Very pure monomers are necessary to generate high molecular weight of any condensation polymerization.
Molecular weights that give good mechanical properties typically require greater than 99.9% conversion based on the Flory-Schultz distribution. Essentially, low molecular weight species vastly outnumber high MW species until monomer conversion is nearly complete, then oligomers have a chance to start condensing with one another.
It's why diacyl chlorides and amines make great polymerizations, the amide bond is hard to break (less back reaction) and HCl is more easily pulled out of the system than water is during a carboxylic acid-based condensation, which drives the reaction forward. Kevlar is typically made this way. Also I've heard the processing of the strands is very nasty, done in neat sulfuric acid or something like that.
Perhaps you could get terepthlatic functionality back by converting to acid chloride and distilling it, provided no plasticizers are volatile. I'm sure someone has thought of this for recycling.
Just curious, you need a diamine, do you plan to nitrate and reduce aniline?