the present invention provides for a novel heat stable class of polyamide - imide polymers having excellent flow properties and hydrolytic stability which are prepared by forming the polycondensation product of one or more aromatic or aliphatic diamines and a comonomer comprising a tri - or hexafluoro - substituted tricarboxylic acid anhydride having the structure : ## str1 ## wherein r is cf 3 or a phenyl radical . in addition to improved flow properties , the polyamide - imide polymers of this invention also exhibit improved solubility properties in most organic solvents , good resistance to attack by chlorinated solvents such as trichloroethylene as compared with polyimides , improved hydrophobic properties as well as good thermal properties , including resistance to thermooxidative degradation .