Optical Activity of Homochiral Polyamides in Solution and Solid State: Structural Function for Chiral Induction.

In this work, we have explored a simple and facile approach to prepare optically active helical polyamides. The hydroxyl groups of l-TA and d-TA were protected by O-alkoyl ester, and the resulting enantiomers, l-2,3-di-O-acetyl-tartaric acid (l-ATA) and d-2,3-di-O-acetyl-tartaric acid (d-ATA) crystals, were obtained. A pair of aliphatic homochiral polyamides of PA-l and PA-d are prepared using l-ATA, d-ATA, and achiral 1,11-undecanediamine as building blocks via interfacial polycondensation. PA-l and PA-d display negative and positive mirror circular dichroism (CD) spectra images in both solution and solid state. Moreover, the polyamides in solid state display different CD signals and stronger optical activity compared to those in ethanol and even the related chiral monomers in solid state, which was due to the helical conformation of the polyamides in solid state. Scanning electron microscopy results indicated that the aggregations of PA-l express left-handed helical sense, whereas those of PA-d express right-handed helix. In addition, the induced CD signals from the chiral conformation of the backbone become weaker when increasing the temperature from 0 to 60 °C in dilute solution. Either of the polyamides displays relatively stable CD images in solid state when elevating the temperature from 0 to 90 °C.

Introduction

The development of artificial optically active helical n class="Chemical">polymers attracts the particular interest of synthetic polymer scientists because of a wide variety of potential applications, including chiral separation,[1−6] asymmetric catalysis,[7−9] and enantioselective crystallization,[10] to name but a few.[11] However, there are several limited factors restricted to the practice applications of the optically active polymers, such as complicated and tightly controlled polymerization processes, inadequate monomers, and expensive chiral catalysts or initiators.[12−15] In addition, the practical applications of optically active polymers are commonly in solid phase.[16−21] Therefore, a facile preparation of the optically active materials and the evaluation of chiral information in solid state are of great significance. Polyamides have been widely utilized in industry due to their high tensile strength, high elongation, excellent abrasion resistance, and high resistance to chemicals.[22] And the interfacial polycondensation is one of the important methodologies used to synthesize polyamides for its tolerance of impurities and nonequivalence of reactants, short reaction time, and mild reaction temperature.[23−26] In fact, optically active polyamides have received considerable attention in recent years, whereas only few reports have characterized the chirality of the polyamides in solid state.[27] Furthermore, the developments of the polymer with desired chirality through introducing chiral atoms into the backbone are very limited, especially aliphatic linear polymers.

Yokoyama and co-workers prepared n class="Chemical">poly(p-benzamide)s bearing a chiral side chain on the nitrogen atom, which exhibited circular dichroism (CD) signals in solution, and the helical conformations were investigated by the study of oligo(benzamides) with different molecular weights and narrow polydispersity.[28] However, the chain-growth condensation polymerization employed in the reports is restricted to construct π-conjugated polymers.[29] Mallakpour and co-workers[30] employed aromatic isophthalic acid composed of m-substituted chiral group and diisocyanates as monomers to synthesize polyamides, which displayed optical activity in solution. In 2014, Akagi and colleagues[31] prepared the films of PA610 and PA6T with one-hand spiral morphologies by interfacial polymerization via a water layer and a chiral nematic liquid crystal layer, whose chirality could affect the swirling directions of the spiral morphology of the polyamides. But no literature was reported to illustrate that the helical films of PA610 and PA6T could retain the one-hand spiral morphologies after reprocessing. Bou and co-workers developed tartaric acid-based polyamides synthesized by polycondensation of bis(pentachlorophenyl)tartrates with diamines activated as N,N′-bis(trimethylilyl) derivatives under rigorous exclusion of moisture conditions and relatively long polycondensation time (3–4 days), and the tartaric acid-based polyamides display specific rotation in solution but does not report about the optical activity in solid phase.[32,33]

In this work, the successful preparation of optically active n class="Chemical">polyamides with no detectable amounts of racemization has been realized by introducing chiral pools in the backbone via a facile method. The linear aliphatic polyamides with optical activity were prepared from naturally occurring l/d-tartaric acid and 1,11-undecanediamine, which can be developed through fermentation from light wax. The optical activities of the resulting polyamides PA-l and PA-d were studied in both solution and solid state and compared to their responding monomers. Moreover, the aggregations of the polymers in solid state were evaluated by scanning electron microscopy (SEM). And the thermostability of the chirality of the polyamides was studied in both solution and solid state.

Results and Discussion

Preparation of Monomers and Polyamides

The preparation of monomers and n class="Chemical">polymers is illustrated in detail in Supporting Information. The monomers l-2,3-di-O-acetyl-tartaric acid (l-ATA) and d-2,3-di-O-acetyl-tartaric acid (d-ATA) are silky white crystals due to the high purity of ATAs obtained in this work. And the two enantiomers display no differences in the melting point, as illustrated by the 1H NMR and IR spectra shown in Supporting Information (Figures S1 and S2).

Schematic illustration of the preparation of n class="Gene">PA-l and PA-d is shown in Scheme . The two monomers d-ATA and l-ATA were used as building blocks to prepare aliphatic linear polyamides with 1,11-undecanediamine. To avoid the change of the configuration of the intermediates and obtain the high stereotactic polyamides, interfacial polycondensation was employed.

Scheme 1

Schematic Illustration of the Preparation of PA-l and PA-d

IR, 1H NMR, and elemental analyses corroborated the chemical structure anticipated for the n class="Chemical">polyamides. The IR spectra of PA-l and PA-d (Figure S3) are very similar, showing the typical acetoxy group (1764 cm–1 for C=O; 1200 and 1060 cm–1 for C–O–C; 1370 and 2931 cm–1 for CH3) as well as amide group (1664 cm–1 for C=O; 3300 and 1550 cm–1 for N–H) and methylene stretching (2850 cm–1). 1H NMR spectroscopy confirmed the chemical structure of PA-l and PA-d, exhibiting the expected area and multiplicity with no trace of any other signal detected in the spectra (Figure S4). And the results of the elemental analysis of the polymers are illustrated in Table .

Table 1

Properties of the Polymers PA-l and PA-d

		elemental analysisa
polymers	yield (%)	C	H	N	M̅w (g/mol)	M̅n (g/mol)	PDI
PA-l	72	59.23 (59.36)	8.28 (8.39)	7.12 (7.29)	165 500	50 100	3.3
PA-d	68	59.25 (59.36)	8.38 (8.39)	7.22 (7.29)	172 400	51 300	3.4

In parentheses, elemental composition calculated for the polymers with the indicated compositions.

In parentheses, elemental composition calculated for the pan class="Chemical">polymers with the indicated compositions.

The molecular weight of the n class="Chemical">polyamides was evaluated by gel permeation chromatography (GPC) (Figure S5), and the results are shown in Table . The number-average molecular weights (M̅n) of PA-l and PA-d are 50 100 and 51 300 g/mol, respectively, and the polydispersity indexes (PDIs) are both less than 3.4.

The solubilities of both n class="Gene">PA-l and PA-d were assessed in an assortment of representative solvents, and the results are compared in Table S1. These polyamides with polar ester side groups favor solubility in organic solvents, due to the side groups disrupting the intermolecular associations. Specifically, PA-l and PA-d display similar solubility patterns irrespective of the configuration of the ATA segments. These polyamides are soluble in dimethyl sulfoxide (DMSO), N-methyl pyrrolidone (NMP), and dimethylformamide (DMF) at room temperature. As a result, DMF can be used as the solvent when the molecular weight is determined via GPC. The solubilities of PA-l and PA-d in ethanol were 9.48 and 10.25 mg/mL at 20 °C, respectively, so that their CD spectra could be tested in dilute ethanol.

Configuration of the Monomers and the Polyamides

Table shows that the specific n class="Disease">optical rotation ([α]D20) of l-ATA in water is −17.8 and −19.8° in ethanol at 20 °C and that of d-ATA is +22.9° in water and +25.4° in ethanol at 20 °C, which indicates that ATAs derived from l-TA and d-TA are of opposite chirality. To make a further study of the configuration of the l-ATA and d-ATA, circular dichroism (CD) spectroscopy was employed to characterize the chirality of the monomers in ethanol and in the solid state at 20 °C. Figure shows that l-ATA displays the same negative Cotton effect with l-TA at about 215 nm, and d-ATA the same positive Cotton effect with d-TA, which indicates that the configuration of the acylated tartaric acid did not change during the reaction. Therefore, we name the monomers l-(−)-2,3-di-O-acetyl-tartaric acid (l-ATA) and d-(+)2,3-di-O-acetyl-tartaric acid (d-ATA), which are chirally pure enantiomers with different configurations. Moreover, the CD spectra of l-ATA in ethanol have the same negative Cotton effect at around 215 nm with that in the solid state. And d-ATA displays similar CD signals in ethanol and solid state. The results demonstrate that the conformations of the monomers d- and l-ATA in solution and in the solid state have not influenced these CD signals significantly. It was reported that the tartrate molecule prefers a planar zigzag conformation, and the conformation with intramolecular hydrogen bonding attached to the same chiral carbon atom is energetically favored.[34] Therefore, the relatively stable CD signals in both solution and solid state, thanks to the contribution of a planar T-conformer of tartrate molecule, do not change much in different phases.

Table 2

Specific Optical Rotation [α]D20 of the Monomers at 20 °C in Different Solvents (c = 1 g/100 mL)

	[α]D20, (deg) dm–1 g–1 cm3
solvent	l-TA	d-TA	l-ATA	d-ATA
water	+15.3	–15.0	–17.8	+22.9
ethanol	+4.0	–5.0	–19.7	+25.4

Figure 1

UV–CD spectra signals of d-TA and l-TA in ethanol (c = 2.08 × 10–6 mol/mL) and in the solid state at 20 °C.

UV–CD spectra signals of n class="Chemical">d-TA and l-TA in ethanol (c = 2.08 × 10–6 mol/mL) and in the solid state at 20 °C.

We measured the specific n class="Disease">optical rotations ([α]D20) of the polymers in different solvents, which are illustrated in Table . The choices of the solvents were restricted to DMSO, NMP, and hexafluoroisopropanol (HFIP) because the polymers could not be soluble in water and slightly soluble in ethanol (Table S1). Table shows that the [α]D20 values of PA-l are −13.0, −6.7, and −1.8° in DMSO, NMP, and HFIP, respectively, and +12.8, +6.9, and +2.0° for PA-d. Essentially, [α]D20 for a chiral species is a composite of the response of the multiple conformations.[35] As a result, optical rotations of chiral molecules in solution states are solvent-dependent. In the case of PA-l and PA-d in solution, hydrogen-bonded interactions between the isotropic solvents and the polymers play critical roles in their optical rotation. When the molecules of the nonchiral isotropic solvents combine with the polymers, the intermolecular and intramolecular interactions among the polymers themselves become weaker and then the possibility of forming different conformations increases obviously, which is responsible for the decrease in the asymmetry of the polymers.[36,37] The polar protonic solvent of HFIP tends to hydrogen-bond with polymers the most among the three solvents, and DMSO the least due to its nonprotonic nature and being less polar than NMP. As a result, the polyamides in HFIP display the least value of [α]D20, and the largest in DMSO among the three solvents. Nevertheless, PA-l and PA-d display opposite specific optical rotation values regardless of the type of solvents.

Table 3

Specific Optical Rotation of PA-l and PA-d at 20 °C in Different Solvents (c = 1 g/100 mL)

	[α]D20, (deg) dm–1 g–1 cm3
solvent	PA-l	PA-d
MDSO	–13.0	+12.8
NMP	–6.7	+6.9
HFIP	–1.8	+2.0

To make a further investigation of the optical activity of the n class="Chemical">polyamides influenced by both intrinsic chirality of the tartrate units and the conformations of the main chain, UV–CD spectra of the polyamides were researched and compared to the corresponding optical activity monomers. Figure shows the CD spectra and PA-l and PA-d at 20 °C in ethanol (2.08 × 10–6 mol/mL, calculated as constitutional repeating unit), which were prepared at the same concentration that of l-ATA and d-ATA (Figure ). Both the polyamides exhibit characteristic mirror positive and negative CD spectra images in ethanol, and the CD signals are weaker than the corresponding monomers. The Cotton effect of the l-ATA is negative at 215 nm, whereas that of the PA-l is negative at 208 nm. The CD signals of polyamides have an opposite reversal at 195 nm, where the corresponding monomers do not show any such absorption (abs) tail. The less intense Cotton effect of polymers compared to the monomers results in flexible linear backbones, which tend to be less ordered in dilute ethanol, leading to intramolecular compensation of asymmetry.[36] Meanwhile, the UV absorption of the polyamides (about 3.1 abs) is much stronger than that of the responsive monomer (about 0.4 abs), which is because the UV absorption of amide groups is stronger than that of ester and carboxyl groups. Again, the absorption of UV–CD spectra of the polyamides in far-ultraviolet regions are dominated by the n−π* (λmax = 220 nm) and π–π* (λmax = 200 nm) transitions of amide groups (Figure S6).[38,39] Tartaric acid derivatives containing the carbonyl group (C=O) were expected to exhibit CD spectra within the range of the n−π* transition (200–230 nm).[40] Thus, the differences of the CD signals of polymers between the corresponding monomers are due to the amide groups of the polymers and the conformations of the polymers. Furthermore, there are other evidences support this claim.

Figure 2

UV–CD spectra signals of PA-d and PA-l at 20 °C in ethanol (c = 2.08 × 10–6 mol/mL of the repeat unit) and solid state prepared from ethanol solution (the thickness of the film was evaluated by the equal UV-abs to that in ethanol).

UV–CD spectra signals of n class="Chemical">PA-d and PA-l at 20 °C in ethanol (c = 2.08 × 10–6 mol/mL of the repeat unit) and solid state prepared from ethanol solution (the thickness of the film was evaluated by the equal UV-abs to that in ethanol).

Interestingly, the CD spectra of the n class="Chemical">polyamides in solid state vary clearly from those in solution state. Taking PA-l for example, Figure shows that the CD signals of PA-l in solid film state display clear splits observed with crossovers at 215 and 197 nm, whereas a weak split with a crossover at 199 nm was observed in solution. Furthermore, the negative ellipticities at 205 nm (−50 mdeg) of PA-l in film state are much more intense than those in ethanol at 210 nm (−13 mdeg) when the similar UV absorption was measured. According to the exciton coupling theory, the split of the CD signal is related to the interaction between the adjacent chromophores.[41] In our case, the chiral pools of ATA segments induced the neighbor amide group to a chiral chromophore.[42] The weak split in dilute ethanol is due to the less intermolecular aggregation and disordered conformation. And the enlarged optical activity in solid state results from the induced chiral amide groups displaying high enantiopurity for the same twisted direct of the amide groups in the backbone.[42] Meanwhile, the chiral PA-d displays mirror opposite CD images to PA-l in both ethanol and solid state (Figure ).

In fact, several reports supported that the amplification of chirality could be associated with the helical structure.[43−46] And the deduction was confirmed by the scanning electron microscopy (SEM) of the morphologies of pan class="Gene">PA-l and pan class="Chemical">PA-d in solid film state obtained from ethanol solution. Figure shows SEM images of the PA-l (a) and PA-d (b) films by casting the polymers in ethanol. Interestingly, the helical sense morphologies with left-handed and right-handed directions for PA-l and PA-d were observed from the SEM images, which indicate the aggregations of the polyamides that preferred helical conformation.

Figure 3

SEM images of the PA-l (a) and PA-d (b) films.

SEM images of the pan class="Gene">PA-l (a) and pan class="Chemical">PA-d (b) films.

The interactions of noncovalent bonds always play critical roles in the stabilization and conformation of the pan class="Chemical">polyamides, where pan class="Chemical">carbonyl oxygen atom and amide hydrogen atom readily form intermolecular hydrogen bonds. It is indicated by X-ray diffraction (XRD) that PA-l and PA-d are partly crystallized (Figure S7). Polymers crystallized because the chains can pack together in a regular manner.[47] Repulsion by the adjacent substituents and the same configuration of the chiral carbon atoms in the main chain are responsible for the single-handed twisted of the backbone, and intermolecular hydrogen bonds maintain the conformation.

Thermostability of the Optical Activity of the Polyamides

UV–CD spectra were used to study the thermostability of n class="Gene">PA-l and PA-d in ethanol when increasing the temperature from 0 to 65 °C. Figure shows that the characteristic λmax of PA-l and PA-d red-shifts from 206 to 210 nm for the first Cotton effect, whereas the CD intensity rather expresses stability in ethanol, which was related to the substituted ester groups. On the other hand, the reversal of the CD signals from 193 to 200 nm for the second Cotton effect varies obviously when increasing the temperature from 0 to 65 °C. Taking PA-d in ethanol for example, the ellipticity is −20 mdeg at 195 nm at 0 °C, whereas it decreases to −4 mdeg at 200 nm when increasing the temperature to 65 °C (Figure ). In a word, the λmax red-shifts about 5 nm and the magnitude of the ellipticity increases about 16 mdeg when heating from 0 to 65 °C. And similar changes occurred in PA-l, in which the ellipticity intensity increased about 20 mdeg and red-shifted about 2 nm around 190 nm. In fact, the variety of CD Cotton effects from 190 to 200 nm may be related to the induced chiral chromophores of the amide groups in the backbone of the polyamides. Meanwhile, the UV absorption improves from 2.7 to 4.1 abs, which is induced from the π–π* electron transition of amide groups.[45]

Figure 4

UV–CD spectra of PA-l and PA-d in ethanol when increasing temperature from 0 to 65 °C.

UV–CD spectra of n class="Gene">PA-l and PA-d in ethanol when increasing temperature from 0 to 65 °C.

A part of UV absorption chromophores and chiral pools are released from the inside of the coiled chain when warming the solution. Meanwhile, the flexible backbones tend to be less ordered, which is responsible for the intramolecular compensation of asymmetry. Combining these factors together, the CD signals derived from the chirality of the ATA segments are rather stable in temperature, whereas the induced CD (pan class="Disease">ICD) signals derived from the chiral conformation of the backbone are themo-related.

The UV–CD images of n class="Gene">PA-l in solid state vary slightly when the environmental temperature was changed in the range of 0–90 °C. Figure shows that CD images kept rather stable in the ellipticity and the characteristic λmax. The characteristic λmax red-shifts slightly from 222 to 223 nm, and the ellipticities at 208 nm increased from −10 to −8.9 mdeg when the temperature elevated from 0 to 90 °C. In a word, both the characteristic λmax at 222 nm red-shifts and the absolute value of ellipticities at 208 nm increases slightly. The results suggest that the chirality of the polymers is thermodynamically stable in solid state. In fact, crystalline polymers could have rather stable conformation when the intermolecular interactions of the noncovalent bonds are not broken at certain elevated temperature.

Figure 5

UV–CD spectra of PA-l and PA-d in solid state when increasing the temperature from 0 to 90 °C.

UV–CD spectra of pan class="Gene">PA-l and pan class="Chemical">PA-d in solid state when increasing the temperature from 0 to 90 °C.

Conclusions

In summary, we employed bio-based monomers n class="Chemical">l-ATA, d-ATA, and 1,11-undecanediamine as building blocks for the polycondensation of novel optically active materials via a facile method. In addition, l-ATA and d-ATA derived from naturally occurring l-TA and d-TA are chiral enantiomers. PA-l and PA-d display opposite and intensive optical activity in both solution and solid phase, which has not been reported in previous studies. The CD spectra were greatly influenced by the secondary structures of the polymers in solution and the film state. Furthermore, the aggregations in solid film increased the optical activity of the polymers resulting from the single-handed helical conformation. The induced CD signals derived from the chiral conformation of the polyamides are related to the temperature in solution. However, the chirality of the polyamides is rather stable in solid state when varying temperature. The polyamides are more soluble in organic solvents than the traditional nylons, which may open applications in the membrane field, such as separation membranes, coatings, polymer blends, and composites, to name but a few. Consequently, the polymers are expected to be highly facile chiral packing materials because of their low cost, high optical activity, and thermostability in solid state.

Experimental Section

Materials

n class="Chemical">l-Tartaric acid (99%) and d-tartaric acid (99%) were purchased from Aladdin Industrial Inc. 1,11-Undecanediamine was provided commercially by Zibo Guangtong Chemical (China). Other chemicals were of analytical grade or higher and used without further purification.

Characterizations

Gel permeation chromatography (GPC) was carried out on a Waters Instruments 515 HPLC pump at 40 °C. pan class="Chemical">Polymer solutions (1 mg/mL, in pan class="Chemical">DMF) were injected, and dimethylformamide (DMF) was used as mobile phase. Molecular weights were estimated against poly(methyl methacrylate) standards. IR spectra ranging from 4000 to 400 cm–1 were recorded on a NICOLET 460 Fourier transform infrared spectrophotometer from KBr disks. 1H NMR spectra were recorded on a Bruker DPX-400 (400 MHz) spectrometer at room temperature. Spectra of intermediate compounds and polymers were taken in dimethyl sulfoxide-d6 (DMSO-d6). Sample concentrations about 1% (w/v) were used for 1H NMR analysis, and tetramethylsilane was used as internal reference. Elemental analysis was performed on a PerkinElmer 2400 CHNS/O elemental analyzer at 700 °C under nitrogen. Optical rotations of the monomers and polyamides in solutions were measured on a PerkinElmer-341 automatic digital polarimeter using sodium D-line (589 nm) at 20 °C. CD and UV absorption spectra of the monomers and polyamides in solution and solid film state were recorded on a Chirascan (Applied Photophysics) circular dichroism (CD) spectrometer, the compound in ethanol (c = 2.08 × 10–6 mol/mL, repeat unit for polymers) was measured in quartz colorimetric dishes with a path length of 1 mm at 20 °C, the thin film was coated on a quartz substrate by evaporation of the solvent, the thickness of the film was evaluated by the UV-abs equal to that coated from other solutions, and the solution of polyamides (c = 1.0 × 10–5 mol/mL, repeating unit for polymers) was measured in a quartz colorimetric dish with a path length of 0.1 mm when elevating the temperature from 0 to 65 °C. And the concentration of the polyamide solution is about 1 g/100 mL for the preparation of the solid film. The accelerating voltage of field emission-SEM (JSM-7500F, JEOL) was 5 kV, and the PA-l and PA-d films were coated with Pt alloy using an ion coater of type JFC-1600 (JEOL) before SEM measurement. The films were coated from ethanol solution at 50 °C for 3 h, and the thickness was evaluated to be about 5 μm. X-ray diffraction (XRD) measurements were carried out at room temperature using a Rigaku XRD Ultima IV diffractometer operated at 40 kV and 40 mA with curved graphite crystal filtration and Cu Kα1 radiation (λ = 0.15406 nm). The data were collected in the 2θ range of 3–55° at 2°/min.