Friedel–Crafts A2 + B4 Polycondensation toward Regioselective

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Friedel−Crafts A2 + B4 Polycondensation toward Regioselective Linear Polymer with Rigid Triphenylmethane Backbone and Its Property as Gas Separation Membrane Lei Zou,† Xiaosong Cao,† Qinnan Zhang,‡ Marcus Dodds,† Ruilan Guo,‡ and Haifeng Gao*,† †

Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556, United States Department of Chemical and Biomolecular Engineering, University of Notre Dame, Notre Dame, Indiana 46556, United States

‡

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S Supporting Information *

ABSTRACT: This report presents an unexpected acid-catalyzed Friedel−Crafts (F−C) hydroxyalkylation polymerization to produce linear polymers containing triphenylmethane repeating units in the backbone. The polymerization of 4-nitrobenzaldehye (NBA, a difunctional A2 monomer) and 1,4-dimethoxybenzene (DMB, a B4 monomer with four identical reactive sites) demonstrated very high regioselective reactions at the 2,5 positions of DMB monomer, leaving 3,6 positions unreacted to avoid any branched structure. The produced linear polymers were always terminated with DMB units at both ends, allowing easy synthesis of high-molecular-weight polymers by using excess amount of NBA. The polymerizations used various inexpensive 4-substituted benzaldehydes and para-disubstituted benzenes as the A2 and B4 monomers, producing a series of linear polymers with tunable side group compositions. These polymers with high molecular weight and rigid triphenylmethane backbone structure were cast into freestanding membranes and demonstrated excellent gas transport properties with the permeability-selectivity combination outperformed relevant commercial polymer membranes for CO2/CH4 separation.

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INTRODUCTION Step-growth polymerization, as one of the two polymerization mechanisms, is contributing to the production of various important polymer materials, such as polyesters, polyamides, and polyurethanes.1,2 The fundamental kinetics in the stepgrowth polymerizations have been well documented in the textbook, in which a balanced stoichiometry of the difunctional A2 + B2 monomer mixture is required to achieve high molecular weight and a slight impurity of multifunctional An or Bm (m, n > 2) monomers could bring up complexity of branched and even cross-linked structure in the final polymer product.3 To pursue efficient polycondensation reactions that bypass the requirement of stoichiometric balance and can freely tune the polymer molecular weights, we recently turn our interest to Friedel−Crafts (F−C) reactions4 that build carbon−carbon bonds and incorporate aryl units into polymer structure. In particular, the strong acid catalyzed F−C hydroxyalkylation reactions between electron-deficient ketones and electron-rich arenes have been applied to construct linear polymers, branched polymers, and dendrimers.5−11 In all these reactions, careful selection of arene monomers as nucleophiles is critical to achieve high regioselectivity and to produce polymers with defined structures. As compared to the popularly used ketone monomers, aldehydes are much less used in these F−C polycondensations for constructing polymers,12,13 except the example of phenol formaldehyde resin from reaction of © XXXX American Chemical Society

formaldehyde with phenol. However, this reaction is not regioselective and often produces cross-linked product. Meanwhile, literature reports have applied acid-catalyzed bisarylation reaction of aryl aldehydes to synthesize small molecule triarylmethanes,14−25 which inspires us to investigate the possibility of preparing polymers with rigid triarylmethane backbone structure using inexpensive aryl aldehydes and substituted benzenes as monomers. Herein, we present an intriguing F−C hydroxyalkylation reaction between commercially available 4-nitrobenzaldehye (NBA) and 1,4-dimethoxybenzene (DMB) to exclusively produce linear polymers (Scheme 1). Since the DMB has four identical reactive sites at the 2, 3, 5, and 6 positions, the production of linear polymers in this reaction was initially unexpected, representing an exceptional example in the stepgrowth polymerization of A2 and B4 monomers. It was further discovered that this F−C polymerization could be applied to a family of benzaldehydes as electrophilic monomers and 1,4disubstituted benzenes as nucleophilic monomers, all producing linear polymers with triphenylmethane units in the backbone. Preliminary pure-gas permeation tests showed that the synthesized polymers held great potential as gas separation membranes, which outperformed relevant commercial polymer Received: July 2, 2018 Revised: August 3, 2018

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DOI: 10.1021/acs.macromol.8b01394 Macromolecules XXXX, XXX, XXX−XXX

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S1 of the Supporting Information, SI), indicating the carbocation nature of the F−C hydroxyalkylation reaction. From the 1H NMR spectroscopy of the crude samples taken out at different times (Figure 1A), the conversions of both NBA and DMB monomers were determined by integrating the characteristic signals for NBA at δ = 8.4 ppm and DMB δ = 3.7 ppm (marked as squared dash lines in Figure 1A). The results showed that the conversion of DMB was always higher than that of NBA, which was difference from the equal conversions of A2 and B2 monomers in traditional step-growth polymerizations (Figure 1B). Meanwhile, the polymers characterized using size exclusion chromatography (SEC) with THF as the mobile phase showed decrease of elution volumes (Figure 1C). Using linear polystyrene standards for SEC calibration, the evolution of number-averaged molecular weights (Mn,RI) exhibited a step-growth polycondensation mechanism with the dispersity of polymers Mw/Mn < 2 (Figure 1D). The unexpected evolution of polymer molecular weight and dispersity indicated that the polymer probably had a linear structure. A control experiment using 1:2 feed ratio of NBA to DMB at less MSA acid, e.g., 0.7 equiv of NBA, was carried out, which had slow kinetics and was intentionally stopped at low conversion to produce oligomers (primarily trimer and pentamer after purification, details in SI and Figure S2). 1H NMR spectroscopy showed that these trimer and pentamer were exclusively terminated by DMB units at both ends (Figure 2A), evidencd by signals at δ = 3.7 and 6.8 ppm for protons from the terminal methoxy groups and phenyl rings, respectively. The NMR peaks of these oligomers completely overlapped with those from the purified polymer product although the intensity of chain-end DMB signals decreased due to the increased polymer molecular weights (Figure 2B). These

Scheme 1. Synthesis of Linear Polymers Using AcidCatalyzed F−C Polycondensation of A2 + B4 Monomers

membranes for important industrial gas separation, such as CO2 removal from CH4.

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RESULTS AND DISCUSSION All polymerizations in the present study used commercially available monomers with affordable price. The reactions were set up at room temperature with no need of anhydrous solvents or deoxygenation procedures. When methanesulfonic acid (MSA) was added into dichloromethane (DCM) solution of NBA and DMB at molar ratios of [DMB]0:[NBA]0:[MSA]0 = 1:1:4, the polycondensation reaction occurred immediately (Scheme 1), evidenced by the increased viscosity of the reaction mixture and the changed solution color from orange to dark brown, then to brown and ultimately to green (Figure

Figure 1. (A) Stacked 1H NMR spectra of the reaction mixture during polymerization with dashed squares indicating the reactions of NBA and DMB monomers; (B) conversions of NBA and DMB monomers over reaction time during polymerization; (C) overlay of SEC elution chromatograms of polymers; and (D) evolution of the number-average molecular weights versus the conversion of NBA; polymerization conditions: [DMB]0:[NBA]0:[MSA]0 = 1:1:4, [DMB]0 = 1.0 M in DCM at room temperature. B

DOI: 10.1021/acs.macromol.8b01394 Macromolecules XXXX, XXX, XXX−XXX

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The results in Figure 2 suggested a significantly higher reactivity of the diaryl carbinol than NBA (Scheme 1), so that the intermediate was immediately consumed via fast reaction with either another DMB monomer or any DMB-terminated oligomer. Meanwhile, the production of linear polymer structure was primarily due to the unequal reactivity of the three nucleophilic sites on DMB unit after one site was first reacted. It is hypothesized that the reaction of the first site, i.e., any of the four identical ortho positions (N1, Scheme 1) next to the methoxy groups on DMB, decreased the reactivity of its ortho and meta sites (N3 and N4, Scheme 1), although the reactivity of the para site (N2) was unknown. To understand the reactivity of the N2, N3, and N4 sites, a model reaction was set up between DMB and diphenylmethanol (an analog of diaryl carbinol) at 1:20 molar ratio using MSA as the acid catalyst (Scheme 2). By using 20 equiv of diphenylmethanol, its concentration was almost constant throughout the reaction, so that the kinetics of this two-step consecutive reaction was simplified as a pseudo-first-order consecutive reaction A → B→C, in which A, B, and C stand for DMB, MC1, and MC2, respectively (Scheme 2). With the increase of reaction time, the molar fraction of unreacted DMB quickly decreased. The molar fraction of intermediate MC1 based on the feed amount of DMB increased first to about 30% before decreasing gradually, while the final product disubstituted MC2 increased quickly and ultimately reached 100% in molar fraction among the three species (Figure S3). Fitting the evolution curves of the molar fraction of each species using first-order consecutive reaction kinetics equations26,27 indicated that the two kinetic constants were k1 = 0.041 ± 0.004 min−1, and k2 = 0.058 ± 0.007 min−1, which were neither in significant difference, nor the same. Considering there are four N1 sites in DMB monomer and only one N2 site in MC1 intermediate, the reactivity of N2 site over N1 site could be 2− 3 times higher. Intriguingly, there was no slight detection of any product by reacting at the N3 or N4 site of MC1, indicating nonreactive 3,6 position of the DMB monomer in the polymerization. Overall, the reactivity order among these different reacting sites from DMB was N2 > N1 ≫ N3 (N4), which was essential to form linear polymer structure. As an intriguing feature, the polymers in the present study were always terminated with the disubstituted benzenes due to the much higher reactivity of diaryl carbinol intermediate than the aldehyde monomer. Thus, available NBA monomers are

Figure 2. 1H NMR spectra of (A) purified trimer and pentamer produced from [DMB]0:[NBA]0:[MSA]0 = 2:1:0.7, [DMB]0 = 1 M in DCM at room temperature and (B) purified final polymers obtained from the polycondensation at [DMB]0:[NBA]0:[MSA]0 = 1:1:4, [DMB]0 = 1.0 M in DCM at room temperature.

results demonstrated that the produced polymers had linear structure, in which the DMB units were connected by NBA units at 2,5 positions with the 3,6 positions unreacted, except the two DMB terminal units (Scheme 1). By integrating the signal peaks from the DMB repeating units and terminal units, the number-averaged molecular weight (Mn,NMR = 15 300) was calculated, which was smaller than the SEC result Mn,RI = 30 200. Since the apparent molecular weight Mn,RI in SEC was based on linear polystyrene standards, the discrepancy between these two molecular weights suggested the rigid polymer backbone structure in this triphenylmethane-containing polymers (Table 1, entry 4). Table 1. Friedel-Crafts Polycondensation of A2 + B4 Monomers a

1 2a 3a 4a 5a 6a 7a 8a 9a 10b

A2

B4

[A2]0:[B4]0

time (h)

Mn,NMRc

Mn,RId

Mw/Mnd

NBA NBA NBA NBA NBA EBA FBA tBBA MeOBA NBA

DMB DMB DMB DMB DMB DMB DMB DMB DMB pXY

0.9:1 0.95:1 0.98:1 1:1 1.2:1 1.2:1 1.2:1 1.2:1 1.2:1 1:1

48 48 120 120 48 24 48 72 72 100

1750 4770 10 620 15 300

2090 4960 9590 30 200 529 900 396 000 7510 5650 3820 6,770

1.44 1.69 1.79 1.46 2.05 1.90 1.64 1.65 2.57 1.54

2960 2560 1080 5,450

a

[A2]0 = 1 M, catalyst: 4 equiv MSA, room temperature in DCM. b[A2]0 = 1 M, catalyst: 1 equiv TFSA, room temperature in DCM. cNumberaverage molecular weight calculated by 1H NMR spectroscopy after polymer purification (Figures S5−S8). dApparent number-average molecular weight and molecular weight distribution (Mw/Mn) measured by THF SEC with the RI detector, calibrated with linear polystyrene standards. C

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Scheme 2. Illustration of the Two-Step Consecutive Model Reaction: [DMB]0:[Diphenylmethanol]0:[MSA]0 = 1:20:9, [Trimer]0 = 0.1 M in DCM at Room Temperature

Figure 3. (A) SEC elution chromatograms of final polymers obtained from the polycondensation of NBA and DMB at varied feed ratios and (B) digital picture of the purified final polymer from polymerization of NBA and DMB at 1.2:1 feed ratio.

functioning as linkers to connect the DMB terminals from two polymer chains and build up high molecular weight.28−31 Tuning the Molecular Weight of Polymers. On the basis of the mechanism discussed above, it was hypotheiszed that the use of excess NBA monomer in the polymerization could accelerate the reaction and produce polymers with higher molecular weight. In comparison to the polymerization of NBA and DMB at 1:1 ratio that required about 100 h to reach complete conversion and Mn,NMR = 15 300 (Table 1, entry 4), the polymerization using 20% excess amount of NBA produced exhibited polymer with Mn,RI > 500 000 in 48 h (Table 1 entry 5, Figure 3A). This polymer in such a high molecular weight could be easily precipitated out in nonsolvent, such as methanol, into fibrous textile (Figure 3B), which is critical for later solution casting into freestanding film. On the other side, substoichiometric amount of NBA in the polycondensation decreased the polymer molecular weights (Table 1, entries 1−3). Monitored by 1H NMR spectroscopy, all reactions stopped after aldehyde was fully consumed, producing polymers exclusively terminated with DMB structural units (Figure S4). Scope of Monomer Species. It is known that both polarity and steric bulkiness of substituent groups affect the monomer reactivity in the F−C hydroxyalkylation reaction although the polar effect could be more significant.4 From many commercially inexpensive chemicals, the current study focused on the 1,4-disubstituted DMB and para-xylene (pXY) as the B4 monomers and para-substituted benzaldehydes as the A2 monomers (Scheme 3). Series of polymerizations were carried out using various A2 monomers with DMB under similar conditions, e.g., [A2]0:[DMB]0 = 1:1 in DCM at room temperature with MSA as catalyst (Table 1, entries 6−9, Figures S5−S7). The para-substituted group could be electron withdrawing group (EWG) such as NO2 group, COOMe

Scheme 3. Structure Illustration of the Used Benzaldehyde A2 Monomers and the Disubstituted Benzene B4 Monomers

group, fluorine group, or electron donating group (EDG) such as tert-butyl group and methoxy group. It was found that benzaldehydes with para-EWG produced polymers with higher molecular weights in faster polymerizations than monomers with para-EDGs. Meanwhile, the use of excess benzaldehydes always improved the polymer’s molecular weights. The illustrated substituted groups on benzaldehydes provided numerous choices to incorporate functional groups onto the polymer backbone. Meanwhile, pXY was explored as the nucleophile monomer for polymerization with NBA (Table 1, entry 10, Figure S8). Initial tests that reacted pXY and BBA using 4 equiv of or more MSA as catalyst did not produce any polymers in a week, probably due to the fact that the methyl substituent groupsin pXY are weaker EDGs than the methoxy groups on DMB. Therefore, stronger trifluoromethanesulfonic acid (TFSA) was used as the catalyst for polmerization of 1:1 molar ratio of NBA to pXY at room temperature. The polymerization essentially completed within 100 h and produced poly(NBApXY) polymers with number-average molecular weight Mn,NMR= 5450 and dispersity Mw/Mn = 1.54. Gas Transport Properties of Polymers. The produced linear polymers contained rigid triphenylmethane backbone D

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Figure 4. (A) Digital picture of the polymer film cast from NMP solution of polymer (entry 5, Table 1) at concentration of 6.1 wt %; (B) the film’s permeability-selectivity trade-off relationship for CO2/CH4 separation in comparison with commercially relevant polymer membranes.35,36

structural units and are expected to have high gas permeability. To demonstrate the potential application, a selected polymer (Table 1, entry 5) dissolved in N-methyl-2-pyrrolidone (NMP) at 6.5 wt % was cast into defect-free thin film (∼50 μm thickness, 7.5 × 3.7 cm2 in size, Figure 4A) before testing its pure-gas permeation properties on a custom-built gas permeation system using constant-volume/variable-pressure method32 with five ultrahigh-purity (UHP) gases (H2, CH4, N2, O2, and CO2). Gas permeability (P) was determined at various feed pressures at 35 °C and the ideal selectivity of gas A (more permeable gas) over gas B was determined by α(A/B) =PA/PB. A straightforward yet effective way to gauge the separation performance of a polymer of interest is to place its performance in the context of the permeability-selectivity trade-off, known widely as “upper bound”,33,34 which manifests as log−log plots of selectivity versus permeability of the more permeable gas. While the most desired scenario is to have innovative materials surpassing the upper bound limits. Figure 4B shows the membrane’s upper bound relationship for CO2/ CH4 in this work. Clearly, the newly synthesized polymer outperformed all relevant commercial polymer membranes as judged by the distance between the data points and the upper bound. In particular, the new polymer showed significantly higher gas permeability by an order of magnitude (except PPO), indicating large fractional free volume of the polymer likely due to its rigid triphenylmethane backbone structural that effectively disrupt chain packing. Moreover, the new polymer was highly comparable in terms of size sieving capability (i.e., selectivity), which, in combination with its high permeability, surpassed the upper bound slope moving its performance rather laterally toward the upper bound. Although the current result was less permeable than PPO, the ideal selectivity of the new polymer (α = 19) was more than 35% higher than that of PPO (α = 14), making it overall closer to the upper bound. As such, this new series of polymers are highly promising candidate materials for gas separation membranes. Considering the highly diverse chemical structure of this new family of monomers and feasible synthesis of polymer, superior separation performance can be achieved via exquisitely tailoring the substituent groups.

substituted benzaldehyde monomers. Although the disubstituted benzene as a B4 monomer had four reactive sites, the polymerization exclusively produced linear polymers with no branched structure because of the specific steric and electronic effect of the monomers. More importantly, the significantly higher reactivity of diaryl carbinol intermediate than benzaldehyde allowed the production of linear polymers with freely tuned molecular weights Mn = 103 to 5 × 105 by simply using less or more benzaldehyde (A2) monomers. Varying the substituent groups on both monomers produced a series of linear polymers all with rigid triphenylmethane backbone units, but different dangling side groups. The polymer with high molecular weight could be easily cast into freestanding membrane and demonstrated excellent permeability-selectivity combinations as gas separation membrane materials. Considering the inexpensive monomer sources, the facile polymerization conditions and the easy access to high molecular weights, the current polymerization method represents a convenient strategy to produce polycondensation polymers with various structural and compositional tunability to meet different membrane applications.

CONCLUSIONS In summary, we developed a novel polycondensation method for synthesis of linear polymers using an acid-catalyzed F−C polycondensation of 1,4-disubstituted benzene and para-

ACKNOWLEDGMENTS The authors thank the National Science Foundation (CHE1554519) and the University of Notre Dame for financial support. X.C. thanks the support from American Cancer

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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/acs.macromol.8b01394.

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Detailed experimental procedures and additional characterization data (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Ruilan Guo: 0000-0002-3373-2588 Haifeng Gao: 0000-0001-9029-5022 Notes

The authors declare no competing financial interest.

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Society (ACS) Institutional Research Grants (IRG) from Notre Dame. M.D. acknowledges partial financial support from the Baden-Württemberg Stiftung gGmbH.

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