All-cis and all-trans isomers of poly[(arylenevinylene)-alt-(5-octyloxy-1,3-phenylenevinylene)}s having three kinds of arylene groups (PmPVs: arylene = m-phenylene (1a), p-phenylene (1b), 4,4'-biphenylene (1c)) have been synthesized in almost perfect stereoregularity by two types of palladium-catalyzed polycondensation reactions, respectively. Suzuki-Miyaura-type polycondensation of (Z,Z)-bis(2-bromoethenyl)arenes with 5-octyloxy-1,3-benzenediboronic acid pinacolate affords all-cis 1a-c, whereas Hiyama-type polycondensation of (E,E)-bis(2-silylethenyparenes with 5-octyloxy-1,3-diiodobenzene forms all-trans 1a-c. The resulting polymers with relatively short effective pi-conjugation lengths interrupted by mphenylene units undergo two-way photoisomerization to give PmPVs containing cis- and trans-vinylene groups in nearly 1:1 ratios irrespective of the arylene groups of staring polymers.