Grignard metathesis polymerization was used to synthesize a series of poly(3-hexylselenophene)-block-poly(3-hexylthiophene) copolymers with two different molecular weights and varying selenophene content. These polymers were characterized by optical absorption spectroscopy (film and solution), differential scanning calorimetry, powder X-ray diffraction, variable temperature absorption spectroscopy, and atomic force microscopy (on self-assembled polymer nanofibers). The selenophene to thiophene ratio has a large influence on optical properties, and absorption is tunable across the range of both homochromophores. We observe phase separation in the solid state in both pristine and annealed samples. When allowed to slowly assemble in solution, high molecular weight copolymers have a very sharp transition from the molecularly dissolved to the aggregated state. Most interestingly, increasing polyselenophene content induces the polymer to assemble more readily (at a higher temperature) but also appears to hinder the degree of ordered assembly when the thiophene block is not sufficiently long. This study furthers the understanding of the differences between these structurally similar conjugated polymer building blocks and provides insight into the factors that control heterocycle-induced phase separation.