The presence of cis-vinylene bonds in Gilch-polymerized poly[2-methoxy-5(2 '-ethyl-hexyloxy)-1,4-phenylene vinylene] is reported. Through fractionation, species with a weight-average molecular weight of less than 37,000 exhibited an abnormal blueshift of photoluminescence spectra in toluene solutions, and this was attributed to the presence of cis-vinylene bonds, as verified by NMR spectroscopy. Surprisingly, the fractionated species (similar to 1 wt %) with a weight-average molecular weight of 5000 were mostly linked by the cis-vinylene bonds. The concentration decreased with the molecular weight until a molecular weight of 37,000 was reached; at that point, the polymer chains contained mainly trans-vinylene bonds. Obviously, the formation of cis-vinylene bonds strongly inhibited the growth of polymer chains during Gilch polymerization. (c) 2005 Wiley Periodicals, Inc.