THERMAL-STABILITY AND DEGRADATION OF AROMATIC POLYAMIDES .2. STRUCTURE-REACTIVITY RELATIONSHIPS IN THE PYROLYSIS AND HYDROLYSIS OF BENZAMIDES

An experimental study to determine the effect of ring and nitrogen sub stitution on the rate and product distribution of the pyrolytic and hy drolytic degradation of polyamides was carried out. Two families of be nzamides were chosen as models of the amide functionality obtained fro m the reaction of an aromatic acid and an aliphatic amine. The first f amily included benzamide and other primary amides with ring substituen ts. The second family comprised secondary or tertiary amides. The benz amides were reacted in an inert argon atmosphere and in the presence o f added water at 350 degrees C. The influence of nitrogen substitution was dramatic; the rate of disappearance of the primary amides was an order of magnitude faster than that for secondary or tertiary amides. The major products formed from primary amides were the corresponding b enzonitrile and benzoic acid, which was a secondary product of pyrolys is and a primary product of hydrolysis. Pyrolysis of secondary and ter tiary benzamides led to the corresponding lesser-substituted benzamide . A combination of a free radical and a concerted mechanism was propos ed that accounted for the conversion and product selectivities for bot h families of benzamides. A nonspecific free radical mechanism with sh ort chain length combined with a [1,3] sigmatropic shift and subsequen t fission described the reaction of primary benzamides and methyl subs tituted benzamides that lacked a beta C-H. The higher selectivity to b enzamide and benzonitrile for benzamides witha beta C-H suggested beta -carbon directed chemistry. Appliction of a free radical mechanism wit h nonzero chain length afforded benzamide and benzonitrile, but failed to account for the observed product selectivities. A concerted reacti on mechanism with a six-membered cyclic transition state led to high s electivity of benzamide and benzonitrile.