Reinvestigation of thermal isomerization of cis-stereoregulated poly(phenylacetylene) by spectroscopic study and DFT calculation

Abstract

Cis-stereoregulated poly(phenylacetylene) [poly(PA)] was synthesized by polymerization of PA using Rh catalysts, and the thermal isomerization behavior was analyzed by DSC, 1H NMR, IR and Raman spectroscopies. Cis-poly(PA) exhibited DSC exothermic peaks around 170 °C corresponding to isomerization of main chain from cis to trans. The cis content decreased to 12% after heating at 170 °C, and became almost zero after heating at 180 °C, which was determined by 1H NMR spectroscopy. Vibration frequencies of 8-mer models of cis- and trans-poly(PA) were calculated by DFT method to simulate IR and Raman spectra. DFT calculations indicated that an IR absorption peak at 737 cm−1 of poly(PA) originates from out-of-plane bending vibrations of C–H and C–Ph bonded to cis-main chain, and a peak at 692 cm−1 originates from out-of-plane deformation vibrations of C–H in phenyl moieties substituted with trans-main chain. These two IR absorption peaks were used to determine the geometrical structures of poly(PA) main chain. The present study confirmed that DFT calculations provide useful evidences for characterizing geometrical structures of poly(PA).