Copolymers of phenylacetylene (PA) and para-nitrophenylacetylene (pNPA), named poly(PA-co-pNPA), were obtained in different PA/pNPA ratios and different reaction conditions with Rh(l) catalysts. The structure of the copolymers was investigated with IR, laser Raman, H-1 NMR, electron spin resonance (ESR), and diffuse reflective ultraviolet-visible (DRUV) light spectroscopies. The pristine polymers had a cis-transoidal structure as the predominant conformation with some trans sequences. Detailed ESR studies supported by computer simulation and conformation analysis have suggested that the trans sequences were due to pNPA sequences and that the cis-C C bond sequences of pNPA were associated with a stabilized cis radical formed by four to five of pNPA monomers. This particular stabilization was probably the reason for the higher reactivity of pNPA as compared with PA. These cis sequences were preferentially cleaved to generate pi radicals. The compression and, to a minor extent, thermal treatment of poly(PA-co-pNPA) samples induced a cis-to-trans isomerization, leading to a trans-transoidal form with a planar zigzag structure and with a conjugation length up to n = 4 repeat units, determined by DRUV and Raman experiments. (C) 2004 Wiley Periodicals, Inc.