Polymers with high dielectric permittivity are promising for the construction of next-generation transducers and energy storage devices with improved energy density. Here, we report the synthesis, glass-transition temperature (T-g), and dielectric properties of polysiloxanes with different polarities. For this, we set out from poly(dimethyl-co-methylvinyl)siloxanes with different vinyl group content. The vinyl groups were then transformed into polar groups of various nature by an efficient one-step thiol-ene addition postpolymerization modification. We used the resulting collection of materials to establish structure-property relationships, side group design, and thermal and dielectric properties. Our results show that the T-g increases with the polar group content and the strength of the polar group. A similar trend is observed for the dielectric permittivity as long as the T-g of the polymer is well below 0 degrees C. Smaller polar groups tend to show a smaller increase in T-g, and an increased linker length helps to decreases T-g, which is generally favorable for high permittivity. Our findings guided us to design polysiloxanes with a permittivity as high as 27.7 and a T-g of -18.2 degrees C. To the best of our knowledge, this is the highest dielectric permittivity of a polymer with a T-g well below room temperature.