Pressure-Induced Cis to Trans Isomerization of Aromatic Polyacetylenes. 2. Poly((<i>o</i>-ethoxyphenyl)acetylene) Stereoregularly Polymerized Using a Rh Complex Catalyst

Abstract

Stereoregular polymerization of (o-ethoxyphenyl)acetylenes (OEPA) was performed using a [Rh(norbornadiene)Cl]2 catalyst in the presence of triethylamine to give poly((o-ethoxyphenyl)acetylene) (POEPA) bearing a cis-transoid form under mild conditions. The polymer underwent isomerization from the predominant cis form to the trans form when compressed at 200 kg/cm2 at room temperature. The polymers, obtained before and after the compression, were characterized in detail using laser Raman, diffuse reflective UV, ESR, and X-ray diffraction. The data suggested that the isomerization of the resulting polymer from the cis-transoid to the trans-transoid form was induced to give rise to the conjugated trans-planar zigzag structures, where unpaired electrons are stabilized as mobile electrons called solitons. The formation of the soliton radicals was confirmed by the motional narrowing of the line width observed in the ESR spectra of the compressed polymer. The trans conjugation length, n, (CC)n generated by the compression was estimated as n = ca. 22−45 using UV and laser Raman. The studies show that the compression of the cis-transoid POEPA form is the preferred method to isomerize it to the trans-transoid form. The compressed polymer contains a higher degree of conjugation compared with that of the thermal isomerization of the pristine polymer.