Stereoregular polymerization of (p-nitrophenyl)acetylene (pNPA) was performed using [Rh(norbornadiene)Cl](2) and [Rh(cyclooctadiene)Cl](2) catalysts to give poly((p-nitrophenyl)acetylene) (PpNPA), with a cis-transoid form under mild conditions. The polymer underwent isomerization from the predominant cis to trans form when compressed at 200 kg/cm(2) at room temperature. The polymers, obtained before and after compression, were characterized using IR, CP MAS C-13 NMR, ESR, and diffuse reflective UV methods. The data suggest that the isomerization of the resulting polymer from cis-transoid to trans-transoid form gives a conjugated trans planar zigzag structure. The trans conjugation lengths, n, in the polymer chain (C=C)(n) after the compression were estimated as ca. 29 or 50, when polymers obtained with methanol or triethylamine as the polymerization solvents were used. The trans sequence length obtained by thermal treatment of the cis-transoid polymer was also found to have less conjugated sequences than that of the compressed polymer. It is concluded, therefore, that the compression of the predominant cis-transoid PpNPA form gives longer conjugated trans sequences compared with that of the thermal isomerization of the pristine polymer.