Thermal transitions in a new main chain liquid crystal polymer, poly[tris(oxymethylethylene) oxycarbonyl-1,4-phenyleneoxy-terephthaloyloxy-1,4-phenylenecarbonyl], PTOPTO, have been studied by differential scanning calorimetry, optical microscopy and wide-angle X-ray diffraction, employing synchrotron radiation. The thermal behaviour of this polymer is very complex and includes four main transitions at 36, 70, 141 and 176 degrees C, associated with the glass transition, a smectic S-C to S-A transition, a change from smectic S-A to the nematic phase and the isotropization temperature, respect ively. Changes in the structure from fan-shaped to focal conic structures confirm the transition from S-C to S-A and the appearance of Williams domains confirm the transition from S-A to nematic. These observations coincide with the data obtained by X-ray diffraction, in which a shift in the peak at 2 theta=3.35 degrees is detected. Reflections associated with three-dimensional order were not observed, due to the asymmetry of the spacer.