In this contribution, alkyne metathesis of 1,4-dipropynylated benzenes 3 is reported. High-molecular weight poly(p-phenyleneethynylenes) 4 form from 3 under concomitant evolution of butyne. The highly active catalyst system employed in this polymerization reaction forms from commercially available Mo(CO)(6) and 4-chlorophenol or 4-trifluoromethylphenol in situ at 140 degrees C in off-the-shelf quality 1,2-dichlorobenzene. Introduction of dry nitrogen into the reaction vessel serves to remove the byproduct, 2-butyne. Several dipropynylated benzenes 3 carrying solubilizing alkyl (hexyl, nonyl, dodecyl, 2-(ethyl)hexyl, ethylbutyl, isopentyl, 3,7-dimethyloctyl, 2,5,5-trimethylhexyl, cyclohexyl, isopentyloxy) or alkoxy substituents were prepared starting from 1,4-dichlorobenzene (alkyl substituted) or hydroquinone (alkoxy substituted). Polymerization of the monomers 3 furnished the yellow and highly fluorescent poly(p-phenyleneethynylene)s 4 in excellent to quantitative yields and high purity. For soluble derivatives, the degree of polymerization (P-n) is observed in the range 1 x 10(2) less than or equal to P-n less than or equal to 13 x 10(2), while the polydispersities (M-w/M-n) appear in a bracket, 2 less than or equal to M-w/M-n, less than or equal to 4.5. Miscellaneous optimizations (time, temperature) were performed. Maximum molecular weights are obtained at reaction temperatures around 130-150 degrees C and reaction times of 25-30 h.