There is growing interest in designing and developing high dielectric constant (epsilon(r)) organic semiconductors because they have the potential to further enhance device performance by promoting exciton dissociation, reducing bimolecular charge carrier recombination, and potentially enhancing charge carrier mobility via charge screening. In this study, a new class of semiconducting polymers with high epsilon(r), i.e., sulfinylated and sulfonylated poly(3-alkylthiophene)s (P3ATs), were synthesized. Because of efficient rotation of highly polar methylsulfinyl and methylsulfonyl side groups (i.e., orientational polarization), high epsilon(r) values were achieved for these functionalized P3ATs based on an accurate capacitance measurement using a gold/semiconducting polymer/SiO2/n-doped Si configuration. For example, the epsilon(r) at megahertz and room temperature increased from 3.75 for the regioregular poly(3-hexylthiophene) (P3HT) to 7.4 for the sulfinylated and 8.1-9.3 for sulfonylated P3AT polymers. These values are among the highest epsilon(r) reported for conjugated polymers so far. Grazing-incident wide-angle X-ray diffraction results showed that these polar groups decreased the crystallinity for the polythiophene backbones and interfered with the pi-pi stacking in the crystalline structure. Consequently, their optical properties, including UV-vis absorption and fluorescence, changed in thin films. From this study, the sulfinylated polymer may be promising to provide a balance between high epsilon(r) and preserving favorable polythiophene pi-pi stacking structure for device applications.