We report a new semiaromatic poly(thioester) obtained through the copolymerization of phthalic thioanhydride and propylene oxide. The reaction was catalyzed by a chromium-based complex in conjunction with [PPN]Cl, where PPN = bis(triphenylphosphine)iminium. The reaction temperature exerted a critical influence over catalytic activity as well as the structure of the resulting polymer chain. NMR spectroscopy revealed that the resultant copolymers contained multiple repeating units, including thioester, ester, and thioether-ester linkages, in their main chains due to transesterification, particularly when they were produced at elevated reaction temperatures. The thioester linkage content affected the thermal properties of the polymers. A relatively high glass transition temperature of 69.5 degrees C was observed in the copolymer containing a large number of thioester linkages obtained at 25 degrees C. In addition, this sulfur-containing polymer exhibited desirable optical properties, with a refractive index of 1.60.