Facile Synthesis and Enhanced Aggregation-Induced Circular Dichroism of Novel Chiral Polyamides.

A series of optically active polyamides, PAnLATs (n = 9-12), were prepared by polycondensation of l-tartrate-derived diacid with achiral aliphatic diamines in which the number of the methylene is from 9 to 12. The monomers could be obtained easily, and the synthesis of the polyamides is facile. The number-average molecular weight (M̅ n) of PA9LAT, PA10LAT, PA11LAT, and PA12LAT is 35 500, 24 400, 35 800, and 20 900 g/mol, respectively, and the related polydispersity index is 3.4, 3.3, 3.4, and 3.0. These polyamides display intense optical activity in solution. Particularly, the stronger ellipticities and different circular dichroism (CD) images were presented when the polymers were evaluated in the solid state. The crystalline properties of the polymers were studied to illuminate the enhanced aggregation-induced CD. Moreover, PA9LAT and PA11LAT have similar crystal parameters, and PA10LAT is similar to PA12LAT, which revealed why the chirality of the polyamides displays an odd-even effect. On the other hand, the solubility of the polymers in organic solvents was studied, and thermogravimetric-differential thermal analyzer was utilized to characterize the thermal properties.

Introduction

Complicated and tightly controlled polymerizations, difficulty in obtaining monomers, and expensive chiral catalysts or initiators are the limiting factors for the achievement of n class="Chemical">polymers with optical activity, such as polyacetylenes,[1,2] polyisocyanates,[3,4] and polyethylenes,[5] which have been obtained by additional polymerization of unsaturated monomers substituted with chiral pendants. If the chiral polymers with desired optical activity can be achieved directly by introducing chiral monomers into the backbone, the preparation will be simplified significantly. In fact, natural macromolecules such as proteins, peptides, and polysaccharides consist of chiral constitutional units, and their formation processes occur in the specific living system. Moreover, if the chiral monomers occur naturally, the methodology will be of further significance because the monomers are renewable, easily available, inexpensive, and environmentally friendly, to name a few. Nevertheless, the naturally occurring chiral sources such as amino acids and aldaric acids are always saturated, which are unsuitable for additional polymerization, but suitable for polycondensates, such as polyamides and polyesters.[6−8] Furthermore, there are few reports related to the development of the polymers with desired chirality through the introduction of chiral atoms into the backbone. In fact, it is a challenge to avoid the racemization and intramolecular compensation when introducing the chiral pools into the backbone, especially for linear aliphatic polyamides. The preparation of polyamides with optical activity has received considerable attention in recent years.[9] However, success has been achieved so far only in a limited number of aromatic polyamides bearing chiral side chains on the nitrogen atoms,[10,11] and the linear aliphatic polyamides with optical activity are very scarce. In addition, the practical applications of optically active polymers are commonly in the solid phase. Therefore, the evaluation in the solid state could directly provide chiral information about the optically active materials and make it more reliable for practical applications. Especially, circular dichroism (CD) is regarded as one of the most powerful and efficient techniques for chirality analysis.[12,13]

Takeishi et al. synthesized aromatic polyamides with a helical structure by introducing axially dissymmetric n class="Chemical">biphenylene into the backbone.[14,15] Yokoyama and coworkers prepared poly(p-benzamide)s bearing chiral side chains on the nitrogen atoms, which exhibited CD signals in solution, and the helical conformations were investigated by the study of oligo(benzamides) with different repeating units.[10] Bou and co-workers have developed tartaric acid-based polyamides synthesized by polycondensation of bis(pentachlorophenyl)-tartarates with activated diamines, N,N′-bis(trimethylilyl) derivatives, under rigorous exclusion of moisture and a relatively long polycondensation time (3–4 days); these polyamides display specific rotation in solution, but there is no report on the optical activity in the solid phase.[16,6,17]

In this work, a series of optically active polyamides (PAnn class="Gene">LATs) were prepared by polycondensation of 1,9-nonamethylenediamine, 1,10-decamethylenediamine, 1,11-undecanediamine, and 1,12-dodecanediamine, respectively, with l-2,3-di-O-acetyltataic dichloride (l-ATC) via interfacial polymerization. To investigate the chirality of the polyamides, we researched the optical activity of these polyamides both in solution and in the solid state. The crystallization of the PAnLATs was characterized by X-ray diffraction (XRD) patterns. In addition, the solubility and thermal stability were investigated to compare the polyamides with different numbers of the methylene in the aliphatic diamine segment.

Results and Discussion

Preparation of PAnLATs

The reaction strategy to obtain PAnLATs is outlined in Scheme . To obtain linear n class="Chemical">polyamides, the hydroxyl groups of l-tartaric acid (l-TA) were protected via O-acetylization to form l-2,3-di-O-acylatedtartaric acid (l-ATA). l-ATC was prepared by using bis(trichloromethyl)carbonate (BTC) as the chlorination agent to enhance its reactive activity to the diamines. 1,9-nonamethylenediamine, 1,10-decamethylenediamine, 1,11-undecanediamine, and 1,12-dodecanediamine were selected as diamines for the synthesis of PA9LAT, PA10LAT, PA11LAT, and PA12LAT, respectively. Fourier transform infrared (FTIR), 1H NMR, and elemental analysis were utilized to corroborate the chemical structure of PAnLATs.

Scheme 1

Synthetic Route Leading to PAnLATs (n = 9–12)

The infrared spectra display sharp absorption peaks at around 1760, 1660, and 1540 cm–1. The first peak arises from the ester side group, and the other two arise from the n class="Chemical">amide group. The absorption at 3320 cm–1 represents the N–H stretching vibration. The absorption of the methoxy side group is observed at 1200 cm–1. The methylene stretching vibration is evidenced by the two sharp absorption peaks at 2930 and 2850 cm–1 (Figure S1). The specific 1H NMR spectroscopies of the four polyamides are shown in Figure S2. The chemical shifts and related protons are described in detail in the Supporting Information. In short, 1H NMR spectra confirm definitely the chemical structure of PAnLATs and exhibit the expected peak area. The elemental analysis data of PAnLATs are listed in Table . The calculated components are consistent with the theoretical values.

Table 1

Synthesis and Properties of PAnLATs

		elemental analysisa
polymers	yield (%)	C	H	N	M̅W (g/mol)	M̅n (g/mol)	polydispersity index (PDI)	X̅n
PA9LAT	78	57.16(57.29)	7.80(7.92)	7.96(7.86)	120 400	35 500	3.4	94.0
PA10LAT	80	58.23(58.36)	8.02(8.16)	7.69(7.56)	81 100	24 400	3.3	65.9
PA11LAT	82	59.23(59.36)	8.28(8.39)	7.12(7.29)	123 900	35 800	3.4	93.2
PA12LAT	76	60.10(60.28)	8.58(8.60)	6.90(7.03)	62 700	20 900	3.0	52.4

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

To achieve polyamides with a high molecular weight, we prepared the same concentration ratio of n class="Chemical">diamine and diacid chloride and operated the removal of films continuously. In addition, to facilitate the solubility of the long-alkylene diamines in the aqueous phase, first, ethanol was used to dissolve the diamines and then a certain amount of water was added to obtain the aqueous phase. The molecular weights for each polymer are shown in Table , and the gel-permeation chromatography (GPC) curves of PAnLATs are shown in Figure S3. M̅n values of PA9LAT, PA10LAT, PA11LAT, and PA12LAT are 35 500, 24 400, 35 800, and 20 900 g/mol, respectively. The related PDIs are all around 3.0. As a result, PAnLATs with a large molecular weight were obtained successfully.

Solubility of PAnLATs

The solubility of PAnLATs is shown in Table . PAnn class="Gene">LATs are all soluble in trifluoroacetic acid (TFA) and hexafluoroisopropanol (HFIP), which can dissolve almost all aliphatic polyamides. Moreover, PAnLATs are soluble in dimethyl sulfoxide (DMSO), N-methyl-2-pyrrolidone (NMP), and N,N′-dimethyl formamide (DMF), which cannot dissolve unsubstituted aliphatic polyamides. DMF can be used as the eluent for GPC measurements. However, these polyamides are insoluble in TFE and dimethylacetamide (DMAc). In addition, the polymers are slightly soluble in ethanol. Specifically, the solubility of PA11LAT in ethanol at 20 °C is 9.48 × 10–3 g/mL, whereas for PA9LAT, PA12LAT, and PA10LAT the solubility in ethanol is 9.06 × 10–3, 4.86 × 10–3, and 4.28 × 10–3 g/mL, respectively. Thus, the solubility of these polyamides is enhanced notably with respect to that of unsubstituted polyamides, that is, nylon11,11. PAnLATs with polar ester side groups favor solubility in organic polar solvents, apparently because the side groups disrupt the intermolecular associations.

Table 2

Solubility of PAnLATs

	solubilitya
polyamides	CHCl3	DMSO	NMP	TFE	TFA	ethanol	HFIP	DMF	DMAc
PA9LAT	–	+	+	–	+	±	+	+	–
PA10LAT	–	+	+	–	+	–	+	+	–
PA11LAT	–	+	+	–	+	±	+	+	–
PA12LAT	–	+	+	–	+	–	+	+	–

–, insoluble; ±, slightly soluble; +, soluble at room temperature. DMSO: dimethyl sulfoxide; TFE: trifluoroethanol; TFA: trifluoroacetic acid; HFIP: hexafluoroisopropanol; DMF: N,N′-dimethyl formamide; DMAc: dimethylacetamide.

Thermal Properties of PAnLATs

The thermal stability of PAnLATs was evaluated by thermogravimetry under n class="Chemical">nitrogen flow, and the results are shown in Figure . According to the chemical structure of PAnLATs, the contents of the acyloxy groups in the polymers are 33.13, 31.88, 30.72, and 29.63%, respectively, for PA9LAT, PA10LAT, PA11LAT, and PA12LAT. When heated to 250 °C, the weight loss of the polymers is around 30%, which is consistent with the contents of the acyloxy groups. Thus, the lower-temperature decomposition peaks at around 220 °C correspond to the release of the acyloxy side groups. The main chain scission is thought to take place in the second decomposition step at around 450 °C.

Figure 1

Thermogravimetric-differential thermal analysis traces (weight loss: solid line; decomposition rate: dashed line) of PAnLATs.

Optical Activity of PAnLATs Both in Solution and in Solid State

The specific optical rotations ([α]D20) of the polymers and the ren class="Gene">lated chiral monomer were measured, and the results are listed in Table . The [α]D20 values of these polyamides are not fixed but vary within a certain range. The [α]D20 magnitudes of PAnLATs decrease in the order of PA10LAT (−17.9 to −20.0°), PA12LAT (−15.2 to −16.0°), PA9LAT (−13.3 to −15.9°), and PA11LAT (−11.8 to −13.6°).

Table 3

Specific Optical Rotation of PAnLATs in DMSO

monomer and polymers	[α]D20 (deg, c = 1 g/100 mL, DMSO)
l-ATA	–21.0
PA9LAT	–13.3 to −15.9
PA10LAT	–17.9 to −20.0
PA11LAT	–11.8 to −13.6
PA12LAT	–15.2 to −16.0

All of these four polyamides express a negative value of [α]D20, which are consistent with the corresponding monomer n class="Chemical">l-ATA ([α]D20 = −21.0°). However, the magnitudes of the [α]D20 are not associated strictly with the density of the chiral carbon atom in the backbone. Essentially, the specific optical rotation for a chiral species is a composite of the response of the multiple conformations.[18] The linear aliphatic polyamides containing flexible methylene segments display varying conformations in the dilute solution, which influences the asymmetry of the macromolecules. The specific optical rotation of the odd–even polyamides, PA9LAT and PA11LAT, are weaker than that of the even–even polyamides, PA10LAT and PA12LAT.

To make a further investigation of the reason for the optical activity of the polymers and the influences of the number of methylene groups in the n class="Chemical">diamine segment, UV–CD spectra of the polyamides were researched and compared with the corresponding optical activity of the monomer. Figure a shows that the polyamides display the same negative Cotton effect at about 210 nm, and the intensities are −13.1 mdeg for PA11LAT, −10.8 mdeg for PA12LAT, −8.5 mdeg for PA9LAT, and −7.3 mdeg for PA10LAT. On the other hand, these CD curves exhibit a reversal at around 200 nm, and the strength of the reversal of PA10LAT is the strongest, PA9LAT the second, PA12LAT the third, and PA11LAT the least (Figure a). In addition, the CD intensities of the polymers are less strong than the corresponding monomer (−20 mdeg at 215 nm), but l-ATA does not show any such reversal tail (Figure ).

Figure 3

UV–CD spectra signals of PAnLATs in HFIP (c = 2.08 × 10–6 mol/mL of the repeating unit) at 20 °C (a). Relationship of the ellipticities of PAnLATs at 210 nm and the number of methylene groups in the diamine segment (n) (b). Relationship of n and the reversal strength from 210 to 200 nm (c).

Figure 2

CD signals of l-ATA characterized in HFIP (c = 2.08 × 10–6 mol/mL) and in the solid state.

To clarify the relationship of the optical activity and the number of methylene groups in the n class="Chemical">diamine segment for PAnLATs, we constructed the tendency chart of the magnitudes of CD signals at the specific wavelength (Figure b,c). Figure b shows that the ellipticities of PAnLATs at 210 nm increase in a zigzag pattern with the number of methylene groups in the diamine segment, and Figure c suggests that the reversal intensity from 210 to 200 nm decreases in a zigzag pattern with the number of methylene groups.

UV–CD spectra signals of PAnn class="Gene">LATs in HFIP (c = 2.08 × 10–6 mol/mL of the repeating unit) at 20 °C (a). Relationship of the ellipticities of PAnLATs at 210 nm and the number of methylene groups in the diamine segment (n) (b). Relationship of n and the reversal strength from 210 to 200 nm (c).

In short, these PAnLATs display different optical activities ren class="Gene">lated to the odd–even effect of the methylene unit in the diamine segment, which was conformed both by CD signals and their specific optical rotations. In fact, the flexible linear backbones tend to be less ordered in a dilute solution, which leads to intramolecular compensation of asymmetry and take responsibility for the less intense Cotton effect of polymers compared to that of the monomer.[19] As a result, the optical activity of the polyamides is influenced by both intrinsic chirality of the tartrate units and the conformations of the main chain in solution.

In this work, the CD spectra of these PAnn class="Gene">LATs in the solid state were investigated. To facilitate the research of PAnLATs in different states, the thicknesses of the PAnLAT films were adjusted to achieve equal UV-abs, which is similar to those in solution, when the UV–CD spectra were measured in the solid phase. Figure a shows that the CD signals of PAnLATs in the solid film state display clear splits, with the first positive Cotton effect presenting at 221 nm and then the specific reversals at around 205 nm and the second reversal at about 185 nm, whereas a weak reversal around 200 nm was observed in solution. Therefore, the CD images of PAnLATs in the solid state vary clearly from those in the dilute solution, and the CD signals of PAnLATs in the solid phase are higher than those in solution when the similar UV absorption was measured. However, the chiral monomer, l-ATA, displays similar CD images both in solution and in the solid state (Figure ). On the other hand, the polyamides in the solid phase exhibit an odd–even effect in the strength of the optical activity. For instance, the reversal strength from 221 to 205 nm for PA10LAT is the most intensive (34.6 mdeg), whereas PA11LAT is the least (22.3 mdeg). In general, the intensity of the reversal decreases in a zigzag pattern with the number of the methylene groups (Figure b).

Figure 4

(a) UV–CD spectra signals of PAnLATs at 20 °C in the solid film; the thickness of the films was evaluated by the equal UV-abs to each other. (b) Relationship of the reversal strength from 221 to 205 nm and the number of methylene groups in the diamine segment of PAnLATs.

(a) UV–CD spectra signals of PAnn class="Gene">LATs at 20 °C in the solid film; the thickness of the films was evaluated by the equal UV-abs to each other. (b) Relationship of the reversal strength from 221 to 205 nm and the number of methylene groups in the diamine segment of PAnLATs.

According to the exciton coupling theory, the split of the CD signal is ren class="Gene">lated to the interaction between the adjacent chromophores.[20] In our cases, the chiral pools in the l-ATA segment induced the neighbor amide group to a chiral chromophore.[21] The weak split of the CD image in the dilute solution is due to the less intermolecular aggregation and less ordered conformations. The stronger CD signals and different CD images of PAnLATs compared to those of the corresponding monomer indicate that all of the polyamides adopt a chiral conformation in the solid state. The enlarged optical activity in the solid state is mainly due to the induced chiral amide groups, which display high enantiopurity for the same twisted direction of the amide groups in the backbone. Furthermore, PAnLATs in the solid state display different optical activities related to the odd–even effect of the methylene unit in the backbone, which may be resulted from the different aggregations in the solid phase.

Crystalline Properties of PAnLATs

The CD spectra results indicate that PAnn class="Gene">LATs have specific CD images and display obvious chirality amplification in the solid state when compared to those in solution as well as the related chiral monomer. In addition, the optical activity of polyamides presents an odd–even effect in the solid phase. It is reported that the noncovalent interactions among the polymers play a crucial role in the chiral conformation of the polymer, which is responsible for the chirality amplification.[22,23] In the case of PAnLATs, the crystallization and the intramolecular hydrogen bonding between C=O and N–H may aid in the spontaneous organization of the chiral/helical architectures. We investigated the crystallinity and the intramolecular hydrogen bonding between C=O and N–H via XRD and FTIR spectra for the understanding of the specific chirality of the polyamides.

Figure represents XRD patterns of PA9LAT, n class="Chemical">PA10LAT, PA11LAT, and PA12LAT in the powder state. It indicates that the polyamides are partly crystallized. The powder samples of PA9LAT and PA11LAT possess similar XRD patterns, which reveal strong reflection peaks at diffraction angles 2θ = 20.9 and 20.6°, and the corresponding d-spacings are 0.42 and 0.43 nm, respectively. On the other hand, PA10LAT shows three reflection peaks at 2θ = 17.4° (d-spacing = 0.51 nm), 2θ = 21.2° (d-spacing = 0.42 nm), and 2θ = 11.4° (d-spacing = 0.77 nm). PA12LAT expresses similar XRD patterns, with characteristic reflection peaks at 2θ = 17.6° (d-spacing = 0.50 nm), 2θ = 20.9° (d-spacing = 0.42 nm), and 2θ = 11.6° (d-spacing = 0.76 nm). It is reported that the crystal structure of aliphatic polyamides is greatly related to the odd–even numbers of carbon atoms in the diamine segments and diacid segments.[24] In this case, the different crystal structure of PAnLATs presents the odd–even effect to the number of carbon atoms in the backbone. PA9LAT and PA11LAT are odd–even polyamides whose powders appear crystalline, similar to the typical characteristics of a γ-phase with a diffraction peak at 0.415 nm.[25] In fact, the γ-form structure is always found when hydrogen bonds cannot be well established with the extended conformation, as seen with odd–even or even–odd nylons.[26] The repulsive effects of the vicinal side pendent ester groups would be expected to restrict the aldaric acid moiety in an extended conformation. However, polymers can crystallize because the chains pack together in a regular manner.[27] To further study the intermolecular interaction, FTIR spectra of PAnLATs were obtained to compare the N–H stretching vibration at 3320 cm–1 when the ester groups at 1740 cm–1 have no detectable difference (Figure ). Figure shows that the intensity of the absorption from N–H stretching vibration at 3320 cm–1 decreases in the order of PA10LAT, PA12LAT, PA9LAT, and PA11LAT. Although crystal structures of PA10LAT and PA12LAT are different from the traditional aliphatic nylons, more hydrogen bonding among polymer chains help them aggregate in a more ordered packing.

Figure 5

XRD patterns of the polyamides in the powder state.

Figure 6

FTIR spectra of PA9LAT, PA10LAT, PA11LAT, and PA12LAT.

The results indicate that more hydrogen bonds are formed in even–even n class="Chemical">polyamides than those in odd–even polyamides when the polyamide chains aggregate in the solid phase. Interestingly, the optical activity of PAnLATs is identical to the results of FTIR and XRD measurements, which demonstrates that the higher the crystallinity of the polyamide, the more intense CD signals the polyamide presents. The study of the crystalline properties and the intramolecular hydrogen bonding between C=O and N–H of PAnLATs suggests that the molecular chains pack stereoregularly and contribute to form a stable chiral conformation in the solid state, which avoids the intramolecular compensation of asymmetry and takes responsibility for the chirality amplification.

Conclusions

A series of PAnLATs, n class="Chemical">PA9LAT, PA10LAT, PA11LAT, and PA12LAT, were prepared successfully via a facile method by introducing the chiral pools into the backbone. The mild reaction conditions contribute greatly to avoiding racemization and intramolecular compensation. For the first time, the optical activity of these polyamides with different numbers of methylene groups in the backbone was studied both in a dilute solution and in the solid state. These linear aliphatic polyamides show a similar Cotton effect, analogical CD images, and different ellipticities in the solution and solid state. When the crystalline polymers are packed together in a regular manner, the main chain of the polyamide could adopt a chiral conformation, which is responsible for the enhanced aggregation-induced CD signals. PA10LAT and PA12LAT (even–even polyamides) exhibit a more stable crystal structure than PA9LAT and PA11LAT (odd–even polyamides), which is identical to the variation trend of the optical activity for the polyamides. In brief, the higher the crystallinity of the polyamide, the more intense optical activity the polyamide presents. PAnLATs display intensive chirality in the solid phase, which facilitates the applications in the chiral packing materials for the optical resolution of racemates.

Experimental Details

Materials

1,9-Nonamethylenediamine, n class="Chemical">1,10-decamethylenediamine, 1,11-undecanediamine, and 1,12-dodecanediamine were provided commercially by Zibo Guangtong Chemical Co., Ltd. (China). l-Tartaric acid (99%) was purchased from Aladdin Industrial Inc. Other chemicals were of analytical grade or higher and were used without further purification. Solvents to be used under anhydrous conditions were dried by standard methods.

Characterization

GPC was carried out on Waters instruments with a Waters 515HPLC Pump at 40 °C. Polymer solutions (1 mg/mL, in n class="Chemical">DMF) were injected, and DMF was used as the eluent. Molecular weights were estimated against poly(methyl methacrylate) standards. FTIR spectra ranging from 4000 to 400 cm–1 were recorded on a TENSOR II FTIR spectrophotometer from KBr disks. In the case of PAnLATs, 2 g of PAnLAT and KBr compounds, in which the quantity of repeating units for PAnLAT is 1 mmol, were blended and grinded. A quantity of 150 mg of the mixture was taken to prepare the KBr disk under a pressure of 13 MPa for 2 min. 1H NMR spectra were recorded at room temperature on a Bruker DPX-400 (400 MHz) spectrometer. Tetramethylsilane was used as the internal reference. Elemental analysis was performed on a PerkinElmer 2400 CHNS/O elemental analyzer at 700 °C under a nitrogen atmosphere. Thermogravimetric analysis (TGA) was performed under a nitrogen atmosphere with a METTLER TOLEDO TGA/differential scanning calorimetry thermobalance at a heating rate of 10 °C/min. XRD measurements were carried out at room temperature using a Rigaku XRD Ultima IV diffractometer operated at 40 kV and 40 mA with a filtered curved graphite crystal and Cu Kα1 radiation (λ = 0.15406 nm). The samples were fixed on the sample holder, and the data were collected in a range of 2θ = 10–40° at 4°/min. The solubilities of PAnLATs were evaluated. The solvent (1 mL) was added to the polymers (10 mg, respectively) in vials. The vials were allowed to stand overnight at room temperature, and the solutions were then centrifuged before determining the solubility. Optical rotations of the polyamides in solutions were measured on a PerkinElmer-341 automatic digital polarimeter using sodium D-line (589 nm) at 20 °C. UV–CD absorption spectra of the polyamides in solution and in the solid phase were recorded on a Chirascan (Applied Photophysics) CD spectrometer. The compound in solution (c = 2.08 × 10–6 mol/mL, repeating unit for polymers) was measured in a quartz colorimetric dish with the path length of 1 mm at 20 °C. A thin film was coated on a quartz substrate by evaporation of the solvent, and the thickness of the film was evaluated by UV-abs.

Synthesis of the PAnLAT Polymers

The reaction strategy to obtain PAnLATs is outlined in Scheme . n class="Chemical">l-ATA was prepared according to the literature.[28]l-Tartaric acid (10 g) was stirred in 50 mL of acetic anhydride for 2 h at 60 °C. The mixture was kept at 0 °C overnight, and the precipitated l-2,3-di-O-acetyltartaric anhydride was collected by filtration and washed with anhydrous ether. The anhydride was dissolved in 30 mL of water, stirred for 30 min at 30 °C, and then extracted with ethyl acetate (3 × 30 mL). The organic phase was dried over Na2SO4 and filtered. The filtrate was concentrated under reduced pressure at 35 °C till silky white crystals (l-ATA) were formed. Yield: 11.2 g (72%), mp 120 °C. 1H NMR (D2O, ppm): 2.23 (s, 6H, CH3), 5.76 (s, 2H, CH). FTIR (KBr): 3400 cm–1 (O–H carboxylic acid); 2950, 1380 cm–1 (CH3); 1740 cm–1 (C=O ester); 1070, 1220 cm–1 (C–O–C ester). [α] D20 = −18.1° (c = 1 g/100 mL, H2O).

l-ATA (2.4 g, 0.01 mol), BTC (2.5 g, 0.0075 mol), and toluene (50 mL) were stirred in a three-necked flask, and then DMF (2 mL) was added dropwise while the reaction was kept in the ice-bath. Subsequently, the reaction system was heated to 30 °C and kept for 1 h till unsoluble l-ATA transformed into l-ATC, which was soluble in toluene. The toluene solution containing l-ATC was prepared for the synthesis of the polyamides.

1,9-Nonamethylenediamine, 1,10-decamethylenediamine, 1,11-undecanediamine, and 1,12-dodecanediamine were employed for the preparation of PAnLATs via interfacial polymerization. Taking PA12LAT for example, 0.01 mol of 1,12-dodecanediamine was dissolved in 5 mL of ethanol and then 45 mL distilled water was added slowly under stirring at 40 °C. Triethylamine (0.02 mol) was added to the aqueous phase as the acid acceptor. A volume of 50 mL of toluene containing 0.01 mol of l-ATC was added into the breaker slowly. Then, the interface formed and the polymerization began immediately when adding l-ATC. A PA12LAT film formed at the liquid interface and was collected by a glass rod. The polyamide was washed well with distilled water and then dried in a vacuum oven at 60 °C for 8 h. All of the polymers of PAnLATs were prepared similarly.