Polymerized ionic liquids (PolyILs) are promising candidates for a wide range of technological applications due to their single ion conductivity and good mechanical properties. Tuning the glass transition temperature (T-g) in these materials constitutes a major strategy to improve room temperature conductivity while controlling their mechanical properties. In this work, we show experimental and simulation results demonstrating that in these materials Tg does not follow a universal scaling behavior with the volume of the structural units V-m (including monomer and counterion). Instead, T-g is significantly influenced by the chain flexibility and polymer dielectric constant. We propose a simplified empirical model that includes the electrostatic interactions and chain flexibility to describe T-g in PolylLs. Our model enables design of new functional PolylLs with the desired T-g.