Antimicrobial new Schiff base polyesters: design, thermal, and structural characterizations


In this work, a series of isovanillin-or syringaldehyde-based new Schiff base monomers were synthesized and polymerized using terephthaloyl chloride by solution condensation polymerization technique. Both the monomers and polymers were characterized by FT-IR, 1H-NMR, and 13C-NMR. The thermal and molecular weight properties were determined by thermogravimetric analysis (TGA) and Gel permeation chromatography (GPC). The antimicrobial activities of the polyesters were evaluated based on minimum inhibition concentration against bacteria such as Escherichia coli, Vibrio parahaemolyticus, and staphylococcus aureus as well as fungi such as Candida albicans and Aspergillus niger. The results of inhibition zones showed that new Schiff base polyesters had good antimicrobial activity.


Schiff base polymers which contain –HC=N– functional group in the main chain are anticipative materials due to their good thermal resistance [1], fiber-forming [2], liquid crystal [3], semiconductive [4], photo- and electro-luminescence [5], and biological [6] properties. Especially in the past decade, Schiff base polymers are superior because their nonlinear optical properties [7] found applications in the photovoltaic cell [8], organic light-emitting diodes [9], photorefractive holographic material [10], and organic field-effect transistors [11]. The properties of polyazomethine can be improved by various functional groups [12,13,14]. It has been reported in the literature that polyazomethine containing aromatic and hydroxyl groups have been ideal structures in terms of their exceptional thermal [15, 16], conductivity [17, 18], and antimicrobial properties [15, 19]. Molecular docking analysis can be used to model the interaction between a small molecule and a protein at the atomic level, which allows us to characterize the behavior of small molecules in the binding site of target proteins as well as to elucidate fundamental biochemical processes [20]. Antimicrobial polymers are a class of polymers that link bioactive monomers which contain functional groups such as hydroxyl, amino, and carboxyl groups [21,22,23]. Generally, cationic polymers are employed for antimicrobial activity since microbes have a negative charge at the outer shell of their membrane of the cell [24]. Similarly, a class of polymers that comprises organic antimicrobial agent in their backbone show good antimicrobial activity [25]. Schiff base compounds are well known for their antimicrobial activity [26]. The incorporation of the Schiff base moiety into the backbone of polyester may lead to the antifouling fabric material [27]. In this study, new Schiff base diol monomers were synthesized by the condensation method. Then, the monomers were polymerized using the solution condensation technique in the presence of triethylamine as a catalyst. The structure of all synthesized monomers and polymers was characterized by FT-IR, NMR techniques. The thermal stabilities of synthesized polyesters were determined by TGA and DSC techniques. Also, the antimicrobial activity of polymers was determined by the disk diffusion method.

Materials and method

3-Hydroxy-4-methoxybenzaldehyde, 4-hydroxy-2,6dimethoxybenzaldehyde, terephthaloyl chloride, and sebacoyl chloride were purchased from TCI chemicals. Bentonite clay and PVC powder were purchased from SRL chemicals and used without further purification. 4-Aminophenol, Propane-1,3-diamine, and Ethane-1,2-diamine were purchased from Alfa Aesar and 4,4′-diaminodiphenyl sulfone, 4,4′-oxydianiline were purchased from Sigma-Aldrich. Solvents such as methanol, ethanol, tetrahydrofuran, dimethylformamide, dichloromethane, dimethylsulfoxide, chloroform, dimethylacetamide, and Acetone were purchased from SRL chemicals and they were purified.

Monomer synthesis

In a round bottom flask fitted with a condenser and magnetic stirrer, 3-hydroxy-4-methoxybenzaldehyde (1 mmol) dissolved in 40 ml of methanol and 1 ml of conc. HCl was added. A solution of 1 mmol of 4-aminophenol dissolved in 40 ml of methanol was slowly added to the flask using a dropping funnel. At first, the mixture was stirred at room temperature for 2 h and then refluxed for 5 h for completion of the reaction (monitored by TLC). After cooling, the precipitate obtained was filtered, washed with ethanol, and dried in a vacuum at 70 °C for 3 h (Scheme 1).

Scheme 1

Monomer synthesis

Polyester synthesis

Polyesterification was carried out (Scheme 2) in a 250 ml three-necked round bottom flask equipped with gas inlet and dropping funnel. The flask was charged with a mixture of Schiff base diol monomer (2.0 mmol) dissolved in 10 ml of dry dimethylformamide (DMF) and 0.8 ml of triethylamine under the nitrogen atmosphere. Terephthaloyl dichloride (TPDC) dissolved in 20 ml of DMF was added dropwise to the above-stirred solution at ice cold condition. Then, the reaction mixture was heated using an oil bath at 80 °C for 4 h and the reaction was terminated with cold water. The product was filtered, washed with NaHCO3 solution, and dried under vacuum at 80 °C for 24 h [28].

Scheme 2

General route of synthesis of Schiff Base polyester


The solubility of all synthesized Schiff base polyesters was studied qualitatively with 1% (w/v) concentration in various organic solvents such as tetrahydrofuran, dimethylformamide, dichloromethane, dimethylsulfoxide, chloroform, dimethylacetamide, and acetone.

Molecular weight determination

The molecular weight determination and polydispersity index (PDI) of the polyesters were determined by the SEC–MALLS light scattering instrument (WYATT QELS +). A concentration of 10 mg per ml was utilized. DMF was used as the mobile phase solvent with a flow rate of 0.8 ml/min.

Thermal properties of polyesters

The thermal behavior of the polyesters was recorded using simultaneous thermal analyzer NETZSCH STA 449F3. Each sample was loaded in an alumina pan and the STA thermogram was recorded at a scanning rate of 5 °C/min. The analysis was carried out under argon atmosphere to determine the decomposition temperature (Td) and melting temperature (Tm) of polyesters.

Antimicrobial properties of polyesters

Antibacterial activity of polyesters

The fresh inoculum was made with 106 cfu/ml of each bacterial culture was spread over the nutrient agar plates using a sterile swab dipped in the bacterial inoculum. After swabbing, wells were punched into the agar medium to the diameter of 8 mm, and each well was loaded with 50 μl (1 mg/ml) of polymer samples and stored at room temperature for 2 h to get the sample diffused. Then, the plates were incubated at 37 °C for about 24 h in the upright position.

Wells containing the same volume of acetone and water were served as negative controls and two antibiotic solutions with 25 μg concentration were used as positive controls. After 24 h of incubation, the diameters of the zone of inhibition was measured and noted in mm.

Antifungal activity of polyester

The well diffusion test was performed using nutrient agar. The inoculum used was prepared using the fungi strains from a 24-h culture on nutrient agar; a suspension was made using a sterile saline solution (0.85%). The fungal inoculum was swabbed in nutrient agar plates. Four wells with each of 8 mm in diameter were cut out of the agar, and 50 μl of the polymer samples was loaded into each well, and the plates were incubated at 35 °C for 24 h. Wells containing the same volume of acetone and water served as negative controls, and two antibiotic solutions with 25 μg concentration were used as positive controls. After 24 h the zone of inhibition was measured.

Results and discussion

Structural analysis of synthesized compounds

The FTIR spectrum of monomer SB1 (C28H24N2O5) (Fig. S1) displayed a relatively broad absorption band at 3507 cm−1 due to –OH stretching and a strong absorption band at 1608 cm−1 due to C=N stretching. Also, it showed a band at 1567 cm−1 due to aromatic C=C stretching whereas the aliphatic CH showed its presence at 2815 cm−1. 1H NMR spectrum of monomer SB1 (Fig. S6) showed four different signals of varying intensities. Among them, three signals resonated in the aromatic region 7.0–8.5 ppm whereas one signal resonated in the aliphatic region at 3.9 ppm. A singlet appearing at 8.4 ppm confirms the presence of azomethine proton. Two multiplet signals appeared in the range 7.0–7.5 ppm due to aromatic ring protons. On the other hand, the oxymethylene protons showed their presence at 3.9 ppm.

In the 13C NMR spectrum of SB1 (Fig. S11), eleven peaks of varying intensities were found, in which the signal at 165.45 ppm is due to azomethine carbon. The signal resonated at 100–160 ppm is arising from the aromatic ring carbons, whereas oxymethyl carbons resonated at 57.28 ppm.

Monomer SB2 (C28H24N2O6S) The FTIR spectrum (Fig. S2) showed –OH stretching frequency at 3445 cm−1. Further, it showed peaks at 1610 cm−1 and 1589 cm−1 due to C=N stretching and aromatic C=C stretching, respectively. 1H NMR spectrum (Fig. S7) showed a multiplet at 6.8–7.5 ppm due to the aromatic protons. A singlet at 8.3 ppm due to azomethine proton confirmed its presence in monomer SB2. The methoxy proton appeared at 3.9 ppm as a singlet. 13C NMR spectrum (Fig.S12) showed signals related to hydroxyl, azomethine, and methoxy carbons at 168.97, 159.12, and 56.35 ppm, respectively.

Monomer SB3 (C30H28N2O8S) FTIR (Fig. S3): 1688 cm−1 (C=N stretching), 3439 cm−1 (O–H stretching), 2899 cm−1 (methoxy C–H stretching), 1140 and 1398 cm−1 (–SO2– stretching). 1H NMR (DMSO-d6) in d (ppm) (Fig. S8): 9.8 (singlet, 2H, O–H), 8.25 (singlet, 2H, HC=N), 3.95 (singlet, 6H, aromatic –OCH3), 6.7 (doublet, 2H, aromatic proton), 7.0 (doublet, 4H, aromatic proton), 7.18 (doublet, 4H, aromatic proton), 7.2 (doublet of doublet, 2H, aromatic proton), 7.4 (doublet, 2H, aromatic proton).

13C–NMR (DMSO-d6) (Fig. S13): 162.12 ppm (2C, C–OH), 157.77 ppm (2C, HC=N) 57.24 ppm (2C, –OCH3) and 112–157 ppm (aromatic carbons). The percentage of C, H, N, and S for M8 were 62.48, 4.87, 4.58, and 5.45, respectively.

Monomer SB4 (C30H28N2O7) FTIR (Fig. S4): 1638 cm−1 (C=N stretching), 3503 cm−1 (O–H stretching), 1543 cm−1 (aromatic C=C stretching), 2789 cm−1 (oxymethylene C–H stretching).1H NMR (DMSO d6) (Fig. S9): 9.7 ppm (singlet, 2H, O–H), 8.4 ppm (singlet, 2H, HC=N), 3.9 ppm (singlet, 6H, –OCH3), 7.1 ppm (doublet, 4H), 7.28 ppm (doublet, 4H), 7.32 ppm (doublet of doublet, 2H), 7.5 ppm (doublet, 2H).

13C-NMR (DMSO-d6) (Fig. S14): 163.82 ppm (2C, HC=N), 59.27 ppm (2C, –OCH3) and 110–155 ppm (aromatic carbons).

Monomer SB5 (C21H26N2O6) (Fig. S5) showed peaks at 1643 cm−1 and 3565 cm−1 due to imine and hydroxyl stretching, respectively. The aromatic C=C and methoxy C-H stretching of SB5 was observed at 1536 cm−1 and 2817 cm−1. Along with these stretching frequencies, 2998 cm−1 for SB5 were also observed due to aliphatic C–H stretching. In 1H NMR (Fig. S10), the monomer showed a signal for azomethine linkage at 8–9 ppm and hydroxyl proton at 9–10 ppm. The 13C NMR spectra (Fig. S15) further confirmed the structures by the appearance of the hydroxyl carbon (C–OH) signal at 161.46 ppm. The imine carbons of SB5 was observed at 158.77 ppm.

Structural characterization of Polyester

5-(4-(4-(3,4-dimethoxybenzylideneamino)phenylsulfonyl)phenylimino)methyl)-2-methoxyphenyl-4-acetylbenzoate (P1)

Yield: 72%, FTIR (Fig. S16): 1754 cm−1 (C=O ester, stretching), 1672 cm−1 (C=N, stretching), 1612 cm−1 and 1515 cm−1 (C=C aromatic, stretching) and 1089 cm−1 (C–O ester, stretching). 1H-NMR (ppm) (DMSO) (Fig. S21): δ 8.45 (s, –CH=N), 8.1 (d, 4H, aromatic), 7.9 (d, 4H), 7.45 (d, 4H), 7.14 (s, 1H) and 3.92 (s, 6H). 13C–NMR (ppm) (DMSO) (Fig. S26): 195.76 (ester carbon), 160.25 (C=N carbon), 152.32, 145.11, 138.54, 129.31, 123.62, 111.52 and 59.67 (methoxy carbon).

5-(4-(4-(3,4-dimethoxybenzylideneamino)phenoxy)phenylimino)methyl-)-2-methoxyphenyl-4-acetylbenzoate (P2)

Yield: 65%, FTIR (Fig. S17): 1745 cm−1 (C=O ester, stretching), 1696 cm−1 (C=N, stretching), 1609 cm−1 and 1510 cm−1 (C=C aromatic, stretching) and 1094 cm−1 (C–O ester, stretching). 1H-NMR (ppm) (DMSO) (Fig. S22): δ 8.47 (s, –CH=N), 8.15 (d, 4H, aromatic), 7.9 (d, 4H), 7.45 (d, 4H), 7.14 (s, 1H) and 3.92 (s, 6H). 13C–NMR (ppm) (DMSO) (Fig. S27): 193.25 (ester carbon), 157.74 (C=N carbon), 151.25, 145.10, 138.54, 129.02, 123.58, 110.15 and 60.67 (methoxy carbon).

2,6-dimethoxy-4-(4-(4-(3,4,5-trimethoxybenzylideneamino)phenylsulfonyl)phenylimino)methyl) phenyl-4-acetylbenzoate (P3)

Yield: 60%, FTIR (cm−1) (Fig. S18): 1750 (C=O ester, stretching), 1671 (C=N, stretching), 1614 and 1526 (C=C aromatic, stretching) and 1062 (C–O ester, stretching). 1H-NMR (ppm) (DMSO) (Fig. S23): δ 8.64 (s, –CH=N), 8.24 (d, 4H, aromatic), 7.97 (d, 4H), 7.52 (d, 4H), 7.32(d, 2H), 7.14 (d, 2H) and 3.82 (s, 6H).13C-NMR (ppm) (DMSO) (Fig. S28): 196.34 (ester carbon), 157.74 (C=N carbon), 152.14, 140.24, 137.09, 129.06, 123.58, 103.42 and 55.85 (methoxy carbon).

2,6-dimethoxy-4-(4-(4-(3,4,5-trimethoxybenzylideneamino)phenoxy)phenylimino)methyl)phenyl-4-acetylbenzoate (P4)

Yield: 64%, FTIR (Fig. S19): 1751 cm−1 (C=O ester, stretching), 1670 cm−1 (C=N, stretching), 1612 cm−1 and 1515 cm−1 (C=C aromatic, stretching) and 1094 cm−1 (C–O ester, stretching). 1H-NMR (ppm) (DMSO) (Fig. S24): δ 8.45 (s, –CH=N), 8.1 (d, 4H, aromatic), 7.9 (d, 4H), 7.45 (d, 4H), 7.14 (s, 1H) and 3.92 (s, 6H). 13C–NMR (ppm) (DMSO) (Fig. S29): 195.76 (ester carbon), 160.25 (C=N carbon), 152.32, 145.11, 138.54, 129.31, 123.62, 111.52 and 59.67 (methoxy carbon).

2,6-dimethoxy-4-(3-(-3,4,5-trimethoxybenzylideneamino)propylimino) methyl)phenyl-4-acetylbenzoate (P5)

Yield: 74%, FTIR (Fig. S20): 2976 cm−1 and 2834 cm−1 (–CH2, stretching), 1734 cm−1 (C=O ester, stretching), 1646 cm−1 (C=N, stretching), 1625 cm−1 and 1502 cm−1 (C=C aromatic, stretching) and 1135 cm−1 (C–O ester, stretching). 1H-NMR (ppm) (DMSO) (Fig. S25): δ 8.45 (s, -CH=N), 7.85 (d, 4H), 7.42 (d, 4H), 7.15 (s, 1H), 3.69 (q, 4H), 3.85 (s, 6H) and 2.05 (d, 2H).13C-NMR (ppm) (DMSO) (Fig. S30): 195.82 (ester carbon), 160.08 (C=N carbon), 153.23, 141.92, 134.12, 119.77, 104.12, 59.4 and 32.2 (alkyl methylene carbon), 55.08 (methoxy carbon).

Solubility of synthesized polyester

The solubility study helps in the optimization of processing conditions of a polymer by choosing a suitable solvent. Polymer dissolution takes place in two steps; one is solvent diffusion and another is polymer chain entanglement. Usually, aromatic polymers of high molecular weight do not dissolve readily in most of the solvents. Hence, solubility becomes an important parameter that needs to be specified clearly for the synthesized polymers. The solubility of the synthesized polyesters was determined qualitatively using solvents such as dimethylformamide, Dimethylsulfoxide, dichloromethane, chloroform, dimethylacetamide, tetrahydrofuran, and acetone. About 10 mg of the polymer was taken in 1 ml of solvent in a stoppered glass bottle, shaken well, and kept aside for 24 h. If the polymer is not soluble at room temperature, it is heated up to the boiling point of the solvent taken. The solubility of the polymers is strongly influenced by the degree of crystallinity, temperature, molar volume, and also the similarity between the structure of the solvent and that of the polymer. Generally, poly(azomethine esters) show good thermal properties but poor solubility. The solubility of poly(azomethine esters) can be enhanced by the presence of aromatic diol monomer in the polymer chain [29]. The introduction of the aromatic bulky group increases the solubility of the polymer [30, 31], but some of the synthesized polyesters show moderate solubility due to the presence of methoxy groups in the polymer backbone [32]. The good solubility of P5 may be due to the presence of an alkyl spacer group which is absent in other polymers (Table 1).

Table 1 Solubility of synthesized polyesters

Molecular weight determination of synthesized polyester

The molecular weights of the polymers can be determined by various techniques such as end group analysis, ebullioscopy, viscometry, osmometry, and gel permeation chromatography. Among these methods, gel permeation chromatography is an appropriate method to determine the absolute molar mass of polymers because it is fast and most reliable than other methods and also suitable for polymers that have a wide range of molecular weight distribution. The absolute molar mass of polymers with different topologies can be determined and also different values of molecular weight (Mn, Mw, MZ, and Mz+1) can be measured in a short period simultaneously. The molecular weight and polydispersity index (PDI) of the newly synthesized polymers were determined. The results of GPC analysis are tabulated in Table 2. For instance, the chromatogram (Fig. 1) of polymers P1 and P2 depicts the weighted average molecular weight (Mw) as 9.725 × 103 and 5.290 × 103 g/mol, respectively, and the polydispersity index (PDI) values of P1 and P2 are 1.986 and 1.921, respectively. Among the two polymers, P2 showed relatively lesser molecular weight compared to P1. This could be attributed to fully aromatic diol monomers which require less activation for polymerization than aliphatic–aromatic diol monomers.

Table 2 GPC data of polyesters
Fig. 1

GPC traces of representative polymers

Thermal analysis of polyester

The thermal properties of synthesized polymers were determined by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) analysis. TGA analysis gives information about the thermal stability of the polymer using initial decomposition temperature (Ton) and DSC provides information about a possible thermal transition that occurs in a polymer sample by Endo and Exo peaks. The thermal stability of the polymer was studied in the range between 30 and 600 °C.

For instance, a close examination of the thermogram of isovanillin-based polymer P1 showed initial decomposition temperature (IDE) to be 240.3 °C corresponding to cleavage of ester linkage, whereas the residual decomposition temperature of polymer was found to be 335.7 °C. The initial decomposition temperature and residual decomposition temperature of syringaldehyde-based polymer P3 were found to be 270° and 340.2 °C, respectively. Both the polymers exhibit good thermal stability due to the presence of azomethine linkage. The enhanced thermal stability of polymer P3 over polymer P1 may be due to the meta-orientation linkage of polymer P1 which leads to the bent structure of the polymer chain which may ultimately result in less thermal stability. Similarly, the thermal stabilities of other representative polymers are shown in Fig. 2. Various thermo-physical changes such as softening (Tg) and melting temperatures (Tm) of the polymer can be studied using the DSC technique. The Tg was used to investigate the physical properties of the polymer such as flexibility and response to mechanical stress. The glass transition temperature Tg of synthesized polyester is in the range 77.2–126.2. These values suggest that the polymers were flexible substantially below their decomposition temperatures. The DSC traces of polymers are shown in Fig. 3. The Tg and Td values of synthesized polymers are tabulated in Table 3.

Fig. 2

TGA thermogram of representative polymers

Fig. 3

DSC thermogram of representative polymers

Table 3 TGA and DSC data of synthesized polyesters

Antimicrobial activity of polyester

The Schiff base polymers containing phenol groups showed good antimicrobial activities. The antimicrobial activities of Schiff base polymers were due to inhibition of enzyme production by phenol group in bacteria and fungi [33,34,35]. The N and O donor atoms present in the Schiff base backbone of polymer bind with the free –OH group in microbial enzyme leading to antibacterial activity. The synthesized polyesters consist of both phenol group and donor atoms which are responsible for their antimicrobial activities. The representative polymers were tested against three bacteria, namely E. coli (ATCC 25,922) bacterium, Vibrio parahaemolyticus, and S. aureus (MRSA), and two fungi such as C. Albicans and A. niger to investigate the usability of polymers as antimicrobial agents (Fig. S31). The measured inhibition zones with control A and B are tabulated in Table 4. The antibacterial study indicated that P2 showed maximum zone of inhibition with 15–16 mm relative to other chosen polymers tested. The better activity of P2 compared to other polymers could be due to high molecular weight, since the antimicrobial activity increases with an increase in the molecular weight of polymer [36, 37].

Table 4 Antimicrobial activity of representative polyesters

The antifungal activities of representative polymers P1 and P3 did not show any activity against A. niger but P2 showed better activity against both the fungi. The antimicrobial activity not only depends on the structure of the sample but also on the amount of sample utilized [33, 34].


Five new Schiff base diol monomers were synthesized starting from isovanillin and syringaldehyde. A series of aromatic Schiff base polyester was synthesized using the above-said diol monomers with terephthaloyl chloride by solution condensation technique. The synthesized polyesters were characterized by FTIR and NMR techniques. The weight average molecular weight (Mw) of polyester was in the range 5.290 × 103 −9.725 × 103, indicating that reasonably moderate to low molecular weight polyesters. The synthesized polyester show good thermal stability and exhibit glass transition temperatures 77.2–126.2. The antimicrobial activity of polyesters was tested using the disk diffusion method and found to be polymer P2 showing good activity compared to other polymers.


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Ibrahim, A.M., Shabeer, T.K. Antimicrobial new Schiff base polyesters: design, thermal, and structural characterizations. Polym. Bull. (2021).

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  • Schiff base polyester
  • Synthesis
  • Thermal studies
  • GPC
  • Antimicrobial studies