Process for forming thermally stable thermotropic liquid crystalline polyesters of predetermined chain length

In a process for the formation within a polymerization zone of a wholly aromatic poly(ester-amide) which is capable of forming an anisotropic melt through a polymerization reaction of ester-forming and amide-forming reactants to form a polymer which consists essentially of moieties I, II, III, and optionally IV, wherein in each instance Ar is at least one aromatic ring, and wherein:

• I is ##STR22## II is ##STR23## III is --Y--Ar--Z--, where Y is O, NH, or NR, and Z is NH or NR where R is an alkyl group of 1 to 6 carbon atoms or an aryl group, and
• IV is --O--Ar--O--,
• wherein said poly(ester-amide) comprises approximately 40 to 80 mole percent of moiety I, approximately 5 to 30 mole percent of moiety II, approximately 5 to 30 mole percent of moiety III, and approximately 0 to 25 mole percent of moiety IV;
• the improvement of providing in the polymerization zone during said polymerization reaction an approximately 0.25 to 4 percent molar excess of aromatic dicarboxylic acid monomer which during the polymerization reaction imparts dicarboxyaryl units to the interior of the polymer chains of the resulting polymer and causes the polymer chains to terminate in carboxylic acid end groups wherein the polymer chains achieve a predetermined average chain length through the depletion of other reactants present in the polymerization zone to yield a wholly aromatic poly(ester-amide) product which is substantially incapable of additional chain growth upon subsequent heating.