Two series of constitutional isomers of monosubstituted ethynylbenzoates were synthesized. The series of the constitutional isomers were based on methyl ethynylbenzoates (M1-M3) and pentafluorophenyl ester ethynylbenzoates (M4-M6), respectively. Rhodium-based catalysts as well as a tungsten-based catalyst were used to polymerize the monomers M1-M6. While the tungsten catalyst could successfully polymerize all six monomers, the ortho-substituted active ester monomer M4 could not be polymerized with rhodium based catalysts. The polymers were obtained in variable molecular weights ranging from 7000 to 200 000 g mol(-1) with molecular weight distributions smaller than 3.6 (mostly M-w/M-n < 1.9). The configuration of the polymeric backbone was characterized by H-1 NMR and Raman spectroscopy and it was found that polymers synthesized using the rhodium catalyst possessed a cis-transoid structure, while polymers synthesized with tungsten based catalysts featured a trans-transoid sequence as their major structure. Further, post-polymerization modifications of the reactive polymers with various amines have been investigated and it was found that the polymers showed a different reactivity towards amines depending on the isomer, i.e. the position of the ester moiety at the phenyl ring. Moreover, the UV-vis spectra of the polymers before and after conversion with amines revealed a distinct shift of the UV-vis band maximum of up to 150 nm, accompanied by an apparent colour change indicating a pronounced effect of the side-group onto the conjugated polymer backbone.