Synthesis , Characterization and Biological Studies of New Linear Thermally Stable Schiff Base Polymers with Flexible Spacers

Five new linear Schiff base polymers having azomethine structures, ether linkages and extended aliphatic chain lengths with flexible spacers were synthesized by polycondensation of dialdehyde (monomer) with aliphatic and aromatic diamines. The formation yields of monomer and polymers were obtained within 75–92%. The polymers with flexible spacers of n-hexane were somewhat soluble in acetone, chloroform, THF, DMF and DMSO on heating. The monomer and polymers were characterized by melting point, elemental microanalysis, FT-IR, HNMR, UV-Vis spectroscopy, thermogravimetry (TG), differential thermal analysis (DTA), fluorescence emission, scanning electron microscopy (SEM) and viscosities measurement of their dilute solutions. The studies supported formation of the monomer and polymers and on the basis of these studies their structures have been assigned. The synthesized polymers were tested for their antibacterial and antifungal activities.


Introduction
2][3][4][5] They are synthesized by polycondensation reaction between diamine and dialdehyde or diketone. 6,7They are an important class of compounds and find their application in different fields.][16] The Schiff base polymers demonstrate antimicrobial activity against bacteria, yeast and fungi. 17,18Thus these can be used for the purification of industrial contaminants from heavy metals and microbiological organisms and are significant for environmental applications.0][21][22][23][24] The poly Schiff bases may also form li-quid crystalline melts, the aromatic azomethine blocks being good mesogens. 3,25,26he polymeric Schiff bases are attractive polymers, but they indicate poor solubility in common organic solvents and are difficult to liquate for practical applications in various fields. 27However attempts have been made to improve the solubility of the polymers by polycondensation reactions with some aliphatic-aromatic aldehydes, 15 incorporating phosphorus in the main chain, 28 including oxygen atom in the repeat units, 29 inserting solubility enhancing groups in the backbone 30 and introducing alkyl or alkoxy groups in the ortho position of the aromatic ring. 31The present work examines the effect of increasing the flexible spacer between aromatic aldehydes, the introduction of ether linkage and heterocyclic ring in backbone on the solubility of the polymers.Five new polymers have been synthesized by polycondensation of a monomer with five different diamines and characterized by spectroscopic, thermal analysis and viscometric measurements and scanning electron microscopy (SEM).

Equipment
The elemental microanalysis of the polymers was carried out by Elemental Microanalysis Ltd, Devon, U.K. The mass spectra of the monomer (4,4'-hexamethylenebis(oxybenzaldehyde) (HOB) was recorded at the HEJ Research Institute of Chemistry, University of Karachi on Jeol JMS 600 mass spectrometer.The spectrophotometric studies in DMSO were recorded on double beam Lambda 35 spectrophotometer (Perkin Elmer, Singapur) within 500-200 nm with dual 1cm quartz cuvettes.The spectrophotometer was controlled by the computer with Lambda 35 software.Infrared spectra of the compounds were recorded on Nicolet Avatar 330 FT-IR (Thermo Nicolet Corporation, U.S.A) with attanulated total reflectance, accessory (smart partner) within 4000-600 cm -1 .The 1 HNMR spectra of the dialdehyde and polymers were recorded on a Bruker AVANCE-NMR spectrometers at 300 MHz using DMF as solvent and tetramethylsilane (TMS) as internal reference at HEJ Research Institute of Chemistry, University of Karachi.Spectrofluorimetric studies were carried out on Spectrofluorophotometer RF-5301PC Series (Shimadzu Corporation, Kyoto, Japan) with 1cm cuvettee.Thermogravimetry (TG) and differential thermal analysis (DTA) were carried on thermogravimetric thermal analyzer Pyris Diamond TG/ DTA (Perkin Elmer, Japan) from room temperature to 600 °C with a nitrogen flow rate 100 ml / min.Sample 5 mg was placed in platinum crucible and recorded against alumina as reference with heating rate of 20 °C / min.The morphologies of the polymers were examined by scanning electron microscopy (SEM) using a JEOL JSM-6490LV instrument at Centre for Pure and Applied Geology, University of Sindh.The polymers were ground to powder and were placed on carbon conducting tape before recording their SEM.The SEM images were taken at an accelerating voltage of 20 KV.
The viscosity measurement of dialdehyde and polymers in DMF with 0.02-0.06g/dl were recorded in the tem-perature range 383-323 K with an interval of 10K by using a suspended level viscometer (Technico ASi 445).Each time 15ml of the solution was used and average flow time was noted from atleast three readings (n = 3).The flow time of the solvent was also recorded.A Gallenkamp viscometer water bath was used to control the temperature.The reduced viscosity (η red ) was calculated by dividing specific viscosity (η sp ) by concentration (d/dl).The intrinsic viscosity (η) was calculated by plotting η red against concentration and extraploting to zero concentration.The Huggins constant (K H ) was calculated from the slope.
The antibacterial activity of polymers was measured against Escherichia coli, Shigella flexenari, Staphylococcus aureus and Pseudomonas aeruginosa.For antibacterial assay 2 mg of polymers were separately dissolved in DMSO to get concentration of 50 μg/disk.Percent inhibition of polymers was compared with the percent inhibition of drug ofloxacin.The antifungal activity of polymers was measured against Trichphyton rubrum, Candida albicans, Microsporum canis, Fusarium lini, Candida glabrata.The standard drug Amphotericin B was used for Aspergillus niger and Miconazole for the other fungal species.The concentration of polymers was 200 μg/ml of DMSO.Incubation was at 28° ± 1°C and incubation period was 7 days.

3. Preperation of Monomer 4,4'-hexamethylenebis(oxybenzaldehyde)(HOB)
To 0.2 mol (24.5 g) of 4-hydydroxybenzaldehyde into 250 ml round bottom flask equipped with a condenser was added 0.25 mol (25 g) anhydrous sodium carbonate.The contents were stirred with magnetic bar and added 0.1 mol (15.38 ml) of 1,6-dibromohexane dissolved in 25 ml DMF.The reaction mixture was refluxed (about 150 °C) for 5 h under continuous stirring.After cooling, the product was poured into 2 l of cold distilled water (5 °C) and allowed precipitate to settle.The product was filtered and washed with KOH (0.1M) and then three times with water.The product was dried and then recrystallised from ethanol.M.p = 100 °C, yield 92 %,

4. Preparation of Polymers
The five Schiff base polymers were synthesized by following same general procedure.An equimolar mixture of 5 mmol of diamine (ethylenediamine, 1,3-propyplenediamine, 2,6-diaminopyridine, 4,4'-diaminophenyl ether or thiosemicarbazide) dissolved in 10 ml DMF and 5 mmol dialdehyde (HOB) dissolved in 20 ml DMF were transferred into a 250 ml round bottom flask equipped with a condenser and a magnetic stir bar.Then 3 drops of 0.1 mol hydrochloric acid were added.The reaction mixture was refluxed with continuous stirring for 6 h under nitrogen atmosphere.The product was added to 200 ml water and allowed precipitate to settle.The products was filtered and washed with ethanol and then dried.

1. Synthesis of Monomer and Polymers
The general reaction scheme for the preparation of the monomer HOB and five polymers with their possible structure is given in (Figure 1).
The monomer was easily prepared following a general reaction scheme as reported 32 and was obtained in good yield (92% theoretical).The polymers are also prepared by polycondensation by warming together the equimolar solutions of monomer and diamino-compounds in the presence of a few drops of acid.The compounds were obtained in good yield (76-95%).

2. Solubility
The solubility of the monomer and the polymers were examined in water, ethanol, acetone, chloroform, THF, DMF and DMSO.The monomer HOB was soluble in most of the solvents except water, but the polymers were somewhat soluble in DMF and DMSO (Table 1).Among the polymers PHOBP indicated lowest solubility within the solvents examined, due to the incorporation of aromatic pyridyl ring in the polymer.

E.I Mass Spectrum of Monomer HOB
The mass spectrum of the monomer indicated M + at m/z at 326, followed by fragment peak at m/z 205 corresponding to [

4. FT-IR Spectroscopy
The FTIR of the monomer (HOB) indicated a strong band at 1683 cm -1 for υ C=O 1595 and 1507 cm -1 for υC=C aromatic rings and at 1250, 1069 cm -1 for C-O-C vibrations.The FT-IR of the polymers PHOBen, PHOBPR, PHOBPh, PHOBP and PHOBTSc indicated weak to medium intensity band within 1671-1686 cm -1 due to υC=O contributed from end on group, followed by strong to medium intensity band within 1599-1651 cm -1 due to υC=N vibrations.Two to three bands were visible within 1603-1491 cm -1 due to aromatic rings of the polymers.Two bands were observed in the polymers within 1236-1281 cm -1 and 1008-1018 cm -1 due to asymmetric and symmetric C-O-C vibrations.A number of bands were observed within 997-670 cm -1 due to in plane and out of plane C-H vibrations of aromatic ring systems (See supplementary data)

6. UV-Vis Spectroscopy
The spectrophotometric study of monomer and polymers was carried out in DMSO against the solvent and the monomer HOB indicated a broad band centered at 283.0 nm with molar absorptivity 3.2 × 10 4 L • mole -1 cm -1 due to π-π* transition within aromatic ring systems.The polymers PHOBen and PHOBPR indicated a broad band each with maximum absorbance at 274 nm and 281 nm, with 1% absorptivity 189.4 and 251.3 respectively.The polymers PHOBPh and PHOBTSc indicated two bands and polymer PHOBP three bands within their absorption spectra.The increase in the number of bands in

7. Thermal Analysis
The thermal analysis (thermogravimetry (TGA) and differential thermal analysis) (DTA) of the monomer and the polymers were recorded in nitrogen atmosphere.TG of HOB indicated single stage weight loss of 95% within 250-500 °C with maximum rate of weight loss (T max ) at 362 °C.DTA showed three endotherms, first at 112 °C for melting point and two broad endotherm with their maximum at 365 °C and 475 °C for vaporization/ decomposition of the compound (See supplementary data).TG of PHOBen indicated three stages weight loss with 8% weight loss within 225-328 °C followed by 15% weight loss within 330-445 °C and further loss of 35% within 446-500 °C.The maximum rate of weight loss (T max ) was at 462 °C.DTA showed an endotherm at 125 °C for loss of solvent and melting endotherm at 200 °C.A broad decomposition exotherm was observed at 345 °C.TG of PHOBPR indicated 3 stages weight loss with 5% within 225-310 °C followed by 20% weight loss within 311-440 °C and 35% further loss within 441-500 °C.Derivative thermogravimetry indicated T max at 465 °C.DTA indicated melting endotherm at 125 °C and two decomposition exotherms at 355 °C and 440 °C.TG of PHOBPh indicated a single stage weight loss of 50% within 250-500 °C.DTG showed T max at 446 °C.DTA indicated two decomposition exotherms at 275 °C and 440°C.TG of PHOBP indicated initial loss of 5% within 35-200 °C may be due to the loss of solvent followed by 14% weight loss within 221-425 °C and further loss of 20% within 452-500°C.DTG indicated T max at 452 °C.DTA indicated a decomposition exotherm at 415 °C.TG of PHOBTSc indicated also three stages weight losses with 12% loss within 211-340 °C, followed by 23% loss within 341-440 °C and further loss of 20% within 441-500 °C.DTG showed T max value at 450 °C.
DTA show a decomposition exotherm at 352 °C (See Supplementary data).The results support the enhancement in thermal stability of the polymers with higher T max value as compared to the monomer HOB.

8. Fluorescence Emission
The monomer HOB and its polymers contained aromatic ring system and were examined for the fluorescence properties.The results are summarized in Table 2.
The monomer HOB indicated fluorescence with excitation 313 nm and 378 nm and emission at 362, 413 and 436 nm.The Polymers also indicated 1 to 2 emission bands with verifying relative intensities and the results support that the prepared materials are fluorescent compounds (See Supplementary data).

10. Viscosity Measurement
The monomer HOB and its polymers were examined for viscous flow of their dilute solutions within temperatures 293-333 K to examine the effect of polymerization.HOB indicated reduced viscosity within 0.287-0.377dl/g, which increased to 0.610-0.748dl/g and 0.881-0.984dl/g in PHOBen and PHOBPh respectively.The intrinsic viscosity which is dependent on the size and shape of molecule indicated values for HOB within

11. Antimicrobial Assay
The synthesized polymers were tested for their antifungal and antibacterial activities.The polymers show non-significant antifungal activity.The polymers PHOBen, PHOBPR and PHOBP show some antibacterial activity and the results are summarized in Table 3.

Conclusion
Five new polymers have been synthesized by single stage polycondensation reaction in solution in DMF.The polymers are characterized by elemental microanalysis, FT-IR, UV-Vis, 1 HNMR and dilute solution viscosities measurement.The polymers indicated different morphologies from globular, fibrous, amorphous to rough with rigid structures observed from SEM studies.The polymers

Figure 1 .
Figure 1.Reaction scheme (a) synthesis of monomer HOB and (b) synthesis of polymers

0
.249-0.314 dl/g as compared to 0.570-0.686dl/g and 0.850-0.932dl/g for PHOBen and PHOBPh polymers respectively due to increase in the molecular mass on polymerization.The values of Huggins constant (K H ) depend upon solvent properties for the compounds and were within the range 1.06-1.25,1.01-1.97and 0.86-1.50for HOB, PHOBen and PHOBPh respectively above the values of 0.5 indicating DMF as poor solvent for the compounds.

Table 1 :
Solubility of monomer (HOB) and polymers in different solvents at the concentration of 5mg/ 5ml

Table 3 :
Antibacterial activities of polymers