Mn = 9900, Đ = 1.60) with LDA (10 eq. relative to the repeating unit) and BnBr (20 eq.) in THF at 25 °C for 1 h resulted in benzylation with 9 mol% main chain alkylation and 2 mol% O-alkylation based on the relative intensities of the signals in the 1H NMR spectra of the product (Mn = 6600, Đ = 1.60, yield = 50%; see Figure S6 for the 1H NMR spectrum) (Scheme 7). Although the incorporation was not effective, the results indicated that direct benzylation allowed the preparation of benzylated polymers without the need for the ketene silyl acetal intermediate.
Direct benzylation of pMEDA’
Conclusions
We demonstrated that the repeating units of pMEDA’ and pPEDA’ were efficiently transformed into ketene silyl acetal units via reactions with Me3SiCl/LDA. Compared to those of pEDA’ and pBDA’, the reactivities of the pMEDA’ and pPEDA’ repeating units were higher due to the presence of the additional oxygen atom at the side chain terminal, which resulted in higher silylation proportions of up to 93 mol%. Notably, pACMs with such high ketene silyl acetal contents were obtained via a relatively simple procedure. Although TBAF-mediated benzylation of the silylated polymer did not proceed as far as anticipated, the unique repeating unit with the ketene silyl acetal framework could be utilized for postpolymerization modification or other applications of the pACMs because the ketene silyl acetals can be activated by not only various bases in addition to TBAF but also acids, leading to a variety of chemical structures [44,45,46].
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Acknowledgements
This work was supported by JSPS KAKENHI (Grant Numbers JP18H02021, JP19K05586, JP19K22219, JP21H01988, and JP22K05219). The authors thank the Advanced Research Support Center (ADRES) at Ehime University for its assistance in NMR measurements and elemental analyses.
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Shimomoto, H., Inouchi, S., Itoh, T. et al. Postpolymerization modification of poly(2-alkoxyethoxycarbonylmethylene)s: Efficient formation and reactivity of the ketene silyl acetal repeating units in the polymer main chain. Polym J (2024). https://doi.org/10.1038/s41428-024-00891-z
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DOI: https://doi.org/10.1038/s41428-024-00891-z
