Periodic pi-conjugated polymers of the group 16 heterocycles (furarr, thiophene, and selenophene) were synthesized with controlled chain lengths.,and relatively low dispersities using catalystrtransfer polycondensation. The optical gap and redox potentials of these copolymers were finetuned by altering the hetetocycie sequence, and atomic force microscopy revealed nanofibrillar morphologies for all the materials. Grazing incidence Wide-angle X-ray scattering of the thiophene-selenophene copolymers indicated that the 7r-stacking disfance increased with incorporation of the larger heteroatom (from similar to 3.7-4.0 angstrom), while the lamellar spacing decreased (from similar to 15.8-45.2 angstrom). The-study also revealed that periodic sequences allow electronic properties to be tuned while retaining nanofibrillar morphologies similar to thoSe Observed for poly(3-hexylthiophene).