One-pot synthesis of copoly(aliphatic-aromatic) oxadiazoles: how to control the formation of random or block structures?

The polyoxadiazole synthesis in P2O5/CH3SO3H solution has been improved by precipitation of the reaction mixture in cold NMP. The polyoxadiazole obtai ned by this process did not contain uncyclized hydrazide moieties and was m ore stable in strongly acidic media. This process has been used to prepare some copolyoxadiazoles with dodecanedioic and isophthalic diacids or dihydr azides. The microstructure of the copolymers (alternate versus block) depen ds on the nature of the isophthalic derivative. With isophthalic dihydrazid e, which is soluble in the reaction mixture like the dodecanedioic derivati ve, an alternate structure is obtained. With isophthalic diacid, which exhi bits low solubility, the dodecanedioic derivate first reacts giving an alip hatic oxadiazole sequence and finally a block copolymer is obtained. The mi crostructure was determined by proton NMR spectroscopy allowing us also to calculate the average sequence length. UV-vis spectra of the block copolyme rs show a bathochromic effect compared with the spectra of the alternate co polymers. The block copolymer obtained with 75% dodecanedioic and 25% isoph thalic diacid shows three transitions at -10 degreesC (T-g1), 50 degreesC ( T-m) and 220 degreesC (T-g2). Heated at 150 degreesC, a moulded specimen ha s been stretched up to 100%, and cooled down at room temperature under stre ss. The temporary shape is stable at room temperature and the initial shape is restored by heating at 110 degreesC. The mechanical stress stored in th e temporary shape was about 1.6 MPa.