A process for preparing a regioregular homopolymer or copolymer of 3-substituted thiophene, 3-substituted selenophene, 3-substituted thiazol or 3-substituted selenazol by a) reacting a 3-substituted 2,5-dihalothiophene, 2,5-dihaloselenophene, 2,5-dihalothiazol or 2,5-dihaloselenazol with reactive zinc, magnesium and/or an organomagnesium halide to give an organozinc or organomagnesium intermediate containing one halozinc or one halomagnesium group, b) bringing the organozinc or the organomagnesium intermediate into contact with a Ni(II), Ni(O), Pd(II) or Pd(0) catalyst to initiate the polymerization reaction, and c) polymerizing the organozinc or the organomagnesium intermediate to give a regioregular head-to-tail homopolymer or copolymer of 3-substituted thiophene, 3-substituted selenophene, 3-substituted thiazol or 3-substituted selenazol characterized in that the polymerization reaction is carried out at a temperature rising from a lower temperature T1 to a higher temperature T2 during a time t1, wherein T1 is in the range of from −40 to 5° C. and T2 is in the range of from −20 to 40° C., wherein T2−T1 is at least 10° C. and the average rate of increase (T2−T1)/t1 is in the range of from 0.05° C./min to 1° C./min.