Naphthalene diimide (NDI) copolymers are attractive n-type materials for use in organic electronic devices. Four highly soluble NDI polymers are presented-each differing only in the thiophene content comprising the material. Electron mobilities as high as 0.076 cm(2) V-1 s(-1) for the novel material PNDI-3Th are reported. Polymer crystallinity and general macromolecular order are shown to effectively improve by increasing the number of thiophene units within the polymer backbone. The structure-property relationship of NDI-thiophene copolymers is presented and discussed as it pertains to organic field effect transistor (OFET) performance.