MODEL STUDIES ON THE STRUCTURE OF POLY(AMIDE OXIME)S AND THEIR CYCLODEHYDRATION REACTIONS LEADING TO POLY(1,2,4-OXADIAZOLE)S

To obtain an understanding of the low molecular weight character of wholly aromatic poly(1,4-arylene acyldiamide oxime)s (PAAs) having n-alkyloxymethyl side braches and the additional decrease in molecular weight during their cyclodehydration reaction leading to the corresponding poly(1,4-arylene-1,2,4-oxadiazole)s (PAOs), tautomeric structures of poly [1,4-phenylene-2,5-bis (n-octyloxymethyl) terephthaldiamide oxime] (C-8-PAA) were H-1-NMR-spectroscopically investigated by using two model compounds: O,O'-dibenzoyl terephthaldiamide oxime (H-PAA-M, 4) and O,O'-dibenzoyl-2,5-bis(n-octyloxymethyl)terephthaldiamide oxime (C-8-PAA-M, 5). In solution H-PAA-M existed exclusively as oximino isomer, while C-8-PAA-M consisted only of imino tautomer and C-8-PAA contained both the oximino and imino form, the former being dominant in composition. It was found that the presence of imino tautomer caused serious effects both on the low molecular weight character of C-8-PAA and on the additional decrease in molecular weight during thermal cyclodehydration to C-8-PAO, whereas the presence of oximino tautomer showed practically little effect. A discussion on the results is provided from mechanistic viewpoint. (C) 1993 John Wiley & Sons, Inc.