23. Preparation of new optically active polyamides containing L-phenylalanine, phthalimide side chain via diisocyanate route by microwave energy: Comparison with conventional heating
Phthalic anhydride was reacted with L-phenylalanine in N,N-dimethylformamide solution at refluxing temperature, which led to the formation of imide acid. Treatment of this imide acid with excess thionyl chloride gave acid chloride in high yield, which further reacted with 5-aminoisophthalic acid to yield (2S)-5-(3-phenyl-2-phthalimidylpropanoylamino)isophthalic acid as a bulky chiral aromatic diacid monomer. The polycondensation reaction of this diacid with different aromatic and aliphatic diisocyanates such as 4,4′-methylene bis(4-phenylisocyanate), toluylene-2,4-diisocyanate, hexamethylene diisocyanate and isophorone diisocyanate was developed by two different methods: microwave-assisted and classical heating in the presence of N-methyl-2-pyrrolidone as a solvent. The polymerization reactions proceeded fast and produced a series of new optically active polyamides (PAs) with good yield and moderate inherent viscosity of 0.25–0.46 dl/g. The resulting PAs are optically active and thermally stable. All of the above compounds were fully characterized by means of FT-IR spectroscopy and specific rotation techniques and typical ones by 1H-NMR, elemental analysis and thermogravimetric analysis.