The polymerization reaction and its side reactions were investigated in the synthesis of 4-acetoxybenzoic acid (ABA) and 6-acetoxy-2-naphthoic acid (ANA) copolymers by the measurement of byproducts and polymer molecular weight and the study of polymer end groups and model reactions. A method was developed to isolate and collect the byproducts produced in the polymerization. These byproducts, acetic anhydride, phenol, phenyl acetate, ketene, and carbon dioxide, were detected using HPLC and GC/MS. The existence of acetoxy, carboxy, phenyl ester, and phenolic hydroxyl end groups on the polymer chain was confirmed by direct measurement of end groups and polymer molecular weight and by the study of model reactions, such as the decarboxylation of 4-(benzoyloxy)benzoic acid and the decomposition of ABA methyl ester. For the bulk copolymerization of ABA and ANA at temperatures ranging from 195 to 325 degrees C under nitrogen, the classical acidolysis mechanism changes to a phenolysis mechanism in the late stages of the polymerization.