N-Heterocyclic Carbene Initiated Anionic Polymerization of (E,E

Oct 9, 2017 - Department of Life Science and Applied Chemistry, Graduate School of Engineering, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, ...
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Cite This: J. Am. Chem. Soc. 2017, 139, 15005-15012

N‑Heterocyclic Carbene Initiated Anionic Polymerization of (E,E)‑Methyl Sorbate and Subsequent Ring-Closing to Cyclic Poly(alkyl sorbate) Yuhei Hosoi, Akinori Takasu,* Shin-ichi Matsuoka, and Mikihiro Hayashi Department of Life Science and Applied Chemistry, Graduate School of Engineering, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466-8555, Japan S Supporting Information *

ABSTRACT: A diene-based cyclic polymer has been synthesized by the anionic polymerization of methyl sorbate (MS) by an N-heterocyclic carbene (NHC) in the presence of a bulky aluminum Lewis acid. We first polymerized methyl sorbate (MS) initiated by NHC in N,N-dimethylformamide (DMF) at 25 °C, poly(MS) with a number-average molecular weight (Mn) of 3.5 × 103 (Mw/Mn = 2.1) was obtained with a conversion of 93%. The structure was confirmed by 1H and 13 C NMR and IR spectra, which revealed that the propagation proceeded via 1,2-addition as well as 1,4-addition. Although the polymerization did not occur in toluene in the absence of any additive, quantitative monomer consumption was observed in the presence of methylaluminum bis(2,6-di-tert-butyl-4methylphenoxide) (MAD) to afford the poly(MS) with a 1,4-trans structure, 86% of threo diastereoselectivity, and a Mn of 23.0 × 103 with narrow molecular weight distribution (Mw/Mn = 1.17). From the matrix assisted laser desorption/ionization (MALDITOF) mass spectra of poly(MS) and the hydrogenated analogue, ring-closing occurred by nucleophilic attack of the anionic propagating center into the adjacent carbon of the α-terminal imidazolimium group to afford cyclic poly(MS). The cyclic formation in the present synthesis system was confirmed by DSC and viscosity measurements.

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INTRODUCTION A variety of polymerization of conjugated dienes have been investigated for a long time, as the resulting polymers have been industrially important thermoplastic resins. However, in contrast to the chain polymerization of vinyl monomers, it remains more difficult to control the molecular weight, regioselectivity, and stereochemistry. Furthermore, the relationship between a polymer’s architecture and its various properties has been one of the most important subjects of research, and worthy of investigations for both theoretical study and practical applications. Therefore, the diene based cyclic polymers, termed as polymer rings or macrocycles, should present a unique challenge as species with special characteristics because of the absence of polymer termini. Many cyclic polymers1,2 are synthesized via ring-closing and ring-expansion polymerization3 involving well-known ring-opening metathesis polymerization (ROMP),4 in which the latter is predominant because the former procedure suffers from contamination of the corresponding linear polymers and the purification is tedious. Moreover, there are only a few reports dealing with synthetic approaches to vinyl monomers as well as dienes-based cyclic polymers. Therefore, the exploration of new synthetic procedures to synthesize diene-based cyclic polymers, particularly by a ring-closing procedure, is a still challenging subject. Previously, we have reported the regioselective (1,4-addition) anionic polymerizations of polar diene monomers, i.e., methyl © 2017 American Chemical Society

2-methyl-2,4-hexadienoate (MMHd) and methyl-2,4-dimethyl2,4-pentadienoate (MDMPd).5 The hydrogenation of the obtained polymers allows the first synthesis of head-to-head (H−H) and head-to-tail (H−T) poly(propylene-alt-methyl methacrylate)s, respectively.6 We also performed a stereospecific polymerization of sorbic acid derivatives assisted by bulky aluminum Lewis acid, methylaluminum bis(2,6-di-tertbutyl-4-methylphenoxide) (MAD), and the polymerization mechanism, in which the polymerization proceeded via threo (diastereoselective)7-disyndiotactic (stereospecific)8 polymerization (Figure 1). However, the initiator efficiency was very low (99 >99 0 99 99

2.9 3.5 2.4 3.4 4.2 2.2 1.1 1.5 28.7 5.2

1.2 2.1 1.8 1.7 2.0 1.6 2.0 5.0 1.37 1.28

95 93 88 87 96 80 79 77 100 100

23.0 4.5

1.17 1.31

100 100

a

Feed molar ratio of methyl sorbate to NHCtBu. b[NHCtBu]0/[additive]0 = 1/3. cMonomer conversions measured by the 1H NMR. dDetermined by SEC in CHCl3 using polystyrene standards. eMethylaluminum bis(2,6-di-tert-butyl-4-methylphenoxide) (MAD).

(geometric structure was not regulated).20 In the report, methine carbons of CHCH3 (40.34 and 40.54 ppm) and CHCOOCH3 (55.71 and 56.46 ppm) in the trans structure were distinguishable from those in the cis structure [CHCH3 (35.48 ppm), CHCOOCH3 (50.43 and 50.97 ppm)].20 Poly(MS)s obtained in our study exhibited resonances at 38.8−39.5 (CHCH3) and 54.9−56.1 (CHCOOCH3), and the peaks assigned to the cis structure were not observed at all. In order to accomplish controlled polymerization, we decreased the [M]0/[I]0 ratio from 100 (run 15) to 20 (rum 16) using THF as the solvent, and the Mn = 23.0 × 103 (Mw/ Mn = 1.17) decreased to 4.5 × 103 (Mw/Mn = 1.31), which coincided well with the calculated Mn (12.6 × 103 and 2.5 × 103, respectively) and indicated improved initiator efficiencies (54−55%). The results also indicated that this polymerization mechanism is controlled anionic polymerization. Cyclic Poly(MS) Initiated by NHC/MAD and Successive Ring-Closing. We now consider the possibility of chaintransfer reaction via backbiting of the growing ester enolate to the electrophilic carbon directly bonded to the NHC, accompanied by formation of the cyclic polymer and regeneration of NHCtBu as shown in Scheme 1, because any 1 H NMR signals ascribed to the α-terminus [peaks at δ 7.10 ppm (d, 1H, J = 3.2 Hz, N−CHCH-N) and 6.46 ppm (d, 1H, J = 3.1 Hz, N−CHCH−N)] were not observed (please see Figure 2b). The MALDI-TOF mass spectrum of the poly(MS) produced by anionic polymerization using NHCtBu as the initiator, was used to determine the absolute molecular weights and the structure of the poly(MS) termini (Figure 3). In the spectra, there is one set of peaks with a repeat of 126.07 m/z, which corresponds to the molecular weight of the MS unit. When we examined the peak at 1409.71, we found that this peak coincided with the calculated peak [MS (126.07 × 11 Da) + [Na+](23.0 Da) = 1409.77] (see also Figure 3, right structure), and the presence of α-(NHCtBu-) terminal groups (m/z = 180 Da) was not confirmed. However, the spectra pattern unfortunately eliminate the possibility of the linear polymer prepared via a sequence of events involving abstraction of a α-proton to the NHC by the enolate chain end to form an enamine intermediate, which reacts with another MS to form a linear polymer, accompanied by release of the NHC (see also

Kitayama et al. already reported that the polymerization of methyl methacrylate by t-BuLi/organoaluminum compounds in toluene affords stereoregular poly(methyl methacrylate) with narrow molecular weight distributions.16,17 In our previous study of anionic polymerization of MS using t-BuLi as the initiator,7,8 when the ratio of aluminum compound, i.e., methylaluminum bis(2,6-di-tert-butyl-4-methylphenoxide) (MAD),18 and initiator ([Al]0/[Li]0) was 3, the 1,4-trans polymers were synthesized in excellent yields, although the initiator efficiencies were low (ca. 10%).7,8 When we carried out the polymerization of MS at the ratio ([Al]0/[NHCtBu]0 = 3) in toluene at −20 °C (run 12), the conversion was quantitative within 24 h, and the obtained poly(MS) consisted of complete 1,4-adducts (Figure 2b), even though no polymerization was confirmed in the absence of MAD under the same conditions (runs 1−3). It seems that the coordination of MAD to MS promotes the 1,4-addition accompanied by suppression of carbonyl attack by the propagating anion (Figure 2b), in which the initiator efficiencies [M n (calcd)/M n (exptl), where Mn(calcd) = MW(MS) × [M]0/[I]0 × conversion (%) + MW(α-terminus: carbonyl group)] were much improved (43%−55%). The coordination of various bulky aluminum compounds with MS monomer was confirmed by 13C NMR measurement. The δ-carbon (138.8 ppm) of MS monomer (in toluene-d8) shifted toward lower field in the 1:1 mixture (mol/ mol) of MS/aluminum compound, MS/MAD7 (145.1 ppm in toluene-d8). These results indicate that MAD did not react with NHCtBu but selectively coordinated with the carbonyl group of MS similar to anionic polymerization of MS initiated by t-BuLi in the presence of MAD.7,8 The IR absorption of the residual double bond in the main chain was observed at 971 (δC−H) cm−1, indicating a trans geometry (Supporting Information, SI, Figure S1). The 13C NMR also supported the contention that the main addition form is 1,4-addition. Although it was reported that the signals of carbons 1 and 4 in cis-2-hexene up-shifted (5.1−5.9 ppm) compared with those of trans-2-hexene,19 the proton-decoupled resonance signals of carbons were all single, differing by 1.4 ppm in the 13C NMR measurement. Michael addition-type group-transfer polymerization of MS was also reported to give polymer composed of cis (22%) and trans (78%) structure 15007

DOI: 10.1021/jacs.7b06897 J. Am. Chem. Soc. 2017, 139, 15005−15012

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different from that of the linear polymer prepared via a sequence of events involving abstraction of a α-proton to the NHC by the enolate chain end to form an enamine intermediate, because the peak at 1560.00 clearly shows a discrepancy with the calculated peak [CH3−CH−CH−CH− C(COOCH3)-(α-terminus) (129.07 Da) + (propylene-altmethyl acrylate) unit (128.07 × 11 Da) + H(ω-terminus) (1.0 Da) + [Na+](23.0 Da) = 1561.84] (Figure 4, right structure). The spectra suggested that formation of well-defined cyclic poly(MS) produced via NHC-initiated anionic polymerization of MS and subsequent ring-closing as the termination reaction occurred after monomer consumption, accompanied by regeneration of NHCtBu (Scheme 1). As far as we know, this is the first example of cyclic vinyl polymers as well as diene polymer produced from ring-closing procedure. Although Chen et al. reported NHC-catalyzed polymerization of methacrylates including MMA and discussed the possibility of presence of the corresponding cyclic structure, they concluded that the synthesized polymers have linear structures.13 Unlike the NHC-derived enamine intermediates involved in the dimerization of MMA,12a the enamine precursor from NHCtBu and MS is not formed due to high-energy barrier for H-transfer because abstraction of methine proton adjacent to imidazolinium group by ω-terminal enolate is suppressed by coordination to MAD, which well-coincided with our experimental data for ring-closing by SN2 nucleophilic substitution. Matsuoka et al. also examined the reaction βsubstituted vinyl monomers such as crotonates with TPT, but tail-to-tail dimers were not formed because of the α-substituent adjacent to the imidazolinium group (NHC),12e which wellcoincided with our results because MS also has the αsubstituent adjacent to imidazolinium group (NHC). We speculated that in this anionic polymerization, like a chainexpansion polymerization, α-terminal imidazolinium group acts as a countercation neighboring at propagating anion, in which the cyclic propagating chain preferred the ring-closing than Htransfer as shown in Schemes 1 and 2. Diastereoselectivity of Cyclic Poly(MS). In the highresolution (100 MHz) 13C NMR spectrum of poly(MS) initiated by t-BuLi/MAD,7,8 a splitting of the resonance assigned to −COOCH3 is observed at 51.6 ppm, which is ascribed to a threo configuration, because we confirmed that all geometries of the internal double bonds are trans. The 13C NMR spectrum of the polymer obtained by NHCtBu MAD showed only a sharp signal at 51.6 ppm, indicating that anionic polymerization of MS assisted by MAD proceeds with threo diastereoselectively.7 However, it is still difficult to quantify the ratio of erythro/threo structures because of overlapping of the peaks; i.e., the difference of their chemical shift (Δδ) was only 0.2 ppm. The stereochemistry can be evaluated in detail by the hydrogenation of the internal double bonds.7,8 The poly(MS)s prepared by NHCtBu/MAD (run 12, Table 1) were converted to the saturated polymers, i.e., head to head poly(propylene-altmethyl acrylate)s by diimide generated by thermal decomposition of p-toluenesulfonylhydrazide (TSH)5−7 (yield, 84− 98%). The 13C NMR spectrum of the hydrogenated poly(MS) prepared by NHCtBu/MAD at −20 °C is shown in Figure 5 (run 12 in Table 1). The resulting chemical shifts reflected the different diastereoselectivities clearly (the Δδ value became higher), and the erythro/threo structure was distinguishable from the signals due to −CH2− (26.1−26.5 ppm for erythro and 26.8−27.5 ppm for threo, Δδ = 1.35 ppm) and carbonyl carbons (175.8−176.2 ppm for erythro and 175.2−175.7 ppm

Figure 2. 1H NMR spectra of poly(MS) prepared using (a) NHCtBu in DMF (run 5 in Table 1) and (b) NHCtBu/MAD in toluene (run 12 in Table 1) (CDCl3, 400 MHz, 25 °C).

Scheme 2), because the peak at 1409.71 did not show any discrepancy with the calculated peak [CH3OOC−CHCH− CHC(CH3)-(α-terminus)(125.07 Da) + MS (126.07 × 10 Da) + H(ω-terminus) (1.0 Da) + [Na+](23.0 Da) = 1409.77] (Figure 3, right structure). The last step shown in Scheme 2 is analogous to tail-to-tail dimerization (umpolung) of methacrylates.12a Therefore, in order to eliminate the possibility of the presence of linear polymer, we performed hydrogenation of poly(MS) in toluene at 115 °C for 72 h under N2, leading to head-to-head poly(propylene-alt-methyl acrylate). In the spectra, there is one set of peaks with a repeat of 128.07 m/ z, which corresponds to the molecular weight of the (propylene-alt-methyl acrylate) unit (Figure 4). When we examined the peak at 1560, we found that this peak coincided with the calculated peak [(propylene-alt-methyl acrylate) unit (128.07 × 12 Da) + [Na+](23.0 Da) = 1559.84], and the presence of α-(NHCtBu-) terminal groups (m/z = 180 Da) was not confirmed. Furthermore, the spectra pattern is definitely 15008

DOI: 10.1021/jacs.7b06897 J. Am. Chem. Soc. 2017, 139, 15005−15012

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Scheme 2. Possible Termination Pathway via H-Transfer and Enamine Intermediate in NHC-Initiated Anionic Polymerization of Alkyl Sorbates

Figure 3. Expanded MALDI-TOF mass spectrum of poly(MS) initiated by NHCtBu/MAD with chemical structures of cyclic and linear poly(MS).

for threo, Δδ = 0.55 ppm). The erythro and threo 13C NMR assignments were identified with those of poly(MS)s made by anionic7,8 and group-transfer20 polymerizations. From the results, we concluded that the ratio of threo/erythro of hydrogenated poly(MS) from NHCtBu/MAD system was 86/ 14 (Figure 5), which indicated threo diastereoselectivie polymerization catalyzed by NHCtBu. Stereoregularity of Cyclic Poly(MS). There are two types (diisotactic and disyndiotactic) of possible stereoregular structures for the trans-threo-1,4-polymer of poly(sorbic acid) derivatives.8 The resonance of threo carbonyl carbon of the hydrogenated poly(MS) was split into two peaks clearly at

175.5 and 175.7 ppm (Figure 5), which is assigned to the two stereoregularities. From the intensity ratio of the peaks in the threo structure (I175.5 ppm/I175.7 ppm), the threo-poly(MS) possesses 55% of diisotacticity. Thermal Property of Cyclic Poly(MS). We performed DSC measurements to evaluate thermal properties of prepared cyclic poly(MS) (Mn = 22.9 × 103, Mw/Mn = 1.38, threo 84%, diisotaciticity 50%) compared with linear poly(MS)7,8 (Mn = 14.2 × 103, Mw/Mn = 1.28, threo 77%, diisotaciticity 51%). The glass transition temperatures of the cyclic and the linear poly(MS)s were 20 and 13 °C, respectively (Figure 6). Note that the molecular weights of both samples are enough higher 15009

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Figure 4. Expanded MALDI-TOF mass spectrum of hydrogenated poly(MS) prepared by TSH with chemical structures of hydrogenated cyclic and linear poly(MS)s.

Figure 5. 13C NMR spectra of hydrogenated poly(MS) prepared using NHCtBu/MAD (run 12 in Table 1) (CDCl3, 400 MHz, 25 °C), accompanied by the expanded spectrum at carbonyl region and chemical structure of threo-diisotactic (upper) and threo-disyndiotactic poly(MS).

well-known that cyclic polymers show higher Tg than the linear counterparts because the lack of chain-ends in the cyclic one reduces segmental mobility of polymer chains.23 Therefore, the above results, that is, cyclic poly(MS) showed a Tg of 7 °C higher than the corresponding linear poly(MS), can be a clear evidence of formation of cyclic chains for poly(MS) initiated by NHCtBu in this study. Viscosity Measurement of Bulk Cyclic Poly(MS). Viscosity check was also made by rheometer to verify the cyclic chain formation since cyclic chains typically show lower viscosity than the linear counterparts under the same molecular weight condition at the comparable temperatures.24 Note that the measurement temperatures for the comparison were set under consideration of the 7 °C Tg difference between cyclic and linear poly(MS)s for the purpose of tentative fair comparison. In the viscosity plots in Figure S3, both samples behaved as Newtonian liquids within the measurement shear rate range. The viscosity values at each temperature are summarized in Tables S1 and S2. It has been known that

Figure 6. DSC curves of linear poly(MS) (black line) and cyclic poly(MS) (red line). Heating rate, 10 °C/min.

than 10 × 103 and the tacticity is very similar to each other, and thus the comparison of these Tgs is significant.21,22 It has been 15010

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viscosity increases with an increase in M with following the equation, η ≈ M3.4, in the entanglement regime.21 Considering that entanglement molecular weight (Me) for amorphous flexible polymers possessing unsaturated bonds in the mainchain is approximately 5 × 103 or lower,21 Mn of the linear poly(MS) with Mn = 14.2 × 103 should be enough high to form entanglements. In addition, we confirmed the tacticity of cyclic and linear poly(MS)s are almost similar to each other, so that the effects of tacticity are not necessary to be considered.25 Then, by taking the viscosity value of the linear poly(MS) with Mn ≈ 14.2 × 103 as the reference, the viscosity of same sample at Mn ≈ 22.9 × 103 can be predicted from the above M − η relationship. From the comparison between the predicted values in case for linear poly(MS) and experimental viscosity values of the actual cyclic poly(MS) (Table S2), the cyclic poly(MS) was found to have less than half of the predicted values at all measurement temperatures (please see Table S2). Furthermore, the flow activation energy, which can be used as an universal flow properties independent of the sample Tg, was lower for the cyclic poly(MS) (104.8 kJ/mol) than for the linear poly(MS) (112.4 kJ/mol) (see Figure S4), meaning the cyclic poly(MS) is easier to flow.26,27 These results again indicate the poly(MS) prepared using NHCtBu/MAD as the initiator in this study has the cyclic chain formation.

CONCLUSIONS We have demonstrated NHC-catalyzed anionic polymerization of a sorbic acid derivative, MS, and a subsequent ring-closing process. Addition of an bulky aluminum Lewis acid, MAD, into NHC-catalyzed anionic polymerization of MS not only accelerates quantitative monomer consumption but also inhibits the proton transfer (first step shown in Scheme 2) by coordination with propagating anion as well as monomer molecules, which shift the balance from enamine formation (linear polymer) to ring-closing (cyclic polymer). The formation of cyclic poly(MS)s was supported by not only MALDI-TOF-MS of the hydrogenated poly(MS) but also by thermal and viscosity measurements. This fundamental study is the first example of NHC-catalyzed anionic polymerization of diene monomers as well as the formation of diene-based cyclic polymers using ring-closing procedure and will be useful for designing new cyclic polymers using polar diene monomers. ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b06897. Representative spectral and analytical data (PDF)

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AUTHOR INFORMATION

Corresponding Author

*[email protected] ORCID

Akinori Takasu: 0000-0003-3059-4463 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support to A.T. from the Ministry of Education, Science, and Culture of Japan (Grant-in-Aid for Development Scientific Research, No. 15K04872) is gratefully acknowledged. 15011

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