Conformation Shift Switches the Chiral Amplification of Helical Copoly

Jun 16, 2017 - Beijing National Laboratory for Molecular Sciences, Key Laboratory of Polymer Chemistry and Physics of Ministry of Education, College o...
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Conformation Shift Switches the Chiral Amplification of Helical Copoly(phenylacetylene)s from Abnormal to Normal “Sergeantsand-Soldiers” Effect Sheng Wang, Junxian Chen, Xuanyu Feng, Ge Shi, Jie Zhang, and Xinhua Wan* Beijing National Laboratory for Molecular Sciences, Key Laboratory of Polymer Chemistry and Physics of Ministry of Education, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China S Supporting Information *

ABSTRACT: Optically active helical random copoly(phenylacetylene)s were prepared from structurally similar monomers, (+)-3-hexadecylcarbamoyl-5-[((S)-1-phenylethyl)carbamoyl]phenylacetylene and 3hexadecylcarbamoyl-5-(benzylcarbamoyl)phenylacetylene, under the catalysis of Rh[C(C6H5)C(C6H5)2](nbd)(4-FC6H4)3P (nbd = 2,5-norbornadiene). The chiral amplification followed either normal or abnormal “sergeants-and-soldiers” rule depending on the solvent nature. In apolar solvents (i.e., CHCl3 and THF), contracted cis-cisoid M- and P-helices were selectively induced for the copolymers containing chiral units below or above 74 mol %, respectively. In polar solvent (i.e., CHCl3/CH3OH, 70/30, v/v), a cis-cisoid to cis-transoid transition occurred and stretched M-helices were dominantly formed, the optical activity of which scaled up nonlinearly with increasing chiral component. This unusual phenomenon was rationalized by the distinct competing interactions between the vicinal chiral/achiral and chiral/chiral unit pairs in the contracted and stretched helices according to the modified Ising model. It offers a promising design strategy to control macromolecular helicity.

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chiral−achiral (CA) unit pairs. These findings greatly enrich the synthesis and functionalization of helical polymers. However, most of the helical copolymers display just one kind of chiral amplification, i.e., either obeying or violating “sergeants-andsoldiers” rule. Switching chiral amplification in a single copolymer system is highly desired but has been rarely achieved.27,28 The underlying mechanism and the macromolecular design strategy thereof remain undeveloped. Recently, we reported reversible contracted cis-cisoid to stretched cis-transoid helical transition of poly(3,5-disubstituted phenylacetylene)s.29,30 Since the distance and the concomitant chiral interactions between the neighboring monomer units are distinct in two kinds of helices, it was expected that both normal and abnormal chiral amplifications could be achieved in a single copolymer system by means of the helix−helix transition. In the present work, we designed and synthesized a series of random cis-copoly(3,5-disubstituted phenylacetylene)s, sMx-co-aM1−x (x denotes the molar fraction of chiral unit sM). These copolymers adopted either cis-cisoid or cis-transoid helical conformation depending on the nature of solvent. Composition-driven helical sense inversion was observed in the compacted helices while monotonic chiral amplification was observed in the stretched ones. This is the

INTRODUCTION One characteristic that polymers outstand small molecules in optical activity lies in chiral amplification. By taking a predominant helical conformation, the chiral monomer units act cooperatively and enlarge the bias from the stereocenter that casts the left- and right-handed helical sense into a diastereomeric relationship.1 The copolymerization of chiral and achiral monomers pushes such kind of cooperation to the limit and represents one of the most convenient and economic ways to prepare optically active polymers.2−16 In this instance, a small amount of chiral monomer can endow the copolymer with the optical activity similar to that of the homopolymer of the chiral monomer, i.e., the strength of which increases nonlinearly with the chiral component. On the basis of their pioneering works on helical polyisocyanates, Green et al. proposed a famous “sergeants-and-soldiers” rule to rationalize and applied one-dimensional Ising model to quantitatively analyze this chiral, nonlinear relationship.1,2,17,18 Contrasting to the conventional “sergeants-and-soldiers” rule, some helical copolymers were reported to inverse their screw sense as the chiral unit content increases.19−25 A modified Ising model was then developed by Sato and co-workers to explain this composition-driven helical sense inversion.26 In terms of the modified Ising model, both P- and M- helices could be induced by an identical chiral unit, the chiral discrimination of which depends on the competing contributions of two types of unit pairs along the copolymer chain, i.e., chiral−chiral (CC) and © XXXX American Chemical Society

Received: May 17, 2017 Revised: June 7, 2017

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DOI: 10.1021/acs.macromol.7b01028 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules first chiral copolymer system that behaves both normal and abnormal “sergeants-and-soldiers” effects due to the backbone conformation shift.

Table 1. Homo-/Copolymerization Results and Properties of (Co)polymersa

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RESULTS AND DISCUSSION A chiral monomer, sM, and an achiral monomer, aM, were synthesized according to the synthetic route outlined in Scheme S1. Their structures were identified by 1H/13C NMR spectroscopy as well as high-resolution mass spectrometry (Figures S1−S12). Homo/copolymerizations were carried out in THF at 25 °C by using Rh[C(C6H5)C(C6H5)2](nbd)(4FC6H4)3P (nbd = 2,5-norbornadiene) as the catalyst with a constant monomer/catalyst molar ratio of 230/1 (Scheme 1).31−35 The corresponding copolymers, sMx-co-aM1−x, were Scheme 1. Homo- and Copolymerizations of Chiral and Achiral Monomers

run

xaddb

xcalc

convd (%)

Mn × 10−4 e

PDIe

Ntotf

cisg (%)

[α]D25 h

1 2 3 4 5 6 7 8 9 10 11 12

0.00 0.05 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00

0.00 0.07 0.13 0.23 0.33 0.41 0.52 0.62 0.72 0.81 0.89 1.00

95 97 96 98 97 99 98 96 97 96 96 99

−i 11.4 11.2 11.3 11.1 11.3 11.3 11.3 11.5 11.4 11.5 11.8

−i 1.78 1.58 1.51 1.53 1.76 1.80 1.62 1.64 1.65 1.80 1.85

−i 227 222 224 219 223 222 221 225 222 224 229

94 99 99 98 99 99 99 96 99 95 94 98

−i +338 +713 +963 +1075 +1024 +1034 +763 +400 −657 −1022 −1050

Carried out in THF at 30 °C under a N2 atmosphere for 3 h; [M]/ [cat.] = 230. bFeed ratio of [sM] to [aM]. cMolar fraction of sM determined by 1H NMR analysis. dEthyl acetate-insoluble part. e Estimated by GPC in THF against a polystyrene calibration. f Number-averaged total number of structural units. gCis-structure content. hSpecific optical rotation measured in CHCl3 at a concentration of 0.02 g/dL. iInsoluble in THF and CHCl3. a

weight loss above 390 °C under an inert atmosphere (Figure S25). Figure 1 shows the UV−vis absorption and CD spectra of chiral sM and its homopolymer sM1.00-co-aM0 in CHCl3 and

obtained with high yields and controllable molar masses (Ntot = 219−229) after the precipitation into large amount of ethyl acetate, filtration, and drying under vacuum at 50 °C for 3 h (Table 1 and Figure S18). Except for sM0-co-aM1.00, all the (co)polymers were soluble in apolar solvents, such as CHCl3, THF, and dichlorobenzene, but insoluble in polar solvents, such as DMF and DMSO. Their cis-structure contents were estimated as larger than 94% by 1H NMR spectroscopy according to the method proposed by Percec and co-workers (Figures S13 and S14)36,37 and qualitatively supported by solidstate Raman spectroscopy (Figure S19). Because of the similar structures of sM and aM, the copolymer composition was close to the feed ratio (Figures S14 and S15). The product of monomer reactivity ratio (rsM × raM) for the copolymerization of sM and aM was estimated as 0.88 by using Kelen−Tüdös method,38,39 further confirming the random distribution of two units along the copolymer main chain (Table S1, Figures S16 and S17). Strong intramolecular hydrogen bonds existed in CHCl3 but were disrupted in CHCl3/CH3OH mixture, as evidenced by FTIR (Figures S20−S24). All the (co)polymers were thermally stable and displayed the temperature of 5%

Figure 1. UV−vis absorption and CD spectra of sM and sM1.00-coaM 0 in CHCl 3 and CHCl 3 /CH 3 OH mixture (v/v, 70/30), respectively; c = 5.0 × 10−5 mol/L.

CHCl3/CH3OH mixture (v/v, 70/30), respectively. Although no electronic transition above 325 nm was observed for sM in CHCl3, intense absorption peaked at 357 nm was observed for sM1.00-co-aM0, suggesting the presence of contracted, cis-cisoid helical polyene backbones.29,30 Moreover, sM was CD-inactive at the wavelength longer than 280 nm, whereas sM1.00-co-aM0 presented strong negative Cotton effects in the backbone absorption region, implying the selective induction of P-helices. sM1.00-co-aM0 behaved much differently in CHCl3/CH3OH mixture (Figure 1 and Figure S26). The absorption at 357 nm in CHCl3 was completely absent when 30 vol % CH3OH was added, and a new absorption at 450 nm appeared. Accompanied by this change, the negative Cotton effects at 355 nm vanished and a relatively weak but obvious positive CD signal appeared at 450 nm. The large bathochromic shift in absorption spectrum was attributed to the transition from a B

DOI: 10.1021/acs.macromol.7b01028 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules compacted cis-cisoid to a stretched cis-transoid helical structure, which increased effective conjugation length of polyene backbones, and the opposite sign in Cotton effects suggested the inversion of helical sense. The UV−vis absorption spectra of the copolymers in CHCl3 and CHCl3/CH3OH mixture (v/v, 70/30) were similar to those of sM1.00-co-aM0 (Figure 2), indicating the adoption of

vol % methanol was added. Its optical activity was not measured. sM0.10-co-aM0.90 was soluble in CHCl3/CH3OH mixture. However, its cis-cisoid to cis-transoid transition was not completed since both of the Cotton effects at 357 and 440 nm were present, indicating the coexistence of contracted and stretched helices (Figure 2b). The above-mentioned abnormal and normal “sergeants-andsoldiers” effects were quantitatively explained by the onedimensional Ising model first developed by Green et al. and lately modified by Sato et al.1,2,17,18,26 gabs = tanh( −N ΔG h /2RT ) × gmax

(1)

ΔG h = x 2 × ΔG h,CC + x(1 − x) × ΔG h,CA

(2)

where gabs (= [θ]/3300/ε) is Kuhn’s dissymmetry factor, gmax represents the gabs of the homopolymer of chiral monomer, N is the average number of chiral and achiral monomer units, ΔGh denotes the free energy difference (per monomer unit) between the M and P helical states, and ΔGh,CC and ΔGh,CA denote the ΔGh of a chiral unit interacts with the preceding chiral or achiral unit along the chain, respectively. According to Sato, the competing contributions of ΔGh,CC and ΔGh,CA determines both the extent and the direction of helical induction. Figure 3 displays the relationships between the

Figure 2. UV−vis absorption and CD spectra of sMx-co-aM1−x (x = 0.05, 0.10, 0.20, 0.30, 0.40, 0.50, 0.60, 0.70, 0.80, 0.90, and 1.00) in CHCl3 (a, c = 5.0 × 10−5 mol/L) and CHCl3/CH3OH (v/v, 70/30) (b, c = 2.0 × 10−4 mol/L).

cis-cisoid and cis-transoid helical structures in these two solvents, respectively. However, the dependences of sign and strength of Cotton effects on the copolymer composition were quite distinct in two solvents. In CHCl3, negative Cotton effects peaked at 355 nm were observed for sM0.80-co-aM0.20 to sM1.00co-aM0, the intensity of which increased nonlinearly with the chiral unit content. For the copolymers containing less than 74 mol % chiral units, i.e., sM0.05-co-aM0.95 to sM0.70-co-aM0.30, positive Cotton effects were presented in the same region, implying selective adoption of M-helices (Figure 2a). With an increase of chiral unit content, the intensity of Cotton effects first increased quickly and then leveled off in the chiral unit content range of 20−50 mol %. Increasing the chiral unit content further led to the decrease in the strength of Cotton effects, showing an abnormal “sergeants-and-soldiers” effect. In CHCl3/CH3OH mixture (v/v, 70/30), all the copolymers containing 20−100 mol % chiral units exhibited positive Cotton effects, implying the presence of stretched, cis-transoid helices. The strength of the Cotton effects at 450 nm increased nonlinearly with increasing chiral unit content, implying a normal “sergeants-and-soldiers” effect (Figure 2b). The optical activity of sM0.05-co-aM0.95 was not measured because its solution was unstable and became cloudy within 24 h after 30

Figure 3. Plots of gabs values at indicated wavelength of sMx-co-aM1−x against copolymer composition in CHCl3 (a) and CHCl3/CH3OH (v/ v, 70/30) (b), respectively. Black circles and squares denote observed data, while red regression lines represent fitting results based on eqs 1 and 2.

molar fraction of chiral unit and the gabs values of copolymers at 340 and 450 nm, respectively, which were derived from gabs spectra of sMx-co-aM1−x in two solvents (Figures S28 and S29). Black data points are experimental results, and red regression lines were obtained by a nonlinear, least-squares fitting of the parameters ΔGh,CC, ΔGh,CA, and gmax with N = 224.40 The parameters ΔGh,CC, ΔGh,CA, and gmax in CHCl3 and CHCl3/ CH3OH are compiled in Table 2. The opposite sign of ΔGh,CA (−0.200 kJ/mol) to that of ΔGh,CC (+0.069 kJ/mol) in CHCl3 indicated that the helical sense induced by CA interaction was the opposite of that induced by the CC interaction. Thus, the screw sense was modulated by changing the copolymer composition. In CHCl3/CH3OH mixture, ΔGh,CC and ΔGh,CA C

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Macromolecules Table 2. Free Energy Parameters of the Copolymersa solvent

gmax

ΔGh,CC (kJ/mol)

ΔGh,CA (kJ/mol)

THF CHCl3 CHCl3/CH3OH (v/v, 70/30)

0.0045 0.0049 0.0003

+0.098 +0.069 +0.042

−0.257 −0.200 +0.036

a The fitting results were based on eqs 1 and 2, and N value was set as 224.

are +0.042 and +0.036 kJ/mol, respectively. Both of them were positive, evincing that ΔGh,CC and ΔGh,CA tended to induce the same sense of helical structure in cis-transoid copolymers, in accord with the observed results. The copolymers in THF exhibited the chiral amplification (Figures S33−S35) similar to that in CHCl3, and the corresponding ΔGh,CC and ΔGh,CA values were +0.098 and −0.257 kJ/mol (Table 2), respectively, consistent with abnormal “sergeants-and-soldiers” effect. However, the cis-cisoid to cis-transoid transition could not be realized by adding methanol probably due to the hydrogen bond acceptor (HBA) character of THF.29,41,42 The overwhelming hydrogen bonding interactions between THF and methanol molecules would forbid methanol molecules to enter the interior of contract ciscisoid helix and destroy the intramolecular hydrogen bonds. The chiral amplification of cis-cisoid and cis-transoid sMx-coaM1−x evaluated by optical rotation gave a consistent trend (Figure S36). Besides the distinct “sergeants-and-soldiers” effects observed in CHCl3 and CHCl3/CH3OH mixture, the optical activity strength of the copolymers was also remarkably different in two solvents. For example, gmax of the cis-transoid system was less than one-tenth of that of the cis-cisoid system. This was explained by the extension of helical structure and the presence of helical reversals along the polyene backbones caused by the disruption of intramolecular hydrogen bonding. Because of the strong interactions with methanol molecules, the copolymers could accommodate more solvent molecules into the helical structures and favored extended helical conformations. The increased isolation between neighboring pendant groups disfavored a single-handed helical structure and led to a frustrated helical conformation. A combinatory analyses of dynamic light scattering (DLS) and static light scattering (SLS) proved this speculation. The prefactors (ρ = Rg/Rh, Rg is the root-mean-square radius of gyration while Rh is the effective hydrodynamic radius) of sM1.00-co-aM0 and sM0.50-co-aM0.50 were 2.63 and 2.53 in CHCl3, but 1.64 and 1.78 in CHCl3/ CH3OH (v/v, 70/30) (Figures S37−S44), respectively. The larger ρ values in CHCl3 indicated rigid, rod-like cis-cisoid helical chains while the smaller ρ values in CHCl3/CH3OH suggested frustrated stretched cis-transoid helical chains. These results support the following mechanism (Figure 4). In the solvent favorable for hydrogen bonding, sMx-co-aM1−x adopts contracted cis-cisoid helical structures. The strong intramolecular interactions between amide groups counteract the steric repulsion of neighboring pendants and compensate for the entropy penalty. In polar solvent, the intramolecular hydrogen bonds are disrupted, and the transition to stretched cis-transoid helical structure is triggered. Since the pendant groups of vicinal units are closer in contracted helices than in stretched ones, the chiral interactions of chiral/achiral and chiral/chiral unit pairs are different in two cases. This is

Figure 4. Conformation shift switches the chiral amplification of helical copoly(phenylacetylene)s from abnormal to normal “sergeantsand-soldiers” effect.

reflected by the sign of energy difference ΔGh,CC and ΔGh,CA, which discriminates the P- and M-helices. It is noteworthy that abnormal “sergeants-and-soldiers” effect is usually observed in the copolymer systems consisting of structurally unmatched monomer pairs.19−25 The chemical structures of sM and aM in this work are very similar. Random poly(quinoxaline-2,3-diyl) copolymers consisting of dissimilar chiral (S)-octyloxymethyl and achiral propoxymethyl pendant chains were recently reported to follow solvent dependent “sergeants-and-soldiers” rule.28 However, regardless of the solvent nature, the homopolymer of chiral monomer shows identical helicity. Here, sMx-co-aM1−x formed M-helices in CHCl3/CH3OH, whereas sM0.80-co-aM0.20 to sM1.00-co-aM0 formed P-helices in CHCl3, indicative of a different switching mechanism. These results emphasized again that it is the competing contributions of the interactions between the vicinal CA and CC unit pairs that discriminate helical sense. Upon tuning these interactions, such as helix−helix transition, could switch the screw sense from a bidirectional to a monotonic induction.

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CONCLUSIONS In summary, we have designed and synthesized a series of cis random copoly(phenylacetylene)s consisting of structurally similar chiral and achiral monomer units. Contracted cis-cisoid helices were stabilized by the strong intramolecular hydrogen bonds in apolar solvents and transferred to stretched cis-transoid ones in hydrogen bonds disrupting solvent. This transition led to solvent switching of the relationship between the chiral unit content and the optical activity of the derived copolymers. Inversion of screw sense was observed in cis-cisoid helical copolymers depending on its composition, while monotonic increase of optical activity with chiral unit contents was observed in cis-transoid helical copolymers, which can be explained by the difference in the competing inductions of chiral unit to neighboring chiral and achiral unit based on the modified Ising model. This finding may be applicable to other chiral macromolecular and supramolecular systems and offer new design strategy for novel stimulus-responsive materials.21,43−45 D

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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.7b01028. 1 H NMR, 13 C NMR, and FTMS of important compounds and polymers, TGA, FTIR, Raman, CD, and UV−vis absorption spectra, DLS and SLS data (PDF)

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AUTHOR INFORMATION

Corresponding Author

*(X.W.) Tel 86-10-62754187, Fax 86-10-62751708, e-mail [email protected]. ORCID

Jie Zhang: 0000-0002-6509-8614 Xinhua Wan: 0000-0003-2851-6650 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support of the National Natural Science Foundation of China (No. 21274003) and the Research Fund for Doctoral Program of Higher Education of MOE (No. 20110001110084) is greatly appreciated.

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REFERENCES

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DOI: 10.1021/acs.macromol.7b01028 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.7b01028 Macromolecules XXXX, XXX, XXX−XXX