Poly(butylene/thiodiethylene naphthalate) copolymers (PBN-PTDEN) were synthesized in bulk according to the usual polycondensation procedure and examined by NMR, GPC, TGA, DSC and XRD techniques. At room temperature they appeared as semicrystalline materials; the copolymerization caused a lowering in the T-g value, a decrement of T-m and of the crystallization rate. Pure alpha- or beta-form was obtained at low and high MEN unit content, respectively; crystalline form transition never occurred in the solid state, analogously to PBN. After cooling from the melt, the pure alpha-form was always evidenced in PBN-PTDEN10, whereas the pure beta' crystal phase develops in the copolymers containing 30 and 40 mol% MEN units, independently on the cooling rate. In the case of PBN-PTDEN20 a pure alpha- or beta'-form was obtained at low and high cooling rate, respectively. (C) 2009 Elsevier Ltd. All rights reserved.