An investigation into the synthesis and characterization of new optically active poly(ester-imide) thermoplastic elastomers, derived from N,N′-(pyromellitoyl)-bis-l-leucine, synthetic diols and polyethyleneglycol-diol (PEG-200)
Introduction
Due to the increasing demands for high-performance polymers as a replacement for ceramics or metals in the microelectronic, aerospace and automotive industries, thermally stable polymers have received much interest over the past decade. Polyimides and their copolymers are certainly one of the most useful classes of high-performance polymers, which have found many applications in industries [1], [2]. Aromatic polyimides are an important class of heterocyclic polymers with remarkable heat resistance and superior mechanical and electrical properties, and also durability [3], [4], [5]. Poor thermoplastic fluidity and solubility are the major problems in wide application of polyimides. This makes it impossible for most polyimides to be directly processed in their imidized forms; thus, their applications have been restricted in some fields. Processable engineering plastics possessing moderately high softening temperatures and/or solubility in some organic solvents are required for practical use. Therefore, various efforts have been focused on the preparation of soluble and/or thermoplastic polyimides, while still maintaining the excellent thermal and mechanical properties. Typical approaches have been employed to improve the processability of these polyimides include the incorporation of flexible links [6], bulky pendant or cardo groups [7], kinked or unsymmetrical structures [8], and spiro-skeletons [9] into the polymer chain. These modifications lower the melting temperature and lead to soluble and amorphous polymers. In general, amorphous polymers have a lower softening temperature (Tg) and improved solubility with respect to their crystalline analogues. Some of the block copolymers composed of polyethers and polyamides have already been commercialized as thermoplastic elastomers [10]. A number of synthetic routes for polyether-polyimide block copolymers have been known [11]. Ether linkages inserted in the main chains provide them with significantly lower energy of internal rotation.
The synthesis and application of optically active polymers are the newly considerable topics which have been paid more attention recently [12]. Most of the natural polymers are optically active and have special chemical activities, such as catalytic properties that exist in genes, proteins and enzymes. Some other applications are construction of chiral media for asymmetric synthesis, chiral stationary phases for resolution of enantiomers in chromatographic techniques [13], [14], [15], [16], [17], chiral liquid crystals in ferroelectrics and nonlinear optical devices [18], [19], [20], [21]. These synthetic polymers based on optically pure aminoacids can induce crystallinity with their ability to form higher ordered structures that exhibit enhanced solubility characteristics [22]. These properties have caused them to be good candidate for drug delivery systems, biomimetic systems, biodegradable macromolecules, biomaterials, and also as chiral purification media [23]. So, more considerations to improve different synthetic procedures of optically active polymers exist. Recently, we have synthesized optically active polymers by different methods [24], [25], [26]. In compare to our last project on optically active polymers [26], these polymers showed improved optical activity (+15.5 to +20.2) and also because of the presence of benzophenone moiety, the polymers containing it, can potentially be photolabile [27]. The photolabile polymers are potentially able to be used as affinity columns for protein purification [27]. The outstanding characteristics of these polymers include thermal stability, good solubility, improved optical activity and being photolabile.
Here we have also investigated the effect of different reaction conditions such as solvent, catalyst, reaction temperature, and reaction time on optical activity and viscosity of polymers.
Section snippets
Experimental
N-Methyl-2-pyrrolidone (NMP, Merck), N,N-dimethyl formamide (DMF, Merck) and pyridine (Py, Merck) were purified by distillation under reduced pressure over calcium hydride and then stored over 4 Å molecular sieve. O-dichlorobenzene (Merck) was dried and stored over 4 Å molecular sieve. Pyromellitic dianhydride (1) (Merck) and 3,3′,4,4′-benzophenonetetracarboxylic-3,3′,4,4′-dianhydride (Merck) were recrystalized from acetic anhydride. Reagent grade triethylamine (TEA, Merck), l-leucine (2)
Monomer synthesis
We synthesized the diimide-diacid [N,N′-Pyromelliticdiimido-di-l-leucine(3)] by the condensation reaction of dianhydride (1) with l-leucine (2) in 1:2 molar ratio in refluxing acetic acid/pyridine (3/2). Washing the residue with cold water yields a gummy layer that breaks by adding concentrated HCl into a white solid. The 1H NMR spectrum of diacid (3) showed peaks that confirm its chemical structure (Fig. 1). The chlorination was completed after 2 h, when the mixture was completely dissolved in
Summary and conclusions
A series of optically active PEIs and Co-PEIs, having inherent viscosities of 0.10–0.33 dl g−1 was synthesized for the first time by indirect polycondensation of optically active N,N′-Pyromelliticdiimido-di-l-leucine(3) as a diacid having a preformed imide rings as an “enlarged” monomer containing two chiral l-leucine groups with polyethyleneglycol-diol (PEG-200) and/or three synthetic aromatic diols. These polymers showed improved optical activity (+15.5 to +20.2) and also because of the
Acknowledgements
We gratefully acknowledge the funding support received for this project from the Isfahan University of Technology (IUT), IR Iran (ARH) and Grant GM 33138 (AER) from the National Institutes of Health, USA. Further financial support from Center of Excellency in Chemistry Research (IUT) is gratefully acknowledged.
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