Process for forming an aramid copolymer

1. A polymer comprising residues of 2-(4-amino phenyl)-5 (6) amino benzimidazole (DAPBI), paraphenylene diamine (PPD), and terephthaloyl dichloride (TCl), said polymer produced by process comprising the steps of:

(a) forming a slurry of b mole percent DAPBI and y mole percent PPD in a solvent system comprising organic solvent and c weight percent of an inorganic salt, wherein the inorganic salt is present in an amount of at least 5 weight percent of the organic solvent, DAPBI and PPD being present in an amount sufficient for providing a polymer solution having a weight percent solids of 12 percent or greater on a polymer basis; and

(b) contacting the slurry of step a) with a stoichiometric amount of terephthaloyl dichloride to form a product comprising the polymer

wherein the sum of y and b is 100 and the product of b×c is 225 or greater, and

wherein the weight percent solids in the product is 14-25% on a monomer basis.

2. The polymer of claim 1 , wherein b×c is 300 or greater.

3. The polymer of claim 2 , wherein b×c is 350 or greater.

4. The polymer of claim 1 , wherein the addition of terephthaloyl dichloride in step b) occurs in at least 3 steps.

5. The polymer of claim 1 , wherein after each addition of terephthaloyl dichloride in step b) prior to the final addition, the product is cooled to a temperature of 15° C. or below.

6. The polymer of claim 1 , wherein the inorganic salt is LiCl or CaCl 2 .