A series of eight low band gap polymers based on benzodithiophene - diketopyrrolopyrrole (BDT-DPP) skeleton were designed with side chain variation on the benzodithiophene unit. The effect of different side chains, including -H, alkyl, alkoxy, and aryl moieties, over polymer stability is examined. Thin films were processed and submitted to photooxidative degradation. We experimentally confirm that alkoxy side chains are the most promising candidates for designing soluble and photo-resistant polymers. This conclusion is modulated by the polymer dispersity which also plays an important role: a low dispersity value being synonymous of higher photostability. The reasons why the side chain structure and dispersity value drive the polymer photostability are discussed. For the design of photo-resistant polymers and devices, side chain selection must go hand in hand with the control of the synthesis meaning a low dispersity.