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Synthesis and characterization of new optically active polyesters by step-growth polymerization of novel aromatic (2S)-4-[(4-methyl-2-phthalimidyl-pentanoylamino)benzoylamino]isophthalic acid with aromatic diols

A new diacid monomer 9 was synthesized in five steps. Starting from the reaction of l-leucine with phthalic anhydride in acetic acid provided (2S)-4-methyl-2-phthalimidylpentanoic acid (3) in good yield. The compound 3 was converted to (2S)-4-methyl-2-phthalimidylpentanoyl chloride (4) by reaction with thionyl chloride. The resulting acid chloride 4 was reacted with p-aminobenzoic acid (5) in dry tetrahydrofuran and (2S)-4-(4-methyl-2-phthalimidylpentanoyl-amino)benzoic acid (6) was obtained. Then, it was converted to (2S)-4-(4-methyl-2-phthalimidylpentanoylamino)benzoyl chloride (7) by treatment with thionyl chloride. The reaction of acid chloride 7 with 5-aminoisophthalic acid in dry N,N-dimethylacetamide (DMAc) gave (2S)-4-[(4-methyl-2-phthalimidylpentanoylamino)-benzoylamino]isophthalic acid (9) in high yield. The step-growth polymerization of this diacid with several aromatic diols, such as bisphenol-A (10a), phenolphthalein (10b), 1,5-naphthalenediol (10c), hydroquinone (10d), bisphenyl-2,2′-diol (10e), 4,4′-dihydroxydiphenyl sulfone (10f), 4,4′-dihydroxydiphenyl sulfide (10g), and 1,8-dihydroxyanthraquinone (10h), was carried out in a system of tosyl chloride, pyridine, and N,N-dimethylformamide (DMF) to give a series of new optically active polyesters (PE)s with good yield and moderate inherent viscosity ranging from 0.29 to 0.43 dL/g. All of the aforementioned polymers were fully characterized by FTIR and specific rotation. Some structural characterization was performed by 1H NMR and elemental analysis. These polymers are readily soluble in amide-type solvents such as DMF, DMAc, dimethyl sulfoxide, and protic solvents such as sulfuric acid. Thermal gravimetry analysis showed that the 10% weight loss temperature in a nitrogen atmosphere was more than 360°C, which indicates that the resulting PEs have a good thermal stability as well as excellent solubility.

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