Nickel-catalyzed chain-growth copolymerizations of thiophene and selenophene derivatives afforded well-defined pi-conjugated copolymers with narrow molecular weight distributions, defined end-groups, and specific sequences. In particular, a pi-conjugated copolymer with a novel linear gradient sequence was prepared. Compared to the analogous block and random copolymers, the gradient copolymer displayed unique optical and thermal properties as well as thin-film morphology. Moreover, the gradient copolymer exhibited an intermediate extent of phase separation into thiophene-rich and selenophene-rich domains compared to the block and random copolymers. Because this gradient sequence provides access to new solid-state properties, these materials should be further explored in applications where phase-separated morphology is important (e.g., solar cells, transistors, and light-emitting diodes).