The solubility and thermal properties of poly(ether ether ketone) (PEEK) have been modified by utilizing various ratios of the traditional electrophilic component, 4,4'-difluorobenzophenone, 1, utilized to synthesize PEEK and a comonomer, 3,5-difluorobenzophenone, 2, which is simply a geometric isomer of 1. The resulting polymers ("m-PEEK") have the same chemical composition as PEEK, allowing for accurate structure-property relationships to be determined. The thermal properties were investigated by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The ratio of monomer 1 to monomer 2 had a dramatic influence on the thermal properties and solubility characteristics of PEEK. At higher ratios, 90:10, 85:15, and 80:20, the materials were semi-crystalline and their solubility was very limited in solvents such as N-methyl-pyrrolidinone (NMP) while at lower ratios, 75:25 and 50:50, the PEEK derivatives were completely amorphous and soluble in NMP. TGA analysis indicated excellent thermal stability as most of the materials displayed 5% weight loss temperatures (T-d_5%) greater than 450 degrees C. (C) 2012 Elsevier Ltd. All rights reserved.