Synthesis and properties of novel polyimides derived from 2,6-bis(4-aminophenoxy-4′-benzoyl)pyridine with some of dianhydride monomers
Introduction
It is well known that the polyimides with aromatic heterocyclic structures play important roles in the development of advanced technologies because of their excellent thermal and thermooxidative stabilities, outstanding mechanical and electrical properties, so they have been employed in the adhesives, matrices of composites, fibers, films, foams, microelectronic materials and so on [1], [2], [3], [4]. However, the further applications of the polyimides are often limited by their poor solubility and high processing temperature partly due to the rigidity and strong interaction among their chains. In order to overcome these limitations, some significant and synthetic efforts were focused, and improving solubilities in organic solvents without sacrificing the above excellent properties should be research objectives regarded, especially, the design and synthesis of new monomers would be one of approaches improving the processability and solubility of the polyimides [5], [6], [7], [8], [9], [10].
Pyridine is a heteroaromatic molecule with rigidity and polarizability. New kinds of heteroaromatic diamine, dianhydride or other monomers holding pyridine unit have been designed and synthesized, and the novel heteroaromatic polymers with good thermostability and processability have been obtained derived from those monomers containing pyridine nucleus structures at the same time [11], [12], [13], [14], [15], [16], [17]. Considering the rigidity based on symmetry and aromaticity of pyridine ring would have contributions for the thermal stability, chemical stability, retention of mechanical property of the resulting polymer at elevated temperature, as well as polarizability resulting from nitrogen atom in pyridine ring could be suitable to improve their solubility in organic solvents [18], [19], the polyimides with good themostability and processability have been prepared by polycondensation of 2,6-bis(3-aminobenzoyl) pyridine with some aromatic dianhydride monomers [20].
In this paper, the synthesis and characterization on a new kind of aromatic diamine containing pyridine was described, i.e. 2,6-bis(4-aminophenoxy-4′-benzoyl)pyridine, which was derived from 2,6-bis(4,4′-dihydroxybenzoyl)pyridine. Meanwhile, the resulting diamine monomer was employed in preparation of the polyimides with several aromatic dianhydrides by a tow-step procedure, and the polyimides obtained were characterized detailedly.
Section snippets
Materials
2,6-Pyridinedicarboxyl chloride (TCI). Phenyl ethyl ether (Chemspec Inc. Shanghai, China) was purified by distillation from calcium hydride prior to use. 4,4′-Oxydiphthalic anhydride (ODPA, Shanghai Nanxiang Chemical Co., China), 3,3′,4,4′-benzophenonetetracarboxylic dianhydride (BTDA, Beijing Chemical Reagents Corp., China) and 4,4′-(hexaflouroisopropylidene)diphthalic anhydride (6FDA, Aldrich) were recrystallized from acetic anhydride before use. Pyromellitic dianhydride (PMDA, Beijing
Monomer synthesis
As shown in Scheme 1, 2,6-bis(4-aminophenoxy-4′-benzoyl)pyridine, i.e. novel diamine monomer BABP, was synthesized via consecutive reactions by three step procedures. Firstly, 2,6-bis(4,4′-dihydroxybenzoyl)pyridine (BHBP) was formed by Friedel–Crafts acylation of phenyl ethyl ether with 2,6-pyridinedicarbonyl chloride using anhydrous aluminum chloride as a catalyst, secondly, 2,6-bis(4-nitrophenoxy-4′-benzoyl)pyridine, i.e. dinitro compound BNBP, was obtained by the nucleophilic substitution
Conclusions
A new heteroaromatic diamine monomer containing pyridine unite, i.e. 2,6-bis(4-aminophenoxy-4′-benzoyl)pyridine (BABP), was successfully prepared in high purity and high yields in this work, and the resulting BABP could be employed in preparation of the novel polyimides with aromatic dianhydride monomer PMDA, BTDA, ODPA and 6FDA by two-step procedures and thermal or chemical imidization methods. Experimental results indicate that the resulting diamine monomer BABP holds a good polymerizability,
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