The Grignard metathesis (GRIM) polymerization of 3-alkylthiophenes proceeds by a quasi"living" chain growth mechanism, not by a step growth process. Kinetic studies of the Grignard metathesis polymerization of 2,5-dibromo-3-alkylthiophenes showed that the molecular weight of poly(3-alkylthiophenes) is a function of the molar ratio of the monomer to nickel initiator, and conducting polymers with predetermined molecular weights and relatively narrow molecular weight distributions (PDIs = 1.2-1.5) can be made. Sequential monomer addition resulted in new block copolymers containing different poly(3-alkylthiophene) segments.