Two disparate modes of operation can occur in an electrolyte-gated transistors (EGTs) depending on the permeability of the semiconductor to ions that are opposite in charge to the induced carriers. High permeability to ions promotes volumetric doping whereas a low permeability promotes electric double layer (EDL) formation and field-effect doping at the semiconductor/dielectric interface. Here, we present a generalized method to control the mode of charge accumulation in an EGT with a constant semiconducting layer by gating with anhydrous polymeric ionic liquids (PILs) of opposite polarity. The polarity of the acrylate-based PILs was controlled by tethering ionic units of positive or negative charge to the backbone. In situ optical spectroscopy of EGTs with poly(3-hexylthiophene), P3HT, reveals that selectively tethering either the anion or the cation dictates whether ions infiltrate P3HT. Gating with both dielectric materials results in similar trends in the dependence of the charge carrier mobility on carrier concentration, despite the difference in doping mechanisms. The EDL (interfacial) doping of the anion-tethered PIL leads to higher mobilities at low carrier concentrations in P3HT with lower onset voltages. Optical measurements during gating show that the anion-tethered PIL gate results in a more even distribution of carriers between ordered and less ordered domains, promoting the formation of a percolated network in the film.