Pressure-induced cis to trans isomerization of poly((p-nitrophenyl)acetylene) prepared using Rh complex catalysts. Extension of pi conjugation length

Stereoregular polymerization of (p-nitrophenyl)acetylene (pNPA) was perform ed using [Rh(norbornadiene)Cl](2) and [Rh(cyclooctadiene)Cl](2) catalysts t o give poly((p-nitrophenyl)acetylene) (PpNPA), with a cis-transoid form und er mild conditions. The polymer underwent isomerization from the predominan t cis to trans form when compressed at 200 kg/cm(2) at room temperature. Th e polymers, obtained before and after compression, were characterized using IR, CP MAS C-13 NMR, ESR, and diffuse reflective UV methods. The data sugg est that the isomerization of the resulting polymer from cis-transoid to tr ans-transoid form gives a conjugated trans planar zigzag structure. The tra ns conjugation lengths, n, in the polymer chain (C=C)(n) after the compress ion 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-t ransoid PpNPA form gives longer conjugated trans sequences compared with th at of the thermal isomerization of the pristine polymer.