Semiconducting conjugated polymers have promising applications in field-effect transistors, organic light emitting diodes and organic photovoltaics as both electron-accepting (n-type) and electron donating (p-type) materials. These polymers offer a valuable alternative to inorganics due to their excellent processability, low cost and relatively low toxicity. A crucial point in achieving good device performance is the precise control over both the HOMO and LUMO energy levels of the conjugated polymer. To this end we propose a synthetic methodology that allows efficient conversion of ethynylene moieties in the backbone of precursor polymers into vinyl sulfide groups using highly efficient and orthogonal thiol-yne click chemistry. We envision that with our methodology the electronic structure of a conjugated polymer can simply be tuned by introducing an appropriate thiol after polymerization. This modular approach allows the introduction of functionalities that are incompatible with polymerization conditions and can give rise to a multitude of conjugated polymers based on a single precursor. Here we investigate the effect of a library of thiols, ranging from electron-withdrawing to electron-donating, on the energy levels of poly(p-phenylene vinylene) using cyclic voltammetry and UV-Vis absorption and emission spectroscopies. With these post-polymerization functionalizations the energy levels of conjugated polymers can be tuned in a simple and controlled manner, which may significantly contribute to the improvement of organic electronic devices.