One-Pot Synthesis of Copoly(Aliphatic–Aromatic)Oxadiazoles: How to Control the Formation of Random or Block Structures?

Abstract

The polyoxadiazole synthesis in P₂O₅/CH₃SO₃H solution has been improved by precipitation of the reaction mixture in cold NMP. The polyoxadiazole obtained by this process did not contain uncyclized hydrazide moieties and was more stable in strongly acidic media. This process has been used to prepare some copolyoxadiazoles with dodecanedioic and isophthalic diacids or dihydrazides. The microstructure of the copolymers (alternate versus block) depends on the nature of the isophthalic derivative. With isophthalic dihydrazide, which is soluble in the reaction mixture like the dodecanedioic derivative, an alternate structure is obtained. With isophthalic diacid, which exhibits low solubility, the dodecanedioic derivate first reacts giving an aliphatic oxadiazole sequence and finally a block copolymer is obtained. The microstructure was determined by proton NMR spectroscopy allowing us also to calculate the average sequence length. UV–vis spectra of the block copolymers show a bathochromic effect compared with the spectra of the alternate copolymers. The block copolymer obtained with 75% dodecanedioic and 25% isophthalic diacid shows three transitions at −10°C (T _g1), 50°C (T _m) and 220°C (T _g2). Heated at 150°C, a moulded specimen has been stretched up to 100%, and cooled down at room temperature under stress. The temporary shape is stable at room temperature and the initial shape is restored by heating at 110°C. The mechanical stress stored in the temporary shape was about 1.6 MPa.