Five different poly(amic dialkyl ester) precursors with various chain rigidities were synthesized and their powder and film samples were prepared in residual solvent free: poly(4,4＇-oxydiphenylene biphenyltetracarboxamic diethyl ester); poly(p-phenylene biphenyltetracarboxamic diethyl ester); poly(p-phenylene biphenyltetracarboxamic dimethyl ester); poly(4,4＇-oxydiphenylene pyromellitamic diethyl ester); and poly(p-phenylene pyromellitamic diethyl ester). Glass transition temperatures (T(g)s) of these precursors, which have never been reported so far, were successfully determined in the present study by the new oscillating differential scanning calorimetry (os.c.). In addition, their imidization behaviours and morphological structures were investigated by os.c. and X-ray diffraction, respectively. The measured T(g)s are in the range 179.2-235.4 degrees C, depending upon the precursor backbone structure as well as the history of sample preparation: these T(g)s are very close to the imidization temperatures (T(i)s), causing difficulties in the T-g measurements. The measured T(i)s are in the range 198.1-235.4 degrees C, depending upon the precursor backbone structure as well as the history of sample preparation. The T(g)s are dependent on the chain rigidity and the morphological structure, whereas the T(i)s are influenced by the chemical nature in addition to the chain rigidity and the morphological structure.