With the aid of in-situ absorption spectroscopy we performed a comparative study of the precursor route poly(p-phenylene-vinylene) homopolymer and an acetoxy substituted copolymer derivative as a function of conversion temperature. It is shown that assuming a trans-cisoid conformation both polymers can be treated as arrays of trans-stilbene-like chromophores rendering a consistent description in the framework of molecular excitons. During conversion the intrachain geometry is retained and the energies of vibronic transitions are unchanged, while the respective intensities depend on the interchain distance. We find that the acetoxy substituent of the copolymer acts as interchain spacer supressing aggregation. (C) 2003 Elsevier Science B.V. All rights reserved.