Preparation of Salt Monomers

the nylon-salt-type monomers such as XPME [20], XTPE [21, 22], and ODPME [23] with exactly 1 1 composition were prepared from a series of aliphatic diamines (X standing for the number of methylene unit) or aromatic bis(4-aminophenyl) ether (ODA) and the corresponding aromatic tetracarbox-ylic acid half diesters (diacid-diesters) such as pyromellitic acid diethyl ester (PME) and 3,3 ,4,4 -p-terphenyltetracarboxylic acid diethyl ester (TPE). 

These salt monomers were prepared readily as white crystalline solids by dissolving an equimolar amount of each individual diamine and tetracarboxylic acid half diester in hot ethanol (or methanol), and subsequently cooling the resultant solution. The ease of preparation of the salt monomers is based on ready accessibility of the precise 1 1 stoichiometric balance due to the diamine-dicar-boxylic acid functionalities, just like the well-known formation of nylon salts from aliphatic diamines and dicarboxylic acids. 

Interestingly, aromatic tetracarboxylic acids themselves such as pyromellitic acid (PMA),3,3,4,4,-biphenyltetracarboxylic acid (BPA),4,4 -oxydiphthalic acid 

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