Synthesis and characterization of new poly(azomethine ester)s having phenylthiourea units

Abstract

New polyesters having azomethine and phenylthiourea groups in the polymer backbone were synthesized by interfacial polycondensation method. The dihydroxy monomer N-(4-hydroxy-3-methoxybenzal) N′-(4′-hydroxyphenyl)thiourea was condensed with six diacid chlorides: terephthaloyl, isophthaloyl, azeloyl, suberoyl, pimeloyl and adipolyl chlorides. The resulting polyesters were characterized by viscosity, IR, NMR and TGA analysis. The wholly aromatic poly(azomethine ester) derived from terephthaloyl chloride when blended with polyaniline/NH₄OH, polyaniline/HCl and pure polyaniline shows conductance in the range 3.2 × 10⁻³–0.91 × 10⁻¹ S cm⁻¹.

Introduction

Polyesters have always been an attraction from the early days of Carothers [1]. Purely aromatic polyesters are well known for their fibre and film forming nature and some of them are biodegradable, hence their use in both medical and non medical areas are widely accepted [2], [3], [4]. Consequently, it was of interest to investigate the thermal stability of aromatic polyesters with various linking groups in their backbone. The azomethine linkage is of special interest due to its interesting properties such as syn–anti isomerism [5], good thermal stability [6], non-linear optical activity [7], ability to form metal chelates [8], fibre-forming ability [9], [10], liquid crystalline property [11], [12], [13] and semiconductivity [14], [15]. The introduction of azomethine moiety in the polymer backbone will incorporate the above mentioned properties in the newly synthesized polymer.

Further the introduction of thiourea group into the poly(azomethine ester) backbone is relatively an unexplored area. The thiourea group introduces additional features like biomedical activity [16], corrosion inhibition [17], [18], [19], [20], [21], as organic thermal stabilisers [22] and semiconductivity [23], [24]. The major drawback of poly(phenylthiourea azomethine ester)s is their insolubility in common organic solvents due to their chain stiffness introduced by the imine group and the intermolecular hydrogen bonding introduced by the phenylthiourea group [25], [26]. Some of the general methods to improve the solubility are insertion of flexible bonds between mainchain aromatic rings, introduction of structural irregularities as kinks, bends, crankshaft-shaped, etc. [27]. The need to solubilize these compounds is to enable their utilization in the preparation of optical devices, films, fibers, dielectric and semiconductor materials, for biomedical purposes and in corrosion inhibition.

In poly(Schiff base)s, in addition to the p_z orbitals of carbon the p_z orbitals of nitrogen also contribute to the conjugation. This extended spatial conjugation offered by the π electrons will help in conductivity and the use of conducting polymers in various fields are note worthy [28], [29], [30]. Recent reports from our laboratory involved the synthesis, characterization and electrical conducting properties of certain poly(azomethine ester)s/polyaniline blends [31] and synthesis of new polyesters containing azo and phenylthiourea groups [32].

In this paper, we report the synthesis and characterization of six new poly(phenylthiourea azomethine ester)s by interfacial polycondensation method and conducting properties of one of the wholly aromatic poly(phenylthiourea azomethine ester) with different polyaniline blends.