A series of copolymers, poly[2-(dodecyloxy)-5-methyl-m-phenyleneethynylene] and poIy[2-(dodecyloxy)-5-methyl-m-bis(ethynyl)phenyleneoligothienylene]s, in which ethynylene units are inserted into both of the meta positions of every phenylene for the elimination of the steric hindrance around the meta linkages, was synthesized by a Heck type coupling reaction. The resulting polymers were characterized by infrared and nuclear magnetic resonance spectroscopy and by elemental analysis and gel permeation chromatography. Their optical spectra show that the absorption and emission maxima regularly shift to lower energy with increasing number of the thiophene units between the consecutive m-phenylene linkages. It is confirmed that the role of the m-phenylene as an interrupting block does not come from the steric hindrance around the Linkage but from the intrinsic property, e.g., the MO of the m-phenylene moiety. The quantum yields of their emissions gradually decrease with a decrease in concentration of the ethynylated m-phenylene units on the polymer backbone and the Stockes shifts increase. Both of the trends indicate that a nonradiative decay of the excited states increases with an increase in the possible configurations of thiophene rings within the repeating active block.